OsCYP51H9 integrates brassinosteroid biosynthesis and nitrogen sensing to enhance grain development and nitrogen utilization in rice.
Nitrogen use efficiency (NUE) and grain development are pivotal for rice yield improvement, particularly under low-nitrogen (LN) conditions. Cytochrome P450 CYP51 family members are conserved obtusifoliol 14α-demethylases essential for phytosterol and brassinosteroid (BR) biosynthesis; however, the biological roles of the rice CYP51H subfamily remain largely unclear. Here, we characterized OsCYP51H9, a gene encoding an endoplasmic reticulum-localized protein highly expressed in reproductive and vascular tissues. Loss-of-function mutants (oscyp51h9) and RNAi lines exhibited BR-deficient phenotypes, including reduced plant height, impaired root growth, smaller grains, and erect leaves, which were associated with disrupted phytosterol and BR biosynthesis. While preliminary metabolite profiling indicated a potential link between OsCYP51H9 and triterpene metabolism via β-amyrin, this requires further validation. Notably, under LN conditions, OsCYP51H9-overexpressing plants displayed enhanced root growth, increased grain yield, and higher nitrogen accumulation. This improvement coincided with the upregulation of key nitrogen-responsive transcription factors, including OsWRKY69 and OsDREB1C/B. Collectively, our results suggest that OsCYP51H9 participates in the phytosterol-BR pathway and plays a positive role in rice adaptation to low-nitrogen environments, providing a potential target for molecular breeding.