| Summary: | Summary: Plant growth and development are defined by environmental cues. The transcription factor PHYTOCHROME INTERACTING FACTOR 4 (PIF4) is the central signaling hub that integrates environmental cues, including light and temperature, to regulate growth and development. The thermosensory mechanisms controlling the PIF4-mediated temperature response, and its integration with other environmental responses, remain poorly understood. DE-ETIOLATED 1 (DET1) and CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1), key regulators of light signaling, have been proposed to control thermosensory growth by transcriptional regulation of PIF4, through ELONGATED HYPOCOTYL 5 (HY5). Here, we show that DET1/COP1 and HY5 regulate thermosensory elongation through distinct mechanisms. DET1 and COP1 are essential for promoting PIF4 expression and stabilizing PIF4 protein. Furthermore, HY5 inhibits elongation growth through competitive chromatin binding to PIF4 targets, not through transcriptional regulation of PIF4. Our findings reveal a mechanistic framework in which DET1/COP1 and HY5 regulatory modules act independently to regulate growth through the environmental signal integrator PIF4. : Gangappa and Kumar show that integration of light and temperature signaling is critical for growth. They show that major light signaling components DET1, COP1, and HY5 control PIF4-mediated thermosensory growth through distinct mechanisms. DET1/COP1 promote temperature-responsive growth by stabilizing PIF4 protein. HY5 antagonizes PIF4-mediated elongation through competitive chromatin binding. Keywords: thermosensory growth, environmental signal integration, elongation growth, Arabidopsis, PIF4, DET1, COP1, HY5
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