Submitted to: Plant Physiology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/26/1996
Publication Date: N/A
Citation: Interpretive Summary: Italian ryegrass is a serious weed in the wheat-growing regions of northwestern U.S. Resistance to the herbicide diclofop has appeared in this weed in wheat fields where the herbicide has been used for several years. Previous investigations established that resistance was due to diclofop being less effective in blocking acetyl-CoA carboxylase (ACCase), the protein that serves as the target site of this herbicide. It was not known whether Italian ryegrass contained multiple ACCase proteins and whether herbicide resistance was due to a change in one or all of these proteins. We isolated and purified ACCase proteins from the leaves of herbicide-resistant and -susceptible Italian ryegrass. We found that the leaves of both herbicide-susceptible and resistant Italian ryegrass contain two different types of ACCase proteins (ACCase I and ACCase II) and that the relative amount of each is influenced by the environment and leaf age. Resistance to diclofop was due to a change in herbicide sensitivity of ACCase I, the major ACCase protein found in young leaves. In the resistant biotype, diclofop does not block the activity of ACCase I and hence does not kill the weed. This research has increased basic knowledge regarding the mechanism of diclofop resistance in Italian ryegrass. This knowledge can be used to develop strategies to prevent the development and limit the spread of this herbicide-resistant weed.
Technical Abstract: Two multifunctional acetyl-CoA carboxylase (ACCase, EC 220.127.116.11) isoforms were isolated from etiolated and mature leaves of Italian ryegrass (Lolium multiflorum Lam.). ACCase I and ACCase II eluted from a TMAE anion exchange column at 275 and 195 mM KCl, respectively. Both isoforms were dimers with 206 kD subunits. Total and specific activities of ACCase I were greater in 3-week-old leaves compared to 6-week-old leaves and decreased during greening of etiolated leaves. 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 = 7.0 uM) while ACCase I from the susceptible biotype was sensitive (I50 = 0.2 uM). Total and specific activities of ACCase II were greater in 6-week-old leaves compared to 3-week-old leaves and exhibited a pronounced decrease upon greening of etiolated leaves. ACCase II activity was relatively insensitive to diclofop (I50 > 100 uM) in both diclofop-resistant and -susceptible biotypes. Polyclonal antiserum to the plastid-localized ACCase isoform in maize, which was affinity-purified to Lolium ACCase I, cross-reacted with the 206-kD polypeptide of 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).