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Arabidopsis primary root growth response to phosphate (Pi) deficiency is mainly controlled by changes in apoplastic iron (Fe). Upon Pi deficiency, apoplastic Fe deposition in the root apical meristem activates pathways leading to the arrest of meristem maintenance and inhibition of cell elongation. Here, we report that a member of the uncharacterized cytochrome b561 and DOMON domain (CYBDOM) protein family, named CRR, promotes iron reduction in an ascorbate-dependent manner and controls apoplastic iron deposition. Under low Pi, the crr mutant shows an enhanced reduction of primary root growth associated with increased apoplastic Fe in the root meristem and a reduction in meristematic cell division. Conversely, CRR overexpression abolishes apoplastic Fe deposition rendering primary root growth insensitive to low Pi. The crr single mutant and crr hyp1 double mutant, harboring a null allele in another member of the CYDOM family, shows increased tolerance to high-Fe stress upon germination and seedling growth. Conversely, CRR overexpression is associated with increased uptake and translocation of Fe to the shoot and results in plants highly sensitive to Fe excess. Our results identify a ferric reductase implicated in Fe homeostasis and developmental responses to abiotic stress, and reveal a biological role for CYBDOM proteins in plants.
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Crop yield loss due to flooding is a threat to food security. Submergence-induced hypoxia in plants results in stabilization of group VII ETHYLENE RESPONSE FACTORs (ERF-VIIs), which aid survival under these adverse conditions. ERF-VII stability is controlled by the N-end rule pathway, which proposes that ERF-VII N-terminal cysteine oxidation in normoxia enables arginylation followed by proteasomal degradation. The PLANT CYSTEINE OXIDASEs (PCOs) have been identified as catalysts of this oxidation. ERF-VII stabilization in hypoxia presumably arises from reduced PCO activity. We directly demonstrate that PCO dioxygenase activity produces Cys-sulfinic acid at the N terminus of an ERF-VII peptide, which then undergoes efficient arginylation by an arginyl transferase (ATE1). This provides molecular evidence of N-terminal Cys-sulfinic acid formation and arginylation by N-end rule pathway components, and a substrate of ATE1 in plants. The PCOs and ATE1 may be viable intervention targets to stabilize N-end rule substrates, including ERF-VIIs, to enhance submergence tolerance in agriculture.