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Photoperiodic flowering is one of the most important pathways to gove flowering in rice (Oryza sativa),in which Heading date 1 (Hd1),an ortholog of theArabidopsis CONSTANS gene,encodes a pivotal regulator.Hd1 promotes flowering under short-day conditions (SD) but represses flowering under long-day conditions (LD) by regulating the expression of Heading date 3a (Hd3a),the FLOWERING LOCUS T (FT) ortholog in rice.However,the molecular mechanism of how Hd1 changes its regulatory activity in response to day length remains largely unknown.In this study,we demonstrated that the repression of flowering in LD by Hd1 is dependent on the transcription factor DAYS TO HEADING 8 (DTH8).Loss of DTH8 function results in the activation of Hd3a by Hd1,leading to early flowering.We found that Hd1 directly interacts with DTH8 and that the formation of the DTH8-Hd1 complex is necessary for the transcriptional repression of Hd3a by Hd1 in LD,implicating that the switch of Hd1 function is mediated by DTH8 in LD rather than in SD.Furthermore,we revealed that DTH8 associates with the Hd3a promoter to modulate the level of H3K27 trimethylation (H3K27me3) at the Hd3a locus.In the presence of the DTH8-Hd1 complex,the H3K27me3 level was increased at Hd3a,whereas loss of DTH8 function resulted in decreased H3K27me3 level at Hd3a.Taken together,our findings indicate that,in response to day length,DTH8 plays a critical role in mediating the transcriptional regulation of Hd3a by Hd1 through the DTH8-Hd1 module to shape epigenetic modifications in photoperiodic flowering.