A weak OsBRI1 allele in Zhonghua 11 as a genetic tool for brassinosteroid signaling research in rice

• Zhonghua 11 and Taichung 65 exhibit distinct physiological and molecular responses to brassinosteroids despite their close japonica relationship. • A near-isogenic d61-zh11 line was developed to transfer the classical BR-insensitive mutation into a modern functional genomics background. • d61-zh11 provides a standardized reference mutant for reproducible and systems-level dissection of BR signaling in rice. Brassinosteroids (BRs) are essential plant hormones that regulate rice architecture, development, and yield. Although both belong to the japonica subspecies, rice cultivars such as Zhonghua 11 (ZH11) and Taichung 65 (TC65) exhibit marked differences in BR-related traits, reflecting divergent genetic backgrounds and breeding histories. ZH11 has become a model genotype for functional genomics due to its compact growth, rapid flowering, high transformation efficiency, and complete T2T genome. However, the absence of key BR signaling mutants, such as d61 ( OsBRI1 ), limits its utility for BR-related studies. Here, we performed detailed morphological, physiological, and molecular comparisons between ZH11 and TC65, revealing substantial variation in their responses to exogenous BRs. To establish a BR-deficient platform in a modern genetic background, we introgressed a weak d61-1 allele from TC65 into ZH11 via backcrossing and marker-assisted selection. The resulting line, d61-zh11 , displays partial BR insensitivity, a compact stature, and stable inheritance, while avoiding severe pleiotropic defects. This non-transgenic mutant enables precise dissection of BR signaling in ZH11 and serves as a versatile resource for forward/reverse genetic screens. Our work bridges classical hormone genetics with modern genomics, offering a valuable tool for BR-related research in rice improvement.