Flagellar motility of Pseudomonas syringae pv. actinidiae biovar 3 contributes to bacterial infection through stomata

Highly virulent Pseudomonas syringae pv. actinidiae biovar 3 (Psa3) causes kiwifruit bacterial canker, which seriously damages kiwifruits worldwide. We previously screened Psa3-transposon-inserted mutants and obtained reduced-virulence mutants with genes encoding the flagellar protein. Flagella are involved in the motility of bacteria and contribute to P. syringae invasion of plant leaves. However, whether flagella are required for the ability to enter stomata and then multiply in the plant apoplast is not known. Here, we sprayed kiwifruit leaves with Psa3-flagellar-defective mutants and found that their virulence was reduced compared to that of the Psa3 wild type, but their virulence was not reduced when the leaves were infiltrated with Psa3-flagellar-defective mutants using a syringe. Motility and bacterial entry into apoplastic space of the flagellar-defective mutants were also significantly reduced. These results indicate that flagella contribute to Psa3 motility and stomata-mediated entry, leading to disease development, but not multiplication in apoplast space after Psa3 entry. This study provides new insight into bacterial motility in host plants.