Cold Acclimation in Arabidopsis thaliana: Function and Regulation of COR Genes

Abstract

Arabidospis thaliana alters gene expression and increases in freezing tolerance in response to low non-freezing temperatures. One of our long range goals has been to determine whether certain COR (cold-regulated) genes that encode hydrophilic, “boiling-soluble” polypeptides have a role in freezing tolerance; another has been to determine the mechanism(s) by which A. thaliana senses low temperature and alters gene expression. Most of our work on COR gene function has focused on COR15. We have found that this gene encodes a 15 kD polypeptide that is targeted to the stromal compartment of chloroplasts. During import, the polypeptide is processed to a hydrophilic 9 kD polypeptide, designated COR15am, that shares a low degree of amino acid sequence similarity with group III LEA (late-embryogenesis-abundant) proteins Transgenic A. thaliana plants that constitutively produce COR15am have been created and tested for freezing tolerance. The data indicate that COR15am enhances the freezing tolerance of chloroplasts in non-acclimated plants by abundant 2°C, nearly one-third of the increase that occurs upon cold acclimation of both transgenic and wild type plants. Constitutive expression of COR15a was also found to affect freezing tolerance at the cellular level: at freezing temperatures between -6 and -8 °C, the survival of protoplasts isolated from leaves of non-acclimated plants expressing CORl5a was greater than that of protoplasts isolated from leaves of plants not expressing CORl5a. The implications of these data regarding possible COR15a functions are discussed. In regarding to gene regulation, the promoters of COR15a and COR78 were found to be up-regulated in response to low temperature, drought and ABA. An examination of COR gene expression in the A. thaliana abi (ABA-insensitive) mutants indicated that low temperature and ABA regulation involve independent pathways. Gene fusion experiments established that COR15a has a functional DRE (drought-regulatory-element) that imparts cold- and drought-regulated gene expression in tobacco. Gel retardation studies indicated that nuclear extracts prepared from both cold-acclimated and non-acclimated Arabidopsis plants contain a protein(s) that binds to the COR15a DRE and that the avidity of the protein(s) for the element is not significantly altered with cold acclimation.