Phosphatidylcholine Synthesis in Plants

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The synthesis of phosphatidylcholine in plants has been thought to occur primarily through phosphorylation of choline, followed by conversion to CDPcholine, and then transfer of a phosphatidate group to the choline to give the final lipid product. An alternate pathway has been proposed which involves three sequential methylations, utilizing S- adenosyl-L-methionine, of the amino group of phosphatidylethanolamine. Recent research has indicated that in some tissues, such as castor bean endosperm, the choline produced from a commonly assumed precursor, L-serine, may not be available for phosphatidylcholine synthesis. In addition, evidence now is available that in several tissues phosphatidylethanolamine does not act as a substrate for methylation, unlike the case in yeast and mammalian tissues. Thus, either plants differ in some respects from yeast and mammalian systems in the exact pathways of methylation and their regulation, or some aspects of the methylation scheme have not been detected and examined in these other systems. The goals of this proposal are to examine these newly described initial methylation enzymes in detail, to try to determine a source of choline for the nucleotide (CDP- derivative) pathway (and for regulation of the functioning of the methylation pathway), and to continue our investigations into the enzymes of the nucleotide pathway. We will characterize the methylation enzymes, work on their purifications, and test for regulation through allosteric and/or membrane-enzyme interactions as well as phosphorylation/dephosphorylation. Various plant organs, particularly leaves and roots, will be tested for a capacity to synthesize and export choline or choline-P. Phospholipids are important components of membranes of all living organisms. They play crucial structural and metabolic role; yet their pathways of synthesis are incompletely known, especially in plants. This project affords the opportunity to elucidate important pathways of phospholipid synthesis and regulation in a model plant system.***//