ABSTRACT
The effect of the fungi is to both alter the nutrient content of the host plant and the production of secondary metabolites that act as deterrents to the animals. For example, species of the genus Baccharis have been shown to produce high concentrations of macrocyclic tricothecenes that are toxic to cattle (Jarvis et al., 1987). Indeed, Clay (1990) suggests that there are complex interactions among endophytes and host plants that vary along a continuum between herbivory defense and pathogenicity, which strongly influences the fitness of the host plant (Table 5.12). The close association between plant and endophyte has been shown to have evolved to a great extent in grasses, in which the two partners are inseparable. Examples of this are the fungi Acremonium lolii and A. coenophialum, associated with ryegrass and tall fescue, respectively. Wilson (1993) suggests that the improved fitness of alkaloid production by the endophyte has led to vertical transmission of the endophyte from plant to plant via the seed. In this way, all new plants can start life with the endophyte and the advantages the fungus confers. In contrast, Wilson suggests that the endophytes of many other plants, such as the fungus Discula quercina of oak, are transmitted horizontally via rain splash from tree to tree. The chances of a plant becoming colonized by the appropriate fungus is therefore, dependent upon environmental conditions. This will result in a mosaic of both colonized and uncolonized plants, in such a way that the resulting community will be dependent upon the competitive and fitness advantages conferred on the plant by the endophytic fungi. Additionally, these fungi infect seeds where the pericarp is missing or damaged, in such a way that the fungus acts as an antagonist against pathogenic fungi. Fitness of the host plant is also enhanced by the presence of endophytes in its seeds. Clay (1990) cites a number of examples in which grass seeds were rendered ineffective in germination due to insect damage in the absence of endophytic fungi in the seed. Where the endophyte was present, the level of seed germination was normal. The degree of endophyte development within the host plant and proportion of seeds infected thus can be an important aspect of actual
