Proteobacteria Contain Diverse flg22 Epitopes That Elicit Varying Immune Responses in Arabidopsis thaliana.

Jacqueline Monaghan,Melissa Bredow,George C Dicenzo,Janis H T Cheng

doi:10.1094/mpmi-11-20-0314-sc

Jacqueline Monaghan, Melissa Bredow + Show 2 more

Open Access

https://doi.org/10.1094/mpmi-11-20-0314-sc

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Abstract

Bacterial flagellin protein is a potent microbe-associated molecular pattern. Immune responses are triggered by a 22-amino-acid epitope derived from flagellin, known as flg22, upon detection by the pattern recognition receptor FLAGELLIN-SENSING2 (FLS2) in multiple plant species. However, increasing evidence suggests that flg22 epitopes of several bacterial species are not universally immunogenic to plants. We investigated whether flg22 immunogenicity systematically differs between classes of the phylum Proteobacteria, using a dataset of 2,470 flg22 sequences. To predict which species encode highly immunogenic flg22 epitopes, we queried a custom motif (11[ST]xx[DN][DN]xAGxxI21) in the flg22 sequences, followed by sequence conservation analysis and protein structural modeling. These data led us to hypothesize that most flg22 epitopes of the γ- and β-Proteobacteria are highly immunogenic, whereas most flg22 epitopes of the α-, δ-, and ε-Proteobacteria are weakly to moderately immunogenic. To test this hypothesis, we generated synthetic peptides representative of the flg22 epitopes of each proteobacterial class, and we monitored their ability to elicit an immune response in Arabidopsis thaliana. The flg22 peptides of γ- and β-Proteobacteria triggered strong oxidative bursts, whereas peptides from the ε-, δ-, and α-Proteobacteria triggered moderate, weak, or no response, respectively. These data suggest flg22 immunogenicity is not highly conserved across the phylum Proteobacteria. We postulate that sequence divergence of each taxonomic class was present prior to the evolution of FLS2, and that the ligand specificity of A. thaliana FLS2 was driven by the flg22 epitopes of the γ- and β-Proteobacteria, a monophyletic group containing many common phytopathogens.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Highlights

504 / Molecular Plant-Microbe Interactions evolution of FLS2, and that the ligand specificity of A. thaliana FLS2 was driven by the flg22 epitopes of the g- and bProteobacteria, a monophyletic group containing many common phytopathogens
The observations made here (Fig. 4H) and elsewhere (Chinchilla et al 2006) that nonimmunogenic flg22 peptides can compete for FLS2 binding raise the exciting possibility that community dynamics within a plant’s microbiota, whose composition differs across tissues (Turner et al 2013) and environments (Dastogeer et al 2020), may modulate FLS2-dependent immune responses
FLS2-dependent immune responses may be locally suppressed by high densities of microbes with nonimmunogenic flg22 epitopes that remain capable of interacting with FLS2/BAK1, while plants or tissues lacking these microbes may display more sensitive FLS2-dependent pathogen detection

Summary

Introduction

504 / Molecular Plant-Microbe Interactions evolution of FLS2, and that the ligand specificity of A. thaliana FLS2 was driven by the flg22 epitopes of the g- and bProteobacteria, a monophyletic group containing many common phytopathogens. The above suggestion was further supported by analysis of flg22 amino acid sequence conservation both within and across taxonomic classes (Fig. 2), and by in silico modeling of the interactions between representative flg22 peptides of each proteobacterial class ( defined as consisting of the most commonly occurring amino acid at each position [Fig. 2], and termed flg22-g, flg22-b, flg22-a, flg22-d, and flg22-e), in complex with FLS2 or BAK1 (Fig. 3; Supplementary Fig. S3).

Results

Conclusion