13-HODE, a phytotoxic metabolite produced by Epicoccum nigrum, a fungal pathogen from the rhizosphere of Bromus tectorum

Bromus tectorum (cheatgrass) is one of the most diffused invasive plants in North America. The fungus Epicoccum nigrum has been isolated from the rhizosphere of cheatgrass. Pathogenicity tests revealed the capacity of this strain to kill germinating seeds of the host plant under conditions of water stress. The organic extract obtained from liquid culture of the pathogen showed high toxicity on seeds of Portulaca oleracea. The main secondary metabolite (13S,9Z,11E)-13-hydroxyoctadeca-9,11-dienoic acid (i.e. 13-HODE, 1) produced by this strain has been isolated and identified, together with its methyl ester. In hypocotyl/radicle elongation bioassays on P. oleracea, 13-HODE showed a moderate phytotoxicity at 6.6 mM, reducing hypocotyl length by almost half and radicle length by a factor of ten, compared to the control. The bioactivity of this fungal compound has been compared with methyl and acetyl derivatives to determine the relevance of the main functional groups of 13-HODE in the observed toxicity.