“Sink” regulation of photosynthetic metabolism in transgenic tobacco plants expressing yeast invertase in their cell wall involves a decrease of the Calvin-cycle enzymes and an increase of glycolytic enzymes

Abstract

Leaves on transgenic tobacco plants expressing yeast-derived invertase in the apoplast develop clearly demarcated green and bleached sectors when they mature. The green areas contain low levels of soluble sugars and starch which are turned over on a daily basis, and have high rates of photosynthesis and low rates of respiration. The pale areas accumulate carbohydrate, photosynthesis is inhibited, and respiration increases. This provides a model system to investigate the "sink" regulation of photosynthetic metabolism by accumulating carbohydrate. The inhibition of photosynthesis is accompanied by a decrease of ribulose-1,5-bisphosphate and glycerate-3-phosphate, and an increase of triosephosphate and fructose-1,6-bisphosphate. The extracted activities of ribulose-1,5-bisphosphate carboxylase, fructose-1, 6-bisphosphatase and NADP-glyeraldehyde-3-phosphate dehydrogenase decreased. The activity of sucrose-phosphate synthase remained high or increased, an increased portion of the photosynthate was partitioned into soluble sugars rather than starch, and the pale areas showed few or no oscillations during transitions between darkness and saturating light in saturating CO2. The increased rate of respiration was accompanied by an increased level of hexose-phosphates, triose-phosphates and fructose-1,6-bisphosphate while glycerate-3-phosphate and phosphoenolpyruvate decreased and pyruvate increased. The activities of pyruvate kinase, phosphofructokinase and pyrophosphate: fructose-6-phosphate phosphotransferase increased two- to four-fold. We conclude that an increased level of carbohydrate leads to a decreased level of Calvin-cycle enzymes and, thence, to an inhibition of photosynthesis. It also leads to an increased level of glycolytic enzymes and, thence, to a stimulation of respiration. These changes of enzymes are more important in middle- or long-term adjustments to high carbohydrate levels in the leaf than fine regulation due to depletion of inorganic phosphate or high levels of phosphorylated metabolites.

A Novel Mechanism, Linked to Cell Density, Largely Controls Cell Division in Synechocystis

“…This disaccharide is described as a compatible solute with no net charge, and it is proposed that it can be accumulated without interfering in cellular homeostasis (Brown, 1976;Mackay et al, 1984;Mikkat et al, 1997;Miao et al, 2003;Hagemann, 2011). However, our data suggest that Suc might have an effect on cellular homeostasis, and in particular photosynthesis, possibly by sugar feedback inhibition of photosynthesis (Stitt et al, 1991). Indeed, in contrast to ML, cells grown under HL could maintain a high level of photosynthesis while exporting larger amounts of Suc in the medium and accumulating much lower amounts of Suc within their cells.…”

Effects of Short- and Long-Term Elevated CO2 on the Expression of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Genes and Carbohydrate Accumulation in Leaves of Arabidopsis thaliana (L.) Heynh.1

“…An alternative possibility for hexose sensing in the absence of any detectable cytosolic hexoses at any time during the day could involve futile cycling of Suc between the cytosol and vacuole and/or between the cytosol and apoplast (Foyer, 1987;Huber, 1989;Stitt et al, 1990). Exposure to elevated CO 2 may result in increased carbon flux to Suc.…”

Regulation of Photosynthesis during Leaf Development in RbcS Antisense DNA Mutants of Tobacco

“…photosynthesis may be inhibited earlier in the WT because it has higher carbohydrate levels. Consistent with this possibility is the well-established finding that source leaves adjust their photosynthetic rates in response to the demand for photoassimilates in the rest of the plant, inhibiting photosynthesis when the demand is low and increasing photosynthesis when the demand is high (sink regulation of photosynthesis) (reviewed by Turgeon, 1989;von Schaewen et al, 1990;Stitt et al, 1990;Stitt, 1991;Sonnewald and Willmitzer, 1992). It has also been observed that the rate of leaf senescence is affected by the sinksource status of the plant (Christensen et al, 1981;Bin Lazan et al, 1983;Wittenbach, 1983;Shibles et al, 1987;Guitman et al, 1991) and that photosynthetic gene expression can be modulated by carbohydrates at the transcriptional level (Sheen, 1990;Krapp et al, 1993).…”