Triaziflam and diaminotriazine derivatives affect enantioselectively multiple herbicide target sites

Enantiomers of triaziflam and structurally related diaminotriazines were sy nthesized and their herbicidal mode of action was investigated. The compoun ds caused light and dark-dependent effects in multiple test systems includi ng heterotrophic cleaver and photoautotrophic algal cell suspensions, the H ill reaction of isolated thylakoids and germinating cress seeds. Dose-respo nse experiments revealed that the (S)-enantiomers of the compounds preferen tially inhibited photosystem II electron transport (PET) and algae growth w ith efficacies similar to that of the herbicide atrazine. In contrast, the (R)-enantiomers of the diaminotriazines were up to 100 times more potent in hibitors of growth in cleaver cell suspensions and cress seedlings in the d ark than the (S)-enantiomers. The most active compound, the (R)enantiomer o f triaziflam, inhibited shoot and root elongation of cress and maize seedli ngs at concentrations below 1 mum. The meristematic root tips swelled into a club shape which is typical for the action of mitotic disrupter herbicide s and cellulose biosynthesis inhibitors. Microscopic examination using hist ochemical techniques revealed that triaziflam (R)-enantiomer blocks cell di vision in maize root tips 4 h after treatment. The chromosomes proceeded to a condensed state of prometaphase but were unable to progress further in t he mitotic cycle. Disruption of mitosis was accompanied by a loss of spindl e and phragmoplast micotubule arrays. Concomitantly, cortical microtubules decreased which could lead to isodiametric cell growth and consequently to root swelling. In addition, a decline in cellulose deposition in cell walls was found 24 h after treatment. Compared to the (R)-form, triaziflam (S)-e nantiomer was clearly less active. The results suggest that triaziflam and related diaminotriazines affect enantioselectively multiple sites of action which include PET inhibitory activity, mitotic disruption by inhibiting mi crotubule formation and inhibition of cellulose synthesis.