Possesses glucomannan synthase and mannan synthase activities in vitro. Mannan synthase consists of a 4-beta-mannosyltransferase activity on mannan using GDP-mannose. The beta-1,4-mannan product is the backbone for galactomannan synthesis by galactomannan galactosyltransferase. Galactomannan is a noncellulosic polysaccharides of plant cell wall (PubMed:15647349, PubMed:17307900). Required for lateral root development (PubMed:14612582). GDP-mannose + (glucomannan)n = GDP + (glucomannan)n+1. Expressed in cotyledons at the base of the hypocotyls, in root elongation zone, lateral root primordia, vascular system of young leaves, abscission zone of the pedicle,. Plants develop approximately half the number of lateral roots without affecting significantly aerial parts development, and are resistant to A.tumefaciens transformation. Belongs to the glycosyltransferase 2 family. Plant cellulose synthase-like A subfamily.