Genetically modified herbicide tolerance to bromoxynil

Bromoxynil is a contact herbicide that kills plants by inhibiting photosynthesis at photosystem II. It is used for selective weed control in a number of crops including wheat, barley and oats, but is not suitable for use in oilseed rape. GM-HT oilseed rape has been produced using the oxy gene from Klebsiella ozaenae that encodes a nitrilase enzyme (Pallett et al., 1997). The K. ozaenae strain containing this gene, isolated from bromox-ynil-contaminated soil, can utilise bromoxynil as its sole carbon source, indicating that the strain contains enzymes that can catabolise bromoxynil. It has subsequently been shown that the nitrilase is responsible for the breakdown of bromoxynil to the non-phytotoxic 3,5-dibromo-4-hydroxybromobenzoic acid (Figure 13.7). To date this method of producing herbicide tolerance has only had limited commercialisation including oilseed rape in Canada (under the trade name BXN). Due to the poor performance of bromoxynil

HSCoA

Acetyl-CoA

W-Acetyl phosphinothricin

Bromxynil

3,5-Dibromo-4-hydroxy benzoic acid

CO2H

Bromxynil

3,5-Dibromo-4-hydroxy benzoic acid

Figure 13.7 The detoxification of bromoxynil by bromoxynil nitrilase.

against grass weeds and certain broad-leaved weeds, it is unlikely that this method will ever prove as popular as those used for creating tolerance to glyphosate or glufosinate.

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