CONSTITUTIVE EXPRESSION OF THE COLD-REGULATED ARABIDOPSIS-THALIANA COR15A GENE AFFECTS BOTH CHLOROPLAST AND PROTOPLAST FREEZING TOLERANCE

Cold acclimation in plants is associated with the expression of COR (f old-regulated) genes that encode polypeptides of unknown function. It has been widely speculated that products of these genes might have rol es in freezing tolerance. Here we provide direct evidence in support o f this hypothesis. We show that constitutive expression of COR15a, a c old-regulated gene of Arabidopsis thaliana that encodes a chloroplast- targeted polypeptide, enhances the in vivo freezing tolerance of chlor oplasts in nonacclimated plants by almost 2 degrees C, nearly one-thir d of the increase that occurs upon cold acclimation of wild-type plant s. Significantly, constitutive expression of COR15a also affects the i n vitro freezing tolerance of protoplasts. At temperatures between -5 and -8 degrees C, the survival of protoplasts isolated from leaves of nonacclimated transgenic plants expressing COR15a was greater than tha t of protoplasts isolated from leaves of nonacclimated wild-type plant s. At temperatures between -2 and -4 degrees C, constitutive expressio n of COR15a had a slight negative effect on survival. The implications of these data regarding possible modes of COR15a action are discussed .