Bioprotective Alkaloids of Grass-Fungal Endophyte Symbioses.

Symbiotic interactions of C3 grasses with fungal endo-phytes, Epichloe species and their asexual relatives Neoty-pkodium, often provide the grass hosts with major fitness enhancements (for review, see Siegel and Bush [1994]). The endophytes protect host plants from both biotic and abiotic environmental stresses. Documented enhancements to host fitness include greater resistance to mammalian and insect herbivores, pathogens, and nematodes, as well as increased drought tolerance and competitiveness. Our understanding of the mechanisms responsible for a11 of these effects on host fitness is not well established. However, the antiher-bivore fitness enhancements are largely attributable to the accumulation of four groups of alkaloids: lolines, peram-ine, ergot alkaloids, and lolitrems. The objective of this Update is to present the current status of knowledge on these alkaloids, focusing on their biological activities and the potential for genetically manipulating their expression in grasslendophyte symbiota. Grass/ endophyte symbiota are extremely intimate and perennial, and exhibit close matching of the host and sym-biont life cycles (Fig. 1). Each symbiotum exhibits one of three possible life history strategies, resulting in: (a) pure vertical transmission of the endophyte, (b) pure horizontal transmission of the endophyte, or (c) a mixture of the two life cycles (Schardl et al., 1997). The asexual Neotypkodium species are restricted to pure vertical transmission, a highly efficient clonal propagation in flowering meristems, ovules, seeds, and, ultimately, progeny seedlings of infected mother plants. In contrast, the pure horizontal transmission strategy of some Epichloe species relies on the production of contagious sexual spores. These spores can only be produced on a fungal structure (stroma) surrounding the grass flagleaf sheath, but as soon as stroma are produced, the inflorescence of the affected tiller ceases 1 development (it seems likely that this is due to fungal products mimicking plant growth regulators). The resulting suppression of seed production is termed " choke disease. " The most highly pathogenic Epickloe species completely suppress host seed production, and, because seeds cannot be a vehicle for their dissemination, the species are obligately sexual and horizontally transmitted. Many Epi-ckloe species use a third, remarkably balanced strategy of mixed transmission (both cycles in Fig. 1). These fungi choke some flowering tillers and produce sexual spores but leave other tillers (usually a majority) unaffected and fertile. Like the asexual endophytes they are transmitted in nearly a11 seeds produced by infected mother plants. Symbiota involving asexual, vertically transmitted endo-phytes are analogous to individuals with maternally transmitted endophytes. …