Figure 7.

Model of the Cold Stress Response in Arabidopsis during the Day and Night.

Plants recognize cold stress as two different signals and induce the three DREB1 genes encoding main switches of the cold stress response. The DREB1 genes are tandemly arrayed in the Arabidopsis genome in the order of DREB1B, DREB1A, and DREB1C. One of the signals is caused by a rapid drop in temperature. In this signaling pathway, CAMTA3 and CAMTA5 activate expression of DREB1B and DREB1C. The other signal is caused by low temperatures in general and includes both rapid and slow decreases in temperature. This signal activates expression of DREB1A and DREB1C strongly and that of DREB1B weakly. Activation of this signaling pathway may be controlled by the central oscillator of the circadian clock, involving CCA1 and LHY. During the day, both signaling systems efficiently function in response to slow and rapid temperature decreases. During the evening, only the signaling system involving CAMTAs is activated in response to a rapid temperature decrease. The arrows indicate positive regulatory pathways, and the question marks denote factors that remain to be confirmed.