For much of the past 30 years, tall fescue research has focused on the detrimental effects of toxins produced by a naturally symbiotic endophyte (Neotyphodium coenophialum) on livestock performance and metabolism (see Chapter 1). Early efforts to mitigate the toxic effects of endophyte infected (E+) tall fescue included eradicating the endophyte from seedstocks and eliminating existing E+ stands and reseeding with endophyte free (E-) tall fescue. It seemed that in many pasture and turf plantings, E+ plants persisted better than E- plants where drought stress and insect predation occurred. At the same time, scientists in New Zealand observed that perennial ryegrass (L. perenne L.) infected with a similar endophyte (N. lolii Latch, M.J. Christensen & Samuels) showed superior resistance to Argentine stem weevil [Listronotus bonariensis (Kuschel)]. Evidence suggested that host plants had competitive and adaptive advantages when infected with endophytes. This had tremendous implications for management and cultivar development for pastures and turf and, on a basic level, for understanding the ecology of plant communities (i.e., plant autecology).

A number of plant physiological and ecological processes are influenced by host-endophyte symbiosis; however, a simple, unifying mechanism by which the endophyte promotes host fitness is not apparent. In general, Neotyphodium endophytes confer resistances to biotic stresses (i.e., herbivory by insects and mammals, including grazing livestock) and abiotic stresses (e.g., drought and soil deficiencies) and possibly even promote growth. Biotic and abiotic stresses can interact to influence host-endophyte responses. For example, when the presence of toxic alkaloids deters grazing and allows plants to thrive with high leaf area and tiller density, the plants can acquire resources (i.e., water and nutrients) and persist in stressful conditions. The complex relationship between the host plant and endophyte is a function of direct and indirect effects of the endophyte on metabolic and developmental processes influencing plant production and persistence (Malinowski and Belesky, 2000; Malinowski et al., 2005a). We review the attributes of host plant-endophyte interactions with emphasis on tall fescue responses to abiotic stresses associated with drought and soil mineral deficiencies. The reader is referred to in-depth reviews of water relations (Frank et al., 1996) and mineral nutrition (Mayland and Wilkinson, 1996) of cool-season grasses to complement this chapter.

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