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. GDP-mannose + (glucomannan)n = GDP + (glucomannan)n+1. Belongs to the glycosyltransferase 2 family. Plant cellulose synthase-like A subfamily.