|De Prado, Rafael|
Submitted to: Pesticide Biochemistry and Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/1999
Publication Date: N/A
Citation: N/A Interpretive Summary: Propaquizafop is a herbicide that is used for controlling grass weeds in broadleaf crops. This herbicide kills grass weeds because it binds to a target site protein in the weed called acetyl-CoA carboxylase (ACCase) and blocks it from functioning. ACCase is an important plant protein that is involved in the production of oils needed for plant growth. Herbicide mode of action research involves investigating whether a herbicide is converted to other compounds in the plant and whether it has any affect on the target site protein. The mode of action of propaquizafop in the broadleaf crop chickpea had not been previously investigated. We found that when propaquizafop was applied to chickpea leaves, it was rapidly converted to propaquizafop acid which moved from the treated leaves to other plant parts. In order to examine the effects of this herbicide on ACCase, a new protocol was developed to assay for this protein. Three different types of ACCase proteins were found in chickpea leaves. Propaquizafop did not bind to any of these proteins and hence did not block oil synthesis which explains why this herbicide does not cause injury to chickpea. This research has advanced understanding of the mode of action of the herbicide propaquizafop. In addition, the research has resulted in the development of a new assay for the important plant protein ACCase. This assay will be useful to other scientists investigating the role of ACCase in regulating the amount of oil produced in the seeds of broadleaf crops such as soybean.
Technical Abstract: Propaquizafop absorption, translocation, metabolism, and effect on acetyl- CoA carboxylase (ACCase) isoforms were examined in chickpea (Cicer arientinum L.). Maximum foliar absorption of propaquizafop, approximately 35% of recovered herbicide, occurred 48 h after treatment. Of the absorbed propaquizafop, approximately 30% was rapidly metabolized to the acid form followed by a slow conversion of the acid to a polar metabolite. Approximately 4% of foliar-applied 14C-propaquizafop was translocated from the treated leaf within 72 h after application. Chloroplast stromal and cytosolic fractions were isolated from chickpea leaves. Proteins from both fractions were separated by SDS-PAGE and probed with avidin-alkaline phosphatase to detect biotinylated polypeptides. The cytosolic fraction contained a multi-functional ACCase as indicated by the presence of a biotinylated polypeptide of 200 kDa. The chloroplast stromal fraction contained the 36 kDa biotinylated subunit of the multi-subunit ACCase and 200-kDa biotinylated protein which suggested the presence of a multi- functional ACCase. The 200-kDa biotinylated polypeptide in the chloroplast stromal fraction did not represent contamination by the cytosolic, multi- functional ACCase because it was not removed by pretreating isolated chloroplasts with thermolysin. I50 values for ACCase inhibition by propaquizafop acid in the chloroplast stromal and cytosolic fractions were approximately 700 uM and 200 uM, respectively. The results of this study indicate that ACCase activity measured in chickpea chloroplast stromal fractions is highly tolerant to graminicides and that total ACCase activity in this fraction appears to be composed of activities contributed by both multi-subunit and multi-functional ACCases.