RESISTANCE TO ACETYL-COENZYME-A CARBOXYLASE-INHIBITING HERBICIDES ENDOWED BY A SINGLE MAJOR GENE ENCODING A RESISTANT TARGET SITE IN A BIOTYPE OF LOLIUM-RIGIDUM

The mechanism and mode of inheritance of resistance to acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicides was investigated in a bi otype of Lolium rigidum that has evolved resistance following selectio n with diclofop-methyl for 10 consecutive years. ACCase extracted from the resistant biotype is > 6.9 times more resistant to inhibition by diclofop than enzyme from a susceptible biotype. Similar or greater le vels of resistance were found to other related herbicides. There is no difference in absorption or metabolism of diclofop-methyl or haloxyfo p-methyl between the resistant and susceptible biotypes, hence differe ntial absorption or metabolism of these herbicides does not contribute to resistance. F-1 families from reciprocal crosses between the resis tant biotype and a susceptible biotype respond similarly to the herbic ide and are nearly as resistant as the resistant parent, indicating th at the resistance trait is nuclearly located and has incomplete domina nce over susceptibility. F-2 families treated with 26 and 208 g ai ha( -1) of haloxyfop-ethoxyethyl reveal only two phenotypes: resistant pla nts showing no injury and susceptible plants showing no growth. At bot h rates of haloxyfop-ethoxyethyl, the segregation of resistance to sus ceptibility follows a ratio of 3:1 (R:S) that fits the predicted ratio for a single nuclear gene with high dominance. From the F-1 and F-2 d ata, it is concluded that resistance to haloxyfop in this resistant bi otype of L. rigidum is inherited as a single nuclear incompletely domi nant gene coding for a resistant form of the target enzyme ACCase.