Abstract
Type III secretion systems (T3SSs) play an essential role in the pathogenicity of many bacterial pathogens. Phytopathogenic bacteria use the T3SS to deliver effector proteins directly into host plant cells to promote pathogenicity. To counteract, plants have evolved resistant proteins to monitor the effectors and halt pathogen invasion and disease. We have investigated the T3SS of Mesorhizobium loti, a microsymbiont of model legume Lotus japonicus. Expression analysis showed that T3SS is expressed in response to host-derived flavonoids, suggesting that it plays a role during infection. Depending on host species, T3SS affected symbiosis either positively or negatively. By inoculating mutants of various effector candidates, an ORF mlr6361 was identified as a major determinant of nodulation restriction. Mlr6361 is a protein of 3,056 amino acids containing 15 repetitions of a sequence motif of 40 to 45 residues and a shikimate kinase-like domain at its carboxyl-terminus. Homologues of Mlr6361 are present in several plant pathogens. These results suggested that some legume recognize Mlr6361 as potentially pathogen-derived and subsequently halts the infection of symbiotic partner.
