[Identification of a new type III effector XC3176 in Xanthomonas campestris pv. campestris].

OBJECTIVE: Type III secretion system (T3SS) is essential for many phytopathogenic bacteria to cause disease in susceptible host plants and to elicit a hypersensitive response in resistant host and non-host plants. Xanthomonas campestris pv. campestris (Xcc) uses T3SS to deliver T3SS effectors (T3SEs) directly into host cells, where they play important roles in pathogenesis. The aim of this study was to identify a new T3SE in Xcc.

METHODS: To validate if XC3176 is a T3S effector translocated into plant cells, the promoter and signal region of XC3176 were fused to the plasmid pLJB of harboring HR-inducing AvrBs1 C-terminal domain lack of 58 N-terminal amino acid residues. The recombinant plasmid pLJB3176 was introduced by triparental conjugation into ΔavrBs1 and ΔhrcV. Hypersensitive response induced by the obtained strains ΔavrBs1/pLJB3176 and ΔhrcV/pLJB3176 were examined on the pepper ECW-10R. To determine transcription of XC3176, GUS fusion report strains were constructed. The virulence of Xcc strains was investigated on the Chinese radish by the leaf-clipping method.

RESULTS: Hypersensitive response was elicited on the pepper ECW-10R by the strain ΔavrBs1/pLJB3176, but not ΔhrcV/pLJB3176. The GUS activities in the mutant strains ΔhrpX and ΔhrpG were significantly lower than that in the wild type Xcc strain. The mutant of XC3176 reduced virulence significantly and the complementary strain C3176 could restore the virulence as the wild-type strain.

CONCLUSION: XC3176 is a T3SS-dependent effector of Xanthomonas campestris pv. campestris. The expression of XC3176 is regulated by hrpG and hrpX. XC3176 is required for the full virulence of Xcc 8004.