MODE OF ACTION OF THE COR15A GENE ON THE FREEZING TOLERANCE OF ARABIDOPSIS-THALIANA

Constitutive expression of the cold-regulated COR15a gene of Arabidops is thaliana results in a significant increase in the survival of isola ted protoplasts frozen over the range of -4.5 to -7 degrees C. The inc reased freezing tolerance is the result of a decreased incidence of fr eeze-induced lamellar-to-hexagonal II phase transitions that occur in regions where the plasma membrane is brought into close apposition wit h the chloroplast envelope as a result of freeze-induced dehydration. Moreover, the mature polypeptide encoded by this gene, COR15am, increa ses the lamellar-to-hexagonal II phase transition temperature of diole oylphosphatidylethanolamine and promotes formation of the lamellar pha se in a lipid mixture composed of the major lipid species that compris e the chloroplast envelope. We propose that COR15am, which is located in the chloroplast stroma, defers freeze-induced formation of the hexa gonal II phase to lower temperatures (lower hydrations) by altering th e intrinsic curvature of the inner membrane of the chloroplast envelop e.