TRANSCRIPTIONAL REGULATION OF PROLINE BIOSYNTHESIS

Plants are subjected to various kinds of abiotic and biotic stresses throughout their life cycles which include salinity, drought, temperature extremes, infection by pathogens, nutrient deficiency and UV radiation. A general response of plants to various kinds of stresses is the accumulation of compatible osmolytes such as proline, glycine betaine, proline betaine, glycerol, mannitol and sorbitol etc. which protect cells against damage caused by stress. Among them, proline plays a pivotal role and accumulates in a large number of species under salinity, drought, cold, nutrient deficiency, pathogen attack and high acidity. The core enzymes in this reaction are pyrroline5- carboxylate synthetase (P5CS) and pyrroline5- carboxylate reductase (P5CR). In another pathway, proline synthesis occurs via deamination of ornithine which is transaminated to P5C by ornithine-delta-aminotransferase (OAT). Plant cells have a potential to accumulate proline rapidly and break it down quickly when needed. Considerable evidence confirmed that proline synthesis under osmotic stress is driven by both ABA-dependent and ABA-independent signaling. Emerging data suggest that the expression of proline biosynthetic genes is regulated by many TFs that are related to almost all plant hormones. Several unique predicted elements were found in AtP5CR, including putative bZIP, HD-HOX, MYB and C2C2 (Zn) DOF binding sites. Thus, it could be concluded that proline regulation takes place through complex interrelation of different TFs and helps in generating tolerance in plants against abiotic stress.