Abstract
Basic leucine zipper (bZIP) transcription factors control important developmental and physiological processes in plants. In Arabidopsis thaliana, the three gene F-bZIP subfamily has been associated with zinc deficiency and salt stress response. Benefiting from the present abundance of plant genomic data, we performed an evolutionary and structural characterization of plant F-bZIPs. We observed divergence during seed plant evolution, into two groups and inferred different selective pressures for each. Group 1 contains AtbZIP19 and AtbZIP23 and appears more conserved, whereas Group 2, containing AtbZIP24, is more prone to gene loss and expansion events. Transcriptomic and experimental data reinforced AtbZIP19/23 as pivotal regulators of the zinc deficiency response, mostly via the activation of genes from the ZIP metal transporter family, and revealed that they are the main regulatory switch of AtZIP4. A survey of AtZIP4 orthologs promoters across different plant taxa revealed an enrichment of the Zinc Deficiency Response Element (ZDRE) to which both AtbZIP19/23 bind. Overall, our results indicate that while the AtbZIP24 function in the regulation of the salt stress response may be the result of neo-functionalization, the AtbZIP19/23 function in the regulation of the zinc deficiency response may be conserved in land plants (Embryophytes).
Highlights
Regulatory networks and modulation of gene expression by transcription factors (TFs) provide cells and organisms with a dynamic and broad response to developmental and environmental cues
Group 1 includes A. thaliana bZIP19/AT4G35040 and bZIP23/AT2G16770, and Group 2 includes bZIP24/AT3G51960. These results suggest a monophyletic origin for F-Basic leucine zipper (bZIP) with subsequent expansion during seed plant evolution
To obtain additional information on the functional association between AtbZIP19/23 and their putative target genes from the ZIP family of metal transporters in the context of different zinc supply, we studied the expression of AtZIP4 and the regulatory impact of AtbZIP19/23 loss of function in an in vivo reporter assay
Summary
Regulatory networks and modulation of gene expression by transcription factors (TFs) provide cells and organisms with a dynamic and broad response to developmental and environmental cues. AtbZIP19 and AtbZIP23 were shown to play a pivotal role as positive transcriptional regulators of the zinc deficiency response[32] They were identified, in a yeast-one-hybrid screening, to associate to promoter regions of the zinc deficiency-induced AtZIP4 gene of the Zrt, Irt-like protein (ZIP) family of metal transporters. Transcriptomic profiling of the mutant revealed deregulation of only a small set of genes, including genes associated with zinc homeostasis and comprising additional AtZIP genes These were characterized by the presence of the cis-element Zinc Deficiency Response Element (ZDRE) in their promoter region, to which both bZIP19 and −23 proteins can bind[32, 33]. All three F bZIP proteins display a characteristic His/Cys-rich motif which is a signature of this bZIP subfamily[24, 35]
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