Pyrroline-5-carboxylic acid mediates some physiological processes during osmotic stress in rice

Osmotic stress, often caused by drought and salinity, is a common problem in agriculture. Continued growth and productivity of crops depends on their ability to adapt to adverse environmental conditions. Osmotic adjustment by accumulation of osmolytes is a major adaptive mechanism exhibited by all organisms. Sucrose, proline and betaine are some of the common osmolytes accumulated by plants. The amino acid proline is perhaps one of the most widely distributed osmolytes across phylogenetic lines. Rice like many flowering plants under stress is known to accumulate proline to high levels in leaf tissues. Despite intensive research during the last several years, the reason for proline accumulation is still disputed. Results from in vitro experiments have led to the belief that proline acts as an osmoprotectant by protecting cellular macromolecules and membrane structures that are sensitive to dehydration from denaturation. But the role of proline has not been proven conclusively.; My research focused on investigating the effects of increased endogenous proline in plant response to osmotic stress. The biochemical and physiological responses of rice plants was studied after growing them in the presence of proline. Results from the study provides evidence that {dollar}{bsol}Delta{bsol}sp1{dollar}-pyrroline-5-carboxylic acid (P5C), a metabolic intermediate and a precursor to proline, is able to regulate several osmotically-induced rice genes including salT and dehydrin genes such as dhn4 and rab16. One millimolar P5C was a much better inducer of these genes than similar concentrations of D- or L-proline or 75 mM NaCl. P5C had no effect on another osmotically-induced gene, Em, but slightly altered the expression of two metabolically important genes S-adenosylmethionine synthetase (sam) and hsp70. Unlike NaCl, induction of gene expression by P5C did not require de novo protein synthesis or altered intracellular ABA levels. Specific metabolic pathways were also affected. Treated plants had markedly decreased respiratory activity and levels of NADH and NADPH. One other oxidized form of proline, dehydro-DL-proline (DHP), acted like P5C. DHP appeared to be a more potent inducer of salT gene expression than P5C, but unlike P5C, DHP demonstrated a requirement for NAD{dollar}{bsol}sp+{dollar} for gene induction. For example, respiratory inhibitors significantly reduced the effectiveness of DHP as an inducer of gene expression, indicating that exogenously provided DHP may require a source of NAD{dollar}{bsol}sp+{dollar} to first be converted to proline and then oxidized to become the inducing molecule. In addition, P5C and DHP treated plants accumulated many of the same solutes that are known to be involved in osmotic adjustment. However, their effectiveness varied with the region of the plant. P5C increased solute levels mostly in the root whereas DHP primarily affected the blade solute pools. These results indicate that stress-induced increases in levels and distribution of P5C might be used by rice plants to alter the levels of expression of specific sets of genes and the activity of metabolic pathways involved in osmotic adjustment. Since the effects of P5C on rice metabolism closely resembles that of NaCl, it is possible that the effects of salt stress are being mediated by P5C that is continuously made during the synthesis of proline.