Graduate Research Assistant University of Mississippi Oxford, Mississippi
Body of Abstract: Plants are sensitive to temperature changes and thus expend valuable resources and even alter their developmental fates to cope with them. Their morphological and architectural changes in response to high ambient temperatures are termed thermomorphogenesis, which are regulated by thermosensory transcription factors such as PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and coactivators such as HEMERA (HMR). We previously demonstrated that PIF4 and HMR recruit Mediator subunit 14 (MED14), a tail component of the Mediator coactivator complex, to activate growth-promoting genes at warmer temperatures. However, the molecular basis of such recruitment during thermo-induced transcriptional regulation is unclear. Here, we provide in vitro and in vivo evidence showing that PIF4 and HMR form biomolecular condensates with MED14 to activate thermomorphogenetic genes. We determined the transcriptional activation domain (TAD) of PIF4 and demonstrated its significance in promoting thermomorphogenetic stem elongation. We further identified intrinsically disordered regions (IDRs) in PIF4 and MED14 and validated their liquid-liquid phase separation capacity in vitro. Moreover, we pinpointed the effects of PIF4 and HMR’s TADs in forming biomolecular condensates. This work demonstrates that the phase-separation ability of plant transcription factors and Mediator subunits is crucial for their target gene expression, a mechanism conserved in other eukaryotes, including yeast and mammals. Our data also unravel a missing link in thermo-responsive transcriptional regulation and sets the foundation for future studies on the relationship between other thermosensory transcription factors and the Mediator complex.