PLANT PHYSIOLOGY , Vol 109, Issue 4 1395-1403, Copyright © 1995 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Plasma Membrane Sterols Are Essential for Sensing Osmotic Changes in the Halotolerant Alga Dunaliella
A. M. Zelazny, A. Shaish and U. Pick
Department of Biochemistry, The Weizmann Institute of Science, Rehovot 76100, Israel
The halotolerant alga Dunaliella responds to hyperosmotic stress by
synthesis of massive amounts of glycerol. The trigger for this osmotic
response is the change in cell volume, but the mechanism that senses volume
changes is not known. Preincubation of Dunaliella salina with tridemorph, a
specific inhibitor of sterol biosynthesis, inhibits glycerol synthesis and
volume recovery. The inhibition is associated with suppression of
[14C]bicarbonate incorporation into sterols and is correlated with
pronounced depletion of plasma membrane sterols. Incubation of
sterol-depleted cells with cholesterol hemisuccinate restores the capacity
for volume regulation in response to hyperosmotic stress. Tridemorph as
well as lovastatin also inhibit volume changes that are induced by high
light in Dunaliella bardawil, a species that responds to high light
intensity by synthesis of large amounts of [beta]-carotene. These volume
changes result from accumulation of glycerol and are associated with de
novo synthesis of sterols. The major plasma membrane sterol in D. salina
and the high-light-induced sterol in D. bardawil co-migrate with ergosterol
on thin-layer chromatography and on reversed-phase high-performance liquid
chromatography. These results suggest that the osmosensory mechanism in
Dunaliella resides in the plasma membrane, and that sterols have an
important role in sensing osmotic changes.
