Three Proteins (Hpa2, HrpF and XopN) Are Concomitant Type III Translocators in Bacterial Blight Pathogen of Rice.

Xuyan Mo,Liyuan Zhang,Kai Lu,Hansong Dong,Xuan Wang,Yan Liu,Lei Chen,Shenshen Zou,Jiaqi Bai

doi:10.3389/fmicb.2020.01601

Abstract

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 SWEET11, 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.

Highlights

Many pathogenicity-determinative proteic effectors of Gram-negative bacteria, which are plant (Alfano and Collmer, 2004; Nissinen et al, 2007; White et al, 2009), or animal (Chatterjee et al, 2013; Domingues et al, 2016; Piscatelli et al, 2016) pathogens, are secreted by the bacterial type III (T3) secretion system
In order to assess the individual roles of Hpa2, HrpF, and XopN in PXO99A-bacterial virulence on Nipponbare plants, virulence levels were compared when the hpa2, hrpF, and xopN genes were present canonically in the PXO99A genome, individually deleted from and backfilled to the genome
In seven of eight previous reports that we have scrutinized with appreciation, bacterial effector translocation was measured by experiments performed on tobacco, the common non-host plant of almost all plant-pathogenic microbes, rather than host plants of the bacteria (Table 1)

Summary

Introduction

Many pathogenicity-determinative proteic effectors of Gram-negative bacteria, which are plant (Alfano and Collmer, 2004; Nissinen et al, 2007; White et al, 2009), or animal (Chatterjee et al, 2013; Domingues et al, 2016; Piscatelli et al, 2016) pathogens, are secreted by the bacterial type III (T3) secretion system. The translocon-dependent model is conceived as a nanomachinery called “T3 translocon” (Büttner and Bonas, 2002), which is hypothetically formed by interactions of bacterial T3 translocators with each other and with specific recognizing constituents (Ji and Dong, 2015b; Büttner, 2016), both lipids (Büttner et al, 2002; Haapalainen et al, 2011), and proteins (Oh and Beer, 2007; Li et al, 2015, 2019), associated with eukaryotic PMs. In the translocon-dependent model, recognition of the hydrophilic translocator by a PM constituent is the first step toward the translocon assembly (Goure et al, 2004; Mueller et al, 2008). Lipid binding of hydrophobic translocators finalize the translocon with an inner channel to accommodate effector translocation (Mueller et al, 2008; Büttner, 2012; Ji and Dong, 2015b; Zhang et al, 2019a)

Methods

Results

Conclusion