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

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.