InnB, a Novel Type III Effector of USDA61, Controls Symbiosis With Species.

USDA61 is incompatible with mung bean ( cv. KPS1) and soybean ( cv. BARC2) and unable to nodulate either plant. This incompatibility is due to the presence of a functional type III secretion system (T3SS) that translocates effector protein into host cells. We previously identified five genes in that are responsible for its incompatibility with KPS1 plants. Among them, a novel gene designated as exhibited some characteristics associated with the T3SS and was found to be responsible for the restriction of nodulation on KPS1. In the present study, we further characterized by analysis of gene expression, protein secretion, and symbiotic phenotypes. The gene was found to encode a hypothetical protein that is highly conserved among T3SS-harboring rhizobia. Similar to other rhizobial T3SS-associated genes, the expression of was dependent on plant flavonoids and a transcriptional regulator TtsI. The InnB protein was secreted via the T3SS and was not essential for secretion of other nodulation outer proteins. In addition, T3SS-dependent translocation of InnB into nodule cells was confirmed by an adenylate cyclase assay. According to inoculation tests using several species, InnB promoted nodulation of at least one cultivar. These results indicate that encodes a novel type III effector controlling symbiosis with species.