Florigen and cytokinin signaling antagonistically regulate FLOWERING LOCUS T-LIKE1 to drive a florigen relay that facilitates inflorescence development in rice

Plant reproductive development involves a series of transitions, with the shoot apical meristem (SAM) transitioning into the inflorescence meristem (IM) and then the floral meristem (FM). In rice (<i>Oryza sativa</i>), the florigens Heading date 3a (Hd3a) and RICE FLOWERING LOCUS T1 play central roles in these transitions. The plant hormones auxin and cytokinin shape the morphologies associated with each transition, but little is known about the factors that integrate florigen function with phytohormone signaling during these transitions. Here, we show that in rice, Hd3a activates the transcription of <i>FT-LIKE1</i> (<i>OsFTL1</i>), which encodes a mobile FT-like protein that promotes the transition from the IM to the FM, whereas cytokinin signaling suppresses its transcription. We performed single-cell resolution three-dimensional imaging to elucidate the spatiotemporal distribution of Hd3a, cytokinin, and auxin signaling during the SAM-to-IM and IM-to-FM transitions, finding that Hd3a accumulation and cytokinin signaling occupy opposite domains in the IM. Hd3a accumulated at the center of the meristem and activated the <i>OsFTL1</i> promoter, while OsFTL1 was present throughout the IM, suggesting that a "florigen relay" consisting of Hd3a from the leaves moves to the SAM and induces <i>OsFTL1</i> expression within the SAM. We propose an antagonistic mechanism mediated by Hd3a and cytokinin that modulates the abundance of OsFTL1 and thus regulates reproductive development in rice.