Ultrastructure of a Symptomless Fungal Endophyte of Festuca arundinacea

Abstract

Festuca arundinacea Schreb., commonly infected by a fungal endophyte of the genus Acremonium, may also contain a second, referred to as the Phialophora-like, endophyte. Hyphae of this fungus are intercellular in tissues of the host leaf blade, sheath, and culm, reaching their greatest numbers in outermost leaf sheaths. They also become intracellular in senescing host tissues. Vegetative hyphae have apparently homogeneous walls, heavy cytoplasmic electron opacity, and a high lipid content. Utilization of lipid reserves in the form of large globules in green sheaths and blades results in the formation of heavy electronopaque bodies; these occur in reduced amount in senescent blades. In senescent sheaths, lipid globules are occasionally associated with numerous mitochondria covered in a close layer of conspicuous ribosomes. During early winter, when hyphae are most common in host tissues, reproductive structures of the Phialophora-like endophyte are found on and in outermost-senescing blades. Conidiomata take the form of penicillate tufts of mucilage-secreting conidiophores whose ultimate branches (phialides) bear enteroblastic conidia. Phialide cytoplasm contains much endoplasmic reticulum as both rough fenestrated cisternae and smooth cisternae of a proliferating form, often associated with the plasmalemma and connected to the nuclear envelope. Mitochondria are numerous throughout the cells of the conidioma and are conspicuous due to a coating of ribosomes. Conidia are borne at the apex of an elongated neck and are encircled by an upstanding or reflexed collarette. After schizolytic secession, conidia enlarge and continue maturation with elongation and contortion of the nucleus and depletion of reserve material. A zone of parallel-lying short lengths of peg- or rod-like structures, from which heterochromatin is excluded, lies within the inner membrane of the nuclear envelope. Ultrastructural similarities between the Gliocladium- and Phialophora-like endophytes of grasses suggest that these two fungi may be congeneric.