Abscisic Acid Metabolism in Salt-Stressed Cells of Dunaliella salina: Possible Interrelationship with beta-Carotene Accumulation.

The interrelationship between abscisic acid (ABA) production and beta-carotene accumulation was investigated in salt-stressed cells of the halotolerant green alga Dunaliella salina var bardawil. Cells were supplied with either R-[2-(14)C]mevalonolactone or [(14)C] sodium bicarbonate for 20 hours and then exposed to increased salinity (1.5 to 3.0 molar NaCl) for various lengths of time. Incorporation of label into abscisic acid and phaseic acid and the distribution of [(14)C]ABA between the cells and incubation media were monitored. [(14)C]ABA and [(14)C]phaseic acid were identified as products of both R-[2-(14)C]mevalonolactone and [(14)C]sodium bicarbonate metabolism. ABA metabolism was enhanced by hypersalinity stress. Actinomycin D, chloramphenicol, and cycloheximide abolished the stress-induced production of ABA, suggesting a role for gene activation in the process. Kinetic analysis of both ABA and beta-carotene production demonstrated two stages of accelerated beta-carotene production. In addition, ABA levels increased rapidly, and this increase occurred coincident with the early period of accelerated beta-carotene production. A possible role for ABA as a regulator of carotenogenesis in cells of D. salina is therefore discussed.