Fitness consequences and the evolution of R gene resistance to pathogen infection

Plants have an induced resistance to pathogen infection called R gene resistance. R genes tend to be polymorphic; some plants in a species do not have an R gene to match a particular strain and are susceptible. We address whether in some growth conditions susceptible plants are more fit that R gene resistant plants when infected with a pathogen. We call this potential outcome a net cost of resistance in the presence of disease.; We compared the fitness of nearly isogenic R gene resistant and susceptible Arabidopsis thaliana plants. We found net costs of R gene resistance with disease for two different R genes and show that the net costs occurred by different mechanisms. For the R gene RPS2, in the absence of competition, resistant plants produced less seed than susceptible, rps2 mutant plants when infected with Pseudomonas syringae. This net cost was due to overcompensation by susceptible plants associated with changes in branch architecture in response to infection. For the R gene RPP5, we compared resistant and susceptible introgression lines. In the presence of competition, RPP5 resistance was associated with a net cost in the presence of Peronospora parasitica. This net cost was due to both a decrease in the fitness of RPP5 lines with P. parastica infection relative to mock-treated plants, and compensation in the rpp5 lines. In a second A. thaliana accession, we found no evidence of a net cost in the presence of P. parasitica, either with introgression or transgenic lines. We present a mathematical model showing that a trade-off between net benefits of resistance with disease in some environments, and net costs of resistance with disease in others, could maintain resistance-susceptibility polymorphism.; We also report that three different pathogens similarly alter A. thaliana development. P. syringae, P. parasitica, and Xanthomonas campestris reduced time to flowering and the number of branches on the primary inflorescence above the rosette. P. syringae and P. parasitica infection also increased basal branching in high resource conditions, and seeds per silique on the primary and basal branches. We suggest that these changes constitute a general developmental response to pathogen infection.