OsFD4 promotes the rice floral transition via florigen activation complex formation in the shoot apical meristem

In rice, the florigens Heading Date 3a (Hd3a) and Rice Flowering Locus T 1 (RFT1), OsFD-like basic leucine zipper (bZIP) transcription factors, and Gf14 proteins assemble into florigen activation/repressor complexes (FACs/FRCs), which regulate transition to flowering in leaves and apical meristem. Only OsFD1 has been described as part of complexes promoting flowering at the meristem, and little is known about the role of other bZIP transcription factors, the combinatorial complexity of FAC formation, and their DNA-binding properties. Here, we used mutant analysis, protein-protein interaction assays and DNA affinity purification (DAP) sequencing coupled to in silico prediction of binding syntaxes to study several bZIP proteins that assemble into FACs or FRCs. We identified OsFD4 as a component of a FAC promoting flowering at the shoot apical meristem, downstream of OsFD1. The osfd4 mutants are late flowering and delay expression of genes promoting inflorescence development. Protein-protein interactions indicate an extensive network of contacts between several bZIPs and Gf14 proteins. Finally, we identified genomic regions bound by bZIPs with promotive and repressive effects on flowering. We conclude that distinct bZIPs orchestrate floral induction at the meristem and that FAC formation is largely combinatorial. While binding to the same consensus motif, their DNA-binding syntax is different, suggesting discriminatory functions.