Many plants increase in freezing tolerance upon exposure to low nonfreezing
temperatures, a phenomenon known as cold acclimation. A fundamental goal o
f cold acclimation research is to identify genes with key roles in this res
ponse. Here we review results from our laboratory regarding the discovery o
f a family of transcriptional activators in Arabidopsis (Ara-bidopsis thali
ana) that regulates the expression of freezing tolerance genes. Specificall
y, we have identified 3 genes that encode nearly identical transcriptional
activators that bind to the CRT (C-repeat)/DRE (dehydration responsive elem
ent) DNA regulatory element present in the promoters of many cold- and drou
ght-inducible genes, including those designated COR (cold-regulated). These
regulatory genes, CBF1, CBF2 and CBF3 (CRT/DRE binding factor), are locate
d in tandem array on chromosome 4. Overexpression of the CBI:genes in Arabi
dopsis induces expression of the entire battery of known COR genes and incr
eases freezing tolerance without a low temperature stimulus. We have, there
fore, proposed that the CBF genes are 'master switches' that activate a reg
ulon of genes involved in cold acclimation. Significantly, the CBF: genes t
hemselves are responsive to low temperature; the levels of CBF transcripts
begin increasing within 15 min of transferring plants to low temperature fo
llowed by accumulation of COR gene transcripts at 2-4 h. The CBF genes do n
ot appear to be subject to autoregulation as the promoter regions have no e
vident CRT/DRE elements and overexpression of CBF1 does not induce expressi
on 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 activator
s.