Identification of Aerotaxis Receptor Proteins Involved in Host Plant Infection by Pseudomonas syringae pv. tabaci 6605

Pseudomonas syringae pv. tabaci 6605 (Pta6605) is a foliar plant pathogen that causes wildfire disease on tobacco plants. It requires chemotaxis to enter plants and establish infection. While chemotactic signals appear to be the main mechanism by which Pta6605 performs directional movement, the involvement of aerotaxis or energy taxis by this foliar pathogen is currently unknown. Based on domain structures and similarity with more than 50 previously identified putative methyl-accepting chemotaxis proteins (MCPs), the genome of Pta6605 encodes three potential aerotaxis transducers. We identified AerA as the main aerotaxis transducer and found that it possesses a taxis-to-serine-and-repellent (Tsr)-like domain structure that supports a periplasmic 4HB-type ligand-binding domain (LBD). The secondary aerotaxis transducer, AerB, possesses a cytosolic PAS-type LBD, similar to the Aer of Escherichia coli and Pseudomonas aeruginosa. Aerotaxis ability by single and double mutant strains of aerA and aerB was weaker than that by wild-type Pta6605. On the other hand, another cytosolic PAS-type LBD containing MCP did not make a major contribution to Pta6605 aerotaxis in our assay system. Furthermore, mutations in aerotaxis transducer genes did not affect surface motility or chemotactic attraction to yeast extract. Single and double mutant strains of aerA and aerB showed less colonization in the early stage of host plant infection and lower biofilm production than wild-type Pta6605. These results demonstrate the presence of aerotaxis transducers and their contribution to host plant infection by Pta6605.