Identification of differentially expressed genes in the mutualistic association of tall fescue with Neotyphodium coenophialum

Tall fescue (Lolium arundinaceum), an agronomically important forage grass, is typically associated with a mutualistic asexual fungus Neotyphodium coenophialum. Plant colonization is endophytic with no symptoms, and fungal growth is confined to the intercellular spaces. The endophyte enhances host fitness by providing protection from various abiotic and biotic stresses and by improving nutrient acquisition. By suppression subtractive hybridization (SSH) we identified 29 genes that are up-regulated or down-regulated in endophyte-infected tall fescue as compared to endophyte-free tall fescue. Of the genes that had matches to known genes present in the NCBI databases (approximately 50%), several had roles related to plant defense and stress tolerance. Differential expression of these genes was confirmed by semi-quantitative RT-PCR, competitive RT-PCR, and northern hybridization. Endophyte-associated changes in gene expression patterns were consistent among cultivars of tall fescue but differed in some other grass-endophyte associations. Our 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). Further analyses of the differentially expressed genes should aid in understanding the fundamental nature of this mutualistic symbiosis and provide insight into the mechanisms of documented endophyte-enhanced plant improvements.