Bioactivity of the fungal metabolite (8R,16R)-(-)-pyrenophorin on graminaceous plants.

A secondary metabolite was isolated from cultures of a Drechslera avenae pathotype with host specificity to Avena sterilis and identified as the macrodiolide (8R,16R)-(-)-pyrenophorin (8,16-dimethyl-1,9-dioxa-cyclohexadeca-3,11-diene-2,5,10,13-tetraone). A considerable yield of the substance was obtained after 8-12 days of incubation at temperatures of 15-20 degrees C. The compound at a concentration of 60 microM inhibited seed germination of wild oats (Avena sterilis, A. fatua), oat (A. sativa), wheat (Triticum aestivum), and barley (Hordeum vulgare). Root growth of pregerminated seeds of the graminaceous plants was stimulated, remained unaffected, or was inhibited by pyrenophorin at 10-30, 31-50, and >51 microM, respectively. The metabolite caused abnormal chlorophyll retention in leaf sections of all five graminaceous plants, but seedling cuttings partially immersed in 1000 microM solutions remained unaffected. The rate of chlorophyll dissipation was decreased by half in leaf sections treated with pyrenophorin at 320 microM compared with the control. These findings are discussed and compared with data on the production and bioactivity of the macrodiolide (5S,8R,13S,16R)-(-)-pyrenophorol, which has a similar stereochemical configuration.