Involvement of CBF in the fine-tuning of litchi flowering time and cold and drought stresses.

Litchi () is an economically important fruit tree in southern China and is widely cultivated in subtropical regions. However, irregular flowering attributed to inadequate floral induction leads to a seriously fluctuating bearing. Litchi floral initiation is largely determined by cold temperatures, whereas the underlying molecular mechanisms have yet to be identified. In this study, we identified four () homologs in litchi, of which , and showed a decrease in response to the floral inductive cold. A similar expression pattern was observed for the homolog () in litchi. Furthermore, both LcCBF2 and LcCBF3 were found to bind to the promoter of to activate its expression, as indicated by the analysis of yeast-one-hybrid (Y1H), electrophoretic mobility shift assays (EMSA), and dual luciferase complementation assays. Ectopic overexpression of and in caused delayed flowering and increased freezing and drought tolerance, whereas overexpression of in had no significant effect on flowering time. Taken together, we identified LcCBF2 and LcCBF3 as upstream activators of and proposed the contribution of the cold-responsive CBF to the fine-tuning of flowering time.