STARCH DEGRADATION IN INTACT AMYLOPLASTS ISOLATED FROM CAULIFLOWER FLORAL BUDS (BRASSICA-OLERACEA L)

Abstract

Isolated amyloplasts from cauliflower buds are capable of mobilizing starch. This mobilization is strongly dependent upon the intactness of the plastids and is linear with time for up to 30 min. The degradation of starch occurs via a hydrolytic breakdown and is stimulated by ATP-dependent phosphorylation of products of this hydrolysis. The rate of phosphorolytic stimulation of starch degradation is negligible. Carbohydrates derived from starch degradation do not appear to enter the oxidative pentose-phosphate pathway. Phosphorylation of hydrolytically solubilized intermediates leads to the synthesis of dihydroxyacetone phosphate and hexose phosphates. 3-Phosphoglyceric acid acts as an inhibitor of starch mobilization. The export of labelled phosphorylated intermediates from amyloplasts containing C-14-labelled starch implies the presence of an amyloplastic phosphate translocator in these plastids. The physiological role of varying concentrations of 3-phosphoglyceric acid is discussed with respect to the regulation of a metabolic cycle of simultaneous starch synthesis and degradation.