Photoperiodic flowering regulators are required for nitrogen-dependent flowering delay in rice under long-day condition

Plants recognize environmental information as external signals, which they use to coordinate developmental processes for their survival and reproduction. In the agricultural production of rice, the primary staple crop worldwide, soil nitrogen (N) is an important environmental factor. Heading date, a key trait for rice quality and yield, is modulated by N fertilisation; however, the molecular mechanisms underlying N-dependent flowering regulation remain unclear. Here, we conduct a genome-wide association study using differences in heading dates of Japanese rice cultivars grown under different N conditions. We identify Hd6, which is known to be involved in photoperiodic flowering regulation, as a key signalling component in the phenotypic variation observed under various N conditions. Further analyses using near-isogenic lines reveal that not only Hd6 but also Hd2 and Hd1 are required for the delayed flowering caused by N fertilisation. We also discover that Hd6 regulates floral inducer genes by stabilizing Hd2 through phosphorylation in response to N conditions and that the Hd6-Hd2 module can counteract the transcriptional regulation of Hd1. This study elucidates the molecular pathway directly linking N responses to flowering regulation in rice, providing insights for novel breeding and cultivation strategies.