STRESS INDUCTION OF ZEAXANTHIN FORMATION IN THE BETA-CAROTENE-ACCUMULATING ALGA DUNALIELLA-SALINA TEOD

The formation of zeaxanthin from vioalaxanthin via antheraxanthin is considered 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 exposed to either high light intensity or hypersalinity. Exposure to high light intensity caused cell swelling whereas exposure to salt stress 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 compartments whereas salt stress caused alkalinization. Intracellular pH changes coincided with alterations in xanthophyll cycle pigment composition and stress-induced de novo synthesis of beta,beta-carotenoids. Production of beta-carotene was similar for both salt-stressed and high light-treated cells during the early part of the initial stage of beta-carotene accumulation in Dunaliella. Likewise, production of xanthophyll cycle pigments was stoichiometrically identical for salt-stressed and high light-treated cells during this stage. Major differences were however observed in the composition of xanthophyll cycle pigments. High light-treated cells accumulated zeaxanthin whereas salt-stressed cells formed violaxanthin at the expense of zeaxanthin. Pulse-chase studies revealed that cells exposed to high light intensity had higher turnover rates of xanthophylls and accumulated zeaxanthin via violaxanthin. Cessation of zeaxanthin and violaxanthin accumulation coincided with restoration of compartmental pH and re-establishmend of cell volume. The subsequent decline in violaxanthin in salt-stressed cells occurred coincident with a decline in zeaxanthin and beta-carotene content, Sustained zeaxanthin content of high light-treated cells occurred coincident with continued accumulation of beta-carotene.