Endophytes and Host-Plant Herbivore Relationships
Perhaps the most well-studied attribute of endophyte-infected grasses is their ability to produce alkaloids. These endophytic alkaloids fall into four general classes: 1) pyrrolopyrazine alkaloids; 2) lolines or pyrrolizidine alkaloids; 3) ergot alkaloids; and 4) lolitrems or indole dipterpene alkaloids. These alkaloids have been shown to have deterrent and toxic effects on invertebrate and vertebrate herbivores, as well as anti-pathogen effects, depending on the type and level of alkaloid produced. Hence, protection of grass against herbivores has been viewed as the major mechanism of mutualistic benefit to the host and endophytes have been known as defensive mutualists. However, this chapter shows that there is not a simple relationship between endophytes, alkaloids, and resistance to herbivores. Whereas bioassays and tests with generalized agronomic pests show that alkaloids may indeed increase herbivore resistance, this is not always the case, or may not even be typical in natural grass communities. In more complex communities, alkaloids may have more negative effects on natural enemies of herbivores than the herbivores themselves, thus increasing rather than decreasing herbivory. Endophyte infection can change tolerance rather than resistance to herbivores. In the case of increasing tolerance, infected grasses may compensate for increased herbivory without concomitant loss, or even with increases, in fitness. Finally, recent evidence suggests that asexual endophytes promote herbivory during early host ontogeny which, in turn, increases seed transmission. Because most hosts of systemic endophytes are perennial grasses, resistance and tolerance to herbivores may not be an all-or-none phenomenon, but instead change with host ontogeny.
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