Myo-inositol is a precursor of many plant metabolites, including polyols, cell wall components and phosphoinositides. The first committed step in the de novo myo-inositol synthetic pathway is catalysed by the enzyme 1D-myo-inositol 3-phosphate synthase (MIPS; EC 220.127.116.11), which converts D-glucose 6-phosphate to 1D-myo-inositol 3-phosphate. Suppression of MIPS activity by an antisense RNA approach in transgenic potato (Solanum tuberosum L.) plants to below 20% of the wild-type level in leaves resulted in strongly reduced levels of inositol, galactinol and raffinose (approximately 7%, 5% and 12%, respectively, of wild-type values). In contrast, increases were observed for concentrations of hexose phosphates (up to 1.7-fold), sucrose (twofold) and starch (two- to fourfold). Transgenic plants exhibited reduced apical dominance, altered leaf morphology, precocious leaf senescence and a decrease in overall tuber yield. These observations indicate a crucial role for myo-inositol in plant physiology and development.