Xanthomonas oryzae pv. oryzicola is an X. oryzae pathovar that causes bacterial leaf streak in rice. In this study, we performed functional characterization of a nine-gene his operon in X. oryzae pv. oryzicola. Sequence analysis indicates that this operon is highly conserved in Xanthomonas spp. Auxotrophic assays confirmed that the his operon was involved in histidine biosynthesis. We found that two genes within this operon, trpR and hisB, were required for virulence and bacterial growth in planta. Further research revealed that trpR and hisB play different roles in X. oryzae pv. oryzicola. The trpR acts as a transcriptional repressor and could negatively regulate the expression of hisG, -D, -C, -B, -H, -A, and -F. hisB, which encodes a bifunctional enzyme implicated in histidine biosynthesis, was shown to be required for xanthomonadin production in X. oryzae pv. oryzicola. The disruption of hisB reduced the transcriptional expression of five known shikimate pathway-related genes xanB2, aroE, aroA, aroC, and aroK. We found that the his operon in X. oryzae pv. oryzicola is not involved in hypersensitive response in nonhost tobacco plants. Collectively, our results revealed that two genes in histidine biosynthesis operon play an important role in the pathogenicity of X. oryzae pv. oryzicola Rs105.

Xanthomonas oryzae pv. oryzae, an economically important bacterium, causes a serious disease in rice production worldwide called bacterial leaf blight. How X. oryzae pv. oryzae infects rice and causes symptoms remains incompletely understood. Our earlier works demonstrated that the twin-arginine translocation (Tat) pathway plays an vital role in X. oryzae pv. oryzae fitness and virulence but the underlying mechanism is unknown. In this study, we used strain PXO99A as a working model, and identified 15 potential Tat-dependent translocation proteins (TDTP) by using comparative proteomics and bioinformatics analyses. Combining systematic mutagenesis, phenotypic characterization, and gene expression, we found that multiple TDTP play key roles in X. oryzae pv. oryzae adaption or virulence. In particular, four TDTP (PXO_02203, PXO_03477, PXO_02523, and PXO_02951) were involved in virulence, three TDTP (PXO_02203, PXO_03477, and PXO_02523) contributed to colonization in planta, one TDTP (PXO_02671) had a key role in attachment to leaf surface, four TDTP (PXO_02523, PXO_02951, PXO_03132, and PXO_03841) were involved in tolerance to multiple stresses, and two TDTP (PXO_02523 and PXO_02671) were required for full swarming motility. These findings suggest that multiple TDTP may have differential contributions to involvement of the Tat pathway in X. oryzae pv. oryzae adaption, physiology, and pathogenicity.