The relationship between cellular Zn status and regulation of Zn homeostasis genes in plant cells

Zinc is one of the most widespread transition metals in plants, and Zn deficiency has many adverse effects on plant productivity and human health. The zinc homeostasis system is actively studied; however, the mechanisms underlying regulation of Zn homeostasis have not been fully elucidated. Two potential ways to assess cellular Zn homeostasis are to sense changes in the free Zn2+ levels or disturbances in protein folding due to zinc deficiency. We found that changes in the cellular free Zn2+ level regulated the transcriptional activity of a set of primary Zn homeostasis genes in a dose-dependent manner and with fast kinetics. We propose that in addition to the well-known bZIP19/bZIP23 transcription factors, another Zn-sensing system exists in the cell and regulates free Zn2+-dependent changes in MTP2 gene expression. We first demonstrated that changes in the free Zn2+ levels in the cells likely influenced not only the transcriptional activation but also the mRNA degradation rates of Zn homeostasis genes, which were accelerated for several genes during recovery of the cellular Zn status. Zn deficiency clearly disturbed the protein folding processes in the cytosol of plant cells, but the cellular response system to these disturbances did not play a significant role in the regulation of Zn homeostasis genes.