Mode of Action of Herbicides Affecting Acetyl-CoA Carboxylase and Fatty Acid Biosynthesis

The mode o f action o f cyclohexane-l,3-dione-type (cycloxydim, clethodim , sethoxydim, tralkoxydim ) and aryloxyphenoxypropanoate-type herbicides (diclofop, fenoxaprop, haloxyfop, fluazifop) is sum m arized in this review. Both herbicide classes, though structurally com ­ pletely different, specifically block the same target enzyme i.e. the plastid acetyl-CoA carboxy­ lase (ACC) (EC 6.4.1.2). M ost m em bers o f the Poaceae are sensitive tow ards both herbicide groups, whereas other m onocotyledonous p lan ts as well as the dicotyledonous plants appear to be resistant. This resistance, which can be found on the level o f whole plants, in isolated chloroplasts and also on the level o f A CC-enzym e p reparations, is apparently due to a m odifi­ cation o f the target enzyme A CC. W ithin the sensitive grass family some m em bers (Festuca and Poa species) are partially to leran t against bo th gram inicide groups. In the case o f cyclohexanedione herbicides the tolerance seems to be due to a reduced sensitivity o f the target en­ zyme. In the case o f aryloxyphenoxypropionic acid herbicides the tolerance is apparently based on a com bined action o f cytoplasm ic factors (m etabolization?) and a slightly reduced sensitivity o f the target enzyme. F rom differences in the sensitivity o f certain grasses against the two herbicide classes it is concluded tha t bo th gram inicide groups bind to the same binding dom aine o f the ACC enzyme but possess different subsites. The consequences o f the block o f de novo fatty acid biosynthesis in the plastids o f sensitive p lants is the lack o f glycerolipid and biom em brane form ation which finally causes cell death in the m eristem atic tissues.