Plants maintain their inteal temperature under environments with fluctuating temperatures by adjusting their morphology and architecture,an adaptive process termed thermomorphogenesis.Notably,the rhythmic pattes of plant thermomorphogenesis are goveed by day-length information.However,it remains elusive how thermomorphogenic rhythms are regulated by photoperiod.Here,we show that warm temperatures enhance the accumulation of the chaperone GIGANTEA (GI),which thermostabilizes the DELLA protein,REPRESSOR OF ga1-3 (RGA),under long days,thereby attenuating PHYTOCHROME INTERACTING FACTOR 4 (PIF4)-mediated thermomorphogenesis.In contrast,under short days,when GI accumulation is reduced,RGA is readily degraded through the gibberellic acid-mediated ubiquitination-proteasome pathway,promoting thermomorphogenic growth.These data indicate that the GI-RGA-PIF4 signaling module enables plant thermomorphogenic responses to occur in a day-length-dependent manner.We propose that the GI-mediated integration of photoperiodic and temperature information shapes thermomorphogenic rhythms,which enable plants to adapt to diel fluctuations in day length and temperature during seasonal transitions.