A transcription factor OsbHLH156 regulates Strategy II iron acquisition through localising IRO2 to the nucleus in rice

Abstract

Summary Plants have evolved two strategies to acquire ferrous (Strategy I) or ferric (Strategy II) iron from soil. The iron‐related bHLH transcription factor 2 (IRO2) has been identified as a key regulator of iron acquisition (Strategy II) in rice. However, its mode of action, subcellular localisation and binding partners are not clearly defined. Using RNA‐seq analyses, we identified a novel bHLH‐type transcription factor, OsbHLH156. The function of OsbHLH156 in Fe homeostasis was analysed by characterisation of the phenotypes, elemental content, transcriptome, interaction and subcellular localisation of OsbHLH156 and IRO2. OsbHLH156 is primarily expressed in the roots and transcript abundance is greatly increased by Fe deficiency. Loss of function of OsbHLH156 resulted in Fe‐deficiency‐induced chlorosis and reduced Fe concentration in the shoots under upland or Fe(III) supplied conditions. Transcriptome analyses revealed that the expression of most Fe‐deficiency‐responsive genes involved in Strategy II were not induced in the osbhlh156‐1 mutant. Furthermore, OsbHLH156 was required for nuclear localisation of IRO2. We conclude that OsbHLH156 is required for a Strategy II uptake mechanism in rice, partnering with a previously identified ‘master’ regulator IRO2. Mechanistically it is required for the nuclear localisation of IRO2.

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

“…OsIRO2 is preferentially expressed in the cytoplasm, and the upregulated expression of OsFIT results in the increased nuclear accumulation of OsIRO2 (Figure 2C). A similar result was also reported by Wang et al (2020). A latest study found that AtFIT promotes the nuclear accumulation of AtbHLH39, an Arabidopsis homolog of OsIRO2, in Arabidopsis (Trofimov et al, 2019).…”

“…Similarly, Arabidopsis bHLH IVc TFs promote the nuclear accumulation of AtbHLH121 which is required for the maintenance of Arabidopsis Fe homeostasis (Kim et al, 2019; Gao et al, 2020; Lei et al, 2020). Although we did not observe the localization change of OsIRO2 in response to Fe deficiency in our transient expression assays, the increased nuclear accumulation of OsIRO2 under Fe deficiency condition was observed by Wang et al (2020). Further investigations are required to reveal the biological relevance of OsIRO2 localization change with the Fe deficiency response.…”

Mobility and localization of the iron deficiency‐induced transcription factor bHLH039 change in the presence of FIT

“…A recent study in rice identified OsbHLH156 as a rice ortholog of FIT, albeit with low sequence similarity and controlling Strategy II, instead of Strategy I, Fe uptake. Interestingly, this transcription factor was found necessary for the nuclear localization of OsIRO2, the rice ortholog of bHLH039 (Wang et al, ). This underlines the importance of the here described phenomenon in Arabidopsis as an integral part of the plant Fe deficiency response.…”

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

“…Very recently, Wang et al (2020) also reported the function of OsbHLH156/OsFIT in rice. Here, we not only characterizes the biological role of OsFIT and OsIRO2 and their genetic relationship in Fe homeostasis, but also reveals their molecular regulation mechanism.…”