The long-term maintenance of a resistance polymorphism through diffuse interactions.

Area of Science:

• Plant pathology
• Evolutionary genetics
• Molecular biology

Background:

• Plant resistance (R) genes are vital for plant defense against pathogens but often lack durability in agriculture.
• R genes frequently persist as balanced polymorphisms in natural populations, typically explained by coevolutionary dynamics between host resistance and pathogen virulence.
• The maintenance of such polymorphisms in species with diffuse interactions remains poorly understood.

Purpose of the Study:

• To investigate the maintenance mechanisms of balanced polymorphisms in plant R genes within natural populations.
• To identify and characterize the R gene-effector interactions driving polymorphism in Arabidopsis thaliana and Pseudomonas syringae.
• To explore the role of diffuse community-wide interactions in maintaining R gene polymorphisms.

Main Methods:

• Identification of a naturally interacting R gene (RPS5) and effector (AvrPphB2) pair in Arabidopsis thaliana and Pseudomonas syringae.
• Analysis of the evolutionary history and selection pressures on the RPS5 gene polymorphism.
• Investigation of the fitness costs and benefits associated with RPS5 alleles in the presence and absence of specific pathogen effectors.
• Assessment of the frequency and role of different pathogen effectors and species in selecting for RPS5.

Main Results:

• The RPS5 R gene and its recognized effector AvrPphB2 exhibit a balanced polymorphism maintained for over 2 million years.
• RPS5 confers a benefit against P. syringae carrying avrPphB2 but incurs a significant cost in uninfected plants, with alleles maintained at intermediate frequencies globally.
• The low frequency of avrPphB homologues and limited virulence benefit in P. syringae suggest this pair alone is insufficient to explain the RPS5 polymorphism.
• Evidence indicates selection on RPS5 involves multiple non-homologous effectors and pathogen species, pointing to community-wide interactions.

Conclusions:

• The balanced polymorphism of the RPS5 R gene in Arabidopsis thaliana is likely maintained by complex, diffuse community-wide interactions rather than a single coevolved R gene-effector pair.
• This finding challenges standard models of R gene polymorphism maintenance, highlighting the importance of broader ecological contexts.
• Understanding these community-level dynamics is crucial for predicting the evolution of plant defense and pathogen virulence.