The rsmA‐like gene rsmAXoo of Xanthomonas oryzae pv. oryzae regulates bacterial virulence and production of diffusible signal factor

SUMMARY The plant‐pathogenic prokaryote Xanthomonas oryzae pv. oryzae ( Xoo ) causes bacterial leaf blight, one of the most destructive diseases of rice. A nonpolar mutant of the rsmA ‐like gene rsmA Xoo of the Xoo Chinese strain 13751 was constructed by homologous integration with a suicide plasmid. Virulence tests on a host plant, namely the hybrid rice cultivar Teyou 63, showed that the mutant had lost its virulence almost completely, whereas tests on a nonhost, namely castor‐oil plant ( Ricinus communis ), showed that the mutant had also lost the ability to induce a hypersensitive response in the nonhost. In addition, the rsmA Xoo mutant produced significantly smaller amounts of the diffusible signal factor, extracellular endoglucanase, amylase and extracellular polysaccharide, but showed significantly higher glycogen accumulation, bacterial aggregation and cell adhesion. The expression of most hrp genes, genes encoding AvrBs3/PthA family members, rpfB , xrvA , glgA , eglXoB and XOO0175 (encoding an α‐amylase) was down‐regulated in the rsmA Xoo mutant. All phenotypes and expression levels of the tested genes in the rsmA Xoo mutant were restored to their levels in the wild‐type by the presence of rsmA Xoo in trans . These results indicate that rsmA Xoo is essential for the virulence of Xoo .