Role of the Arabidopsis CBF transcriptional activators in cold acclimation

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Abstract
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 (Arabidopsis 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 CBF 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.