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Summary Plants form mutualistic symbioses with a variety of microorganisms including endophytic fungi that live inside the plant and cause no overt symptoms of infection. Some endophytic fungi form defensive mutualisms based on the production of bioactive metabolites that protect the plant from herbivores in exchange for a protected niche and nutrition from the host plant. Key elements of these symbioses are vertical transmission of the fungus through seed of the host plant, a narrow host range, and production of bioactive metabolites by the fungus. Grasses frequently form symbioses with endophytic fungi belonging to the family Clavicipitaceae. These symbioses have been studied extensively because of their significant impacts on insect and mammalian herbivores. Many of the impacts are likely due to the production of four classes of bioactive alkaloids – ergot alkaloids, lolines, indole-diterpenes and peramine – that are distributed in different combinations among endophyte taxa. Several legumes, including locoweeds, are associated with a toxic syndrome called locoism as a result of their accumulation of swainsonine. Species in two genera were recently found to contain previously undescribed endophytic fungi (Undifilum spp., family Pleosporaceae) that are the source of that toxin. The fungi are strictly vertically transmitted and have narrow host ranges. Some plant species in the morning glory family (Convolvulaceae) also form symbioses with endophytic fungi of the Clavicipitaceae that produce ergot alkaloids and, perhaps in at least one case, lolines. Other species in this plant family form symbioses with undescribed fungi that produce swainsonine. The swainsonine-producing endophytes associated with the Convolvulaceae are distinct from the Undifilum spp. associated with locoweeds and the Clavicipitaceous fungi associated with Convolvulaceae. In the establishment of vertically transmitted symbioses, fungi must have entered the symbiosis with traits that were immediately useful to the plant. Bioactive metabolites are likely candidates for such pre-adapted traits which were likely useful to the free-living fungi as well. With future research, vertically transmitted fungi from diverse clades with narrow host ranges and that produce bioactive compounds are likely to be found as important mutualists in additional plants.

Deployment of novel endophytes [Neotyphodium coenophialum (Morgan-Jones & Gams) Glenn, Bacon & Hanlin] may expand the use of tall fescue [Lolium arundinaceum (Schreb.) S.J. Darbyshire] in the south-central United States. Our objectives were to compare agronomic performance of, and health of lambs (Ovis aries) grazing, several tall fescue-novel endophyte combinations in the south-central United States. Two tall fescue populations (97TF1 and PDF) were inoculated with four novel endophytes (AR514, AR524, AR542, and AR584). At most locations, persistence of the eight combinations under grazing by beef cattle (Bos taurus) was similar to Jesup MaxQ. However, the combination PDF AR584 had greater persistence than Jesup MaxQ at Booneville, AR (24 vs. 14% stands, respectively) and Burneyville, OK (31 vs. 8% stands, respectively). Forage yields of the eight combinations were comparable to Jesup MaxQ, with PDF AR584 having greater yields on occasion. Lambs grazing PDF AR584, 97TF1 AR584, PDF AR542, PDF AR524, and Jesup MaxQ gained an average of >139 g d -1 , more than twice the 68 g d -1 gained by animals grazing endophyte-infected Kentucky-31. Prolactin and urinary alkaloid data indicated no detrimental health effects for animals grazing the novel endophyte combinations. Several tall fescue-novel endophyte combinations, and in particular PDF AR584, appear to be suited for the south-central United States, based on competitive agronomic performance and health of grazing lambs.

Neotyphodium and Epichloe species (Ascomycota: Clavicipitaceae) are fungal symbionts (endophytes) of grasses. Many of these endophytes produce alkaloids that enhance their hosts' resistance to insects or are toxic to grazing mammals. The goals of eliminating from forage grasses factors such as ergot alkaloids that are responsible for livestock disorders, while retaining pasture sustainability, and of developing resistant turf grasses, require better understanding of how particular alkaloids affect insect herbivores. We used perennial ryegrass Lolium perenne L. (Poaceae) symbiotic with Neotyphodium lolii x Epichloe typhina isolate Lp1 (a natural interspecific hybrid), as well as with genetically modified strains of Lp1 with altered ergot alkaloid profiles, to test effects of ergot alkaloids on feeding, growth, and survival of the black cutworm, Agrotis ipsilon (Hufnagel) (Lepidoptera: Noctuidae), a generalist grass-feeding caterpillar. Neonates or late instars were provided clippings from glasshouse-grown plants in choice and rearing trials. Wild-type endophytic grass showed strong antixenosis and antibiosis, especially to neonates. Plant-endophyte symbiota from which complex ergot alkaloids (ergovaline and lysergic acid amides such as ergine) or all ergot alkaloids were eliminated by endophyte gene knockout retained significant resistance against neonates. However, this activity was reduced compared to that of wild-type Lp1, providing the first direct genetic evidence that ergot alkaloids contribute to insect resistance of endophytic grasses. Similarity of larval response to the two mutants suggested that ergovaline and/or ergine account for the somewhat greater potency of wild-type Lp1 compared to the knockouts, whereas simpler ergot alkaloids contribute little to that added resistance. All of the endophyte strains also produced peramine, which was probably their primary resistance component. This study suggests that ergot alkaloids can be eliminated from an endophyte of perennial ryegrass while retaining significant insect resistance.