Summary Inositol-(1,4,5)-trisphosphate (InsP3) is a second messenger in plants that increases in response to many stimuli. The metabolic consequences of this signalling pathway are not known. We reduced the basal level of InsP3 in tomato (Solanum lycopersicum cv. Micro-Tom) by expressing the human type I inositol polyphosphate 5-phosphatase (InsP 5-ptase) gene. Transgenic lines producing InsP 5-ptase protein had between 15% and 30% of the basal InsP3 level of control plants. This increased hydrolysis of InsP3 caused dramatic increases in drought tolerance, vegetative biomass and lycopene and hexose concentrations in the fruits. Transcript profiling of root, leaf and fruit tissues identified a small group of genes, including a cell-wall invertase inhibitor gene, that were differentially regulated in all tissues of the InsP 5-ptase expressing plants. Significant differences were found in the amounts of carbohydrates and organic phosphate in these plants. Plants with increased hydrolysis of InsP3 in the cytosol also showed increased net CO2-fixation and sucrose export into sink tissue and storage of hexoses in the source leaves. The increase in biomass was dependent on the supply of inorganic phosphate in the nutrient medium. Uptake and storage of phosphate was increased in the transgene expressing lines. This suggests that in tomato, increased flux through the inositol phosphate pathway uncoupled phosphate sensing from phosphate metabolism. Altering the second messenger, InsP3, revealed multiple coordinated changes in development and metabolism in tomato that have potential for crop improvement. © 2010 Blackwell Publishing Ltd.