Starch synthesizing enzymes in chloroplasts of developing leaves of spinach (Spinacea oleracea L.)

In experiments in which RuDP carboxylase activity was used as a marker for the integrity of isolated chloroplasts, more than 90% of the starch synthase activity and more than 80% of the ADP-glucose pyrophosphorylase activity of spinach leaves was found to be located in chloroplasts. Less than 2% of the UDP-glucose pyrophosphorylase was associated with chloroplasts. The activity of starch synthase per chloroplast remained almost constant during plastid replication in developing leaves on whole plants and in leaf discs cultured for 7 d on agar under different light conditions. The ADP-glucose pyrophosphorylase activity of chloroplasts increased during leaf development and was much lower in dark-grown tissues. The results suggest that the synthesis of starch is controlled by the synthesis of ADP-glucose pyrophosphorylase as well as by the previously known control of activity by metabolites such as 3-phosphoglyceric acid and inorganic phosphate.