Understanding How Plants Respond to Drought Stress at the Molecular and Whole Plant Levels

Depending on duration and severity, drought stress causes many morphological, physiological, biochemical, and molecular changes in plants. At the morphological level, drought in general, reduces leaf expansion and growth and results in smaller plant size, lower leaf area index, and lower plant biomass and yield. The consequence of the drought stress on the reduction of grain yield depends on the growth stage at which the stress occurs. Drought stress is more sensitive when stress occurs during the reproductive growth stage, especially prior to or at the flowering stage. Stomatal closure, osmotic adjustment (OA), redox regulation through scavenging of reactive oxygen species (ROS), and synthesis of inducible proteins (dehydrins) are processes that help plants to tolerate drought stress. These processes are mediated in plants via signaling pathways and their crosstalk which culminates in the expression of drought-responsive genes. Several signaling pathways that operate under drought in plants include ABA, strigolactone, lipid-derived signal, ROS, soluble sugars, and others. These signaling pathways are primarily ABA-dependent or ABA-independent. Major transcription factors such as ABREB/ABF, DREB1/CBF, DREB2, NAC, and so on play important roles in the regulatory network of the induction of drought-related genes, the products of which are either functional proteins such as antioxidant enzymes and dehydrins, or regulatory proteins such as transcription factors. Several efforts are being made in recent times where some of these important genes involved in plant response to drought are transferred through a genetic engineering approach to produce transgenic plants. Overexpression of some of these essential drought-responsive genes in the transgenic plants enhanced their tolerance to drought stress, mainly under controlled conditions. However, more comprehensive work needs to be directed toward testing these transgenic plants under natural growing conditions in the field. Even though a single-gene transfer enhanced drought tolerance (DT), multigene transfer might have a greater improvement in DT.