Epichloë Fungal Endophytes and Plant Defenses: Not Just Alkaloids

Abstract

Although the role of fungal alkaloids in protecting grasses associated with Epichloë fungal endophytes has been extensively documented, the effects of the symbiont on the host plant's immune responses have received little attention. We propose that, in addition to producing protective alkaloids, endophytes enhance plant immunity against chewing insects by promoting endogenous defense responses mediated by the jasmonic acid (JA) pathway. We advance a model that integrates this dual effect of endophytes on plant defenses and test its predictions by means of a standard meta-analysis. This analysis supports a role of Epichloë endophytes in boosting JA-mediated plant defenses. We discuss the ecological scenarios where this effect of endophytes on plant defenses would be most beneficial for increasing plant fitness. To achieve a well-established symbiosis, plants and beneficial microorganisms engage in a complex molecular dialog that affects several plant functions, including resource allocation and the expression of plant defenses. Beneficial microorganisms such as rhizobia and mycorrhizal fungi can enhance the defenses of their host plants, thereby increasing plant resistance to a broad spectrum of attackers. Recent studies reveal that Epichloë fungal endophytes can also induce changes in molecular components associated with defense signaling in the host plant. These changes could have impacts on the plant's resistance to attackers.