PIF4-mediated thermomorphogenesis relies on its oligomerization ability, not DNA-binding or transactivation activity

Plants tailor their architecture to warm ambient temperatures through the central thermosensory transcription factor PHYTOCHROME-INTERACTING FACTOR 4 (PIF4), yet the sequence features that confer this activity remain poorly defined. Here, we combine targeted mutagenesis, phase-separation assays, and transgenic complementation to 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 gel-like condensates in vitro and in planta. 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 merely 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, replacing three conserved basic residues in the first helix of the HLH domain disrupts PIF4 oligomerization and abolishes thermo-induced hypocotyl growth. We conclude that the ability of PIF4 to oligomerize, rather than to bind DNA or recruit the transcriptional machinery, is the primary determinant of its thermomorphogenetic activity.