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

Zinc (Zn) is an essential microelement for plants as well as for human beings, and it regulates numerous metabolic process and protein activity. Zn deficiency not only impairs plant growth and development, but also affects human health. Rice (Oryza sativa L.) is a staple food for more than half of the worlds population, but how Zn homeostasis in rice is maintained is largely unknown. Here, we demonstrate that two F-group bZIP transcription factors, OsbZIP48 and OsbZIP50, are important for metal homeostasis in rice. Mutation of OsbZIP48 and OsbZIP50 impairs plant growth and reduces Zn, Fe, and Cu content in shoots. The N-terminus of either OsbZIP48 or OsbZIP50 contains two cysteine- and histidine-rich (CHR) domains, deletion or mutation of these CHR domains increases nucleus localization of OsbZIP48 and OsbZIP50. Both OsbZIP48 and OsbZIP50 have transcriptional activation activity, and the expression of 1117 genes involved in metal uptake, phenylpropanoid biosynthetic process, cell wall organization, et al., is reduced in OsbZIP48 and OsbZIP50 double mutant than that in wild-type ZH11 plant under Zn deficiency. Both OsbZIP48 and OsbZIP50 bind to the promoter region of the ZIP family transporter gene OsZIP10 in rice, and activate the promoter activity of ZDRE cis-element derived from the OsZIP10 promoter in effector-reporter assays. Mutation of the CHR domain of OsbZIP48 in OsbZIP50 mutant background increases the content of Zn/Fe/Cu in brown rice seeds and leaves. Thus, this study reveals that OsbZIP48 and OsbZIP50 regulate metal homeostasis, especially under Zn deficiency in rice, and provides candidate target genes for biofortification of micronutrients in future. significance statementZinc (Zn) is an essential microelement not only for plants but also for human beings. This paper shows that the N-terminal cysteine- and histidine-rich domain of OsbZIP48/50 is important for their nucleus localization, therefore transcriptional activity, and reveals the downstream genes of OsbZIP48/50 involved in metal homeostasis under Zn deficiency in rice.