Defensive strategies against high light stress in wild and D1 protein mutant biotypes of Erigeron canadensis

The Ser264ÆGly substitution on the D1 protein is accompanied by a higher photosensitivity of the mutant plant. This may be due to an increased D1 protein turnover and/or to a lower xanthophyll cycle activity in vivo. The relative importance of these two photoprotective mechanisms in wild and D1 protein mutant biotypes of Erigeron canadensis L. was established by using dithiothreitol and streptomycin. Moreover, the interconversion of violaxan-thin to zeaxanthin via antheraxanthin was studied in isolated thylakoids and in intact leaves treated with paraquat. Streptomycin caused a more severe decrease in the optimal quantum yield (Fv/Fm) of PS II and a large increase in the initial fluorescence yield (Fo) in the mutant compared to the wild biotype. In the fluorescence-quenching parameters of the wild-type leaves, dithiothreitol caused alterations similar to those observed in the mutant plant without dithiothreitol. A lowered activity of the xanthophyll cycle was detected in the mutant biotype compared to the wild-type in vivo. However, under in vitro, conditions which were optimal for violaxanthin de-epoxidation, or when paraquat was used on intact leaves to accelerate the electron transport, violaxanthin could readily be converted to zeaxanthin even in the mutant plants. This demonstrates that neither the decrease in the enzymatic activity of violaxanthin de-epoxidase nor the low availability of violaxanthin is responsible for the low zeaxanthin formation under in vivo conditions. It is presumed that, in vivo, the D1 protein mutation results in slower electron transport, a smaller DpH and lower zeaxanthin formation, and thereby in alterations in the defensive strategies against high light illumination.