HERBICIDE MULTIPLE-RESISTANCE IN A LOLIUM-RIGIDUM BIOTYPE IS ENDOWED BY MULTIPLE MECHANISMS - ISOLATION OF A SUBSET WITH RESISTANT ACETYL-COA CARBOXYLASE

The development of herbicide multiple-resistance in weed species repre sents a major threat to current agricultural practices. The mechanisti c basis for herbicide multiple-resistance has been investigated in a p opulation of the annual grass weed Lolium rigidum Gaud. (annual ryegra ss) resistant to herbicides affecting 6 target sites. A subset of the resistant population (R(2) subset) has been isolated by germination on a medium containing the acetyl-CoA carboxylase (ACCase, EC 6.4.1.2) i nhibiting herbicide, sethoxydim {(2-[1-(ethoxyimino)butyl]-5-[2-(ethyl thio)propyl] -3-hydroxy-2-cyclohexen-1-one)}. This 12% R(2) subset of the population is 600 times more resistant to sethoxydim and between 3 0 to 200 times more resistant to other ACCase inhibitors than the bulk of the R population. The subset has a form of ACCase which is 6 to 55 times less sensitive to inhibition by these herbicides than the enzym e present in the bulk of the resistant or in the susceptible populatio n. There was no difference in the uptake and metabolic degradation of [4-C-14]sethoxydim between the R(2) subset and the unselected R popula tion. These results show the accumulation of different resistance mech anisms in that single population. Furthermore we propose that this acc umulation of multiple resistance mechanisms is the basis for herbicide multiple-resistance in this biotype.