Galactomannan galactosyltransferase (GMGT) involved in galactomannan biosynthesis in seed endosperm. GMGT specificity is an important factor regulating the distribution and amount of alpha-1,6-galactose (Gal) substitution of the beta-1,4-linked mannan backbone. 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 nonfood 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 34 family.