Role of Abscisic Acid under Water Deficit Stress in Plants

Abscisic Acid (ABA) and water deficit stress are interlinked to each other. A deficit of water induces the synthesis of ABA for an increase of osmoprotectants, antioxidants and membrane stability as well as a decrease in water loss. The phytohormone that induces the factor for the protection of plant under stress is known as an antistress phytohormone and Abscisic acid (ABA) is a well-known antistress phytohormone that saves the plant from adverse water deficit stress in cells. Its concentration increases when plants sense water deficit stress and its signal transduction pathway activates leading to cellular responses to activate the genes. It stimulates stomatal closure, change in gene expression and adaptive morpho-physiological responses. Light regulates stomata opening and ABA promotes partial or complete closure of stomata. The closure of stomata reduces the loss of water by transpiration which accounts for approximately 90% through its pore. At the same time, complete stomatal closure prevents the exchange of gases through its pore resulting in the decrease of the Calvin cycle and eventually reduction of biomass. Moreover, the efflux of water decreases and influx increases in the cell due to the synthesis of water-capturing compatible solutes viz. proline, glycine betaine, sugars (sucrose, fructose, trehalose), polyols (sorbitol, mannitol, arabinitol, glycerol), etc. which are induced by ABA. However, proline and glycine betaine are prominent among all these compatible solutes. Thus, ABA regulates the factors that help the plant cope up the water deficit stress. Proline biosynthesis in chloroplasts reached up to 80% during stress as compared to 5% in normal which facilitates the electron flow between photosynthetic excitation centers and maintains a low NADPH:NADP ratio in cells.