Joanne G. Romagni

Investigating the Mode of Action of Natural Phytotoxins

The potential use of natural phytotoxins (including allelochemicals) to develop novel tools for weed management is enhanced by the elucidation of their modes of action. This approach has not been emphasized by the agrochemical industry, although the possibility of discovering new target sites may be promising, since natural products tend to have modes of action different from synthetic herbicides. The approach of testing a compound on all known herbicide molecular target sites for commercial herbicides and other potent phytotoxins is feasible. However, this would preclude the discovery of new mechanisms of action. Discovering new target sites requires more challenging holistic approaches, initiated with physiological and biochemical studies that use whole plant assays. Studying basic plant responses to a compound may yield important clues to the specific physiological processes affected by the compounds and uncover novel mechanisms of action.

Allelopathic effects of volatile cineoles on two weedy plant species.

The volatile monoterpene analogs, 1,4-cineole and 1,8-cineole, have been identified as components of many plant essential oils, but relatively little is known about their biological activities. We compared the effects of 1,4- and 1,8-cineole on two weedy plant species by monitoring germination, mitosis, root and shoot growth, chlorophyll content, and photosynthetic efficiency. 1,4-Cineole severely inhibited growth of roots and shoots, causing cork-screw shaped morphological distortion, whereas 1,8-cineole caused a decrease in root growth and germination rates. Chlorophyll fluorescence data (yield and Fv / Fm) indicated that 1,4-cineole caused significantly higher stress (P ≤ 0.001) to photosynthesis when compared to controls. Mitotic index data showed that 1,8-cineole severely decreased (P ≤ 0.001) all stages of mitosis when compared with controls, while 1,4-cineole only caused a decrease in the prophase stage (P ≤ 0.05). Although superficially similar in structure, these two cineoles appear to have different modes of action.

Invited Paper: Chemicals from nature for weed management

Abstract Natural products represent a vast repository of materials and compounds with evolved biological activity, including phytotoxicity. Some of these compounds can be used directly or as templates for herbicides. The molecular target sites of these compounds are often unique. Strategies for the discovery of these materials and compounds are outlined. Numerous examples of individual phytotoxins and crude preparations with weed management potential are provided. An example of research to find a natural product solution of a unique pest management problem (blue-green algae in aquaculture) is described. Finally, the problems associated with natural products for pest control are discussed.

Natural products as sources for new mechanisms of herbicidal action.

Abstract New mechanisms of action for herbicides are highly desirable to fight evolution of resistance in weeds, to create or exploit unique market niches, and to cope with new regulatory legislation. Comparison of the known molecular target sites of synthetic herbicides and natural phytotoxins reveals that there is little redundancy. Comparatively little effort has been expended on determination of the sites of action of phytotoxins from natural sources, suggesting that intensive study of these molecules will reveal many more novel mechanisms of action. Examples of natural products that inhibit unexploited steps in the amino acid, nucleic acid, and other biosynthetic pathways are given. AAL-toxin, hydantocidin, and various plant-derived terpenoids are discussed. Strategies and potential pitfalls of using natural products as leads for new herbicide classes are summarized.

The inhibitory activity of natural products on plant p-hydroxyphenylpyruvate dioxygenase.

The inhibitory activity of 34 natural products of various structural classes on hydroxyphenylpyruvate dioxygenase (HPPD), the target site for triketone herbicides, and the mode of interaction of selected natural products were investigated. Recombinant HPPD from arabidopsis is sensitive to several classes of natural compounds including, in decreasing order of sensitivity, triketones, benzoquinones, naphthoquinones and anthraquinones. The triketone natural products acted as competitive tight-binding inhibitors, whereas the benzoquinones and naphthoquinones did not appear to bind tightly to HPPD. While these natural products may not have optimal structural features required for in vivo herbicidal activity, the differences in their kinetic behavior suggest that novel classes of HPPD inhibitors may be developed based on their structural backbones.

Lichens as a potential source of pesticides

Franck E. Dayan and Joanne G. Romagni of the USDA-ARS Natural Products Utilization Research Unit (National Center for Natural Products Research, Mississippi, USA) and St. Thomas University (Department of Biology, Texas, USA), respectively, review the future prospects for using lichen metabolites as a source of natural products in pest management.

Structural Diversity of Lichen Metabolites and Their Potential Use

Lichens are symbiotic organisms composed of a fungal partner, the mycobiont, usually in association with one or more photosynthetic partners, the photobiont(s). The photobionts can be green alga, cyanobacteria or both. There are approximately 17,000 species of mycobionts whereas there are only ca. 40 species ofphotobionts. Therefore, the taxonomic name of the lichen is traditionally determined by the species of the fungal partner. The mycobiont, being the host partner for the photobiont, generally determines the morphology of the lichen species. Most lichen mycobionts are not known to exist naturally without the photobiont, whereas many of the photobionts, like the cyanobacteria Nostoc, Scytonema, and the green algae, Trentepohlia, are known to flourish alone in nature. However, because the symbiosis is so complex, lichens are normally referred to as individual organisms rather than separate organisms.

CHAPTER 68 – Protoporphyrinogen Oxidase Inhibitors

This chapter explores the protoporphyrinogen oxidase (protox), commercially available protox inhibitors, and agricultural uses. Nitrofen was the first protox-inhibiting herbicide to be introduced for commercial use. This diphenyl ether (DPE) herbicide was eventually recognized as a relatively weak inhibitor of protox, but was a lead compound of an entire class of structurally related herbicides that were much more active. Protox inhibitors have historically been used for broad-spectrum weed control in soybean fields; protox-inhibiting herbicides control both monocotyledonous and dicotyledonous weeds. Protox-inhibiting herbicides may play a more important role in the future agrochemical market for several reasons. These compounds are effective at very low application rates and have generally good ecotoxicology and human toxicology profiles at recommended application rates. Many are highly compatible with the trend toward no-tillage agriculture. Furthermore, unlike with some of the other herbicide classes, weeds have not been able to evolve resistance at this particular site of action. The success of protox inhibitors is dependent on broadening their use to include other major crops such as maize and in enhancing the resistance of crops for which these herbicides are currently being marketed.