Modifications of host cells and tissues by the biotrophic endophyte Epichloe amarillans (Clavicipitaceae; Ascomycotina)

The Clavicipitaceae (Ascomycotina) are unique among biotrophs in that they do not produce haustoria to extract nutrients from hosts. Stromata are usually formed on developing parts of the host and are composed of a mixture of host and fungal tissues. An ultrastructural examination of stromata of Epichloe amarillans was conducted to assess changes in host tissues associated with development of stromata on grasses. Notable changes were observed in the epidermal layers of leaves and inflorescence primordia embedded within stromata. Epidermal cells within stromata showed progressive disintegration of the cuticle and wall layers. These alterations effectively broke down the epidermal barrier to the flow of nutrients and water to fungal tissues that develop outside the plant tissues. Mycelium was observed to associate closely with host cells. No penetrations of cells of the mesophyll, phloem, or xylem were observed; however, some alteration of the host cell walls was detected. Host cell walls proximal to hyphae were often seen to be slightly thicker than walls adjacent to other plant cells. Unseen alterations to host cell membranes are hypothesized. A mechanism of nutrient flow from host to stromal mycelium is discussed. The modification of host cells and tissues to enhance nutrient flow to fungal reproductive structures, without the development of haustoria, is suggested to be a basic mechanism of nutrient acquisition among biotrophic fungi.