Molecular Mechanisms of Zinc Deficiency Sensing and Regulation in Arabidopsis thaliana

Research output: Book/ReportPh.D. thesisResearch

Zn is an essential micronutrient for all forms of life because it plays key catalytic and structural roles in a wide variety of proteins. However, Zn deficiency is widespread in agricultural soils globally, adversely affecting yield and nutritional quality of crops. Zn is, therefore, of major interest in agronomy and in human and animal nutrition. Hence, understanding how plants sense and adapt to changes in Zn availability is crucial in order to develop Zn-efficient and biofortified crops. In Arabidopsis, two members of the basic-leucine zipper (bZIP) family of dimerizing transcription factors (TFs), bZIP19 and bZIP23, have been identified as essential regulators of the response to Zn deficiency. Under Zn deficient conditions, the bZIP19 and bZIP23 TFs bind to a unique cis element, the Zinc-deficiency response element (ZDRE), on the promoters of Zn transporter and chelator genes activating their expression, leading to enhanced plant Zn uptake and distribution. Moreover, bZIP19 and bZIP23, which are partially redundant, act in a Zn-dependent manner. However, the mechanism behind this functional partial redundancy and the mode of action by which cellular Zn modulates the activity of these TFs is still unknown. Interestingly, bZIP19 and bZIP23, together with a third member, bZIP24, belong to the F subfamily of Arabidopsis bZIPs (hereafter referred to as F-bZIPs), and they all possess a characteristic His/Cys-rich motif. Because His and Cys-rich domains are known to bind to divalent metal ions, it is hypothesized that the bZIP19 and bZIP23 TFs, via the His/Cys-rich motif, may directly bind to and sense cellular Zn. The main aim of this PhD project was, therefore, to investigate the molecular mechanisms of Zn deficiency sensing and regulation in Arabidopsis thaliana. In this context, a detailed functional characterization of the bZIP19 and bZIP23 TFs was performed, including analysis of their tissue specific expression patterns, transcript levels and subcellular localization in response to varying Zn supply. In addition, the functional significance of the conserved His/Cys-rich motif in the mode of action of the bZIP19/23 activity and their Zn deficiency sensing was thoroughly investigated. Finally, the functional conservation of the Zn deficiency response regulatory network was explored in a translational approach to Rice. A phylogenetic analysis of F-bZIP homologs indicated that F-bZIPs and the Zn deficiency response regulatory network are conserved across land plants. In addition, the His/Cys-rich motif, which is characteristic to F-bZIPs, is also conserved across all F-bZIP members. Functional characterization of all three Arabidopsis F-bZIP members, using complementation lines, showed that the activity of bZIP19 and bZIP23 is post-translationally modulated by Zn at the protein level, in the nucleus, where cellular Zn sufficiency represses their activity. In addition, it showed that the third F-bZIP member, bZIP24, does not play a major role in the Zn deficiency response. The analysis of tissue expression patterns, using promoter-GUS reporter lines, showed that bZIP19 and bZIP23 have differential, somewhat non-overlapping, tissue-specific expression patterns, which might explain their functional partial redundancy. Furthermore, detailed in vitro and in vivo analysis showed that the His/Cys-rich motif, which is a conserved characteristic motif of F-bZIPs, is required for direct binding to Zn and that it acts as a Zn sensor in planta, revealing the molecular mechanisms of how plants sense Zn. Finally, translational work in Rice showed that Rice F-bZIP homologues regulate the Zn deficiency response in a similar way to the Arabidopsis F-bZIPs providing functional evidence that the Zn deficiency response regulatory mechanism is conserved across land plants. Overall, these results reveal the molecular mechanisms and role of F-bZIPs in Zn deficiency sensing and regulation providing potential targets for improving Zn-use efficiency and biofortification of crops.
Original languageEnglish
PublisherDepartment of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen
Publication statusPublished - 2019

ID: 223566744