Natural phytotoxins

Dayan, F.E., Romagni, J.G. and Duke, S.O. (2000). Investigating the mode of action of natural phytotoxins. Journal of Chemical Ecology 26 2079-2094. 

In relation with resistance of weeds to herbicides, Duke et al. (2000) mentioned that 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. These authors gave some examples of natural products that inhibit unexploited steps in the amino acid, nucleic acid, and other biosynthetic pathways AAL-toxin, hydantocidin, and various plant-derived terpenoids. 

Natural Phytotoxins with Potential for Development in Weed Management Strategies... 

The need exists to expand the repertoire of tools available for weed management systems. What little we know of the molecular target sites of natural phytotoxins indicates that they are inhibitors of a broad array of enzymes and other molecular targets that have not been the focus of herbicide discovery efforts [1]. At present, commercially available herbicides target a relatively limited number of enzymes and metabolic pathways. The discovery of new target sites is a growing emphasis of pesticide companies, especially since the U.S. Food Quality Protection Act has combined food tolerance levels of pesticides with the same molecular target sites. 

Modem molecular biology is transforming the pesticide industry. We will discuss how it can be used in both the search for new molecular target sites and elucidation of modes of action of natural phytotoxins, as well as in modification of crops to fight weeds with allelochemicals. 

Table I. Modes of Action of Commercial Herbicides and Natural Phytotoxins (Adapted in part from references 116 and 120).

Catalytic cycloadditions of carbenes to alkenes is a straightforward method for synthesizing cyclopropanes [40]. In fact, cyclopropanes are present in a variety of natural products [41-43]. For example, they occur in some unusual amino acids, in natural phytotoxins such as coronatine, as well as in marine terpenes [44], sesquiterpenes [45], cyclosteroids [46-47] (as part of the A-cycle) or in the side chain of steroids [48]. 

Allelopathy may be invoked with confidence when soil concentrations of an allelochemical are at or above levels applied to soil that cause phytotoxicity. When concentrations are below such levels, allelopathy may still be taking place due to the continuous input of the compounds by the producing plant. This would be very difficult to imitate by exogenous application of the compounds. Furthermore, some have invoked synergism between natural phytotoxins to support the view that weakly phytotoxic phytochemicals are allelochemicals (e.g. [13]), but others have disputed such claims (e.g. [14,15]). This topic is discussed in more detail in the Section on Common Phenolic Acids. 

Nevertheless, the molecular target sites of natural phytotoxins, including allelochemicals, are being discovered at an increasing rate (e.g. [17, 18]), so the approach of linking an allelochemical to a particular physiological, transcriptional, or translational response is a reasonable one however, we are unaware of... 

FIg.l Chemical structures of the natural phytotoxins mentioned in the text from Sections Artemisinin to Rice Allelochemicals... 

It is not known if m-tyrosine can be converted to l-DOPA by a cell-free extract of a species susceptible to m-tyrosine. If so, would the process be highly efficient in vivol Synthetic proherbicides, such as diclofop-methyl [11], that are inactive at the molecular target site are much more effective when applied to intact plants than the active molecule to which they are converted in vivo. This is due to superior cuticular and cellular uptake of the proherbicide. Some potent natural phytotoxins from microbial origin, such as hydantocidin and 2,5-anhydro-D-glucitol, are protoxins [170-171]. 

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