Pulse-Chase Experiments with [35S]Methionine Show D1 Reaction II Protein Turnover in Variously Herbicide Tolerant Species

Dl protein of Photosystem II, the photosynthetic herbicide target, is rapidly turned over compared to the other chloroplastic proteins. However, the importance of Dl turnover for herbicide action is unclear. In short- and long-term experiments we treated various plant species with atrazine herbicide in combination with chloramphenicol (CAP), an inhibitor of Dl protein synthesis. A synergistic effect Which inhibited plant growth was seen primarily in moderately resistant plants treated with herbicide and CAP. Dl protein turnover was studied by [35S]methionine incorporation in seedlings treated in the light with these inhibitors. The effect of the two inhibitors on Dl content varied: CAP inhibited Dl synthesis and retarded its degradation, resulting in a depletion of Dl from the thylakoid membranes of about 50% in all tested species. The herbicide altered both Dl synthesis and degradation but the extent of inhibition varied significantly among species and resulted in either a reduction or an accumulation of Dl protein. Simultaneous treatment with both CAP and herbicide resulted in a significant decrease in Dl content in moderately tolerant plants only. It is supposed that tolerance may be due to faster degradation or overproduction of the Dl herbicide binding protein.