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Abstract
Tall fescue has formed a mutualistic relationship with the fungal endophyte Acremonium coenophialum. Endophyte-infected plants can have increased growth and survival when compared with endophyte-free plants. Responses to endophyte infection vary and may be host genotype and fungal biotype specific. The mechanism(s) by which endophyte infection confers increased growth and survival is not understood. This research determined the occurrence, distribution, and ultrastructure of endophyte hyphae in the roots of axenically grown tall fescue seedlings. Acremonium coenophialum was recovered from excised roots of endophyte-infected seedlings grown on diverse agar media and from sterile coarse sand. Successful recovery of the isolate varied with seedling age, root type, and growth medium. Overall, 67% of 218 endophyte-infected seedlings contained endophyte in their roots. Root-fungal ultrastructure of agar grown seedlings revealed epiphytic hyphae bearing conidiophores with typical conidia and an electron dense matrix that adhered hyphae to each other and to the root epidermal cell wall. Hyphae were found in regions previously occupied by root epidermal cells, but no direct penetration of intact cells was observed. Electron dense granules within the fungal cytoplasm contained high amounts of phosphorous relative to adjacent tissues. This may benefit endophyte-infected plants in low soil phosphate conditions.