**Progress and Challenges in Improving Nutritional Quality in Wheat**

Mantshiuwa C. Lephuthing, Timmy A. Baloyi, Nondumiso Z. Sosibo and Toi J. Tsilo

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67230

#### **Abstract**

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CC-83-0370

344 Wheat Improvement, Management and Utilization

Wheat (*Triticum aestivum* L.) houses a wide range of nutritional components such as iron (Fe), zinc (Zn), vitamins and phenolic acids, which are important for plant metabolism and human health. The bioavailability of these nutritional components is low due to their interaction with other components and low quantity in the endosperm. Biofortification is a more sustainable approach that could improve the bioavailability of essential nutritional components. Substantial progress has been made to improve nutritional quality through the application of conventional, technological and transgenic approaches. This has led to the discovery, cloning and introgression of the *Gpc-B1* gene; the invention of online databases with minimally characterized biosynthetic, metabolic pathways and biological processes of wheat-related species; the establishment of genetic variation in grain Fe and Zn content and the biofortification of wheat with Zn by the HarvestPlus organization. Nonetheless, the biofortification of wheat with micronutrients and phenolic acids is still a challenge due to incomplete understanding of the wheat genome, biosynthesis and translocation of selected nutritional components into different wheat grain compartments. There is a need to integrate selected omics technologies to obtain a holistic overview and manipulate key biological processes involved in the remobilization and biosynthesis of nutritional components into desired wheat grain compartments.

**Keywords:** bioavailability, biofortification, nutritional quality, omics, wheat endosperm

### **1. Introduction**

Wheat (*Triticum aestivum* L.) is a major crop grown in many countries. It is predominantly used for the production of products, such as bread, pasta, cereals and cakes, which are consumed on a regular basis [1, 2]. Thus, wheat has the potential to contribute to the reduction

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of malnutrition and deficiency-related ailments by contributing to food security and the daily required intake of essential macro- and micro-nutrients in individuals [3].

Nutritional quality, in context of this book chapter, is a collective term that refers to the bioavailability or concentration of desirable nutritional components for human health such as iron (Fe), zinc (Zn), vitamins and selected phenolic acids found in the wheat grain. These key nutritional components are found in different wheat grain compartments at varying concentrations [2]. The wheat germ and bran region contain the highest levels of these nutritional components [1]. Micronutrient deficiencies, especially those arising from Fe, Zn and vitamins pose a serious threat to human health as they affect more than 2 billion people worldwide especially women and children under the age of 5 years [4, 5]. Cardiovascular diseases, diabetes, cancer and malnutrition are among the most dreadful diseases. These diseases could be prevented through regular consumption of selected nutritionally important components, such as wholegrain products and antioxidants, which are acquired as phenolic acids from various foods including the wheat grain [6–11].

There are several challenges encountered in attempts to enhance the levels and bioavailability of some micronutrients, vitamins and phenolic acids in the wheat endosperm [12, 13]. The major challenge is that the complete wheat genome sequence is not available yet. Moreover, biological processes such as nutrient assimilation, translocation and biosynthesis pathway of wheat are not completely understood, in that some pathways have not yet been characterized. In addition, there are a few or no studies aimed at characterizing the process pertaining to micronutrient, vitamin and phenolic acid translocation into the wheat endosperm. Thus, it is difficult to manipulate biological processes involved in the accumulation of micronutrients, vitamins and phenolic acids into the wheat endosperm [14–16]. Therefore, there is a need to characterize the timing at which micronutrients, vitamins and phenolic acids are assimilated, translocated and synthesized in the endosperm. Some wheat grain constituents reduce the bioavailability of Fe through their inhibitory activity [17]. Furthermore, transporter molecules and chelators are mainly localized in the apoplast region, which leave the outer grain compartment layers more concentrated. This prevents desirable nutritional components essential nutrients from being loaded into the endosperm [18].

The NAC transcription factors that are involved in the acceleration of senescence and nutrient remobilization into the grain have also been identified as selected agents that could be used to improve nutritional quality [19]. Gene editing may potentially improve a number of identified traits of interest, thereby resulting in the improvement of the value of wheat [20]. In addition, foliar application has more advantage over other application methods as far as nutritional quality is concerned. Consequently, nitrogen application has enhanced the accumulation of Fe and Zn. Furthermore, soil and foliar application has also been shown to result in an enhancement of Fe and Zn [21]. The bioavailability of Ca2+, Zn2+ and Fe2+ was increased through breaking down phytate by the expression of phytase in transgenic wheat [22].

Additional online resources in the form of databases are also available and have been made public. These databases, including omics viewers for comparative analysis, mainly contain important information regarding various cellular processes, which have been acquired from more than 37,000 publications [23]. The resulting databases include the MetaCyc database of metabolic pathways and enzymes, and the BioCyc database is a collection of pathway/ genome databases that are currently available to the public [23]. These databases also made a huge contribution to our current understanding of many biological processes involved in nutrient assimilation, accumulation and translocation across different species. Nonetheless, not all pathways on *T. aestivum* are available. In addition, much progress has been made to understand several key biological processes facilitating the uptake of nutrients from the soil, through vascular tissues, then into the grain [14].

of malnutrition and deficiency-related ailments by contributing to food security and the daily

Nutritional quality, in context of this book chapter, is a collective term that refers to the bioavailability or concentration of desirable nutritional components for human health such as iron (Fe), zinc (Zn), vitamins and selected phenolic acids found in the wheat grain. These key nutritional components are found in different wheat grain compartments at varying concentrations [2]. The wheat germ and bran region contain the highest levels of these nutritional components [1]. Micronutrient deficiencies, especially those arising from Fe, Zn and vitamins pose a serious threat to human health as they affect more than 2 billion people worldwide especially women and children under the age of 5 years [4, 5]. Cardiovascular diseases, diabetes, cancer and malnutrition are among the most dreadful diseases. These diseases could be prevented through regular consumption of selected nutritionally important components, such as wholegrain products and antioxidants, which are acquired as phenolic acids from

There are several challenges encountered in attempts to enhance the levels and bioavailability of some micronutrients, vitamins and phenolic acids in the wheat endosperm [12, 13]. The major challenge is that the complete wheat genome sequence is not available yet. Moreover, biological processes such as nutrient assimilation, translocation and biosynthesis pathway of wheat are not completely understood, in that some pathways have not yet been characterized. In addition, there are a few or no studies aimed at characterizing the process pertaining to micronutrient, vitamin and phenolic acid translocation into the wheat endosperm. Thus, it is difficult to manipulate biological processes involved in the accumulation of micronutrients, vitamins and phenolic acids into the wheat endosperm [14–16]. Therefore, there is a need to characterize the timing at which micronutrients, vitamins and phenolic acids are assimilated, translocated and synthesized in the endosperm. Some wheat grain constituents reduce the bioavailability of Fe through their inhibitory activity [17]. Furthermore, transporter molecules and chelators are mainly localized in the apoplast region, which leave the outer grain compartment layers more concentrated. This prevents desirable nutritional components essential

The NAC transcription factors that are involved in the acceleration of senescence and nutrient remobilization into the grain have also been identified as selected agents that could be used to improve nutritional quality [19]. Gene editing may potentially improve a number of identified traits of interest, thereby resulting in the improvement of the value of wheat [20]. In addition, foliar application has more advantage over other application methods as far as nutritional quality is concerned. Consequently, nitrogen application has enhanced the accumulation of Fe and Zn. Furthermore, soil and foliar application has also been shown to result in an enhancement of Fe and Zn [21]. The bioavailability of Ca2+, Zn2+ and Fe2+ was increased through breaking down phytate by the expression of phytase in transgenic wheat [22].

Additional online resources in the form of databases are also available and have been made public. These databases, including omics viewers for comparative analysis, mainly contain important information regarding various cellular processes, which have been acquired from more than 37,000 publications [23]. The resulting databases include the MetaCyc database

required intake of essential macro- and micro-nutrients in individuals [3].

various foods including the wheat grain [6–11].

346 Wheat Improvement, Management and Utilization

nutrients from being loaded into the endosperm [18].

Although the strategies deployed to improve the levels and bioavailability of selected nutrients in wheat have been successful, biofortification is a more sustainable approach for improved nutritional quality [12, 13, 24]. It has been rendered sustainable in that it has been used to improve human health through ensuring that the required dietary intake of essential nutrients can reach poor individuals in a more sustainable and cost-effective manner [13]. Biofortification is a process of enhancing the dietary bioavailability or concentration of desirable nutritional components in plants genetically [25, 26]. This process has been used to successfully improve the bioavailability or levels of β-carotene in rice, Zn and Fe in wheat grain as well as levels of other nutrients in other crops [4]. As a result, a number of strategies have been deployed to improve nutritional quality in wheat, such as conventional, technological and transgenic approaches that were undertaken in efforts to improve the levels and bioavailability of micronutrients and phenolic acids, mainly through the biofortification route [17, 21, 27, 28]. This includes several efforts that managed to successfully increase the total grain nutrient content and bioavailability of some micronutrients through genetic biofortification, agronomic biofortification, the use of bioavailability enhancers, including genetic modification through transforming the plants with the ferritin gene, which may not be desirable by the public. Constitutive expression of ferritin, a gene that encodes an iron-rich soybean storage protein reported to be abundant in the endosperm amyloplast region, has largely contributed to Fe bioavailability enhancement [29].

However, there are still some challenges with the biofortification of wheat. The major challenge is enhancing the levels and bioavailability of selected nutritional components in the endosperm region as opposed to increasing the total grain micronutrient or phenolic acid content [15]. This is mainly because most micronutrients and phenolic acids are mainly loaded in the outer layers, which are removed upon milling of the wheat grain [14]. In addition, wheat grain yield, grain protein content and disease resistance are important traits that should not be compromised during new variety development or improvement. Nonetheless, there is little research aimed at characterising the process involved in enhancing the bioavailability and/or loading of micronutrients and phenolic acids into the endosperm region.

The era and deployment of omics technologies has largely contributed to our current understanding of biological functions of many traits in various crops including wheat [30, 31]. This has led to the manipulation and sustainable development of crops with improved traits of interest. These technologies have been widely applied in wheat research and resulted in improved understanding, manipulation and improvement of various complex traits in wheat [31]. Consequently, there is a need to integrate selected omic technologies to improve our current understanding of nutrient loading into different wheat grain compartments further. This will allow further manipulation of the nutrient loading pathway without affecting other traits of importance [31, 32]. Thereof, the selected omics technological platforms would bring about data outputs that would allow the establishment of a good balance in the expression of selected traits of interest in desired grain compartments [33]. The integration of these technologies would allow researchers to identify novel genes or pathways that could be activated to improve the bioavailability of desired nutritional components in wheat. This chapter aims to highlight the progress and challenges encountered in attempts to improve nutritional quality in wheat in order to recommend strategies that could be deployed to improve nutritional quality in a more sustainable and efficient way. The most important research question that needs to be addressed is, what is the source or origin of the total grain nutrient content of minerals or phenolic acids found in different grain compartments? Thus, there is still a need to conduct a quantitative assessment of the total mineral nutrient use efficiency and the type of mineral used for plant metabolism and seed production.
