The mechanical strength of rice culms plays a critical role in plant growth and development and is directly associated with lodging resistance. Brittle culm mutants serve as valuable genetic resources for dissecting the molecular mechanisms underlying culm strength regulation. In this study, a brittle culm mutant, named ebc (extreme brittle culm), was isolated from a T-DNA insertion population of the indica variety ‘Guangluai 4’ (GLA4). The causal gene was mapped using TAIL-PCR and MutMap⁺ approaches, and its identity and expression pattern were further characterized by RNA-seq and qRT-PCR. The results showed that, compared with the wild type, ebc was characterized by brittleness in all examined organs, accompanied by abnormal secondary cell wall structure and composition; meanwhile, no significant differences were observed in key agronomic traits including plant height, per-plant grain yield, seed setting rate, and grain shape between ebc and wild-type GLA4. Genetic analysis indicated that the brittle phenotype is controlled by a single recessive nuclear gene. TAIL-PCR analysis confirmed that the T-DNA insertion was located in an intergenic region, with no causal relationship to the brittle phenotype. MutMap⁺ mapping identified a candidate causal mutation in BRITTLE CULM 1 (BC1): a single guanine (G) deletion at codon 427 of its third exon (CGG→CG). This mutation triggers a frameshift mutation and disrupts the C-terminal functional domain including the conserved GPI modification site. qRT-PCR showed that the expression level of BC1 in the culms and leaves of ebc was significantly decreased. Complementation of the mutant with the BC1 whole genomic region restored the wild-type phenotype, confirming that ebc is a novel allelic mutant of BC1. Transcriptome analysis revealed that differentially expressed genes (DEGs) were primarily involved in biological processes such as transmembrane transport and carbohydrate metabolism, were significantly associated with cellular components including the plasma membrane, and were enriched in pathways related to secondary metabolite biosynthesis and phenylpropanoid biosynthesis. qRT-PCR validation showed that the expression levels of multiple key enzyme-encoding genes involved in lignin biosynthesis within the phenylpropanoid pathway were significantly altered in ebc, consistent with the trends observed in the transcriptome data. In conclusion, the brittle culm gene ebc is a novel allele of BC1. BC1 may participate in the coordinated assembly of cell wall components through feedback regulation of the phenylpropanoid pathway, providing a new basis for further elucidating its molecular mechanism.