Phosphatidylcholine turnover during senescence of rose petals

Abstract

Senescence of rose ( Rosa hyb., cv. Mercedes) petals is accompanied by intensive modifications in membrane properties, where a decline in phospholipid content is a major change. Pulse-chase experiments with [ 14C]choline chloride revealed that the decline in phosphatidylcholine (PC) with age is a result of a lower biosynthetic capacity of older petals and not of an increased degradation. The enzyme cytidine-diphospho-choline diacylglycerol phosphorylcholine phosphotransferase (CDP-cholinephosphotransferase, EC 2.7.8.2) was therefore studied in detail. This enzyme is involved in PC synthesis and its activity was demonstrated in microsomal fractions and found to be similar to that reported for other plant systems. The enzyme's V max was higher in microsomes of young petals than of old ones. This, coupled with the reduction in protein content, resulted consequently in a decline in the total activity of the enzyme during the petal senescence. Modifications in the thiol groups of the membrane proteins resulted in changes in the enzyme activity. The activity of the enzyme was enhanced by the addition of egg lecithin and partially inhibited by cholesterol. In addition, it was altered by detergents known to influence the enzyme's membranous microenvironment. Senescence of petals has previously been shown to be accompanied by a decrease in membrane lipid fluidity and protein thiol content. Therefore it is possible that the observed age-related changes in enzyme activity, which lead in turn to the overall reduction in petal phospholipid content, are a result of these membrane changes.