The transcription elongation factor IWS1 promotes thermomorphogenesis by integrating the brassinosteroid and gibberellin pathways in Arabidopsis.

In plants, multiple hormone pathways orchestrate thermomorphogenesis in response to warm temperatures, yet the mechanism that coordinates these pathways remains elusive. Here, we identify INTERACT WITH SPT6 (IWS1) as an essential transcription elongation factor that coordinates the brassinosteroid (BR) and gibberellin (GA) pathways to promote thermomorphogenesis in Arabidopsis. During prolonged exposure to warm temperatures, IWS1 transcript and protein levels increase; under these conditions, the iws1 loss-of-function mutant exhibits severe hypocotyl elongation defects comparable to those of pif4-2, a mutant of the central thermomorphogenesis regulator PIF4. Mechanistically, IWS1 forms a complex with the BR transcription factor BRASSINAZOLE RESISTANT 1 (BZR1) and directly binds to the PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) promoter in a temperature-dependent manner, thereby facilitating BZR1 chromatin occupancy and activating PIF4 transcription. This IWS1-BZR1 module also enhances BR biosynthesis by upregulating the BR biosynthetic gene BR6ox2. IWS1 also interacts with the GA-associated transcription factors TCP14 and TCP15 to activate expression of the GA biosynthetic gene GA20ox2 and promotes DELLA degradation to alleviate GA-mediated growth repression. Thus, IWS1 coordinates the BR and GA pathways by concurrently promoting PIF4-mediated transcription and relieving DELLA-mediated growth repression, thereby enhancing thermomorphogenesis. Our findings identify IWS1 as a central coordinator that integrates hormonal signals to positively regulate thermal acclimation in plants.