bZIP TFs involved in regulating ABA signaling pathway
As a ‘emergency hormone’ in plants, ABA is an important signaling molecule in plants. When plants encounter abiotic stress such as salt, drought, or low temperature, they will activate both ABA-dependent and ABA-independent signaling pathways (Bray et al., 1997; Shinozaki et al., 1996; Thomashow et al., 1998; Verslues et al., 2005). Genes involved in the ABA-dependent pathway not only induce ABA biosynthesis, but also regulate the expression of genes containing ABRE elements (Shinozaki et al., 2007; Zhu et al., 2002). The bZIP transcription factor family can bind to ABRE elements and is called ABA response element binding factors (AREBs) or ABRE binding factors (ABF) (Choi et al., 2000; Uno et al., 2000). So far, bZIP transcription factors are proven to participate in ABA-dependent stress signaling in various plants, including Arabidopsis thaliana , rice, soybean, wheat. (Casaretto et al., 2003; Fujita et al., 2005; Kobayashi et al., 2008; Lu et al., 2009).
The A subfamily bZIP transcription factor in Arabidopsis thaliana is a major regulator of ABA-dependent responses (Satoh et al., 2004). AtbZIP1 regulates ABA signal transduction by binding to the ABREs and alters the expressions of the ABA responsive genes to tolerate the cold stress (Sun et al., 2011). In rice, OsbZIP23 and OsbZIP46 can directly target the expression of multiple stress genes through the ABA pathway, thereby significantly improving drought- and salt-resistance of rice (Dey et al., 2016; Tang et al., 2012a; Xiang et al., 2008; Zong et al., 2016). In the transgenic plants over-expressing OsbZIP42 , it showed a rapid rise of transcriptional expression of ABA responsive LEA3 andRab16 and increased tolerance to drought stress (Joo et al., 2019). In soybeans, GmbZIP44, GmbZIP62, and GmbZIP78 can positively regulate the expression of ABI1 and ABI2 genes and further induce the expression of downstream genes such as ERF5 ,KIN1 , COR15A , and COR78 in response to ABA treatment (Liao et al., 2008).
Recent years, bZIPs are also found with increasing contributions in regulating ABA responses in other plants. Overexpression of the ABA-depended grapevine VvABF2 gene could enhance osmotic stress tolerance in Arabidopsis thalianaand thereby reduce the cell membrane damage (Liu et al., 2019b). Wang et al. (2019) found that sweet potato IbABF4 gene, encodes a bZIP transcription factor, overexpression in Arabidopsis thaliana and sweet potato could enhance their tolerance to multiple abiotic stresses through the ABA signaling pathway.
In short, bZIP family members play an important role in the abscisic acid signaling pathway under various stresses. A large number of studies have shown that bZIP transcription factors affect ABA biosynthesis through the ABA-mediated signal transduction pathways and thus improve plant stress resistances.
To reveal the relevance between bZIP subfamilies and stress types, the functional annotated bZIPs were also classified into 13 verified clades followed the approach used by Corrêa et al. (2008) (Table II and Figure I). There is yet not any functional report on bZIPs in subfamilies H, J and L on abiotic stresses. Among the rest 10 subfamilies, there are 8, 7, 6 and 3 of which involved in salinity, drought, cold and osmotic stress, respectively. The bZIPs for regulating salinity tolerance are most frequently found in subgroups A, D, G and S; while for modulating resistances to both drought and osmotic stress are most members in subgroup A; and for controlling cold responses are most those from subgroups A, C and S (Table II).