水稻条斑病菌hrcC、hpa3和hrpE基因表达不依赖hrpG和hrpX基因调控

Expression of the hrcC, hrpE and hpa3 Genes Is not Regulated by the hrpG and hrpX Genes in a Rice Pathogen Xanthomonas oryzae pv. Oryzicola

摘要:

摘要：【目的】决定水稻条斑病菌（Xanthomonas oryzae pv. oryzicola）在非寄主植物上激发过敏反应（hypersensitive response, HR）和在寄主水稻上致病性（pathogenicity）的hrp基因簇是受hrpG和hrpX基因调控的，但还不清楚hrpG和hrpX基因是否共同决定着所有hrp基因的表达。【方法】本文通过基因敲除方式获得了水稻条斑病菌的hrpG和hrpX基因的双突变体。【结果】烟草和水稻上测定结果显示，双突变体与单突变体一样，均在烟草上失去HR激发能力和丧失在水稻上的致病性；相应地，功能互补后双突变体恢复至野生表型。细菌在水稻悬浮细胞、hrp诱导培养基XOM3和营养丰富的培养基NB中生长后的RT-PCR结果显示，NB中hrp基因低水平表达，XOM3和水稻细胞能够高水平诱导hrp基因表达。无论何种生长条件，hrpG单突变体中hrcC、hrcT、hpa3和hrpE基因表达，而hpa1、hpa2、hpaB、hrcJ和hrpG基因不表达；hrpX单突变体中hpa2、hrcC、hpa3、hrpE和hrpG基因表达，而hpa1、hrcT、hpaB和hrcJ基因不表达；hrpG和hrpX双突变体中hrcC、hpa3和hrpE基因表达，而hpa1、hpa2、hpaB、hrcT、hrcJ和hrpG基因不表达。【结论】这提示，水稻条斑病菌的hrcC、hrpE和hpa3基因不受hrpG和hrpX基因单独或同时调控，而hrcT基因受HrpG调控。由此推测，水稻条斑病菌III型分泌系统关键组份的表达有可能通过另外的信号途径进行调控，这为进一步分析III型分泌途经的形成提供了线索。

Abstract:

Abstract: [Objective] 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. [Methods] We constructed a mutant with both hrpG and hrpX genes knocked out. [Results] 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. [Conclusion] 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.