Pseudomonas syringae pv. tabaci 6605 Requires Seven Type III Effectors to Infect Nicotiana benthamiana

Type III effectors (T3Es), virulence factors injected into plant cells via the type III secretion system (T3SS), play essential roles in the infection of host plants. Pseudomonas syringae pv. tabaci 6605 (Pta 6605) is the causal agent of wildfire disease in tobacco and harbours at least 22 T3Es in its genome. However, the specific T3Es required by Pta 6605 to infect Nicotiana benthamiana remain unidentified. In this study, we investigated the T3Es that contribute to Pta 6605 infection of N. benthamiana. We constructed Pta 6605 poly-T3E-deficient mutants (Pta DxE) and inoculated them into N. benthamiana. Flood assay, which mimics natural opening-based entry, showed that mutant strains lacking 14-22 T3Es, namely, Pta D14E-D22E mutants, exhibited reduced disease symptoms. By contrast, infiltration inoculation, which involves direct injection into leaves, showed that the Pta D14E to Pta D20E mutants developed disease symptoms. Notably, the Pta D20E, containing AvrE1 and HopM1, induced weak but observable symptoms upon infiltration inoculation. Conversely, no symptoms were observed in either the flood assay or infiltration inoculation for Pta D21E and Pta D22E. Taken together, these findings indicate that the many T3Es such as AvrPto4/AvrPtoB, HopW1/HopAE1, and HopM1/AvrE1 in Pta 6605 collectively contribute to invasion through natural openings and symptom development in N. benthamiana. This study provides the basis for understanding virulence in the host by identifying the minimum T3E repertoire required by Pta 6605 to infect N. benthamiana.