| Summary: | Iron (Fe) homeostasis is essential for plant growth and development, and it is strictly regulated by a group of transcriptional factors. Iron-related transcription factor 3 (OsIRO3) was previously identified as a negative regulator for Fe deficiency response in rice. However, the molecular mechanisms by which OsIRO3 regulate Fe homeostasis is unclear. Here, we report that OsIRO3 is essential for responding to Fe deficiency and maintaining Fe homeostasis in rice. OsIRO3 is expressed in the roots, leaves, and base nodes, with a higher level in leaf blades at the vegetative growth stage. Knockout of <i>OsIRO3</i> resulted in a hypersensitivity to Fe deficiency, with severe necrosis on young leaves and defective root development. The <i>iro3</i> mutants accumulated higher levels of Fe in the shoot under Fe-deficient conditions, associated with upregulating the expression of <i>OsNAS3</i>, which lead to increased accumulation of nicotianamine (NA) in the roots. Further analysis indicated that OsIRO3 can directly bind to the E-box in the promoter of <i>OsNAS3</i>. Moreover, the expression of typical Fe-related genes was significantly up-regulated in <i>iro3</i> mutants under Fe-sufficient conditions. Thus, we conclude that OsIRO3 plays a key role in responding to Fe deficiency and regulates NA levels by directly, negatively regulating the <i>OsNAS3</i> expression.
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