Glycosyltransferases (GTs) are one of the largest enzyme groups required for the synthesis of complex wall polysaccharides and glycoproteins in plants. However, due to the limited number of related mutants that have observable phenotypes, the biological function(s) of most GTs in cell-wall biosynthesis and assembly have remained elusive. We report here the isolation and in-depth characterization of a brittle rice mutant, brittle culm 10 (bc10). bc10 plants show pleiotropic phenotypes, including brittleness of the plant body and retarded growth. The BC10 gene was cloned through a map-based approach, and encodes a Golgi-located type II membrane protein that contains a domain designated as 'domain of unknown function 266' (DUF266) and represents a multiple gene family in rice. BC10 has low sequence similarity with the domain to a core 2 beta-1,6-N-acetylglucosaminyltransferase (C2GnT), and its in vitro enzymatic activity suggests that it functions as a glycosyltransferase. Monosaccharide analysis of total and fractioned wall residues revealed that bc10 showed impaired cellulose biosynthesis. Immunolocalization and isolation of arabinogalactan proteins (AGPs) in the wild-type and bc10 showed that the level of AGPs in the mutant is significantly affected. BC10 is mainly expressed in the developing sclerenchyma and vascular bundle cells, and its deficiency causes a reduction in the levels of cellulose and AGPs, leading to inferior mechanical properties.