Characterization of Two Acetyl-CoA Carboxylase Isoforms in Diclofop-MethylResistant and -Susceptible Biotypes of Alopecurus myosuroides,

Previous results indicated that resistance to acetyl-CoA carboxylase-inhibiting herbicides in a Spanish diclofop-methyl-resistant biotype of Alopecurus myosuroides could be due in part to the presence of a less sensitive form of acetyl-CoA carboxylase (ACCase, EC 6.4.1.2). In this study, partial purification and separation of two multifunctional acetyl-CoA carboxylase isoforms (ACCase I and ACCase II) did not reveal any differences between resistant and susceptible biotypes of A. myosuroides in terms of purification, activity, elution profile in a trimethylaminoethyl (TMAE)-anion-exchange column, and molecular mass of ACCase subunits. ACCase I and ACCase II eluted from a TMAE-anion-exchange column at approximately 280 and 220 mM KCl, respectively. Both isoforms were composed by subunits of 200-202 kDa. ACCase I represented about 90% of the total activity recovered from the TMAE column and was diclofop sensitive, with I50 values (herbicide concentration resulting in 50% inhibition of ACCase activity) within the range of other susceptible grasses. ACCase II was diclofop tolerant, with I50 values 100-fold greater than those observed for ACCase I. The differences in susceptibility observed between resistant and susceptible crude extracts disappeared in further purification steps, the I50 values not being significantly different between R and S biotypes after TMAE-anion-exchange chromatography. These results indicate that the presence of two distinct, multifunctional ACCase isoforms in grasses (only reported in maize and Lolium multiflorum) is also observed in both the resistant and the susceptible biotypes of A. myosuroides. However, it also confirms that diclofop resistance in the resistant biotype is not due to a mutation at the target site, which is the predominant ACCase I isoform.