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

A new phytotoxic sesquiterpenoid penta-2,4-dienoic acid, named pyrenophoric acid, was isolated from solid wheat seed culture of <i>Pyrenophora semeniperda</i>, a fungal pathogen proposed as a mycoherbicide for biocontrol of cheatgrass (<i>Bromus tectorum</i>) and other annual bromes. These bromes are serious weeds in winter cereals and also on temperate semiarid rangelands. Pyrenophoric acid was characterized as (2<i>Z</i>,4<i>E</i>)-5-[(7<i>S</i>,9<i>S</i>,10<i>R</i>,12<i>R</i>)-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

Pyrenophoric Acids B and C, Two New Phytotoxic Sesquiterpenoids Produced by <i>Pyrenophora semeniperda</i>

Two new phytotoxic sesquiterpenoid acids, named pyrenophoric acids B and C, were isolated together with the related pyrenophoric and abscisic acids from solid <i>Bromus tectorum</i> (cheatgrass) seed culture of the seed pathogen <i>Pyrenophora semeniperda</i>. This fungus has been proposed as a mycoherbicide for biocontrol of cheatgrass (<i>Bromus tectorum</i>), a Eurasian annual grass that has become invasive in rangelands and is also a serious agricultural weed in the western U.S. Pyrenophoric acids B and C were characterized by spectroscopic methods (NMR and HR ESIMS) as (2<i>Z</i>,4<i>E</i>)-5-[(1<i>R</i>*,4<i>R</i>*,6<i>R</i>*)-1,4-dihydroxy-2,2,6-trimethylcyclohexyl]-3-methylpenta-2,4-dienoic and (2<i>Z</i>,4<i>E</i>)-5-[(1<i>S</i>*,3<i>S</i>*,4<i>R</i>*,6<i>S</i>*)-3,4-dihydroxy-2,2,6-trimethylcyclohexyl]-3-methylpenta-2,4-dienoic acids, respectively. Cytochalasins A, B, F, and Z3, as well as deoxaphomin and pyrenophoric acid, all previously isolated from <i>P. semeniperda</i> grown on wheat seed, were also isolated from cheatgrass seed culture. In a cheatgrass seedling bioassay at 10^–3 M, pyrenophoric acid B showed higher coleoptile toxicity than pyrenophoric acid, while pyrenophoric acid C showed lower phytotoxicity. Abscisic acid was by far the most active compound

Chloromonilinic Acids C and D, Phytotoxic Tetrasubstituted 3‑Chromanonacrylic Acids Isolated from <i>Cochliobolus australiensis</i> with Potential Herbicidal Activity against Buffelgrass (<i>Cenchrus ciliaris</i>)

The fungal pathogen <i>Cochliobolus australiensis</i> isolated from infected leaves of the invasive weed buffelgrass (<i>Pennisetum ciliare</i>) was grown in vitro to evaluate its ability to produce phytotoxic metabolites that could potentially be used as natural herbicides against this weed. Two new tetrasubstituted 3-chromanonacrylic acids, named chloromonilinic acids C (<b>1</b>) and D (<b>2</b>), were isolated from the liquid cultures of <i>C. australiensis</i>, together with the known chloromonilinic acid B. Chloromonilinic acids C and D were characterized by spectroscopic and chemical methods as (<i>E</i>)-3-chloro-3-[(5-hydroxy-3-(1-hydroxy-2-methoxy-2-oxoethyl)-7-methyl-4-oxo-4<i>H</i>-chromen-2-yl)]­acrylic acid and (<i>Z</i>)-3-chloro-3-[(5-hydroxy-3-(2-methoxy-2-oxoethyl)-7-methyl-4-oxo-4<i>H</i>-chromen-2-yl)]­acrylic acid, respectively. The stereochemistry of chloromonilinic acids C and D was determined using a combination of spectroscopic and computational methods, including electronic circular dichroism. The fungus produced these compounds in two different liquid media together with cochliotoxin, radicinin, radicinol, and their 3-epimers. The radicinin-related compounds were also produced when the fungus was grown in wheat seed solid culture, but chloromonilinic acids were not found in the solid culture organic extract. All three chloromonilinic acids were toxic to buffelgrass in a seedling elongation bioassay, with significantly delayed germination and dramatically reduced radicle growth, especially at a concentration of 5 × 10^–3 M

Phytotoxic activity against <i>Bromus tectorum</i> for secondary metabolites of a seed-pathogenic <i>Fusarium</i> strain belonging to the <i>F. tricinctum</i> species complex

<p>The winter annual grass <i>Bromus tectorum</i> (cheatgrass) has become highly invasive in semiarid ecosystems of western North America. In these areas, a natural phenomenon, complete cheatgrass stand failure (‘die-off’), is apparently caused by a complex interaction among soilborne fungal pathogens. Several <i>Fusarium</i> strains belonging to the <i>Fusarium tricinctum</i> species complex were isolated from these soils and found to be pathogenic on <i>B. tectorum</i> seeds. One of these strains was produced in cheatgrass seed culture to evaluate its ability to produce phytotoxins. Six metabolites were isolated and identified by spectroscopic methods (essentially 1D and 2D NMR and ESIMS) as acuminatopyrone (<b>1</b>), blumenol A (<b>2</b>), chlamydosporol (<b>3</b>), isochlamydosporol (<b>4</b>), ergosterol (<b>5</b>) and 4-hydroxybenzaldehyde (<b>6</b>). Upon testing against <i>B.</i> <i>tectorum</i> in a seedling bioassay, (<b>6</b>) the coleoptile and radicle length of cheatgrass seedlings were significantly reduced. Compounds <b>1</b> and <b>2</b> showed moderate activity, while <b>3</b>–<b>5</b> were not significantly different from the control.</p