Is the pathogenic ergot fungus a conditional defensive mutualist for its host grass?

PLoS One. 2013 Jul 10;8(7):e69249. doi: 10.1371/journal.pone.0069249. Print 2013.

Abstract

It is well recognized, that outcomes of mutualistic plant-microorganism interactions are often context dependent and can range from mutualistic to antagonistic depending on conditions. Instead, seemingly pathogenic associations are generally considered only harmful to plants. The ergot fungus (Claviceps purpurea) is a common seed pathogen of grasses and cereals. Ergot sclerotia contain alkaloids which can cause severe toxicity in mammals when ingested, and thus the fungal infection might provide protection for the host plant against mammalian herbivores. Theoretically, the net effect of ergot infection would positively affect host seed set if the cost is not too high and the defensive effect is strong enough. According to our empirical data, this situation is plausible. First, we found no statistically significant seed loss in wild red fescue (Festuca rubra) inflorescences due to ergot infection, but the seed succession decreased along increasing number of sclerotia. Second, in a food choice experiment, sheep showed avoidance against forage containing ergot. Third, the frequency of ergot-infected inflorescences was higher in sheep pastures than surrounding ungrazed areas, indicating a protective effect against mammalian grazing. We conclude that, although ergot can primarily be categorized as a plant pathogen, ergot infection may sometimes represent indirect beneficial effects for the host plant. Ergot may thus serve as a conditional defensive mutualist for its host grass, and the pathogenic interaction may range from antagonistic to mutualistic depending on the situation.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animal Feed / microbiology
  • Animals
  • Claviceps / pathogenicity
  • Claviceps / physiology*
  • Festuca / microbiology
  • Festuca / physiology
  • Food Contamination
  • Genetic Fitness / physiology
  • Herbivory / physiology
  • Host-Pathogen Interactions / physiology*
  • Plant Diseases / microbiology
  • Poaceae / microbiology*
  • Sheep / physiology
  • Symbiosis / physiology*

Grant support

This study was supported by grants from the Marjatta and Eino Kolli Foundation, Olvi Foundation, Oskar Öflund’s Foundation and Thule Institute (PPW), and the Academy of Finland (project nro 127140) and Ella and Georg Ehrnrooth Foundation (PRW). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.