CARK3-mediated phosphorylation of bHLH106 inhibits iron uptake by repressing the expression of IRT1 in Arabidopsis.

The maintenance of iron homeostasis is fundamental to plant survival, preventing both iron deficiency and toxicity. This equilibrium is orchestrated by transcription factors (TFs), including those from the basic helix-loop-helix (bHLH) family. Numerous bHLH TFs have been identified as positive regulators of plant iron uptake, but negative regulators remain largely unknown. In this study, we demonstrated that bHLH106 can bypass the FIT-centered transcriptional regulatory network and directly bind to the promoter of IRT1 (Iron Regulated Transporter 1), inhibiting its expression. The FIT-IRT1-mediated iron-uptake pathway is activated under iron deficiency, promoting the accumulation of intracellular iron. As intracellular iron levels rise, CARK3 (Cytosolic ABA Receptor Kinase 3) phosphorylates bHLH106 to promote its recruitment to the IRT1 promoter, enhancing the transcriptional repression of IRT1 by bHLH106 and thus limiting iron uptake. Under iron-deficient conditions, bHLH106 expression is suppressed, and bHLH106 protein levels are reduced. The transcriptional repression of IRT1 by bHLH106 is alleviated, facilitating FIT-mediated upregulation of IRT1 expression and promoting iron uptake. In summary, the CARK3-bHLH106-IRT1 module enables plants to adapt rapidly to fluctuations in iron levels.