REGULATION OF BZIPS ON METABOLISM OF FLAVONOIDS
Recently, a plenty of flavonoids show significant contributions to plant tolerances to abiotic stresses (Agati et al., 2012; Pi et al., 2016, 2018, 2019; Yamasaki et al., 1997; Yan, et al., 2014). Interestingly, many bZIP transcription factors usually play a key regulatory roles in the process of flavonoid biosynthesis. They regulate the expression of key enzyme genes in the synthetic pathway, thereby regulating the metabolism and synthesis of flavonoids.
Matousek et al. (2010) found that both hop HlbZIP1 and HlbZIP2 could activate the expression of chalcone synthase chs_H1 and the O-methyl transferase 1 genes to regulate the accumulation of flavonoid glycosides and anthocyanins. Akagi et al. (2012) found that ectopic DkbZIP5 overexpression in persimmon calluses could induced the up-regulation of DkMyb4 and then affect the seasonal biosynthesis of proanthocyanidins in persimmon fruit. Malacarne et al. (2016) showed that VvibZIPC22, a member of clade C of the grapevine bZIP family, was able to activate the transcriptional expression of specific genes of the flavonoid pathway includingVviCHS3 , VviCHI , VviFLS1 and VviANR , alone or together with other factors to participate in the biosynthesis of flavonols during flowering and UV light-mediated induction. Dash et al. (2017) found that the poplar PatbZIP1 transcription factor regulated the expression of two flavonol synthase genes PtaFLS2 andPtaFLS4 in the flavonoid synthesis pathway to promote the synthesis of related flavonoids and thus promotes the lateral root formation. bZIP transcription factor HY5 plays a multifaceted role in plant growth and development. Apple MdHY5 gene, induced by light and abscisic acid treatments, promoted anthocyanin accumulation by regulating expression of the MdMYB10 gene and downstream anthocyanin biosynthesis genes (An et al., 2017a). Zhang et al. (2011b) found that the protein levels of two bZIP transcription factors AtbZIP56/HY5 and AtbZIP64/HYH in Arabidopsis thaliana induced the accumulation of anthocyanins under low temperature induction. In addition, ABA can induce the expression of Artemisia annuaAabZIP1 to activate the expression of downstream gene ADSand CYP71AV1 , thereby regulating the biosynthesis of artemisinin (Zhang et al., 2015a). Fan et al. (2019) showed that the expression ofRsbZIP011 and RsbZIP102 was significantly up-regulated in radish tissue with higher anthocyanin content under heat and salt stress.
So far, the bZIPs that involve in flavonoid synthesis varies from plant species and their target genes (coding for different enzymes in flavonoid metabolism). To uncover the relationship between bZIP subfamilies and flavonoid synthesis, all the functional annotated bZIPs were also categorized into the 13 known subgroups according to CorrĂȘa et al. (2008) (Table III and Figure I). It seems that only bZIPs in subfamilies A, H and S could regulate flavonoid metabolism.