Molecular and Morphophysiological Analysis of Drought Stress in Plants

Drought is a major environmental stress factor that affects the growth and development of plants. Most of the physiological traits associated with drought tolerance are quantitative in nature. An important research strategy that has been widely used to deal with such complexity is to use molecular markers to identify quantitative trait loci (QTLs) in appropriate mapping populations. In response to drought brought about by soil water deficit, plants can exhibit either drought escape or drought resistance mechanisms, with resistance further classified into drought avoidance and drought tolerance. Drought escape is the ability of plants to complete the life cycle before severe stress arrives. Drought avoidance is the maintenance of high tissue water potential in spite of soil water deficit. Drought avoidance is consequence of improved water uptake under stress and the capacity of plant cells to hold acquired water that reduces water loss. Drought tolerance is the ability to withstand water deficit with low tissue water potential. Plant water status that includes leaf water potential, osmotic potential and relative water content (RWC) represents an easy measure of water deficit and provides best sensor for stress. Genomics‐assisted breeding (GAB) approaches, such as marker‐ assisted selection (MAS), can greatly improve precision and efficiency of selection in crop breeding. Molecular markers can facilitate indirect selection for traits that are difficult or inconvenient to score directly, pyramiding genes from different sources and combining resistance to multiple stresses. Conventional breeding for developing drought‐tolerant crop varieties is time‐consuming and labor intensive due to the quantitative nature of drought tolerance and difficulties in selection for drought tolerance. The identification of genomic regions associated with drought tolerance would enable breeders to develop improved cultivars with increased drought tolerance using marker‐assisted selection (MAS). This requires integration of knowledge from plant physiology and biotechnology into plant breeding. The availability of a large number of molecular markers, dense genetic maps and markers associated with traits and transcriptomics resources have made it possible to integrate genomics of mild drought and recovery revealed a photosynthetic acclimation pattern in response to mild drought in contrast to photosynthesis inhibition under severe drought. A comprehensive understanding of the response of plants to mDr with physiological and molecular tools provided a better understanding of the acclimation process. A semi‐automat‐ ed, controlled mDr testing system was employed to compare with pDr treatment for physio‐ logical and molecular responses. This revealed differential gene reprogramming under the two drought treatments. The dissection of mDr treatment is presented using a time‐course study to provide a picture of physiological and molecular responses toward acclimation in plant growth. of parallel pathways in other crop plants such as in rice (Oryza sativa) as a model for monocot plants. A number of drought treatments have been used to test the response of plants for improved tolerance/resistance. One method is progressive drought (pDr), in which water is with held for a certain period of time until symptoms of wilting are observed. Usually, this method of drought treatment has been used to determine survival rate or to monitor gene expression changes of wild‐type plants or of plant genotypes overexpressing candidate genes for drought tolerance. These studies have helped to study plant responses to drought at both the physiological and molecular levels. However, one of the drawbacks of pDr treatment is that it cannot be used to compare the performance of different genotypes with different growth characteristics. In nature, drought often develops during a growing season and occurs for a short period, which tolerant plants can manage to survive and complete their growth cycle. Soil water deficit causing drought stress in crop plants has been tested in Arabidopsis using controlled soil moisture treatment. Controlled drought treatment, exposing plants to constant levels of soil moisture deficit, enables the evaluation between genotypes/ecotypes for plant responses to drought. coupled with identification of several large‐effect QTLs for grain yield under drought in different crops. Independent and epistatic QTLs for grain yield and other traits of agronomic importance were studied in different crops. Only a few studies reported major QTLs affecting yield advantage under both drought stress and non‐stress environments. Drought is normally associated with increased incidence of diseases such as blast, brown spot, and bacterial blight. Few studies have been undertaken to understand the genetics of these abiotic and biotic stresses simultaneously in a mapping population. Identification of QTLs is paving the way to MAS and assisted pyramiding of the beneficial QTL alleles. Markers can be used in marker‐assisted selection (MAS) for improving the desired trait. Isolation and validation of genes underlying the QTL/genes for the traits of interest is an essential step to determine gene function. QTLs for drought tolerance have been identified for several major and important crop species such as rice, maize, wheat, barley, sorghum, pearl millet, soybean and chickpea. These QTLs were identified for a variety of important traits including: (1) yield and yield‐ contributing traits under water‐deficit conditions (in the case of wheat, maize, rice, soybean