Ak. Cowan et al., STRESS INDUCTION OF ZEAXANTHIN FORMATION IN THE BETA-CAROTENE-ACCUMULATING ALGA DUNALIELLA-SALINA TEOD, Journal of plant physiology, 146(4), 1995, pp. 554-562
The formation of zeaxanthin from vioalaxanthin via antheraxanthin is c
onsidered to play a major role in photoprotection in higher plants and
green algae, Stress induction of zeaxanthin formation was studied in
cells of the beta-carotene-accumulating green alga Dunaliella salina e
xposed to either high light intensity or hypersalinity. Exposure to hi
gh light intensity caused cell swelling whereas exposure to salt stres
s caused cell shrinkage. Changes in cell. volume occurred concomitant
with changes in compartmental pH. Analysis of P-31 in vivo NMR spectra
of Dunaliella cells encapsulated in agarose beads revealed that high
light intensity induced rapid acidification of intracellular compartme
nts whereas salt stress caused alkalinization. Intracellular pH change
s coincided with alterations in xanthophyll cycle pigment composition
and stress-induced de novo synthesis of beta,beta-carotenoids. Product
ion of beta-carotene was similar for both salt-stressed and high light
-treated cells during the early part of the initial stage of beta-caro
tene accumulation in Dunaliella. Likewise, production of xanthophyll c
ycle pigments was stoichiometrically identical for salt-stressed and h
igh light-treated cells during this stage. Major differences were howe
ver observed in the composition of xanthophyll cycle pigments. High li
ght-treated cells accumulated zeaxanthin whereas salt-stressed cells f
ormed violaxanthin at the expense of zeaxanthin. Pulse-chase studies r
evealed that cells exposed to high light intensity had higher turnover
rates of xanthophylls and accumulated zeaxanthin via violaxanthin. Ce
ssation of zeaxanthin and violaxanthin accumulation coincided with res
toration of compartmental pH and re-establishmend of cell volume. The
subsequent decline in violaxanthin in salt-stressed cells occurred coi
ncident with a decline in zeaxanthin and beta-carotene content, Sustai
ned zeaxanthin content of high light-treated cells occurred coincident
with continued accumulation of beta-carotene.