Fig. 6.

Schematic representation of epigenetic changes at low temperature. Increased levels of ROS and antioxidants under low temperature are associated with chromatin structure which triggers DNA and histone modifications, thereby activating COR genes. For example, in normal conditions, the HOS15 and HD2C complex represses COR gene expression due to hypoacetylation of H3 on COR chromatin. Under cold conditions, histone deacetylase 2C (HD2C) degradation triggers the hyperacetylation of H3 on COR chromatin and, thus, activates COR genes through CBF TFs. Using the high expression of the osmotically responsive gene 15 (HOS15) as a substrate receptor, the CULLIN4-based ubiquitin E3 ligase complex (CUL4) promotes HD2C degradation. The association of HOS15 with COR gene chromatin and HD2C degradation requires Powerdress (PWR), a HOS15-interacting protein. The HOS15/PWR complex and CBF TFs bind to the promoters of COR genes [81], [131]. Likewise, chromatin changes also activate miRNAs that influence COR gene expression through post-transcriptional modification. Cold stress also stimulates calcium influx which triggers protein kinases to activate ICE1. Activated ICE1 suppresses MYB15 and activates the expression of CBFs, thus regulating COR genes. A small star circle represents post-transcriptional modification, such as phosphorylation. ROS, reactive oxygen species; COR, cold-responsive genes; miRNAs, microRNAs; ICE1, an inducer of CBF expression 1; CBF, C-repeat binding factor; CRT, C-repeat elements; DRE, dehydration-responsive elements; ABRE, ABA-responsive element; KIN, cold-induced genes [125].