Proline Protects Plants Against Abiotic Oxidative Stress

Plants are constantly exposed to various abiotic and biotic stresses in nature. Abiotic stresses such as salinity, drought and extreme temperatures (heat, cold and freezing) are critical factors that reduce crop yields for most major crops worldwide. A widespread phenomena attributed to responses to both abiotic and biotic stresses is the unrestrained accumulation of reactive oxygen species (ROS) in plant cells, which can lead to irreparable metabolic dysfunction and death. To survive under stress conditions, plants have evolved intricate mechanisms to perceive external signals, allowing optimal responses to environmental conditions. Metabolic acclimation via the accumulation of proline is often regarded as a basic strategy for the protection and survival of plants under abiotic stress. Recent proteomic, genomic and metabolomic studies have revealed that proline, produced under stressful conditions, can act as a compatible solute in osmotic adjustment, a free radical scavenger, a metal chelator, an activator of ROS detoxification pathways, a cell redox balancer, a cytosolic pH buffer, a source of energy, a source of nitrogen and carbon, a stabilizer of subcellular structures and membranes including photosystem II, and can act as a signaling molecule. Moreover, the accumulation of proline – either naturally, generated by the expression of transgenes, or by exogenous application – can induce the expression of stress-responsive genes, including those encoding enzymes that scavenge ROS. In this chapter we will discuss and summarize the current understanding of ROS formation, proline biosynthesis and its accumulation in plants under various abiotic stresses. New insights gained about the molecular mechanisms of proline mediated oxidative stress tolerance will also be discussed.