Phytophthora phospholipase D genes and their role in plant cell degradation

Plant pathogens secrete a large repertoire of proteins that may have a role in pathogenicity such as modulation of host defense (e.g. effectors) or degradation of host tissue. Phytophthora infestans, the potato late blight pathogen, possesses genes encoding secreted proteins with a phospholipase D (PLD) catalytic domain and this suggests that the Phytophthora secretome comprises this class of enzymes. PLD catalyzes the hydrolysis of membrane phospholipids leading to the production of phosphatidic acid and a free headgroup. Phosphatidic acid is a key player in the arena of cellular signalling. It is involved in many processes, including G-protein regulation, protein phosphorylation, transcription, cell proliferation and growth. PLD activity and phosphatidic acid production in plants is also correlated with membrane degradation during senescence and wounding. Phytophthora infestans, has 18 genes encoding PLDs. One PLD is a universal PLD that is present in all eukaryotes. A second one has homology to a novel class of PLD(-like) proteins. The other sixteen, divided over 4 subfamilies, are more diverged from known PLDs in eukaryotes and unique for Phytophthora. Two of these sub- families are small, secreted proteins. We detected PLD activity in exudates of Phytophthora and showed that these exudates are capable of degrading lipid vesicles. Transient in planta expression of the genes encoding secreted PLDs resulted in a calcium dependent induction of cell death responses. This could point to a function in host membrane modification and/or degradation. Further molecular and biochemical characterization of the various Phytophthora PLDs is in progress.