Genetic Analysis of Biosynthesis and Roles of Anti-Herbivore Alkaloids Produced by Grass Endophytes

Abstract

A characteristic of seed-borne clavicipitaceous symbionts (endophytes) that mediates their mutualisms with host grasses is production of anti-herbivore metabolites. Ergot alkaloids and indolediterpenes are potent neurotoxins in vertebrates ; saturated 1-aminopyrrolizidines (lolines) and the pyrrolopyrazine alkaloid, peramine, are active against insects. All except lolines are reported to be produced in cultures of fungal endophytes free of plant material. We identified lolines in defined-medium cultures of Neotyphodium uncinatum. We have also developed Epichloë festucae and Epichloë typhina as Mendelian genetic models to test the effects on aphids of lolines and peramine, respectively. In each case, the phenotypic difference of expression or non-expression was apparently governed by a single locus. Genotypes of E. typhina expressing peramine caused killing of greenbug aphid (Schizaphis graminum) on the host plants. Lolines were associated with killing of both greenbug and bird-cherry oat aphid (Rhopalosiphum padi). Statistically, the anti-aphid activities of the endophytes were entirely attributable to their alkaloids. Recent progress on genetic control of ergot alkaloid and indolediterpene expression holds promise for analogous tests for roles of these alkaloids in host benefits. Mendelian segregation and molecular knockouts can be used eventually to test the ecological importance of all known endophyte alkaloids in the many established endophyte effects, including increased drought tolerance, competitiveness, resistance to nematodes, and resistance to vertebrate and insect herbivores. Genetic knockouts in endophytes of genes for anti-vertebrate alkaloids will likely become an integral part of forage cultivar development.