The distribution and ultrastructure of the endophyte of toxic tall fescue

Abstract

The symptomless relationship of tall fescue grass (Festuca arundinacea Schreber) and its endophytic parasitic fungus, Sphacelia typhina (Fr.) Tul. (= Acremonium coenophialum fide Morgan-Jones et Gams), was studied with light and electron microscopy. The fungus was intercellular and was distributed throughout the plant with the exception of roots and leaf blades. The highest concentration of the fungus occurred in the base of the leaf sheath. In the seed the fungus was sequestered between the epithelial cells of the scutellum and the starchy endosperm. The seed embryo was not infected; infection was also not evident during the early stages of seed germination. At the ultrastructural level, the parasitic relationship was established in tissues where there were intercellular spaces. Micrographs indicated that the fungus did not separate the middle lamella of host cells, nor did it produce any obvious alteration in host cell morphology. Ultrastructural characteristics of the fungus depended on the age and type of fescue tissue. Hyphal cells possessed numerous vesicular and multivesicular structures, and in some sections, the hyphal walls were inordinately thickened and with or without an extracellular matrix. The fungus appeared to colonize the innermost areas of tissues in all infected organs and no signs of egress were ever detected.