Regulation of metal homoeostasis by two F-group bZIP transcription factors bZIP48 and bZIP50 in rice

Zinc (Zn) deficiency not only impairs plant growth and development but also has negative effects on human health. Rice (Oryza Sativa L.) is a staple food for over half of the global population, yet the regulation of Zn deficiency response in rice remains largely unknown. In this study, we provide evidence that two F-group bZIP transcription factors, OsbZIP48/50, play a crucial role in Zn deficiency response. Mutations in OsbZIP48/50 result in impaired growth and reduced Zn/Fe/Cu content under Zn deficiency conditions. The N-terminus of OsbZIP48/OsbZIP50 contains two Zn sensor motifs (ZSMs), deletion or mutation of these ZSMs leads to increased nuclear localization. Both OsbZIP48 and OsbZIP50 exhibit transcriptional activation activity, and the upregulation of 1117 genes involved in metal uptake and other processes by Zn deficiency is diminished in the OsbZIP48/50 double mutant. Both OsbZIP48 and OsbZIP50 bind to the promoter of OsZIP10 and activate the ZDRE cis-element. Amino acid substitution mutation of the ZSM domain of OsbZIP48 in OsbZIP50 mutant background increases the content of Zn/Fe/Cu in brown rice seeds and leaves. Therefore, this study demonstrates that OsbZIP48/50 play a crucial role in regulating metal homoeostasis and identifies their downstream genes involved in the Zn deficiency response in rice.

Regulation of the Zinc Deficiency Response in the Legume Model Medicago truncatula

The zinc deficiency response in Arabidopsis thaliana is regulated by F-group basic region leucine-zipper (F-bZIP) transcription factors, and there is evidence of evolutionary conservation of this regulatory network in land plants. Fundamental knowledge on the zinc homeostasis regulation in crop species will contribute to improving their zinc nutritional value. Legumes are protein-rich crops, used worldwide as part of traditional diets and as animal forage, being therefore a good target for micronutrient biofortification. Here, we identified F-bZIP transcription factors in representative legume species and functionally characterized the two F-bZIPs from Medicago truncatula. Results indicate that MtFbZIP1 is the functional homolog of A. thaliana bZIP19 and bZIP23, while MtFbZIP2 does not play a role in the zinc deficiency response. Additionally, analysis of M. truncatula genes from the Zrt/Irt-like protein (ZIP) family of zinc transporters or encoding nicotianamine synthase enzymes that produce the zinc ligand nicotianamine, support the conservation of the F-bZIP-regulated zinc deficiency response in M. truncatula. Phylogenetic analysis of F-bZIP homologs enriched in legume species reinforces the branching into two groups, with MtFbZIP1 and MtFbZIP2 mapping in Groups 1 and 2, respectively. This phylogeny combined with the functional characterization of MtFbZIPs supports the suggested conservation of the zinc deficiency response associated with Group 1 F-bZIPs, and the more variable evolutionary paths associated with Group 2. Overall, we provide novel insight on the mechanisms of response to zinc deficiency in M. truncatula, which contributes to developing strategies for improving zinc content in legume crops.