Graduate Thesis Or Dissertation
 

Studies of the fungal endophyte Acremonium coenophialum in tall fescue

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  • Tall fescue is thought to have co-evolved with the fungal endophyte Acremonium coenophialum to form a mutualistic relationship. Endophyte-infected (EI) plants can have increased growth and survival when compared with endophyte-free (EF) plants. Responses to endophyte-infection vary and are host-genotype and fungal-biotype specific. 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 tall fescue seedlings. Acremonium coenophialum was isolated from excised roots of EI seedlings grown on diverse agar media and from sterile coarse sand. Frequency of isolation varied with seedling age, root type, and growth medium. Sixty-seven percent of the EI seedlings contained endophyte in their roots. Root-fungal ultrastructure of agar grown seedlings revealed epiphytic hyphae bearing conidiophores with 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. These poly-phosphate reserves may benefit EI plants in low soil phosphate conditions. The effects of plant genotype, endophyte presence, and soluble soil-PO4 concentrations on the growth and physiology of tall fescue were also investigated. Two morphologically diverse clones (numbered 7 and 11) with paired EI and EF ramets were grown in soil containing known concentrations of soluble soil-PO4. Growth responses to soluble soil-PO4 concentrations varied with clone genotype and endophyte presence. In general, optimal growth was attained when the ramets were grown in soluble soil PO4 concentrations between 0.20-1.00 ppm. In moderate light (400 pmol/m2/s) photosynthetic rates (PSN) of EI ramets of both clones were reduced when compared to their EF counterparts. This reduction was due in part to increased stomatal resistance in the El ramets. However, the presence of endophyte had no effect on PSN when the ramets were placed in saturated light (1000 pmol/m2/s). El clone 7 had reduced numbers of tillers, leaves, and total leaf lengths per ramet when compared with EF clone 7. In contrast, EI clone 11 had increased number of tillers, leaves, and total leaf lengths per ramet when compared with EF clone 11. Biomass distribution and ramet ortho-phosphate ratios also differed with endophyte infection in the two clones. These observed changes in plant growth, biomass distribution, and ortho-phosphate ratios induced by endophyte infection may increase both clones ability to adapt to a wider range of habitats, thus increasing survival and persistence.
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