Melibiose, A Suitable, Non-Permeating Osmoticum for Suspension-Cultured Tobacco Cells

Dracup, M., Gibbs, J. and Greenway, H. 1986. Melibiose, a suitablenon-permeating osmoticum for suspension-cultured tobacco cells.—J.exp. Bot. 37: 1079–1089. A neutral, non-permeating osmoticum with low molecular weightwas required for studies involving responses to water deficitand salinity by suspension-cultured cells of tobacco. For thispurpose, raffinose, sorbitol, mannitol and melibiose were evaluated. Raffinose was hydrolysed by cells which were then able to useone of the products, namely fructose, for growth. Sorbitol andmannitol were not used for growth but were taken up by cells.After 96 h in media containing 50 mol m^–3 of sorbitolor mannitol as the carbon source, cells contained 85 mol m^–3sorbitol or 45 mol m^–3 mannitol. At least part of theuptake of sorbitol would have been active as sorbitol was transportedagainst a concentration gradient. Melibiose was one of the products of hydrolysis of raffinoseand proved to be an effective osmoticum. When supplied as thesole source of sugar for cells, melibiose was neither hydrolysednor taken up by cells. Furthermore, melibiose was not toxicsince adding 50 mol m^–3 to a culture medium containingglucose did not affect growth of cells. Key words: Sorbitol, mannitol, uptake