Probable catalytic subunit of cellulose synthase terminal complexes ('rosettes'), required for beta-1,4-glucan microfibril crystallization, a major mechanism of the 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. Expressed in young plants, stems, flowers and inflorescences. Not found in embryos. Belongs to the glycosyltransferase 2 family. Plant cellulose synthase subfamily.