Oryza sativa POSITIVE REGULATOR OF IRON DEFICIENCY RESPONSE 2 (OsPRI2) and OsPRI3 are involved in the maintenance of Fe homeostasis

Abstract

Iron (Fe) is an essential micronutrient for plant growth development and plays a key role in regulating numerous cellular processes. In rice, OsHRZ1, an Fe‐binding ubiquitin ligase, is a putative sensor of Fe homeostasis that negatively regulates iron acquisition. Despite its apparent importance, only a single basic‐Helix–Loop–Helix (bHLH) transcription factor, OsPRI1, has been identified as a direct target of OsHRZ1. In this study, we identified and functionally characterized OsPRI2 and OsPRI3, two paralogs of OsPRI1, observing that they directly interact with OsHRZ1. Additional analyses suggested that OsHRZ1 promotes the degradation of OsPRI2 and OsPRI3. The translocation of Fe from roots to shoots was impaired in plants with loss‐of‐function mutations in OsPRI2 or OsPRI3, causing the downregulation of Fe‐deficiency‐responsive genes. In contrast, overexpression of OsPRI2 and OsPRI3 promotes Fe accumulation and activates the expression of Fe‐deficiency‐responsive genes. We also provide evidence that OsPRI2 and OsPRI3 bind to the promoters of OsIRO2 and OsIRO3, two key regulators of Fe homeostasis. Moreover, OsPRI2 and OsPRI3 directly induce expression of the metal‐nicotianamine transporter, OsYSL2, by associating with the promoter in response to Fe deficiency. Our results provide insights into the complex network regulating Fe homeostasis in rice.

Oryza sativa FER‐LIKE FE DEFICIENCY‐INDUCED TRANSCRIPTION FACTOR (OsFIT/OsbHLH156) interacts with OsIRO2 to regulate iron homeostasis

“…We observed that the OsIRO2 transcript levels increased in the fit‐1 mutant (Table 1), implying that OsIRO2 expression is not positively regulated by OsFIT. To determine whether OsFIT is positively regulated by OsIRO2, we examined its expression in the iro2‐1 mutant (Zhang et al 2020). Interestingly, the OsFIT transcript level increased in the iro2‐1 mutant (Figure S6).…”

“…Although OsFIT and OsIRO2 positively regulate the expression of many Fe‐deficiency‐responsive genes, the transcription of OsFIT and OsIRO2 is also induced by Fe deficiency. We previously revealed that OsPRI1, OsPRI2, and OsPRI3 directly activate OsIRO2 expression (Zhang et al 2017, 2020). Therefore, we evaluated whether OsFIT expression is also positively regulated by OsPRI1, OsPRI2, and OsPRI3.…”

“…OsHRZ1 is a putative Fe‐binding sensor which negatively regulates Fe homeostasis in rice (Kobayashi et al 2013). Our recent work revealed that OsHRZ1 interacts with and degrades OsPRI1, OsPRI2, and OsPRI3 which directly activate the expression of OsIRO2 and OsIRO3 (Zhang et al 2017, 2020). OsIRO2 positively and OsIRO3 negatively regulate Fe homeostasis, respectively, (Ogo et al 2007; Zheng et al 2010).…”

“…As the homologs of BTS, OsHRZ1, and OsHRZ2 were identified as putative rice Fe sensors that also contain hemerythrin motifs and a RING domain (Kobayashi et al 2013). Previous studies revealed that OsHRZ1 negatively regulates Fe homeostasis by mediating the degradation of OsPRI1, OsPRI2, and OsPRI3 (orthologs of Arabidopsis bHLH IVc), which positively regulate Fe homeostasis by directly targeting OsIRO2 (homolog of Arabidopsis bHLH Ib) and OsIRO3 (homolog of AtPYE ) (Zhang et al 2017, 2020; Kobayashi et al 2019). Both OsIRO2 and OsIRO3 exhibit upregulated expression in response to Fe deficiency and their products positively and negatively regulate Fe homeostasis, respectively (Ogo et al 2006, 2007; Zheng et al 2010).…”

FER-LIKE FE DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (OsFIT) interacts with OsIRO2 to regulate iron homeostasis

“…As the homologs of BTS, OsHRZ1 and OsHRZ2 were identified as putative rice Fe sensors that also contain hemerythrin motifs and a RING domain (Kobayashi et al, 2013). Previous studies revealed that OsHRZ1 negatively regulates Fe homeostasis by mediating the degradation of OsPRI1, OsPRI2, and OsPRI3 (orthologs of Arabidopsis bHLH IVc), which positively regulate Fe homeostasis by directly targeting OsIRO2 (homolog of Arabidopsis bHLH Ib) and OsIRO3 (homolog of AtPYE ) (Zhang et al, 2017, 2020; Kobayashi et al, 2019). Both OsIRO2 and OsIRO3 exhibit upregulated expression in response to Fe deficiency and their products positively and negatively regulate Fe homeostasis, respectively (Ogo et al, 2006, 2007; Zheng et al 2010).…”