Fungal LysM effectors perturb chitin-triggered host immunity

Cladosporium fulvum is a biotrophic fungal pathogen that causes leaf mould of tomato. The in planta abundantly secreted C. fulvum effector Ecp6 (for extracellular protein 6) acts as a potent virulence factor. The Ecp6 protein contains three lysin motifs (LysMs), protein domains that were also identified in plant cell surface receptors that activate host immunity upon perception of chitin oligosaccharide PAMPs, breakdown products of fungal cell walls that are released during plant invasion. Affinity precipitation assays showed that Ecp6 is a chitin-binding protein and three binding sites per molecule were detected. We found that Ecp6 does not protect fungal cell walls against hydrolysis by plant chitinases. Rather, Ecp6 appears to prevent the activation of chitin-triggered immunity through scavenging of chitin oligosaccharide PAMPs. Interestingly, homologues of Ecp6 were identified in many fungal species. These effectors are collectively referred to as LysM effectors. A number of LysM effectors from other fungal plant pathogens have been produced. Similar to Ecp6, most of these bind chitin and are able to suppress chitin-triggered immunity. However, in contrast to Ecp6, some LysM effectors protect fungal cell walls against chitinases, while others appear to have different substrates than chitin