Multiple hormone pathways orchestrate thermomorphogenesis in response to warm temperatures, yet the mechanism coordinating these pathways remains elusive. Here, we identified INTERACT WITH SPT6 (IWS1) as an essential transcription elongation factor that integrates brassinosteroid (BR) and gibberellin (GA) pathways to promote thermomorphogenesis in Arabidopsis. Under prolonged warm temperatures, IWS1 transcription and protein levels increase, and its loss-of-function mutant exhibits severe hypocotyl elongation defects comparable to pif4-2, a mutant of the central thermomorphogenesis regulator. Mechanistically, IWS1 forms a complex with the BR transcription factor BZR1 to directly bind the PIF4 promoter in a temperature-dependent manner, facilitating BZR1 chromatin occupancy, and activating PIF4 transcription. This IWS1-BZR1 module further enhance BR biosynthesis by upregulating the BR biosynthetic gene BR6ox2. Simultaneously, IWS1 interacts with the GA-associated transcription factor TCP14/15 to activate expression of the GA biosynthetic gene GA20ox2 and promotes DELLA degradation, thereby releasing GA-mediated growth repression. Together, IWS1 orchestrates BR and GA pathways by concurrently enabling PIF4-mediated transcription and relieving DELLA-mediated growth repression, thereby enhancing thermomorphogenesis. Our findings reveal IWS1 as a central coordinator that integrates hormonal signals to positively regulate thermal acclimation in plants.
Chen et al. (Sun,) studied this question.