Export of Carbon from Chloroplasts at Night

Abstract

Hexose export from chloroplasts at night has been inferred in previous studies of mutant and transgenic plants. We have tested whether hexose export is the normal route of carbon export from chloroplasts at night. We used nuclear magnetic resonance to distinguish glucose (Glc) made from hexose export and Glc made from triose export. Glc synthesized in vitro from fructose-6-phosphate in the presence of deuterium-labeled water had deuterium incorporated at C-2, whereas synthesis from triose phosphates caused C-2 through C-5 to become deuterated. In both tomato (Lycopersicon esculentum L.) and bean (Phaseolus vulgaris L.), Glc from sucrose made at night in the presence of deuterium-enriched water was deuterated only in the C-2 position, indicating that >75% of carbon is exported as hexoses at night. In darkness the phosphate in the cytosol was 28 mM, whereas that in the chloroplasts was 5 mM, but hexose phosphates were 10-fold higher in the cytosol than in the chloroplasts. Therefore, hexose phosphates would not move out of chloroplasts without the input of energy. We conclude that most carbon leaves chloroplasts at night as Glc, maltose, or higher maltodextrins under normal conditions.

Chloroplast protrusions in leaves of Ranunculus glacialis L. respond significantly to different ambient conditions, but are not related to temperature stress

“…As in darkness, the metabolic exchange through the chloroplast envelope does not completely stagnate, it is conceivable that CP formation in darkness may be connected to the export of photosynthetic products, for example hexoses from the chloroplast (Schleucher et al . 1998) during the degradation of transitory starch, and related transport processes.…”

Visualization of maltose uptake in living yeast cells by fluorescent nanosensors

“…In plants, hydrolytic degradation of transient starch in leaf chloroplasts takes place at night but also plays a crucial role during reserve mobilization in germinating seedlings. Maltose can be exported from chloroplasts (29), a process which is currently not fully understood. For the analysis of these physiological processes, a maltose nanosensor was developed on the basis of the bacterial MBP.…”

“…Thus, in vivo imaging will allow determination of the presence of maltose and MOS. The ability to detect both maltose and MOS will be interesting, e.g., with respect to determining the presence of soluble starch and maltose in mesophyll cytosol or to follow starch turnover in chloroplasts (29). Mutagenesis and fluorescence screening may be means for identifying FLIPmals selective for maltose.…”

Antisense Repression of Hexokinase 1 Leads to an Overaccumulation of Starch in Leaves of Transgenic Potato Plants But Not to Significant Changes in Tuber Carbohydrate Metabolism

“…This hypothesis was supported by experiments with wild-type Arabidopsis leaves indicating that there is a net glycolytic flux at night, which would be an impossibility if the primary export of carbon was at the triose-P level. Third, Schleucher et al (1998) have recently reported the use of NMR to distinguish between Glc synthesized from hexose export and that derived from triose-P; starch degradation was allowed to occur in the presence of 2 H-enriched water, and the ratio of labeling in Glc at the different carbon atoms was used to determine the relative contribution of the two routes of export. These authors concluded that in tomato and bean leaves, more than 75% of the carbon exported from chloroplasts at night is in the form of hexose.…”