MYOINOSITOL PHOSPHATES AS IMPLICATED IN METABOLIC AND SIGNAL-TRANSDUCTION PATHWAYS IN PLANTS

An account of the establishment of a novel metabolic cycle involving myoinositol phosphates and glucose-6-P in plants elucidating the probable pathway of synthesis and degradation of myoinositol hexakis phosphate has been documented. Glucose-6-P is used during seed formation by myoinositol(1)P synthetase for the production of myoinositol(1)P which is subsequently phosphorylated to myoinositol pentakis-phosphate, i.e, Ins(1,3,4,5,6)P-5 by phosphoinositol kinase and untimately to InsP(6) by another enzyme, i.e. InsP(6)-ADP-phosphotransferase. InsP(6) is stored in the seeds as phosphate reservoir. During germination InsP(6) is hydrolysed by an enzyme phytase ultimately to myoinositol which is required for cell wall biosynthesis. As in the early phase of germination Ins(1,3,4,5,6)P-5 is also formed by the reversible reaction of InsP(6)-ADP-phosphotransferase and hydrolysed by phytase to Ins(1)P prior to finally giving rise to myoinositol. This in turn can be converted to ribulose-5-P by myoinositol(1) P-dehydrogenase giving the feed-back to the production of glucose-6-P during early phase of germination through pentose shunt pathway. During the operation of this cycle ATP and NADH are generated, providing necessary energy at the early germination period. Corollary to this metabolic cycle an intermediary phytase product, i.e. Ins(2,4,5)P-3 has been implicated to a pathway leading to Ca2+ mobilsation in plant cells. This is demonstrated when InsP(6)-phytase complex was added after a definite time of hydrolysis which coincides with the time of optimal production of Ins(2,4,5)P-3 bound to phytase. The in vitro constituted Ins(1,4,5)P-3- or Ins(2,4,5)P-3 phytase complex has also been found effective in releasing Ca2+ from cellular stores, the release being 45% more as compared to that by free InsP(3) under identical conditions. Thus the alternative pathway of specific InsP(3) generation and its involvement in Ca2+ mobilisation has been proposed and elucidated.