Fig. 1 Protein ubiquitination regulates the CBF-dependent low temperature signaling pathway CBFs transcription factors are responsible for the regulation of COR genes containing CRT/DRE(CCGAC)motifs in their promoters. CBFs are activated by ICE1 and CAMTA transcription factors,whereas repressed by MYB15. HOS1 and SIZ1 encode RING E3 ligase and SUMO E3 ligase,respectively,which antagonistically regulate the abundance of ICE1 protein. OST1 is a kinase that positively regulates the abundance of the ICE1 protein. Crooked arrows indicate post-translational regulation,and solid arrows indicate activation. T-shaped bars represent repression;the solid line represent direct interaction. The same below

Fig. 2 Schematic diagram depicts the involvement of E3s in regulation of freezing stress tolerance by the ethylene(ET)pathway in Arabidopsis In the biosynthesis of ethylene,the ETO1,EOL1 and EOL2 modulate ET biosynthesis by interacting with the ET biosynthetic enzyme ACS5 and inhibiting its activity by proteasome-dependent degradation. In the signal transductions of ethylene,in the absence of ethylene,the highly phosphorylated ethylene receptors activate CTR1 kinase activity,which in turn phosphorylates EIN2,likely causing the degradation of EIN2 by F-box proteins EPT1 and EPT2. Meanwhile,EIN3/EILs are also subjected to proteasomal degradation mediated by F-box proteins EBF1 and EBF2. In the presence of ethylene,ethylene binding inactivates the receptors by suppressing its phosphorylation,which consequently leads to deactivation of CTR1. The un-phosphorylated EIN2 is thus cleaved and its C-terminal domain is translocated into the nucleus,resulting in activation of EIN3/EILs and downstream transcriptional cascades. Finally,they regulate the freezing stress tolerance response