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Micronutrients play an indispensable role in human and animal growth. In the world, many people are suffering from malnutrition and micronutrient deficiency mainly due to lack of zinc (Zn) and iron (Fe). Several crops are grown, such as wheat, rice, maize, and legumes, to address the challenges of micronutrient deficiency. Wheat landraces are evidently proven to be a rich source of genetic variability as against modern cultivated varieties due to thousands of years of their cultivation under low input farming systems. Landraces serve as a potential reservoir of desirable allelic forms of valuable traits but are low in traits like Zn and Fe. Wheat is a major cereal consumed worldwide and could be a good source to provide these micronutrients. Biofortification in wheat can be an effective way to solve the problem of malnutrition. Biofortification in wheat varieties may be enhanced by the application of molecular breeding approaches, such as genome editing, transgenic technology, and marker-assisted selection. These biofortified wheat varieties show better adaptation to environments. In this chapter, we included the recent advances in quantitative trait loci (QTLs) in biofortified wheat and the techniques used to develop biofortified wheat varieties.
School of Agricultural Sciences and Technology, RIMT University, Mandi Gobindgarh, India
Department of Genetics and Plant Breeding, Chandra Shekhar Azad University of Agriculture and Technology, Kanpur, India
Prashant Kaushik*
Institute of Conservation and Improvement of Valencian Agrodiversity, Polytechnic University of Valencia, Spain
*Address all correspondence to: prakau@doctor.upv.es
1. Introduction
In the world, many people are suffering from malnutrition. Malnutrition occurs due to the nonavailability of proper diet including primary deficiency of zinc (Zn) and iron (Fe) to the living being. Nutritional quality and food quality must be improved by biofortification using the breeding program and it is a sustainable solution to the global malnutrition problem [1]. The Biofortification concept was started at the time of green revolution (1966–1985) [2]. In 1990, the work on micronutrients was started by the American economist by the name of Howarth E-Howdy. But, the term, Biofortification, was coined by Steve Beebe in 2001 [3]. Biofortification provides cost-effective sustainable micronutrients for human beings. This approach will help us to maintain the nutritional status of a living being. In addition, the biofortification process provides micronutrients in wheat varieties, and these wheat varieties are preferred for agronomic practices [4]. These wheat varieties have sufficient micronutrients, especially Zn and Fe. For a better life, Zn is the major micronutrient for human beings as well as plants and its role is observed in both plants and humans [5]. An approximate 17% population has not used up the benefit of Zn [6]. To provide the availability of Zn and Fe, wheat landraces are developed and cultivated globally [7].
Primitive cultivars and landraces are evidently proven to be a rich source of genetic variability as against modern cultivated varieties due to thousands of years of their cultivation under low input farming systems [8]. Landraces serve as a potential reservoir of desirable allelic forms of valuable traits and therefore, could help in biodiversity enrichment and subsequently [9]. A large collection of wheat landraces with enormous variability for different traits still remain within gene banks, without being explored for their utility [10]. With the advancement of modern technologies, applications of molecular breeding approaches, such as genome editing, transgenic technology, and marker-assisted selection, are developing biofortified wheat varieties [9].
Wheat is one of the most popular cereal crops grown and is placed in the second position after rice worldwide and consumed by the people for food and feeding purpose globally [11]. On the earth, the population is increasing alarmingly, however for the purpose of food consumption, wheat production should be increased for food security [12]. Wheat is grown for a better yield under different climatic conditions and is adopted in every diverse area worldwide. It is having to constitute about 10–12% protein and 20% dietary energy. Improving the role of biofortified wheat varieties is the main objective [13]. At the global level, there are many organizations that are playing a key role in biofortification breeding, especially for developing higher zinc and Fe micronutrient varieties.
In the world, many people are suffering from malnutrition. Malnutrition occurs due to the nonavailability of a proper diet, including primary deficiency of zinc (Zn) and iron (Fe). In the present study, it is observed that the deficiency of zinc and iron is found in dietary food. For a better life, both Zn and Fe are the major micronutrients for humans [14]. The nonavailability of Zn and Fe may be increased using conventional breeding along with molecular breeding [15]. For humans, the availability of Zn was established in 1961 [14]. Human body has a zinc content of about 1.4 to 2.3 g. Because of its nature, its rank is good in periodic numbers in the Periodic Table, and zinc has chemical properties that make it important in biological systems. Zinc plays an indispensable role in cellular function, cell division, cell growth, cellular transport, immune deficiency, transcription, translation, synthesis, platelets, blood cells, immune system, muscles, and liver bones. The quantity of Zn is present in the cell membrane (10%), nucleus (30–40%), and in the cytoplasm (50%) [16]. Zinc also has a good effect on the immune system of a human being.
The deficiency of zinc causes dangerous effects and damage to immunity cellular mediators, such as natural killer cell activity and the production of cytokines [17]. Zinc is an inflammatory and anti-oxidative stress agent. Zn pills may be used for the treatment of diarrhea, and bouts of the condition [18]. Zinc also has an effect on the brain and increases the mind’s ability for proper learning through neurons. The common cold may be treated with zinc by reducing the duration and severity of cold in healthy people. Zinc also plays an important role in wound healing, skin creams for treating diaper rash or other skin irritations and may increase the development of sperm in humans [19]. In addition, in 2020, humans faced more problems with Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) (Coronavirus Disease 2019 (COVID-19)) [20].
On the other hand, iron is an essential nutrient; it helps in regulating the flow of oxygen from the lungs to the body tissue. Iron is very necessary for hematopoiesis, regulation of metabolic energy, normal brain development, and muscle development, conversion of blood sugar to energy, and proper growth and development [21]. The deficiency of iron in diet causes severe harmful effects on growth development and muscle metabolism of the body [22]. Therefore, iron is beneficial for a human being, and it differs from gender to gender. Females require more iron than adult males, because females lose blood during the monthly menstrual cycle. However, preadolescent males require more iron and prevalence of higher anemia than preadolescent females. A high quantity of iron is recommended for females aged 14–50 years than the same quantity required for males. This report is given by the Food and Nutrition Board at the National Academies of Sciences, Engineering, and Medicine (USA) [23]. Physiological requirements of iron at the time of pregnancy and lactation show that a high iron requirement is needed during pregnancy and less iron during lactation than in nonpregnant and nonlactating females [23]. Iron is a micronutrient for the proper function of living organisms.
Both iron and zinc play good roles in living beings and respond to regulatory roles of the body as cofactors for producing energy and lipid metabolism [24]. The absence of these elements produces disorder in body involving degenerative nerve cells and disorder in cells.
Quality improvement is one of the most important aspects of wheat. But the question is how to improve the quality of wheat. A prerequisite that specifies quality of the target gene is needed for the improvement of micronutrients (Fe and Zn) [25]. Getting the knowledge about micronutrient genes Fe and Zn in the crop is a very difficult task and searching where these micronutrient genes in the crop are present in chromosomes [26]. These micronutrient genes depend upon the various physiological, biological, and molecular processes. The main aim of plant breeders is to get the additive effect, transgressive segregation, and heterosis for iron and zinc concentrations in wheat grain [27]. The genetics of iron and zinc’s enormous contribution to wheat have been studied. Several studies on wheat landraces have been conducted, including conventional breeding, transgenic, genome-wide association studies (GWAS), clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9), mutagenesis, genomic selection, etc., for biofortification (Figure 1) [21, 28].
Figure 1.
Schematic representation of the development of biofortified wheat.
3.1 Conventional breeding approach
Conventional breeding approach is a good tool for the improvement of biofortification through conservative manipulation of plant genome in a limited area without harm to the environment. Despite this, conventional breeding approach is less preferable [29]. Development of biofortified varieties using traditional methods, such as mass selection, pure line selection, pedigree method, bulk method, backcrossing, single seed descent, synthetics, and composite, is very popular [30], but these methods have some problems, such as taking more time and being labor consumable. In wheat landraces, screenings of germplasm for both yield and quality improvement are a tedious job for a plant breeder, but these aforesaid programs (Genome-wide association studies, CRISPR/Cas9, mutagenesis, genomic selection, etc.) are very useful to overcome problems of a plant breeder [28].
3.2 Quantitative trait loci (QTL) mapping
Another approach to QTL mapping is molecular breeding, which is one of the most important tools by which quality control research work in wheat is done successfully. In this approach, marker-assisted breeding or marker-assisted selection is a very potent tool for bringing about quality improvement in wheat [31]. The QTL mapping in wheat for Zn and Fe has been applied to the population; these populations may be mortal (segregating) and immortal (nonsegregating). The mortal population comprised F1 and backcross and immortal population such as recombinant inbred lines (RIL) (getting after 6–8 generations by single seed descent), and many QTLs have been identified in wheat now [32, 33].
The first QTLs for iron and zinc were identified in 1997 on chromosome 6BS by working on recombinant inbred lines, which were developed by the cross between wheat and wild emmer (Triticum turgidum) [34]. This quantitative trait locus (QTL) is the gene Gpc-B1, which conferred that the increments of iron and zinc are 18 and 12%, respectively, and that this gene has the NAC (NAM (no apical meristem), ATAF (Arabidopsis transcription activation factor), and CUC (cup-shaped cotyledon)) transcription factor NAM-B1 [35, 36]. In addition, several QTLs have been identified for iron and zinc which have been given in the table and some other reviewers presented them elsewhere in their papers [37]. In each study, different QTLs have been identified due to the use of different populations, environments, and marker data sets, which is affected by the location [38]. The use of reference sequence makes it easier to detect the physical position for identification of QTLs with the help of markers on to the genome sequence, and to help in the detection of QTLs for iron and zinc [39]. In these studies, it may be useful to detect the small-effect QTLs for iron and zinc that were not previously possible [40]. However, detecting the QTLs with the help of marker-assisted selection in a large population due to practical limits is a tedious job. However, a small effect of QTLs is being identified using marker-assisted selection from a haplotype and has multiple beneficial QTLs [41]. This haplotype is done as a work of a single unit using a marker-assisted breeding program and reducing the number of markers in later stages [42].
On the other hand, an increment in both traits of iron and zinc may facilitate a significant and positive relationship with several environments; it may have happened by colocalization of the aforesaid QTLs that are underpinned by the pleiotropic effect of a single gene. For instance, by altering the rate of senescence, both iron and zinc content are affected by transcription factor NAM-B1 which takes a step for remobilization of multiple micronutrients [36, 42].
3.3 Identification of a candidate gene for biofortification
In wheat, an annotated chromosome genome sequence has a total assembly size of 14.5 GB which represents the total 94% whole genome and was published in 2018 [43]. The high level of assembly of the genome helps to build up in the genome mapping of loci which are involved in iron and zinc biofortification. In total, 107,891 high-level confidence gene models have been annotated that helped in genome-wide analysis for biofortification of related gene family and identification of loci in QTL. For example, by orthology of rice, the members of the wheat natural resistance-associated macrophage protein (NRAMP) family have been identified by Ali and Borill, 2020 [42] (For further details, see review paper). The NRAMP family illustrates the idea that analyzing a gene family with up-to-date references is necessary. In the previous assembly, several NRAMP gene models were missed out but now they are complete [44]. It has been studied that the RefSeq v 1.1 gene annotation is not perfect while it has significant improvement, for example, 24 NARMP genes were studied, in which two genes were incomplete [42]. This problem is overcome by an ongoing work for improving the annotation of Chinese genome assembly.
3.4 Genome-wide association studies
Even though marker-assisted breeding is a good tool to develop biofortification varieties, the small effect of QTL is not identified by marker-assisted breeding and it also requires a biparental population [45]. Therefore, this restriction is broken by genome-wide association studies. Genome-wide association studies are a powerful tool, because they do not require biparental mating, which is required for QTL mapping [46]. Recently, genome-wide association mapping is a prominent tool that has been utilized by plant breeders, which involves natural variation in elite lines and diverse lines by the production of a dense linkage map, whereas marker-assisted breeding requires a biparental population [47]. In genome-wide association studies, stable marker-trait association and multitraits as well as a large number of genes are responsible for iron concentration in wheat [48, 49]. GWAS have more wide adaptability and are less time consumable than QTL mapping because of which, segregating population is not required. In GWAS, natural variations were exhibited that led to higher resolution mapping than QTL mapping [42].
Moreover, Zhou et al. (2020) used GWAS for increasing the zinc content in wheat with a diversity panel of 207 bred wheat varieties in three locations. In this panel, a total of 29 zinc QTLs that are associated are identified. On chromosomes 1B, 3B, 3D, 4A, 5A, 5B, and 7A, seven nonredundant loci are located in at least two environments. Six coincident known QTLs were identified out of these studies on QTL, in that, 3D chromosome that was previously not identified showed the highest QTL effect in this study [50]. Liu et al. 2019 determined that nine loci are responsible for iron concentration in grain and some other genes were also identified [51]. A total of 35,648 high-quality genotyping and sequencing were used in 123 synthetic hexaploid wheat with the help of single nucleotide polymorphism (SNP) across all 21 chromosomes [48].
3.5 Genomic selection
Genomic selection is another method that is used by the plant breeder for bringing about the improvement of a complex trait like yield in wheat. Applied genomic selection in wheat would accelerate genetic gains in the development of a nutritionally enhanced variety [52]. It also states about the minor gene QTLs. And genomic selection can increase genetic gain by early selection. Velu et al. (2016) evaluated a set of 330 entries from the harvest plus association mapping (HPAM) panel which are derived from diverse wheat genetic resources in which the panel was genotyped with 90 K illumina SNP chip to examine the potential of genomic selection for Zn in two different sets of the environment in India and Mexico [53]. In this, 39 marker-trait associations were discovered using GWAS, and chromosome numbers 2 and 7 showed a larger effect QTL region [54].
Quantitative trait loci and GWAS are more utilized to dissect iron and zinc content in wheat, but these techniques take more time and are labor intensive. In GWAS, only natural variations can be exploited, and we cannot develop biparental mapping [55]. Hence, these techniques are overcome by the transgenic approach. The transgenic approach is a good rapid method to increase micronutrients in grains. In addition, the transgenic approach not only increases the iron and zinc content in grains but also elevates the micronutrient content in the endosperm of wheat [42]. A few studies have been conducted on wheat for increasing transformation efficiency and novel techniques to transform elite lines of wheat [56, 57], which will help in rapid characterization. Transgenic technology will help us to elucidate the function of candidate genes, in that either these genes are identified by mapping approaches or by the ortholog genes. The first transgenic approach was aimed to increase iron not only in grain but also in the storage of iron [56, 57].
3.6 CRISPR/Cas9
Genome editing technology, such as zinc finger nuclease (ZFNs), transcription activator-like effector nucleases (TALEN), and clustered regularly interspaced short palindromic repeats (CRISPR) and Cas9, serve as good and potential tools for biofortification in several crops without altering the whole genome. It alters and knocks out interested genes by the induction of small deletions that lead to the frameshift mutation, whereas it targets only the specific gene of interest [58, 59, 60]. Moreover, it is preferred by nongovernmental organizations and governmental organizations, unlike transgenic crops. Therefore, it is highly recommended by eminent plant breeders and scientists. These techniques employed in crops have been widely used for the improvement of some specific characteristics, such as biotic and abiotic stresses and biofortification, in crops [61]. The first-generation technology ZFN and TALEN are less preferred due to low target specificity and labor-intensive nature and develop many off-target cleavages in the genome. While TALEN required a thymine base at starting time that possesses a large size and is repetitive in nature [62, 63].
On the other hand, the use of CRISPR/Cas9 may bring in rapid advancement for Zn and Fe biofortification in elite wheat cultivars and saves the time that is taken by the backcross to remove the unwanted gene that came from conventional breeding. Simultaneously, the editing of homologous genes has been studied; however, a low percentage of transgenic lines have all three homologously edited genes [60]. In addition, little work has progressed using CRISPR for biofortification.
3.7 Mutagenesis
Mutagenesis is an alternative route to manipulation of gene functions, but to find the gene of interest in genome using target-induced local lesions in genome (TILLING) is a tedious and laborious work for plant breeders. Sequence mutant population is developed in hexaploid wheat and tetraploid wheat, which enable the rapid identification of mutation of the gene of interest in silico [64]. With the use of a 2014 IWGSC (International Wheat Genome Sequencing Consortium) reference genome, the mutations were identified and these mutations were analyzed using the RefSeqv1.0 genome which is available on the Ensembl Plants website [65].
Moreover, in durum wheat, this approach has been successfully applied to enhance the provitamin A content and it may be applied to increase the iron and zinc biofortification, if suitable genes are present [66]. And some mutagenesis population has been identified for iron and zinc biofortification, which is possible in nature in a limited/restricted manner, and it has also been observed in gamma-irradiated wheat lines [67]. Therefore, this mutagenesis approach may be utilized for increasing zinc and iron content in wheat crop without using a genetically modified (GM) approach (Table 1) [42].
Schematic representation of quantitative trait loci (QTL) of biofortified wheat.
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Written By
Anand Kumar and Prashant Kaushik
Submitted: 25 October 2022Reviewed: 25 May 2023Published: 23 July 2023
Open access peer-reviewed chapter
