Why FIT and bHLH Ib interdependently regulate Fe-uptake

FIT (FER-LIKE IRON DEFICIENCY- INDUCED TRANSCRIPTION FACTOR) and four bHLH Ib transcription factors (TFs) bHLH38, bHLH39, bHLH100 and bHLH101, are the master regulators of Fe uptake genes, and they interact with each other to activate the Fe uptake systems. However, it remains unclear why FIT and bHLH Ib depend on each other to regulate the Fe deficiency response. By analyzing Fe deficiency phenotypes and Fe uptake genes, we found that the quadruple bhlh4x mutants (bhlh38 bhlh39 bhlh100 bhlh101) mimic the fit mutant. Subcellular localization analyses indicate that bHLH38 and bHLH39 are preferentially expressed in the cytoplasm whereas bHLH100 and bHLH101 in the nucleus. Transcriptome data show that the genes involved in Fe signaling pathway show the same expression trends in bhlh4x and fit. Genetic analyses suggest that FIT and bHLH Ib depend each other to regulate the Fe deficiency response. Further biochemical assays indicate that bHLH Ib TFs possess the DNA binding ability and FIT has the transcription activation ability. This work concludes that FIT and bHLH Ib form a functional transcription complex in which bHLH Ib is responsible for target recognition and FIT for transcription activation, explaining why FIT and bHLH Ib interdependently regulate Fe uptake.