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

Abstract

Several nuclear proteins undergo condensation. The question remains often whether this property is coupled to a functional aspect of the protein in the nucleus. The basic helix-loop-helix (bHLH) FER-LIKE IRON DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT) integrates internal and external signals to control the amount of iron that is acquired in accordance with growth. The previously described C-terminal Ser271/272 allows FIT to form active complexes with subgroup Ib bHLH factors such as bHLH039. FIT has lower nuclear mobility than mutant FITmSS271AA, but this behavior has remained mechanistically and functionally obscure. Here, we show that FIT undergoes a light-inducible subnuclear partitioning into nuclear condensates that we termed FIT nuclear bodies (NBs). The characteristics of FIT NBs could be examined using a standardized FIT NB analysis procedure coupled with different types of quantitative and qualitative microscopy-based approaches. We found that FIT condensates were 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. Hence, the inducible highly dynamic FIT condensates link active transcription factor complexes with posttranscriptional regulation processes. Active transcription factor complexes for Fe signaling may form as nuclear condensates to integrate environmental signals and Fe nutrition.

HY5: a key regulator for light‐mediated nutrient uptake and utilization by plants

“…BRUTUS is involved in negative regulation of Fe uptake genes, and PYE is known to act as a repressor of genes involved in Fe transport, storage, and assimilation (Long et al ., 2010; Selote et al ., 2015). A recent study has also demonstrated that blue light positively regulates the Fe uptake responses in Arabidopsis (Trofimov et al ., 2022). Given that HY5 and blue light both positively regulate Fe deficiency responses in Arabidopsis, it is most likely that HY5 regulates Fe deficiency responses by acting downstream of blue light.…”