Monomeric GTPases, which belong to the Ras superfamily, are small proteins involved in many biological processes. They are fine-tuned regulated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Several families have been identified in organisms from different kingdoms. Overall, the most studied families are Ras, Rho, Rab, Ran, Arf, and Miro. Recently, a new family named Big Ras GTPases was reported. As a general rule, the proteins of all families have five characteristic motifs (G1–G5), and some specific features for each family have been described. Here, we present an exhaustive analysis of these small GTPase families in fungi, using 56 different genomes belonging to different phyla. For this purpose, we used distinct approaches such as phylogenetics and sequences analysis. The main functions described for monomeric GTPases in fungi include morphogenesis, secondary metabolism, vesicle trafficking, and virulence, which are discussed here. Their participation during fungus–plant interactions is reviewed as well.

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.