Multi-generational biotic stress increases the rate of spontaneous epimutations in a ROS1-dependent manner

Mistakes in the maintenance of CG methylation are a source of spontaneous epimutations in plants that can be inherited across generations. The extent to which these stochastic events are affected by prolonged exposure to biotic and abiotic stress remains poorly characterized. Here, we grew Arabidopsis Mutation Accumulation (MA) lines for 12 generations in the presence of two biotic stressors: Pseudomonas syringae (Pst) and salicylic acid (SA). We found that multi-generational exposure to Pst and SA led to an 18%-32% and 23%-61% increase in the genome-wide epimutation rate, respectively. These rate increases were mainly targeted to subsets of genes characterized by low steady-state methylation (LM), on average, and a lack of transcriptional responsiveness to stress. We show that these effects are mediated by the DNA demethylase Repressor of Silencing 1 (ROS1). Loss of ROS1 not only buffers transcriptional responses to biotic stress, but also stabilizes epimutation rates, particularly in LM genes, rendering them insensitive to environmental perturbations. Taken together, our data demonstrates that stress can induce heritable epimutations and highlights a ROS1-mediated link between transcriptional plasticity and DNA methylation maintenance fidelity over generations.