FER-LIKE IRON DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT) accumulates in homo- and heterodimeric complexes in dynamic and inducible nuclear condensates associated with speckle components

Some nuclear proteins undergo condensation, but the functional importance remains often unclear. The basic helix-loop-helix (bHLH) FER-LIKE IRON DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT) integrates internal and external signals to control iron acquisition and growth. The previously described C-terminal residues Ser271/272 allow FIT to form active complexes with subgroup Ib bHLH factors such as bHLH039. FIT has lower nuclear mobility than mutant FITmSS271AA. Here, we show that FIT undergoes a light-inducible subnuclear partitioning into nuclear condensates that we termed FIT nuclear bodies (NBs). FIT NB characteristics were examined using a standardized FIT NB analysis procedure coupled with different types of quantitative and qualitative microscopy-based approaches. FIT condensates were reversible and likely formed by liquid-liquid phase separation. FIT accumulated preferentially in FIT NBs versus nucleoplasm when engaged in protein complexes with itself and with bHLH039. FITmSS271AA, instead, localized to NBs with different dynamics. FIT colocalized with splicing and light signaling NB markers. The NB-inducing light conditions were linked with active FIT and elevated FIT target gene expression in roots. Hence, we conclude that inducible, highly dynamic FIT condensates form preferentially when transcription factor complexes are active. Inducible FIT nuclear condensates may affect nuclear mobility and integrate environmental and Fe nutrition signals.