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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

Involvement of CBF Transcription Factors in Winter Hardiness in Birch^1,[W],[OA]

Viikki Biocenter, Department of Biological and Environmental Sciences, Division of Plant Biology (A.W.), and Viikki Biocenter, Department of Biological and Environmental Sciences, Division of Genetics (E.T.P.), University of Helsinki, Helsinki FIN–00014, Finland

Cold acclimation of plants involves extensive reprogramming of gene expression. In Arabidopsis (Arabidopsis thaliana), three cold-inducible transcriptional activators designated CBF1 to -3/DREB1a to -c have been shown to play an important regulatory role in this acclimation process. Similarly to Arabidopsis, boreal zone trees can increase their freezing tolerance (FT) in response to low temperature during the growing season. However, maximal FT of these trees requires short daylength-induced dormancy development followed by exposure to both low and freezing temperatures. To elucidate the molecular basis of FT in overwintering trees, we characterized the role of birch (Betula pendula) CBF transcription factors in the cold acclimation process. We identified four putative CBF orthologs in a birch expressed sequence tag collection designated BpCBF1 to -4. Ectopic expression of birch CBFs in Arabidopsis resulted in constitutive expression of endogenous CBF target genes and increased FT of nonacclimated transgenic plants. In addition, these plants showed stunted growth and delayed flowering, typical features for CBF-overexpressing plants. Expression analysis in birch showed that BpCBF1 to -4 are low temperature responsive but differentially regulated in dormant and growing plants, the expression being delayed in dormant tissues. Freeze-thaw treatment, simulating wintertime conditions in nature, resulted in strong induction of BpCBF genes during thawing, followed by induction of a CBF target gene, BpLTI36. These results suggest that in addition to their role in cold acclimation during the growing season, birch CBFs appear to contribute to control of winter hardiness in birch.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: E. Tapio Palva (tapio.palva{at}helsinki.fi).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.117812

^* Corresponding author; e-mail tapio.palva{at}helsinki.fi.

Received February 17, 2008; accepted April 29, 2008; published May 8, 2008.