Enzyme In Sucrose Synthesis Research Articles

Overexpression of Sucrose Phosphate Synthase Enhanced Sucrose Content and Biomass Production in Transgenic Sugarcane.

Sucrose phosphate synthase (SPS) is a key enzyme in sucrose synthesis, which controls sucrose content in plants. This study was designed to examine the efficacy of the overexpression of SoSPS1 gene on sucrose accumulation and carbon partitioning in transgenic sugarcane. The overexpression of SoSPS1 gene increased SPS activity and sucrose content in transgenic sugarcane leaves. More importantly, the overexpression enhanced soluble acid invertase (SAI) activity concomitant with the increase of glucose and fructose levels in the leaves, whereas sucrose synthase activity exhibited almost no change. In the stalk, a similar correlation was observed, but a higher correlation was noted between SPS activity and sugar content. These results suggest that SPS overexpression has both direct and indirect effects on sugar concentration and SAI activity in sugarcane. In addition, SPS overexpression resulted in a significant increase in plant height and stalk number in some transgenic lines compared to those in non-transgenic control. Taken together, these results strongly suggest that enhancing SPS activity is a useful strategy for improving sugarcane yield.

At-harvest fruit maturity affects sucrose metabolism during cold storage and is related to chilling injury in peach.

We investigated the effects of at-harvest maturity of 'YuLu' peach fruit on soluble sugar metabolism and their relationship with chilling injury susceptibility. Peaches were sorted into four maturity groups at harvest by I AD (index of the absorbance difference between 670 and 720nm) then stored at 5°C for 28days. Fruit quality parameters, flesh browning index, malondialdehyde (MDA) content, soluble sugar content, gene expression, and enzyme activities associated with sucrose metabolism were measured. The more mature fruit groups had significantly (p < 0.05) lower firmness, higher soluble solid content, a* values of background color, sorbitol and sucrose content at harvest. During the cold storage, the higher flesh browning index in the mature groups (M3 and M4) maybe due to the double stress of senescence and chilling injury because there was concomitant sharp increase in MDA content. However, the most immature at-harvest group (M1) had the significantly (p < 0.05) higher MDA content after 14days of cold storage, and a flesh browning index significantly (p < 0.05) higher than the M2 group (the next more mature group), late in the storage period. Moreover, the M1 group had lower sucrose content at postharvest and higher activities and transcript levels of sucrose degrading enzymes and lower levels of sucrose synthesis enzymes, which was responsible for the lower sucrose levels than M2 group during storage. It was concluded that the more immature peach fruit with lower sucrose content, have a higher chilling susceptibility than more mature fruit.

The functions of cucumber sucrose phosphate synthases 4 (CsSPS4) in carbon metabolism and transport in sucrose- and stachyose-transporting plants

Sucrose phosphate synthases (SPSs) are rate-limiting sucrose synthesis enzymes present in photosynthetic and non-photosynthetic tissues. The cucumber genome contains three SPSs that can be grouped into families A, B, and C. CsSPS1 and CsSPS2 are highly expressed in flowers and mature leaves, while the expression level of CsSPS4 increased gradually after leaf unfolding in our study and reached its peak after 20 days. In CsSPS4-overexpression tobacco plants, sucrose content and sucrose/starch ratio were increased significantly and resulted in improved leaf yield. By contrast, in CsSPS4-overexpression (CsSPS4-OE) cucumber lines, contents of sucrose and starch were unchanged, and raffinose was increased in transgenic cucumber leaves. The expression of cucumber raffinose family oligosaccharide (RFO)-synthesis-related genes increased obviously in cucumber CsSPS4-OE plants, and the sucrose, raffinose, and stachyose contents increased significantly in the petioles of CsSPS4-OE lines. In CsSPS4-antisense (CsSPS4-A) cucumber lines, decreases occurred in mRNA expression, enzyme activity, sucrose content, sucrose/starch ratio, and stachyose transport, but the RFO-synthesis-related genes were nearly unchanged. Together, these results suggest that overexpression of CsSPS4 can lead to carbon metabolism prioritizing sugar transport in cucumber, and suppression of CsSPS4 likely promotes carbon metabolism to accumulate starch, showing a more complicated carbon distribution model than in transgenic tobacco plants.

The chromosome regions for increasing early growth in rice: role of sucrose biosynthesis and NH4 + uptake

To elucidate the mechanism of plant growth we investigated chromosome region affecting traits (CRATs) using the chromosome segment substitution lines derived from a cross between japonica ‘Koshihikari’ and indica ‘Kasalath’ using rice (Oryza sativa L.). Four CRATs associated with increased plant growth rate were identified which were derived from ‘Kasalath’ alleles on chromosomes 1, 6, 8 and 12. This was achieved by evaluating the net dry weight gain from seed germination to 30 day-old seedlings. Physiological analysis of substitution lines containing a ‘Kasalath’ allele chromosome segment in the ‘Koshihikari’ background revealed that the CRAT on chromosome 1 (EGR1) increased the relative growth rate by increasing the net assimilation rate (NAR), and increased expression of the OsSPS1 gene that encodes the rate-limiting enzyme in sucrose synthesis. In contrast, CRATs on chromosomes 6, 8 and 12 (EGR6, EGR8 and EGR12) were associated with an increased uptake of NH4 +, the major nitrogen source for rice. These results suggest that early growth of rice is controlled by multiple traits and sucrose biosynthesis and NH4 + uptake play key roles.

Copper-induced alteration in sucrose partitioning and its relationship to the root growth of two Elsholtzia haichowensis Sun populations

ABSTRACTHydroponic culture was used to comparatively investigate the copper (Cu)-induced alteration to sucrose metabolism and biomass allocation in two Elsholtzia haichowensis Sun populations with one from a Cu-contaminated site (CS) and the other from a non-contaminated site (NCS). Experimental results revealed that biomass allocation preferred roots over shoots in CS population, and shoots over roots in NCS population under Cu exposure. The difference in biomass allocation was correlated with the difference in sucrose partitioning between the two populations. Cu treatment (45 μM) significantly decreased leaf sucrose content and increased root sucrose content in CS population as a result of the increased activities of leaf sucrose synthesis enzymes (sucrose phosphate synthetase and sucrose synthase) and root sucrose cleavage enzyme (vacuolar invertase), which led to increased sucrose transport from leaves to roots. In contrast, higher Cu treatment increased sucrose content in leaves and decreased sucrose content in roots in NCS population as a result of the decreased activities of root sucrose cleavage enzymes (vacuolar and cell wall invertases) that led to less sucrose transport from leaves to roots. These results provide important insights into carbon resource partitioning and biomass allocation strategies in metallophytes and are beneficial for the implementation of phytoremediation techniques.

Tissue specificity and diurnal change in gene expression of the sucrose phosphate synthase gene family in rice

The rice genome contains 5 isogenes for sucrose phosphate synthase (SPS), the key enzyme in sucrose synthesis; however, little is known about their transcriptional regulation. In order to determine the expression patterns of the SPS gene family in rice plants, we conducted an expression analysis in various tissues and developmental stages by real-time quantitative RT-PCR. At the transcript level, the rice SPS genes, particularly SPS1, were preferentially expressed in source tissues, whereas SPS2, SPS6, and SPS8 were expressed equally in source and sink tissues. We also investigated diurnal changes in SPS gene expression, SPS activity, and soluble sugar content in leaf blades. Interestingly, the expression of all the SPS genes, particularly that of SPS1 and SPS11, tended to be higher at night when the activation state of the SPS proteins was low, and the mRNA levels of SPS1 and SPS6 were negatively correlated with sucrose content. Furthermore, the temporal patterns of SPS gene expression and sugar content under continuous light conditions suggested the involvement of endogenous rhythm and/or sucrose sensing in the transcriptional regulation of SPS genes. Our data revealed differential expression patterns in the rice SPS gene family and part of the complex mechanisms of their transcriptional control.

Overexpression of a Maize SPS Gene Improves Yield Characters of Potato under Field Conditions

We analyzed the yield characters of field-grown transgenic potato plants (Solanum tuberosum) carrying a maize gene for sucrose-phosphate synthase (SPS), the key enzyme in sucrose synthesis. The SPS activity in the leaves of transgenic plants (line Ag1203) was 2 times that of the control (cv. May Queen). There was no difference in the photosynthetic CO2 uptake rates between Ag1203 and May Queen plants, and the leaf starch content of Ag1203 was lower. These observations indicate that the introduction of a foreign SPS gene improved the supply of photosynthate from source (leaves) to sink (tubers). Additionally, leaf senescence of the transgenic potato plants was delayed relative to that of May Queen. The average tuber weight and total yield of Ag1203 plants were at least 20% higher, and the tuber sucrose content, which is related to eating quality, was also higher. Increased translocation of photosynthate and longer period of photosynthetic activity in the leaves may have increased the yield of Ag1203. These results suggest that introduction of the SPS gene improved the yield characters and quality of potato tubers under field conditions.

Quantitative trait loci for sucrose, starch, and hexose accumulation before heading in rice

We studied the storage of sucrose, starch, and hexose before heading in rice ( Oryza sativa L.) plants by quantitative trait locus (QTL) analysis with a population of backcross inbred lines (BILs) of japonica cv. Nipponbare × indica cv. Kasalath. Carbohydrates are accumulated in the rice plant before heading and are translated to the panicle after heading. A higher capacity for accumulation is thus a main target for improvement in yield. The form of carbohydrate (sucrose, starch, or hexose) differs depending on the organ in which it is stored. There was no correlation between starch and sucrose or hexose contents in BILs, and the positions of QTLs controlling starch differed from those for sucrose and hexose accumulation. These results suggest that the genetic control of accumulation differs between starch and sugars. QTLs that control the ratio of sucrose to starch content were detected, suggesting the existence of a mechanism(s) that determines this ratio. On chromosome 1, sucrose-phosphate synthase 1, the key enzyme in sucrose synthesis was close to the peaks of the likelihood odds ratios in QTLs for sucrose or hexose content. These results suggest that SPS1 is related to conversion of carbohydrate to sucrose as accumulated form in a plant before heading.

CO2 Assimilation, Photosynthetic Enzymes, and Carbohydrates of `Concord' Grape Leaves in Response to Iron Supply

Own-rooted 1-year-old `Concord' grapevines (Vitis labruscana Bailey) were fertigated twice weekly for 11 weeks with 1, 10, 20, 50, or 100 μm iron (Fe) from ferric ethylenediamine di (o-hydroxyphenylacetic) acid (Fe-EDDHA) in a complete nutrient solution. As Fe supply increased, leaf total Fe content did not show a significant change, whereas active Fe (extracted by 2,2′-dipyridyl) content increased curvilinearly. Chlorophyll (Chl) content increased as Fe supply increased, with a greater response at the lower Fe rates. Chl a: b ratio remained relatively constant over the range of Fe supply, except for a slight increase at the lowest Fe treatment. Both CO2 assimilation and stomatal conductance increased curvilinearly with increasing leaf active Fe, whereas intercellular CO2 concentrations decreased linearly. Activities of key enzymes in the Calvin cycle, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), NADP-glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoribulokinase (PRK), stromal fructose-1,6-bisphosphatase (FBPase), and a key enzyme in sucrose synthesis, cytosolic FBPase, all increased linearly with increasing leaf active Fe. No significant difference was found in the activities of ADP-glucose pyrophosphorylase (AGPase) and sucrose phosphate synthase (SPS) of leaves between the lowest and the highest Fe treatments, whereas slightly lower activities of AGPase and SPS were observed in the other three Fe treatments. Content of 3-phosphoglycerate (PGA) increased curvilinearly with increasing leaf active Fe, whereas glucose-6-phosphate (G6P), fructose-6-phosphate (F6P), and the ratio of G6P: F6P remained unchanged over the range of Fe supply. Concentrations of glucose, fructose, sucrose, starch, and total nonstructural carbohydrates (TNC) at both dusk and predawn increased with increasing leaf active Fe. Concentrations of starch and TNC at any given leaf active Fe content were higher at dusk than at predawn, but both glucose and fructose showed the opposite trend. No difference in sucrose concentration was found at dusk or predawn. The export of carbon from starch breakdown during the night, calculated as the difference between dusk and predawn measurements, increased as leaf active Fe content increased. The ratio of starch to sucrose at both dusk and predawn also increased with increasing leaf active Fe. In conclusion, Fe limitation reduces the activities of Rubisco and other photosynthetic enzymes, and hence CO2 assimilation capacity. Fe-deficient grapevines have lower concentrations of nonstructural carbohydrates in source leaves and, therefore, are source limited.

CO2 Assimilation, Photosynthetic Enzymes, and Carbohydrates of Grape Leaves (Vitis labrusca L. cv. Concord) in Response to Iron Supply

Own-rooted one-year-old `Concord' grapevines were fertigated twice weekly for 11 weeks with 1, 10, 20, 50, OR 100 μmol iron (Fe) from ferric ethylenediamine di (o-hydroxyphenylacetic) acid in a complete nutrient solution. As Fe supply increased, leaf total Fe content did not change, whereas active Fe (extracted by 2, 2'-dipyridyl) and total chlorophyll content increased curvilinearly. CO2 assimilation and stomatal conductance increased curvilinearly with increasing active Fe, whereas intercellular CO2 concentrations decreased linearly. Activities of key Calvin cycle enzymes, Rubisco, NADP-glyceraldehyde-3-phosphate dehydrogenase, phosphoribulokinase, stromal fructose-1,6-bisphosphatase (FBPase), and a key enzyme in sucrose synthesis, cytosolic FBPase, all increased linearly with increasing active Fe. No difference was found in the activities of ADP-glucose pyrophosphorylase and sucrose phosphate synthase of leaves between the lowest and the highest treatments, whereas slightly lower activities were observed in the middle Fe treatments. Content of 3-phosphoglycerate increased curvilinearly with increased active Fe, whereas glucose-6-phosphate and fructose-6-phosphate did not change. Glucose, fructose, sucrose, starch, and total non-structural carbohydrates at both dusk and pre-dawn increased with increasing active Fe. Carbon export from starch breakdown during the night, calculated as the difference between dusk and predawn levels, increased as active Fe increased. In conclusion, Fe limitation reduces the activities of Rubisco and other photosynthetic enzymes, and hence CO2 assimilation capacity. Fe-deficient grapevines have lower concentrations of non-structural carbohydrates in source leaves, and therefore, are source limited.

Heterologous expression of a ketohexokinase in potato plants leads to inhibited rates of photosynthesis, severe growth retardation and abnormal leaf development.

In the present paper we investigated the effect of heterologous expression of a rat liver ketohexokinase in potato (Solanum tuberosum L.) plants with the aim of investigating the role of fructose 1-phosphate in plant metabolism. Plants were generated that contained appreciable activity of ketohexokinase but did not accumulate fructose 1-phosphate. They were, however, characterised by a severe growth retardation and abnormal leaf development. Studies of (14)CO(2) assimilation and metabolism, and of the levels of photosynthetic pigments, revealed that these lines exhibited restricted photosynthesis. Despite this fact, the levels of starch and soluble sugars remained relatively constant. Analysis of intermediates of starch and sucrose biosynthesis revealed large increases in the triose phosphate and fructose 1,6-bisphosphate pools but relatively unaltered levels of inorganic phosphate and 3-phosphoglycerate, and these lines were also characterised by an accumulation of glyceraldehyde. The transformants neither displayed consistent changes in the activities of Calvin cycle enzymes nor in enzymes of sucrose synthesis but displayed a metabolic profile partially reminiscent of that brought about by end-product limitation, but most likely caused by an inhibition of photosynthesis brought about by the accumulation of glyceraldehyde. Analysis of the metabolite contents in lamina and vein fractions of the leaf, and of the enzymes of carbohydrate oxidation indicate that the phloem-enriched veins of ketohexokinase-expressing leaves tend toward hypoxia and indicate a problem of phloem transport.

Effects of Chilling Temperature on the Activity of Enzymes of Sucrose Synthesis and the Accumulation of Saccharides in Leaves of Three Sugarcane Cultivars Differing in Cold Sensitivity

The effects of short-term exposure to chilling temperature (10 °C) on sucrose synthesis in leaves of the cold-tolerant sugarcane cultivars Saccharum sinense R. cv. Yomitanzan and Saccharum sp. cv. NiF4, and the cold-sensitive cultivar S. officinarum L. cv. Badila were studied. Plants were grown at day/night temperatures of 30/25 °C, and then shifted to a constant day/night temperature of 10 °C. After 52-h exposure to the chilling temperature, sucrose content in the leaves of NiF4 and Yomitanzan showed a 2.5- to 3.5-fold increase relative to that of the control plants that had been left on day/night temperatures of 30/25 °C. No such increase was observed in Badila leaves. Similarly, starch content in the leaves of NiF4 and Yomitanzan was maintained high, but starch was depleted in Badila leaves after the 52-h exposure. During the chilling temperature, sucrose phosphate synthase (SPS; E.C.2.4.1.14) activity was relatively stable in the leaves of NiF4 and Yomitanzan, whereas in Badila leaves SPS activity significantly decreased. There was no significant change in cytosolic fructose-1,6-bisphosphatase activity for the three cultivars at the chilling temperature. This supports the hypothesis that: (1) on exposure to chilling temperature, sucrose content in sugarcane leaves is determined by the photosynthetic rate in the leaves, and is not related to SPS activity; (2) SPS activity in sugarcane leaves at chilling temperature is to be determined by sugar concentration in the leaves.

Contrasting responses of photosynthesis and carbon metabolism to low temperatures in tall fescue and clovers.

Growth, photosynthesis and carbohydrate metabolism in plants of two grassland species, clover (Trifolium subterraneum L. cv. Areces and Gaitan) and tall fescue (Festuca arundinacea Schreb.), shifted from 25 to 12 degrees C for 1 day or developed at 12 degrees C were compared with controls kept at 25 degrees C. Cold development produced a larger inhibition of growth in fescue than in clovers. In contrast, transferring plants from high to low temperature inhibited photosynthesis to a lesser extent in fescue than in clovers, this difference being associated with an increase in the activation state of Calvin cycle enzymes in fescue, but not in the clovers, a decreased cytosolic fructose-1,6-bisphosphatase (cFBPase, EC 3.1.3.11) activity in clovers, and an accumulation of hexose phosphates only in fescue. Development at 12 degrees C partly relieved the inhibition of photosynthesis in clovers, in contrast with fescue, which correlated with increases in total ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco, EC 4.1.1.39) activity only in clovers, and with greater increases in total stromal FBPase (sFBPase) activity in clovers than in fescue. The activity of sucrose synthesis enzymes was increased in the two clovers and fescue developed in the cold, while carbohydrate accumulation was much bigger in cold-developed fescue than in clovers because of a 5-fold increase in fructan contents in the former. The contents of phosphorylated intermediates increased in clovers but decreased in fescue grown at 12 degrees C. Our results suggest that restricted ribulose-1,5-bisphosphate (RuBP) regeneration limited the recovery of photosynthetic capacity in cold-developed fescue.

Gene Expression of Enzymes for Starch and Sucrose Metabolism and Transport in Leaf Sheaths of Rice (Oryza sativa L.) during the Heading Period in Relation to the Sink to Source Transition

Leaf sheaths of rice plants are known to temporarily accumulate starch prior to heading, which is subsequently remobilized and transported into the panicle after heading. We investigated the time course for both carbohydrate content and steady state mRNA levels of enzymes related to starch and sucrose metabolism in the rice leaf sheath (Oryza sativa L. cv. Nipponbare). Leaf sheaths from the second leaf below the flag leaf accumulated high levels of starch before heading but they rapidly decreased after heading. In contrast, the flag leaf sheath did not accumulate as much starch. In the second leaf sheath, the mRNA levels of enzymes involved in starch synthesis, ADP glucose pyrophosphorylase (EC 2. 7. 7. 27), soluble starch synthase (EC 2. 4. 1.21) and branching enzyme (EC 2. 4. 1. 18) were high before heading, which coincided with rapid accumulation of starch. The mRNA levels of sucrose synthesis enzymes, cytosolic FBPase (EC 3. 1. 3. 11) and sucrose phosphate synthase (EC 2. 4. 1. 14), and the sucrose transporter (OsSUTI) increased at the time of heading, which was largely coincident with a decrease in the mRNA levels of starch synthesis enzymes. In the flag leaf sheaths, changes in mRNA levels of starch synthesis enzymes were not pronounced, however mRNA levels of sucrose synthesis enzymes and the sucrose transporter showed a clear increase throughout the heading period. The different characteristics observed between the two leaf sheaths will be discussed in relation to the sink to source transition.

Photosynthesis, carbon partitioning and metabolite content during drought stress in peach seedlings

Photosynthesis, metabolic carbon partitioning and the contribution of sorbitol to the osmotic potential of mature peach (Prunus persica (L.) Batsch) leaves and phloem sap were examined in plants undergoing two levels of a short term drought stress. The relationship between osmotic potential at full turgor and water potential showed that neither mild nor severe drought stress induced a significant active osmotic adjustment in mature leaves. The osmotic potential of leaves at full turgor was -1.9 MPa; sorbitol was the major organic component (20%). Leaf sucrose and starch contents were significantly reduced by drought. The partitioning of newly-fixed carbon was also affected by stress. These changes appeared to originate from the inhibition of photosynthesis induced by drought stress. At low photosynthetic rates, the turnover of organic ions was low, and sorbitol synthesis was favoured over that of sucrose. Water stress did not affect the in vitro activity of sucrose phosphate synthase (EC 2.4.1.14), the key enzyme in sucrose synthesis. The in vitro activity of aldose-6-phosphate reductase (EC 1.1.1.200), the key enzyme in sorbitol synthesis, tended to increase linearly in response to drought stress. It is concluded that, contrary to some other polyol-synthesising species, peach did not seem to benefit from sorbitol synthesis during short term drought stress for active osmotic adjustment in mature leaves. However, in phloem sap, increases in sucrose and especially sorbitol concentration were observed in stressed plants.

The effect of open‐air fumigation with SO2 and O3 on carbohydrate metabolism in Scots pine (Pinus sylvestris) and Norway spruce (Picea abies)

ABSTRACTThe carbohydrate metabolism of the needles of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) has been examined in trees that were exposed to SO2, and O3, in an open‐air fumigation experiment located in the Liphook forest in southern England. Two‐year‐old seedlings were planted in 1985 in seven experimental plots. Five plots received fumigation treatments of SO2, O3 or a combination of these gases to give a 2 × 3 factorial design with one additional ambient plot Fumigation with SO2, occurred from May 1987 to December 1990 and O3, fumigation occurred from March to December 1988, May to December 1989 and February to December 1990. Five samples of needles for investigation of carbohydrate metabolism were taken between February and July 1989.The concentrations of soluble carbohydrates (including sucrose and hexoses) were greatly reduced in the needles taken from Scots pine growing in the treated plots, and were also reduced, but to a lesser extent, in the needles taken from Norway spruce. Little variation in the concentration of starch in the needles of either species was detected. The activities of the two final enzymes of sucrose synthesis, sucrose phosphate synthase and sucrose 6‐phos‐phate phosphatase, were greatly reduced in the needles of Scots pine and were also reduced, but to a lesser extent, in the needles of Norway spruce in the fumigated plots. These reductions could be correlated with decreases in rates of photosynthetic CO2 assimilation determined by independent groups of researchers working on the Liphook site.

Effects of Growth Temperature on the Responses of Ribulose-1,5-Biphosphate Carboxylase, Electron Transport Components, and Sucrose Synthesis Enzymes to Leaf Nitrogen in Rice, and Their Relationships to Photosynthesis.

Effects of growth temperature on the photosynthetic gas-exchange rates and their underlying biochemical properties were examined in young, fully expanded leaves of rice (Oryza sativa L.). The plants were grown hydroponically under day/night temperature regimes of 18/15[deg]C, 23/18[deg]C, and 30/23[deg]C and all photosynthetic measurements were made at a leaf temperature of 25[deg]C and an irradiance of 1800 [mu]mol quanta m-2 s-1. Growth temperature affected the photosynthetic CO2 response curve. The relative ratio of the initial slope to the CO2-saturated photosynthesis increased with rising growth temperature. This was caused mainly by an increase in CO2-limited photosynthesis for a given leaf nitrogen content with rising growth temperature. However, there was no difference in ribulose-1,5-bisphosphate carboxylase (Rubisco) content at any given leaf nitrogen content among temperature treatments. In addition, the activation state and catalytic turnover rate of Rubisco were not affected by growth temperature. The increase in CO2-limited photosynthesis with rising growth temperature was the result of an increase in the CO2 transfer conductance between the intercellular airspaces and the carboxylation sites. The amounts of total chlorophyll and light-harvesting chlorophyll a/b protein II increased for the same leaf nitrogen content with rising growth temperature, but the amounts of cytochrome f and coupling factor 1 and the activities of cytosolic fructose-1,6-bisphosphatase and sucrose-phosphate synthase were the same between plants grown at 23/18[deg]C and those grown at 30/23[deg]C. Similarly, CO2-saturated photosynthesis was not different for the same leaf nitrogen content between these treatments. For the 18/15[deg]C-grown plants, a slight decrease in the amounts of cytochrome f and coupling factor 1 and an increase in the activities of cytosolic fructose-1,6-bisphosphatase and sucrose-phosphate synthase were found, but these were not reflected in CO2-saturated photosynthesis.

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.

Sucrose metabolism in mango fruit during ripening

The amounts of sucrose, starch, fructose and glucose were determined in ripening mango fruits (cv. Haden) grown in Venezuela. Samples were taken at two-day intervals, over a ripening period of 16 days commencing 90 days after flowering. The level of sucrose increased rapidly after day 6 and then remained high. The amount of starch decreased slightly during the ripening period but was insufficient to account for the observed increase in the level of sucrose. Reducing sugars, comprising mainly of fructose, increasec slightly during ripening. Sucrose phosphate synthase activity increased about 10-fold during the phase of rapid sucrose accumulation. This activity was more than sufficient to account for the maximum observed rate of sucrose synthesis. The proportion of sucrose phosphate synthase activity that was sensitive to inhibition by inorganic phosphate changed during ripening. Maximum calatytic activity of sucrose synthase remained constant throughout the ripening period and was sufficient to contribute sunificantly to sucrose metabolism. The activities of neutral and acid invertases were very low in comparison with those of the other enzymes of sucrose synthesis. However, acid invertase activity did increase and later decrease during ripening.

Starch‐Sugar Transformation During Banana Ripening: The Behavior of UDP Glucose Pyrophosphorylase, Sucrose Synthetase and Invertase

ABSTRACTStarch concentration, sequence of appearance of sucrose, glucose and fructose and activity of some enzymes of sucrose synthesis were followed during ripening of preclimacteric bananas (Musa acuminatd). As starch was degraded sucrose content increased and preceded formation of glucose and fructose. At the same time, while UDP‐glucose pyrophosphorylase activity remained constant, activity of sucrose synthetase and invertase increased. The observed sugar and enzyme changes indicated that starch to sucrose transformation via glucose‐1‐phosphate‐UDP‐glucose may be an important mechanism for starch disappearance during ripening. The hypothesis was confirmed with thin banana slices infiltrated with [14C(U)]glucose‐1‐phosphate: the label was incorporated into sucrose three times faster for climacteric than for preclimacteric fruits. Pyrophosphorylase activity and climacteric induced activation of sucrose synthetase could be inhibited by protein and nucleic acids synthesis inhibitors.