Maintaining symbiotic homeostasis: how do plants engage with beneficial microorganisms while at the same time restricting pathogens?

That plants recruit beneficial microbes while simultaneously restricting pathogens is critical to their survival. Plants must exclude pathogens; however, the majority of land plants are able to form mutualistic symbioses with arbuscular mycorrhizal fungi. Plants also associate with the complex microbial communities that form the microbiome. The outcome of each symbiotic interaction-whether a specific microbe is pathogenic, commensal or mutualistic-relies on the specific interplay of host and microbial genetics and the environment. Here, we discuss how plants use metabolites as a gate to select which microbes can be symbiotic. Once present, we discuss how plants integrate multiple inputs to initiate programs of immunity or mutualistic symbiosis, and how this paradigm may be expanded to the microbiome. Finally, we discuss how environmental signals are integrated with immunity to fine tune a thermostat that determines whether a plant engages in mutualism, resistance to pathogens, and shapes associations with the microbiome. Collectively, we propose that the plant immune thermostat is set to select for and tolerate a largely non-harmful microbiome while receptor-mediated decision making allows plants to detect and dynamically respond to the presence of potential pathogens or mutualists.