Cloning and expression of a wheat (Triticum aestivum L.) phosphatidylserine synthase cDNA - Overexpression in plants alters the composition of phospholipids

We describe the cloning of a wheat cDNA (TaPSS1) that encodes a phosphatidy lserine synthase (PSS) and provides the first strong evidence for the exist ence of this enzyme in a higher eukaryotic cell, The cDNA was isolated on i ts ability to confer increased resistance to aluminum toxicity when express ed in yeast, The sequence of the predicted protein encoded by TaPSS1 shows homology to PSS from both yeast and bacteria but is distinct from the anima l PSS enzymes that catalyze base-exchange reactions. In wheat, Southern blo t analysis identified the presence of a small family of genes that cross-hy bridized to TaPSS1, and Northern blots showed that aluminum induced TaPSS1 expression in root apices, Expression of TaPSS1 complemented the yeast cho1 mutant that lacks PSS activity and altered the phospholipid composition of wild type yeast, with the most marked effect being increased abundance of phosphatidylserine (PS). Arabidopsis thaliana leaves overexpressing TaPSS1 showed a marked enhancement in PSS activity, which was associated with incr eased biosynthesis of PS at the expense of bo th phosphatidylinositol and p hosphatidylglycerol, Unlike mammalian cells where PS accumulation is tightl y regulated even when the capacity for PS biosynthesis is increased, plant cells accumulated large amounts of PS when TaPSS1 was overexpressed. High l evels of TaPSS1 expression in Arabidopsis and tobacco (Nicotiana tabacum) l ed to the appearance of necrotic lesions on leaves, which may have resulted from the excessive accumulation of PS, The cloning of TaPSS1 now provides evidence that the yeast pathway for PS synthesis exists in some plant tissu es and provides a tool for understanding the pathways of phospholipid biosy nthesis and their regulation in plants.