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Roots and leaves comprise two of the largest microbial habitats on Earth, particularly in tropical forests where root and leaf surface areas are extremely high and microbes are abundant and diverse. Fungal and bacterial endophytes are primarily acquired via contagious spread from the surrounding environment. The soil is an important reservoir for both fungal and bacterial endophytes; we term this a soil microbial bank and suggest that it functions similarly to a soil seed bank. Because most (~ 75%) studies have found a strong positive relationship between plant diversity and soil microbial diversity, we predict that as plant diversity increases so will endophyte taxonomic and functional diversity. Once inside plant host tissues, endophytes can act as mutualists and increase plant performance directly by producing plant hormones, or indirectly by decreasing fungal or insect damage by up to 80%. Recent studies, however, have demonstrated that there are costs associated with hosting “beneficial” endophytes for tropical trees. This is important because it challenges more traditional dichotomies (eg, beneficial or deleterious) about endophytes and suggests that there are highly complex and context-dependent trade-offs and costs involved in plantendophyte interactions. Though they comprise a cryptic component of tropical forests, plant-microbe interactions may typically regulate tree diversity, composition, and forest function at neighborhood and even regional scales. For example, pathogens may maintain tree diversity by reducing the fitness of common species in areas where plant host density is high or where hosts are close to reproductive …