HexR Transcription Factor Contributes to pv. Virulence by Coordinating Type Three Secretion System Genes.

pv. () causes bacterial blight on cabbage. We previously conducted a screening for reduced virulence using Tn transposon mutants and identified one of the transcriptional factors, HexR, as a potential virulence factor. However, the role of HexR in plant pathogenic virulence has not been investigated well. Here, we demonstrated that the mutant showed reduced disease symptoms and bacterial populations on cabbage, indicating that HexR contributes to virulence. We used RNA-seq analysis to characterize the genes regulated by HexR. We found that several type three secretion system (T3SS)-related genes had lower expression of the mutant. Five genes were related to T3SS machinery, two genes were related to type three helper proteins, and three genes encoded type three effectors (T3Es). We also confirmed that T3SS-related genes, including , , , and , were also down-regulated in the mutant both in culture and in vivo by using RT-qPCR. T3SS functions to suppress plant defense in host plants and induce hypersensitive response (HR) cell death in non-host plants. Therefore, we investigated the expression profiles of cabbage defense-related genes, including and , and found that the expressions of these genes were greater in the mutant. We also demonstrated that the mutant did not induce HR cell death in non-host plants, indicating that HexR contributes in causing HR in nonhost plants. Together, these results indicate that the mutation in leads to a reduction in the T3SS-related gene expression and thus an impairment in plant defense suppression, reducing virulence.