ON THE QUESTION OF DIRECT TRANSPORT OF URIDINE-DIPHOSPHATE GLUCOSE INTO PLASTIDS OF THE STORAGE ORGAN OF SUGAR-BEET

We investigated uptake of UDP[C-14]-glucose in fractions of highly intact plastids isolated from the storage organ of sugar beet Beta vulgaris L. It is demonstrated that plastids are capable of metabolizing exogenous UDP-glucose (UDPG) with formation of substances of a polymeric nature whose synthesis is not associated with any admixture of other cellular components, but rather reflects metabolic activity of the organelles themselves. Synthesis of polysaccharides is curtailed after breaking of the organelle envelope by 0.1% Triton X-100 or by osmotic shock. After incubation of plastid fractions with UDP[C-14]-glucose, neither this compound nor other sugar phosphates was detected in the stromal space of the organelles, i.e., UDPG did not cross the membrane envelope in unaltered form. It is hypothesized that a UDPG translocator built into the membrane performs guided transport of the activated glucosyl from exogenous UDPG with synthesis of the polysaccharide inside the plastids. Products synthesized include substances hydrolyzable by alpha-amyloglucosidase (starch) and beta-glucosidase (apparently callose). Chase experiments showed redistribution of radioactivity taken up from beta-glucan into the composition of starch. It is hypothesized that callose preliminarily synthesized in the plastid stroma from exogenous UDPG can act as an intermediate compound in biosynthesis of starch, the main storage compound of the organelles.