Catalytic subunit of cellulose synthase terminal complexes ('rosettes'), required for beta-1,4-glucan microfibril crystallization, a major mechanism of the cell wall formation. Involved in the secondary cell wall formation. Required for the xylem cell wall thickening. [(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. Interacts with CESA4 and CESA7. Assembly with CESA4 and CESA7 is required for functional complex and localization in secondary cell wall deposition sites. Interacts with STL1 and STL2, but not with GOT1 (PubMed:27277162). Confined to secondary cell wall developing tissues such as xylems and interfascicular regions. Expressed in young plants, stems, flowers and inflorescences, but not in leaves. Not found in embryos. Increasing amount as stems mature. S-acylated. Enhanced resistance to the pathogens Ralstonia solanacearum and Plectosphaerella cucumerina. Belongs to the glycosyltransferase 2 family. Plant cellulose synthase subfamily.