Antagonistic Transcription Factor Complexes Modulate the Floral Transition in Rice

Plants measure day or night lengths to coordinate specific developmental changes with a favorable season. In rice (Oryza sativa), the reproductive phase is initiated by exposure to short days when expression of HEADING DATE 3a (Hd3a) and RICE FLOWERING LOCUS T 1 (RFT1) is induced in leaves. The cognate proteins are components of the florigenic signal, and move systemically through the phloem to reach the shoot apical meristem (SAM). In the SAM, they form a transcriptional activation complex with the bZIP transcription factor OsFD1, to start panicle development. Here, we show that Hd3a and RFT1 can form transcriptional activation or repression complexes also in leaves, and feed-back to regulate their own transcription. Activation complexes depend upon OsFD1 to promote flowering. However, additional bZIPs, including Hd3a BINDING REPRESSOR FACTOR 1 (HBF1) and HBF2 form repressor complexes that reduce Hd3a and RFT1 expression to delay flowering. We propose that Hd3a and RFT1 are also active locally in leaves to fine-tune photoperiodic flowering responses.