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

Iron (Fe) homeostasis is essential for plant growth and development. Many transcription factors (TFs) play pivotal roles in the maintenance of Fe homeostasis. bHLH11 is a negative TF that regulates Fe homeostasis. However, the underlying molecular mechanism remains elusive. Here, we generated two loss-of-function bhlh11 mutants in Arabidopsis (Arabidopsis thaliana), which display enhanced sensitivity to excess Fe, increased Fe accumulation, and elevated expression of Fe deficiency responsive genes. Levels of bHLH11 protein, localized in both the cytoplasm and nucleus, decreased in response to Fe deficiency. Co-expression assays indicated that bHLH IVc TFs (bHLH34, bHLH104, bHLH105, and bHLH115) facilitate the nuclear accumulation of bHLH11. Further analysis indicated that bHLH11 represses the transactivity of bHLH IVc TFs toward bHLH Ib genes (bHLH38, bHLH39, bHLH100, and bHLH101). The two ethylene response factor-associated amphiphilic repression motifs of bHLH11 provided the repression function by recruiting the TOPLESS/TOPLESS-RELATED (TPL/TPRs) corepressors. Correspondingly, the expression of Fe uptake genes increased in the tpr1 tpr4 tpl mutant. Moreover, genetic analysis revealed that bHLH11 has functions independent of FER-LIKE IRON DEFICIENCY-INDUCED TRANSCRIPTION FACTOR. This study provides insights into the complicated Fe homeostasis signaling network.