Tall Fescue Online Monograph

Abstract

Tall fescue [Lolium arundinaceum (Schreb.) Darbysh.] and many other grasses belonging to the subfamily Pooideae form seemingly symptomless partnerships with endophytic fungi belonging to the genus Neotyphodium. We describe aspects of the delicately balanced relationship between the fungus and the host plant, including the location and structure of the intercellular hyphae in the apical meristem, leaves, and seeds. Also described is a new mechanism of endophyte colonization of plants, intercalary growth of hyphae. This mode of growth is an adaptation that enables the endophyte to colonize expanding host leaves, even though attached to plant cells, and explains how hyphae can extend as fast as leaves are growing. Hyphal branching and extension cease when leaf growth ceases, even though the hyphae have an ongoing supply of nutrients. The hyphae remain metabolically active, producing alkaloids that enhance the robustness and persistence of the host grass. Their close sexual relatives, the Epichloë, share these characteristics when colonizing vegetative host tissues, but unlike Neotyphodium species, which are solely vertically transmitted in the seed of host plants, may be either fully or occasionally horizontally transmitted via ascospores produced in stromata formed on reproductive tillers.

Keywords: Grass endophytes, Neotyphodium, Epichloë, symbiosis, fungal ultrastructure, seed transmission, intercalary growth.

Abbreviations: GFP, green fluorescent protein; SA, shoot apex; SAM, shoot apical meristem.