Plant toxins resistance mechanisms

CYP6D1 of the housefly (Musca domestica) has been found to hydroxylate cyper-methrin and thereby provide a resistance mechanism to this compound and other pyrethroids in this species (Scott et al. 1998 see also Chapter 12). Also, this insect P450 can metabolize plant toxins such as the linear furanocoumarins xanthotoxin and bergapten (Ma et al. 1994). This metabolic capability has been found in the lepi-dopteran Papilio polyxenes (black swallowtail), a species that feeds almost exclusively on plants containing furanocoumarins. 

As explained in Chapter 1, the toxicity of natural xenobiotics has exerted a selection pressure upon living organisms since very early in evolutionary history. There is abundant evidence of compounds produced by plants and animals that are toxic to species other than their own and which are nsed as chemical warfare agents (Chapter 1). Also, as we have seen, wild animals can develop resistance mechanisms to the toxic componnds prodnced by plants. In Anstralia, for example, some marsupials have developed resistance to natnrally occnrring toxins produced by the plants upon which they feed (see Chapter 1, Section 1.2.2). 

Manduca sexta and Leptinotarsa decemllneata. For 2-trldecanone to be effective as a resistance mechanism, larvae on the resistant plant must be exposed to lethal quantities of the toxin. The role... 

Biodegradation of mycotoxins has become an area of great interest. Biological detoxification involves the enzymatic degradation or transformation of toxins to less toxic componnds and is often a detoxification or resistance mechanism nsed by microbes or plants for protection from adverse impacts of toxins. It has been shown that S. cerevisiae and lactic acid bacteria are potential candidates for mycotoxin decontamination (Halady Shetty Jespersen, 2006). 

The effect of plant root exudation and exudation patterns on root colonization and expression of toxin production must be considered. For example, it may be important to determine the effect of root exudates from cold-stressed plants on these organisms, since the exudates apparently first appear just after the plants break winter dormancy (34). These data should provide information on root colonization potential, possible stimulation or reduction of toxin production, and mechanisms of plant resistance to the organisms. 

Earlier studies indicated that wheat cultivars responded differently to these organisms (15). If necessary, it should be possible to develop resistant varieties, especially When we know the mechanisms of plant uptake and mode of action of the toxin within the plant. One biological control may be the development of a TOX-negative inoculum for seed treatment. These bacteria may act like other rhizobacteria, which are known to increase plant growth, apparently by displacing nonbeneficial bacteria in the plant rhlzosphere (14). 

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. 

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