Ultrastructure of the Gliocladium‐like endophyte of perennial ryegrass (Lolium perenne L.)

summary Vegetative hyphae of the Gliocladium‐like endophyte of Lolium perenne L. were characterized by apparently unstructured walls and high lipid content. Hyphae were intercellular during their active phase from late summer to winter, at which point they reached their highest density and became also intracellular in the tissues of the senescent outermost leaf. When numbers of hyphae in host tissue were greatest, a marked cytoplasmic change was noted in the endophyte present in blades of the outermost leaves of tillers. This involved depletion of lipid and deposition of electron‐opaque material within vacuoles. These modifications occurred shortly before the formation of reproductive structures and defined a pre‐sporulation phase. Aggregates of hyphae near the epidermis in senescent outer sheaths resembled sclerotia, but lacked indurated rinds. Concentric bodies were observed in pre‐sporulation hyphae. Conidiomata formed in the senescent outermost leaf blade and on its surface during winter. Conidiophores developed from pre‐sporulation hyphae and produced mucilage‐secreting, penicillate conidiomata bearing cylindrical phialides with enteroblastic conidia at their apices. Phialide cytoplasm contained many ribosomes, small vacuoles and enlarged vesicles. The envelope of the prominent nucleus showed connections with an extensive and variable system of dilated rough endoplasmic reticulum stacks close to the nucleus and, further removed, areas of smooth fenestrated cisternae. With formation of the first conidium, the outer mucilaginous wall of the phialide ruptured to form a recurved collarette. Conidia continued maturation after schizolytic secession, with enlargement, disappearance of reserve materials, and elongation and contortion of the nucleus. A layer of parallel‐lying short lengths of peg‐ or rod‐like structures lay within the inner membrane of the envelope and heterochromatin was excluded from this zone.