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Two structurally different oomycete MAMPs induce distinctive plant immune responses

Plants recognize a variety of external signals and induce appropriate mechanisms to increase their tolerance to biotic and abiotic stresses. Precise recognition of attacking pathogens and induction of effective resistance mechanisms are critical functions for plant survival. Some molecular patterns unique to a certain group of microbes (MAMPs, microbe-associated molecular patterns) are sensed by plant cells as non-self molecules via pattern recognition receptors. While a variety of MAMPs of bacterial and fungal origin have been identified, reports on MAMPs of oomycete pathogens are relatively limited. This study aimed to identify unique MAMP elicitors from the oomycete pathogen Phytophthora infestans, the causal agent of potato late blight. Using reactive oxygen species (ROS) production and phytoalexin production in potato as markers for the purification of oomycete elicitors, we identified two structurally different groups of elicitors, namely ceramides and diacylglycerols. P. infestans ceramide (Pi-Cer) elicitors induced ROS production, while diacylglycerol (Pi-DAG) elicitors, containing eicosapentaenoic acid (EPA) as a substructure, induced the formation of phytoalexins in potato. Pi-Cer and Pi-DAG are also contained in the mycelia of another oomycete pathogen Pythium aphanidermatum, indicating that they are MAMPs of oomycetes. When Arabidopsis was treated with Pi-Cer and EPA, partially overlapping but different sets of genes were induced. Furthermore, simultaneous treatment with Pi-Cer and EPA did not have a cumulative effect on induced genes, but rather the expression of some genes induced by EPA was attenuated by the co-treatment with Pi-Cer. These results indicate that plants may combine the signals from simultaneously recognized MAMP elicitors to specifically adapt the defense response to a particular pathogen.