Induction of Hexose-Phosphate Translocator Activity in Spinach Chloroplasts

Abstract

Many environmental and experimental conditions lead t o accumulation of carbohydrates in photosynthetic tissues. This situation is typically associated with major changes in the mRNA and protein complement of the cell, including metabolic repression of photosynthetic gene expression, which can be induced by feeding carbohydrates directly t o leaves. I n this study we examined the carbohydrate transport properties of chloroplasts isolated from spinach (Spinacia oleracea L.) leaves fed with glucose for severa1 days. lhese chloroplasts contain large quantities of starch, can perform photosynthetic 3-phosphoglycerate reduction, and surprisingly also have the ability t o perform starch synthesis from exogenous glucose-6-phosphate (Clc-6-P) both in the light and in darkness, similarly to heterotrophic plastids. Clucose-1 -phosphate does not act as an exogenous precursor for starch synthesis. Light, ATP, and 3-phosphoglyceric acid stimulate Clc-6-P-dependent starch synthesis. Short-term uptake experiments indicate that a nove1 Clc-6-P-translocator capacity is present in the envelope membrane, exhibiting an apparent K, of 0.54 mM and a V,,, of 2.9 pmol Clc-6-P mg-' chlorophyll h-'. Similar results were obtained with chloroplasts isolated from glucose-fed potato leaves and from water-stressed spinach leaves. l h e generally held view that sugar phosphates transported by chloroplasts are confined to triose phosphates is not supported by these results. A physiological role for a Clc-6-P translocator in green plastids is presented with reference t o the source/ sink function of the leaf.

In Vivo Respiratory Metabolism of Illuminated Leaves

“…So, Glc fed 0.045/ 0.84 ; 5% of starch synthesis in the leaf during the feeding experiment. The rate of starch synthesis from 13 C-enriched Glc is consistent with that found in potato by Quick et al (1995): chloroplasts extracted from intact leaves and illuminated with 14 C-Glc (in a medium containing HCO 3…”

“…These observations might be paralleled with the presence of a diffusion-driven Glc phosphate translocator on the inner chloroplast membrane (Schäfer et al, 1977;Quick et al, 1995), which can feed starch synthesis with cytoplasmic hexose molecules, although the associated flux appears to be quantitatively minor in this study.…”

Proteome Analysis of the Rice Etioplast

“…It is also possible that the BY-2 plastid metabolism reflects the long term adaptation of the BY-2 cell culture to the carbohydrate supply from the growth medium. Glucose feeding can induce transport of glucose 6-phosphate into plastids (38), whereas the expression of TPT in tobacco seedlings decreased after external sucrose supply (39). Our results clearly show that the etioplasts have a specific metabolism that differs from other heterotrophic plastid types.…”

ADP-glucose drives starch synthesis in isolated maize endosperm amyloplasts: characterization of starch synthesis and transport properties across the amyloplast envelope

“…In contrast with control chloroplasts, the organelles isolated from the glucose-fed leaves had gained the ability to import Glc6P. This suggests that a new type of hexose phosphate translocator was induced in addition to the typical triose phosphate translocator found in typical chloroplasts [28].…”

“…The observed discrepancies between the rates of homo and hetero exchange indicate that the same may hold true for maize endosperm amyloplasts. The assumption that different types of phosphate translocator can be present in the same envelope membrane is further supported by the recent observation that after detached spinach and potato leaves had been fed with glucose, chloroplasts with new transport properties were isolated [28]. In contrast with control chloroplasts, the organelles isolated from the glucose-fed leaves had gained the ability to import Glc6P.…”