The hrp (hypersensitive response on nonhost plants and pathogenicity in host plants) gene cluster, which conforms Xanthomonas oryzae pv. oryzicola, the causal agent of bacterial leaf streak in rice, to the ability to elicit a hypersensitive response (HR) in nonhost tobacco and pathogenicity in host rice, is thought to be regulated by the hrpG and hrpX genes. However, it is unclear whether the hrpG and hrpX genes regulate all the hrp gene expression of the cluster coordinately or not. We constructed a mutant with both hrpG and hrpX genes knocked out. which, as the same as the single mutant at the hrpG or hrpX gene, lost the ability to trigger HR in tobacco and pathogenicity in rice, respectively. Correspondingly, the hrpG and hrpX genes together could restore such the ability to the double mutant at the hrpG and hrpX loci. After grew in rice suspension cells, hrp-inducing medium XOM3 and nutrient-rich medium NB for 16 hours, respectively, reverse transcriptional polymerase chain reaction (RT-PCR) revealed that: the expression of hrp genes in NB medium was lower that that in rice cells and XOM3 medium; in whatever growth media, the hrcC, hrcT, hrpE and hpa3 genes expressed, while the hpa1, hpa2, hpaB, hrcJ, and hrpG genes did not in the hrpG mutant; the hpa2, hrcC, hpa3, hrpE and hrpG genes expressed, while the hpa1, hrcT, hpaB and hrcJ genes did not in the hrpX mutant; the hrcC, hrpE and hpa3 genes expressed, but the hpa1, hpa2, hpaB, hrcT, hrcJ and hrpG genes did not in the hrpG and hrpX double mutant. This indicated that the expression of the hrcC, hrpE and hpa3 genes was not regulated by the hrpG and/or hrpX genes, but the expression of the hrcT gene was negatively controlled by the hrpG gene. Thus, we postulated that the expression of key type- III secretion (T3S) apparatus components was controlled by an unknown signaling pathway, which may facilitate our further understanding on the formation of the T3S machine.