Synthetic herbicides

The ban of synthetic herbicides, fungicides, insecticides, and other pesticides is... 

Organic farming systems rely more on mechanical weed control and in certain crops on considerably more intensive soil tillage as the use of synthetic herbicides is prohibited. This can have negative effects on other key species of soil meso-fauna, i.e. a reduction of population of Collembola with organic cultivation (Krogh 1994). 

Natural triketones and other natural products also inhibit The natural products tend to be competitive inhibitors of HPPD, whereas the synthetic herbicides have been optimized and bind irreversibly to the enzyme. 

Natural products represent an immense reservoir of new molecules for the development of pesticide agents. In particular, natural products based herbicides have become attractive in recent years for a few reasons. One of the reasons is that they are environmentally benign. Furthermore, the novelty of their structures offers the possibility of finding compounds with new mode of actions, thus diminishing the induction of plant resistance, a problem associated with commonly used synthetic herbicides. 

Many alkaloids have been implicated in plant-animal chemical interactions but few have been associated with allelopathy (83). Several alkaloids were demonstrated by Evenari (73) to be strong inhibitors of seed germination. Little recent work has been done on alkaloids except for caffeine (78). a-Picolinic acid is a microbial alkaloid with toxic action on plants (82). One of the more active synthetic herbicides on the market, piclorariv(Dow s Tordon), is a chlorinated picolinic acid derivative. 

The limited work on mode of action of allelochemicals suggests that they affect a variety of sites and biochemical processes, many of which are similar to those affected by synthetic herbicides. Many novel sites and mechanisms probably remain to be discovered. 

Herbicide-resistant and pesticide-resistant crops should avoid soil erosion and limit the spread of synthetic herbicides and pesticides. Modified crops may also provide heat-stable monounsaturated oleic acid, avoiding the problem of heat-unstable poljainsaturated fetty acids that give unhealthy trans-fatty acids as side products of industrial hydrogenation (Mazur 1999). On the longer term, biotechnology may also provide renewable fuel and raw chemicals that may replace petroleum. 

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. 

Adapted species may have developed, however, strategies which enable them to survive allelopathic attacks. One of those strategies certainly includes detoxification of absorbed allelochemicals by constitutive or inducible pathways. Metabolization and detoxification are known reactions in a number of crops upon application of diverse synthetic herbicides.38 Enhanced herbicide detoxification is an important factor in the development of nontarget-site cross-resistance and multiple resistance. It is reasonable to expect comparable strategies in plants that are relatively resistant to allelochemicals such as DIBOA, DIMBOA, and their derivatives. Especially in ecosystems where co-existing species have to be adapted to each other, detoxification of absorbed allelochemicals may play a crucial role under defined circumstances. 

Discovery and Screening of Synthetic Herbicides. Microbial Herbicides, and Microbial Phvtotoxins... 

Synthetic Herbicides. Most commercial phytotoxins (herbicides) have been discovered through random screening programs, followed by derivative synthesis and activity studies when a potential chemical candidate Is discovered. Seldom has a bloratlonal approach for herbicide discovery been achieved or used. Over the past 4S-50 years, perhaps close to a million compounds have been screened for herbicldal activity. The number of compounds screened has continually Increased and estimates for the 1980 s suggest that 12 to 15 thousand compounds per year were synthesized and screened for each compound reaching the market (28). In the 1990 s, the ratio may Increase 5-fo1d. 

Iroexll and Benzadox. A wood-degrading basldlomycete, Iroex Dachvodon. produces Irpexll [47], which had antibacterial activity (1891. Later, the compound was found to have phytotoxic properties and to be structurally related to several synthetic herbicides. Including benzadox (benzamido-oxyacetic acid) [48]... 

Benzadox (and presumably, Irpexll) acts via metabolic conversion within plants to form amlno-oxyacetic acid, which Is a potent pyridoxyl phosphate-requiring enzyme Inhibitor (1901 and also a non-commercial synthetic herbicide (191. 1921. 

Phytotoxins from nonpathogenic organisms also have potential as weed-control agents. Numerous phytotoxic compounds have been isolated and bioassayed for phytotoxic activity, and their chemical structures have been determined. Generally, these naturally occurring compounds have unique chemistries that differ from those of commercial synthetic herbicides. Some of these compounds—or others yet to be discovered—may be used directly as herbicides or provide templates for the development of new, efficacious weed control agents that are less persistent and more environmentally compatible than some compounds currently being used. 

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