Brittle Culm15 Encodes a Membrane-Associated Chitinase-Like Protein Required for Cellulose Biosynthesis in Rice

Plant chitinases, a class of glycosyl hydrolases, participate in various aspects of normal plant growth and development, including cell wall metabolism and disease resistance. The rice ( Oryza sativa ) genome encodes 37 putative chitinases and chitinase-like proteins. However, none of them has been characterized at the genetic level. In this study, we report the isolation of a brittle culm mutant, bc15 , and the map-based cloning of the BC15 / OsCTL1 (for chitinase-like1) gene affected in the mutant. The gene encodes the rice chitinase-like protein BC15/OsCTL1. Mutation of BC15 / OsCTL1 causes reduced cellulose content and mechanical strength without obvious alterations in plant growth. Bioinformatic analyses indicated that BC15/OsCTL1 is a class II chitinase-like protein that is devoid of both an amino-terminal cysteine-rich domain and the chitinase activity motif H-E-T-T but possesses an amino-terminal transmembrane domain. Biochemical assays demonstrated that BC15/OsCTL1 is a Golgi-localized type II membrane protein that lacks classical chitinase activity. Quantitative real-time polymerase chain reaction and β-glucuronidase activity analyses indicated that BC15 / OsCTL1 is ubiquitously expressed. Investigation of the global expression profile of wild-type and bc15 plants, using Illumina RNA sequencing, further suggested a possible mechanism by which BC15/OsCTL1 mediates cellulose biosynthesis and cell wall remodeling. Our findings provide genetic evidence of a role for plant chitinases in cellulose biosynthesis in rice, which appears to differ from their roles as revealed by analysis of Arabidopsis ( Arabidopsis thaliana ).