REGULATION OF LEVELS OF PROLINE AS AN OSMOLYTE IN PLANTS UNDER WATER-STRESS

Compatible osmolytes are potent osmoprotectants that play a role in co unteracting the effects of osmotic stress. Proline (Pro) is one of the most common compatible osmolytes in water-stressed plants, The accumu lation of Pro in dehydrated plants is caused both by activation of the biosynthesis of Pro and by inactivation of the degradation of Pro, In plants, L-Pro is synthesized from L-glutamic acid (L-Glu) via Delta(1 )-pyrroline-5-carboxylate (P5C) by two enzymes, P5C synthetase (P5CS) and P5C reductase (P5CR), L-Pro is metabolized to L-Glu via P5C by two enzymes, proline dehydrogenase (oxidase) (ProDH; EC 1.5.99.8) and P5C dehydrogenase (P5CDH; EC 1.5.1.12), Such metabolism of Pro is inhibit ed when Pro accumulates during dehydration and it is activated when re hydration occurs, Under dehydration conditions, when expression of the gene for P5CS is strongly induced, expression of the gene for ProDH i s inhibited, By contrast, under rehydration conditions, when the expre ssion of the gene for ProDH is strongly induced, the expression of the gene for P5CS is inhibited, Thus, P5CS, which acts during the biosynt hesis of Pro, and ProDH, which acts during the metabolism of Pro, appe ar to be the rate-limiting factors under water stress, Therefore, it i s suggested that levels of Pro ape regulated at the level of transcrip tional the genes of these two enzymes during dehydration and rehydrati on. Moreover, it has been demonstrated that Pro acts as an osmoprotect ant and that overproduction of Pro results in increased tolerance to o smotic stress of transgenic tobacco plants, Genetically engineered cro p plants that overproduce Pro might, thus, acquire osmotolerance, name ly, the ability to tolerate environmental stresses such as drought and high salinity.