The Transcription Factor bHLH121 Interacts with bHLH105 (ILR3) and its Closest Homologs to Regulate Iron Homeostasis in Arabidopsis

Iron (Fe) is an essential micronutrient for plant growth and development. Any defects in the maintenance of Fe homeostasis will alter plant productivity and the quality of their derived products. In Arabidopsis thaliana, the ILR3 transcription factor plays a central role in the control of Fe homeostasis. In this study, bHLH121 was identified as an ILR3 interacting transcription factor. Interaction studies showed that bHLH121 interacts also with the three closest homologues of ILR3 (i.e. bHLH34, bHLH104 and bHLH115). bhlh121 loss-of-function mutants display severe Fe homeostasis defects that could be reverted by exogenous Fe supply. bHLH121 acts as a direct transcriptional activator of key genes involved in the Fe regulatory network, including bHLH38, bHLH39, bHLH100, bHLH101, PYE, BTS, BTSL1 as well as IMA1 and IMA2. In addition, bHLH121 is necessary for the activation of FIT expression in response to Fe deficiency via an indirect mechanism. bHLH121 is expressed throughout the whole plant body and its expression is not affected by Fe availability. In contrast, Fe availability conditions the cellular localization of bHLH121 protein in roots. Altogether these data show that bHLH121 is a master regulator of Fe homeostasis that acts upstream from FIT in concert with ILR3 and its closest homologues.