OsCYP51H9 integrates brassinosteroid biosynthesis and nitrogen sensing to enhance grain development and nitrogen...

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.