The rice <i>BRITTLE CULM 4</i> gene encodes a membrane protein affecting cellulose synthesis in the secondary cell wall

The formation of secondary cell walls, which provide mechanical strength to the plant body, depends on numerous factors. Studies on rice brittle culm (bc) mutants allow us to identify these factors and gain insights into the mechanisms of secondary cell wall formation. Rice bc4 is a recessive bc mutant with fragile culms and leaves, similar to other bc mutants. We found that the bc4 mutant exhibited reduced cellulose content in the culm cell walls compared to the japonica cultivar Taichung 65 and the indica cultivar Kasalath, while hemicellulose content remained unchanged. Transmission electron microscopy revealed reduced cell wall thickness in the sclerenchyma cells of the bc4 culm, indicating that BC4 contributes to normal cellulose synthesis or deposition in secondary cell walls. Positional cloning and subsequent genome sequencing revealed that the BC4 gene encodes a four α-helical transmembrane protein with 205 amino acids, and that the bc4 mutation results in a premature termination codon in this gene. Four bc4 mutants generated from the japonica cultivar Nipponbare, using genome editing with the CRISPR/Cas9 system, exhibited reduced cellulose content along with bc phenotypes. Gene clustering analysis based on expression patterns and metabolomic analysis suggested that BC4 functions independently from secondary cell wall cellulose synthase catalytic subunits and COBRA-like protein. These results suggest that the BC4 protein is a newly identified factor involved in cellulose synthesis or deposition in the secondary cell walls of rice.