Figure 2. Signaling pathways involved in the crosstalk between light and cold acclimation.
Upon cold temperatures, the circadian clock-regulated CBF genes are induced resulting in cold acclimation. Upon light perception, HY5 is activated which induces the expression of anthocyanin biosynthesis and cold-responsive genes through the Z-box/LTRE thereby reducing reactive oxygen species (ROS) and results in cold acclimation. Changes in light quality (R:FR) are sensed by phyB which exists in an active PfrB and an inactive PrB form. Under high R:FR ratios (> 1), PfrB represses CBF gene expression, while low R:FR ratios (< 1) caused by e.g. increased twilight during autumn causes cold acclimation by decreasing the amount of active PfrB. PIF7 which represses CBF gene expression is under control of TOC1, a central component of the circadian clock, and under the control of phyB. Under SD photoperiod, CBF genes are strongly induced causing cold acclimation. Under warmer LD, PIF4 and PIF7 which are under the control of phyB are higher expressed resulting in an inhibition of CBF gene expression. As days shorten, e.g. during autumn, this repression falls away resulting in cold acclimation. CBF proteins interact with phyB and PIF3 causing degradation of PIF1, 4 and 5 which releases COR genes from PIF repression. For more information concerning the different pathways, please refer to section 3.1. Abbreviations: R, red light; FR, far-red light, SD, short day; LD, long day; ROS, reactive oxygen species; LTRE, low temperature responsive element.