Ultrastructure of Acremonium coenophialum, Acremonium lolii, and Epichloe typhina endophytes in host and nonhost Festuca and Lolium species of grasses

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Abstract
Ultrastructural studies were made of Acremonium fungal endophytes of tall fescue and perennial ryegrass and two isolates of Epichloe typhina. Hyphae of these fungi were observed in leaf sheaths of their natural host plant grown from infected seed and in artificially inoculated nonhost Festuca and Lolium species of grasses. Hyphae were also studied from fungal colonies grown on cellophane on potato dextrose agar. Crystalline inclusions were found in all endophyte-host combinations with the exceptions of E. typhina from perennial ryegrass in its own host and in a nonhost tall fescue, and Acremonium coenophialum in a nonhost perennial ryegrass. Tubular complexes were seen only in Acremonium lolii when growing in both host and in a non-host tall fescue. Three structures not seen before in endophytes (bundles of microtubules, 55 to 60 nm diameter "tubes," and bilayered cell walls) were found in E. typhina growing only in perennial ryegrass, its natural host. None of the distinguishing ultrastructural components described were found in hyphae grown in culture. Our observations suggest that endophytes express specific ultrastructural features only when they are grown in the grasses. However, it was not readily discernible what function the components serve and what role, if any, the natural versus the nonhost grass plays in the expression of endophyte ultrastructure.