Phytic Acid Suspension Induced by Phosphate Synthesis and Vacuolar Accumulation in Suspension-Cultured Cells of Catharanthus roseus Induced by High Concentration of Inorganic Phosphate and Cations.

We have established a new system for studying phytic acid, myo-inositol hexakisphosphate (InsP₆) synthesis in suspension-cultured cells of Catharanthus. InsP₆ and other intermediates of myo-inositol (Ins) phosphate metabolism were measured using an ion chromatography method. The detection limit for InsP₆ was less than 50 nM, which was sufficient to analyze Ins phosphates in living cells. Synthesis of Ins phosphates was induced by incubation in high inorganic phosphate medium. InsP₆ was mainly accumulated in vacuoles and was enhanced when cells were grown in high concentration of inorganic phosphates with the cations K⁺ Ca²⁺, or Zn²⁺. However, there was a strong tendency for InsP₆ to accumulate in the vacuole in the presence of Ca²⁺ and in nonvacuolar compartments when supplied with Zn²⁺, possibly due to precipitation of InsP₆ with Zn²⁺ in the cytosol. A vesicle transport inhibitor, brefeldin A, stimulated InsP₆ accumulation. The amounts of both Ins(3)P₁ myo-inositol monophosphate synthase, a key enzyme for InsP₆ synthesis, and Ins(1,4,5)P₃ kinase were unrelated to the level of accumulation of InsP₆. The mechanisms for InsP₆ synthesis and localization into vacuoles in plant cells are discussed. [ABSTRACT FROM AUTHOR]