Possesses 4-beta-mannosyltransferase activity on mannan using GDP-mannose. The beta-1,4-mannan product is the backbone for galactomannan synthesis by galactomannan galactosyltransferase. The galactomannan is a hemicellulosic storage polysaccharide accumulated in the form of secondary wall thickenings in the seed endosperm. GDP-mannose + (glucomannan)n = GDP + (glucomannan)n+1. Expressed exclusively in the endosperm of developing seeds, 25 to 30 days after flowering. Not expressed in the embryo or seed coat. Guar gum also called guaran is a galactomannan extracted from guar seed endosperm. Gums are most commonly used as food additives to provide stiffness and texture, prevent ice crystal formation, maintain crispiness, and retain moisture. They have many other non-food applications such as dying and printing aids in the textile industry, thickeners for shampoos and conditioners, binders and hardeners for paper, rheological facilitators for concrete, and drilling agents for oil and gas wells. Galactomannan is accumulated only in the endosperm and constitutes more than 90% of this tissue at maturity. Galactomannan is made by the combined actions of two enzymes: mannan synthase (ManS), which makes beta-1,4-linked mannan backbone, and alpha-galactosyltransferase, which adds galactosyl residues to the mannan backbone. The mannose/galactose ratio of guar galactomannan is 2. The degree of galactosyl substitution on the mannan backbone determines the quality of galactomannan as a gum. A mannose/galactose ratio of 4 provides a gum of higher quality than a ratio of 2. Belongs to the glycosyltransferase 2 family. Plant cellulose synthase-like A subfamily.