Pyrenophoric Acid, a Phytotoxic Sesquiterpenoid Penta-2,4-dienoic Acid Produced by a Potential Mycoherbicide,Pyrenophora semeniperda

A new phytotoxic sesquiterpenoid penta-2,4-dienoic acid, named pyrenophoric acid, was isolated from solid wheat seed culture of Pyrenophora semeniperda, a fungal pathogen proposed as a mycoherbicide for biocontrol of cheatgrass (Bromus tectorum) and other annual bromes. These bromes are serious weeds in winter cereals and also on temperate semiarid rangelands. Pyrenophoric acid was characterized as (2Z,4E)-5-[(7S,9S,10R,12R)-3,4-dihydroxy-2,2,6-trimethylcyclohexyl)]-3-methylpenta-2,4-dienoic acid by spectroscopic and chemical methods. The relative stereochemistry of pyrenophoric acid was assigned using 1H,1H couplings and NOESY experiments, while its absolute configuration was determined by applying the advanced Mosher’s method. Pyrenophoric acid is structurally quite closely related to the plant growth regulator abscisic acid. When bioassayed in a cheatgrass coleoptile elongation test at 10−3 M, pyrenophoric acid showed strong phytotoxicity, reducing coleoptile elongation by 51% relative to the control. In a mixture at 10−4 M, its negative effect on coleoptile elongation was additive with that of cytochalasin B, another phytotoxic compound found in the wheat seed culture extract of this fungus, demonstrating that the extract toxicity observed in earlier studies was due to the combined action of multiple phytotoxic compounds.