Catalytic subunit of cellulose synthase terminal complexes ('rosettes'), required for beta-1,4-glucan microfibril crystallization, a major mechanism of the cell wall formation (By similarity). Involved in the secondary cell wall formation. [(1->4)-beta-D-glucosyl](n) + UDP-alpha-D-glucose = [(1->4)-beta-D-glucosyl](n+1) + H(+) + UDP Binds 2 Zn(2+) ions per subunit. Glycan metabolism; plant cellulose biosynthesis. Plants develop a brittle culm (bc) phenotype with a reduction of up to 75% percent of cellulose content in culm. Belongs to the glycosyltransferase 2 family. Plant cellulose synthase subfamily.