E-Mail IMAGE: Mechanisms of symbiotic engagement -- three principles by which plants may select for or restrict potential mutualists or pathogens. view more Credit: David Thoms, Yan Liang, and Cara H. Haney Plants are constantly exposed to microbes: pathogens that cause disease, commensals that cause no harm or benefit, and mutualists that promote plant growth or help fend off pathogens. For example, most land plants can form positive relationships with arbuscular mycorrhizal fungi to improve nutrient uptake. How plants fight off pathogens without also killing beneficial microbes or wasting energy on commensal microbes is a largely unanswered question. In fact, when scientists within the field of Molecular Plant-Microbe Interactions were asked to come up with their Top 10 Unanswered Questions, the #1 question was "How do plants engage with beneficial microorganisms while at the same time restricting pathogens?" Put more simply, how do plants tell good microbes from bad and what do they do about it? As part of a Top10MPMI review series in the open access