Iron (Fe) is vital for plant growth and development, with numerous transcription factors playing a key role in maintaining Fe homeostasis. In the context of rice Fe signaling, the basic helix-loop-helix (bHLH) IVc proteins—OsPRI1/OsbHLH060, OsPRI2/OsbHLH058, OsPRI3/OsbHLH059, and OsPRI4/OsbHLH057—serve as positive regulators. Conversely, Oryza sativa IRON-RELATED BHLH TRANSCRIPTION FACTOR 3 (OsIRO3/OsbHLH063) operates as a negative regulator. Moreover, HEMERYTHRIN MOTIF-CONTAINING REALLY INTERESTING NEW GENE AND ZINC-FINGER PROTEIN1 (OsHRZ1) is implicated 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 provided new insights into the molecular mechanisms regulating Fe homeostasis in rice (Oryza sativa), highlighting the critical roles of transcription factors OsbHLH062 and OsIRO3 in balancing Fe uptake and preventing toxicity.
The researchers used gene editing technology to mutate OsbHLH062 and OsIRO3, utilizing transgenic rice strains for experimentation. By analyzing the expression levels of OsPRI genes and proteins, they observed changes in growth phenotypes and physiological indicators under iron deficiency conditions in different mutants.
They found that OsbHLH062 directly binds to the promoter of OsIRO2—a master regulator of Fe uptake—recruiting corepressors OsTPL/OsTPRs to suppress its activity. Unlike Fe-responsive OsIRO3, OsbHLH062 expression remains stable under Fe-deficient conditions, suggesting a constitutive regulatory role.
Double mutations in OsbHLH062 and OsIRO3 result in severe developmental defects: stunted roots, reduced plant height, and Fe overaccumulation. These mutants exhibit dramatic upregulation of Fe deficiency-responsive genes regardless of Fe availability, indicating overlapping roles in repressing Fe signaling.
Both OsbHLH062 and OsIRO3 physically interact with OsHRZ1 for degradation. This interaction enhances OsHRZ1’s ability to ubiquitinate OsPRIs, reducing their stability and suppressing Fe uptake pathways. OsbHLH062 uniquely shuttles between the nucleus and cytoplasm. Its nuclear accumulation is promoted by binding to OsPRIs, enabling rapid transcriptional repression of OsIRO2 under high Fe conditions.
“Our findings reveal how plants balance Fe acquisition with detoxification,” said LIANG Gang of XTBG. “Targeting these regulatory nodes could help engineer crops resilient to Fe-deficient soils or excess Fe environments—critical for food security in changing climates.”
First published: 10 May 2025
