Introductory Chapter: Current Trends in Wheat Research

1. Introduction

Wheat (Triticum aestivum) is known as one of the most important cereal crops and is extensively grown worldwide [1]. Wheat contributes to 50% and 30% of the global grain trade and production respectively [2]. Wheat is also known as a staple food in more than 40 countries of the world. Wheat provides 82% of basic calories and 85% of proteins to the world population [3, 4]. Wheat-based food is rich in fiber contents than meat-based food. Dough produced from bread wheat flour has different viscoelastic properties than other cereals. It is considered a higher fiber food. Therefore, its positive effects on controlling cholesterol, glucose, and intestinal functions in the body were observed [5]. Primarily, wheat is being used to make Chapatti (Bread) but it also contributes to other bakery products. Wheat utility and high nutritional value made it the staple food for more than 1/3rd population of the world. Wheat grain is separated from the chaff and stalks after the harvesting of wheat. Stalks of wheat are further used in animal bedding and construction material. Globally, the need for wheat production is enhancing even in countries having unfavorable climates for its production. Global climate changes are badly affecting the production of wheat and it raised the concern for food security.

It is estimated that annual cereal production should be increased by 1 billion tons to feed the expected population of 9.1 billion by 2050 [1]. The current scenario demands an increase in crop productivity to meet the increased requirements of food supply [6]. Wheat is grown in tropical and subtropical regions which experiences a lot of stress. These stresses result in a reduction of yield [7]. Major environmental stresses include cold, salinity, heat, and drought which are drastically affecting its yield. However, water and heat are considered as the key environmental stresses which caused in reduction of the wheat yield globally [8, 9]. So, genetic improvements related to yield and stress tolerance are mandatory to enhance the production of wheat [10, 11].

2. Genetically modified wheat plants

Genetically modified wheat plants have been produced by the use of bacteria. Wheat plants were inoculated with the plant-growth-promoting bacteria (PGPB) which resulted in the higher expression of abiotic stress (mainly drought and salinity) tolerant genes [12]. PGPB inoculated wheat cultivars also showed the higher expression of genes encoding antioxidant-enzymes, such as catalase (CAT), peroxidase, ascorbate peroxidase (APX), and glutathione peroxidase (GPX). So, it was concluded that PGPB used in wheat plants resulted in increased tolerance to abiotic stresses [12]. Cold shock proteins increase the survival of bacteria in severe environmental conditions. CspA and CspB genes from bacteria were transformed into wheat. Transgenic wheat plants expressing SeCspA and SeCspB were observed to have decreased water loss rate, increased proline and chlorophyll contents under salinity, and less water-stress conditions [13]. It was further investigated that SeCsp transgenic wheat plants resulted in enhanced weight and yield of grain than the control plants. SeCspA transgenic wheat plants were observed to have an improved water-stress tolerance than the control plants (Table 1, [13]).

S. No.	Gene Name	Trait/Phenotype	Reference
1.	ZmDof1	Increased yield	[14]
2.	TaNFY-A-B1	Increased Nitrogen and Phosphorus uptake	[15]
3.	TaNF-YB4	More grain yield	[16]
4.	TaNAC2-5A	More root growth	[17]
5.	OsSS-I	Increased heat tolerance	[18]
6.	TaGS2	More yield	[19]
7.	ZmAGPase	More yield	[20]
8.	NtNR	More yield, More seed protein contents	[21]
9.	TaVIT2	Iron biofortification	[22]
10.	HD-ZipI	Drought and frost tolerance	[23]
11.	DREB1A	Drought tolerant	[24]
12.	CspA & CspB	Drought-stress tolerance	[25]
13.	HaHB4	Abiotic-stress tolerance	[26]
14.	TaDREB3	Drought-stress tolerance	[27]

Table 1.

Development of transgenic wheat having various traits/phenotypes.

Gluten is a protein comprised of gliadins found in wheat. Gluten is the main cause of coeliac disease in individuals. Bread-making quality of wheat is determined by the gluten proteins. Wheat varieties with less gliadin contents were produced using gene-editing technologies and RNAi (RNA interference). Wheat lines lacking immunogenic gluten were produced. Low immunogenic gluten and more nutritional values were added in one wheat line named E82. A better microbiota profile (protection microorganisms available in the gut) was observed in the NCWS patients using the bread made with E82 [28]. Plant cuticle has a positive role in the protection of plant against biotic and abiotic stresses. Wheat plants transformed with TaSHN1 resulted in increased water-stress tolerance by reducing the leaf stomatal density and changing the composition of the cuticle [29].

3. Biotic stress tolerance in wheat

Wheat is considered an excessive contributor toward the human calorie intake [30]. Pests and pathogens cause yield losses in wheat up to 21.5% of the total losses and could be reached to 28.1% [31]. Wheat is affected by the fungal disease, powdery mildew caused by Blumeria graminis f. sp. tritici (Bgt). Powdery mildew is a damaging disease that resulted in greater loss of wheat [32]. Broad-spectrum resistant genes (BSR) are considered to have the most significant role to control powdery mildew. CMPG1-V gene was cloned from the Hynaldia villosa and it was observed that higher expression of CMPG1-V gene resulted in the Broad-spectrum resistance against powdery mildew [33, 34]. Barley chi26 gene could also be used to enhance the resistance against powdery mildew and rust through genetic modification [35]. Some epigenetic regulators were determined to have a role in wheat powdery mildew resistance. TaHDT701 is a histone deacetylase that was found as a negative regulator of wheat defense against powdery mildew. TaHDT701 was observed to be associated with the one repeat protein (TaHOS15) and RPD3 type histone deacetylase TaHDA6. Knockdown of this histone deacetylase complex (TaHDT701, TaHDA6, TaHOS15) in wheat resulted in increased powdery mildew tolerance [36].

Fusarium graminearum is a plant fungal pathogen that causes a devastating disease called Fusarium head blight in wheat. It results in the reduction of wheat production. Genetic techniques were used to increase the FHB (Fusarium head blight) resistance in wheat. Transgenic wheat plants expressing barley class II chitinase gene 2 were observed to have a higher resistance against Fusarium graminearum [37]. Lr10 and Lr21 were cloned and transformed into wheat. The transgenic plants were reported to be resistant to leaf rust disease. Evolution and diversification of HIPPs (heavy metal-associated isoprenylated plant proteins) genes were studied in Triticeae [38]. HIPPs genes of Hynaldia villosa were cloned through homology-based cloning. Transgenic wheat having HIPP1-V was developed and the role of HIPP1-V in cadmium stress was characterized. It was observed that higher expression of this gene resulted in increased tolerance to cadmium stress. Therefore, HIPP1-V could be used to increase the tolerance in wheat against cadmium [39].

4. Abiotic stress tolerance in wheat

Grain number, weight, and size are greatly reduced under the negative effects of environmental stresses. However, the timing, duration, and intensity of stress determine the severity of the negative effects [40, 41]. Wheat is a major source of protein and calories for the human diet. High temperature is badly affecting the yield of wheat which is a main concern worldwide. Drought and heat stresses are the two main abiotic stresses which are playing a greater role in the reduction of wheat yield. Reduction in starch contents, photosynthetic activity, grain number, and chlorophyll contents in the endosperm is caused due to rise in temperature. Heat stress results in the accumulation of reactive oxygen species (ROS) which is the main reason for higher oxidative damage to the plant. Heat stress also results in the variation of wheat biochemistry, morphology, and physiology. Tolerance, avoidance, and escape are known as the three major mechanisms that support the plant to grow in a heat-stress environment. Major heat tolerance mechanisms in wheat are known as stay green, heat shock proteins, and antioxidant defense [42]. Protein synthesis and folding were observed to be interrupted during heat stress. Heat stress also resulted in the production of several stress agents badly affecting transcription, translation, and DNA replication in plants [43]. Plants speed up the production of heat shock proteins as a defense mechanism [44]. Higher activity of antioxidants, such as peroxidases, catalase, and superoxide dismutase, was observed under heat stress. Wheat cultivar showing greater tolerance to heat stress was observed to have higher activity of catalase, ascorbate peroxidase, and S-transferase [45].

Salt stress greatly affects the growth of wheat plants. Salinity stress has a higher impact on the morphology and physiology of wheat plants. Plants having less tolerance to salinity are not suitable for cropping. Potassium transporter (HKT) genes have a greater role in achieving salinity tolerance in wheat. Sodium (Na⁺) exclusion through HKT genes is a major mechanism in wheat to have a salinity tolerance. OsMYBSs and AtAB14 are the transcription factors having a role in regulating HKT genes, which are considered as the candidate targets for increasing salinity tolerance in wheat [46]. Wheat transformed with a mutated transcription factor, HaHB4 showed higher water-use efficiency and was more yielding under drought stress [26]. Transgenic wheat expressing GmDREB1 gene from soybean was also observed to have higher drought tolerance under water-stress conditions [47]. DREB1A gene from Arabidopsis thaliana was introduced to bread wheat and increased tolerance against water stress in the transgenic wheat was observed. Bread wheat under drought stress was observed to have a higher level of WRKY proteins [48]. Higher expression of AtHDG11 gene in transgenic wheat resulted in increased water-stress tolerance during drought-stress conditions. Enhanced TaNAC69 expression in root and leaf of wheat during drought stress was observed [49]. Researchers are working to develop transgenic wheat having various traits/phenotypes by using advanced approaches of biotechnology for the last several decades (Table 1). Numbers of transgenic wheat cultivars are being grown in the fields and several more are under trial.

5. CRISPR/Cas9 system in wheat

Gliadins and glutenins are known as the gluten proteins and ingestion of these proteins from barley, rye, and wheat could cause the disease called coeliac disease in humans. The only remedy is to develop gluten-free food. Transgenic wheat which retains baking quality and is safe for coeliac could not be produced using conventional methods because of the complexity of the wheat genome. Coeliac disease (CD) is activated by the immunogenic isotopes mainly gliadins. Gliadin families were downregulated by the use of RNA interference. CRISPR/Cas9 is a targeted gene manipulation tool considered to have a potential role in genetic modification (Table 2, [60, 61]). CRISPR/Cas9 system was recently used for gene editing of gliadins. Offsprings with deleted, edited, or silenced gliadins were produced by CRISPR/Cas9. They helped to decrease the exposure of the patient to the CD epitopes [62]. This technology has been used to develop wheat cultivars having gluten genes with inactivated CD epitopes [62, 63].

S. No.	Gene Name	Trait/Phenotype	Reference
1.	TaMLO	Powdery mildew resistance	[50]
2.	TaPHO2-A1	Improved Phosphorus uptake	[51]
3.	TaGASR7	Improved yield	[52]
4.	TaEDR1	Powdery mildew resistance	[53]
5.	TaGW2	Improved yield	[54]
6.	TaMs1	Male sterility	[55]
7.	TaSBEIIa	High amylase contents	[56]
8.	TaLOX2	Improved quality	[57]
9.	TaALS	Herbicide tolerance	[58]
10.	TaACC	Herbicide tolerance	[59]

Table 2.

Genome edited wheat developed by CRISPR/Cas9 system.

CRISPR/Cas9 system and TALENS (transcription activator-like effector nuclease) were used in the bread wheat to generate the mutations in three homoeoalleles that encode MLO locus proteins against mildew. Mutations in all three TaMLO were generated by using TALENS which resulted in resistance against powdery mildew. The MLO homoeoalleles (TaMLOA1, TaMLOB1, and TaMLOD1) of bread wheat contributed to the mildew infection. Mutation of MLO alleles resulted in powdery mildew tolerance in wheat [50]. Genome editing was reported in which pds (phytoene desaturase) and inox (inositol oxygenase) genes in the cell suspension-culture of wheat were targeted. It was demonstrated that the genome-editing technique could also be applied in the cell suspension of wheat [64]. Very recently, various research groups are involved to develop transgenic wheat by using genome-editing technology. Some of the experiments are listed in Table 2.

6. Wheat computational analysis

A comprehensive resource for wheat reference genome was developed by International Wheat Genome Sequencing Consortium. The URGI portal (https://wheat-urgi.versailles.inra.fr/) was developed for the breeders and researchers to access the genome sequence data of bread-wheat. InterMine tools, genome browser, and BLAST were established for the exploration of genome sequences together with the additional linked datasets, including gene expression, physical maps, and sequence variation. Portal provided the higher browser and search features that facilitated the use of the latest genomic resources required for the upgradation of wheat [65].

DNA binding with one finger (Dof) transcription factors is known to have an important role in abiotic stress tolerance as well as the growth of plants. Ninety-six TaDof members of the gene family have been studied using computational approaches. By qPCR analysis, it was revealed that TaDof genes were upregulated under heavy metal and heat stress in wheat. Consequently, it could be concluded that detection of amino acid sites, genome-wide analysis, and identification of the Dof transcription factor family could provide us the new insight into the function, structure, and evolution of the Dof gene family [66].

Acknowledgments

This work was supported by funds from the Higher Education Commission of Pakistan.