Figure 9.

Model for the involvement of ZAT12 in the regulation of Fe uptake. Fe deficiency leads to induced expression of FIT, as part of the FIT autoregulatory loop. Active FIT protein confers induction of the genes for the Fe deficiency response (indicated in green; summarized in Brumbarova et al., 2015). The presence of FIT protein is a prerequisite for the accumulation of H₂O₂ in Fe-deficient roots. After 10 d of Fe starvation, the presence of H₂O₂ leads to the induction of ZAT12 gene expression and enhanced ZAT12 protein levels. Both FIT and ZAT12 protein levels are controlled in the cell through their degradation. We propose that ZAT12 engages FIT through its EAR motif in a protein complex. H₂O₂ causes an enhanced FIT protein level despite the decreased gene expression. We suggest that FIT protein could be stabilized but inactivated by H₂O₂, possibly through interaction with ZAT12. Inactive FIT may eventually also contribute to reduced FIT gene expression levels. Such a control mechanism may allow the plant to keep a backup of FIT protein molecules ready to use in case more favorable conditions arise. Upon Fe resupply, the system is shifted and FIT is active again.