PLASMA-MEMBRANE LIPID-METABOLISM OF PETUNIA PETALS DURING SENESCENCE

The specific activities of 6 enzymes, which are involved in the synthe sis and catabolism of membrane lipids, were monitored in plasma membra nes isolated from petunia petals during senescence. These included pho sphatidylinositol (PI) kinase (EC 2.7.1.67), phosphatidylinositol mono phosphate (PIP) kinase (EC 2.7.1.68), diacylglycerol (DAG) kinase (EC 2.7.1.107), phospholipase A (EC 3.1.1.4) and PIP- and PIP2-phospholipa se C- (EC 3.1.4.3). Using endogenous substrate, the [P-32] PA and [P-3 2]PIP2 formation increased to 140 and 200%, respectively, of the day 1 value by 4 days after harvest. There was no significant change in [P- 32]PIP formation during the same time period. On the fifth day the pet als wilted and the [P-32]PA and [P-32]PIP formation declined significa ntly. In contrast, the [P-32]PIP, formation remained high in the day 5 petals. When the lipid kinase activities were assayed in the membrane s in the presence of exogenous substrate the specific activity of all of the enzymes increased, and the changes in [P-32P]PA production over the 5-day period were similar to those observed with endogenous subst rate. When exogenous PI and PIP were added, however, there was no long er an increase in [P-32]PIP2 formation by plasma membranes of day 4 pe tals and [P-32]PIP formation significantly decreased. The relative dec rease in PIP and PIP2 formation by day 4 membranes when exogenous subs trate was added may have resulted from differences in the lipase activ ities in the day 1 and day 4 membranes. The plasma membrane A-type pho spholipase activity increased throughout the 5 day period, and phospho lipase C activity increased two-fold between day 1 and day 4. Such cha nges in the metabolism of the plasma membrane lipids during flower sen escence would affect the ability of the petals to use inositol phospho lipid-based signal transduction pathways.