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Abstract While moderately elevated ambient temperatures do not trigger stress responses in plants, they do substantially stimulate the growth of specific organs through a process known as thermomorphogenesis. The basic helix–loop–helix transcription factor PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) plays a central role in regulating thermomorphogenetic hypocotyl elongation in various plant species, including Arabidopsis (Arabidopsis thaliana). Although it is well known that PIF4 and its co-activator HEMERA (HMR) promote plant thermosensory growth by activating genes involved in the biosynthesis and signaling of the phytohormone auxin, the detailed molecular mechanism of such transcriptional activation is not clear. In this report, we investigated the role of the Mediator complex in the PIF4/HMR-mediated thermoresponsive gene expression. Through the characterization of various mutants of the Mediator complex, a tail subunit named MED14 was identified as an essential factor for thermomorphogenetic hypocotyl growth. MED14 was required for the thermal induction of PIF4 target genes but had a marginal effect on the levels of PIF4 and HMR. Further transcriptomic analyses confirmed that the expression of numerous PIF4/HMR-dependent, auxin-related genes required MED14 at warm temperatures. Moreover, PIF4 and HMR physically interacted with MED14 and both were indispensable for the association of MED14 with the promoters of these thermoresponsive genes. While PIF4 did not regulate MED14 levels, HMR was required for the transcript abundance of MED14. Taken together, these results unveil an important thermomorphogenetic mechanism, in which PIF4 and HMR recruit the Mediator complex to activate auxin-related growth-promoting genes when plants sense moderate increases in ambient temperature.
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PIF4-mediated thermomorphogenesis relies on its oligomerization ability, not DNA-binding or transactivation activity
Abstract 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 andin 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.
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- PAR ID:
- 10668911
- Publisher / Repository:
- Research Square
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript
- Posted Content:
- https://doi.org/10.21203/rs.3.rs-7294532/v1
