Mutational analysis of type III effector genes from Xanthomonas citri subsp. citri

Abstract

Xanthomonas citri subsp. citri, the causal agent of citrus canker, relies extensively on a type III secretion system for infection by delivering type III effectors into host cells. In the genus Xanthomonas, two major regulators, HrpG and HrpX, are involved in the expression of genes encoding the type III secretion system. Twenty-three candidate type III effectors were identified as targets for analysis. The involvement in pathogenicity of 20 candidate effector genes in X. citri strain 306 (Xcc-306) was investigated using site-directed mutagenesis. Pathogenicity assays in grapefruit of 19 genes using site-directed mutagenesis revealed that none of the mutants demonstrated to have reduced ability to cause canker disease. A mutation in the TAL effector pthA4– resulted in loss of hypertrophy although no changes were observed in bacterial growth in leaves. Mutations in hrpG, hrpX, or hrpA genes displayed a complete loss of pathogenicity. Moreover, all mutants maintained the ability to trigger a hypersensitive response (HR) in non-host tomato. In contrast to previous studies, hrpG–, hrpX– and hrpA– mutants also retained the ability to elicit an HR in tomato, indicating the presence of an Hrp independent elicitor in Xcc-306.