Epichloë
Epichloë is a genus of ascomycete fungi forming an endophytic symbiosis with grasses. Grass choke disease is a symptom in grasses induced by some Epichloë species, which form spore-bearing mats on tillers and suppress the development of their host plant's inflorescence. For most of their life cycle however, Epichloë grow in the intercellular space of stems, leaves, inflorescences, and seeds of the grass plant without incurring symptoms of disease. In fact, they provide several benefits to their host, including the production of different herbivore-deterring alkaloids, increased stress resistance, and growth promotion.
Within the family Clavicipitaceae, Epichloë is embedded in a group of endophytic and plant pathogenic fungi, whose common ancestor probably derived from an animal pathogen. The genus includes both species with a sexually reproducing stage and asexual, anamorphic species. The latter were previously placed in the form genus Neotyphodium but included in Epichloë after molecular phylogenetics had shown asexual and sexual species to be intermingled in a single clade. Hybrid speciation has played an important role in the evolution of the genus.
Epichloë species are ecologically significant through their effects on host plants. Their presence has been shown to alter the composition of plant communities and food webs. Grass varieties, especially of tall fescue and ryegrass, with symbiotic Epichloë endophyte strains, are commercialised and used for pasture and turf.
Taxonomy
, in 1849, first defined Epichloë as a subgenus of Cordyceps. As type species, he designated Cordyceps typhina, originally described by Christiaan Hendrik Persoon. The brothers Charles and Louis René Tulasne then raised the subgenus to genus rank in 1865. Epichloë typhina would remain the only species in the genus until the discovery of fungal grass endophytes causing livestock intoxications in the 1970s and 1980s, which stimulated the description of new species. Several species from Africa and Asia that develop stromata on grasses were split off as a separate genus Parepichloë in 1998.Many Epichloë species have forms that reproduce sexually, and several purely asexual species are closely related to them. These anamorphs were long classified separately: Morgan-Jones and Gams collected them in a section of genus Acremonium. In a molecular phylogenetic study in 1996, Glenn and colleagues found the genus to be polyphyletic and proposed a new genus Neotyphodium for the anamorphic species related to Epichloë. A number of species continued to be described in both genera until Leuchtmann and colleagues included most of the form genus Neotyphodium in Epichloë. Phylogenetic studies had shown both genera to be intermingled, and the nomenclatural code required since 2011 that one single name be used for all stages of development of a fungal species. Only Neotyphodium starrii, of unclear status, and N. chilense, which is unrelated, were excluded from Epichloë.
Species
As of 2020, there are 35 accepted species in the genus, with 3 subspecies and 6 varieties described. 14 species, 3 subspecies and 5 varieties are haploid. 21 species and 1 variety are hybrids. Several taxa are only known as anamorphic forms, most of which have previously been classified in Neotyphodium.| Haploid Taxa | Known Distribution | Sexual Reproduction | Vertical Transmission | Known Host Range | Reference to Species Description |
| Epichloë amarillans J.F. White | North America | Observed | Present | Agrostis hyemalis, Agrostis perennans, Calamagrostis canadensis, Elymus virginicus, Sphenopholis nitida, Sphenopholis obtusata, Sphenopholis × pallens, Ammophila breviligulata | |
| Epichloë aotearoae Leuchtm. & Schardl | New Zealand, Australia | Not observed | Present | Echinopogon ovatus | |
| Epichloë baconii J.F. White | Europe | Observed | Absent | Agrostis capillaris, Agrostis stolonifera, Calamagrostis villosa, Calamagrostis varia, Calamagrostis purpurea | |
| Epichloë brachyelytri Schardl & Leuchtm. | North America | Observed | Present | Brachyelytrum erectum | |
| Epichloë bromicola Leuchtm. & Schardl | Europe, Asia | Observed on Bromus erectus, Elymus repens and Roegneria kamoji | Present in Bromus benekenii, Bromus ramosus and Hordelymus europaeus, Hordeum brevisubulatum, Leymus chinensis and Roegneria kamoji; absent in Bromus erectus and Elymus repens | Europe: Bromus benekenii, Bromus erectus, Bromus ramosus, Elymus repens, Hordelymus europaeus, Hordeum brevisubulatum. Asia: Leymus chinensis, Roegneria kamoji | |
| Epichloë elymi Schardl & Leuchtm. | North America | Observed | Present | Bromus kalmii, Elymus spp. | |
| Epichloë festucae Leuchtm., Schardl & M.R. Siegel | Europe, Asia, North America | Observed | Present | Festuca spp., Koeleria spp., Schedonorus spp. | |
| Epichloë festucae var. lolii C.W. Bacon & Schardl | Europe, Asia, North Africa, introduced in New Zealand, Australia and elsewhere | Not observed | Present | Lolium perenne subsp. perenne | |
| Epichloë gansuensis Schardl | Asia | Not observed | Present | Achnatherum inebrians, Achnatherum sibiricum, Achnatherum pekinense | |
| Epichloë gansuensis var. inebrians Schardl | Asia | Not observed | Present | Achnatherum inebrians | |
| Epichloë glyceriae Schardl & Leuchtm. | North America | Observed | Absent | Glyceria spp. | |
| Epichloë mollis Leuchtm. & Schardl | Europe | Observed | Present | Holcus mollis | |
| Epichloë sibirica Tadych | Asia | Not observed | Present | Achnatherum sibiricum | |
| Epichloë stromatolonga Leuchtm. | Asia | Not observed | Present | Calamagrostis epigejos | |
| Epichloë sylvatica Leuchtm. & Schardl | Europe, Asia | Observed | Present | Brachypodium sylvaticum, Hordelymus europaeus | |
| Epichloë sylvatica subsp. pollinensis Leuchtm. & M. Oberhofer | Europe | Observed | Present | Hordelymus europaeus | |
| Epichloë typhina Tul. & C. Tul. | Europe, introduced in North America and elsewhere | Observed | Present in Puccinellia distans; absent in other hosts | Anthoxanthum odoratum, Brachypodium phoenicoides, Brachypodium pinnatum, Dactylis glomerata, Lolium perenne, Milium effusum, Phleum pratense, Poa trivialis, Poa silvicola, Puccinellia distans | |
| Epichloë typhina subsp. clarkii Leuchtm. & Schardl | Europe | Observed | Absent | Holcus lanatus | |
| Epichloë typhina subsp. poae Tadych | Europe, North America | Observed on Poa nemoralis and Poa pratensis | Present in Poa nemoralis, Poa secunda subsp. juncifolia; absent in Poa pratensis | Europe: Poa nemoralis, Poa pratensis. North America: Poa secunda subsp. juncifolia, Poa sylvestris | |
| Epichloë typhina subsp. poae var. aonikenkana Iannone & Schardl | Argentina | Not observed | Present | Bromus setifolius | |
| Epichloë typhina subsp. poae var. canariensis Leuchtm. | Canary Islands | Not observed | Present | Lolium edwardii | |
| Epichloë typhina subsp. poae var. huerfana Tadych & Leuchtm. | North America | Not observed | Present | Festuca arizonica |
| Hybrid Taxa | Progenitor Species | Known Distribution | Sexual Reproduction | Vertical Transmission | Known Host Range | Reference to Species Description |
| Epichloë alsodes T. Shymanovich, C.A. Young, N.D. Charlton & S.H. Faeth | Epichloë amarillans × Epichloë typhina subsp. poae | North America | Not observed | Present | Poa alsodes | |
| Epichloë australiensis Leuchtm. & Schardl | Epichloë festucae × Epichloë typhina complex | Australia | Not observed | Present | Echinopogon ovatus | |
| Epichloë cabralii Iannone, M.S. Rossi & Schardl | Epichloë amarillans × Epichloë typhina complex | Argentina | Not observed | Present | Phleum alpinum | |
| Epichloë canadensis N.D. Charlton & C.A. Young | Epichloë amarillans × Epichloë elymi | North America | Not observed | Present | Elymus canadensis | |
| Epichloë chisosa Schardl | Epichloë amarillans × Epichloë bromicola × Epichloë typhina complex | North America | Not observed | Present | Achnatherum eminens | |
| Epichloë coenophiala C.W. Bacon & Schardl | Epichloë baconii × Epichloë festucae × Epichloë typhina complex | Europe, North Africa, introduced in North America and elsewhere | Not observed | Present | Schedonorus arundinaceus | |
| Epichloë danica Leuchtm. & M. Oberhofer | Epichloë bromicola × Epichloë sylvatica | Europe | Not observed | Present | Hordelymus europaeus | |
| Epichloë disjuncta Leuchtm. & M. Oberhofer | 'Epichloë typhina complex × second unknown ancestor | Europe | Not observed | Present | Hordelymus europaeus | |
| Epichloë funkii J.F. White | Epichloë elymi × Epichloë festucae | North America | Not observed | Present | Achnatherum robustum | |
| Epichloë guerinii Leuchtm. & Schardl | Epichloë gansuensis × Epichloë typhina complex | Europe | Not observed | Present | Melica ciliata, Melica transsilvanica | |
| Epichloë hordelymi Leuchtm. & M. Oberhofer | Epichloë bromicola × Epichloë typhina complex | Europe | Not observed | Present | Hordelymus europaeus | |
| Epichloë hybrida M.P. Cox & M.A. Campell | Epichloë festucae var. lolii × Epichloë typhina | Europe | Not observed | Present | Lolium perenne | |
| Epichloë liyangensis Z.W. Wang, Y. Kang & H. Miao | Epichloë bromicola × Epichloë typhina complex | Asia | Observed | Present | Poa pratensis subsp. pratensis | |
| Epichloë melicicola Schardl | Epichloë aotearoae × Epichloë festucae | South Africa | Not observed | Present | Melica racemosa, Melica decumbens | |
| Epichloë novae-zelandiae Leuchtm. & A.V. Stewart | Epichloë amarillans × Epichloë bromicola × Epichloë typhina subsp. poae | New Zealand | Not observed | Present | Poa matthewsii | |
| Epichloë occultans Schardl | Epichloë baconii × Epichloë bromicola | Europe, North Africa, introduced in New Zealand and elsewhere | Not observed | Present | Lolium multiflorum, Lolium rigidum u.a. | |
| Epichloë pampeana Iannone & Schardl | Epichloë festucae × Epichloë typhina complex | South America | Not observed | Present | Bromus auleticus | |
| Epichloë schardlii Leuchtm. | Epichloë typhina complex | North America | Not observed | Present | Cinna arundinacea | |
| Epichloë schardlii var. pennsylvanica T. Shymanovich, C.A. Young, N.D. Charlton & S.H. Faeth | Epichloë typhina complex | North America | Not observed | Present | Poa alsodes | |
| Epichloë siegelii Leuchtm. & Schardl | Epichloë bromicola × Epichloë festucae | Europe | Not observed | Present | Schedonorus pratensis | |
| Epichloë sinica Leuchtm. | Epichloë bromicola × Epichloë typhina complex | Asia | Not observed | Present | Roegneria spp. | |
| Epichloë sinofestucae Leuchtm. | Epichloë bromicola × Epichloë typhina complex | Asia | Not observed | Present | Festuca parvigluma | |
| Epichloë tembladerae Iannone & Schardl | Epichloë festucae × Epichloë typhina complex | North America | Not observed | Present | North America: Festuca arizonica. South America: Bromus auleticus, Bromus setifolius, Festuca argentina, Festuca hieronymi, Festuca magellanica, Festuca superba, Melica stuckertii, Phleum alpinum, Phleum commutatum, Poa huecu, Poa rigidifolia | |
| Epichloë uncinata Leuchtm. & Schardl | Epichloë bromicola × Epichloë typhina complex | Europe | Not observed | Present | Schedonorus pratensis'' | A1:G25 |
Life cycle and growth
Epichloë species are specialized to form and maintain systemic, constitutive symbioses with plants, often with limited or no disease incurred on the host. The best-studied of these symbionts are associated with the grasses and sedges, in which they infect the leaves and other aerial tissues by growing between the plant cells or on the surface above or beneath the cuticle. An individual infected plant will generally bear only a single genetic individual clavicipitaceous symbiont, so the plant-fungus system constitutes a genetic unit called a symbiotum.Symptoms and signs of the fungal infection, if manifested at all, only occur on a specific tissue or site of the host tiller, where the fungal stroma or sclerotium emerges. The stroma is a mycelial cushion that gives rise first to asexual spores, then to the sexual fruiting bodies. Sclerotia are hard resting structures that later germinate to form stipate stromata. Depending on the fungus species, the host tissues on which stromata or sclerotia are produced may be young inflorescences and surrounding leaves, individual florets, nodes, or small segments of the leaves. Young stromata are hyaline, and as they mature they turn dark gray, black, or yellow-orange. Mature stromata eject meiotically derived spores, which are ejected into the atmosphere and initiate new plant infections. In some cases no stroma or sclerotium is produced, but the fungus infects seeds produced by the infected plant, and is thereby transmitted vertically to the next host generation. Most Epichloë species, and all asexual species, can vertically transmit.
The taxonomic dichotomy is especially interesting in this group of symbionts, because vegetative propagation of fungal mycelium occurs by vertical transmission, i.e., fungal growth into newly developing host tillers. Importantly, many Epichloë species infect new grass plants solely by growing into the seeds of their grass hosts, and infecting the growing seedling. Manifestation of the sexual state — which only occurs in Epichloë species — causes "choke disease", a condition in which grass inflorescences are engulfed by rapid fungal outgrowth forming a stroma. The fungal stroma suppresses host seed production and culminates in the ejection of meiospores that mediate horizontal transmission of the fungus to new plants. So, the two transmission modes exclude each other, although in many grass-Epichloë symbiota the fungus actually displays both transmission modes simultaneously, by choking some tillers and transmitting in seeds produced by unchoked tillers.
While being obligate symbionts in nature, most epichloae are readily culturable in the laboratory on culture media such as potato dextrose agar or a minimal salts broth supplemented with thiamine, sugars or sugar alcohols, and organic nitrogen or ammonium.
Epichloë species are commonly spread by flies of the genus Botanophila. The flies lay their eggs in the growing fungal tissues and the larvae feed on them.
formed on the grass Agrostis stolonifera, showing eggs, brood chambers and larval feeding tracks of Botanophila'' flies.
Evolution
The epichloae display a number of central features that suggest a very strong and ancient association with their grass hosts. The symbiosis appears to have existed already during the early grass evolution that has spawned today's pooid grasses. This is suggested by phylogenetic studies indicating a preponderance of codivergence of Epichloë species with the grass hosts they inhabit. Growth of the fungal symbiont is very tightly regulated within its grass host, indicated by a largely unbranched mycelial morphology and remarkable synchrony of grass leaf and hyphal extension of the fungus; the latter seems to occur via a mechanism that involves stretch-induced or intercalary elongation of the endophyte's hyphae, a process so far not found in any other fungal species, indicating specialized adaptation of the fungus to the dynamic growth environment inside its host. A complex NADPH oxidase enzyme-based ROS-generating system in Epichloë species is indispensable for maintenance of this growth synchrony. Thus, it has been demonstrated that deletion of genes encoding these enzymes in Epichloë festucae causes severely disordered fungal growth in grass tissues and even death of the grass plant.Molecular phylogenetic evidence demonstrates that asexual Epichloë species are derived either from sexual Epichloë species, or more commonly, are hybrids of two or more progenitor Epichloë species.
Bioactive compounds
Many Epichloë endophytes produce a diverse range of natural product compounds with biological activities against a broad range of herbivores. The purpose of these compounds is as a toxicity or feeding deterrence against insect and mammalian herbivores. Ergoline alkaloids are characterized by a ring system derived from 4-prenyl tryptophan. Among the most abundant ergot alkaloids in epichloë-symbiotic grasses is ergovaline, comprising an ergoline moiety attached to a bicyclic tripeptide containing the amino acids L-proline, L-alanine, and L-valine. Key genes and enzymes for ergot alkaloid biosynthesis have been identified in epichloae and include dmaW, encoding dimethylallyl-tryptophan synthase and lpsA, a non-ribosomal peptide synthetase.Another group of epichloë alkaloids are the indole-diterpenoids, such as lolitrem B, which are produced from the activity of several enzymes, including prenyltransferases and various monooxygenases. Both the ergoline and indole-diterpenoid alkaloids have biological activity against mammalian herbivores, and also activity against some insects. Peramine is a pyrrolopyrazine alkaloid thought to be biosynthesized from the guanidinium-group-containing amino acid L-arginine, and pyrrolidine-5-carboxylate, a precursor of L-proline, and is an insect-feeding deterrent. The loline alkaloids are 1-aminopyrrolizidines with an oxygen atom linking bridgehead carbons 2 and 7, and are biosynthesized from the amino acids L-proline and L-homoserine. The lolines have insecticidal and insect-deterrent activities comparable to nicotine. Loline accumulation is strongly induced in young growing tissues or by damage to the plant-fungus symbiotum. Many, but not all, epichloae produce up to three classes of these alkaloids in various combinations and amounts. Recently it has been shown that Epichloë uncinata infection and loline content afford × Festulolium grasses protection from black beetle.
Many species in Epichloë produce biologically active alkaloids, such as ergot alkaloids, indole-diterpenoids, loline alkaloids, and the unusual guanidinium alkaloid, peramine.