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

Dracup, M., Gibbs, J. and Greenway, H. 1986. Melibiose, a suitable non-permeating osmoticum for suspension-cultured tobacco cells.—J. exp. Bot. 37: 1079–1089. A neutral, non-permeating osmoticum with low molecular weight was required for studies involving responses to water deficit and salinity by suspension-cultured cells of tobacco. For this purpose, raffinose, sorbitol, mannitol and melibiose were evaluated. Raffinose was hydrolysed by cells which were then able to use one of the products, namely fructose, for growth. Sorbitol and mannitol were not used for growth but were taken up by cells. After 96 h in media containing 50 mol m−3 of sorbitol or mannitol as the carbon source, cells contained 85 mol m−3 sorbitol or 45 mol m−3 mannitol. At least part of the uptake of sorbitol would have been active as sorbitol was transported against a concentration gradient. Melibiose was one of the products of hydrolysis of raffinose and proved to be an effective osmoticum. When supplied as the sole source of sugar for cells, melibiose was neither hydrolysed nor taken up by cells. Furthermore, melibiose was not toxic since adding 50 mol m−3 to a culture medium containing glucose did not affect growth of cells.