From Guard to Decoy: A New Model for Perception of Plant Pathogen Effectors

TLDR

The Guard Model proposes that plant resistance proteins monitor pathogen effector targets, while recent evidence supports a Decoy Model in which evolved decoy targets perceive effectors. The study argues that guarded effector targets are evolutionarily unstable in R‑gene polymorphic populations and seeks to compare Guard and Decoy models, hypothesize decoy evolution, and suggest tests. They analyze how the presence or absence of R genes creates opposing selection pressures on effector targets, and how gene duplication or target mimicry can relax constraints to produce decoys that solely perceive effectors.

Abstract

The Guard Model for disease resistance postulates that plant resistance proteins act by monitoring (guarding) the target of their corresponding pathogen effector. We posit, however, that guarded effector targets are evolutionarily unstable in plant populations polymorphic for resistance (R) genes. Depending on the absence or presence of the R gene, guarded effector targets are subject to opposing selection forces (1) to evade manipulation by effectors (weaker interaction) and (2) to improve perception of effectors (stronger interaction). Duplication of the effector target gene or independent evolution of a target mimic could relax evolutionary constraints and result in a decoy that would be solely involved in effector perception. There is growing support for this Decoy Model from four diverse cases of effector perception involving Pto, Bs3, RCR3, and RIN4. We discuss the differences between the Guard and Decoy Models and their variants, hypothesize how decoys might have evolved, and suggest ways to challenge the Decoy Model.