Iron (Fe) is vital for plant growth and development with numerous transcription factors playing a key role in maintaining Fe homeostasis. In rice Fe signaling, the basic helix-loop-helix (bHLH) IVc proteins act as positive regulators, while Oryza sativa IRON-RELATED BHLH TRANSCRIPTION FACTOR 3 (OsIRO3/OsbHLH063) functions as a negative regulator. Moreover, HEMERYTHRIN MOTIF-CONTAINING REALLY INTERESTING NEW GENE AND ZINC-FINGER PROTEIN1 (OsHRZ1) plays a role in targeting OsPRIs for degradation.

In a study published in New Phytologist, researchers from Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences revealed the molecular mechanisms regulating Fe homeostasis in rice (Oryza sativa), and highlighted the critical role of transcription factors OsbHLH062 and OsIRO3 in balancing Fe uptake and preventing toxicity.

Transgenic rice strains were used for experimentation. Researchers mutated OsbHLH062 and OsIRO3 through gene editing. By analyzing the expression levels of OsPRI genes and proteins, they observed changes in growth phenotypes and physiological indicators under Fe deficiency conditions in different mutants.

Researchers found that OsbHLH062 directly bound to the promoter of OsIRO2—a master regulator of Fe uptake—recruiting corepressors OsTPL/OsTPRs to suppress its activity. And they found that unlike Fe-responsive OsIRO3, OsbHLH062 expression remained stable under Fe-deficient conditions, suggesting a constitutive regulatory role.

Besides, researchers found that double mutations in OsbHLH062 and OsIRO3 resulted in severe developmental defects: stunted roots, reduced plant height, and Fe overaccumulation. These mutants exhibited dramatic upregulation of Fe deficiency-responsive genes regardless of Fe availability, indicating overlapping roles in repressing Fe signaling.

Moreover, researcher revealed that both OsbHLH062 and OsIRO3 physically interact with OsHRZ1 for degradation, which enhanced OsHRZ1’s ability to ubiquitinate OsPRIs, reducing their stability and suppressing Fe uptake pathways. OsbHLH062 uniquely shuttled between the nucleus and cytoplasm, and its nuclear accumulation was promoted by binding to OsPRIs, which enables the rapid transcriptional repression of OsIRO2 under high Fe conditions.  

“Our findings reveal how plants balance Fe acquisition with detoxification,” said LIANG Gang from XTBG. “Targeting these regulatory nodes could help engineer crops resilient to Fe-deficient soils or excess Fe environments, which is critical for food security in changing climates.”