Epigenetic regulation plays a central role in modulating plant immune responses and interactions with beneficial microbes. In this study, we investigated the contribution of RNA-directed DNA methylation (RdDM) components—DCL3; AGO9; DCL1; and the de novo DNA methyltransferases CMT3, DRM1, and DRM2—to the interaction between Arabidopsis thaliana, Trichoderma atroviride, and foliar pathogens. We show that DCL3 and AGO9 differentially regulate basal and inducible immunity, negatively affecting resistance to the necrotrophic fungus Botrytis cinerea, while promoting defense against the hemibiotrophic bacterium Pseudomonas syringae pv. tomato DC3000. Transcriptional analyses revealed that RdDM components modulate the balance between jasmonic acid/ethylene (JA/ET) and salicylic acid (SA) signaling pathways, influencing the amplitude and coordination of defense responses. In addition, DCL3 and DCL1 appear to be required for the full expression of T. atroviride-mediated systemic resistance, whereas AGO9 and DNA methyltransferases contribute to efficient root colonization. Notably, mutants in these pathways displayed enhanced basal resistance but impaired responsiveness to beneficial microbial signals, revealing a trade-off between constitutive defense activation and inducible systemic protection. Consistent with this, alterations in RdDM components were also associated with changes in plant growth dynamics under specific conditions, supporting a role for epigenetic regulation in coordinating growth–defense trade-offs. Together, our findings support a model in which epigenetic regulation controls defense responsiveness, enabling plants to balance immune activation, growth and compatibility toward beneficial microbes.
Rosendo‐Vargas et al. (Sat,) studied this question.