bZIP TFs involved in cold stress response
Low temperature stimulation will disturb the normal physiological and metabolic activities and further affect the plant growth and development. The plant mainly responds to low temperature stress through the ICE-CBF-COR pathway. When the plant is exposed to low temperature, it induces CBFs (C-repeat-binding Factors) expression by ICE (inducer of CBF expression), which recognizes CRT/DRE (C-repeat/dehydration responsive element) located on the target gene promoter and combines with COR (cold regulated genes) genes to regulate transcriptional expression, thereby enhancing plant cold stress resistance (Shi et al., 2018). bZIP transcription factors also play an indispensable role in regulating plant cold stress responses.
According to incomplete statistics, the first rice bZIP-like transcription factor identified and reported was OsbZIP38/LIP19 of the H subfamily. Induced by low temperature, as a Fos-like molecular switch, it is involved in the plant’s response to cold signal pathways (Aguan et al., 1991; Aguan et al., 1993; Shimizu et al., 2005). OsbZIP38/LIP19 and subfamily member OsbZIP87/OsOBF1 are able to bind DNA and form homodimers, and they are more likely to interact with OsbZIP38/LIP19 and form heterodimers to participate in the plant’s response to cold signaling (Shimizu et al., 2005). In addition, OsbZIP52/RISBZ5, OsbZIP68/ROS-bZIP1 and OsbZIP73/OsTFX1 in the rice bZIP transcription factor family were also involved in cold resistance. OsbZIP52/RISBZ5 is a member of the G subfamily. It is not induced by drought, salt, PEG, and ABA, but by low temperature. It can form homodimers and specifically bind G-box. However, the test of the cold tolerance of rice showed that the survival rate of over-expressed OsbZIP52/RISBZ5 rice plants was significantly lower than that of wild type. It can be seen that the expression of OsbZIP52/RISBZ5 is inversely related to low temperature tolerance (Liu et al., 2012). Cheng et al. (2007) found that OsbZIP68/ROS-bZIP1 could be induced and responded quickly within 24 hours when rice was treated at 10 °C. Liu et al. (2018) identified a total of 8 bZIP genes in rice, OsbZIP08 , OsbZIP35 ,OsbZIP38 , OsbZIP46 , OsbZIP63 , OsbZIP72 ,OsbZIP73 and OsbZIP76 , which are associated with low temperature resistance at seedling stage. In addition, Liu et al. (2018, 2019a) also found and revealed the molecular mechanism ofOsbZIP73/OsTFX1 adapting to the cold climate in northern China by comparing the whole genome sequences of Japonica and Indica rice.
Except for rice, carrot, soybean, wheat, tomato and other crops have been successively excavated bZIP transcription factors in response to low temperature stress. For example, Ito et al. (1999) found that the expression of bZIP-like protein Lip (Low temperature-Induced protein) in the roots of radish was up-regulated under low temperature treatment, thereby enhancing the cold resistance of radish. Soybeans GmbZIP44, GmbZIP62 and GmbZIP78 can regulate and promote the synthesis of proline (plant cold tolerance osmotic regulator) to enhance the tolerance of plants to cold stress. At the same time, GmbZIP44, GmbZIP62, and GmbZIP78 can enhance their ability to respond to cold damage and high salt stress by activating the expression of their downstream genes ERF5 , KIN1 ,CORl5A , and COR78 (Liao et al., 2008). Hwang et al. (2014) treated Brassica rapa with low temperature stress and found that the expression of 27 BrbZIPs were significantly up-regulated, among which Bra000256 , Bra003320 , Bra004689 ,Bra011648 , Bra020735 and Bra023540 may be the key genes involved in the response to low temperature stress. Compared with wild-type Arabidopsis thaliana , heterologous expression ofTabZIP6 in wheat under cold treatment significantly reduced the expression of CBFs , key CORs and other genes in transgenic plants, making the transgenic plants sensitive to low temperature (Cai et al., 2018). However, the over-expressed wheat TabZIP14-B ,TaAREB3 and TabZIP60 in Arabidopsis thaliana can significantly enhance the ability of plants to resist cold stress, and the expression of corresponding stress-responsive genes in transgenic plants was significantly up-regulated. In addition, transgenic plants are more sensitive to ABA than wild type, indicating that TabZIP14-B , TaAREB3 , and TabZIP60 all enhance the cold resistance of plants through the ABA pathway (Wang et al., 2016a; Zhang et al., 2015b; Zhang et al., 2017b). Xu et al. (2014) found that over-expression of wheat bZIP transcription factor TaABL 1 (ABI-like) elevated cold tolerance in wheat. Apple bZIP transcription factor MdHY5 can respond to low temperature stress at both the transcriptional and protein levels. Overexpression of MdHY5 can significantly enhance cold stress resistance in apple callus and transgenic Arabidopsis thaliana . EMSA results indicate that MdHY5 can bind to G-Box on theMdCBF1 promoter, thereby increasing its transcription level and regulating the expression of COR genes independent of CBF (An et al., 2017b). Wang et al. (2017b) found that transgenic Arabidopsis thaliana plants showed reduced survival, increased electrical conductivity, increased malondialdehyde content, and reduced soluble sugar content when overexpressed Camellia sinensis CsbZIP6in it. Transcriptome analysis found that the expression of low-temperature and drought-responsive genes in over-expressed plants was significantly lower than that of wild type, indicating thatCsbZIP6 plays a negative regulatory role in low-temperature stress response.