Effect of Inorganic Phosphate on the Levels of Amino Acids in Suspension-cultured Cells of Catharanthus roseus*

The levels of free amino acids in suspension cultures of Catharanthus roseus were monitored for 96 h after stationary-phase cells were transferred to fresh complete (‘+Pi’)‡ or phosphate-deficient (‘−Pi’) Murashige-Skoog medium. The levels of most amino acids in the cells in the ‘+Pi’ medium decreased, although levels of a few amino acids increased transiently. In contrast, levels of all free amino acids increased, but to different extents, in the cells grown in the ‘−Pi’ medium. A greater than 30-fold increase was found in the levels of glycine, glutamine, arginine and γ-aminoadipic acid during the experimental period. The radioactivity of [U−14C]sucrose was incorporated into at least seven amino acids in the cells grown in the ‘+Pi’ and the ‘−Pi’ medium, and the most heavily labelled free amino acids were glutamine and glycine. The level of protease (endopeptidase) decreased rapidly when the cells were transferred to the fresh ‘+Pi’ medium, while its level remained constant in the cells in ‘−Pi’ medium. The results suggest that the decrease of the levels of free amino acids in the cells in ‘+Pi’ medium is associated with rapid protein synthesis. Accumulation of free amino acids in the Pi-deficient cells seems to originate both from de novo biosynthesis of amino acids and from net degradation of proteins.