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Phytoactive substances were present in a culture broth of Drechslera gigantea, a pathogenic fungus of several grasses. Twelve eremophilane sesquiterpenes (1, 2, 3, 4, 6, 7, 10, 11, 12, 13, 14, and 15) were isolated and structurally characterized by a combination of single-crystal X-ray diffraction and spectroscopic analyses. Most of these sesquiterpenoids were phytotoxic; however, compounds 1 and 3 caused chlorophyll retention, an activity previously associated with phytohormones.

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A rationale for the study of phytotoxins from fungal pathogens of plants is presented. Structural chemistries and biological data are given for numerous, recently discovered phytotoxins in such diverse chemical classes as polyketides, terpenoids, diketopiperazines, and isocoumarins. The biological activities of these compounds range from broadly toxic (curvulin) to host specific (maculosin-1). Phytotoxicology offers a viable supplement to organic synthesis as a means of developing and implementing new, biorational, and economical herbicides.

Summary. Gigantenone, a new eremophilane diepoxide, was isolated from the fungal plant pathogen Drechslera gigantea, it displays unique biological activity on higher plants. On most graminaceous species, the application of 18 nanomoles to a leaf surface results in the formation of 'green islands' localized areas of chlorophyll retention. Gigantenone is structurally unrelated to the cytokinins, yet induces the green island effect associated with these phytohormones at comparable concentrations. However, on dicotyledonous species it generally causes necrotic lesions. On host plants of D. gigantea, gigantenone induced lesions closely resembling those appearing in natural infections. Gigantenone also induces root formation in mung bean hypocotyls and shows a high level of activity in several plant tissue culture systems.Key words. Phytohormone; fungal plant pathogen; chlorophyll retention; green island effect; senescence.The fungus Drechslera gigantea causes zonate eyespot disease on numerous grasses3. This leaf disease is a serious problem in many commonly cultivated turfgrasses and also occurs in weeds such as crabgrass (Digitaria spp.), quackgrass (Agropyron repens), and Bermuda grass (Cynodon dactylon). The lesions of zonate eyespot first appear as brown flecks. These flecks eventually enlarge and coalesce, although they are sometimes surrounded by reddish brown or dark green borders. The dark green borders are reminiscent of the 'green island' effect that commonly occurs in mildew and leaf rust infections 4. Green islands have been attributed to the action of the cytokinin family o1~ plant hormones. Since many phytohormones are produced by plant-pathogenic fungi, we investigated the possible link between D.gigantea metabolites, disease symptoms, and phytohormone activity.Isolates of D. gigantea, kindly supplied by E. S. Luttrell of the University of Georgia, were maintained on potato dextrose agar containing 15 % (v/v) V-8 juice. The fungus was grown in a defined liquid medium 5 at room temperature for 4 weeks. The culture was filtered through 4 6 layers of cheesecloth and the filtrate extracted with ethyl acetate (3 times with 1/3 the volume of filtrate). The extract was dried under vacuum at 40 ~ The residue was further purified by flash chromatography on silica gel with chloroform/methanol, 9/I, followed by chromatography on Sephadex LH-20 with methanol. The final chromatography was HPLC on a RP-18 (4x 250 mm) column using acetonitrile/water, 1/1, with a flow rate of 1 ml/min. The column eluant was monitored at 254 nm to detect fractions, and biological activity was monitored by a needle-puncture-droplet overlay technique on leaves of various plant species 5. Specifically, 5 gl of 5% ethanolic H20 containing 10 30 nmoles of compound were placed over a fresh needle puncture on the adaxial surface of test leaves. This was the optimum range for observing biological effects. Control leaves were treated with 4 gl of 5 % ethanolic H20 and never exhibited any adverse effects. Leaves were placed in petri dishes over ...