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

## TL;DR

Rice flowering time responds to nitrogen in a U-shaped pattern, controlled by a feedback loop between Nhd1 and Ghd7 genes, which could help breed rice that is more resilient to nitrogen levels.

## Contribution

Discovery of an Nhd1–Ghd7 regulatory module that explains the U-shaped flowering response to nitrogen in rice.

## Key findings

- Deficient and superior nitrogen levels delay flowering through opposing effects of the Nhd1-Ghd7 feedback loop.
- Natural variation in Nhd1 and Ghd7 alleles correlates with geographic patterns of soil nitrogen and rice domestication.
- Nhd1 and Ghd7 regulate flowering time via Hd3a/RFT1, with distinct transcriptional and protein functions fine-tuning nitrogen sensitivity.

## Abstract

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.

The “seesaw effect” of Nhd1‐Ghd7 module in regulating flowering time (days to heading in rice) responses to nitrogen (N) via Hd3a/RFT1: deficient‐N (DN) delays flowering via Ghd7‐mediated suppression of Nhd1; moderate‐N (MN) impairs this suppression, enhancing Nhd1 to promote flowering; whereas superior‐N (SN) delays flowering by Nhd1‐activated expression of Ghd7.