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

Abstract

D1 protein of Photosystem II, the photosynthetic herbicide target, is rapidly turned over compared to the other chloroplastic proteins. However, the importance of D1 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 D1 protein synthesis. A synergistic effect which inhibited plant growth was seen primarily in moderately resistant plants treated with herbicide and CAP. D1 protein turnover was studied by [ 35 S]methionine incorporation in seedlings treated in the light with these inhibitors. The effect of the two inhibitors on D1 content varied; CAP inhibited D1 synthesis and retarded its degradation, resulting in a depletion of D1 from the thylakoid membranes of about 50% in all tested species. The herbicide altered both D1 synthesis and degradation but the extent of inhibition varied significantly among species and resulted in either a reduction or an accumulation of D1 protein. Simultaneous treatment with both CAP and herbicide resulted in a significant decrease in D1 content in moderately tolerant plants only. It is supposed that tolerance may be due to faster degradation or overproduction of the D1 herbicide binding protein.