Galactomannan gums are widely used in the food and oil industries, and there is considerable interest in applying biotechnological approaches to improve their physical properties. A mannan synthase from guar (Cyamopsis tetragonoloba) was expressed under the control of a bean beta-phaseolin promoter in transgenic Medicago truncatula. Although the expression of exogenous mannan synthase caused a slight decrease in galactomannan levels in Medicago, the molecular weight and viscosity of the polymer were significantly increased, although the mannose to galactose ratio and degree of polydispersity remained unchanged. At the same time, expression of about 2.8% of the genes was altered significantly in the seeds of transgenic Medicago lines analysed by Affymetrix genome chip, with a particularly striking induction of putative trehalose phosphate synthase genes. Mannan synthase expression also caused large alterations in the levels of a number of sugars and sugar alcohols, suggesting that over-expression of a processive glycosyltransferase perturbs the mechanisms of sugar sensing and/or homeostasis, possibly involving signalling via trehalose-6-phosphate.