Abstract
Iron is an essential micronutrient for plant growth and development. In Arabidopsis thaliana, an intricate regulatory network involving several bHLH transcription factors (TFs) controls the homeostasis of iron. Among these TFs, bHLH121 plays a crucial role. bHLH121 interacts in vivo with clade IVc bHLH TFs and activates the expression of FIT and clade Ib bHLH TFs to stimulate the uptake of iron. How bHLH121 and clade IVc bHLH TFs function collectively and efficiently to maintain iron homeostasis is still unclear. Herein, we found that double loss-of-function mutants involving bhlh121 and one of the clade IVc bHLH displayed more severe iron deficiency associated growth defects than each of the single ones. We also found that among the four clade IVc bHLH TFs, solely bHLH34 and bHLH105 could partially complement the iron-associated growth defects of bhlh121 when overexpressed. These data, together with protein localization analysis, support that bHLH121 and clade IVc bHLH TFs act synergistically to regulate iron homeostasis and that different bHLH121/clade IVc and clade IVc/clade IVc protein complexes are involved in this process.