Abstract
Like several pathogenic bacteria, Xanthomonas infect host plants through the secretion of effector proteins by the Hrp pilus of the Type Three Protein Secretion System (T3SS). HrpE protein was identified as the major structural component of this pilus. Here, using the Xanthomonas citri subsp. citri (Xcc) HrpE as a model, a novel role for this protein as an elicitor of plant defense responses was found. HrpE triggers defense responses in host and non-host plants revealed by the development of plant lesions, callose deposition, hydrogen peroxide production and increase in the expression levels of genes related to plant defense responses. Moreover, pre-infiltration of citrus or tomato leaves with HrpE impairs later Xanthomonas infections. Particularly, HrpE C-terminal region, conserved among Xanthomonas species, was sufficient to elicit these responses. HrpE was able to interact with plant Glycine-Rich Proteins from citrus (CsGRP) and Arabidopsis (AtGRP-3). Moreover, an Arabidopsis atgrp-3 knockout mutant lost the capacity to respond to HrpE. This work demonstrate that plants can recognize the conserved C-terminal region of the T3SS pilus HrpE protein as a danger signal to defend themselves against Xanthomonas, triggering defense responses that may be mediated by GRPs.
Highlights
Phytopathogens of the Xanthomonas genus are bacteria that produce devastating diseases in a diversity of monoand dicotyledonous plants worldwide, comprising important crop plants[1]
The interactions of HrpE with both highly conserved C-terminal regions of AtGRP-3 (HrpE-BD/C-AtGRP-AD) and citrus Glycine-Rich Protein (CsGRP) (HrpE-BD/C-CsGRP-AD) were tested by Yeast Two-Hybrid (YTH) assays and positive interactions were observed (Fig. 5c). These results indicate that AtGRP-3 can recognize HrpE, defense responses to HrpE were analyzed in A. thaliana wild type Col-0 and in a single T-DNA homozygous knockout line in the AtGRP-3 gene. quantitative reverse transcription-PCR (qRT-PCR) assays corroborated that this atgrp-3 mutant line lacks AtGRP-3 expression and represents a null mutant (Fig. S4)
In Xcv, the HrpE protein was identified as the main structural component of the Hrp pilus and its role in the secretion of effector proteins has been well established[8]
Summary
Phytopathogens of the Xanthomonas genus are bacteria that produce devastating diseases in a diversity of monoand dicotyledonous plants worldwide, comprising important crop plants[1]. Plant-pathogenic bacteria carrying mutations in genes encoding the T3SS neither elicit Hypersensitive Response (HR), a plant defense response that restricts bacterial growth in resistant plants, nor cause pathogenicity in susceptible host plants. These genes are referred as HR and pathogenicity (hrp) genes[4]. HrpE lost the capacity to elicit defense responses in atgrp-3 knockout mutant Arabidopsis plants This is the first report of a plant protein involved in direct recognition of a Xanthomonas Hrp pilus protein, with a role in signaling mechanisms mediating plant responses against pathogens
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