U-Shaped Response of Flowering Time to Low and High Nitrogen via a Transcription Feedback Loop in Rice.

Nitrogen (N) availability regulates flowering time (heading date) in rice through a U-shaped response, where both deficient and superior N delay flowering. This N-dependent plasticity of flowering time impacts productivity, N use efficiency and rotation schedules, while the underlying mechanisms remain unclear. Here, a reciprocal feedback loop between two transcript factors is identified, N-mediated heading date 1(Nhd1) and Ghd7, that orchestrates this U-shaped response under long-day condition. Deficient-N delays flowering via repressing Nhd1 regulated by Ghd7, while superior-N delays flowering by activating Ghd7 mediated through glutamine-induced Nhd1. Genetic and molecular evidence further demonstrates that Heading date 3a (Hd3a) is mainly required for the U-shape response regulated by the Nhd1-Ghd7 module. Notably, natural variation analysis reveals that antagonistic combinations of Nhd1 and Ghd7 alleles are selected during rice domestication and correlate with geographic patterns of soil N deposition. Furthermore, Nhd1 alleles differ in both transcriptional activity and protein function, enabling fine-tuning of flowering sensitivity to N availability in weak/none alleles of Ghd7. Collectively, this study identifies an Nhd1-Ghd7 regulatory module that regulates the U-shaped flowering response to N, offering mechanistic insight and potential targets for breeding N-resilient rice.