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. Required during embryogenesis for cell elongation, orientation of cell expansion and complex cell wall formations, such as interdigitated pattern of epidermal pavement cells, stomatal guard cells and trichomes. Plays a role in lateral roots formation, but seems not necessary for the development of tip-growing cells such as root hairs. The presence of each protein CESA1 and CESA6 is critical for cell expansion after germination. [(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 CESA3 and CESA6. Assembly with CESA3 and CESA6 is required for functional complex in primary cell wall cellulose synthesis. Interacts with STL1 and STL2, but not with GOT1 (PubMed:27277162). Binds to CSI1 (PubMed:20616083). Expressed in germinating seeds, seedlings, roots, stems, shoots leaves and flowers, but not in mature flowers. Expressed throughout the embryo during all steps of embryogenesis, and decrease toward the bent-cotyledon stage. 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.