Semester

Fall

Date of Graduation

2002

Document Type

Dissertation

Degree Type

PhD

College

Davis College of Agriculture, Natural Resources and Design

Department

Horticulture

Committee Chair

Daniel G. Panaccione.

Abstract

Tall fescue (Lolium arundinaceum), an agronomically important forage grass, is typically associated with a mutualistic asexual fungus Neotyphodium coenophialum. Endophytic fungal growth is confined to the intercellular spaces of the plant and induces no symptoms. The endophyte partner enhances its host's fitness by improving nutrient acquisition as well as by providing protection from various abiotic and biotic stresses. However, grass-endophyte partnerships are detrimental to grazing livestock, due to the production of an array of fungal alkaloids. To investigate this interaction at the molecular level, suppressive subtractive hybridization was used to identify differentially expressed genes from the symbiosis. In total, twenty-nine transcripts that may be fundamentally important for the effective functioning of this association have been identified. Two of these (Nc12 and Nc25) were novel endophyte genes highly up-regulated in planta. (Nc12 is also up-regulated by phosphorus supplementation, and by infection with mutualistic arbuscular mycorrhizal fungi. The high expression of both fungal genes in planta suggests that they play crucial roles in the tall fescue-endophyte association. Confirmation of differential plant gene expression (either up- or down-regulated) was confirmed for 13 tall fescue genes by a combination of RT-PCR and Northern analysis. Of the genes that had matches to known genes present in the NCBI databases (approximately 40%), many had roles related to plant defense and stress tolerance. Evidence for the functional role of one of the down-regulated tall fescue genes (TFR53), putatively encoding a pathogenesis-related (PR-10) protein, was obtained by showing that TFR53 gene expression could be induced by infection with a fungal pathogen. Additionally, the expression of selected tall fescue and fungal genes were studied in immature tissues. Not all genes were differentially expressed at this early stage of the symbiosis. Overall these results indicate that both partners in this symbiosis are active participants, and that the endophyte may be suppressing at least one plant defense gene (putatively encoding PR-10). Analyses of genes with altered transcription in grass-endophyte symbiotic associations should aid in understanding the fundamental processes of establishment and maintenance of mutualistic symbioses, as well as provide insight into genes important for the documented endophyte-enhanced plant improvements.

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