Mobility and localization of the iron deficiency-induced transcription factor bHLH039 change in the presence of FIT

Regulation of iron (Fe) acquisition and homeostasis is critical for plant survival. In Arabidopsis, Fe deficiency-induced bHLH039 forms a complex with the master regulator FIT and activates it to upregulate Fe acquisition genes. FIT is partitioned between cytoplasm and nucleus, whereby active FIT accumulates more in the nucleus than inactive FIT. At the same time, there is so far no information on the subcellular localization of bHLH039 protein and how it is controlled. We report here that the bHLH039 localization pattern changes depending on the presence of FIT in the cell. When expressed in cells lacking FIT, bHLH039 localizes predominantly in the cytoplasm, including cytoplasmic foci in close proximity to the plasma membrane. The presence of FIT enhances the mobility of bHLH039 and redirects the protein toward primarily nuclear localization, abolishing its accumulation in cytoplasmic foci. This FIT-dependent change in localization of bHLH039 found in transient fluorescent protein expression experiments was confirmed in both leaves and roots of Arabidopsis transgenic plants, stably expressing hemagglutinin-tagged bHLH039 in wild-type or fit mutant background. This posttranslational mechanism for intracellular partitioning of Fe-responsive transcription factors suggests a signaling cascade that translates Fe sensing at the plasma membrane to nuclear accumulation of the transcriptional regulators.