Ergot Alkaloids of the Family Clavicipitaceae

Abstract

Ergot alkaloids are highly diverse in structure, exhibit diverse effects on animals, and are produced by diverse fungi in the phylum Ascomycota, including pathogens and mutualistic symbionts of plants. These mycotoxins are best known from the fungal family Clavicipitaceae and are named for the ergot fungi that, through millennia, have contaminated grains and caused mass poisonings, with effects ranging from dry gangrene to convulsions and death. However, they are also useful sources of pharmaceuticals for a variety of medical purposes. More than a half-century of research has brought us extensive knowledge of ergot-alkaloid biosynthetic pathways from common early steps to several taxon-specific branches. Furthermore, a recent flurry of genome sequencing has revealed the genomic processes underlying ergot-alkaloid diversification. In this review, we discuss the evolution of ergot-alkaloid biosynthesis genes and gene clusters, including roles of gene recruitment, duplication and neofunctionalization, as well as gene loss, in diversifying structures of clavines, lysergic acid amides, and complex ergopeptines. Also reviewed are prospects for manipulating ergot-alkaloid profiles to enhance suitability of endophytes for forage grasses.

FIGURE 1

Examples of Clavicipitaceae that produce ergot alkaloids. A, An ergot of Claviceps purpurea on a tall fescue inflorescence. B, Mycelia of Periglandula ipomoeae on the adaxial side of a young leaf of Ipomoea pes-caprae. C, A fertilized stroma on Epichloë festucae on a grass tiller. D, Aniline blue-stained endophytic hyphae of Epichloë coenophiala between epidermal cells of a tall fescue leaf sheath. Arrows indicate fungal structures.

FIGURE 2

Summary of ergot-alkaloid biosynthetic pathways with emphasis on Clavicipitaceae. Colors indicate precursors (black), early steps (blue), middle steps (green), and late steps in Trichocomaceae (purple) and Clavicipitaceae (red). Examples of fungal species are given for ergot alkaloids that are pathway end products in those fungi. Several metabolites are end products in some fungi and intermediates in others, though even intermediates often accumulate to substantial levels in the fungus or host plant. Other metabolites (ergotryptamine and setoclavine) are spur products derived from early intermediates, and a question regarding elymoclavine biosynthesis in Claviceps fusiformis is discussed in the text. Enzyme designations are based on gene names (Table 1). Cofactors (except O₂, H₂O, and H₂O₂) and L-amino acids are indicated with standard abbreviations. Other abbreviations are as follow: DMAPP =dimethylallyl diphosphate, PhP = 4′-phosphopantetheine, and 2-OG = 2-oxoglutarate. Some cofactors are confirmed, whereas others are predicted based on signature sequences of the enzymes. In some cases, FAD can act in place of FMN, NADP in place of NAD⁺, and NADPH in place of NADH. Amino acids specified by the three LpsA modules vary, so they are generically designated aa1, aa2, and aa3.

FIGURE 3

Ergopeptines. A, Ergopeptines with aa2 = valine, phenylalanine, methionine, isoleucine, homoisoleucine, or homoalanine. Only the ergopeptines in square brackets (β-ergosine and β-ergoptine) remain unknown as natural compounds. B, Ergopeptines with aa2 = norleucine or leucine. Amino acid precursors are indicated as aa1, aa2, and aa3 in the order that they are added by the LpsA subunit of lysergyl peptide synthetase (LPS). Note the variation in aa3, and the dihydrolysergic acid moiety of dihydroergosine.

FIGURE 3

Ergopeptines. A, Ergopeptines with aa2 = valine, phenylalanine, methionine, isoleucine, homoisoleucine, or homoalanine. Only the ergopeptines in square brackets (β-ergosine and β-ergoptine) remain unknown as natural compounds. B, Ergopeptines with aa2 = norleucine or leucine. Amino acid precursors are indicated as aa1, aa2, and aa3 in the order that they are added by the LpsA subunit of lysergyl peptide synthetase (LPS). Note the variation in aa3, and the dihydrolysergic acid moiety of dihydroergosine.

FIGURE 4

Evolutionary hypothesis for ergot-alkaloid diversification. Evolutionary events are mapped onto a phylogeny based on concatenated coding sequences (CDS) for dmaW, easF, easC, and easE (Supplementary File), annotated from whole genome sequences, aligned by MUSCLE (Edgar 2004), and inferred by maximum likelihood implemented in PhyML (Dereeper et al. 2008). Internal branches drawn with thick lines received ≥0.99 ALR support (Anisimova and Gascuel 2006). Evolutionary inventions of major groups of ergot alkaloids are indicated in yellow boxes, gene neofunctionalizations are indicated in red boxes, shifts between subterminal locations (near telomeres) and internal locations are indicated in blue boxes, and gene recruitments and losses are indicated in unshaded boxes. Asterisks (*) indicate the presence of gene remnants. Alkaloids listed at right are confirmed (bold text), predicted but not tested on the strains indicated (regular type), or predicted from gene contents but undetected (in parentheses).

FIGURE 5

Maps of ergot-alkaloid biosynthesis (EAS) gene clusters in some Clavicipitaceae and Neosartorya fumigata (Trichocomaceae). Black box-arrows indicate EAS genes, which are labeled with the full name or an abbreviation with the last letter of the gene name, or with B for cloA, followed by a numeral for those with multiple copies in the respective genome. White boxes indicate pseudogenes (designated with a superscript Ψ), and gray boxes indicate genes for functions other than ergot-alkaloid biosynthesis. Breaks in horizontal lines on the maps indicate breaks in the genome sequence assemblies, where linkages and gap lengths are currently unknown. End-product ergot alkaloids are labeled by species name in Figure 2.