The biosynthesis of starch in the proplastids of germinating Ricinus communis endosperm tissue

Abstract

Electron micrographs of endosperm tissue from germinating seedlings of Ricinus communis L. cv Hale show the presence of proplastids which contain prominent starch grains. The content of starch in the tissue was found to increase from 500 [mu]g per seed in imbibed seed to 1100 [mu]g per seed in 5-day-old germinated seedlings. The maximal rate of starch deposition was 1.1 nmoles glucose incorporated per min per seed. The amount of starch remaining in the endosperm 9 days after imbibition was 200 [mu]g per seed. The accumulation of starch followed the same developmental pattern as the synthesis of sucrose, free reducing sugars, and other metabolic processes found in this tissue. The two key enzymes of starch synthesis, adenosine diphosphoglucose (ADPG) pyrophophorylase and ADPG-starch glucosyl transferase exhibited maximal activities in the endosperm at 4 and 5 days after germination, respectively. The maximal activity of ADPG pyrophosphorylase was 8.17 nmoles ADPG formed per min per seed, while starch synthetase exhibited an activity of 124 nmoles glucose incorporated per min per seed. These levels of enzyme activity are sufficient to account for the amount of starch synthesis. Other enzymes which may be involved in starch synthesis include 3-phosphoglycerate kinase which showed an activity of 8.76 nmoles per min per seed, triose-P isomerase (2.56 [mu]moles per min per seed, fructose-1,6-bisphosphate aldolase (0.99 [mu]moles per min per seed), fructose-1.6-bisphosphatase (0.23 [mu]moles per min per seed), phosphoglucose isomerase (12.6 [mu]moles per min per seed), and phosphoglucomutase (9.72 [mu]moles per min per seed). The activities of these enzymes compare well with literature values from other tissues, as re-reported by Latzko and Gibbs (52), Simcox et al. (69), Nishimura and Beevers (59), and Benedict (11)...