Plant Innate Immunity Induced by Flagellin Suppresses the Hypersensitive Response in Non-Host Plants Elicited by Pseudomonas syringae pv. averrhoi

Chia-Fong Wei,Hsiou-Chen Huang,Shih-Tien Hsu,Wen-Ling Deng,Yu-Der Wen,Ching-Hong Yang

doi:10.1371/journal.pone.0041056

Chia-Fong Wei, Hsiou-Chen Huang + Show 4 more

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https://doi.org/10.1371/journal.pone.0041056

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Abstract

A new pathogen, Pseudomonas syringae pv. averrhoi (Pav), which causes bacterial spot disease on carambola was identified in Taiwan in 1997. Many strains of this pathovar have been isolated from different locations and several varieties of hosts. Some of these strains, such as HL1, are nonmotile and elicit a strong hypersensitive response (HR) in nonhost tobacco leaves, while other strains, such as PA5, are motile and elicit a weak HR. Based on the image from a transmission electron microscope, the results showed that HL1 is flagellum-deficient and PA5 has normal flagella. Here we cloned and analyzed the fliC gene and glycosylation island from Pav HL1 and PA5. The amino acid sequences of FliC from HL1 and PA5 are identical to P. s. pvs. tabaci (Pta), glycinea and phaseolicola and share very high similarity with other pathovars of P. syringae. In contrast to the flagellin mutant PtaΔfliC, PA5ΔfliC grows as well as wild type in the host plant, but it elicits stronger HR than wild type does in non-host plants. Furthermore, the purified Pav flagellin, but not the divergent flagellin from Agrobacterium tumefaciens, is able to impair the HR induced by PA5ΔfliC. PA5Δfgt1 possessing nonglycosylated flagella behaved as its wild type in both bacterial growth in host and HR elicitation. Flagellin was infiltrated into tobacco leaves either simultaneously with flagellum-deficient HL1 or prior to the inoculation of wild type HL1, and both treatments impaired the HR induced by HL1. Moreover, the HR elicited by PA5 and PA5ΔfliC was enhanced by the addition of cycloheximide, suggesting that the flagellin is one of the PAMPs (pathogen-associated molecular patterns) contributed to induce the PAMP-triggered immunity (PTI). Taken together, the results shown in this study reveal that flagellin in Pav is capable of suppressing HR via PTI induction during an incompatible interaction.

Highlights

Plants utilize a refined network of defense mechanisms to protect themselves from the invasion of microorganism
A collection of P. syringae pv. averrhoi (Pav) strains isolated from different area and host varieties in Taiwan were surveyed for their swimming ability and hypersensitive response (HR) elicitation on nonhost plants in this study
By cloning and sequence analysis of Pav fliC gene and its related glycosylation island, our results reveal that the amino acid sequence of FliC from Pav PA5 is identical to P. s. pvs. tabaci, glycinea and phaseolicola, and that the gene organization and sequence of glycosylation island are highly conserved in P. syrinage pathovars

Summary

Introduction

Plants utilize a refined network of defense mechanisms to protect themselves from the invasion of microorganism. A successful bacterial pathogen has to overcome at least two levels of plant defense for its survival. Bacterial PAMPs include flagellin, cold-shock protein, elongation factor Tu (EF-Tu), and peptidoglycan [2] which are recognized by pattern recognition receptor (PRR)-like kinases [3]. Phytobacteria overcome this defense by injecting type III effectors (T3Es) via type III secretion system (T3SS) to suppress PTI. Some plants have evolved a second level of defense by expressing resistance (R) proteins to recognize some of these effectors, thereby induce effector-triggered immunity (ETI) [4]. The HR inhibition by a PTI inducer refers to restriction of T3E injection and a successful pathogen ought to have active T3Es to overcome the restriction [13]

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