Constitutive expression of bZIP19 with the Zn sensor motif deleted in Arabidopsis leads to Zn-specific accumulation and no visible developmental penalty

Aims: The transcription factors bZIP19 and bZIP23 function as central regulators of the Zn deficiency response, and also as sensors of intracellular Zn concentration through their protein Zn-Sensor Motif (ZSM). While under Zn deficiency the target genes of bZIP19/23 are transcriptionally activated, under Zn sufficiency the binding of Zn ions to the ZSM halts gene expression. Mutations, including deletions, in the ZSM affect the activity of bZIP19/23 and leads to a Zn-insensitive and constitutive activation of target gene expression. Here we investigated the effects of such deregulation of the Zn deficiency response on plant growth and Zn accumulation, and evaluate whether this deregulation influences Cd accumulation. Methods: We analysed Arabidopsis lines constitutively expressing bZIP19 with the ZSM deleted and measured developmental traits and ionomics in soil-grown plants, comparing control and Cd-spiked soils. Results: Results indicated that deletion of the ZSM, and the consequent deregulation of the Zn deficiency response, does not cause visible penalties in plant growth, development or reproduction. Compared with the wild-type, bZIP19-ZSM deletion increased Zn accumulation in leaves and seeds, and such an increase was mostly limited to Zn. In seeds, the increased Zn content appears distributed evenly throughout the embryo. Exposure of bZIP19-ZSM deletion to a low-level Cd contamination did not cause enhanced Cd accumulation, which is important given that Cd uptake is a concern in crop Zn biofortification. Finally, we verified that the bZIP19-ZSM deletion represents a gain-of-function dominant mutation. Conclusion: Together, results support that modulation of F-bZIP transcription factor’s activity may be a promising avenue for Zn biofortification in crops.