Fungal Endophytes: Common Host Plant Symbionts but Uncommon Mutualists

TLDR

Fungal endophytes are ubiquitous, diverse, and typically asymptomatic symbionts of plant tissues, traditionally viewed as mutualists that reduce herbivory through toxin production, yet most, especially horizontally transmitted ones, appear to have little or no effect on herbivores. Most endophytes, particularly horizontally transmitted ones, exert minimal or no influence on herbivores, while vertically transmitted grass endophytes can be mutualistic, neutral, or pathogenic depending on host genotype and environmental conditions.

Abstract

Fungal endophytes are extremely common and highly diverse microorganisms that live within plant tissues, but usually remain asymptomatic. Endophytes traditionally have been considered plant mutualists, mainly by reducing herbivory via production of mycotoxins, such as alkaloids. However, the vast majority of endophytes, especially horizontally-transmitted ones commonly found in woody plants, apparently have little or no effect on herbivores. For the systemic, vertically-transmitted endophytes of grasses, mutualistic interactions via increased resistance to herbivores and pathogens are more common, as predicted by evolutionary theory. However, even in these obligate symbioses, endophytes are often neutral or even pathogenic to the host grass, depending on endophyte and plant genotype and environmental conditions.