Although some genes that encode sensory or regulatory elements for photoperiodic flowering are conserved between the long-day (LD) plant Arabidopsis thaliana and the short-day (SD) plant rice, the gene networks that control rice flowering, and particularly flowering under LD conditions, are not well understood. We show here that RICE FLOWERING LOCUS T 1 ( RFT1 ), the closest homolog to Heading date 3a ( Hd3a ), is a major floral activator under LD conditions. An RFT1:GFP fusion protein localized in the shoot apical meristem (SAM) under LD conditions, suggesting that RFT1 is a florigen gene in rice. Furthermore, mutants in OsMADS50 , a rice ortholog of Arabidopsis SUPPRESOR OF OVEREXPRESSION OF CONSTANS 1 ( SOC1 ) did not flower up to 300 days after sowing under LD conditions, indicating that OsMADS50 , which acts upstream of RFT1 , promotes flowering under LD conditions. We propose that both positive ( OsMADS50 and Ehd1 ) and negative ( Hd1, phyB and Ghd7 ) regulators of RFT1 form a gene network that regulates LD flowering in rice. Among these regulators, Ehd1 , a rice-specific floral inducer,integrates multiple pathways to regulate RFT1 , leading to flowering under appropriate photoperiod conditions. A rice ortholog of Arabidopsis APETALA1, OsMADS14 , was expressed in the floral meristem in wild-type but not in RFT1 RNAi plants, suggesting that OsMADS14 is activated by RFT1 protein in the SAM after the transition to flowering. We have thus exposed a network of genes that regulate LD flowering in rice.
RICE FLOWERING LOCUS T 1 ( RFT1/FT-L3 ) is the closest homologue of Heading date 3a ( Hd3a ), which is thought to encode a mobile flowering signal and promote floral transition under short-day(SD) conditions. RFT1 is located only 11.5 kb from Hd3a on chromosome 6. Although RFT1 RNAi plants flowered normally, double RFT1 - Hd3a RNAi plants did not flower up to 300 days after sowing (DAS), indicating that Hd3a and RFT1 are essential for flowering in rice. RFT1 expression was very low in wild-type plants, but there was a marked increase in RFT1 expression by 70 DAS in Hd3a RNAi plants, which flowered 90 DAS. H3K9 acetylation around the transcription initiation site of the RFT1 locus had increased by 70 DAS but not at 35 DAS. In the absence of Hd3a and RFT1 expression, transcription of OsMADS14 and OsMADS15 , two rice orthologues of Arabidopsis APETALA1, was strongly reduced, suggesting that they act downstream of Hd3a and RFT1 . These results indicate that Hd3a and RFT1 act as floral activators under SD conditions, and that RFT1 expression is partly regulated by chromatin modification.