Oligomerization-competent PIF4 drives thermomorphogenesis through functional redundancy in transactivation and DNA binding

Plants tailor their architecture to warm temperatures through the central transcription factor PHYTOCHROME-INTERACTING FACTOR 4 (PIF4). Here, we dissect how structured and disordered regions of PIF4 contribute to its function in thermomorphogenesis. A long N-terminal intrinsically disordered region (IDR) enables PIF4 to form low-mobility condensates. Within this IDR, we identify an acidic transactivation domain (TAD) and an extended basic segment that carries a nuclear-localization signal and the canonical basic motif of the basic helix-loop-helix (bHLH) domain. The basic segment is both necessary and sufficient to drive PIF4 condensate formation, while the TAD tunes condensate properties. Strikingly, alanine substitutions that abolish TAD-mediated transactivation, disrupt DNA binding, or greatly reduce phase-separation propensity have no significant effect on thermomorphogenetic hypocotyl elongation. By contrast, substituting twelve basic residues within the basic segment, which disrupts both DNA binding and HLH-mediated oligomerization, abolishes thermo-induced hypocotyl growth. These findings suggest that PIF4's oligomerization competence contributes significantly to thermomorphogenesis by enabling partner recruitment, allowing DNA-binding and transactivation functions to be supplied in trans.