Neotyphodium

Neotyphodium is a form genus containing species of endophytic fungi. These endophytes are asexual, seed-borne symbionts of cool-season grasses, and grow intercellularly throughout the aerial tissues of their hosts, including shoot apical meristems, leaf sheaths and blades, inflorescences, seeds and embryos.^[1]

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Taxonomic considerations

Neotyphodium species are closely related to teleomorphic species of the genus Epichloë, from which many have evolved by processes involving interspecific hybridization.^[2]. Molecular phylogenetic evidence demonstrates that asexual Neotyphodium species are derived either from individual Epichloë species, or more commonly, from hybrids with at least two ancestral Epichloë species.^[2][3] Hence, the form genus Neotyphodium is very closely associated with the teleomorphic genus Epichloë.^[4] In keeping with the code of botanical nomenclature, the form genus refers to the asexual spore or vegetative state, and the teleomorphic genus refers to the sexual state.

Life cycle

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 (=individual grass plants). Importantly, all Neotyphodium and some Epichloëspecies infect new grass plants solely by growing into the seeds of their grass hosts, and infecting the growing seedling. ^[5] Manifestation of the sexual state - which only occurs in Epichloë fungi — 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 (ascospores) that mediate horizontal (contagious) transmission of the fungus to new plants.^[5].

Effects on the grass plant and on herbivores

It has been proposed that vertically transmitted symbionts should evolve to be mutualists since their reproductive fitness is intimately tied to that of their hosts.^[6] In fact, some positive effects of Neotyphodium species on their host plants include increased growth, drought tolerance, and herbivore and pathogen resistance.^[5][7] Resistance against herbivores has been attributed to endophyte-produced alkaloids.^[8] Although grass-endophyte symbioses have been widely recognized to be mutualistic in many wild and cultivated grasses, the interactions can be highly variable and sometimes antagonistic, especially under nutrient-poor conditions in the soil.^[9]

Neotyphodium alkaloids

Ergoline alkaloids (which are ergot alkaloids, named after the ergot fungus, Claviceps purpurea, a close relative of the Neotyphodium/Epichloë endophytes) are characterized by a ring system derived from 4-prenyl tryptophan.^[10] Among the most abundant ergot alkaloids in endophyte-symbiotic grasses is ergovaline, comprising an ergoline moiety attached to a bicyclic tripeptide containing the amino acids, L-proline, L-alanine, and L-valine. Another group of endophyte alkaloids are the indole-diterpenoids, such as lolitrem B.^[11] 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,^[12] 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.^[13] The lolines have insecticidal and insect-deterrent activities comparable to nicotine. Many, but not all, Neotyphodium species produce up to three classes of these alkaloids.

Species

• Neotyphodium aotearoae
• Neotyphodium australiense
• Neotyphodium chilense
• Neotyphodium chisosum
• Neotyphodium coenophialum
• Neotyphodium gansuense
• Neotyphodium huerfanum
• Neotyphodium lolii
• Neotyphodium melicicola
• Neotyphodium occultans
• Neotyphodium siegelii
• Neotyphodium starrii
• Neotyphodium tembladerae
• Neotyphodium typhinum
• Neotyphodium uncinatum

References

1. ^ Roberts CA, West CP, Spiers DE, eds (2005). Neotyphodium in Cool-Season Grasses. Blackwell. ISBN 978-0813801896. 
2. ^ ^{a b} Tsai HF, Liu JS, Staben C, Christensen MJ, Latch GC, Siegel MR, Schardl CL (1994). "Evolutionary diversification of fungal endophytes of tall fescue grass by hybridization with Epichloë species". Proc. Natl. Acad. Sci. USA 91: 2542-2546. PMID 8172623.
3. ^ Moon CD, Craven KD, Leuchtmann A, Clement SL, Schardl CL (2004). "Prevalence of interspecific hybrids amongst asexual fungal endophytes of grasses". Molec Ecol 13: 1455-1467. PMID 15140090.
4. ^ Glenn AE, Bacon CW, Price R, Hanlin RT (1996). "Molecular phylogeny of Acremonium and its taxonomic implications". Mycologia 88: 369-383.
5. ^ ^{a b c} Schardl CL, Leuchtmann A, Spiering MJ (2004). "Symbioses of grasses with seedborne fungal endophytes". Annu Rev Plant Biol 55: 315-340. PMID 15377223.
6. ^ Ewald PW (1987). "Transmission modes and evolution of the parasitism-mutualism continuum". Ann NY Acad Sci 503: 295-306. PMID 3304078.
7. ^ Malinowski DP, Belesky DP (2000). "Adaptations of endophyte-infected cool-season grasses to environmental stresses: mechanisms of drought and mineral stress tolerance.". Crop Sci. 40: 923-940.
8. ^ Bush LP, Wilkinson HH, Schardl CL (1997). "Bioprotective Alkaloids of Grass-Fungal Endophyte Symbioses". Plant Physiol. 114: 1-7. PMID 12223685.
9. ^ Saikkonen K, Ion D, Gyllenberg M (2002). "The persistence of vertically transmitted fungi in grass metapopulations". Proc Biol Sci 269: 1397-1403. PMID 12079664.
10. ^ Schardl CL, Panaccione DG, Tudzynski P (2006). "Ergot alkaloids – biology and molecular biology". The Alkaloids: Chemistry and Biology 63: 45-86. PMID 17133714.
11. ^ Young CA, Felitti S, Shields K, Spangenberg G, Johnson RD, Bryan GT, Saikia S, Scott B (2006). "A complex gene cluster for indole-diterpene biosynthesis in the grass endophyte Neotyphodium lolii". Fung Genet Biol 43: 679-693. PMID 16765617.
12. ^ Tanaka A, Tapper BA, Popay A, Parker, EJ, Scott B (2005). "A symbiosis expressed non-ribosomal peptide synthetase from a mutualistic fungal endophyte of perennial ryegrass confers protection to the symbiotum from insect herbivory". Mol. Microbiol. 57: 1036-1050. PMID 16091042.
13. ^ Blankenship JD, Houseknecht JB, Pal S, Bush LP, Grossman RB, Schardl CL (2005). "Biosynthetic precursors of fungal pyrrolizidines, the loline alkaloids". Chembiochem 6: 1016-1022. PMID 15861432.