Action mechanism of a herbicide, thiobencarb

The action mechanism of thiobencarb was studied by examining the inhibitory effects of this herbicide on the biosynthesis of very-long-chain fatty acids (VLCFAs). Thiobencarb treatment decreased VLCFAs, such as C20:0, C20:1, C22:0, C24:0, C24:1 and C26:0 fatty acids, and increased long-chain-fatty acids and medium-chain-fatty acids, such as C14:0, C15:0, C18:0 and C18:1 fatty acids, which are precursors of VLCFAs, in barnyard millet cultured cells. Thiobencarb sulfoxide and sulfone potently inhibited VLCFA elongase (VLCFAE) activity in the microsomal fraction of etiolated barnyard millet seedlings, although thiobencarb itself slightly inhibited it. These results suggested that thiobencarb is a VLCFAE-inhibiting herbicide whose active forms are its oxidized metabolites, such as sulfoxide and sulfone. Thiobencarb sulfoxide inhibited the VLCFAE activity of the microsomal fraction of etiolated barnyard millet seedlings in a time-independent manner. This time-independent inhibition proposed a reversible inhibition mechanism of the VLCFAE by thiobencarb sulfoxide, likely with isoxazoline-type herbicides, such as pyroxasulfone, which have been classified into group K3 of the Herbicide Resistance Action Committee (HRAC) in the U.S. It is assumed that the time-independent reversible inhibition of VLCFAE is applicable to other thiocarbamate herbicides presently classified into group N of the HRAC.