The epichloae: alkaloid diversity and roles in symbiosis with grasses

Abstract

Epichloae (Epichloë and Neotyphodium species; Clavicipitaceae) are fungi that live in systemic symbioses with cool-season grasses, and many produce alkaloids that are deterrent or toxic to herbivores. The epichloae colonize much of the aerial plant tissues, and most benignly colonize host seeds to transmit vertically. Of their four chemical classes of alkaloids, the ergot alkaloids and indole-diterpenes are active against mammals and insects, whereas peramine and lolines specifically affect insects. Comparative genomic analysis of Clavicipitaceae reveals a distinctive feature of the epichloae, namely, large repeat blocks in their alkaloid biosynthesis gene loci. Such repeat blocks can facilitate gene losses, mutations, and duplications, thus enhancing diversity of alkaloid structures within each class. We suggest that alkaloid diversification is selected especially in the vertically transmissible epichloae.

Figure 1

Structures of alkaloids and proposed pathways for their biosynthesis in Epichoë, Neotyphodium and Periglandula species. Panel A: Proposed pathway for lolines. Panel B: Proposed pathway for ergot alkaloids. Panel C: Structure of peramine. Panel D: Proposed pathway for indole-diterpenes. Enzymes are designated according to names of genes that encode them. Enzyme designations are in blue type for early steps, purple type for intermediate steps, and red type for late steps. Substrates and cofactors indicated beneath or to the right of arrows are abbreviated as follows: AcCoA = acetyl-CoA, AdoMet = S-adenosylmethionine, Ala = l-alanine, DMAPP = dimethylallyl diphosphate, Leu = l-leucine, Pro = l-proline, Trp = l-tryptophan, Val = l-valine. Major pathway end products are designated in bold, and species and strains that produce them in symbio are given beneath each.

Figure 2

Structures of biosynthesis gene loci for four classes of alkaloids in several sequenced genomes of Epichoë, Neotyphodium and Periglandula species. Panel A: Loline alkaloid biosynthesis gene loci (LOL). Tracks from top to bottom of each map represent genes (arrows and letters), repeats (cyan bars), miniature inverted-repeat transposable elements (MITEs) (vertical bars), and graphs of AT (red) and GC (blue) contents. Genes are color coded according to Figure 1. Additionally, gray arrows indicate housekeeping and other genes not known to be involved in alkaloid biosynthesis, and open arrows indicate pseudogenes. Gene names are abbreviated as follows: A = lolA, C = lolC, D = lolD, E = lolE, F = lolF, M = lolM, N = lolN, O = lolO, P = lolP, T = lolT, and U = lolU. Panel B: Ergot alkaloid biosynthesis gene loci (EAS). Gene names are abbreviated as follows: A = easA, B = cloA, C = easC, D = easD, E = easE (= ccsA), F = easF, G = easG, H = easH, O = easO, P = easP, and W = dmaW. Tracks and color coding are as in panel A. Panel C: The peramine synthetase gene (perA) and flanking genes. Domains of the perA gene product are indicated, and are discussed in references [15•,21••]. The perA gene of E. festucae E2368 lacks the 3′ region encoding the reductase domain (R*), thought to be required for peramine biosynthesis, but the possibility that it may encode an enzyme for biosynthesis of a related alkaloid has not been explored. Tracks and color coding are as in panel A. Panel D: Indole-diterpene biosynthesis gene loci (IDT). Gene names are abbreviated as follows: B = idtB/ltmB, C = idtC/ltmC, F = idtF/ltmF, G = idtG/ltmG, K = idtK/ltmK, M = idtM/ltmM, P = idtP/ltmP, Q = idtQ/ltmQ, S = idtS/ltmS. Tracks and color coding are as in panel A.