Rethinking the Roles of Pathogens and Mutualists: Exploring the Continuum of Symbiosis in the Context of Microbial Ecology and Evolution

Systems of classification are important for guiding research activities and providing a common platform for discussion and investigation. One such system is assigning microbial taxa to the roles of mutualists and pathogens. However, there are often challenges and even inconsistencies in reports of research findings when microbial taxa display behaviors outside of these two static conditions (e.g., commensal). Over the last two decades, there has been some effort to highlight a continuum of symbiosis, wherein certain microbial taxa may exhibit mutualistic or pathogenic traits depending on environmental contexts, life stages, and plant host associations. However, gaps remain in understanding how to apply the continuum approach to host-microbe pairs across a range of environmental and ecological factors. This commentary presents an alternative framework for evaluating the continuum of symbiosis using dominant archetypes that define symbiotic ranges. We focus particularly on fungi and bacteria, though we recognize that archaea and other microeukaryotes play important roles in host-microbe interactions that may be described by this approach. This framework is centered in eco-evolutionary theory and aims to enhance communication among researchers, as well as prioritize holistic consideration of the factors shaping microbial life strategies. We discuss the influence of plant-mediated factors, habitat constraints, coevolutionary forces, and the genetic contributions which shape different microbial lifestyles. Looking to the future, using a continuum-of-symbiosis paradigm will enable greater flexibility in defining the roles of target microbes and facilitate a more holistic view of the complex and dynamic relationship between microbes and plants.