REGULATION MECHANISM OF PLANT BZIPS TO VARIOUS STRESSES
Previous studies have found that bZIPs play important roles in response to a variety of plant stresses, such as salinity, drought and cold damage (Table II). Their regulation mechanism varies depending on species of plant and types of stress.
bZIP TFs involved in salt stress response
Under salt stress, plant cell should successively face challenges of osmotic stress, ion toxicity and oxidative stress (Rozema et al., 2008; Munns et al., 2005). In these responses, bZIP transcription factors play key roles in various physiological processes in Arabidopsis thaliana , tomato, tobacco, rice, and soybeans, etc.
In Arabidopsis thaliana , AtbZIP17 was proven as a positive regulator in the processes salt stress responses, it activates both the expression of salt stress response gene ATHB-7 and SES1(Liu et al., 2007, 2008); while the AtbZIP24 was revealed as a negative regulator in plant tolerance to salinity by RNAi interference technology (Yang et al., 2009). Tang et al. (2012b) found that heterologously expressing Arabidopsis thaliana AtbZIP60 could increase salt resistance and superoxide dismutase (SOD) activity of tobacco, rice, and Pinus elliottii . In Glycine max , overexpression of the GmbZIP1 enhances salt tolerance in transgenic plants (Gao et al., 2011). Besides, heterologously expressing GmbZIP44 ,GmbZIP62 , and GmbZIP78 could significantly increase salt resistance of transgenic Arabidopsis thaliana plants (Wang et al., 2015). In maize, the ABP9 was found as a salinity responsible bZIP gene by Zhang et al. (2011a). Then, Wang et al. (2017a) heterologously expressed it to improve the salt tolerance of transgenic cotton. In Oryza sativa , the OsbZIP05/OSBZ8 firstly found with a higher transcriptional level in salt tolerant cultivar than in salt sensitive cultivar, indicate that OsbZIP05/OSBZ8 might play as a positive role in this stress responses (Mukherjee et al., 2006). After that, OsbZIP12/OsABF1, OsbZIP23, OsbZIP46/OsABF2, OsBZIP71 and OsbZIP72 were successively proven to act as positive regulators in the process of salt tolerance (Amir Hossain et al., 2010; Chang et al., 2017; Hossain et al., 2010; Liu et al., 2014a; Lu et al., 2009; Tang et al. 2012a; Xiang et al., 2008; Zhang et al., 2017a). OsbZIP71 can form both homodimers and heterodimers with Group C members of the bZIP gene family, and overexpression of OsbZIP71 can significantly enhance the salt tolerance of transgenic rice (Liu et al., 2014a). On the contrary, the plants overexpressing OsbZIP10/OsABI5 showed more obvious chlorosis than wild type under high salt concentration, indicating that OsbZIP10/OsABI5 participates in the salt stress tolerance response of rice as a negative regulator, and gene silencing can reduce the sensitivity of transgenic rice to salt (Zou et al., 2008).
Recent years, bZIPs in other plants have also been revealed to participate salinity responsive processes. Cheng et al. (2013) isolated a salt responsive transcriptional factor LrbZIP in lotus root and found that transgenic lotus with LrbZIP overexpression could grow with normal root biomass, chlorophyll content, and electrolyte exudation rate under NaCl treatment. Zhao et al. (2016) revealed that Brassica napus bZIP transcription factor BnaABF2 enhanced salt tolerance of plants through the ABA pathway.
To sum up, many bZIP genes have been excavated in different plants and confirmed that they can significantly enhance the salt tolerance of plants, making the bZIP gene family a gene treasure house for improving the salt tolerance of crops. Therefore, the use of bZIP transcription factors to improve the salt tolerance of crops and breed new salt-tolerant varieties is of great significance for improving agricultural productivity and improving saline soils.