Effects On The Grass Plant and On Herbivores
It has been proposed that vertically transmitted symbionts should evolve to be mutualists since their reproductive fitness is intimately tied to that of their hosts. In fact, some positive effects of epichloae on their host plants include increased growth, drought tolerance, and herbivore and pathogen resistance. Resistance against herbivores has been attributed to alkaloids produced by the symbiotic epichloae. Although grass-epichloë symbioses have been widely recognized to be mutualistic in many wild and cultivated grasses, the interactions can be highly variable and sometimes antagonistic, especially under nutrient-poor conditions in the soil.
Due to the relatively large number of grass species harboring epichloae and the variety of environments in which they occur, the mechanisms underlying beneficial or antagonistic outcomes of epichloë-grass symbioses are difficult to delineate in natural and also agricultural environments. Some studies suggest a relationship between grazing by herbivores and increased epichloë infestation of the grasses on which they feed, whereas others indicate a complex interplay between plant species and fungal symbionts in response to herbivory or environmental conditions. The strong anti-herbivore activities of several bioactive compounds produced by the epichloae and relatively modest direct effects of the epichloae on plant growth and physiology suggest that these compounds play a major role in the persistence of the symbiosis.
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