Role and Regulation of Osmolytes and ABA Interaction in Salt and Drought Stress Tolerance

Abiotic stress conditions lead to the defects in plant growth and development and also reduction in flowering and fertility. Under prolonged stresses, imminent death of the plants has been observed. To cope with such stress conditions, plants accumulate a wide variety of organic solutes called osmolytes. Osmolytes are accumulated in bacteria, lower, and higher plants as a response primarily to abiotic stress. They encompass amino acids such as proline, tertiary sulfonium, and quaternary ammonium compounds like beatines, sugars (trehalose), and polyhydric alcohols (mannitol, sorbitol, pinitol, etc.). Osmolytes are accumulated in the cytoplasm as well as in chloroplasts in certain cases for osmotic adjustment under stress conditions. This enables the plants to absorb water and survive under stress. Out of the many phytohormones that play diverse roles during abiotic stress, abscisic acid (ABA) is an important one and perceived by plants by a core signaling module. As an integral part of signal transduction during stress conditions, ABA and other hormones regulate not only stomatal closure, but also a wide array of gene expressions including osmolyte biosynthetic pathway genes. Many signal molecules like nitric oxide, carbon monoxide, and hydrogen sulfide also play a vital role in osmolyte biosynthesis. Osmolytes appear to have multiple functions during stress such as osmotic adjustment and scavenging of reactive oxygen species (ROS). Thus, generation of ROS and osmolyte accumulation are linked together. This review summarizes the role played by ABA in signal transduction, the role of hormones to regulate osmolyte biosynthesis, and various functions carried out by them

Proline suppresses apoptosis in the fungal pathogen Colletotrichum trifolii

The role of reactive oxygen species (ROS) in cell communication, control of gene expression, and oxygen sensing is well established. Inappropriate regulation of ROS levels can damage cells, resulting in a diseased state. In Colletotrichum trifolii, a fungal pathogen of alfalfa, the mutationally activated oncogenic fungal Ras (DARas) elevates levels of ROS, causing abnormal fungal growth and development and eventual apoptotic-like cell death but only when grown under nutrient-limiting conditions. Remarkably, restoration to the wild-type phenotype requires only proline. Here, we describe a generally unrecognized function of proline: its ability to function as a potent antioxidant and inhibitor of programmed cell death. Addition of proline to DARas mutant cells effectively quenched ROS levels and prevented cell death. Treating cells with inhibitors of ROS production yielded similar results. In addition, proline protected wild-type C. trifolii cells against various lethal stresses, including UV light, salt, heat, and hydrogen peroxide. These observations appear to be general because proline also protected yeast cells from lethal levels of the ROS-generating herbicide methyl viologen (paraquat), suggesting a common protective role for proline in response to oxidative stress. The ability of proline to scavenge intracellular ROS and inhibit ROS-mediated apoptosis may be an important and broad-based function of this amino acid in responding to cellular stress, in addition to its well established role as an osmolyte