Is cold-induced nuclear import of CBF4 regulating freezing tolerance?
C-repeat binding factors (CBFs) are crucial transcriptional activators in plant responses to low temperature. CBF4 differs by a slower, but more persistent regulation and its role in cold acclimation. Cold acclimation is of accentuated relevance for the tolerance to late spring frosts as they become progressively common as consequence of blurred seasonality in the context of global climate change. In the current study, we explore the functions of CBF4 from grapevine, VvCBF4. Overexpression of VvCBF4 fused to GFP in tobacco BY-2 cells confers cold tolerance. Furthermore, this protein shuttles from the cytoplasm to the nucleus in response to cold stress, associated with accumulation of transcripts for other CBFs and the cold responsive gene ERD10d. This response differs for chilling as compared to freezing and is regulated differently by upstream signalling involving oxidative burst, proteasome activity and jasmonate synthesis. This difference between chilling and freezing is also seen in the regulation of CBF4 transcripts in leaves from different grapevines differing in their cold tolerance. We propose the quality of cold stress is transduced by different upstream signals inducing nuclear import to regulate other CBF factor and activate COR genes.