Several bacterial genera, including Bacillus and Pseudomonas species produce cyclic lipopeptides (CLPs), compounds composed of a fatty acid tail linked to a short cyclic oligopeptide. CLPs inhibit the growth of a range of plant and human pathogens, including oomycetes. Research in the Bacterial Ecology and Genomics group is focused on the synthesis of CLPs in bacteria and their role in biological control. When zoospores of P. infestans are exposed to the CLP massetolide A (massA) they lyse within minutes. MassA also inhibits expansion of late blight lesions on tomato, mycelial growth, sporangia formation and cyst germination. It causes increased branching and hyphal swellings and adversely affects autoaggregation of zoospores. Autoaggregation of zoospores of a P. infestans transformant, that constitutively produces the active Gα-subunit, was less affected by massA (Van de Mortel et al. 2009 AEM) suggesting that G-protein signalling plays a role in the perception of CLPs by P. infestans. Because of their zoosporicidal activity and their growth-inhibitory effects, CLPs are promising as a substitute or supplement in the protection of crops against oomycete pathogens. To unravel the mode of action of CLPs we analysed Mass A induced responses in P. infestans at the transcriptional level using an Affymetrix@ GeneChip containing over 15,000 P. infestans unigenes. Several of the up- and down regulated genes encode membrane transporters, including MFS- and ABC-transporters, which may act as a first line of defence. The aim of this project is to analyse the functions MFS transporters in defence of P. infestans against the deleterious effects of Mass A.