PRODUCTS OF PROLINE CATABOLISM CAN INDUCE OSMOTICALLY REGULATED GENESIN RICE

Many plants accumulate high levels of free proline (Pro) in response t o osmotic stress. This imino acid is widely believed to function as a protector or stabilizer of enzymes or membrane structures that are sen sitive to dehydration or ionically induced damage. The present study p rovides evidence that the synthesis of Pro may have an additional effe ct. We found that intermediates in Pro biosynthesis and catabolism suc h as glutamine and Delta(1)-pyrroline-5-carboxylic acid (P5C) can incr ease the expression of several osmotically regulated genes in rice (Or yza sativa L.), including salT and dhn4. One millimolar P5C or its ana log, 3,4-dehydroproline, produced a greater effect on gene expression than 1 mM L-Pro or 75 mM NaCl. These chemicals did not induce hsp70, S -adenosylmethionine synthetase, or another osmotically induced gene, E m, to any significant extent. Unlike NaCI, gene induction by P5C did n ot depend on the normal levels of either de novo protein synthesis or respiration, and did not raise abscisic acid levels significantly. P5C - and 3,4-dehydroproline-treated plants consumed less O-2, had reduced NADPH levels, had increased NADH levels, and accumulated many osmolyt es associated with osmotically stressed rice. These experiments indica te that osmotically induced increases in the concentrations of one or more intermediates in Pro metabolism could be influencing some of the characteristic responses to osmotic stress.