Dual mutualist-antagonist dynamics of grass endophytes
S. H. FAETH (1), S. Saari (1). (1) University of North Carolina-Greensboro, Greensboro, NC, U.S.A.
Systemic, seed-borne endophytic fungi, such as <i>Neotyphodium</i> species, inhabiting cool season grasses have been traditionally viewed as plant mutualists. The main mechanism for mutualistic effects is the fungal production of alkaloids that increase resistance to herbivores, pathogens and seed predators, and generally increase host competitive abilities. Yet accumulating evidence shows that the effects of endophytes can range from mutualistic to antagonistic, especially in native grass populations. This disparity in the effects of seed borne endophyte on their hosts has several explanations. First, most studies of endophyte effects on host are short term. Recent studies show that the direction and strength of endophyte interactions with perennial hosts changes ontogenetically and lifetime fitness effects are challenging to measure. Second, recent molecular genetic evidence shows that even asexual endophytes like <i>Neotyphodium</i> are remarkably variable across and within populations. For example, variation in alkaloid genes among endophytes leads to highly variable interaction outcomes depending on the presence and type of herbivores and abiotic conditions such as soil nutrients. Third, endophytic alkaloids often cascade from herbivores upward to higher trophic levels. Natural enemies may be more affected by the alkaloids than are their herbivore prey, and thus plant defense via endophytic alkaloids may be thwarted. I provide examples of these three scenarios.
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