Molecular characterisation of the EAS gene cluster for ergot alkaloid biosynthesis in epichloe endophytes of grasses

Abstract

Clavicipitaceous fungal endophytes of the genera Epichloë and Neotyphodium form symbioses with grasses of the family Pooideae in which they can synthesise an array of bioprotective alkaloids. Some strains produce the ergot alkaloid ergovaline, which is implicated in livestock toxicoses caused by ingestion of endophyteinfected grasses. Cloning and analysis of a plant-induced non-ribosomal peptide synthetase (NRPS) gene from Neotyphodium lolii and analysis of the E. festucae E2368 genome sequence revealed a complex gene cluster for ergot alkaloid biosynthesis. The EAS cluster contained a single-module NRPS gene, lpsB, and other genes orthologous to genes in the ergopeptine gene cluster of Claviceps purpurea and the clavine cluster of Aspergillus fumigatus. Functional analysis of lpsB confirmed its role in ergovaline synthesis and bioassays with the lpsB mutant unexpectedly suggested that ergovaline was not required for black beetle (Heteronychus arator) feeding deterrence from epichloë-infected grasses. Southern analysis showed the cluster was linked with previously identified ergot alkaloid biosynthetic genes, dmaW and lpsA, at a subtelomeric location. The ergovaline genes are closely associated with transposon relics, including retrotransposons, autonomous DNA transposons and miniature inverted-repeat transposable elements (MITEs), which are very rare in other fungi. All genes in the cluster were highly expressed in planta but expression was very low or undetectable in mycelia from axenic culture, including under nitrogen-, carbonor phosphate-limited conditions. Comparative analysis of the EAS gene cluster in four different epichloë strains showed marked differences in gene expression and ergot alkaloid synthesis. Gene order is conserved in each strain although evidence for recombination between two MITEs and expansion or reduction of a simple sequence repeat (SSR) at a single intergenic region was observed. Heterologous expression of a candidate regulatory gene, laeA, from Aspergillus nidulans, which is a global regulator of secondary metabolism in aspergilli, did not affect eas gene expression. This, along with phylogeny and microsynteny analysis, suggests there is not an orthologue of this gene in epichloë. This work provides a genetic foundation for elucidating biochemical steps in the ergovaline pathway, the ecological role of individual ergot alkaloid compounds, and the regulation of their synthesis in planta.