Probable mannan synthase which 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). Required for synthesis of a cell wall polysaccharide essential for pollen tube growth, for cell wall structure, or for signaling during plant embryo development (PubMed:12586879). GDP-mannose + (glucomannan)n = GDP + (glucomannan)n+1. Ubiquitous. Embryonic lethality when homozygous due to defective pollen tube growth and disruption of embryonic development. Belongs to the glycosyltransferase 2 family. Plant cellulose synthase-like A subfamily.