The HECT-family protein UPL3 suppresses thermomorphogenesis by modulating BZR1 accumulation

Brassinosteroid (BR) promotes hypocotyl growth at warm temperatures through BZR1 by directly regulating the expression of PIF4, or by interacting with PIF4 to enhance its downstream gene expression; however, how BZR1 level is modulated by warm temperature is not known. We found that the Homology to E6-Associated Carboxy-Terminus (HECT) E3 ubiquitin ligase UPL3 plays an important role in regulating the accumulation of BZR1 under warm temperature conditions. UPL3 interacted with BZR1 both in vitro and in vivo; mutations in UPL3 led to increased BZR1 accumulation levels and accelerated hypocotyl growth under warm temperature conditions, which was inhibited by the BR biosynthesis inhibitor BRZ. The expression of PIF4 and its downstream genes was found to be increased in upl3 mutants, and PIF4 is epistatic to UPL3, further supporting that UPL3 is an upstream regulator of PIF4 in suppressing hypocotyl growth at warm temperatures. Overall, our findings highlight the importance of UPL3 in modulating BZR1 accumulation and, consequently, regulating thermo-responsive hypocotyl growth in Arabidopsis.