The transcription factor POPEYE negatively regulates the expression of bHLH Ib genes to maintain iron homeostasis

Iron (Fe) is an essential trace element for plants. When suffering from Fe deficiency, plants modulate the expression of Fe deficiency-responsive genes to promote Fe uptake. POPEYE (PYE) is a key bHLH (basic helix-loop-helix) transcription factor involved in Fe homeostasis. However, the molecular mechanism of PYE regulating the Fe deficiency response remains elusive in Arabidopsis. We found that the overexpression of PYE attenuates the expression of Fe deficiency-responsive genes. PYE directly represses the transcription of bHLH Ib genes (bHLH38, bHLH39, bHLH100, and bHLH101) by associating with their promoters. Although PYE contains an ethylene response factor-associated amphiphilic repression (EAR) motif, it does not interact with the transcriptional co-repressors TOPLESS/TOPLESS-RELATED (TPL/TPRs). Sub-cellular localization analysis indicated that PYE localizes in both the cytoplasm and nucleus. PYE contains a nuclear export signal (NES) which is required for the cytoplasmic localization of PYE. Mutation of the NES amplifies the repression function of PYE, resulting in down-regulation of Fe deficiency-responsive genes. Co-expression assays indicated that three bHLH IVc members (bHLH104, bHLH105/ILR3, and bHLH115) facilitate the nuclear accumulation of PYE. Conversely, PYE indirectly represses the transcription activation ability of bHLH IVc. Additionally, PYE directly negatively regulates its own transcription. This study provides new insights into the Fe deficiency response signalling pathway and enhances the understanding of PYE functions in Arabidopsis. POPEYE (PYE) negatively regulates bHLH Ib genes via two different mechanisms, and the conditional nuclear localization of PYE is crucial to maintain Fe homeostasis in Arabidopsis.

Automated summary

Iron (Fe) is important for plants, and the expression of Fe deficiency-responsive genes is modulated by the transcription factor POPEYE (PYE). This study shows that PYE directly represses the transcription of bHLH Ib genes and negatively regulates its own transcription. The nuclear localization of PYE and its interaction with bHLH IVc members are crucial for maintaining Fe homeostasis in Arabidopsis.