Doug S. Kenfield

Phytotoxins as potential herbicides

Phytotoxins are produced in various culture media by many fungi that are pathogenic to weeds. These phytotoxins belong to a wide array of chemical substances including sesquiterpenoids, sesterterpenoids, diketopiperazines, peptides, spirocyclic lactams, isocoumarins, and polyketides. In most cases, the phytotoxin belongs to a family of related compounds produced by the fungus. These related compounds may or may not be phytotoxins. Phytotoxin production, in some cases, is optimized by the addition of a host extract to the culture medium. Biological activity is usually observed in a range of concentrations from 10−3 to 10−6 M. The concept of using these molecules, derivatives thereof, or related compounds as herbicides should be explored.

Phytotoxins from Alternaria cassiae

Abstract The isolation, structure determination, and phytotoxicity of stemphyperylenol, stemphyltoxin II, alterperylenol, and altertoxin I from Alternaria cassiae are reported.

Phytoactive Eremophilanes Produced by the Weed Pathogen Drechslera gigantea

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.

Gigantenone, a novel sesquiterpene phytohormone mimic

Gigantenone, a new eremophilane diepoxide, was isolated from the fungal plant pathogenDrechslera 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 ofD. 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.

The incredible fungal genus —Drechslera — and its phytotoxic ophiobolins

ManyDrechslera species that haveCochliobolus sp. as a perfect stage produce a related group of ophiobolins, sesterterpenoids (C-25’s), which are phytotoxic to a wide range of plants. One of the most toxic ophiobolins is 6-epiophiobolin A and it is produced by all of theDrechslera species thus far studied. On the other hand, some novel ophiobolins have been found inD. oryzae: ophiobolin J, 6-epiophiobolin I, and 8-deoxyophiobolin J. Potentially, genetic crosses made between these fungi could yield novel organisms, producing novel combinations of the ophiobolins, and thus new pathotypes. Ophiobolins are being used in tissue culture systems to screen plants for sensitivity to these toxins.

Curvulin and O-Methylcurvulinic acid: Phytotoxic metabolites of Drechslera indica which cause necroses on purslane and spiny amaranth

Abstract Curvulin was isolated from Drechslera indica and identified by X-ray crystallography. O -Methylcurvulinic acid was also isolated from this source and identified by NMR and HREIMS. Both compounds are phytotoxic to 2 of the normal hosts of this fungus when applied to detached leaves. Some plants are unaffected by these compounds. Host leaf constituents have some role in regulating the production of these phytotoxins by the fungus.

Triticone A: a novel bioactive lactam with potential as a molecular probe.

Triticone A is one member of a family of novel compounds which are spirocyclic lactams produced by several plant pathogenic fungi including Drechslera tritici repentis on wheat. It undergoes racemization to form triticone B and when tested, the enantiomeric mixture causes chlorosis and necrosis on a wide range of plants. Fluorescein diacetate treated protoplasts in conjunction with various triticone treatments allowed for accurate quantitation of the biological activity of the toxin. Various physiological functions of the wheat cell are impaired including the Hill and CO2 fixation reactions in photosynthesis. In addition, triticone A inhibits enzymes that have SH functional groups as part of their active site, eg., the protease-ficin. Neither triticone C or D had any activity in the enzyme or protoplast assays. It is apparent that triticone A has some potential as a molecular probe in a variety of biological systems.

Production of petasol by Drechslera gigantea in liquid culture

Drechslera gigantea , the causative agent of zonate eyespot disease on grasses, produces at least twelve bioactive eremophilanes. Their structures have been recently elucidated using conventional spectroscopy and X-ray crystallography. A study to examine the production of one of these eremophilanes, petasol, by D. gigantea grown in liquid culture was undertaken. A procedure using high performance liquid chromatography to quantify petasol levels in culture filtrates of D. gigantea was developed. This procedure was used to study petasol production by D. gigantea under different cultural conditions. Leaf material from quackgrass ( Agropyron repens ), a host of the fungus, stimulated petasol production. Amendments such as l -leucine or the sterol biosynthesis inhibitor chloro-choline chloride (CCC) also exhibited stimulatory activity.

Structure-activity relationships of the eremophilanes produced by Drechslera gigantea

Abstract The structure-activity relationships of the eremophilanes produced by Drechslera gigantea were studied on leaf puncture wounds of several different plant species, on stimulation of rhizogenesis in mung bean hypocotyls and on larvae of brine shrimp. The activity appeared to be dependent on degree of oxidation of the eremophilane ring system. Molecules containing epoxide functionalities were as active as compounds without an epoxide group. However, molecules with increased hydroxyl groups on the ring system generally had lower activity in all three assays. This suggests that the oxidation state of the eremophilane molecules may play a role in their activity.

Fungal Phytotoxins — Potential New Herbicides

The desire for selective, less persistent herbicides has generated interest in Phytotoxins as potential chemicals with applicability as agrochemicals (Cutler, 1986; Duke, 1986; Strobel et. al., 1987). Phytotoxins are natural compounds which have a deleterious effect on plants. Of particular interest in the present context are those toxins produced by microbes, usually pathogens, which make their living on weedy plants. These pathogens are first noticed because they cause dramatic symptoms such as necrosis, chlorosis, or wilt in their hosts. In laboratories such as ours, attempts are then made to isolate and identify toxic chemicals produced by the pathogens in hopes of establishing molecular leads for the herbicide industry. One spin-off of such studies is the elucidation of intriguing, often novel chemicals which perturb the normal physiological processes of plants in ways not found in the usual approach of organic synthesis followed by screening. Numerous Phytotoxins have been identified in recent decades and the reader is encouraged to consult the overviews cited above for more information.