Figure 4.

Outline of the integration of environmental stimuli that putatively influence the action of CBF genes. White boxes—environmental stimuli that influence (induce or repress) CBF genes; colored boxes—the cascade or putative cascade pathways activated by CBF genes; black arrows—induction or up-regulation of gene expression; black lines with blunt ends—repression of gene expression; ?—hypothetical interactions still not proven in Triticeae crops. Letters in bold—stimuli that influence the CBF expression. (A)—Hypothetical model for the phytochrome-dependent CBF14 regulation in barley and wheat suggested by Novák et al. [155]. Under white light illumination, HvCBF14 is repressed by the active form of Phytochrome B (phyB), and the expression of Phytochrome A (PHYA) is inhibited by light and by phyB. Under supplemental FR illumination (low R:FR ratio), most of the phyB proteins are in inactive form and HvCBF14 or PHYA are released under repression. The active form of phyA may then enhance the expression of HvCBF14. The original model was proposed by Wang et al. [233] for tomato plants. (B)—In Arabidopsis, Phytochrome-interacting factors PIF4 and PIF7 are involved in the photoperiodic regulation of AtCBF2. In long-day conditions, PIF4 and PIF7 down-regulate the CBF pathway [150]. (C)—Circadian-clock-associated1 (CCA1) and late elongated hypocotyl (LHY) are positive regulators of CBF expression and plant freezing tolerance [148]. The CBF expression peak was observed before the sunset [149]. No circadian-clock-component was found in the promoter region, suggesting an intermediate role by other genes [117]. (D)—Inducers of CBF-expression ICE and Calmodulin-binding transcription activators CAMTA transcription factors are the CBF genes expression promoter under the cold stimuli [81,147]. (E)—Concerning the drought, in barley, it has been shown that HvCBF1- and HvCBF3-subgroup proteins bind the DRE/CRT motif promoter either at warm or low temperatures, and several drought responsive genes related to DRE/CRT motif are present in their promoter regions [148]. Nevertheless, which CBF genes and which pathway are still unknown. In colored rectangles, the CBF’s cascade pathways. Red rectangles—the role of the CBF genes in the drought tolerance is still unclear; however, it might have a putative connection with the ABA pathway that induces root modification [94,234]. Green rectangles—CBF genes have a role in connecting plant hormones like gibberellic acid and DELLA proteins, increasing freezing tolerance and showing stunted growth and delayed flowering [235,236]. This mechanism was established in Arabidopsis (for a recent review, see [237]); however, there have been no reports for Triticeae. Yellow rectangle—for the vernalization, VRN-1 can bind CBF gene promoters, reducing their transcription [72]; however in the first phase of cold acclimation, the CBF protein may have a putative role to down-regulate VRN-1.