Proline accumulation in developing grapevine fruit occurs independently ofchanges in the levels of Delta(1)-pyrroline-5-carboxylate synthetase mRNA or protein

Mature fruit of grapevine (Vitis vinifera) contains unusually high levels o f free proline (Pro; up to 24 mu mol or 2.8 mg/g fresh weight). Pro accumul ation does not occur uniformly throughout berry development but only during the last 4 to 6 weeks of ripening when both berry growth and net protein a ccumulation have ceased. In contrast, the steady-state levels of both the m RNA encoding V. vinifera Delta(1)-pyrroline-5-carboxylate synthetase (VVP5C S), a key regulatory enzyme in Pro biosynthesis, and its protein product re main relatively uniform throughout fruit development, in addition, the stea dy-state protein levels of Pro dehydrogenase, the first enzyme in Pro degra dation, increased throughout early fruit development but thereafter remaine d relatively constant. The developmental accumulation of free Pro late in g rape berry ripening is thus clearly distinct from the osmotic stress-induce d accumulation of Pro in plants. It is not associated with either sustained increases in steady-state levels of P5CS mRNA or protein or a decrease in steady-state levels of Pro dehydrogenase protein, suggesting that other phy siological factors are important for its regulation.