Osmolytes and their Role in Abiotic Stress Tolerance in Plants

Plants are sessile organisms which encounter a variety of stresses at every developmental stage of their life. The phylosphere as well as the rhizosphere encounter abiotic stresses such as cold, heat, UV, submergence, wounding, temperature, drought, salinity, drought, high metal concentrations, waterlogging, and nutrient deficiency stress. Salinity, drought, and high metal level stresses cause some overlapping effects on plants which include high reactive oxygen species level, antioxidant system activation and accumulation of inert solutes such as sugars, polyamines, secondary metabolites and amino acids. Among amino acids, proline is the most important and is well known for its role in salinity stress tolerance. These inactive metabolites or inert solutes are known as osmolytes because of their important function in combating osmotic stress caused by salinity and high metal level stresses. The synthesis and accumulation of various osmolytes are among the earliest responses of host plants for combating osmotic and oxidative stress caused by various stressers. The osmolytes counterbalances osmotic pressure imposed by salinity and high metal level stress. Various studies have shown that the expression of the metabolite biosynthesis pathway genes is regulated by phytohormones, calcium signaling, and MAP kinase pathways under abiotic stress conditions. Thus, the accumulation of osmolytes is the result of various stress signaling pathways, which is necessary for the survival of plants in abiotic stress environments. Their increased level protects, minimizes, and rescues plants from stress-caused oxidative damage, growth diminution, and loss of photosynthesis efficiency. In this chapter, we analyze the different osmolytes and their roles in improving the salinity, oxidative, and high metal level stress tolerance of plants in extreme environments in different plants.