E. Darko et al., Defensive strategies against high light stress in wild and D1 protein mutant biotypes of Erigeron canadensis, AUST J PLAN, 27(4), 2000, pp. 325-333
The Ser(264)-->Gly substitution on the D1 protein is accompanied by a highe
r 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 violaxant
hin 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 (F-v/F-m) of PS II and a large increa
se in the initial fluorescence yield (F-o) in the mutant compared to the wi
ld biotype. In the fluorescence-quenching parameters of the wild-type leave
s, dithiothreitol caused alterations similar to those observed in the mutan
t plant without dithiothreitol. A lowered activity of the xanthophyll cycle
was detected in the mutant biotype compared to the wild-type in vivo. Howe
ver, under in vitro, conditions which were optimal for violaxanthin de-epox
idation, or when paraquat was used on intact leaves to accelerate the elect
ron transport, violaxanthin could readily be converted to zeaxanthin even i
n the mutant plants. This demonstrates that neither the decrease in the enz
ymatic activity of violaxanthin de-epoxidase nor the low availability of vi
olaxanthin is responsible for the low zeaxanthin formation under in vivo co
nditions. It is presumed that, in vivo, the D1 protein mutation results in
slower electron transport, a smaller Delta pH and lower zeaxanthin formatio
n, and thereby in alterations in the defensive strategies against high ligh
t illumination.