Integration of cold tolerance and growth retardation in plants conferred by CbCBF from Capsella bursa-pastoris
Mingqi Zhou, Lihua Wu, Chen Shen, Ming Xu, Juan Lin* 

State Key Laboratory of Genetic Engineering, Institute of Plant Biology, School of Life Sciences, Fudan University, Shanghai 200433, People’s Republic of China。
^*Corresponding author: Tel: +86-21-65642949; E-mail: linjuan@fudan.edu.cn

Abstract
Plant cells respond to cold stress in a regulatory mechanism leading to enhanced cold acclimation accompanied by growth retardation. The C-repeat binding factor (CBF) signaling pathway plays a key role in cold response of higher plants. Our previous report documented a novel CBF-like gene named CbCBF (GenBank accession no. AY391121) from Capsella bursa-pastoris, which was responsive to chilling temperature. Here, we show that CbCBF protein subcellularly localizes to the nucleus. Analysis of expression pattern and characterization of promoter reveal that CbCBF is obviously responsive to chilling, freezing, ABA, GA, IAA and MeJA treatments. Compared with control plants, transgenic tobacco lines overexpressing CbCBF exhibit both enhanced chilling and freezing tolerance and elevated expression levels of endogenous cold induced genes. Meanwhile, growth retardations in CbCBF transformants including dwarf phenotype and delayed flowering are observed. Microscopy and flow cytometry analysis show that leaf cells of CbCBF overexpression lines undergo delayed cycles in G1/S stage and attain smaller sizes, in which the transcription levels of some related cyclin genes are altered. In addition, the dwarf phenotypes of CbCBF transformants can be partially restored by exogenous GA treatments. Accordingly, CbCBF overexpression reduces the endogenous bioactive gibberellin and auxin contents and disturbs the expression levels of gibberellin metabolic related genes in tobacco leaf cells. We conclude that expression of CbCBF is induced by cold and phytohormone, which causes increased cold resistance and growth suppression via activating cold responsive function genes and affecting endogenous gibberellin biosynthesis and cell cycle pathway in plant cells.

Key Words: Capsella bursa-pastoris, cell cycle, cold tolerance, gibberellin, growth retardation, transgenic tobacco.