Abstract
The interiors of plants are colonized by a diverse group of microorganisms. Many of these microbes do not harm their hosts in obvious ways for at least a portion of their life history and are referred to as endophytes. Because of their capacity to influence host phenotypes, endophytes have received a great deal of attention over the past few decades, yet basic questions of endophyte biogeography, ecology, and evolution remain unanswered. To determine the state of endophyte biodiversity exploration—at multiple spatial scales and across the plant phylogeny—we synthesized results from nearly 600 published studies. Our survey revealed a global interest in endophyte biology and highlighted several pressing gaps in knowledge. For instance, of the seventeen biomes encompassed by our survey, seven had fewer than 50 studies (including the boreal, alpine, and tropical grasslands biomes, among others) and together composed only 7% of the studies we considered. We found that fungal endophyte diversity has been characterized in at least one host from 31% of embryophyte families, while bacterial endophytes have been surveyed in hosts from only 10.5% of families. We complimented our broad survey with a meta-analysis and vote counting procedure to determine endophyte richness and diversity patterns at a small spatial scale—among plant tissue types. We found that variation in fungal endophyte richness and diversity among above-ground tissues differed as a function of host growth habit. Stems were the richest tissue in woody plants, whereas roots were the richest tissue in graminoids. For forbs, we observed no clear pattern of one tissue type harboring the most endophytic taxa. We propose a series of future directions and guidelines to fill the gaps in knowledge we uncovered and inspire further research.