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. Stromat a are usually formed on developing parts of the host and are composed of a mixture of host and fungal tissues. An ultrastructural examinatio n of stromata of Epichloe amarillans was conducted to assess changes i n host tissues associated with development of stromata on grasses. Not able changes were observed in the epidermal layers of leaves and inflo rescence primordia embedded within stromata. Epidermal cells within st romata showed progressive disintegration of the cuticle and wall layer s. These alterations effectively broke down the epidermal barrier to t he flow of nutrients and water to fungal tissues that develop outside the plant tissues. Mycelium was observed to associate closely with hos t cells. No penetrations of cells of the mesophyll, phloem, or xylem w ere observed; however, some alteration of the host cell walls was dete cted. Host cell walls proximal to hyphae were often seen to be slightl y 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 stru ctures, without the development of haustoria, is suggested to be a bas ic mechanism of nutrient acquisition among biotrophic fungi.