bHLH11 inhibits bHLH IVc proteins by recruiting the TOPLESS/TOPLESS-RELATED corepressors in Arabidopsis

Abstract

Iron (Fe) homeostasis is essential for plant growth and development. Although tremendous progress has been made in understanding the maintenance of Fe homeostasis in plants, the underlying molecular mechanisms remain elusive. Recently, bHLH11 was reported to function as a negative regulator. However, the molecular mechanism by which bHLH11 regulates Fe homeostasis is unclear. Here, we generated two bhlh11 loss-of-function mutants which displayed the enhanced sensitivity to excessive Fe. bHLH11 is located in the cytoplasm and nucleus due to lack of a nuclear location signal sequence, and its interaction partners, bHLH IVc transcription factors (TFs) (bHLH34, bHLH104, bHLH105 and bHLH115) facilitate its nuclear accumulation. bHLH11 exerts its negative regulation function by recruiting the corepressors TOPLESS/TOPLESS-RELATED. Moreover, bHLH11 antagonizes the transactivity of bHLH IVc TFs towards bHLH Ib genes (bHLH38, bHLH39, bHLH100 and bHLH101). This work indicates that bHLH11 is a crucial component of Fe homeostasis signaling network, playing a pivotal role in the fine-tuning of Fe homeostasis.

POPEYE directly regulates bHLH Ib genes and its own expression

“…bHLH121, one member of bHLH IVb subgroup, is also required for the upregulation of bHLH Ib genes, and bHLH121 directly binds to their promoters (Kim et al, 2019;Gao et al, 2020;Lei et al, 2020). By contrast, bHLH11 is a negative regulator of bHLH Ib genes since it inhibits the transcription activation activity of bHLH IVc towards bHLH Ib genes (Li et al, 2021b). Here, we suggest that PYE can directly represses bHLH Ib genes by binding to their promoters.…”

“…In addition to bHLH121, bHLH IVb subgroup also contains bHLH11 and POPEYE (PYE/bHLH47). bHLH11 is a negative regulator containing an EAR motif which can recruit the transcriptional corepressors TOPLESS/TOPLESS-RELATED (TPL/TPRs) (Tanabe et al, 2019;Li et al, 2021b). Moreover, bHLH11 interacts with bHLH IVc proteins and interferes with their transactivation to bHLH Ib genes (Li et al, 2021b).…”

“…Gene copy number was normalized to that of ACT2 and PP2A. Primers used for qRT-PCR were described previously (Li et al, 2021b). promoter and BD domain were described previously (Li et al, 2016).…”

“…bHLH11 is a negative regulator containing an EAR motif which can recruit the transcriptional corepressors TOPLESS/TOPLESS-RELATED (TPL/TPRs) (Tanabe et al, 2019;Li et al, 2021b). Moreover, bHLH11 interacts with bHLH IVc proteins and interferes with their transactivation to bHLH Ib genes (Li et al, 2021b). In contrast, PYE negatively regulates the expression of NICOTIANAMINE SYNTHASE 4 (NAS4), FRO3, and ZINC-INDUCED FACILITATOR1 (ZIF1) by directly binding to their promoters (Long et al, 2010).…”

Annotation and Molecular Characterisation of the TaIRO3 and TaHRZ Iron Homeostasis Genes in Bread Wheat (Triticum aestivum L.)

“…In contrast, the EAR motif in monocot IRO3 proteins was conserved at amino acid position 286, and had the sequence LxLxL (in ZmIRO3, SbbHLH063 and OsIRO3 proteins) or RxLxL (all other monocots, including the TaIRO3 proteins). A subgroup IVb bHLH TF in Arabidopsis , AtbHLH11, was also recently described to have a functional LxLxL EAR motif near amino acid position 286 [ 54 ]. Given that RxLxL is an atypical EAR motif sequence, we have labelled this sequence as “EAR-like” in the TaIRO3 proteins, and further research is required to confirm whether this motif is functional in bread wheat.…”