METABOLIC-FATE OF MYOINOSITOL IN SOYBEAN CALLUS CELLS

For cell division, soybean callus cells in suspension culture require an intracellular level of myo-inositol (Ins) in excess of that required to maintain viability and growth (M. Biffen and D.E. Hanke, Biochem. J., 265 (1990) 809-814). In pursuit of the cause of this requirement we have investigated Ins metabolism at different intracellular levels of Ins generated by incubating cells with 2-deoxyglucose or varying concentrations of Ins. Incorporation of Ins into phospholipid was constant over the range of intracellular Ins concentrations, whereas when intracellular Ins levels were raised there was a proportionately higher incorporation of Ins into pectin, the result of an increase in the relative contribution of Ins to pectin monomers. Oxidation of Ins to uronic acid residues of pectin was inhibited by the Ins-antagonist 2-O, C-methylene myo-inositol (MMO). The inhibition was prevented by the addition of Ins. MMO treatment did not affect cell division, showing that a high rate of Ins oxidation is not necessary for cell division. MMO treatment reduced cell fresh mass and the incorporation of [H-3]Ins into phospholipid and both of these effects were not prevented by exogenous Ins. It follows that where effects of MMO are detected they may not be due specifically to inhbition of Ins oxidation. In conclusion, we have largely eliminated the possibility that the requirement for Ins in cell division is to sustain Ins incorporation into pectin. Instead our results suggest that the function of Ins oxidation and incorporation into pectin is the irreversible elimination of Ins in excess of the cells immediate requirements.