Coexpression of Methyltransferase Gene <i>dmt50</i> and Methylene Tetrahydrofolate Reductase Gene Increases <i>Arabidopsis thaliana</i> Dicamba Resistance

Dicamba, a broad-spectrum and highly efficient herbicide, is an excellent target herbicide for the engineering of herbicide-resistant crops. In this study, a new tetrahydrofolate (THF)-dependent dicamba methyltransferase gene, <i>dmt50</i>, was cloned from the dicamba-degrading strain <i>Rhizorhabdus dicambivorans</i> Ndbn-20. Dmt50 catalyzed the methyl transfer from dicamba to THF, generating the herbicidally inactive product 3,6-dichlorosalicylic acid (3,6-DCSA) and 5-methyl-THF. A <i>dmt50</i> transgenic <i>Arabidopsis thaliana</i> clearly showed dicamba resistance (560 g/ha, the normal field application rate). However, Dmt50 demethylation activity was inhibited by the product 5-methyl-THF. Mthfr66, encoded by the 5,10-methylene-THF reductase gene <i>mthfr66</i> could relieve the inhibition by removing 5-methyl-THF in vitro. Compared with expression of <i>dmt50</i> alone, simultaneous expression of <i>dmt50</i> and <i>mthfr66</i> further improved the dicamba resistance (1120 g/ha) of transgenic <i>A. thaliana</i>. This study provides new genes for dicamba detoxification and a strategy for the engineering of dicamba-resistant crops.