TYPE Review
PUBLISHED 08 November 2022
DOI 10.3389/fgene.2022.1045955
Genomic approaches for
OPEN ACCESS improving grain zinc and iron
EDITED BY
Zeba I. Seraj, content in wheat
University of Dhaka, Bangladesh
REVIEWED BY
Chandan Roy1*, Sudhir Kumar2, Rakesh Deo Ranjan2
Zhengyu Wen, ,
KeyGene (USA), United States Sita Ram Kumhar1 and Velu Govindan3*
Imran Sheikh,
Eternal University, India 1Department of Genetics and Plant Breeding, Agriculture University, Jodhpur, Rajasthan, India,
2
*CORRESPONDENCE Department of Plant Breeding and Genetics, Bihar Agricultural University, Bhagalpur, Bihar, India,
3
Chandan Roy, International Maize and Wheat Improvement Center (CIMMYT), Mexico City, Mexico
chandan.roy43@gmail.com
Velu Govindan,
velu@cgiar.org
SPECIALTY SECTION More than three billion people worldwide suffer from iron deficiency associated
This article was submitted to Plant
Genomics, anemia and an equal number people suffer from zinc deficiency. These
a section of the journal conditions are more prevalent in Sub-Saharan Africa and South Asia. In
Frontiers in Genetics developing countries, children under the age of five with stunted growth
RECEIVED 16 September 2022 and pregnant or lactating women were found to be at high risk of zinc and
ACCEPTED 24 October 2022
PUBLISHED 08 November 2022 iron deficiencies. Biofortification, defined as breeding to develop varieties of
staple food crops whose grain contains higher levels of micronutrients such as
CITATION
Roy C, Kumar S, Ranjan RD, Kumhar SR iron and zinc, are one of the most promising, cost-effective and sustainable
and Govindan V (2022), Genomic ways to improve the health in resource-poor households, particularly in rural
approaches for improving grain zinc and
areas where families consume some part of what they grow. Biofortification
iron content in wheat.
Front. Genet. 13:1045955. through conventional breeding in wheat, particularly for grain zinc and iron,
doi: 10.3389/fgene.2022.1045955 have made significant contributions, transferring important genes and
COPYRIGHT quantitative trait loci (QTLs) from wild and related species into cultivated
© 2022 Roy, Kumar, Ranjan, Kumhar wheat. Nonetheless, the quantitative, genetically complex nature of iron and
and Govindan. This is an open-access
article distributed under the terms of the zinc levels in wheat grain limits progress through conventional breeding,
Creative Commons Attribution License making it difficult to attain genetic gain both for yield and grain mineral
(CC BY). The use, distribution or
reproduction in other forums is concentrations. Wheat biofortification can be achieved by enhancing mineral
permitted, provided the original uptake, source-to-sink translocation of minerals and their deposition into
author(s) and the copyright owner(s) are grains, and the bioavailability of the minerals. A number of QTLs with major
credited and that the original
publication in this journal is cited, in and minor effects for those traits have been detected in wheat; introducing the
accordance with accepted academic most effective into breeding lines will increase grain zinc and iron
practice. No use, distribution or
reproduction is permitted which does concentrations. New approaches to achieve this include marker assisted
not comply with these terms. selection and genomic selection. Faster breeding approaches need to be
combined to simultaneously increase grain mineral content and yield in
wheat breeding lines.
KEYWORDS
malnutrition, QTL mapping, GWAS-genome-wide association study, speed breeding,
new breeding techniques (NBTs), biofortification, genomic selection
Frontiers in Genetics 01 frontiersin.org
Roy et al. 10.3389/fgene.2022.1045955
Introduction brief/food-security-update). Increasing nutrient density and
bioavailability in staple food crops, as well as their production
Minerals are important components for physical and mental and distribution at affordable prices, can greatly improve the
health development in humans. Malnutrition from nutrition of low-income groups.
micronutrient deficiencies, also known as “hidden hunger,” is “Biofortification” refers to increasing essential mineral and
one of the most challenging health issues globally and vitamin content and bioavailability in edible parts of staple food
particularly in developing countries. Dietary deficiencies of crops, either through conventional breeding and/or
zinc (Zn), iron (Fe), iodine, and vitamin A are most prevalent biotechnological interventions, as well as through fertilizer.
among children and women. Worldwide, more than three billion “Golden rice” is a successful example of improving the beta-
people are affected by zinc (Zn), iron (Fe), and vitamin-A carotene content in rice through the transformation of three bio-
deficiencies (Raemaekers, 1988; Beal et al., 2017), with such synthetic pathway genes: phytoene synthase (psy), phytoene
nutrient deficiencies being particularly high among people in desaturase (crtI) and lycopene β-cyclase (lcy) (Ye et al., 2000).
Asia and Sub-Saharan Africa. Fe is an essential part of Biofortification in staple food crops has progressed through
hemoglobin and myoglobin and being directly involved in initiatives such as Harvest Plus, the Grand Challenge in
oxygen transport, enzymatic functions, energy production and Global Health, the India Biofortification Programme, Scaling
DNA synthesis. Fe deficiency causes anemia (https://www.who. Up Nutrition (SUN), and Global Alliance for Improved
int/health-topics/anaemia); children below 5 years of age (40%) Nutrition (GAIN), among others, and has gained global and
and women at reproductive or lactating stages (30%) are more local recognition. Numerous countries (Bangladesh, Brazil,
anemic. Likewise, Zn is a ubiquitous element for every living China, Colombia, India, Indonesia, Malawi, Nigeria, Pakistan,
organism including human beings, acting as a co-factor for more Panama, Rwanda, Uganda, and Zambia) have included
than 300 enzymes and proteins at the cellular and sub-cellular biofortification in their national health and development
levels during nucleic acid production, metabolism, cell division policies (Virk et al., 2021). Efforts also have been made to
and differentiation, and the immune system. Zn deficiency speed the seed production and distribution of biofortified crop
impairs physical growth and development and the proper varieties to reach resource-poor farmers in remote areas. The
functioning of the immune and reproductive systems and government of India has proposed the inclusion of fortified
mental acuity, as well as increasing child mortality. On wheat, rice, edible oil in mid-day meal programs, the public
average 20% of women and children are Zn deficient, with a distribution system and integrated child development program
high prevalence in low- and middle-income groups and even (https://www.livemint.com/Politics/91RAsPJFJLykDywTPHRCeI/
among adult men (Gupta et al., 2020b). Another study reported Fortified-MidDay-meals-by-December-2019-to-fight-malnutriti.
31.3% Zn deficiency among the children of higher income groups html).
in western Europe and no significant difference was observed Wheat provides 25% of calories in human diets worldwide
between children of the different socio-economic groups and 60% in Central and West Asian countries (Cakmak, 2008).
(Vreugdenhil et al., 2021). Food supplements and fortification The wheat endosperm is rich in starch but poor in minerals,
and more diverse diets can help to address micronutrient particularly Zn, Fe, and vitamins (Cakmak and Kutman, 2018).
malnutrition. Iron and zinc supplements, for example, have Moreover, minerals and bioactive components of the wheat grain
substantially reduced diarrhea and anaemia in affected are concentrated in the aleurone layer, which is removed during
segments of the population (Gupta et al., 2020b). Food milling, so the remaining flour generally contains only small
fortification for vitamin-A and iodine deficiency through amounts of minerals and vitamins. The average Zn concentration
pharmaceutical products and iodized salt have reduced related in the white flour is around 8 mg per kg, falling short of the daily
deficiencies, but such products may be unaffordable for low- recommended dietary allowance (RDA) of from 9–19 mg per day
income families, particularly in developing countries. Reduced for adults (Cakmak, 2007; Gillies et al., 2012; Cakmak and
consumption of vegetables and fruits is observed due to lower Kutman, 2018). The low mineral contents in rice and wheat is
purchasing power in many farm households in Asia and Sub- due to the deficiencies of these substances, particularly of Zn, in
Saharan Africa; now the COVID-19 outbreak substantially the soils of most of the areas where the crops are grown. Finally,
reduced the income of poor households that cut down on breeding for high yielding wheat genotypes since the mid-20th
their intake of fruits, vegetables, and animal-based foods and century has reduced genetic variation for grain Zn and Fe in
increased their dependence on staple grains (Heck et al., 2020). In modern cultivars from the levels found in landraces and
2022, restrictions in the movement of farm produce, disrupted genotypes released during Green Revolution (Debnath et al.,
supply systems, and continued economic fall-out due to the 2021).
COVID-19 pandemic and Russia-Ukraine conflict jeopardized Through agronomic and genetic interventions, mineral
food supply chains, including distribution to Central Asia and density can be enhanced (Cakmak, 2008). Genetic
Africa, and triggered food prices, augmenting global hunger and biofortification is the most sustainable and cost-effective
malnutrition (https://www.worldbank.org/en/topic/agriculture/ approach to alleviate micronutrient malnutrition among
Frontiers in Genetics 02 frontiersin.org
Roy et al. 10.3389/fgene.2022.1045955
lower-middle income consumers. Based on estimated average uptake due to lack of soil moisture. Foliar applications of ZnSO4
micronutrient requirements; average staple food crop intakes; @ 0.5% and FeSO4 @ 1%were more effective than applying to soil
mineral losses incurred during harvesting, processing, and alone or both soil and foliar applications (Ramzan et al., 2020),
storage; and micronutrient bioavailablility, a minimum target and foliar application during grain filling resulted in a more
levels have been set for different minerals in staple food crops effective translocation of zinc to the grain. Spraying zinc fertilizer
(Bouis et al., 2011). To meet out 30 and 40% of estimated average at the grain development stage improved grain zinc
requirements for Fe and Zn, respectively for children and adult concentration by 68% (Zhang et al., 2010). Unlike for zinc,
women, the target levels of 59 ppm for Fe and 38 ppm for Zn are iron fertilization of soils is not effective at increasing grain
set in wheat grain, considering the baseline of 30 ppm for Fe and iron concentrations (Gupta, 1991), but in several studies the
24 ppm for Zn. Large-scale screening of germplasm, foliar application of iron increased grain Fe concentration up to
identification of genotypes, mapping of underpinning genomic 28% (Zhang et al., 2010); 21% (Pahlavan-Rad and Pessarakli,
regions, and their use to develop high-yielding, disease resistant, 2009), and 14% (Aciksoz et al., 2011b). Application of
biofortified wheat genotypes are the major focuses of genetic N-fertilizers along with FeSO4 increased iron concentrations
biofortification research. Wild relatives carry higher levels of Zn in shoots as well as in the grain (Aciksoz et al., 2011b; Singh
and Fe in their grain than cultivated wheat genotypes. The locus et al., 2018). Previous reports showed that N application
Gpc-B1 for grain protein, Zn, and Fe was detected and fine increased the secretion of phytosederophore that chelate Fe in
mapped in Triticum durum ssp dicoccoides and can serve as a the soil and Yellow Stripe 1 protein act as Fe transporter (Aciksoz
source to enhance these traits in cultivated wheat (Uauy et al., et al., 2011a). Seed priming is another way to deliver minerals to
2006). Genome sequence information of T. aestivum cv. Chinese plants. Zinc also improves coleoptile and radical growth and
Spring (Mayer et al., 2014) and a highly annotated chromosome germination in wheat (Ozturk et al., 2006). The recent practice of
level genome sequence (Ref Seqv1.0; Appels et al., 2018) can be applying nano particles (NP) to the soil is becoming popular as a
used for gene discovery, cloning, and functional analysis of cost effective, environmentally-friendly approach to reduce
genome targeting to grain Zn and Fe. In this article we review fertilizer losses and raise productivity and profitability.
the status of wheat biofortification and how genomic resources Applying ZnO-NP and Fe-NP in wheat enhanced grain yield,
can be used in breeding for biofortified wheat genotypes. biomass, chlorophyll content, and drought tolerance (Dimkpa
et al., 2020). Nano zinc and iron treatments significantly
increased Zn and Fe concentration in the roots, shoots, and
Agronomic vs. genetic biofortification grain (Rizwan et al., 2018; Dimkpa et al., 2020). Applying ZnO-
NP or Fe-NP alone or coated with NPK can reduce farmers’ costs
Initial research on agronomic biofortification to increase and, in case of zinc, it is cheaper than ZnSO4.
grain Zn in wheat took place in Turkey (Cakmak et al., 1996; Whereas, foliar and soil applications of Zn and Fe must be
Cakmak et al., 1999). Mineral density can be modulated by soil repeated in each crop cycle, genetic biofortification provides a long-
Zn and Fe status and their availability for the crop. Soil zinc and term solution against micronutrient malnutrition. Genetic
iron availability depends on edaphic factors such as soil physical biofortification is complex, involving several physiological
and chemical properties, micronutrient availability, pH, the pathways and proteins right from soil uptake of minerals to their
status of soil moisture and organic matter, grain filling accumulation in the grain. Targeting the genes or proteins that play
duration, and the timing of senescence (Cakmak and Kutman, key roles inmineral sequestrationmay increase grain Zn and Fe. The
2018). Irrespective of this, wheat grain zinc and iron two major approaches that may be used are selective plant breeding
concentrations can be increased by applying these elements and transgenic breeding, to develop biofortified crop genotypes.
directly to wheat (Yilmaz et al., 1997). Zn and Fe can be Transgenic approaches and outputs are subject to stringent policies
applied in inorganic or organic forms but the most commonly in many countries and their use is restricted or prohibited, so this
used are ZnSO4 and FeSO4, due to their ready availability and low paper focuses on selective breeding. Mainstreaming biofortification
cost. Soil and foliar applications of zinc fertilizer increase grain as a selection priority in breeding programs—along with yield,
Zn density in wheat and can positively affect agronomic disease resistance, end-use preferences, and environmental
parameters such as the number of tillers, thousand kernel adaption—is paramount, and includes the use of micronutrient
weight, chlorophyll content, and biological yield (Rizwan dense parental lines and setting grain micronutrient target levels.
et al., 2018; Dimkpa et al., 2020). Hussain et al. (2012b) The review consists of three major sections: the first section deals
reported that soil applications of Zn increased whole grain with physiological and molecular mechanism of uptake,
zinc concentrations up to 95% and grain yield by 29%. translocation and grain sequestration of Zn and Fe; second
However, foliar application was found to be more effective section deals with breeding for grain zinc and iron including
than soil application to increase zinc and iron concentrations mapping of gene(s) and QTLs; and the third section deals with
in wheat. Foliar application is also advantageous under stress new breeding techniques which need to be adopted with
conditions, particularly drought, to avoid impairment of mineral conventional breeding.
Frontiers in Genetics 03 frontiersin.org
Roy et al. 10.3389/fgene.2022.1045955
Uptake of soil zinc and iron through TaZIFL5, TaZIFL6.1, and TaZIFL6.2, were upregulated in
roots roots under zinc and Fe deficient conditions (Sharma et al.,
2019).
Complex physiological and metabolic processes are Other thanMA, nicotianamine is produced by roots that also
associated with the uptake and accumulation of Zn and Fe help in chelating soil Fe and Zn. NAS increases the production of
into the grain. Broadly, there are three kinds of mineral NA and DMA, which improves the uptake and translocation of
uptake mechanisms: 1) direct uptake of Zn2+ and Fe2+ chelated metals. Overexpression of rice nicotianamine synthase
molecules via the root system with the help of ZRT-IRT like genes, the OsNAS2 gene in wheat, increases the uptake and
proteins; 2) where Zn2+ and Fe3+ molecules are chelated first and translocation of grain zinc in shoots as well as the grain
then taken up through roots; and 3) a combination of 1 and 2. (Singh et al., 2017).
Cereals and millets (rice, maize, wheat, barley, pearl millet,
among others) use mechanism 2 to extract Fe and Zn from
the rhizosphere into the root system, where secretion of Transport of zinc and iron
phytosiderophores (PS), proteins of the Mugineic acid (MA)
family, help in chelating Fe and Zn and form a complex of Fe3+- A complex mechanism of metal transport, chelation and
MA or Zn2+-MA (Marschner and Romheld, 1994; Nozoye et al., sequestration helps plants to avoid metal toxicity. Plants
2015). The role of MA is well understood in Fe uptake and produce several transporter proteins varying in their
transport in rice and barley, but also showed its affinity towards substrate, expression, and locations. Colangelo and Guerinot
molecules like Zn and Cu. In zinc-deficient wheat plants, more (2006) provide a detailed discussion of the P1B-ATPase family,
MA was released (Cakmak et al., 1994). Methionine acts as a the CDF family as metal efflux proteins, ZIP, yellow stripe-1
precursor of MA synthesis, which is converted to S-adenosyl-L- (YSL), the natural resistance associated macrophage protein
methionine (SAM) with the help of the enzyme SAM synthetase (NRAMP), and the copper uptake proteins (COPT) as metal
(Mori and Nishizawa, 1987). In a subsequent reaction, SAM is uptake proteins. Heavy metal transporting P type ATPase
converted to nicotianamine (NA) by NA synthase (NAS) (HMA), a protein of the P1B-ATPase family, act as a Zn, Cd,
followed by 3″-keto acid and 2′-deoxymugineic acid (DMA) and Pb transporter. Arabidopsis mutants of hma2 and hma4
by NA aminotransferase (NAAT) and DMA synthase (DMAS), showed lower Zn uptake than the wild type (Hussain et al.,
respectively (Shojima et al., 1990). In some species, DMA is 2004). HMA helps Zn to move from roots into shoots and plays
further converted to MA. In graminaceous crops MA is a role in xylem loading and unloading. In wheat, 32 HMA
commonly produced, whereas NA is most common in non- protein-producing genes were detected on chromosomes 2A,
graminaceous crops. In wheat, MA producing genes such as 2B, 2D, 4A, 4D, 5A, 5B, 5D, 6A, 6B, 6D, 7A, 7B, and 7D (Zhou
TaNAS, TaNAAT, and TaDMAS were identified (Pearce et al., et al., 2019). Overexpression of the TaHMA2 gene in wheat
2014). MA secreted in the root zone chelates iron along with zinc, increased the Zn concentration in the shoot as well as in the
manganese (Mn), and copper (Cu). The complexes of Fe3+-MA grain; however, grain Zn concentration was limited to the
or Zn2+-MA are taken up by roots. This mechanism is well embryo and aleurone layer (Tan et al., 2013). Similarly, metal
established for Fe uptake in rice, barley and maize, but little is tolerance protein (MTP) is another transmembrane metal
known about the genetic mechanism of metal uptake and its transporter protein of the CDF family located in the vacuole
transport in wheat. Using the knowledge of model crop species, it membrane of roots, shoots, and leaves and helps in Zn transport
is possible to identify the candidate genes in wheat of potential into the vacuole. Knocking out the mtp1 gene in Arabidopsis
use for breeding. reduced Zn accumulation in various plant tissues (Desbrosses-
The gene family “transfer of MAs” (TOM) plays important Fonrouge et al., 2005). NRAMP is another protein located in the
role in the uptake of soil iron and its translocation. TOM1 was vacuole membrane and which takes active part in Fe transport.
first identified in rice and barley as facilitating MA secretions Eight homologues of TaNRAMP genes have been identified in
from roots to soil (Nozoye et al., 2011) and their transport. TOM2 wheat (Borrill et al., 2014). Other genes known as vacuolar iron
and TOM3 are homologous to TOM1 and transport PS into the transporter (VITs) genes help transport of iron into the grain
plant. Over expression of TOM2 using a GUS-promoter in rice and offer a potential target for iron biofortification. Wheat has
allowed scientists to locate it in roots, shoots and seeds, with the two functional VIT genes, TaVIT1 and TaVIT2 on
highest expression in basal plant tissues (Nozoye et al., 2015). chromosomes 2 and 5, each with three homeologs from the
Several zinc-induced facilitator (ZIFL) genes have been identified A, B, and D genome. Overexpression of TaVIT2 increased grain
and their role in zinc homeostasis assessed. ZIFL acts as an efflux iron 2-fold (Connorton et al., 2017).
transporter of vacuolar NA. In rice, ZIFL transports MA Several proteins belong to the yellow stripe (YSL) family, a
synthesized in the roots. In wheat, 15 TaZIFL proteins complex of transporter proteins that help in the uptake of
distributed on chromosomes 3, 4, and 5 have been chelated metals in the soil into roots and their subsequent
characterized; genes like TaZIFL2.3, TaZIFL4.1, TaZIFL4.2, transport to stems, leaves, and grain. First detected in maize,
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Roy et al. 10.3389/fgene.2022.1045955
TABLE 1 Summary of genes involved in uptake, transport and grain accumulation of Zn and Fe in wheat.
Activity Genes Plant organs References
Minerals uptake NAAT2-D, DMAS1-B, TOM, ZIP1, ZIP3, ZIP6, ZIP7, ZIP9, Roots Ajeesh-Krishna et al. (2020); Gupta et al. (2020a); Kumar et al.
ZIP13, TaVTL1, TaVTL2, TaVTL5, TaZIFL2.3, TaZIFL4.1, (2018); Sharma et al. (2019); Sharma et al. (2020)
TaZIFL4.2, TaZIFL5, TaZIFL6.1 and TaZIFL6.2, TaYS1A,
TaYS1B, TaYSL3, TaYSL5, and TaYSL6, TaNRAMP5
Mineral transport ZIP1, ZIP3, ZIP6, ZIP7, ZIP9, ZIP10, ZIP13, ZIP15, TaHMA2, Stem, leaves Ajeesh-Krishna et al. (2020); Connorton et al. (2017); Kumar
TaNRAMP3, TaNRAMP5, TaCNR2 (Cell number regulator 2), et al. (2018); Peng et al. (2018); Qiao et al. (2019); Tan et al.
TaYSL1A, TaYSL1B, TaYSL5, TaYSL12, and TaYSL19, (2013)
TaVIT1 and TaVIT2
Mineral TaFer1, TaFer2, TaMTP1-8A, TaNRAMP3, ZIP1, ZIP3, ZIP7, Spikletes, grains, Ajeesh Krishna et al. (2020); Borg et al. (2012); Vatansever
accumulation into ZIP10, ZIP15 aleurone layer et al. (2017)
grain
Italic values indicates the gene name
YSL-1 is involved in the uptake of metals that form complexes loading and mobility. In rice, grain Zn concentrations depend
with PS or NA and its role in Fe3+-MA transport has been well on plants’ ability to redistribute Zn from older leaves and stems,
established (Curie et al., 2001). Rice genesOsYSL15 express in the as well as phloemmobilization of Zn (Wu et al., 2010). Significant
seed and are involved in seed germination, whileOsYSL2 plays an contributions of the flag and penultimate leaves to wheat grain
important role in phloem transport and transport into the seed yield have been observed (Roy et al., 2021a), but there are no
(Nozoye et al., 2015). Likewise, zinc-induced transporter family detailed studies concerning the proportionate remobilization of
(ZIP) proteins are involved in metal uptake and transport. In Zn and Fe from different wheat plant parts. Uauy et al. (2006)
wheat, 14 and 19 members of YSL and ZIP gene families have observed an abundance of Zn, Fe and proteins in wheat flag
been characterized in up and down regulation during senescence leaves during grain filling, in plants carrying the Gpc-B1 locus.
and which play key roles in metal transport from the cytoplasm Micro elemental analysis revealed higher concentrations of
to the phloem and the phloem to the grain (Pearce et al., 2014). Zn, Fe and other bioactive elements in the embryo and grain
Several YSL, ZIP, and NRAMP homologs are involved in the aleurone layer; achieving localized increases of mineral
uptake, transport, and remobilization of Fe and Zn in wheat concentrations in the endosperm is challenging. To better
(Table 1). Overexpression of the specific transporter protein understand mineral translocation and deposition in the grain,
using a tissue-specific promoter could be a target area for experiments in model crop plants identified key genes. Ferritin is
research to increase the grain mineral concentrations. a Fe storage protein located in the plastid and readily bio-
available. Enhancing FERRITIN gene expression is important
for Fe-biofortification. Genetically transforming rice using the
Increase Zn and Fe in wheat soybean FERRITIN gene under the seed-specific promoter gene
endosperm Glu-B1 (Goto et al., 1999) and the pea FERRITIN gene under the
Gt-1 promoter (Lucca et al., 1999) increased endosperm iron
Minerals deposition in grain occurs through the direct uptake levels. The soybean FERRITIN gene under the control of the
of Zn/Fe by roots at grain filling stage, or remobilization of stored maize ubiquitin promoter was expressed in wheat, resulting in
Zn/Fe from leaves and stems. Studies in rice to understand the higher iron concentrations in both plant tissue and the grain, but
contribution through continued root uptake and remobilization with much higher levels in the former (Drakakaki et al., 2000).
of stored minerals into the grain observed that minerals are Wheat carries two ferritin genes, TaFer1 and TaFer2, located in
accumulated via both processes. Under conditions of adequate chromosomes 4 and 5 and each with three homoalleles in
minerals in the soil, continued root uptake plays a major role in hexaploid wheat. Overexpression of the endogenous TaFer1-A
Zn/Fe deposition into the grain while, under mineral deficient gene targeting the endosperm raised wheat grain Fe levels by
conditions, remobilization of stored Zn/Fe from leaves and stems 50%–58% (Borg et al., 2012).
contributes more to rice grain mineral content (Sperotto, 2013). The endogenous vacuolar transporter (TaVIT2) gene
A similar result was observed for grain Zn accumulation in using a promoter of the GLU-1D-1 gene achieved a more
winter wheat by Liu et al. (2019a), also reported a critical than 2-fold increase in Fe concentrations in white flour
value for soil Zn availability (7.15 mg kg−1 DTPA Zn (Connorton et al., 2017). In another attempt, the
concentration) for wheat, above which the direct uptake of constitutive expression in bread wheat of the rice OsNAS
soil Zn during grain filling is predominant and below which gene that produces chelators such as nicotianamine (NA)
remobilization is the major source of Zn in the grain. and 2′-deoxymugineic acid (DMA) improved grain Fe and
Remobilization of Zn/Fe depends on the xylem to phloem Zn concentrations (Beasley et al., 2019).
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Roy et al. 10.3389/fgene.2022.1045955
Effective genetic biofortification approaches for wheat should bioavailability by 7%. In comparison, the PA content in T.
focus on increasing mineral uptake from the soil and monococcum, T. turgidum and T. aestivum was less in
translocation and remobilization into grains, combining genes diploids than in tetraploids or hexaploids (Bilgrami et al.,
for higher metal uptake and translocation (NAS, NAAT, DMAS, 2018). Seasonal variation of PA was also observed a
TOM, YSL, ZIP etc.) and genes that affect targeted transport to significant difference in PA between tetraploids and
the endosperm (VIT, FERITTIN etc.). Singh et al. (2017) hexaploids was recorded in the fall but was non-significant in
developed wheat transgenic lines using the rice OsNAS2 and the spring. Likewise, the PA: Zn molar ratio was less in T.
bean PvFERRITIN genes separately and combining both genes monococcum (2.15), than T. turgidum (2.66) and T. aestivum
and observed increased grain Zn and Fe concentrations in all (3.15) (Bilgrami et al., 2018). Understanding the physiological
three cases. The OsNAS2 gene was most effective; a grain of and metabolic pathways their underlying genes related to PA
related transgenic wheat lines contained 93.1 ppm of Fe and biosynthesis allow researchers to mitigate/reduce/control PA
140.6 ppm of Zn (Singh et al., 2017) and there was a more than 2- activity in grains. Phytic acid is produced during grain
fold increase in grain Fe, compared to a 1.6-fold increase with the development and abscisic acid (ABA) and gibberellic acid
combined expression of both genes. This differs from the case for (GA) regulates its accumulation during seed maturation. PA is
rice, where the combined expression of OsNAS2 and FERRITIN synthesized from glucose-6-phosphate through a series of
gave a 6-fold increase in grain Fe content (Trijatmiko et al., phosphorylation reactions by several inositol phosphate
2016), suggesting that the two genes may not be synergistic in kinases (IPK) proteins. Genes such as TaIMP, TaITPK1-4,
wheat and the overexpression of endogenous genes in wheat TaPLC1, TaIPK1, TaIPK2 were identified as being involved in
could be an alternative for increasing endospermic mineral PA biosynthesis in wheat (Aggarwal et al., 2015). A protein,
expression. However, the very low efficiency of transformation TaABCC13, acts as a PA transporter and showed a pleiotropic
in wheat, compared with crops like rice and barley, and of effect on seed germination and root development (Bhati et al.,
transgene expression in hexaploid wheat needs to be considered. 2016). Improving mineral bio-availability is possible by reducing
PA expression in grain and also can be achieved through over-
expression of the phytase enzyme, which degrades phytic acid
Increasing bioavailability of minerals and release minerals.
Approaches such as mutagenic treatment and transgenic
Minerals in cereal and legume grains are less bioavailable. development were adopted to develop low phytic acid mutant
The presence of phytic acid (PA), a phosphorus (P) storage lines of maize, rice, barley and soybean. Maize low phytic acid
protein in seed representing 65%–85% of seed P, acts as a chelator mutants can be grouped into lpa1, lpa2, and lpa3 mutants; lpa1
for cations of Ca2+, Mg2+, Zn2+, Mn2+, and Fe3+ and reduces their affects transporter proteins that packages PA into vacuoles; lpa2
absorption in the intestine (Bhati et al., 2016). Thus, the impairs the production of the inositol phosphate kinase enzyme
bioavailability of grain Zn and Fe depends on proportionate (IPK1) that phosphorylates inositol-5-phosphate to PA and lpa3
phytic acid content in the grain or diet. The molar ratio of impairs myo-inositol kinase that phosphorylates myo-inositol to
phytate: Zn/Fe can be used as a determinant of mineral inositol monophosphate (Singh et al., 2020). Plants with lpa
bioavailability; an increase in the molar ratio indicates lower mutations produced seeds with normal levels of phosphorus but
adsorption of Zn and Fe. Morris and Ellis (1989) determined greatly reduced PA bound phosphorus. Low phytic acid mutants
critical ratios of phytate: Fe > 1 (Hallberg et al., 1989) and of wheat showed a two-fold increase in grain Zn and Fe
phytate: Zn > 15. Genotypes with lower phytic acid lines of PA: concentrations (Guttieri et al., 2004; Kenzhebayeva et al.,
Zn < 0.4 and PA: Fe < 5 or 5–10 are preferable for biofortification 2019). In mutant lines, the distribution of grain P was altered,
(Gupta et al., 2022). Assessment of phytic acid in wheat increasing the P content in the central endosperm and lowering it
genotypes showed higher genetic variability. Average PA in in the aleurone layer. But negative pleiotropic effects in lpa
wheat grain ranges from 7.1 to 11.1 mg g−1, giving a molar mutant line Js-12-LPA included reduced yield and height and
ratio for PA: Zn from 24 to 41, in a set of 65 wheat weak straw, thus limiting the promise of lpa mutation for
genotypes from Pakistan (Hussain et al., 2012a). Another breeding (Guttieri et al., 2004).
study involving 42 durum wheat genotypes showed PA Increasing grain phytase activity can readily increase nutrient
variation from 0.462 to 0.952% and ranges for molar ratios of bioavailablility. Endogenous phytase is produced during seed
PA: Zn and PA: Fe of 16.9–23.6 and 12.1–29.6, respectively germination and releases the P from the PA, but in dry seed and
(Magallanes-Lopez et al., 2017). Wen et al. (2022) evaluated flour, digestive tract phytase activity is very low or absent.
330 wheat genotypes from CIMMYT, Mexico and reported Overexpression of the Aspergillus niger phytase gene (phyA)
0.9%–1.72% range in PA content. Genome wide association resulted in a 4-fold increase in phytase activity in wheat
studies revealed six stable genomic regions and the effect of (Brinch-Pedersen et al., 2000). Detailed studies about
four of the six region could reduce PA content from 1.21% to increasing phytase activity in wheat are not available.
1.13% which could potentially increase grain Zn and Fe Orthologous genetic information from other related species
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Roy et al. 10.3389/fgene.2022.1045955
can be used to identify the gene families involved in PA et al., 2021), indicating improvement in micronutrient levels of
biosynthesis and degradation and facilitate their use to one will lead to improvement in the other. This may be due to
develop wheat cultivars whose grain features low levels of commonly associated proteins and enzymes for Zn and Fe uptake
phytic acid. and translocation to the grains. However, studies have also found
a negative association of grain Zn and Fe with grain yield
(Morgounov et al., 2007; Narendra et al., 2021). Velu et al.
Breeding for improved grain zinc and (2016a) reported a negative association of grain yield with
iron content in wheat grain Zn but no association with grain Fe content.
Simultaneous improvement of both traits is challenging. The
Genetic analysis of grain zinc and iron in significant negative association of grain yield with mineral
wheat concentrations could be due to a dilution effect, where high-
yielding genotypes are contributing less grain Zn and Fe than
Understanding the nature of gene action and inheritance for photosynthates to the grain. In a study comparing modern wheat
micronutrient accumulation in the wheat grain is a prerequisite and rice cultivars with the cultivars released 50 years earlier, it
for improving the trait. Genetic analysis revealed the quantitative was found that modern cultivars have less capacity to sequester
nature of inheritance for grain Zn and Fe contents, making Zn and Fe in the grain than earlier cultivars (Debnath et al.,
improvement through conventional breeding is slow. Few 2021). A study by Hao et al. (2021) found that wheat landraces
studies exist regarding the nature of gene action for grain carry higher grain Zn contents than cultivars and that landraces
micronutrient content in wheat. One of the studies reported accumulated more grain Zn under foliar applications of the
the predominance of additive gene action for grain Fe and mineral.
dominance and duplicate epistasis for grain Zn in bread With intensive breeding efforts, high-yielding, mineral dense
wheat (Amiri et al., 2020). Holasoua et al. (2021) reported wheat genotypes can be developed. An example is BARI Gom 33,
additive gene action as being significant for both grain Zn and developed by CIMMYT and released for commercial cultivation
Fe. Higher heritability is paramount for increased genetic gains in Bangladesh and which offers a 7–8 ppm Zn advantage (Velu
through selection. High heritability for grain Fe and moderately et al., 2019), as well as resistance to wheat blast (Roy et al., 2021b).
low heritability for grain Zn were reported by Amiri et al. (2020). Identification of high-yielding wheat genotypes with higher root
Moderate-to-high heritability was detected in other studies (Velu uptakes or with a higher capacity to remobilize storedminerals or
et al., 2016a; Narendra et al., 2021). Low heritability for grain Zn both is important. Grain Fe was improved with the supply of N in
and Fe content was also reported by Joshi et al. (2010). The soil and foliar application (Aciksoz et al., 2011b), indicating the
existence of moderate-to-low heritability and the predominance presence of useful genes for a higher uptake of N and Fe. In
of dominance and duplicate epistasis suggests advanced addition, a significant positive association of grain protein
generation selection for the traits. High genotype × content with grain Zn and Fe has been reported (Morgounov
environment (G × E) interaction for grain Zn and Fe was et al., 2007). The locus TaNAM-B1 was detected to correspond
reported under multilocation testing (Joshi et al., 2010; Velu higher levels of grain protein, Zn, and Fe (Uauy et al., 2006).
et al., 2016a). Numerous factors affect grain Zn and Fe content,
including soil nutrient availability (Alloway, 2009), soil moisture
and organic matter (Cakmak, 2008; Pal et al., 2009), pH (Kirk Use of wild relatives in genetic
and Bajita, 1995; Rupa and Tomar, 1999), tillage (Stipesevic et al., improvement of grain Zn/Fe
2009), and soil N availability (Singh et al., 2018). Wheat grown
under stress conditions showed higher grain Zn and Fe Interestingly, wild species, landraces and synthetic hexaploid
concentrations than crops grown under optimum conditions. wheats (SHW) are probable sources of grain iron and zinc in
Elevated temperatures and drought stress at post-anthesis wheat. Monasterio and Graham (2000) evaluated around
increased grain Zn concentrations in wheat (Velu et al., 3000 CIMMYT genotypes for grain Zn and Fe concentrations.
2016a; Narendra et al., 2021). This may be a result of the T. tauschii genotypes had average and maximum iron
production of smaller grains and an increased aleurone: concentrations of 76 and 99 ppm, with an average and
endosperm ratio under stress, particularly given that the maximum zinc concentrations of 50 and 68.9 ppm,
overall Zn and Fe yield per unit area was higher in non- respectively. T. monococcum had maximum iron and zinc
stressed environments (Velu et al., 2016a). For iron, there was levels of 70 and 131 ppm, respectively. T. dicoccoides and T.
no significant change in grain concentrations between genotypes dicoccon showed zinc levels of 142 and 135 ppm. A core
grown under stressed or optimum conditions (Narendra et al., collection of Asian bread and durum wheat in the CIMMYT
2021). gene bank showed grain Zn levels ranging from 16.85 to
A significant association between grain Zn and Fe contents 60.77 ppm and, for grain Fe, from 26.26 to 68.78 ppm (Velu
has been observed (Velu et al., 2016a; Velu et al., 2019; Narendra et al., 2011). Ninety diploid and tetraploid wheat progenitors
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Roy et al. 10.3389/fgene.2022.1045955
were screened for grain Zn and Fe content, few accessions of D yellow rust and powdery mildew, as well as providing wide
and S genome species including Aegilops tauschii (D), Ae. adaptation, enhanced yield, and abiotic stress tolerance
kotschyi (US), Ae. speltoides (S), Ae. longissima (S) and Ae. (Kumar et al., 2003). Likewise, the 2NS segment from Aegilops
bicornis (S) were found promising (Chhuneja et al., 2006). ventricosa is associated with cyst nematode resistance
Grain Zn level up to 115.4 ppm (Syn43 [T. durum (Yuk) · Ae. (Williamson et al., 2013), lodging resistance (Singh et al.,
tauschii (864)], 90.4 ppm (Ae. tauschii acc. 14,129) and grain Fe 2019), and resistance to wheat blast, one of the devastating
of 109.4 ppm (Ae. tauschii acc. 14,102) and 104.4 ppm (Ae. diseases of wheat (Roy et al., 2021b; Singh et al., 2021; Phuke
kotschyi acc. 3,573) were recorded. Another study reported et al., 2022). A study using Ae. longissima × T. turgidum ssp
non-progenitor wild species with S, U and M genome durum crosses showed that amphidiploids received genes from
carrying 3–4 times higher grain Zn and Fe than the bread and Ae. longissima for higher grain Zn and Fe contents than their
durum wheat (Rawat et al., 2009b). Tetraploids showed higher durum parents (Tiwari et al., 2008). Considering the importance
concentrations of grain Zn and Fe than hexaploids, with ranges of wild relatives as a source of important genes for cultivated
for Zn of 45–177 ppm in T. boeoticum, 20–159 in T. dococcoides, wheat, CIMMYT started production of SHW during the 1980s
29–89 ppm in T. monococcum and, for grain Fe, 41–92 ppm in T. and many of these synthetics provided enhanced resilience
boeoticum, 28–78 ppm in T. dococcoides, and 34–85 ppm in T. against biotic and abiotic stresses (Mujeeb-Kazi, 1995). SHW
monococcum. This compared to 15–61 ppm for grain Zn and developed from crossed of T. turgidum ssp turgidum x T.
24–51 ppm for grain Fe in T. aestivum and 18–50 ppm for grain tauschii, T. turgidum ssp durum x T. tauschii, and T.
Zn and 10–50 ppm for grain Fe in T. durum (Cakmak et al., turgidum ssp dicoccoides x T. tauschii produced higher grain
2000). Other studies have also reported the high genetic potential Zn and Fe than parental accessions (Zhao et al., 2017). SHW
for grain Zn and Fe contents in diploid and tetraploid wild developed by CIMMYT using T. spelta, T. turgidum var
species such as T. monococcum, T. turgidum ssp dicoccoides, T. diccocoides, and Ae. squarosa were found to be promising for
boeoticum, Aegilops tauschii and landraces (Cakmak, 2007; Velu grain mineral concentration (Calderini and Ortiz-Monasterio,
et al., 2014). Grain Zn in T. turgidum ssp dicoccoides varied from 2003; Crespo-Herrera et al., 2016). Calderini and Ortiz-
69 to 139 ppm; Fe concentrations from 44 to 88 ppm and protein Monasterio (2003) reported SHW carrying 25%–30% higher
from 164 to 382 g/kg (Peleg et al., 2007). A few accessions of T. grain Zn, Fe and manganese (Mn). Using SHW, several novel
turgidum ssp dicoccoides showed both high grain Zn and Fe genes responsible for high grain Zn and Fe have been transferred
concentrations. In most cases, the cultivated wheat genotypes into CIMMYT elite wheat breeding lines; some were tested in
showed lower levels of grain Zn and Fe. A range of grain Zn and Bangladesh, India, and Pakistan and released for commercial
Fe was 21–35 ppm and 22–34 ppm, respectively (Tang et al., cultivation (Khokhar et al., 2018; Velu et al., 2018).
2008), 26–40 ppm and 35–56 ppm, respectively (Peterson et al.,
1986), 29–46 ppm and 34–66 ppm, respectively (Ficco et al.,
2009), 8–12 ppm and 29–38 ppm, respectively (Cakmak, 2000) Effect of GPC-B1 locus on GZn and GFe in
were reported in the cultivated wheat genotypes. wheat
Genomic regions associated with higher micronutrient
density can be transferred from wild relatives to cultivated Attempts to detect the genomic regions associated with
varieties. Wheat secondary and tertiary gene pools can be variation for grain protein, zinc and iron in cultivated and
used to broaden the genetic base for grain mineral density wild relatives of wheat have identified the relevant
through pre-breeding or the production of synthetic hexaploid chromosome locations. For grain protein content, the locus
wheat useful for the breeders. Chromosome addition lines from GPC-B1 was mapped on chromosome 6B (DIC-6B) in the
Ae. peregrina, Ae. longissima and Ae. umbellulata have been durum wheat genotype LND, a D genome disomic
found to carry genes for high Zn and Fe in the grain and roots substitution line of durum wheat variety LANGDON (LND)
and control the release of high levels of mugineic acid in the root from T. turgidum var dicoccoides (accession FA-15-3) (Joppa
zone (Neelam et al., 2012). Synthetic amphidiploids produced et al., 1997). The DIC-6B allele encodes a protein specific to the
from T. aestivum (AABBDD) and Ae. kotschyi (UUS1S1) carrying NAC transcription factor family and was found to be similar to
grain Zn and Fe more than double of the hexaploid parents the ArabidopsisNo Apical Meristem (NAM) protein; therefore, it
(Rawat et al., 2009a). has been named NAM-B1 (Uauy et al., 2006). The NAC
The addition of rye chromosomes 1R and 7R improved the transcription factor is associated with auxin signaling,
Zn efficiency of wheat cultivars. Increased zinc concentrations responses under biotic and abiotic stress, and early leaf
and contents were found in the shoots of chromosome addition senescence. This pleiotropic gene also increases grain protein,
lines (Cakmak et al., 1997). One of the most significant Zn, and Fe and triggers early senescence in the plant. Cultivated
achievements in wheat breeding is the addition to hexaploid wheat carries a non-functional allele of the same gene with a 1 bp
wheat of the 1BL:1RS rye chromosome segment, which has insertion of thymine base at position 11 that caused a frameshift
proved an invaluable source of resistance to stem, leaf, and mutation (Uauy et al., 2006). The presence of the DIC-6B locus
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Roy et al. 10.3389/fgene.2022.1045955
increased durumwheat grain Zn, Fe, andMn levels by 12, 18, and higher levels of genetic variation. One early study has reported the
29%, as well as resulting in a 38% higher protein accumulation GPC-B1 locus, which has been cloned and sequenced (Uauy et al.,
than occurs in durum wheat without the locus (Distelfeld et al., 2006). Peleg et al. (2009) used recombinant inbred lines (RILs)
2007). Discovery of high-throughput, tightly-linked markers for developed from T. durum (cv. Langdon) × wild emmer wheat
GPC-B1 facilitated the marker-assisted transfer into the (accession #G18-16) and identified 82 QTLs for different minerals,
cultivated wheat. Through marker assisted breeding, the GPC- six of which were associated with grain Zn and located on
B1 locus was transferred into the Indian wheat variety HUW 468; chromosomes 2A, 5A, 6B, 7A, 7B, and 11 of which linked to
some derivative lines showed significantly higher levels of grain grain Fe were found on chromosomes 2A, 2B, 3A, 3B, 4B, 5A, 6A,
Zn, Fe, and protein (Vishwakarma et al., 2014). Use of the DIC 6B, 7A, and 7B. Using interspecific crosses between Triticum
allele was a breakthrough for quality improvement in wheat boeoticum (pau5088) × Triticum monococcum (pau14087),
worldwide. Wheat varieties with more than 14% GPC using the 2 QTLs for grain Fe were detected on chromosomes 2A and
DIC locus were developed in Canada (https://www.grainscanada. 7A and 1 QTL for grain Zn on 7A (Tiwari et al., 2009). Genes for
gc.ca/en/grain-research/scientific-reports/pdf/canadian-wheat.pdf) enhanced grain Zn and Fe content were observed on chromosome
and Australia (https://seedworld.com/australian-researchers- 2S and 7U of Ae. kotschyi and substitution of 2S and 7U for the
develop-high-protein-wheat/). Cultivars “Lassik” and “Farnum” homoeologous A genome increased up to 117.4% grain Fe and
in hexaploid wheat and “Westmore” and “Desert King High 136% grain Zn over T. aestivum cv.WH 711 (Tiwari et al., 2010). A
Protein” in durum wheat in United States; and “Lallian,” number of SHW were identified as potential donors and their
“Somerset” and “Burnside” in hexaploid wheat in Canada have underlying QTLs were detected. Using a SHW × T. spelta
been developed (Balyan et al., 2013). Recently, Indian scientists recombinant inbred line population, 12 QTLs for grain Zn and
have developed a biofortified wheat variety MACS 4028 that 7 QTLs for grain Fe were detected (Crespo-Herrera et al., 2017).
contains 14.7% GPC and above 40 ppm zinc and iron (https:// The QTLQgrain Zn.cimmyt-7B_1P2 for grain Zn on chromosome
dst.gov.in/scientists-ari-pune-develop-biofortified-high-protein- 7B, with a maximum phenotypic variation explained (PVE) of
wheat-variety). Major constraints on the expression of GPC-B1 32.7% and QTL Qgrain Fe.cimmyt-4A_P2 on chromosome 4A
appear to be a genetic background and environmental conditions, with 21.14% for grain Fe can be used in marker assisted breeding.
particularly high temperatures (Carter et al., 2012), making Pleiotropic QTLs can be targeted throughmarker assisted selection
breeding to develop high-yielding, biofortified genotypes using for simultaneous improvement of multiple traits including yield.
the locus is a challenge, particularly for areas where terminal QTLs detected on chromosomes 2A, 5A, 6B for grain Zn were co-
heat and drought are constraints and may lead to early senescence. localized for highGPC and 5 loci on chromosomes 2A, 2B, 5A, 6A,
and 7B for high grain Fe and GPC, indicating the potential
correlated improvement for grain content of both minerals and
Mapping of QTLs for high Grain Zn/Fe proteins (Peleg et al., 2009). Other studies have reported common
content in wheat genomic regions. QTL for grain Zn content on 7B was co-localized
with a QTL for grain Fe (Crespo-Herrera et al., 2017). Common
Selecting genotypes for highly quantitative traits that also genetic regions influencing high GPC, Zn and Fe contents were
show high G × E interaction is difficult through phenotypic also found on chromosomes 2A and 5A (Peleg et al., 2009) and on
assessment. QTL detection and their use through marker assisted 2A (Krishnappa et al., 2017). Although few, QTLs were detected in
selection can improve selection efficiency. The high the hexaploid wheat background is advantageous, as they can be
environmental influence, narrow range of variation, and transferred through breeding without linkage drag, cross
tedious phenotyping procedures for estimating grain Zn and compatibility issues, or much time. Using RILs from a cross
Fe have made QTL mapping challenging. Phenotyping through between a Chinese wheat line (with the lineage Hong Hua Mai/
inductively coupled plasma spectrometry (ICP-S) and, recently, . ../Blouk #1) and the commercial bread wheat cultivar Roelfs
use of energy-dispersive X-ray fluorescence spectrometry (ED- F2007, 10 QTLs for grain Zn, 9 for grain Fe, 5 for GPC, and 36 for
XRF) has facilitated the screening of large samples, then the agronomic traits were detected (Liu et al., 2019b). Pleiotropic QTLs
atomic absorption spectrometry (AAS). for grain Zn and Fe content on chromosome 3D and for grain Zn,
Efforts have been made in the last two decades to dissect the GPC and thousand kernel weight were detected on chromosome
genetic components governing grain Zn and Fe content in wheat. 2B. Shi et al. (2008) used a double haploid population derived from
A number of QTLs associated with the genetic variation of grain two winter wheat cultivars and mapped 4 and 7 genomic locations
mineral content have been detected. QTL mapping was performed for grain Zn concentration and grain Zn content, respectively.
on the diverse genetic backgrounds using bi-parental and Four QTLs on chromosomes 4A, 4D, 5A and 7A were identified
association mapping population and several QTLs of major and for grain Zn concentration and content and may facilitate
minor allelic variations were detected (Table 2). Most of the QTL simultaneous improvement of grain Zn content and
mapping studies have focused on the background of SHW or wild concentration. Surprisingly, collocated QTLs for grain Zn and
relatives of tetraploid or diploid species, due to the presence of phosphorus (P) were found on chromosomes 4A and 4D,
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TABLE 2Mapping population, number of QTLs detected alongwith their percent of phenotypic variation explained (PVE%) and chromosome carrying
the QTLs for grain zinc and iron in wheat.
Trait Mapping population Parental details No. PVE (%) Chromosomes References
and number of carrying
QTLs the QTLs
Grain DH (119) Hanxuan10 × Lumai 14 7 4.6–14.6 1A, 2D, 3A, 4A, 4D, 5A, Shi et al. (2008)
Zn and 7A
RILs (93) T. boeoticum accession pau5088 × T. 1 18.8 7A Tiwari et al. (2009)
monococcum accession pau14087
DH (90) RAC875–2 × Cascades 12 92.0 3D, 4B, 6B, and 7A Genc et al. (2009)
(combining all
QTLs)
RILs (152) Durum wheat cv. Langdon × wild emmer 6 1–23.0 2A, 5A, 6B, 7A, and 7B Peleg et al. (2009)
wheat (accession #G18-16)
RILs (185) T. spelta accession H+ 26 (PI348449) × T. 5 4.3–16.5 2A, 2B, 3D, 6A, and 6B Srinivasa et al.
aestivum cv. HUW 234 (2014)
RILs (127 for hexaploid and T. aestivum cv. Adana99 × T. 10 9–31.0 1B, 1D, 2B, 3A, 3D, 6A, 6B, Velu et al. (2016b)
105 for tertraploid) sphaerococum cv. 70,711 and T. durum cv. 7A, and 7B
Saricanak 98 × T. dicoccon cv. MM5/4
RILs (140) Seri M82 × SHW CWI76364 6 8.3–19.6 4BS, 6AL, and 6BL Crespo-Herrera
et al. (2016)
RILs (188) Bubo × Turtur 4 2.86–16.75 1B, 6A, and 7B Crespo-Herrera
et al. (2017)
RILs (188) Louries × Bateleur 12 3.3–32.79 1A, 1B, 3B, 3D, 4A, 5B, 6A, Crespo-Herrera
7B, and 7D et al. (2017)
RILs (286) WH 542 × SHW 5 3.2–14.4 2A, 4A, 5A, 7A, and 7B Krishnappa et al.
(2017)
GWAS (369) European elite wheat varieties including 161 5.5 to 13.7 1A, 1B, 2A, 2B, 3A, 3B, 3D, Alomari et al.
355 genotypes of winter wheat and 4A, 4D, 5A, 5B, 6A, 6B, 7A, (2018)
14 spring wheat genotypes and 7B
GWAS (123) 123 SHWs 13 1.8–14.1 1A, 2A, 3A, 3B, 4A, 4B, 5A, Bhatta et al. (2018)
and 6B
GWAS (330) Harvest Plus Association Mapping panel 39 5–10.5 1A, 2A, 2B, 2D, 5A, 6B, 6D, Velu et al. (2018)
7B, and 7D
RILs (200) Roelfs F2007 × Hong Hua Mai/. 10 2.71–14.22 1B, 2B, 3A, 3B, 3D, 4B, 5A, Liu et al. (2019b)
../Blouk #1 6B, and 7A
GWAS (167) Ae. tauschii accessions 4 2.59–3.39 2D, 4D, 6D, and 7D Arora et al. (2019)
GWAS (246) Chinese wheat mini core collection 11 2.7–6.6 1B, 2B, 2D, 3A, 3D, 4A, 4B, Liu et al. (2020)
5A, 5D, 6B, and 7D
GWAS (330) Harvest Plus Association Mapping (AM) 13 3.7–5.2 1A, 1D, 2A, 2B, 3A, 4A, 5B, Cu et al. (2020)
panel and 7A
RILs (254) Jingdong 8 × Bainong AK58 7 2.2–25.1 1DS, 2AS, 3BS, 4DS, 6AS, Wang et al. (2021)
6DL, and 7BL
RILs (190) Zinc-Shakti × Kachu 27 1.1–8.1 1A, 2A, 4A, 5A, 6A, 7A, 1B, Rathan et al.
2B, 3B, 6B, 1D, 2D, 5D, (2021)
and 7D
RILs (95) AS2407 (Ae. tauschii. 1 13.49 2D Chen et al. (2022)
ssp. strangulate) ×AS65 (Ae. tauschii. ssp.
tauchii)
GWAS (280) Indian wheat germplasm 5 5.7–10.9 2B, 5B, 6A, and 7B Krishnappa et al.
(2022)
Grain RILs (93) T. boeoticum accession pau5088 × T. 2 11.7–12.6 2A, 7A Tiwari et al. (2009)
Fe monococcum accession pau14087
DH (90) RAC875–2 × Cascades 10 47.0 3D Genc et al. (2009)
RILs (152) durum wheat cv. Langdon × wild emmer 11 2–18.0 2A, 2B, 3A, 3B, 4B, 5A, 6A, Peleg et al. (2009)
wheat (accession #G18-16) 6B, 7A, and 7B
RILs (185) T. spelta accession H+ 26 (PI348449) × T. 5 1.8–27.1 1A, 2A, and 3B Srinivasa et al.
aestivum cv. HUW 234 (2014)
(Continued on following page)
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Roy et al. 10.3389/fgene.2022.1045955
TABLE 2 (Continued) Mapping population, number of QTLs detected along with their percent of phenotypic variation explained (PVE%) and
chromosome carrying the QTLs for grain zinc and iron in wheat.
Trait Mapping population Parental details No. PVE (%) Chromosomes References
and number of carrying
QTLs the QTLs
RILs (127 and 105 for the crosses T. aestivum cv. Adana99 × T. 7 9–18.0 1B, 2A, 2B, 3A, 6B, and 7B Velu et al. (2016b)
cv. Adana99 × cv. 70,711 and cv. sphaerococum cv. 70,711 and T. durum cv.
Saricanak98 × cv. MM5/4, Saricanak98 × T. dicoccon cv. MM5/4
respectively)
RILs (140) Seri M82 × SHW CWI76364 10 7.2–14.5 2BL, 2DS, 4BS, 5AL, 5BL, Crespo-Herrera
6Al, 6BL, 6DS, and 7DS et al. (2016)
RILs (188) T. spelta cv. Bubo × Turtur (SHW) 3 5.49–10.35 3A, 4B, 5B Crespo-Herrera
et al. (2017)
RILs (188) Louries (SHW) × T. spelta cv. Bateleur 7 5.79–21.14 2A, 2B, 3B, 4A, 4D, and 5B Crespo-Herrera
et al. (2017)
RILs (286) WH 542 × SHW 4 2.3–6.8 2A, 5A, 7A, and 7B Krishnappa et al.
(2017)
GWAS (369) European elite wheat varieties including 137 5.6–13.9 1A, 1B, 2A, 2B, 3A, 3B, 4A, Alomari et al.
355 genotypes of winter wheat and 5A, 5B, 5D, 6A, 6D, 7B, (2018)
14 spring wheat genotypes and 7D
GWAS (123) 123 SHWs 3 11.2–13.2 1A, 3A Bhatta et al. (2018)
GWAS (369) European elite wheat varieties 137 5.6–13.9 1A, 2A, 3A, 3B, 5A, 5B, Alomari et al.
and 6A (2019)
GWAS (167) Ae. tauschii accessions 5 1.47–4.03 1D, 2D, 3D, 4D, and 7D Arora et al. (2019)
RILs (200) Roelfs F2007 × Hong Hua Mai/. 9 2.10–14.56 1A, 2A, 3B, 3D, 4B, 5A, Liu et al. (2019b)
../Blouk #1 and 6B
RILs (254) Jingdong 8 × Bainong AK58 4 2.3–30.4 3BL, 4DS, 6AS, and 7BL Wang et al. (2021)
RILs (190) Zinc-Shakti × Kachu 23 1.0 to 10.2 1A, 2A, 4A, 6A, and 7A, 1B, Rathan et al.
2B, 4B, 5B, 6B, 1D, 2D, (2021)
and 7D
GWAS (280) Indian wheat germplasm 5 12.7–24.1 1A, 3B, 5A, 6A, and 7B Krishnappa et al.
(2022)
suggesting the potential for their correlated breeding grains. Multiple studies have reported QTLs for high Zn and
improvement, even though the elements share an antagonistic Fe concentrations on chromosomes 2A, 2B, 5A, and 7B
relationship for plant uptake in soils. (Srinivasa et al., 2014; Alomari et al., 2018; Alomari et al.,
Genome wide association studies (GWAS) use potential 2019; Krishnappa et al., 2022). Transfer of these QTLs
natural variations to map significant marker trait associations through marker assisted breeding will certainly improve
(MTA), and provide the advantages of high resolution, high mineral concentrations in wheat grains. However, detection
allele coverage over the bi-parental mapping population, and of QTLs with major effects and stable expression across
elimination of the disadvantages associated with RILs, where environments remains a constraint for marker-assisted
variation is confined within the two parental lines. Using GWAS breeding. Stable QTLs reported in some studies will be initial
in winter and red wheat cultivars, 161 MTAs were detected on targets for breeding, but few QTLs were reported to be stable
15 wheat chromosomes. The most significant regions were on across years or locations. Based on across-location evaluations in
chromosomes 3A and 5B and are related to transporter proteins India and Mexico, Velu et al. (2018) reported 39 stable MTAs
such as those of the ZIP family and signal proteins of theMAPK (significant across at least three environments) for grain Zn.
family (Alomari et al., 2018). GWAS performed using a diverse Crespo-Herrera et al. (2016) reported a QTL on 4BS that
panel of SHW develop from of T. durum, T. dicoccon, T. spelta, appeared across years governing 19.6% of the PVE for grain
pre-breeding derivatives from T. polonicum, and landraces Zn and with pleiotropic effects on grain Zn and grain Fe.
detected 39 QTLs for grain Zn on chromosomes 1A, 2A, 2B, Alomari et al. (2018) reported a single MTA with minor
2D, 5A, 6B, 6D, 7B, and 7D (Velu et al., 2018). QTLs with large effects on chromosome 3B and expressed in all 3 years of the
effects were detected on chromosomes 2B and 7B and are study. Cu et al. (2020) reported five MTAs on chromosome 5B
associated with transcription factors such as zinc finger expressed over the season and having pleiotropic effects on grain
motifs and metal ion binding protein (phosphatase), all of Fe, Mn, Cu, and P contents. Recently, a meta-analysis using
which has a role in the additional loading of Zn in wheat QTLs from seven different studies of diverse parental
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Roy et al. 10.3389/fgene.2022.1045955
FIGURE 1
An integrated approach for eradication of malnutrition including new breeding techniques, supplementation and diversification.
combinations (Shariatipour et al., 2021) detected meta-QTLs Australia, Bangladesh, Bolivia, India, Nepal, Mexico, and
(MQTL) on chromosomes 1B, 2B, 4A, 5A, 7A, and 7B with Pakistan (Table 3). Constraints associated with selective
MQTL-1, MQTL-5 and MQTL-7 on chromosomes 1B, 7A, and breeding include limited genetic variation, low heritability,
7B, respectively, were detected with the highest number of initial crossability barriers, and linkage drag, to name several. Yield
QTLs. Functional analysis of candidate genes confined in the gains through pure line varieties have plateaued and the
MQTL revealed that most of the genes were associated with Zn development of pure line varieties is slow. Working in the
and Fe homeostasis. The meta-QTL analysis is a powerful tool to joint Rockefeller Foundation-Mexican government Office of
reduce the confidence interval for the QTLs by integrating Special Studies in the mid-20th century, Nobel Peace laureate
information from independent QTL analyses performed by Dr. Norman E. Borlaug launched a “shuttle breeding” approach
different authors and helps to identify more reliable QTLs. to speed wheat varietal development, allowing two breeding
Wheat biofortification is challenging, an integrated cycles per year and still used by CIMMYT. Double haploids
approach needs to be constructed integrating conventional represents another method to rapidly fix genotypes from
breeding, biotechnological tools, new breeding techniques, gametospores of an F1 plant, but are also associated with the
and agronomic measures for immediate soil remediation flaws of a single generation of recombination and not exposed
(Figure 1). In populations with diets heavy in staple crop to diverse environments for hardening. Recently, speed
foods and highly susceptible to micronutrient deficiencies, breeding has emerged as an approach to the rapid
supplementation through pharmaceutical products and food advancement of generation. In this approach, plants are
additives may be required and need support through raised in growth chambers under controlled conditions with
governmental policies. extended temperatures and photoperiods (Watson et al., 2018).
Among other things, this modulates flowering genes to
stimulate the early onset of flowering and, as a result, allows
New breeding techniques for 5–6 generations to be grown in a year and the rate of genetic
biofortification gains is greatly increased. For instance, genetic gain is ΔG =
ihσA/L, where i = selection intensity, h = heritability, σA =
Speed breeding standard deviation of additive genetic variance and L = length
of breeding cycle interval or generation. Through modification
A number of wheat genotypes with higher grain Zn and Fe in the selection intensity and accurate phenotyping, genetic
content have been developed and released for cultivation in advance can be enhanced to some extent. Rapid breeding cycles
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Roy et al. 10.3389/fgene.2022.1045955
TABLE 3 List of biofortified wheat varieties released for commercial cultivation with their grain micronutrient level (ppm), protein content (%) and
additional traits (modified from Gupta et al., 2022).
Country Variety Nutrient level Year Pedigree Additional traits
origin of
release
India WB 02 Zn:42.0, Fe:40.0 2017 T.DICOCCON,CI9309/AE.SQUARROSA (409)// Resistance to stem rust (Sr2, Sr7b), leaf
MU-TUS/3/2*MUTUS rust (Lr13), wheat blast
DBW 173 Zn:40.7, Protein: 12.5 2018 KAUZ/AA//KAUZ//PBW602 Resistance to stem rust (Sr31, Sr5), leaf
rust (Lr26, Lr10, Lr3), yellow rust (Yr9),
wheat blast (2NS), heat tolerant
DBW 187 Fe:43.1 2018 NAC/TH.AC//3*PVN/3/MIRLO/BUC/4/ Resistance to stem rust (Sr5, Sr11), leaf
2*PASTOR/5/KACHU/6/KACHU rust (Lr23, Lr10, Lr1), yellow rust (Yr2),
wheat blast (2NS)
DBW 303 Zn:36.9 2020 WBLL1*2/BRAMBLING/4/BABAX/LR42// Resistance to leaf rust (Lr13)
Fe:35.8 BABAX*2/3/SHAMA*2/5/PBW343*2/KUKU
NA*2//FRTL/PIFED
Protein: 12.1
DDW 47 (d) Fe:40.1 2020 PBW34/RAJ1555//PDW314 Resistance to stem rust (Sr11, Sr7b),
Protein: 12.7 Yellow rust (Yr2)
DDW 48(d) Zn:39.7 2020 HI8498/PDW233//PDW291 Resistance to stem rust (Sr7b, Sr2)
Fe:38.8
Protein: 12.1
HPBW 01 Zn:40.6 2017 T.DICOCCON CI 9309/A. SQUARROSA (409)/3/ Resistance to stem rust (Sr2, Sr31), leaf
Fe:40 MILAN/S87230//BAV92/4/2* MILAN/S87230// rust (Lr10, Lr23, Lr26), yellow rust (Yr9)
BAV92
PBW 757 Zn:42.3 2018 PBW550/YR15/6*AVOCET/3/2*PBW550/4/ Resistance to stem rust (Sr8a, Sr5, Sr2),
PBW568 + YR36/3*PBW550 leaf rust (Lr13, Lr10, Lr1)
PBW 752 Zn:38.7 2018 PBW621/4/PBW343//YR10/6*AVOCET/3/ Resistance to stem rust (Sr13, Sr11), leaf
Fe:37.1 3*PBW343/5/PBW621 rust (Lr13), wheat blast (2NS)
Protein: 12.4
PBW 771 Zn:41.4 2020 PBW550//YR15/6*AVOCET/3/2*PBW550 Resistance to stem rust (Sr31), leaf rust
(Lr26, Lr23, Lr1) yellow rust (Yr9)
HI 8777 (d) Zn:43.6 2017 B93/HD4672/HI8627 Resistance to stem rust (Sr7b), yellow
Fe:48.7 rust (Yr2)
HI 1605 Zn:35 2017 BOW/VEE/5/ND/VG9144//KAL//BB/3/YACO/4/ Resistance to stem rust (Sr5, Sr11) leaf rust
Fe:43 CHIL/6/CASKOR/3/CROC_1/AE.SQ (224)// (Lr13), yellow rust (Yr2)
OPATA/7/PASTOR//MILAN/KAUZ/3/BAV92
Protein: 13
HI 8759 (d) Zn:42.8 2017 HI8663/HI8498 Resistance to stem rust (Sr2, Sr11), leaf
Fe:42.1 rust (Lr23)
Protein: 12
HI 1633 Zn:41.1 2020 GW322/PBW498 High gluten strength (5 + 10 subunit of
Fe:41.6 Glu-D1)
Protein 12.4
HI 8805 (d) Fe:40.4 2020 IWP5070/HI8638//HI8663 Resistance to stem rust (Sr13, Sr11), leaf
Protein: 12.8 rust (Lr13)
HD 3171 Zn:47.1 2017 PBW343/HD2879 Resistance to stem rust (Sr11, Sr7b, Sr2),
leaf rust (Lr23, Lr13, Lr10), yellow rust
(Yr2), wheat blast, drought tolerant
HD 3249 Fe:42.5 2020 PBW343*2/KUKUNA//SRTU/3/PBW343*2/ Resistance to stem rust (Sr11, Sr2), leaf
KHVAKI rust (Lr13, Lr10), yellow rust (Yr2), wheat
blast (2NS)
HD 3298 Fe:43.1 2020 CL1449/PBW343//CL882/HD2009 Leaf rust (Lr23), Yellow rust (Yr2)
Protein: 12.1
MACS Zn:40.3 2018 MACS2846/BHALEGAON3*2 Resistance to stem rust (Sr7b)
4028 (d) Fe:46.1
(Continued on following page)
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TABLE 3 (Continued) List of biofortified wheat varieties released for commercial cultivation with their grain micronutrient level (ppm), protein content
(%) and additional traits (modified from Gupta et al., 2022).
Country Variety Nutrient level Year Pedigree Additional traits
origin of
release
Protein: 14.7
MACS 4058 Zn:37.8 2020 MACS3125/AKDW2997-16//MACS3125 Resistance to stem rust (Sr13) Leaf rust
Fe:39.5 (Lr23)
Protein: 14.7
UAS 375 Protein: 13.8 2018 UAS 320/GW322//LOK62 Resistance to stem rust (Sr7b, Sr2), leaf
rust (Lr13), yellow rust (Yr 2)
Pakistan NR- 421 High Zn (>6 ppm Zn 2015 OASIS/SKAUZ//4*BCN/3/2*PASTOR/4/ High yield
(Zincol-2016) advantage compared to T.SPELTA PI348449/5/BACEU#1/6/WBLL1*2/
best local check) CHAPIO
Akbar-2019 High Zn (>7 ppm Zn 2019 Becard/Quaiu Resistant to yellow rust and wheat blast
advantage compared to
best local check)
Nawab 2021 High Zn HGO94.7.1.12/2*QUAIU #1/3/VILLA JUAREZ Resistant to yellow rust and wheat blast
F2009/SOLALA//WBLL1*2/BRAMBLING
Bangladesh BARI Gom 33 Zn: 50–55 2017 Kachu/Solala Wheat Blast (2NS)
Mexico Nohely-F2018 7%–8% Zn advantage 2018 T.DICOCCON (CI 9309)/AE.SQUARROSA Resistance to leaf rust, yield 7.8 t/ha
over check (409)//MUTUS/3/2*MUTUS
Bolivia Iniaf-Okinawa High Zn (>8 ppm Zn 2018 Kachu/Solala Wheat blast (2NS)
advantage than the local
check)
Nepal Zinc Gahun 1 High Zn (>6 ppm Zn 2020 MELON//FILIN/MILAN/3/FILIN/5/CROC_1/ High yield
(NL 1327) advantage than the local AE.SQUARROSA (444)/3/T.DICOCCON
check) PI94625/AE.SQUARROSA (372)//3*PASTOR/4/
T.DICOCCON PI94625/AE.SQUARROSA (372)//
3*PASTOR/6/ATTILA/3*BCN//BAV92/3/TILHI/
5/BAV92/3/PRL/SARA//TSI/VEE#5/4/CROC_1/
AE.SQUARROSA (224)//2*OPAT
Zinc Gahun 2 High Zn 2020 T. DICCOCON CI 9309/AE. SQUARROSA High yield
(NL1369) (409)//MUTUS/3/2*MUTUS
Bheri-Ganga High Zn 2020 MELON//FILIN/MILAN/3/FILIN/5/CROC_1/ High yield
(WK 2748) AE.SQ UARROSA (444)/3/T.DICOCCON
PI94625/AE.SQUARROSA (372)//3*PASTOR/4/
T.DICOCCON PI94625/AE.SQUARROSA (372)//
3*PASTOR
Himganga High Zn 2020 CHONTE*2/SOLALA//2*BAJ #1 High yield
(WK 3026)
Khumal-Shakti High Zn 2020 FRNCLN*2/7/CMH83.1020/HUITES/6/ High yield
(WK 3027) CMH79A.9 55/4/AGA/3/4*SN64/CNO67//
INIA66/5/NAC/8/WB LL1*2/KURUKU//HEILO/
9/WBLL1*2/KURUKU//H EILO
Borlaug 2020 High Zn 2020 ROLF07/4/BOW/NKT//CBRD/3/CBRD/5/ High yield
(NL 1307) FRET2/TUKURU//FRET2
d, indicates durum wheat variety
also lead to higher genetic gain and the early development of Genomic selection and integrating
homozygous lines. For quantitative traits with low heritability genomic selection with speed breeding
where the selection in advanced generations is practiced, speed
breeding can be of great importance to achieve high Genomic selection (GS) is a powerful tool to increase genetic
homozygosity quickly. Speed breeding has been used in gains, shorten breeding cycles, prediction the performance of an
breeding for spring wheat, durum wheat, barley, pea, individual in an untested environment, and increase selection
chickpea and canola (Watson et al., 2018). In wheat, speed accuracy and genetic gain. GS is most effective for the traits where
breeding has been used to generate genotypes resistant to stem phenotyping is costly—particularly, yield and quality traits. GS
rust, yellow rust, and fusarium head blight (Voss-Fels et al., can be used for the selection of quantitative traits involving
2019). complex physiological mechanisms, each underpinned by
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Roy et al. 10.3389/fgene.2022.1045955
multiple genetic variations that require genome-wide selection broadly classified as zinc-finger nucleases (ZFNs),
approaches, rather than targeting single QTLs. GS calculates transcription activator-like effector nucleases (TALENs), and
genomic assisted breeding values (GEBVs) by estimating all clustered regularly interspaced short palindromic repeats
available genetic variations in an individual. It uses the (CRISPR)-CRISPR-associated (Cas) nucleases that are
genomic best linear unbiased prediction (GBLUP) model and designed to effect a targeted, double-stranded break in the
a multi-environment, linear mixed model to estimate correlated DNA (Karmakar et al., 2022). ZFNs consist of a zinc finger
environmental structure, to predict the performance of DNA binding domain attached with nuclease FokI (Carroll,
individuals before phenotyping, based on genotyping and 2011). TALENs are engineered with a TAL effector binding
pedigree information. In GS, a set of individuals are DNA domain attached with FokI nuclease (Joung and Sander,
genotyped and phenotyped as a training population for use to 2013). CRISPR-Cas nuclease consists of a guide RNA and a Cas
develop the prediction model (Juliana et al., 2018). The efficacy of nuclease. Using the Cas9 endonuclease, CRISPR-Cas9 is the most
GS has been tested for multiple traits in several crops, including extensively used system for genome editing, thanks to its simple
wheat, for yield (Juliana et al., 2018), leaf rust and yellow rust design, cost effectiveness, high efficacy, reproducibility and
(Juliana et al., 2017), spot blotch (Juliana et al., 2022b), and wheat engineering feasibility (Karmakar et al., 2022). A single guide
blast (Juliana et al., 2022a). GS to improve grain mineral RNA (sgRNA) hybrid consisting of a CRISPR-RNA and a
concentrations has been employed in maize (Mageto et al., transactivating RNA locate the binding and cleavage site for
2020), rice (Rakotondramanana et al., 2022), and wheat (Velu the Cas9 nuclease. CRISPR-Cas9 genome editing is increasingly
et al., 2018), with reasonable prediction accuracies. Velu et al. used to enhance disease resistance, tolerance to abiotic stresses
(2018) reported prediction accuracies ranging from 0.331 to (drought, heat, salinity) and end-use quality in food crops. The
0.694, with an average of 0.542, for grain Zn content and role of genome editing in crop biofortification is being exploited
0.324 to 0.734, with an average of 0.529, for grain Fe content, for several traits including vitamin-A enrichment, targeted
in the Harvest Plus Association Mapping (HPAM) panel. increases in grain zinc and iron, and reducing anti-nutritional
Prediction accuracy was higher in the environments with high factors in the grain (Kumar et al., 2022). Targeted knockout of the
heritability and soil available Zn. Genetic relatedness increases OsAAP6 and OsAAP10 genes using gene editing has reduced
prediction ability and prediction ability sharply declines as protein content in rice (Wang et al., 2020). The CRISPR-Cas9
relatedness between the training and testing population system was used to create novel OsBADH2 genes in non-
decreases (Crossa et al., 2010) and when evaluations are aromatic rice lines and created aroma (Kumar et al., 2020).
conducted in poor environments and with poor phenotyping Targeted change in OsNRAMP2 gene enhanced iron
(Velu et al., 2018). GS becomes more effective with higher remobilization and distribution in rice (Chang et al., 2022). In
prediction accuracies and applied in early generations of wheat, CRISPR-Cas9 mediated genome change in the α-gliadin
selection to retain lines with high breeding value and gene resulted in low gluten wheat (Sánchez-León et al., 2018).
discarding those not expected to give higher genetic gains in Reduction in phytic acid levels through the disruption of inositol
advanced generations. GS can easily be incorporated with speed pentakisphosphate 2-kinase 1 (TaIPK1) improves grain Zn and
breeding to increase genetic gain. GS has not been studied in Fe bioavailability in wheat (Ibrahim et al., 2021). In the future,
detail for increasing mineral density in wheat. A model has been numerous genes can be targeted through CRISPR-Cas to develop
proposed where GS was accumulated through speed breeding biofortified crops. To promote genome edited technology in crop
and predicted higher genetic gains over conventional breeding improvement, Argentina, Brazil, Colombia, Chile, India, and the
(Voss-Fels et al., 2019). It has been shown that the incorporation United States have established regulatory measures separate from
of GS with speed breeding can significantly increase grain yield, those applied for transgenic crops.
over conventional phenotypic selection.
RNAi technology
Genome editing
The suppression of gene expression through antisense or RNAi
Genome editing is a powerful tool in the field of medicine, technology is a powerful way to modulate biosynthetic pathways.
agriculture and the life sciences, allowing targeted changes in a RNAi is preferred over antisense technology, as it is more stable,
genotype and avoided the random changes that occur in induced efficient, and precise. Using a double stranded RNA (dsRNA)
mutations. Genome editing can produce targeted genetic molecule, RNAi inhibits the expression of a gene at the
variation where none previously existed. Unlike the case of transcription and translational levels. It has been used for biotic
genetically modified crops, gene editing does not require the and abiotic stress tolerance and nutritional quality improvement.
insertion of foreign DNA. Deletions, additions, single nucleotides RNAi mediated suppression of the inositol pentakisphosphate
or DNA segment substitution are used to change a target gene. kinase (IPK1) gene involved in the phytic acid biosynthesis
Researchers have developed artificial site-specific nucleases, pathway reduced phytic acid level of 28%–56% in wheat
Frontiers in Genetics 15 frontiersin.org
Roy et al. 10.3389/fgene.2022.1045955
(Aggarwal et al., 2018). Similarly, downregulation of the rice inositol through conventional breeding is hard. Available genomic
triphosphate kinase (OsITP5/6K-1) gene through RNAi resulted in a resources are needed to be utilized while breeding for high
42% reduction in phytic acid in rice grains (Karmakar et al., 2020). grain zinc and iron content wheat varieties. An integrated
This technology can be used to elucidate biosynthetic pathways and breeding programme including marker assisted selection,
understand the roles of different genes in Zn and Fe sequestration. genomic approaches and gene editing tools will gear up
development of biofortified wheat varieties.
Omics technology
Author contributions
Advancement in next-generation sequencing platforms has
enabled the development of a whole genome sequence for CR conceptualized and drafted the first version; SK collected
Triticum aestivum cv. Chinese Spring (https://www.wheatgenome. the information; SK, RR, SRK, and VG edited the article.
org/News/Latest-news/RefSeq-v1.0-URGI), which is the largest
cereal genome. Genome sequence information of diploid (AA,
DD genome) and tetraploid (AABB) progenitors are also Funding
available (Zhou et al., 2020). Sequence information for wheat
wild relatives will elucidate the genetic mechanisms and variation Part of the research work was supported by a grant from the
for micronutrient levels in wheat grain, paving the way for structural Bill and Melinda Gates Foundation (INV003012) and co-funded
genomics, functional genomics and metabolomics. Integration of by the Foreign, Commonwealth and Development Office
multiple omics data with high-throughput techniques has elucidated (FCDO) of the United Kingdom Government to CIMMYT.
the complex pathways of plant growth and their responses under
biotic and abiotic stress. The availability of other omics technologies
(mutagenomics, pangenomics, transcriptomics, proteomics, Conflict of interest
ionomics, and phenomics) would provide new knowledge on
complex biological systems. Huge amounts of data are generated The authors declare that the research was conducted in the
using these technologies; advances in computing and integrated absence of any commercial or financial relationships that could
system-based analysis to use and manage data will expand our be construed as a potential conflict of interest.
understanding of genotype and phenotype relationships, allowing The reviewer ZW declared a past co-authorship with the
wheat breeders to make targeted changes in the pathways of metal author VG to the handling editor.
homeostasis and develop bio-fortified genotypes.
Publisher’s note
Conclusion
All claims expressed in this article are solely those of the
The review updates about wheat biofortification research authors and do not necessarily represent those of their affiliated
include conventional and molecular breeding. Mainstreaming organizations, or those of the publisher, the editors and the
biofortification is essential along with yield, disease resistance reviewers. Any product that may be evaluated in this article, or
and other parameters. Being quantitative, limited genetic claim that may be made by its manufacturer, is not guaranteed or
variations in the cultivated wheat varieties, improvement endorsed by the publisher.
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