|De prado, R|
Submitted to: Weed Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/7/2000
Publication Date: N/A
Citation: Interpretive Summary: Italian ryegrass is a serious weed in the wheat-growing regions of the U.S. and France. Diclofop is a herbicide that is commonly used to control certain grass weeds, such as Italian ryegrass, in wheat. Italian ryegrass weeds resistant to the herbicide diclofop first appeared in a wheat field in Oregon in 1987. Previous research indicated that diclofop resistance in nItalian ryegrass found in Oregon was due to a change in a plant protein called acetyl-CoA carboxylase (ACCase) which serves as the herbicide target site. Normally, the herbicide binds to this protein which is found in grass weeds and interferes with the synthesis of fats and oils which causes death. The ACCase protein found in the resistant Oregon biotype no longer bound diclofop and hence the herbicide had no effect. In 1995, Italian ryegrass weeds resistant to diclofop were reported in a wheat field in France. The mechanism of diclofop resistance in these weeds was not known. .We investigated the mechanism of resistance and discovered that it was the same as was found for this weed in Oregon. The results suggest that resistance due to a modified target site (ACCase) is a common mechanism whereby Italian ryegrass becomes resistant to diclofop. This research has increased basic knowledge about how herbicide resistance develops in Italian ryegrass. This knowledge can be used to develop integrated weed management strategies to prevent the development and limit the spread of this herbicide-resistant weed.
Technical Abstract: An Italian ryegrass (Lolium multiflorum Lam.) biotype resistant to diclofop-methyl was found in a wheat field in France (Normandy) that had been treated for several years with diclofop-methyl. Based on plant survival evaluated 21 d after herbicide application, the biotype exhibited a high level of resistance to diclofop-methyl and moderate resistance to clodinafop-propargil and tralkoxydim. The mechanism of resistance was investigated for diclofop-methyl. There was little or no difference in diclofop-methyl absorption by leaves of resistant and susceptible biotypes as measured 48 h after treatment. For both resistant and susceptible biotypes, less than 1% of absorbed radiolabel was translocated during a 48 h period following foliar application of 14C-diclofop-methyl. Metabolism of diclofop-methyl was not significantly altered in the resistant biotype. In both biotypes, diclofop-methyl was rapidly metabolized to diclofop acid followed by a slow rate of formation of a polar conjugate. Two multifunctional ACCase isoforms (ACCase I and ACCase II) were isolated from leaf tissue of resistant and susceptible biotypes. Graminicide concentrations required to inhibit ACCase activity by 50% (I50 values) were determined for both ACCase isoforms from resistant and susceptible biotypes. The ACCase II isoform was highly resistant to graminicides in both biotypes. In contrast, the I50 value for diclofop inhibition of ACCase I was 19-fold greater for the enzyme isolated from the resistant biotype compared to the susceptible biotype. It is concluded that diclofop resistance in the Lolium multiflorum biotype from Normandy is due to the presence of a resistant form of the ACCase I isoform.