Role of the Arabidopsis CBF transcriptional activators in cold acclimation

Many plants increase in freezing tolerance upon exposure to low nonfreezing temperatures, a phenomenon known as cold acclimation. A fundamental goal of cold acclimation research is to identify genes with key roles in this response. Here we review results from our laboratory regarding the discovery of a family of transcriptional activators in Arabidopsis (Ara-bidopsis thaliana) that regulates the expression of freezing tolerance genes. Specifically, we have identified 3 genes that encode nearly identical transcriptional activators that bind to the CRT (C-repeat)/DRE (dehydration responsive element) DNA regulatory element present in the promoters of many cold- and drought-inducible genes, including those designated COR (cold-regulated). These regulatory genes, CBF1, CBF2 and CBF3 (CRT/DRE binding factor), are located in tandem array on chromosome 4. Overexpression of the CBI:genes in Arabidopsis induces expression of the entire battery of known COR genes and increases freezing tolerance without a low temperature stimulus. We have, therefore, proposed that the CBF genes are 'master switches' that activate a regulon of genes involved in cold acclimation. Significantly, the CBF: genes themselves are responsive to low temperature; the levels of CBF transcripts begin increasing within 15 min of transferring plants to low temperature followed by accumulation of COR gene transcripts at 2-4 h. The CBF genes do not appear to be subject to autoregulation as the promoter regions have no evident CRT/DRE elements and overexpression of CBF1 does not induce expression of CBF3. Thus, we have suggested that COR gene induction involves a two-step cascade of transcriptional activators: the first step, CBF induction, involving an unknown activator present at normal growth temperature and the second step, COR gene induction, involving the action of the CBF activators.