DROUGHT TOLERANCE MECHANISMS IN GENETICALLY MODIFIED WHEAT

A CRITICAL REVIEW OF GENETIC TARGETS, PATHWAYS, AND EXPERIMENTAL EVIDENCE

Abstract

Drought is one of the most harmful abiotic stresses that limit wheat (Triticum aestivum L.) production worldwide and genetic modification can be a complementary approach to conventional breeding to enhance drought resistance. This review summarizes peer-reviewed primary studies that focus on changes in the molecular, physiological and biochemical underlying processes that allow wheat to withstand water deficit and whether these changes can be shown to actually increase drought tolerance. Literature search was performed in the PubMed, ScienceDirect, Frontiers, MDPI, Wiley, Springer Nature, and PMC databases but a structured search was carried out and transgenic or genome-edited wheat research studies were given more preference than review articles. Experimental systems in which evidence was obtained were classified as: wheat transgenic studies (highest priority); wheat gene-manipulation studies; wheat mechanistic studies; and studies in heterologous model species. Improved drought tolerance in wheat has been reproducibly demonstrated in the synthesis through overexpression of transcription factors (GmDREB1, HaHB4, TaWRKY2, TaMYB31-B), expansins (TaEXPA2) and components of ABA signaling (TaSnRK2.4, TaPYL4). These changes occur via five partially overlapping mechanisms: osmoregulation, antioxidant defense, ABA signaling and stomatal regulation, photosynthetic protection and root architecture. There are only a limited number of field-validated evidence and of these, HaHB4 (HB4) and GmDREB1 have been approved for commercial cultivation in Argentina. Currently, the evidence of the drought tolerance of wheat via CRISPR is weak and mostly indirect. There are several important gaps such as the lack of multi-environment field trials, phenotyping protocols that vary and the lack of knowledge regarding the role of genotype-by-environment-by-transgene interactions. The review finds that genetic modification is able to significantly improve specific measures of drought tolerance in wheat, but the enhancement of yield in the field will need to be carefully tested in a range of environments.