Asexual endophytes of grasses: Invisible symbionts, visible imprints in the host neighborhood

Abstract

Asexual fungi from the genus Neotyphodium, relatives of the sexual epichloë species (Clavicipitaceae, Ascomycota), are symbionts of several cool-season grasses inhabiting virtually all terrestrial ecosystems. The host plants incur carbon costs to sustain this symbiosis, but, in return, they obtain multiple benefits from the fungal partners, above all, protection from herbivores. These endophytes are often considered to be defensive mutualists or private protectors because they produce a considerable range of secondary metabolites which prove to be toxic to livestock or deterrent to insects. Over the past decade, ecologists have begun to recognize the critical role played by this grass-endophyte symbiosis in the structure and functioning of natural and human-made communities. In this chapter, I will identify different pathways through which the presence of endophytic plants or their dead tissues (litter) can alter the fitness of nonsymbiotic plants. Those pathways lead to show how these symbionts impact on the establishment and productivity of nonsymbiotic neighbors and the interaction of the latter with multiple above- and belowground ecosystem components. A set of recent studies performed with plants of Lolium multiflorum associated with Neotyphodium occultans will provide experimental evidence to those effects. Finally, I will discuss the relevance of placing these pathways under the spotlight in order to understand the processes that determine the frequency of symbiotic plants within a population. Estimating endophyte impacts on host fitness must consider advantages or disadvantages transferred to conspecific plants in the neighborhood, coexisting as a consequence of inefficiencies during the transmission from plants to seeds.