RICE BZIP TRANSCRIPTION FACTORS REGULATE THE FLORAL TRANSITION VIA FORMATION OF FLORIGEN ACTIVATION COMPLEXES (FACS)

When external and internal cues are favorable, rice plants stop to produce leaves and start to make flowers. This process is called floral transition and occurs in leaves and at the Shoot Apical Meristem (SAM). In rice, Heading Date 3a (Hd3a) and RICE FLOWERING LOCUS T 1 (RFT1) are two signal molecules, called florigens, which are expressed and translated in leaves. Florigens are small and globular proteins with non-cell autonomous functions, as they can move via the vascular tissue of rice and arrive in the SAM, where they promote the transition from the vegetative to the reproductive phase. In this tissue they bind the FD-like bZIP transcription factor OsFD1 thanks to the help of Gf14c, which bridges their interaction, and they form the Florigen Activation Complex (FAC). OsMADS14 and OsMADS15 are FAC targets and they are required for proper flower development. 5 Because the florigens cannot bind the DNA alone, FD-like bZIPs have a crucial role during the floral transition. For this reason, we focused on FACs formation and we characterized different bZIPs involved in the floral transition. We found that in leaves OsFD1-FAC regulates flowering time via positive feedback loops on Hd3a and RFT1 expression. We then characterized OsHBF1, a FD-like bZIP, which generates a negative feedback loop on florigens production via OsHBF1-FAC formation. We isolated the bZIP OsFD4 which promotes the floral transition in the SAM in parallel to OsFD1. OsFD4 interacts with Hd3a and RFT1 in FACs and regulates the expression of OsMADS14 and OsMADS15, such as OsFD1-FAC does. Finally, we performed DAP-sequencing with OsFD1, OsFD4 and OsHBF1 to analyze differences among these bZIPs and to find their targets. Results indicate that bZIPs bind the same consensus motif, but the distance between two motives varies depending on the bZIP considered, suggesting that it could be causal to their different role during the floral transition.