Different effects of galactose and mannose on cell proliferation and intracellular soluble sugar levels in <Emphasis Type="Italic">Vigna angularis</Emphasis> suspension cultures

Plant cells utilize various sugars as carbon sources for growth, respiration and biosynthesis of cellular components. Suspension-cultured cells of azuki bean (Vigna angularis) proliferated actively in liquid growth medium containing 1% (w/v) sucrose, glucose, fructose, arabinose or xylose, but did not proliferate in medium containing galactose or mannose. These two latter sugars thus appeared distinct from other sugars used as growth substrates. Galactose strongly inhibited cell growth even in the presence of sucrose but mannose did not, suggesting a substantial difference in their effects on cell metabolism. Analysis of intracellular soluble-sugar fractions revealed that galactose, but not mannose, caused a conspicuous decrease in the cellular level of sucrose with no apparent effects on the levels of glucose or fructose. Such a galactose-specific decrease in sucrose levels also occurred in cells that had been cultured together with glucose in place of sucrose, suggesting that galactose inhibits the biosynthesis, rather than uptake, of sucrose in the cells. By contrast, mannose seemed to be metabolically inert in the presence of sucrose. From these results, we conclude that sucrose metabolism is important for the heterotrophic growth of cells in plant suspension-cultures.