Role of Formate Chemoreceptor in Pseudomonas syringae pv. tabaci 6605 in Tobacco Infection

Chemotaxis is essential for infection by plant pathogenic bacteria. The causal agent of tobacco wildfire disease, Pseudomonas syringae pv. tabaci 6605 (Pta6605), is known to cause severe leaf disease and is highly motile. The requirement of chemotaxis for infection has been demonstrated through the inoculation of mutant strains lacking chemotaxis sensory component proteins. Pta6605 possesses 54 genes that encode chemoreceptors (known as methyl-accepting chemotaxis proteins, MCPs). Chemoreceptors are classified into several groups based on the type and localization of ligand-binding domains (LBD). Cache LBD-type chemoreceptors have been reported to recognize formate in several bacterial species. In the present study, we identified Cache_3 Cache_2 LBD-type Mcp26 encoded by Pta6605_RS00335 as a chemoreceptor for formate using a quantitative capillary assay, and named it McpF. Although the deletion mutant of mcpF (ΔmcpF) retained attraction to 1% yeast extract, its chemotactic response to formate was markedly reduced. Swimming and swarming motilities were also impaired in the mutant. To investigate the effects of McpF on bacterial virulence, we conducted inoculations on tobacco plants using several methods. The ΔmcpF mutant exhibited weaker virulence in flood and spray assays than wild-type and complemented strains, highlighting not only the involvement of McpF in formate recognition, but also its critical role in leaf entry during the early stages of infection.