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Endobiotics and Epibiotics Formed by the ClavicIpitales

Introduction

Included in the Clavicipitales (Ascomycota) are several fungi that form associations with grasses. The associations include a pathogenic epibiotic (outside living tissue) fungus (e.g. Claviceps, Myriogenospora), a partly endobiotic fungus with surface fruiting structures (some Balansia spp), and an entirely endobiotic fungus with no known sexual stages (the agronomically important Neotyphodium). The fungi have the following characteristics:

Claviceps

  • ovarian parasite
  • annual infection
  • fruiting body is entirely fungal
  • found on many grasses and a few sedges
  • seed is replaced by a fungal sclerotium.

Claviceps became known in Europe during the 15th century when the accidental inclusion of sclerotia with grain used for making bread resulted in neurotoxicological symptoms called St Vitus Dance in those people eating the bread. The compounds responsible for the symptoms include LSD, which in more recent times has been used as a recreational drug that induces hallucinations. The syndrome, St Vitus Dance, has now largely disappeared in humans because of the use of clean seed in flour. Toxicity is still found in livestock. LINK In a recent case in the central west of NSW, cattle showed signs of toxicosis following grazing of infected pasture grass.

In Australia, Paspalum found in areas where it is not grazed or mown, often has a sticky inflorescence, which is indicative of infection by Claviceps paspalii. Infected flowers become white scerotia, with the mature sclerotia bursting from the perianth. The sclerotia subsequently fall to the ground. The following spring ascocarps emerge from sclerotia, spores are released and then germinate, and commence the parasitic phase of the life cycle. Do not eat sclerotia whatever you do. Claviceps paspalii is toxic. If you handle sclerotia or sticky paspalum flower heads, wash your hands afterwards.

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Balansiae

  • systemic infection, outside plant tissue
  • perennial infection arising from the crown of the host plant
  • fruit body incorporates plant tissue and may be found attached to various plant parts
  • hosts include C3 grasses (genera Parepichloe and Atkinsonella) and C4 grasses (Balansia and Myriogenospora)
  • flowering of plant may be aborted or severely reduced.

In Australia, an unusual fungus in this group attacks a variety of native and introduced C4 plants. The toxicity of the fungus has not been tested but assume the sclerotia are toxic.

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Neotyphodium and Epichloe

  • systemic infection of leaves, stems and seeds
  • perennial
  • no known sexual stage in Neotyphodium
  • plants colonised by either fungus flower successfully
  • vertical transmission in seed.

LINK In all cases, the fungus contains toxic alkaloids. Thus consumption of the plant poisons the herbivore. Toxicity is most common where grass is short and growing rapidly (eg after rain in spring). The signs of consumption include staggering and unsteadiness in mammals (rye grass staggers and fescue toxicosis). Once the toxin has passed through the system, the individual recovers, unless further toxin is ingested. Repeated consumption of the active compounds can have quite severe and long-terms consequences for the herbivore LINK.

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Conclusion

A group of closely related members of the Clavicipitales have evolved a variety of associations with grasses in which the plant is used as a source of organic energy and the site of dispersal of the fungi. The fungi also have in common, the formation of secondary metabolites with neurotoxicological effects. Some of the fungi have considerable historical importance, while others have an enormous, though still unrealised, economic importance to the grazing industry in Australia.

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References

Bacon CW & White JF (2000) Microbial Endophytes, Marcel Dekker.

Bischoff JF & White JF (2005) In The Fungal Community (3rd edit) eds Dighton J, White JF and Oudemans P. Taylor & Francis.

Redlin SC & Carris LM (1995) Endophytic Fungi in Grasses and Woody Plants. APS Press. Chs 7 and 8.

Ruders JA & Clay K (2005) In The Fungal Community (3rd edit) eds Dighton J, White JF and Oudemans P. Taylor & Francis.

Schardl CL & Leuchtmann A (2005) In The Fungal Community (3rd edit) eds Dighton J, White JF and Oudemans P. Taylor & Francis.

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