POSITIVE REGULATOR OF IRON HOMEOSTASIS1, OsPRI1, Facilitates Iron Homeostasis.

Oryza sativa HEMERYTHRIN MOTIF-CONTAINING REALLY INTERESTING NEW GENE AND ZINC-FINGER PROTEIN1 (OsHRZ1) is a putative iron-binding sensor. However, it is unclear how OsHRZ1 transmits signals. In this study, we reveal that POSITIVE REGULATOR OF IRON HOMEOSTASIS1 (OsPRI1) interacts with OsHRZ1. A loss-of-function mutation to OsPRI1 increased the sensitivity of plants to Fe-deficient conditions and down-regulated the expression of Fe-deficiency-responsive genes. Yeast one-hybrid and electrophoretic mobility shift assay results suggested that OsPRI1 binds to the OsIRO2 and OsIRO3 promoters. In vitro ubiquitination experiments indicated that OsPRI1 is ubiquitinated by OsHRZ1. Cell-free degradation assays revealed that the stability of OsPRI1 decreased in wild-type roots but increased in the hrz1-2 mutant, suggesting OsHRZ1 is responsible for the instability of OsPRI1. The hrz1-2 seedlings were insensitive to Fe-deficient conditions. When the pri1-1 mutation was introduced into hrz1-2 mutants, the pri1hrz1 double mutant was more sensitive to Fe deficiency than the hrz1-2 mutant. Additionally, the expression levels of Fe-deficiency-responsive genes were lower in the hrz1pri1 double mutant than in the hrz1-2 mutant. Collectively, these results imply that OsPRI1, which is ubiquitinated by OsHRZ1, mediates rice responses to Fe deficiency by positively regulating OsIRO2 and OsIRO3 expression as part of the OsHRZ1-OsPRI1-OsIRO2/3 signal transduction cascade.