Regulation Of C4 Photosynthesis Research Articles

Increased sedoheptulose-1,7-bisphosphatase content in Setaria viridis does not affect C4 photosynthesis.

Sedoheptulose-1,7-bisphosphatase (SBPase) is one of the rate-limiting enzymes of the Calvin cycle, and increasing the abundance of SBPase in C3 plants provides higher photosynthetic rates and stimulates biomass and yield. C4 plants usually have higher photosynthetic rates because they operate a biochemical CO2-concentrating mechanism between mesophyll and bundle sheath cells. In the C4 system, SBPase and other enzymes of the Calvin cycle are localized to the bundle sheath cells. Here we tested what effect increasing abundance of SBPase would have on C4 photosynthesis. Using green foxtail millet (Setaria viridis), a model C4 plant of NADP-ME subtype, we created transgenic plants with 1.5 to 3.2 times higher SBPase content compared to wild-type plants. Transcripts of the transgene were found predominantly in the bundle sheaths suggesting the correct cellular localization of the protein. The abundance of ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit was not affected in transgenic plants overexpressing SBPase, and neither was leaf chlorophyll content or photosynthetic electron transport parameters. We found no association between SBPase content in S. viridis and saturating rates of CO2 assimilation. Moreover, a detailed analysis of CO2 assimilation rates at different CO2 partial pressures, irradiances, and leaf temperatures showed no improvement of photosynthesis in plants overexpressing SBPase. We discuss the potential implications of these results for understanding the role of SBPase in regulation of C4 photosynthesis.

Major alterations in transcript profiles between C3-C4 and C4 photosynthesis of an amphibious species Eleocharis baldwinii.

Engineering C4 photosynthetic metabolism into C3 crops is regarded as a major strategy to increase crop productivity, and clarification of the evolutionary processes of C4 photosynthesis can help the better use of this strategy. Here, Eleocharis baldwinii, a species in which C4 photosynthesis can be induced from a C3-C4 state under either environmental or ABA treatments, was used to identify the major transcriptional modifications during the process from C3-C4 to C4. The transcriptomic comparison suggested that in addition to the major differences in C4 core pathway, the pathways of glycolysis, citrate acid metabolism and protein synthesis were dramatically modified during the inducement of C4 photosynthetic states. Transcripts of many transporters, including not only metabolite transporters but also ion transporters, were dramatically increased in C4 photosynthetic state. Many candidate regulatory genes with unidentified functions were differentially expressed in C3-C4 and C4 photosynthetic states. Finally, it was indicated that ABA, auxin signaling and DNA methylation play critical roles in the regulation of C4 photosynthesis. In summary, by studying the different photosynthetic states of the same species, this work provides the major transcriptional differences between C3-C4 and C4 photosynthesis, and many of the transcriptional differences are potentially related to C4 development and therefore are the potential targets for reverse genetics studies.

The role of inorganic phosphate in the regulation of C4 photosynthesis

Inorganic phosphate participates in many fundamental processes within the plant cell. Its broad influence on plant metabolism is related to such key operations as metabolite transport, enzyme regulation and carbohydrate metabolism in general. This review discusses these topics with special emphasis on the role assigned to this ubiquitous anion within the C4 pathway of photosynthesis.

Regulation of C4 Photosynthesis A Role for Pyruvate in Regulating Pyruvate,Pi Dikinase Activity in vivo

Results are presented in support of the view that pyruvate levels are critical in the light-dark regulation of pyruvate,PI dikinase in vivo. In an in vitro system containing purified Zea mays pyruvate,PI dikinase, increasing pyruvate reduced both the initial rate and the final extent of ADP-mediated inactivation of the enzyme. Pyruvate also reversed the inhibitory effect of ADP on the initial rate of PI-mediated activation of pyruvate,P*i dikinase. With intact Zea mays leaves, inactivation of pyruvate,PI dikinase in darkened leaves was prevented by anaerobic conditions or reduced temperature; both treatments resulted in elevated steady-state levels of pyruvate. Added pyruvate was shown to substantially increase the level of active pyruvate,PI dikinase in chloroplasts incubated with PI in the dark but not in the light. The likely physiological significance of these results is discussed.

Regulation of C4 photosynthesis: Regulation of pyruvate, Pi dikinase by ADP-dependent phosphorylation and dephosphorylation

Pyruvate, Pi dikinase in extracts of chloroplasts from mesophyll cells of Zea mays is inactivated by incubation with ADP plus ATP. This inactivation was associated with phosphorylation of a threonine residue on a 100 kDa polypeptide, the major polypeptide of the mesophyll chloroplast stroma, which was identified as the subunit of pyruvate, Pi dikinase. The phosphate originated from the β-position of ADP as indicated by the labelling of the enzyme during inactivation in the presence of [β-32P]ADP. During inactivation of the enzyme up to 1 mole of phosphate was incorporated per mole of pyruvate, Pi dikinase subunit inactivated. 32P label was lost from the protein during the Pi-dependent reactivation of pyruvate, Pi dikinase.

Regulation of C4 Photosynthesis : Stability and Other Characteristics of the ADP-ATP-Dependent Inactivation of Pyruvate,PI Dikinase in Maize Chloroplast Extracts

Recovery of the capacity for the ADP plus ATP-dependent inactivation of pyruvate,PI dikinase in maize mesophyll chloroplast extracts was highly variable and the remaining enzyme inactivating activity was rapidly lost during subsequent storage at both 0°C and 25°C. Usually no more than 20% of the original activity remained after about 45 min at 25°C or 120 min at 0°C. These losses were largely or completely prevented by the addition of 0.4 mM orthophosphate or 0.1 mM ATP and probably also ADP. AMP or phosphoenolpyruvate at 0.1 mM had little or no effect. Following these studies an analysis of mesophyll cell extracts showed the protein catalysing inactivation of pyruvate,PI dikinase to be exclusively located in chloroplasts and an assay procedure was developed to allow the monitoring of this activity during fractionation.

Regulation of C4 Photosynthesis: Factors Affecting Cold-Mediated Inactivation and Reactivation of Pyruvate, PI Dikinase

The kinetics and other characteristics of cold-induced inactivation and subsequent reactivation of pyruvate, PI dikinase from Zea mays were examined in leaf extracts and with the partially purified enzyme. Significant inactivation occurred at about 10°C and the rate of inactivation increased as the temperature was reduced below 10°C. The rate of inactivation at 0°C varied with Mg2+ concentration; the half-time for inactivation was about 5 min with 2 mM Mg2+ compared with more than 60 min with 15 mM Mg2+. With 10 mM Mg2+, used routinely, the half-time for pyruvate,P*i dikinase isolated from a variety of species varied from less than 10 min to more than 40 min. Activity was rapidly recovered by rewarming to 25°C the kinetics of reactivation were second order. Cold-induced inactivation was completely prevented by both pyruvate and phosphoenolpyruvate at concentrations of about 2 mM and 0.1 mM, respectively. The molecular weight of cold-inactivated pyruvate,PI dikinase, assessed by mobility on Sepharose 6B, was about a quarter of the active enzyme; the tetrameric active enzyme apparently dissociates to monomers at 0°C. When illuminated leaves were exposed to lower temperature, pyruvate, Pi dikinase was partially inactivated. This activity was rapidly recovered by returning leaves to a temperature of 25°C, or by incubating extracts from the cold-treated leaves at 25°C. The dark-inactivated enzyme was apparently also modified by exposure to temperatures in the range 0-5°C.

Metabolic regulation in C4 photosynthesis: Phosphoenol pyruvate carboxylase and 3C intermediates of the photosynthetic carbon reduction cycle

The effects of 3C intermediates of the photosynthetic carbon reduction (PCR) cycle (glyceraldehyde 3 phosphate [G3P], dihydroxyacetone phosphate and 3 phosphoglyceric acid) on the activity of phosphoenol pyruvate (PEP) carboxylase from Pennisetum purpureum (a C4 plant) have been investigated. Addition of these compounds, at concentrations greater than about 3 mM, to standard assay mixtures (5 mM concentrations of PEP, HCO3 (-) and Mg(2+)) reduced the observed activity. The extent of inhibition was greater at lower (<5 mM) concentration of Mg(2+). G3P was the most potent inhibitor. During studies of saturation kinetics with PEP as variable substrate it was found that addition of these 3C compounds (2mM) reduced the sigmoid nature of Michaelis-Menten plots and reduced the value of n obtained from Hill plots from about 3 to near one. The results are disussed in terms of the possible regulation of C4 photosynthesis by intermediates of the PCR cycle which are easily leaked from chloroplasts.