Innate immune defences of plants include a set of basal responses that can be triggered by the perception of general elicitors that have been termed Microbe Associated Molecular Patterns (MAMPs). Examples of MAMPs include lipopolysaccharides (LPS) from Gram-negative bacteria, peptidoglycans (PGN) from both Gram-positive and Gram-negative bacteria, flagellin, fungal cell wall glucans and chitin. Recognition of MAMPs in both insects and animals leads to activation of defences and is often mediated by LRR (Leucine Rich Repeat) proteins such as Toll in Drosophila and the Toll-like receptors (TLRs) in mammals. LRR proteins also serve as receptors for MAMPs in plants; examples include the flagellin receptor FLS2 and elongation factor Ef-Tu receptor EFR. It is now established that LPS and PGN have myriad effects in plants including elicitation of plant defence responses, induction of the oxidative burst, nitric oxide synthesis, and phosphorylation of mitogen-activated protein (MAP) kinase. I will present data showing that two lysine motif (LysM) containing plasma membrane proteins, LYM1 and LYM3 from the model plant Arabidopsis, interact with PGN; and that the transmembrane LysM receptor kinase CERK1 is involved in transmembrane signalling. Still, little is known about the perception of LPS and PGN by plants or the associated signal transduction pathways that trigger the plant immune responses. I will present our progress towards identification of the plant receptors and the following plant signal transduction pathways induced by LPS and PGN purified from the black rot pathogen Xanthomonas campestris.
