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 CESA7 and CESA8. Assembly with CESA7 and CESA8 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 roots, hypocotyls, leaves, inflorescences and flowers. Not expressed in embryos. In young leaves, localized in transient patches along the vascular system. In young inflorescence stems, observed in vascular bundles of primary xylems. In maturing inflorescence stems, most pronounced in regions of developing interfascicular fibers. S-acylated. Enhanced resistance to the pathogens Ralstonia solanacearum and Plectosphaerella cucumerina. Belongs to the glycosyltransferase 2 family. Plant cellulose synthase subfamily.