An endophyte is an organism, often a bacterium or fungus, that lives within a plant for at least part of its life without causing apparent disease. Endophytes are ubiquitous and have been found in all the species of plants studied to date; however, most of these endophyte/plant relationships are not well understood.
Endophytes may be transmitted either vertically (directly from parent to offspring directly) or horizontally (from individual to unrelated individual). Vertically transmitted fungal endophytes are asexual and transmit via fungal hyphae penetrating the host’s seeds (e.g. Neotyphodium). Since their reproductive fitness is intimately tied to that of their host plant, these fungi are often mutualistic. Conversely, horizontally transmitted fungal endophytes are sexual and transmit via spores that can be spread by wind and/or insect vectors. Since they spread similar to pathogens, often horizontally transmitted endophytes are closely related to pathogenic fungi.
Some endophytes can colonize multiple species of plants.
Endophytes may benefit host plants by preventing pathogenic organisms from colonizing them. Extensive colonization of the plant tissue by endophytes creates a "barrier effect", where the local endophytes outcompete and prevent pathogenic organisms from taking hold. Endophytes may also utilize chemicals which inhibit the growth of competitors, including pathogenic organisms.
Some endophytes can be cultured from their host plant in an appropriate growth medium.