In plants, transcription factors (TFs) are vital for regulating transcription, as they bind to cis-acting elements of their intended target genes (Gonzalez 2016; Jakoby et al. 2002). Of these, the basic region/leucine zipper (bZIP) TFs have conserved domains and one of the largest gene families(Izawa et al. 1993). These were classified and named based on their conserved DNA-binding domains, which utilize a leucine zipper dimerization motif to bind to DNA (Hurst 1995; Wei et al. 2012). The conserved basic region, which is responsible for sequence-specific DNA binding, comprises about 16 amino acid residues containing a conserved N-x7-R/K motif (Nijhawan et al. 2008). In plants, bZIPs TFs exist mainly in the form homo- and heterodimers (Hurst 1995) and bind preferentially to the A-box (TACGTA), C-box (GACGTC), and G-box (CACGTG), which have core ACGT sequences (Baloglu et al. 2014). With the development of whole genome sequencing technology, bZIP transcription factor family members have been comprehensively identified or predicted in many plants (Liao et al. 2008). In Arabidopsis, the bZIP gene family is divided into 10 groups based on the sequence similarity of their basic regions and the presence of additional conserved motifs (Jakoby et al. 2002). Many studies have identified members of the bZIP family in various species including 75 in Arabidopsis (Agarwal et al. 2019), 125 in maize (Jain et al. 2018), 187 in wheat (Li et al. 2015), 89 in rice (Liu et al. 2014), 92 in barley (Zhong et al. 2021), 55 in grapevine (Wang et al. 2011), and 64 in cucumber (Xiao et al. 2011), but only a small number of genes from this family have been functionally characterized. This transcription factor family is associated with numerous developmental processes in plants (CAO et al. 2012), including but not limited to flower development (Gupta et al. 2012), seed maturation (Li et al. 2018), and responses to both biotic (Lim et al. 2015)and abiotic factors (Wigge et al. 2005).
Bread wheat (Triticum aestivum L.) is an inbreeding crop with distinctive evolutionary limitations, which results in comparatively low genetic diversity. Despite this, it is a significant global source of both carbohydrates and proteins. In 2018, the International Wheat Genome Sequencing Consortium (IWGSC) publicly disclosed the genome sequencing map of Chinese Spring, a six-row malting wheat originating from the United States, marking the first time the genome had been fully sequenced and made available. In 2020, Chinese Spring chromatin accessibility data using MNase are available in open access. In, 2021, the IWGSC RefSeq v2.1 assembly and annotation became available to download, BLAST and display in a browser. In 2022, a new version of RepetDB containing 54 species became available. The major features include 36 new genomes annotated using REPET v3.0, manual annotation tags, and old annotations were updated using PASTEClassifier v2. In previous studies, They identified 187 and 191 TabZIP members using the wheat genome (2018 version) (Agarwal et al. 2019; Li et al. 2015).
Starch is an important carbohydrate that is an essential source of energy for humans. Crop quality is dependent upon the type and characteristics of starch. Starch biosynthesis is a complex process conducted by a series of starch synthetases. There is a well-organized system for starch synthesis in cereal, which includes three gene families including starch synthase, granule bound starch synthase (GBSS), starch synthase (SS), and ADP-glucose pyrophosphorylase (AGPase). While another gene family is starch branching enzymes (SBE) including several isomers (Preiss 2018). First, sucrose is decomposed into fructose and UDP glucose in the endosperm cells by sucrose synthetase. Subsequently, G-1-P(1-phosphoglucose) is produced by UDp-GlC pyrophosphorylase. Thereafter, ADP-Glc pyrophosphorylase (AGPase) forms the direct substrate of starch synthesis, adenosine diphosphate glucose (ADPG). ADPG in the cytoplasm is transported to the powder by the adenylate transporter (BT1) on the powder membrane. Under the action of a series of starch synthetases, amylose and amylopectin are formed successively (Soliman et al. 2014). The synthesis of amylose is completed under the action of GBSS, located inside starch granules, causing the synthesized amylose to remain unbranched (Sonnewald and Kossmann 2013). The synthesis of amylopectin is relatively complex, involving the interaction of three enzymes SS, SBE, and DBE (Guzmán and & Alvarez 2016; Liu et al. 2016b; Preiss 2018). Also the isoamylase 1 (ISA1) is involved in amylopectin production via trimming glucan cluster (Huang et al. 2021).
Several studies have found OsbZIP58(Wang et al. 2013) and OsbZIP76 (Niu et al. 2020) to be involved in starch regulation in rice. Studies have also found the following genes to be onvolved in maize starch synthesis: ZmNAC36, ZmNAC128, ZmNAC130 ZmbZIP22, ZmbZIP91, Opaque 2, ZmEREB156 (Chen et al. 2016; Dong et al. 2019; Huang et al. 2016; Zhang et al. 2019). Moreover, TubZIP28 and TabZIP28 have been found to be transcriptional activators of starch synthesis in wheat (Liu et al. 2016a; Liu et al. 2020; Song et al. 2020). One study showed that SUSIBA2 (sugar signaling in barley) from WRKY TF was identified only in barley whereas no evidence for it was found in rice and maize (Sun et al. 2003). The products of SUS-IBA2 bind to the promoter of ISA1 and influence the starch synthesis by regulating gene expression in barley. Studies have been published, which showed various bZIP family members involved in starch synthesis are already indentified in rice, maize and wheat.
Analysis of the bZIP family is necessary to screen the genes related to starch synthesis. We used recently published assembly of the Chinese Spring wheat genome. Compared to previous studies, the study focused on the phylogenetic relationships, structural features, chromosomal location, and gene expression pattern among bZIP genes. We analyzed the gene expression pattern of the TabZIP family across various developmental stages and tissues, focusing on their co-expression with starch synthase genes. Hence, this study showed new insight into TabZIP genes, which may play essential roles in the production of starch synthesis in wheat.