Mo Xuyan, Zhang Liyuan, Liu Yan, Wang Xuan, Bai Jiaqi, Lu Kai, Zou Shenshen, Dong Hansong, Chen Lei
College of Plant Protection, Nanjing Agricultural University, Nanjing, China.
College of Plant Protection, Shandong Agricultural University, Tai'an, China.
Front Microbiol. 2020 Jul 22;11:1601. doi: 10.3389/fmicb.2020.01601. eCollection 2020.
Type III (T3) proteic effectors occupy most of the virulence determinants in eukaryote-pathogenic Gram-negative bacteria. During infection, bacteria may deploy a nanomachinery called translocon to deliver T3 effectors into host cells, wherein the effectors fulfill their pathological functions. T3 translocon is hypothetically assembled by bacterial translocators, which have been identified as one hydrophilic and two hydrophobic proteins in animal-pathogenic bacteria but remain unclear in plant pathogens. Now we characterize Hpa2, HrpF, and XopN proteins as concomitant T3 translocators in rice bacterial blight pathogen by analyzing pathological consequences of single, double, and triple gene knockout or genetic complementation. Based on these genetic analyses, Hpa2, HrpF, and XopN accordingly contribute to 46.9, 60.3, and 69.8% proportions of bacterial virulence on a susceptible rice variety. Virulence performances of Hpa2, HrpF, and XopN were attributed to their functions in essentially mediating from-bacteria-into-rice-cell translocation of PthXo1, the bacterial T3 effector characteristic of transcription factors targeting plant genes. On average, 61, 62, and 71% of PthXo1 translocation are provided correspondingly by Hpa2, HrpF, and XopN, while they cooperate to support PthXo1 translocation at a greater-than-95% extent. As a result, rice disease-susceptibility gene , which is the regulatory target of PthXo1, is activated to confer bacterial virulence and induce the leaf blight disease in rice. Furthermore, the three translocators also undergo translocation, but only XopN is highly translocated to suppress rice defense responses, suggesting that different components of a T3 translocon deploy distinct virulence mechanisms in addition to the common function in mediating bacterial effector translocation.
III型（T3）蛋白效应子占据了真核生物致病性革兰氏阴性菌中的大部分毒力决定因素。在感染过程中，细菌可能会部署一种称为转位子的纳米机器，将T3效应子输送到宿主细胞中，效应子在其中发挥其病理功能。T3转位子据推测是由细菌转运蛋白组装而成的，在动物致病性细菌中，这些转运蛋白已被鉴定为一种亲水性蛋白和两种疏水性蛋白，但在植物病原菌中仍不清楚。现在，我们通过分析单基因、双基因和三基因敲除或基因互补的病理后果，将Hpa2、HrpF和XopN蛋白鉴定为水稻白叶枯病菌中的伴随T3转运蛋白。基于这些遗传分析，Hpa2、HrpF和XopN在感病水稻品种上对细菌毒力的贡献分别为46.9%、60.3%和69.8%。Hpa2、HrpF和XopN的毒力表现归因于它们在本质上介导细菌T3效应子PthXo1从细菌向水稻细胞转运的功能，PthXo1是一种靶向植物基因的转录因子的细菌T3效应子。平均而言，Hpa2、HrpF和XopN分别相应地提供了61%、62%和71%的PthXo1转运，而它们协同作用以大于95%的程度支持PthXo1转运。结果，作为PthXo1调控靶点的水稻感病基因被激活，赋予细菌毒力并诱发水稻白叶枯病。此外，这三种转运蛋白也会发生转运，但只有XopN高度转运以抑制水稻的防御反应，这表明T3转位子的不同组分除了在介导细菌效应子转运方面的共同功能外，还具有不同的毒力机制。
