Abstract
Microbe-associated molecular patterns (MAMPs) are molecules, or domains within molecules, that are conserved across microbial taxa and can be recognized by a plant or animal immune system. Although MAMP receptors have evolved to recognize conserved epitopes, the MAMPs in some microbial species or strains have diverged sufficiently to render them unrecognizable by some host immune systems. In this study, we carried out in vitro evolution of the Arabidopsis thaliana flagellin receptor FLAGELLIN-SENSING 2 (FLS2) to isolate derivatives that recognize one or more flagellin peptides from bacteria for which the wild-type Arabidopsis FLS2 confers little or no response. A targeted approach generated amino acid variation at FLS2 residues in a region previously implicated in flagellin recognition. The primary screen tested for elevated response to the canonical flagellin peptide from Pseudomonas aeruginosa, flg22. From this pool, we then identified five alleles of FLS2 that confer modest (quantitatively partial) recognition of an Erwinia amylovora flagellin peptide. Use of this Erwinia-based flagellin peptide to stimulate Arabidopsis plants expressing the resulting FLS2 alleles did not lead to a detectable reduction of virulent P. syringae pv. tomato growth. However, combination of two identified mutations into a single allele further increased FLS2-mediated responses to the E. amylovora flagellin peptide. These studies demonstrate the potential to raise the sensitivity of MAMP receptors toward particular targets.
Highlights
Microbe-associated molecular patterns (MAMPs) are conserved and often relatively abundant molecular signatures that are present across a broad range of microbes, and trigger defense responses of plant and/or animal innate immune systems [1,2,3]
X22 elicited a slight but statistically significant response only when a very high concentration of peptide (10 μM) was used; no response to X22 could be detected at lower concentrations of peptide (Fig 1). This too was expected; a previous study reported a lack of response of Arabidopsis to full-length flagellin extracts from Xanthomonas campestris pv. campestris (Xcc) strain 186 [44]
While plants are able to recognize a diverse repertoire of molecules from microbes, several plant pathogenic species have evolved such that some of their MAMPs are no longer detectable by the immune systems of some plants
Summary
Microbe-associated molecular patterns (MAMPs) are conserved and often relatively abundant molecular signatures that are present across a broad range of microbes (i.e., multiple genera within a kingdom or phylum), and trigger defense responses of plant and/or animal innate immune systems [1,2,3]. Different MAMPs trigger an overlapping suite of plant responses, the amplitude and duration of these responses differ depending on the host and the particular MAMP stimulus [3, 6]. Plant responses to MAMPs can reduce pathogen growth on the host [3, 12]. During extended exposure to MAMP molecules an inhibition of plant growth often occurs, and this growth/defense tradeoff is often used in the seedling growth inhibition assay, one of the most sensitive and convenient assays for FLS2 receptor activation [2, 13,14,15]
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