A DOMON domain-cytochrome b561 protein acts as a ferric reductase in iron homeostasis and impacts primary root growth under phosphate deficiency

Iron (Fe) and phosphate (Pi) are essential nutrients for plant growth. Several interactions between Fe and Pi homeostasis have been described, such as the Fe-dependent inhibition of primary root growth under Pi deficiency. This response involves the formation of apoplastic Fe+3-malate complexes in the root meristem which implicates the oxidation of Fe+2 by the LPR1 ferroxidase. However, how is the reduced Fe+2 generated in the root meristem and the Fe+3/Fe+2 ratio regulated is unknown. Here, we have identified a gene in Arabidopsis thaliana , named CRR , that is implicated in primary root growth under Pi deficiency. Under low-Pi conditions, the crr mutant showed an enhanced reduction of primary root growth that was associated with increased accumulation of apoplastic Fe in the root meristem and a reduction in meristematic cell division. Conversely, CRR overexpression rendered primary root growth insensitive to low-Pi inhibition, reduced root apoplastic Fe deposition, and impacted the expression of genes involved in Fe and redox homeostasis. CRR is a member of an uncharacterized CYBDOM protein family possessing a cytochrome b561 (CYB561) with an N-terminal DOMON domain. We demonstrated that CRR localizes to the plasma membrane and possesses ascorbate-dependent ferric reductase activity. The crr single mutant and the crr hyp1 double mutant, which harbored a null allele in another member of the CYDOM family, showed increased tolerance to high-Fe stress upon germination and seedling growth. In contrast, CRR overexpression was associated with increased uptake and translocation of Fe to the shoot and resulted in plants highly sensitive to Fe excess toxicity. Our results thus identify a ferric reductase implicated in root Fe acquisition and homeostasis and reveal a biological role for CYBDOM proteins in plants. ### Competing Interest Statement The authors have declared no competing interest.