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 primary cell wall formation. The presence of each protein CESA1 and CESA6 is critical for cell expansion. The hypocotyl elongation is based on a CESA6-dependent cell elongation in dark and a CESA6-independent cell elongation in light. The transition between these two mechanisms requires photosynthesis and PHYB, but not CRY1. The CESA6-dependent cell elongation seems to be independent of gibberellic acid, auxin and ethylene. May be involved in sensitivity to isoxaben. Associates with and moves along cortical microtubules for the process of cellulose deposition. [(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 CESA1 and CESA3. Interacts with STL1 and STL2, but not with GOT1 (PubMed:27277162). Binds to CSI1 and CSI3 (PubMed:20616083, PubMed:24368796). Colocalizes with CSI1 in a dynamic way. Expressed in germinating seeds, seedlings, roots, stems, leaves and flowers. Not present in mature flowers. Not found in embryos. Higher levels in tissues undergoing primary cell wall formation, and drop of expression when secondary wall synthesis takes place. High levels in developing seedlings and elongating stems, with a decrease at later growth stages. S-acylated. Belongs to the glycosyltransferase 2 family. Plant cellulose synthase subfamily. Truncated N-terminus.