Isoforms of acetyl-coenzyme a carboxylase in Lolium multiflorum

Two multifunctional acetyI-CoA carboxylase (ACCase, EC 6.4.1.2) isoforms were isolated from etiolated and mature leaves of Italian ryegrass (Lolium multiflorum Lam.). ACCase I and ACCase II eluted from a TMAE (trimethylaminoethyl) anion exchange column at approximately 275 and 195 mM KCl, respectively. Both isoforms were dimers with 206 kD subunits. ACCase 1 represented 95% and 66% of total ACCase activity in 3-week-old and etiolated leaves, respectively. Total activity of ACCase I and ACCase II decreased by 35% and 90% during greening of etiolated leaves, respectively. There were no differences in total or specific activities of ACCase I isolated from leaves of diclofop-resistant and -susceptible Lolium multiflorum biotypes. However, ACCase I from the resistant biotype was tolerant to diclofop [I₅₀ (herbicide concentration resulting in 50% inhibition of activity) = 7.0 μM] while ACCase I from the susceptible biotype was sensitive (I₅₀ = 0.2 μM). ACCase II activity was relatively insensitive to diclofop (I₅₀ > 125 μM) in both diclofop-resistant and -susceptible biotypes. Polyclonal antiserum to the plastid-localized ACCase I isoform from maize cross-reacted with the 206-kD polypeptides of both ACCase I and ACCase II. ACCase I and II differed in their affinity to monomeric avidin; ACCase I did not bind to monomeric avidin while ACCase II did bind. The results indicate that Lolium leaves contain two distinct, multifunctional ACCase isoforms and that diclofop resistance is due to a change in herbicide sensitivity of the predominant isoform (ACCase I).