Crop selectivity mechanisms sulfonylurea herbicides

The remainder of this chapter will describe the mode of action, crop selectivity mechanisms and environmental properties of the sulfonylurea herbicides. Other issues including resistant weeds and recropping intervals will also be discussed. 

A number of possible crop selectivity mechanisms have been investigated (see Brown, H. M. Pestic. Sci. 1990 (in press)). Differential uptake and/or translocation of the selective herbicide between the tolerant crop and sensitive weeds has been ruled out as the basis for crop selectivities in several specific cases. For example, Sweetser et al. ( found no ccnrelation between chlorsulfuron uptake or translocation and sensitivity to this herbicide in a study of 7 plant species, and similar conclusions were drawn in studies of thifensulfuron methyl tolerance in soybeans (28). Lichtner (29) has shown that sulfonylurea herbicide uptake and translocation in plants is not carrier-mediated, but instead depends on the physical properties of the herbicide (pKa, log P) and proceeds through an acid-trapping mechanism common to higher plants. Given this information, we conclude that differential uptake and/or translocation is unlikely to account for any of the sulfonylurea crop selectivities discovered to date. 

Differential herbicide sensitivity at the site of action is a second possible selectivity mechanism, with good precedence in the aryloxyphenoxy and cyclo-hexanedione grass herbicides (2Q, 51). However, several studies now clearly indicate that the inherent sulfonylurea crop selectivities listed in Table 1 are not based on differential sensitivity at the site of action. ALS preparations isolated fix>m crops and weeds are equally sensitive to several selective sulfonylurea herbicides (28.32.33). Although data have not appeared for all of the compounds shown in Table 1, we conclude that differential active site sensitivity is not a general sulfonylurea selectivity mechanism. The clear exceptions to this generalization are the cases of genetically-altered plants (crops and we s) which have acquired through mutation or deliberate transformation a resistant form of the ALS enzyme (see below). 

As mentioned above, there is a key exception to the rule that sulfonylurea herbicide selectivity is based on metabolic inactivation. This exception is the case of plants which, through mutation, selection, or genetic engineering methods, have acquired a gene coding for a herbicide-resistant form of the ALS enzyme. Crop species which have been selected or engineered for broad resistance to sulfonylureas by this mechanism include soybeans (28), tobacco (17, Ig), and canola (22), with similar efforts proceeding in other crops (40 ... 

Tolerance toward the sulfonylureas is known to occur naturally due either to the presence of a form of ALS that is insensitive to the inhibitors (7) or to a mechanism for detoxification of the inhibitors (8). Another mechanism that could in principle lead to tolerance is the overproduction of the target (ALS) enzyme. We are interested in engineering herbicide tolerance in crop plants in order to increase the margin of safety for the application of existing selective chemicals, to achieve selectivity in crops where selective chemicals do not currently exist, and to reduce damage in rotated crops which is due to the presence of herbicide residues. 

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