The role of enzyme activation state in limiting carbon assimilation under variable light conditions

The mechanisms regulating transient photosynthesis by soybean (Glycine max) leaves were examined by comparing photosynthetic rates and carbon reduction cycle enzyme activities under flashing (saturating 1 s lightflecks separated by low photon flux density (PFD) periods of different durations) and continuous PFD. At the same mean PFD, the mean photosynthetic rates were reduced under flashing as compared to continuous light. However, as the duration of the low PFD period lengthened, the CO₂ assimilation attributable to a lightfleck increased. This enhanced lightfleck CO₂ assimilation was accounted for by a greater postillumination CO₂ fixation occurring after the lightfleck. The induction state of photosynthesis, ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco), fructose 1,6-bisphosphatase (FBPase) and ribulose 5-phosphate kinase (Ru5P kinase) activities all responded similarly and were all lower under flashing as compared to constant PFD of the same integrated mean value. However, the fast phase of induction and FBPase and Ru5P kinase activities were reduced more than were the slow phase of induction and rubisco activity. This was consistent with the role of the former enzymes in the fast induction component that limited RuBP regeneration. Competition for reducing power between carbon metabolism and thioredoxin-mediated enzyme activation may have resulted in lower enzyme activation states and hence lower induction states under flashing than continuous PFD, especially at low lightfleck frequencies (low mean PFD).Abbreviations FBPase fructose 1,6-bisphosphatase (EC 3.1.3.11) - LUE lightfleck use efficiency - P-glycerate 3-phosphoglycerate - PICF post-illumination CO₂ fixation - Ru5P kinase ribulose 5-phosphate kinase (EC 2.7.1.19) - RuBP ribulose 1,5-bisphosphate - rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) - SBpase sedoheptulose 1,7-bisphosphatase (EC 3.1.3.37)     

Regulation of Photosynthetic Induction State by the Magnitude and Duration of Low Light Exposure

This study was undertaken to examine the dependence of the regulatory enzymes of photosynthetic induction on photon flux density (PFD) exposure in soybean (Glycine max L.). The induction state varies as a function of both the magnitude and duration of the PFD levels experienced prior to an increase in PFD. The photosynthetic induction state results from the combined activity of separate processes that each in turn depend on prior PFD environment in different ways. Direct measurement of enzyme activities coupled with determination of in situ metabolite pool sizes indicated that the fast-induction component was associated with the activation state of stromal fructose-1,6-bisphosphatase (FBPase, EC 3.1.3.11) and showed rapid deactivation in the dark and at low PFD. The fast-induction component was activated at low PFD levels, around 70 [mu]mol photons m-2 s-1. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 2.7.1.19) deactivated very slowly in the dark and required higher PFD for activation. Both enzymes saturated at lower PFD than did photosynthesis, around 400 [mu]mol photons m-2 s-1. Ribulose-5-phosphate kinase (EC 2.7.1.19) appeared never to be limiting to photosynthesis, and saturated at much lower PFD than either FBPase or Rubisco. Determination of photosynthetic metabolite pool sizes from leaves at different positions within a soybean canopy showed a limitation to carbon uptake at the stromal FBPase and possibly the sedoheptulose-1,7-bisphosphatase (EC 3.1.3.37) in shade leaves upon initial illumination at saturating PFD levels.     

Acclimation of photosynthesis to low temperature in Spinacia oleracea L. II. Effects of nitrogen supply

The photosynthetic capacity of leaves of N-sufficent plantsof Spinacia oleracea L. increases following transfer a constanttemperature of 10C for 10 d compared to plants maintained at25C. The effects of nitrogen nutrition on this low temperatureacclimation have been investigated in respect of CO₂ assimilation,the activities and activation states of key enzymes and thepartitioning of recently fixed carbon. N-deficiency greatlyrestricted acclimation of photosynthetic CO₂ assimilation tolow temperature at both ambient and at saturating CO₂ concentrations,indicating a restriction on accilmatory changes in both ribulose1,5-bisphosphatecarboxylase-oxygenase (Rubisco) and the reactions of ribulose1,5-bisphosphateregeneration. Nitrogen limitation led to an increase in thepartitioning of recently-fixed carbon into starch. Total proteinincreased during acclimation in both N-sufficient and N-deficientleaves and was much less affected than were the activities ofenzymes. Increases in the activation state of Rubisco and thestromal fructose-1,6-bisphosphatase occurred in response tolow temperature, but increases in the activities of Rubisco,sucrose-phosphate synthase or the cytosolic fructose1,6-bisphosphatasecould not be sustained in N-deficient plants throughout theperiod of acclimation, although the activities of these enzymesdeclined less precipitately than in non-acclimated N-deficientplants. These data are all consistent with the view that increasesin the activities of key enzymes of carbon assimilation area pre-requisite for acclimation to low temperature and thatthese increases are restricted under N-limitation. Key words: Low temperature, nitrogen, photosynthesis, Rubisco, sucrose-phosphate synthase     

The Role of Ribulose-1,5-Bisphosphate Regeneration in the Induction Requirement of Photosynthetic CO(2) Exchange under Transient Light Conditions

The temporally variable light environment of natural plant canopies presents distinct limitations to carbon assimilation, partially as a result of the photosynthetic induction requirement that develops when leaves are shaded. This study was undertaken with soybean (Glycine max L.) leaves to further identify factors contributing to the activation state of the fast component of induction during low photosynthetic photon flux density (PPFD) periods. Determination of pool sizes of carbon reduction cycle intermediates at low light and upon return to saturating light indicated that different limitations to photosynthetic activity arise over the time course of a 10-minute low PPFD period. Photosynthetic activity upon reillumination was limited by the regeneration of ribulose 1,5-P₂. There was an increase in the levels of fructose 1,6-P₂, sedoheptulose 1,7-P₂, triose-P, ribose 5-P, and ribulose 5-P pools, indicating inactivation of stromal enzymes, most notably fructose 1,6-bisphosphatase, sedoheptulose 1,7-bisphosphatase, and ribulose 5-P kinase. The fast-induction component was the most important factor limiting assimilation during rapid, brief light transients, during which the decay of the slow component was minimal. This may be particularly significant for upper leaves in soybean canopies that generally experience very rapid light transients.     

Studies on the regulation of chloroplast fructose-1,6-bisphosphatase. Activation by fructose 1,6-bisphosphate

Chloroplast fructose-1,6-bisphosphatase (D-fructose 1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11) isolated from spinach leaves, was activated by preincubation with fructose 1,6-bisphosphate. The rate of activation was slower than the rate of catalysis, and dependent upon the temperature and the concentration of fructose 1,6-bisphosphate. The addition of other sugar diphosphates, sugar monophosphates or intermediates of the reductive pentose phosphate cycle neither replaced fructose 1,6-bisphosphate nor modified the activation process. Upon activation with the effector the enzyme was less sensitive to trypsin digestion and insensitive to mercurials. The activity of chloroplast fructose-1,6-bisphosphatase, preincubated with fructose 1,6-bisphosphate, returned to its basal activity after the concentration of the effector was lowered in the preincubation mixture. The results provide evidence that fructose-1,6-bisphosphatase resembles other regulatory enzymes involved in photosynthetic CO2 assimilation in its activation by chloroplast metabolites.     

Influences of leaf temperature on photosynthetic carbon metabolism in wheat

Net photosynthetic assimilation rate (A), extractable activities of three photosynthetic enzymes, and the concentrations of six metabolites were determined for wheat (Tricum aestivum L.) leaves as leaf temperature was varied under photorespiring (350 microliters per liter CO₂ and 21% O₂) and under nonphotorespiring conditions (800 microliters per liter CO₂ and 2% O₂). The extractable activity of ribulose-1,5-bisphosphate carboxylase (Rubisco) and fructose-1,6-bisphosphatase declined with increasing leaf temperature from 15 to 45°C. Leaf concentrations of ribulose-1,5-bisphosphate (RuBP) declined slightly between 15 and 25°C but increased to a level which is 4 to 5 times the binding site concentration of Rubisco at leaf temperatures of 35 and 45°C. Leaf concentrations of 3-phosphoglycerate, fructose-6-phosphate, and glucose-6-phosphate all declined with increasing leaf temperature. Outside of the limitations imposed by photorespiration, it is proposed that under high light and at suboptimal temperatures, A is limited by rate of utilization of triose phosphate; at optimal temperatures, by the availability of substrate (CO₂ and RuBP) under photorespiring conditions or utilization of triose phosphate under nonphotorespiring conditions; and at supraoptimal temperatures, by the activation state of Rubisco.     

Light dependent activation of CO2 assimilation and the ratio of Rubisco activase to Rubisco during leaf aging of rice (Oryza sativa)

The effects of the ratio of Rubisco activase to Rubisco (activase/Rubisco ratio) on light dependent activation of CO₂ assimilation were investigated during leaf aging of rice. Changes of photosynthetic CO₂ gas exchange rates in relation to step increases of light intensity from two photon flux densities of 60 µmol m⁻² s⁻¹ (low initial PFD) and 500 µmol m⁻² s⁻¹ (high initial PFD) to saturated PFD of 1 800 µmol m⁻² s⁻¹ were measured. These photosynthetic activation processes were considered to be limited by the Rubisco activation rate when analyzed by the relaxation method. The relaxation time of low initial PFD gradually declined from 3 to 33 days after leaf emergence and showed high and negative correlation to the activase/Rubisco ratio. The initial rate of Rubisco activation under low initial PFD linearly correlated to the amounts of Rubisco activase, whereas these were almost constant from 3 to 23 days after leaf emergence. But these correlations could not be recognized in the case of high initial PFD. Moreover, the relaxation times were more sensitive to intercellular CO₂ concentration (C_i) under high initial PFD than under low initial PFD, especially, at C_i below 300 µl l⁻¹. These results suggest the involvement of the activase/Rubisco ratio in the photosynthetic activation under relatively low initial PFD, and the limitation of photosynthetic activation under relatively high initial PFD by Rubisco carbamylation during leaf aging of rice.     

Chloroplast fructose-1,6-bisphosphatase: structure and function

Redox regulation of photosynthetic enzymes has been a preferred research topic in recent years. In this area chloroplast fructose-1,6-bisphosphatase is probably the most extensively studied target enzyme of the CO₂ assimilation pathway. This review analyzes the structure, biosynthesis, phylogeny, action mechanism, regulation and kinetics of fructose-1,6-bisphosphatase in the light of recent findings on structure–function relationship, and from a molecular biology viewpoint. This revised version was published online in June 2006 with corrections to the Cover Date.     

Limitation of CO(2) Assimilation and Regulation of Benson-Calvin Cycle Activity in Barley Leaves in Response to Changes in Irradiance, Photoinhibition, and Recovery

The response of the Benson-Calvin cycle to changes in irradiance and photoinhibition was measured in low-light grown barley (Hordeum vulgare) leaves. Upon the transition from the growth irradiance (280 micromoles per square meter per second) to a high photoinhibitory irradiance (1400 micromoles per square meter per second), the CO₂ assimilation rate of the leaves doubled within minutes but high irradiance rapidly caused a reduction in quantum efficiency. Following exposure to high light the activities of NADP-malate dehydrogenase and fructose-1,6-bisphosphatase obtained near maximum values and the activation state of ribulose-1,5-bisphosphate carboxylase increased. The activity of the latter remained constant throughout the period of photoinhibitory irradiance, but the increase in the activities of fructose-1,6-bisphosphatase and NADP-malate dehydrogenase was transient decreasing once more to much lower values. This suggests that immediately following the transition to high light reduction and activation of redox-modulated enzymes occurred, but then the stroma became relatively oxidized as a result of photoinhibition. The leaf contents of glucose 6-phosphate and fructose 6-phosphate increased following exposure to high light but subsequently decreased, suggesting that following photoinhibition sucrose synthesis exceeded the rate of carbon assimilation. The ATP content attained a constant value much higher than that in low light. During photoinhibition the glycerate 3-phosphate content greatly increased while ribulose-1,5-bisphosphate decreased. The fructose-1,6-bisphosphate and triose phosphate contents increased initially and then remained constant. During photoinhibition CO₂ assimilation was not limited by ribulose-1,5-bisphosphate carboxylase activity but rather by the regeneration of the substrate, ribulose-1,5-bisphosphate, related to a restriction on the supply of reducing equivalents.     

Amino acid sequence similarity between spinach chloroplast and mammalian gluconeogenic fructose-1,6-bisphosphatase

Chloroplast fructose-1,6-bisphosphatase is an essential enzyme in the photosynthetic pathway of carbon dioxide fixation into sugars and the properties of this enzyme are clearly distinct from cytosolic gluconeogenic fructose-1,6-bisphosphatase. Light-dependent activation via a ferredoxin/thioredoxin system and insensitivity to inhibition by AMP are unique characteristics of the chloroplast enzyme. In the present study, purified spinach chloroplast fructose-1,6-bisphosphatase was reduced, S-carboxymethylated with iodoacetic acid, and cleaved with either cyanogen bromide or trypsin. The resulting peptides were purified by reversed-phase high performance liquid chromatography. Automated Edman degradation of some of the purified peptides showed amino acid sequences highly homologous to residues 72-86, 180-199, and 277-319 of pig kidney fructose-1,6-bisphosphatase. These findings suggest a common evolutionary origin for mammalian gluconeogenic and chloroplast fructose-1,6-bisphosphatase, enzymes catalyzing the same reaction but having different functions and modes of regulation.     

Heterogeneous photochemical and chloroplasts mediated activation of spinach fructose-1,6-bisphosphatase

A heterogeneous photochemical electron relay system was constructed, mimicking the chloroplast electron transport reaction, in order to activate fructose-1,6-bisphosphatase in light. The photocatalyst acridine orange or proflavin sensitizes EDTA dependent reduction of ferredoxin. In a complete system, consisting of a dye-donor couple, ferredoxin, thioredoxin and ferredoxin-thioredoxin reductase, light activation of purified spinach fructose-1,6-bisphosphatase was observed in vitro. The ferredoxin was not essential for activation of fructose-1,6-bisphosphatase using heterogeneous photochemical system while chloroplasts mediated redox activation essentially required ferredoxin. The heterogeneous photochemical system activated fructose-1,6-bisphosphatase by about 6 fold similar to chloroplasts mediated ferredoxin dependent redox activation. These observations suggest that a thiol mediator is essential for the reductive activation of carboxylating enzymes of photosynthesis. The mechanism of activation is discussed.     

Photosynthesis in flashing light in soybean leaves grown in different conditions. I. Photosynthetic induction state and regulation of ribulose-1,5-bisphosphate carboxylase activity

The photosynthetic induction state under conditions of different lightfleck frequencies or durations, or different shade periods was studied in soybean leaves in order to examine how it might limit utilization of sunflecks in leaf canopies. Induction following an increase in photon flux density (PFD) from strongly limiting to saturating PFDs exhibited two phases; a fast-inducing one, requiring about 1 min and a slow one, requiring up to 60 min for completion. Transfer of fully induced leaves to low light resulted in a rapid decrease in the fast-inducing component, a slower decrease in the slow-inducing component and an even slower decrease in stomatal conductance. Therefore, the decreases in extent of induction appeared to be due to biochemical factors and not to stomatal closure. Under flashing light regimes consisting of 1-s lightflecks given at different frequencies for long periods, a constant induction state was achieved, the measure of induction state increased with the frequency of the lightflecks. This constant induction state also depended on the growth conditions, with shade leaves having a higher value than those grown at high light at any particular lightfleck frequency. The measure of induction state was mostly lower in flashing light as compared to constant light of the same mean PFD, particularly in leaves with a low light saturation point and in short lightflecks. Initial activities of ribulose-1,5-bisphosphate carboxylase (rubisco) were also higher in continuous light and were highly correlated with the measure of induction state. The rapid decrease in extent of induction of soybean leaves during shade periods is an important limitation to the ability of the leaves to respond to light increases similar to those occurring with sunflecks. At least part of the limitation on carbon assimilation during sunflecks due to photosynthetic induction is based on regulation of rubisco activity.     

Higher-plant chloroplast and cytosolic fructose-1,6-bisphophosphatase isoenzymes: origins via duplication rather than prokaryote-eukaryote divergence

Full-size cDNAs encoding the precursors of chloroplast fructose-1,6-bisphosphatase (FBP), sedoheptulose-1,7-bisphosphatase (SBP), and the small subunit of Rubisco (RbcS) from spinach were cloned. These cDNAs complete the set of homologous probes for all nuclear-encoded enzymes of the Calvin cycle from spinach (Spinacia oleracea L.). FBP enzymes not only of higher plants but also of non-photosynthetic eukaryotes are found to be unexpectedly similar to eubacterial homologues, suggesting a eubacterial origin of these eukaryotic nuclear genes. Chloroplast and cytosolic FBP isoenzymes of higher plants arose through a gene duplication event which occurred early in eukaryotic evolution. Both FBP and SBP of higher plant chloroplasts have acquired substrate specificity, i.e. have undergone functional specialization since their divergence from bifunctional FBP/SBP enzymes of free-living eubacteria.Abbreviations FBP fructose-1,6-bisphosphatase - SBP sedoheptulose-1,7-bisphosphatase - FBA fructose-1,6-bisphosphate aldolase     

Robustness of activity of Calvin cycle enzymes after high light and low temperature conditions in Antarctic vascular plants

Only two vascular plants have naturally colonized the Maritime Antarctic: Colobanthus quitensis and Deschampsia antarctica. We propose that one of the reasons of their success in this environment is the robustness of their CO₂ assimilation machinery. In order to understand the mechanisms involved in the positive photosynthetic rates under stressful conditions, we analyzed changes in the activity of two key Calvin cycle enzymes: Ribulose bisphosphate carboxylase oxygenase (Rubisco) and stromal Fructose-1,6-bisphosphatase after high light and low temperature treatments. Our results show that the activity of both enzymes does not decrease after 48 h high light/low temperature treatments, a feature usually observed in plants adapted to harsh environments. The activation state of both enzymes remained high throughout the treatments. This feature has been related to the redox state of the chloroplast, suggesting that both plants maintain their redox balance under high light and/or low temperature conditions assayed. Both plants differed in their responsiveness to cold acclimation as observed by gas exchange and enzymatic measurements. We propose that these differences may be related to microclimate adaptations to the environment they naturally develop in the Maritime Antarctic.     

Effect of Chilling on Carbon Assimilation,Enzyme Activation,and Photosynthetic Electron Transport in the Absence of Photoinhibition in Maize Leaves

The relationships between electron transport and photosynthetic carbon metabolism were measured in maize (Zea mays L.) leaves following exposure to suboptimal temperatures. The quantum efficiency for electron transport in unchilled leaves was similar to that previously observed in C3 plants, although maize has two types of chloroplasts, mesophyll and bundle sheath, with PSII being largely absent from the latter. The index of noncyclic electron transport was proportional to the CO2 assimilation rate. Chilled leaves showed decreased rates of CO2 assimilation relative to unchilled leaves, but the integral relationships between the quantum efficiency for electron transport or the index of noncyclic electron transport and CO2 fixation were unchanged and there was no photoinhibition. The maximum catalytic activities of the Benson-Calvin cycle enzymes, fructose-1,6-bisphosphatase and ribulose-1,5-bisphosphate carboxylase, were decreased following chilling, but activation was unaffected. Measurements of thiol-regulated enzymes, particularly NADP-malate dehydrogenase, indicated that chilling induced changes in the stromal redox state so that reducing equivalents were more plentiful. We conclude that chilling produces a decrease in photosynthetic capacity without changing the internal operational, regulatory or stoichiometric relationships between photosynthetic electron transport and carbon assimilation. The enzymes of carbon assimilation are particularly sensitive to chilling, but enhanced activation may compensate for decreases in maximal catalytic activity.     

The mitochondrial CMSII mutation of Nicotiana sylvestris impairs adjustment of photosynthetic carbon assimilation to higher growth irradiance

The CMSII mutant of Nicotiana sylvestris, which lacks a functional mitochondrial complex I, was used to investigate chloroplast-mitochondria interactions in light acclimation of photosynthetic carbon assimilation. CMSII and wild-type (WT) plants were grown at 80 micromol m(-2) s(-1) photosynthetic active radiation (PAR; 80) and 350 micromol m(-2) s(-1) PAR (350). Carbon assimilation at saturating PFD was markedly higher in WT 350 leaves as compared with WT 80 leaves, but was similar in CMS 80 and CMS 350 leaves, suggesting that the mutant is unable to adjust photosynthesis to higher growth irradiance. WT 350 leaves showed several general characteristic light acclimation responses [increases in leaf specific area (LSA), total chlorophyll content, and chlorophyll a/b ratio, and a higher light compensation point]. In contrast, a similar chlorophyll content and chlorophyll a/b ratio were measured for both CMS 80 and CMS 350 leaves, while LSA and the light compensation point acclimated as in the WT. The failure of CMSII to adjust photosynthesis to growth PFD did not result from lower quantum efficiency of PSII, lower whole-chain electron transport rates (ETRs), or lower ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) and sucrose phosphate synthase (SPS) capacities. Excess ETR not used for carbon assimilation was even higher in CMS 350 than in WT 350. Since photochemical fluorescence quenching and the initial activity of NADP malate dehydrogenase (NADP-MDH) were identical in WT 350 and CMS 350 leaves but the activation state of NADP-MDH was different, redox signals from primary ETR are not involved in the signal transduction of light acclimation, while a contribution of stromal redox state cannot be excluded. When mature plants were transferred between 350 and 80 conditions, the mutant showed acclimatory tendencies, although adjustments were not as rapid or as marked as in the WT, and the response of the initial activities of Rubisco and NADP-MDH was impaired or altered. Initial activities of Rubisco and SPS at limiting concentration were also affected in CMS 350 as compared with WT plants when compared at growth irradiance or after in situ activation at 1000 micromol m(-2) s(-1) PAR. The data demonstrate that chloroplast-mitochondria interactions are important in light acclimation, and modulation of the activation state of key photosynthetic enzymes could be an important mechanism in this cross-talk.     

“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

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.Abbreviations Fru 1,6bisP fructose-1,6-bisphosphate - Fru 1,6bisPase fructose-1,6-bisphosphatase - Fru6P fructose-6-phosphate - Glc 1P glucose-1-phosphate - Glc6P glucose-6-phosphate - NADP-GAPDH NADP-dependent glyceraldehyde-3-phosphate dehydrogenase - PFK phosphofructokinase - PEP phosphoenolpyruvate - PFP pyrophosphate:fructose-6-phosphate phosphotransferase - PGA glycerate-3-phosphate - PK pyruvate kinase - Pi inorganic phosphate - Ru1,5bisP ribulose-1,5-bisphosphate - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase - SPS sucrose-phosphate synthase - triose-P triose-phosphates     

Responses of Ribulose-1,5-Bisphosphate Carboxylase,Cytochrome f,and Sucrose Synthesis Enzymes in Rice Leaves to Leaf Nitrogen and Their Relationships to Photosynthesis

The photosynthetic gas-exchange rates and various biochemical components of photosynthesis, including ribulose-1,5-bisphosphate carboxylase (Rubisco) content, cytochrome (Cyt) f content, and the activities of two sucrose synthesis enzymes, were examined in young, fully expanded leaves of rice (Oryza sativa L.) grown hydroponically in different nitrogen concentrations. The light-saturated rate of photosynthesis at an intercellular CO2 pressure of 20 Pa (CO2-limited photosynthesis) was linearly dependent on leaf nitrogen content, but curvilinearly correlated with Rubisco content. This difference was due to a greater than proportional increase in Rubisco content relative to leaf nitrogen content and the presence of a CO2 transfer resistance between the intercellular air spaces and the carboxylation sites. CO2-limited photosynthesis was proportional to Cyt f content, one of the key components of electron transport, but was not proportional to the activities of cytosolic fructose-1,6-bisphosphatase and sucrose phosphate synthase, the two regulatory enzymes of sucrose synthesis. Light-saturated photosynthesis above an intercellular CO2 pressure of 60 Pa (CO2-saturated photosynthesis) was curvilinearly dependent on leaf nitrogen content. This CO2-saturated photosynthesis was proportional to Cyt f content in the low- and normal-nitrogen leaves, and correlated better with the activities of cytosolic fructose-1,6-bisphosphatase and sucrose phosphate synthase in the high-nitrogen leaves. The increase in the activities of these two enzymes with increasing leaf nitrogen was not as great as the increase in Cyt f content. Thus, as leaf nitrogen increased, the limitation caused by the activities of sucrose synthesis enzymes came into play, which resulted in the curvilinear relationship. However, this limitation by sucrose synthesis enzymes did not affect photosynthesis under normal ambient air.     

Modulation of Chloroplast Fructose-1,6-bisphosphatase Activity by Light

Inhibitor experiments indicate that light effect mediator_II which is reductively activated by transfer of electrons from the photosynthetic electron transport system at or beyond ferredoxin, is involved in activation by light of fructose-1,6-bisphosphatase in the pea plant. Activation proceeds optimally when the pH is low and Mg²⁺ is 10 millimolar. Modulation by light results in increases in maximal velocity, apparently as a result of changes in enzyme conformation. Pea leaf thylakoids are effective in modulating the activity of glyceraldehyde-3-phosphate dehydrogenase but not of fructose-1,6-bisphosphatase or glucose-6-phosphate dehydrogenase in Kalanchoë stromal extracts. There is apparently species specificity for modulation of some, but not all, of the modulatable enzymes.