Parallel Loss-of-Function at the RPM1 Bacterial Resistance Locus in Arabidopsis thaliana

Laura Rose,Eric B Holub,Murray Grant,Susanna Atwell

doi:10.3389/fpls.2012.00287

Laura Rose, Eric B Holub + Show 2 more

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https://doi.org/10.3389/fpls.2012.00287

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Abstract

Dimorphism at the Resistance to Pseudomonas syringae pv. maculicola 1 (RPM1) locus is well documented in natural populations of Arabidopsis thaliana and has been portrayed as a long-term balanced polymorphism. The haplotype from resistant plants contains the RPM1 gene, which enables these plants to recognize at least two structurally unrelated bacterial effector proteins (AvrB and AvrRpm1) from bacterial crop pathogens. A complete deletion of the RPM1 coding sequence has been interpreted as a single event resulting in susceptibility in these individuals. Consequently, the ability to revert to resistance or for alternative R-gene specificities to evolve at this locus has also been lost in these individuals. Our survey of variation at the RPM1 locus in a large species-wide sample of A. thaliana has revealed four new loss-of-function alleles that contain most of the intervening sequence of the RPM1 open reading frame. Multiple loss-of-function alleles may have originated due to the reported intrinsic cost to plants expressing the RPM1 protein. The frequency and geographic distribution of rpm1 alleles observed in our survey indicate the parallel origin and maintenance of these loss-of-function mutations and reveal a more complex history of natural selection at this locus than previously thought.

Highlights

Plants use two evolutionarily conserved mechanisms to induce defense responses against potential pathogens
The first mechanism is provided by plant receptor-like proteins in the plasma membrane that detect conserved microbe molecules outside the host cell, often referred to as pathogen associated molecular patterns (PAMPs)
Examination of the susceptible accessions by PCR using primers specific for either the RPM1 gene or the deletion null allele revealed that not all susceptible accessions had a deletion of RPM1

Summary

Introduction

Plants use two evolutionarily conserved mechanisms to induce defense responses against potential pathogens. The first mechanism is provided by plant receptor-like proteins in the plasma membrane that detect conserved microbe molecules outside the host cell, often referred to as pathogen associated molecular patterns (PAMPs). This PAMP-triggered immunity (PTI) is robust and encompasses recognition of a wide variety of molecules such as bacterial flagellin, heat shock proteins and peptidoglycans, and fungal chitin oligomers (reviewed in Schwessinger and Zipfel, 2008). The plant membrane bound and cytosolic receptor-like proteins typically share a structurally conserved leucine-rich repeat (LRR) domain

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