Four IVa bHLH Transcription Factors Are Novel Interactors of FIT and Mediate JA Inhibition of Iron Uptake in Arabidopsis

Plants have evolved sophisticated genetic networks to regulate iron (Fe) homeostasis for their survival. Several classes of plant hormones including jasmonic acid (JA) have been shown to be involved in regulating the expression of iron uptake and/or deficiency-responsive genes in plants. However, the molecular mechanisms by which JA regulates iron uptake remain unclear. In this study, we found that JA negatively modulates iron uptake by downregulating the expression of FIT (bHLH29), bHLH38, bHLH39, bHLH100, and bHLH101 and promoting the degradation of FIT protein, a key regulator of iron uptake in Arabidopsis. We further demonstrated that the subgroup IVa bHLH proteins, bHLH18, bHLH19, bHLH20, and bHLH25, are novel interactors of FIT, which promote JA-induced FIT protein degradation. These four IVa bHLHs function redundantly to antagonize the activity of the Ib bHLHs (such as bHLH38) in regulating FIT protein stability under iron deficiency. The four IVa bHLH genes are primarily expressed in roots, and are inducible by JA treatment. Moreover, we found that MYC2 and JAR1, two critical components of the JA signaling pathway, play critical roles in mediating JA suppression of the expression of FIT and Ib bHLH genes, whereas they differentially modulate the expression of bHLH18, bHLH19, bHLH20, and bHLH25 to regulate FIT accumulation under iron deficiency. Taken together, these results indicate that by transcriptionally regulating the expression of different sets of bHLH genes JA signaling promotes FIT degradation, resulting in reduced expression of iron-uptake genes, IRT1 and FRO2, and increased sensitivity to iron deficiency. Our data suggest that there is a multilayered inhibition of iron-deficiency response in the presence JA in Arabidopsis.