Phospholipids are cellular membrane components that execute many important roles in signalling. Genome mining in Phytophthora uncovered a large number of genes encoding enzymes involved in phospholipid signalling and metabolism, several of which have aberrant combinations of catalytic and regulatory domains (Meijer & Govers 2006 MPMI). This project focuses on phospholipase D (PLD). Phytophthora species have a large repertoire of PLDs divided over six subfamilies: PXPH-PLD, PXTM-PLD, TM-PLD, PLD-likes, and type A and B sPLD-likes. Since the latter have signal peptides we developed a method using metabolically labelled phospholipids to monitor if P. infestans secretes PLD. In extracellular medium of ten P. infestans strains PLD activity was detected as demonstrated by the production of phosphatidic acid and the PLD specific marker phosphatidylalcohol (Meijer et al. 2011 PLoS One). To investigate the role of secreted PLDs in pathogenicity of P. infestans we analyse how plants respond when P. infestans sPLD genes are transiently expressed in leaves, and whether or not this response changes when the PLD catalytic site is mutated.