Fig. 1.

Proposed model of signal-induced cold acclimation. Plants acquire cold acclimation through COR gene-dependent and COR gene-independent responses. Acquisition of COR gene expression is categorized into CBF-dependent and CBF-independent pathways. CBFs have been identified as master transcription factors that regulate the expression of many COR genes, including DEAR1, DREB, ZF, CZF2, ZAT10, and AZF2 whose proteins further regulate many COR genes. Expression of HSFC1, ZAT12, and CZF1 is also rapidly induced by cold stress and is involved in the regulation of COR gene expression. Functional redundancy and likely inter-regulation exist among CBF transcription factors. In turn, CBF expression is controlled by other transcription factors, e.g., ICE1, SOC1, MYB15, and CAMTAs. Upstream events include cold-induced calcium influx, enhanced membrane rigidity, activation of protein kinases, and balanced control between protein activation and degradation. These post-translational mechanisms guarantee rapid activation of the CBF transcriptional pathway during cold acclimation and inactivation of the pathway once COR gene expression has been initiated. AZF2 Arabidopsis zinc-finger protein 2, BES1 brassinosteroid-insensitive 1-EMS-suppressor 1, BZR1 brassinazole-resistant 1, CAM Ca²⁺/calmodulin, CAMTAs calmodulin-binding transcription activators, CBF C-repeat binding factor, CCA1 circadian clock-associated 1, CESTA a bHLH transcription factor, COR cold responsive; CRLK1/2, calcium/calmodulin-regulated receptor-like kinases 1 and 2, CRPK1 cold-responsive protein kinase 1, CZF cold-induced zinc-finger protein 2, DEAR1 DREB and EAR motif protein 1, DREB dehydration-responsive element-binding protein, EIN3 ethylene-insensitive 3, HSFC1 heat-shock factor C 1, ICE1 inducer of CBF expression 1, LHY late elongated hypocotyl, MEKK mitogen-activated protein kinase kinase kinase, MKK mitogen-activated protein kinase kinase, MPK mitogen-activated protein kinase, MYB15 MYB transcription factor 15, PIF3/4/7 phytochrome-interacting factor 3, 4 and 7, SOC1 suppressor of constans overexpression 1, ZAT zinc finger of Arabidopsis