Engineering dicamba selectivity in crops: a search for appropriate degradative enzyme(s)

The biotechnological approaches to conferring crop selectivity to herbicides have been demonstrated for a number of compounds such as glyphosate, glufosinate, imidazolinones and cyclohexanediones. Imidazolinone-resistant and cyclohexanedione-resistant maize lines are already in the market. There are several other effective and environmentally benign herbicides such as dicamba, for high engineering crop selectivity is desirable, to broaden the produce utility in different crops and provide new solutions for weed control. One of the most effective approaches to conferring dicamba selectivity in crops is to incorporate a gene for its rapid metabolism. It is advantageous to have different dicamba-metabolizing enzymes in order to maximize the chances of at least one functioning optimally in a plant environment. Three different metabolizing enzymes are currently available to engineer crop selectivity. The first one is the folate-dependent O-demethylase from Clostridium thermoaceticum, that converts dicamba to herbicidally inactive 3,6-dichlorosalicylate. The second enzyme is the NADH-dependent, multi-component monooxygenase from Pseudomonas maltophilia DI-6 that also converts dicamba to 3,6-dichlorosalicylate. The third enzyme is from corn endosperm cultures that catalyzes the 5-hydroxylation of dicamba. The merits of these three enzymes are discussed with respect to conferring crop selectivity to dicamba. In addition, a rapid microbial screen was conceived for discovery of new dicamba-degrading bacteria, which resulted in identification of Pseudomonas orvilla. This bacteria degraded dicamba by the same pathway, perhaps using a similar enzyme system as Pseudomonas maltophilia DI-6. However, the microbial screen has the potential to identify novel bacteria that degrade dicamba by a different pathway, providing more options for metabolizing enzymes to confer herbicide selectivity in crops.