Mutagenesis of 18 Type III Effectors Reveals Virulence Function of XopZPXO99 in Xanthomonas oryzae pv. oryzae

Xanthomonas oryzae pv. oryzae depends on a type III secretion system (T3SS) to translocate effectors into host cells for its ability to cause bacterial blight of rice. All type III (T3) effectors with known function in X. oryzae pv. oryzae belong to a family of transcription activator-like (TAL) effectors. However, other, non–TAL-related effector genes are present in the genome, although their role in virulence and their mode of action have yet to be elucidated. Here, we report the generation of mutants for 18 non-TAL T3 effector genes and the identification of one that contributes to the virulence of strain PXO99A. XopZPXO99 encodes a predicted 1,414-amino-acid protein of unknown function. PXO99A contains two identical copies of the gene due to a duplication of 212 kb in the genome. Strains with knockout mutations of one copy of XopZPXO99 did not exhibit any visible virulence defect. However, strains with mutations in both copies of XopZPXO99 displayed reduced virulence in terms of lesion length and bacterial multiplication compared with PXO99A. The introduction of one genomic copy of XopZPXO99 restores the mutant to full virulence. Transient expression of XopZPXO99 in Nicotiana benthamiana leaves suppresses host basal defense, which is otherwise induced by a T3SS mutant of PXO99A, suggesting a role for XopZPXO99 in interfering with host innate immunity during X. oryzae pv. oryzae infection. XopZPXO99-related genes are found in all Xanthomonas spp. whose genomic sequences have been determined, suggesting a conserved role for this type of effector gene in pathogenesis of Xanthomonas spp. Our results indicate that XopZPXO99 encodes a novel T3 effector and contributes virulence to X. oryzae pv. oryzae strains for bacterial blight of rice.

Abstract licence: CC BY-NC-ND

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