Overexpression of the Arabidopsis CBF3 transcriptional activator mimics multiple biochemical changes associated with cold acclimation

We further investigated the role of the Arabidopsis CBF regulatory genes in cold acclimation, the process whereby certain plants increase in freezing tolerance upon exposure to low temperature. The CBF genes, which are rapidl y induced in response to low temperature, encode transcriptional activators that control the expression of genes containing the C-repeat/dehydration r esponsive element DNA regulatory element in their promoters. Constitutive e xpression of either CBF1 or CBF3 (also known as DREB1b and DREB1a, respecti vely) in transgenic Arabidopsis plants has been shown to induce the express ion of target COR (cold-regulated) genes and to enhance freezing tolerance in nonacclimated plants. Here we demonstrate that overexpression of CBF3 in Arabidopsis also increases the freezing tolerance of cold-acclimated plant s. Moreover, we show that it results in multiple biochemical changes associ ated with cold acclimation: CBF3-expressing plants had elevated levels of p roline (Pro) and total soluble sugars, including sucrose, raffinose, glucos e, and fructose. Plants overexpressing CBF3 also had elevated P5CS transcri pt levels suggesting that the increase in Pro levels resulted, at least in part, from increased expression of the key Pro biosynthetic enzyme Delta (1 )-pyrroline-5-carboxylate synthase. These results lead us to propose that C BF3 integrates the activation of multiple components of the cold acclimatio n response.