bZIP transcription factors are involved in varied biological processes of plant development, biotic and abiotic stress response and environmental signaling. The characteristic feature of bZIP transcription factors are 1)~16 amino acid basic nuclear localization signal region followed by unvarying N-X7-R/K DNA binding motif. Plant bZIPs DNA binding motif recognizes ACGT core in DNA sequences, which can be a part of A-box (TACGTA), C-box (GACGTC) and G-box (CACGTG), thus binding DNA major groove and 2) leucine zipper consisting of heptad repeats of leucine or other hydrophobic amino acids like valine, isoleucine, methionine, or phenylalanine present 9 amino acids exactly at the C- terminus, forming amphipathic helix. Also, leucine zipper enables bZIP transcription factors dimerization, therefore enhancing DNA binding and TF-TF interaction flexibility (Jakoby et al., 2002, Dröge-Laser et al., 2018).

Segmental duplication has resulted in Maize bZIP family expansion. Maize consists of 125 bZIP members distributed unevenly across 10 chromosomes. In a recent study, 54 maize bZIPs from available drought and rewatering transcriptome were divided into 9 groups (A-S) based on intron/exon and motif analysis, where group A bZIPs play role in ABA response and C, D, S members are induced by other stresses to varying degrees. Gene co-expression network analysis revealed core genes (more connections with other genes) ZmbZIP33, 4, 35, 5, 54, 29, 20. However, bZIP17, 33, 42 and 45 encoding nuclear proteins were strongly induced in response to ABA, PEG, NaCl and high temperature stress (Cao et al., 2019).

A common ZmbZIP60 was identified as a positive regulator linking unfolded protein response in ER and heat stress response in cytoplasm, where ZmbZIP60 induced by heat stress in ER activates heat shock response gene ZmHSFTF13 in cytoplasm which in turn activates other HSF genes in response to elevated temperature(Li et al., 2020). Based on chromatin immunoprecipitation and luciferase assay, ZmbZIP4 was found to be a positive regulator targeting genes in various processes like stress response, abscisic acid synthesis, root development (Ma et al., 2018). Three important traits of maize endosperm- nutrition, calories, and yield are regulated by endosperm specific maize TFs Opaque2 (O2) and Prolamin Binding Factor (PBF) belonging to bZIP and DOF family of TFs respectively, regulating storage zein protein genes and starch biosynthesis genes. O2 and PBF mutant improves maize nutritional quality by reducing the zein proteins and increasing the non-zein proteins and reduces enhanced expression of starch biosynthetic enzymes either directly or indirectly, therefore lowering starch synthesis and in turn kernel yield (Zhang et al., 2016). An additional endosperm specific ZmbZIP22 TF transcriptionally coregulates 27 kD g-zein gene with previously known endosperm specific TFs O2, PBF1 and OHPs determined by dual luciferase transient transcriptional activity assay. DEGs identified from zmbzip22-mu9 mutant belonged to various biological processes like nutrient reservoir, apoplast, plastid and stress response (Li et al., 2018).

In other grasses like rice, overexpression of OsbZIP62 resulted in increased drought and oxidative stress tolerance caused due to increased expression of stress related genes. Also, since interaction between OsbZIP62 and stress/ABA dependent protein kinases was confirmed by yeast two hybrid assay, it is possible that bZIPs are activated by phosphorylation by protein kinases. The post translation modification of bZIP by kinases on reception of ABA signal results in binding of the activated TF to their cis-acting region thus regulating downstream targets (Yang et al., 2019, Banerjee et al., 2017)

Last updated June 2023 by Ankita Abnave

References:

Jakoby M, Weisshaar B, Dröge-Laser W, Vicente-Carbajosa J, Tiedemann J, Kroj T, Parcy F; bZIP Research Group. bZIP transcription factors in Arabidopsis. Trends Plant Sci. 2002 Mar;7(3):106-11. doi: 10.1016/s1360-1385(01)02223-3. PMID: 11906833.

Dröge-Laser W, Snoek BL, Snel B, Weiste C. The Arabidopsis bZIP transcription factor family-an update. Curr Opin Plant Biol. 2018 Oct;45(Pt A):36-49. doi: 10.1016/j.pbi.2018.05.001. Epub 2018 Jun 1. PMID: 29860175.

Li Z, Tang J, Srivastava R, Bassham DC, Howell SH. The Transcription Factor bZIP60 Links the Unfolded Protein Response to the Heat Stress Response in Maize. Plant Cell. 2020 Nov;32(11):3559-3575. doi: 10.1105/tpc.20.00260. Epub 2020 Aug 25. PMID: 32843434; PMCID: PMC7610289.

Ma H, Liu C, Li Z, Ran Q, Xie G, Wang B, Fang S, Chu J, Zhang J. ZmbZIP4 Contributes to Stress Resistance in Maize by Regulating ABA Synthesis and Root Development. Plant Physiol. 2018 Oct;178(2):753-770. doi: 10.1104/pp.18.00436. Epub 2018 Aug 20. PMID: 30126870; PMCID: PMC6181033.

Zhang Z, Zheng X, Yang J, Messing J, Wu Y. Maize endosperm-specific transcription factors O2 and PBF network the regulation of protein and starch synthesis. Proc Natl Acad Sci U S A. 2016 Sep 27;113(39):10842-7. doi: 10.1073/pnas.1613721113. Epub 2016 Sep 12. PMID: 27621432; PMCID: PMC5047157.

Li C, Yue Y, Chen H, Qi W, Song R. The ZmbZIP22 Transcription Factor Regulates 27-kD γ-Zein Gene Transcription during Maize Endosperm Development. Plant Cell. 2018 Oct;30(10):2402-2424. doi: 10.1105/tpc.18.00422. Epub 2018 Sep 21. PMID: 30242039; PMCID: PMC6241260.

Yang S, Xu K, Chen S, Li T, Xia H, Chen L, Liu H, Luo L. A stress-responsive bZIP transcription factor OsbZIP62 improves drought and oxidative tolerance in rice. BMC Plant Biol. 2019 Jun 17;19(1):260. doi: 10.1186/s12870-019-1872-1. PMID: 31208338; PMCID: PMC6580479.

Banerjee A, Roychoudhury A. Abscisic-acid-dependent basic leucine zipper (bZIP) transcription factors in plant abiotic stress. Protoplasma. 2017 Jan;254(1):3-16. doi: 10.1007/s00709-015-0920-4. Epub 2015 Dec 15. PMID: 26669319.

Cao L, Lu X, Zhang P, Wang G, Wei L, Wang T. Systematic Analysis of Differentially Expressed Maize ZmbZIP Genes between Drought and Rewatering Transcriptome Reveals bZIP Family Members Involved in Abiotic Stress Responses. Int J Mol Sci. 2019 Aug 22;20(17):4103. doi: 10.3390/ijms20174103. PMID: 31443483; PMCID: PMC6747360.