**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.
