Biochemical Basis for Partitioning of Photosynthetically Fixed Carbon between Starch and Sucrose in Soybean (Glycine max Merr.) Leaves

The control of photosynthetic starch/sucrose formation in leaves of soybean (Glycine max L. Merr.) cultivars was studied in relation to stage of plant development, photosynthetic photoperiod, and nitrogen source. At each sampling, leaf tissue was analyzed for starch content, activities of sucrose-metabolizing enzymes, and labeling of starch and sucrose (by ¹⁴CO₂ assimilation) in isolated cells. In three of the four varieties tested, nodulated plants had lower leaf starch levels and higher activities of sucrose phosphate synthetase (SPS), and isolated mesophyll cells incorporated more carbon (percentage of total ¹⁴CO₂ fixed) into sucrose and less into starch as compared to nonnodulated (nitrate-dependent) plants. The variation among cultivars and nitrogen treatments observed in the activity of SPS in leaf extracts was positively correlated with labeling of sucrose in isolated cells (r = 0.81) and negatively correlated with whole leaf starch content (r = −0.66). The results suggested that increased demand for assimilates by nodulated roots may be accommodated by greater partitioning of carbon into sucrose in the mesophyll cells. We have also confirmed the earlier report (Chatterton, Silvius 1979 Plant Physiol 64: 749-753) that photoperiod affects partitioning of fixed carbon into starch. Within two days of transfer of nodulated soybean Ransom plants from a 14-hour to a 7-hour photoperiod, leaf starch accumulation rates doubled, and this effect was associated with increased labeling of starch and decreased labeling of sucrose in isolated cells. Concurrently, activities of SPS, sucrose synthase, and uridine diphosphatase in leaves were decreased.     

Nonstructural carbohydrate metabolism during leaf ageing in tobacco (Nicotiana tabacum)

Nonstructural carbohydrate status and activities of ADP-glucose pyrophosphorylase (EC 2.7.7.27, ADPG pyrophosphorylase) and sucrose phosphate synthase (EC 2.4.1.14, SPS) were determined during ageing of tobacco ( Nicotiana tabacum L., cvs KY 14 and Speight G28) leaves sampled from control plants and from plants that had the apical meristem and subsequent axillary growth removed (detopped plants). Over the 30-day period shoot growth increased much more for control compared to detopped plants, but the increase in root growth was similar for both treatments. Dry matter and leaf area of the individual leaf used for enzyme and metabolite analysis were constant over time for controls but increased 5-fold for detopped plants. Ageing of control leaves was indicated by a progressive loss of chlorophyll and ribulose 1, 5-bisphosphate carboxylase (EC 4.1.1.39, Rubisco) activity; loss of these components was diminished for detopped plants. In contrast to chlorophyll and Rubisco activity, activities of ADPG pyrophosphorylase and SPS remained relatively constant over time for controls. Thus, under normal ageing conditions, changes in activities of ADPG pyrophosphorylase and SPS were not closely associated with changes in the standard senescence indicators chlorophyll and Rubisco activity. The activities of ADPG pyrophosphorylase and SPS were enhanced, relative to controls, within 6 days after applying the detopping treatment and activities remained high for the duration of the 30-day period. Detopping also led to increased concentrations of starch and sucrose, but the increases were not well correlated with changes in enzyme activities. The data indicated that the leaves of detopped plants functioned as both source leaves, with enhanced ability to synthesize carbohydrate, and sink leaves, with enhanced growth. Therefore, activities of ADPG pyrophosphorylase and SPS were more responsive to changes within an individual leaf than to changes in whole plant growth.     

Expression of a cyanobacterial sucrose-phosphate synthase from Synechocystis sp. PCC 6803 in transgenic plants

Sucrose-phosphate synthase (SPS) from the cyanobacterium Synechocystis sp. PCC 6803 lacks all of the Ser residues known to be involved in the regulation of higher plant SPS by protein phosphorylation. The Synechocystis SPS is also not allosterically regulated by glucose 6-phosphate or orthophosphate. To investigate the effects of expressing a potentially unregulated SPS in plants, the Synechocystis sps gene was introduced into tobacco, rice and tomato under the control of constitutive promoters. The Synechocystis SPS protein was expressed at a high level in the plants, which should have been sufficient to increase overall SPS activity 2-8-fold in the leaves. However, SPS activities and carbon partitioning in leaves from transgenic and wild-type plants were not significantly different. The maximal light-saturated rates of photosynthesis in leaves from tomato plants expressing the Synechocystis SPS were the same as those from wild-type plants. Tomato plants expressing the maize SPS showed 2-3-fold increases in SPS activity, increased partitioning of photoassimilate to sucrose and up to 58% higher maximal rates of photosynthesis. To investigate the apparent inactivity of the Synechocystis SPS the enzyme was purified from transgenic tobacco and rice plants. Surprisingly, the purified enzyme was found to have full catalytic activity. It is proposed that some other protein in plant cells binds to the Synechocystis SPS resulting in inhibition of the enzyme.     

Transgenic cotton over-producing spinach sucrose phosphate synthase showed enhanced leaf sucrose synthesis and improved fiber quality under controlled environmental conditions

Prior data indicated that enhanced availability of sucrose, a major product of photosynthesis in source leaves and the carbon source for secondary wall cellulose synthesis in fiber sinks, might improve fiber quality under abiotic stress conditions. To test this hypothesis, a family of transgenic cotton plants (Gossypium hirsutum cv. Coker 312 elite) was produced that over-expressed spinach sucrose-phosphate synthase (SPS) because of its role in regulation of sucrose synthesis in photosynthetic and heterotrophic tissues. A family of 12 independent transgenic lines was characterized in terms of foreign gene insertion, expression of spinach SPS, production of spinach SPS protein, and development of enhanced extractable V _max SPS activity in leaf and fiber. Lines with the highest V _max SPS activity were further characterized in terms of carbon partitioning and fiber quality compared to wild-type and transgenic null controls. Leaves of transgenic SPS over-expressing lines showed higher sucrose:starch ratio and partitioning of ¹⁴C to sucrose in preference to starch. In two growth chamber experiments with cool nights, ambient CO₂ concentration, and limited light below the canopy, the transgenic line with the highest SPS activity in leaf and fiber had higher fiber micronaire and maturity ratio associated with greater thickness of the cellulosic secondary wall.     

Reduction of the cytosolic fructose-1,6-bisphosphatase in transgenic potato plants limits photosynthetic sucrose biosynthesis with no impact on plant growth and tuber yield

Sucrose produced in source leaves is the predominant carbon source for developing sink tissues in most higher plants. Consequently the rate of sucrose synthesis is likely to be important for sink development and final crop yield. Two sucrose biosynthetic enzymes are believed to possess regulatory properties with respect to the rate of sucrose synthesis: (i) cytosolic FBPase and (ii) sucrose phosphate synthase. To study the impact of reduced photosynthetic sucrose biosynthesis on plant growth and crop yield a cDNA clone encoding cytosolic FBPase was isolated from a potato leaf cDNA library and used for antisense experiments in transgenic potato plants. The cDNA clone cy-F1, containing an open reading frame of 1020 bp highly homologous (85%) to other known sequences of plant cytosolic FBPases, was cloned in reversed orientation between the 35S CaMV promoter and the octopine synthase polyadenylation signal. Out of 75 independent transformants five transgenic lines having 9 to 55% of the wild-type FBPase activity were chosen for further analysis. A 45% reduction of the cytosolic FBPase activity did not cause any measurable change in metabolite concentrations, growth behaviour or photosynthetic parameters of the transgenic plants. Inhibition of cytosolic FBPase activity below 20% of the wild-type activity led to an accumulation of 3-PGA, triose-phosphates and fructose-1,6bisphosphate in source leaves. This resulted in a reduced light-saturated rate of assimilation measured via gas exchange and a decreased photosynthetic rate under conditions of the leaf disc electrode with saturating light and CO₂. Measuring photosynthetic carbon fluxes by labelling leaf discs with ¹⁴CO₂ revealed a 53–65% reduction of sucrose synthesis whereas starch synthesis decreased only by 18–24%. The flux into the anionic and cationic fraction was not altered. Despite these changes steadystate sucrose concentrations were not effected in source leaves from transgenic plants. Starch accumulated by more than a factor of 3 compared with wild-type leaves and was degraded during the night. This provides strong evidence for the hypothesis that hexoses and/or hexosephosphates are exported out of the chloroplasts, thereby circumventing the limitation of sucrose biosynthesis caused by the inhibition of cytosolic FBPase in the dark. Accordingly, plant growth and potato tuber yield remained unaltered. From these data it can be concluded that a reduced photosynthetic sucrose biosynthetic capacity can be efficiently compensated without any reduction in crop yield under greenhouse or growth chamber conditions by changing carbon export strategy. Whether the same holds true for field conditions remains to be elucidated.     

Effects of Elevated Sucrose-Phosphate Synthase Activity on Photosynthesis,Assimilate Partitioning,and Growth in Tomato (Lycopersicon esculentum var UC82B)

The expression of a sucrose-phosphate synthase (SPS) gene from maize (Zea mays, a monocotyledon) in tomato (Lycopersicon esculentum, a dicotyledon) resulted in marked increases in extractable SPS activity in the light and the dark. Diurnal modulation of the native tomato SPS activity was found. However, when the maize enzyme was present the tomato leaf cells were unable to regulate its activation state. No detrimental effects were observed and total dry matter production was unchanged. However, carbon allocation within the plants was modified such that in shoots it increased, whereas in roots it decreased. There was, therefore, a change in the shoot:root dry weight ratio favoring the shoot. This was positively correlated with increased SPS activity in leaves. SPS was a major determinant of the amount of starch in leaves as well as sucrose. There was a strong positive correlation between the ratio of sucrose to starch and SPS activity in leaves. Therefore, SPS activity is a major determinant of the partitioning of photosynthetically fixed carbon in the leaf and in the whole plant. The photosynthetic rate in air was not significantly increased as a result of elevated leaf SPS activity. However, the light- and CO2-saturated rate of photosynthesis was increased by about 20% in leaves expressing high SPS. In addition, the temporary enhancement of the photosynthetic rate following brief exposures to low light was increased in the high SPS plants relative to controls. We conclude that the level of SPS in the leaves plays a pivotal role in carbon partitioning. Furthermore, high SPS levels have the potential to boost photosynthetic rates under favorable conditions.     

Tobacco plants that lack expression of functional nitrate reductase in roots show changes in growth rates and metabolite accumulation.

When tobacco is provided with a high nitrate supply, only a small amount of the nitrate taken up by the roots is immediately assimilated inside the roots, while the majority is transported to the leaves where it is reduced to ammonium. To elucidate the importance of root nitrate assimilation, tobacco plants have been engineered that showed no detectable nitrate reductase activity in the roots. These plants expressed the nitrate reductase structural gene nia2 under control of the leaf-specific potato promoter ST-LS1 in the nitrate reductase-mutant Nia30 of Nicotiana tabacum. Homozygous T2-transformants grown in sand or hydroponics with 5.1 mM nitrate had approximately 55-70% of wild-type nitrate reductase acivity in leaves, but lacked nitrate reductase acivity in roots. These plants showed a retarded growth as compared with wild-type plants. The activation state of nitrate reductase was unchanged; however, diurnal variation of nitrate reductase acivity was not as pronounced as in wild-type plants. The transformants had higher levels of nitrate in the leaves and reduced amounts of glutamine both in leaves and roots, while roots showed higher levels of hexoses (3-fold) and sucrose (10-fold). It may be concluded that the loss of nitrate reductase acivity in the roots changes the allocation of reduced nitrogen compounds and sugars in the plant. These plants will be a useful tool for laboratories studying nitrate assimilation and its interactions with carbon metabolism.     

Decreased SBPase activity alters growth and development in transgenic tobacco plants

The effects of reduced SBPase activity on growth and development were examined in a set of transgenic tobacco plants produced using an antisense construct driven by the ribulose bisphosphate carboxylase, small subunit promoter. Photosynthetic carbon assimilation rates and carbohydrate levels in source leaves were decreased in the antisense plants. Growth rate and total shoot biomass were reduced in the SBPase antisense plants, even in plants where SBPase activity was reduced by only 25%. Floral biomass also decreased in response to reductions in SBPase activity and the onset of flowering was delayed by 5-10 d. This is the first demonstration of a link between reproductive biomass and reductions in Calvin cycle enzyme activity using antisense plants. Furthermore, unexpected changes in the growth and development of the antisense plants were evident. Small reductions in SBPase activity (above 50% wild type) resulted in shorter plants with only a small decrease in stem biomass and specific leaf area. In contrast, plants with larger reductions in SBPase activity had an increase in specific leaf area and attained heights similar to that of the wild-type plants but with a much reduced stem biomass, largely due to a decrease in xylem tissue. This bi-modal response of growth to reductions in SBPase activity has similarities to changes in leaf and stem anatomy and morphology that accompany light acclimation.     

Reduced sedoheptulose-1,7-bisphosphatase levels in transgenic tobacco lead to decreased photosynthetic capacity and altered carbohydrate accumulation

Transgenic tobacco (Nicotiana tabacum L. cv. Samsun) plants with reduced levels of the Calvin cycle enzyme sedoheptulose-1,7-bisphosphatase (SBPase; EC 3.1.3.37) were produced using an antisense construct in which the expression of a tobacco SBPase cDNA clone was driven by the cauliflower mosaic virus (CaMV) promoter. The reduction in SBPase protein levels observed in the primary transformants correlated with the presence of the antisense construct and lower levels of the endogenous SBPase mRNA. No changes in the amounts of other Calvin cycle enzymes were detected using Western blot analysis. The SBPase antisense plants with less than 20% of wild-type SBPase activity were observed to display a range of phenotypes, including chlorosis and reduced growth rates. Measurements of photosynthesis, using both light-dosage response and CO₂ response curves, of T1 plants revealed a reduction in carbon assimilation rates, which was apparent in plants retaining 57% of wild-type SBPase activity. Reductions were also observed in the quantum efficiency of photosystem II. This decrease in photosynthetic capacity was reflected in a reduction in the carbohydrate content of leaves. Analysis of carbohydrate status in fully expanded source leaves showed a shift in carbon allocation away from starch, whilst sucrose levels were maintained in all but the most severely affected plants. Plants with less than 15% of wild-type SBPase activity were found to contain less than 5% of wild-type starch levels. The results of this preliminary analysis indicate that SBPase activity may limit the rate of carbon assimilation. Received: 23 February 1997 / Accepted: 2 May 1997     

Low sink demand limits photosynthesis under P(i) deficiency.

The role of the demand for carbon assimilates (the 'sink') in regulating photosynthetic carbon assimilation (Pn: the 'source') in response to phosphate (P(i)) deficiency was examined in tobacco (Nicotiana tabacum L.). P(i) supply was maintained or withdrawn from plants, and in both treatments the source/sink ratio was decreased in some plants by darkening all but two source leaves (partially darkened plants). The remaining plants were kept fully illuminated. P(i)-sufficient plants showed little variation in rate of Pn, amounts of P(i) or phosphorylated intermediates. Withdrawal of P(i) decreased Pn by 75% under the growing conditions and at both low and high internal CO2 concentration. Concomitantly, P(i), phosphorylated intermediates and ATP contents decreased and starch increased. RuBP and activity of phosphoribulokinase closely matched the changes in Pn, but Rubisco activity remained high. Partial darkening P(i)-deficient plants delayed the loss of photosynthetic activity; Rubisco and phosphoribulokinase activities and amounts of sucrose and metabolites, particularly RuBP and G6P, were higher than in fully illuminated Pi-deficient plants. Rates of sucrose export from leaves were more than 2-fold greater than in fully illuminated P(i)-deficient plants. Greater sucrose synthesis, facilitated by increased G6P content, an activator of SPS, would recycle P(i) from the cytosol back to the chloroplast, maintaining ATP, RuBP and hence Pn. It is concluded that low sink strength imposes the primary limitation on photosynthesis in P(i)-deficient plants which restricts sucrose export and sucrose synthesis imposing an end-product synthesis limitation of photosynthesis.     

Over-expression of an arabidopsis family A sucrose phosphate synthase (SPS) gene alters plant growth and fibre development

The objective of this study was to manipulate the intracellular pools of sucrose by differentially expressing exogenous sucrose phosphate synthase (SPS) and investigating its role in regulating plant growth and fibre development. Tobacco (Nicotiana tabacum cv. Xanthi) plants were transformed with an arabidopsis SPS gene under the regulation of the ubiquitously expressed tandem repeat of the 35S cauliflower mosaic virus promoter, and subject to growth trials and fibre characterization. It was apparent that over-expression of SPS resulted in substantially elevated concentrations of sink sucrose pools compared to wild-type plants, while source tissue sucrose pools remained the same. All transformed plants had significantly increased stem height, which was ascribed to internode elongation, and greater stem diameters, longer fibers and increased total dry biomass relative to the control plants. Difference in the chemical composition of either the storage or structural carbohydrates of the wild-type and SPS transgenic lines were only minor. The correlation between increased stem sucrose content and plant phenotypes with elevated SPS gene expression confirm a role for sucrose availability in controlling plant growth and fibre elongation.     

Decreased expression of sucrose phosphate synthase strongly inhibits the water stress-induced synthesis of sucrose in growing potato tubers

Water stress stimulates sucrose synthesis and inhibits starch synthesis in wild-type tubers. Antisense and co-suppression potato transformants with decreased expression of sucrose–phosphate synthase (SPS) have been used to analyse the importance of SPS for the regulation of this water-stress induced change in partitioning. (i) In the absence of water stress, a 70–80% decrease in SPS activity led to a 30–50% inhibition of sucrose synthesis and a slight (10–20%) increase of starch synthesis in tuber discs in short-term labelling experiments with low concentrations of labelled glucose. Similar changes were seen in short-term labelling experiments with intact tubers attached to well-watered plants. Provided plants were grown with ample light and water, transformant tubers had a slightly lower water and sucrose content and a similar or even marginally higher starch content than wild-type tubers. (ii) When wild-type tuber slices were incubated with labelled glucose in the presence of mannitol to generate a moderate water deficit (between –0.12 and –0.72 MPa), there was a marked stimulation of sucrose synthesis and inhibition of starch synthesis. A similar stimulation was seen in labelling experiments with wild-type tubers that were attached to water-stressed wild-type plants. These changes were almost completely suppressed in transformants with a 70–80% reduction of SPS activity. (iii) Decreased irrigation led to an increase in the fraction of the dry-matter allocated to tubers in wild-type plants. This shift in allocation was prevented in transformants with reduced expression of SPS. (iv) The results show that operation of SPS and the sucrose cycle in growing potato tubers may lead to a marginal decrease in starch accumulation in non-stressed plants. However, SPS becomes a crucial factor in water-stressed plants because it is required for adaptive changes in tuber metabolism and whole plant allocation.     

Alteration of the Amount of the Chloroplast Phosphate Translocator in Transgenic Tobacco Affects the Distribution of Assimilate between Starch and Sugar

Tobacco plants (Nicotiana tabacum L.) transformed with sense and antisense constructs of a cDNA encoding the tobacco phosphate-triose phosphate-3-phosphoglycerate translocator (phosphate translocator) were shown to contain altered amounts of phosphate translocator mRNA and protein. Phosphate translocator activity in intact chloroplasts isolated from transformed plants showed a 15-fold variation, from 20% of the wild-type activity in antisense transformants to 300% of the wild-type activity in sense transformants. However, the maximal rates of photosynthesis and the rates of photosynthetic carbon assimilation in ambient CO2 showed no consistent differences between transformants. Starch content was decreased by 20% and total soluble sugars were increased by 20% in leaves of antisense transformants compared to sense transformants. The 40% decrease in the ratio of starch to total soluble sugars in antisense transformants relative to sense transformants indicates that distribution of assimilate between starch and sugar had been altered. However, the amount of sucrose in the leaves was unchanged. The changes in total soluble sugars were accounted for completely by changes in glucose and fructose, suggesting the existence of a homeostatic mechanism for maintaining sucrose concentrations in the leaves at the expense of glucose and fructose.     

Accumulation of hexoses in leaf vacuoles: Studies with transgenic tobacco plants expressing yeast-derived invertase in the cytosol,vacuole or apoplasm

The subcellular distribution of hexoses, sucrose and amino acids among the stromal, cytosolic and vacuolar compartments was analysed by a nonaqueous fractionation technique in leaves of tobacco (Nicotiana tabaccum L.) wild-type and transgenic plants expressing a yeast-derived invertase in the cytosolic, vacuolar or apoplasmic compartment. In the wild-type plants the amino acids were found to be located in the stroma and in the cytosol, sucrose mainly in the cytosol and up to 98% of the hexoses in the vacuole. In the leaves of the various transformants, where the contents of hexoses were greater than in wild-type plants, again 97–98% of these hexoses were found in the vacuoles. It is concluded that leaf vacuoles contain transporters for the active uptake of glucose and fructose against a high concentration gradient. A comparison of estimated metabolite concentrations in the subcellular compartments of wild-type and transformant plants indicated that the decreased photosynthetic capacity of the transformants is not due to an osmotic effect on photosynthesis, as was shown earlier to be the case in transformed potato leaves, but is the result of a long-term dedifferentiation of tobacco leaf cells to heterotrophic cells.Abbreviations apo-inv tobacco plant with yeast invertase in the apoplasm - Chl chlorophyll - cy-inv tobacco plant with yeast invertase in the cytosol - vac-inv tobacco plant with yeast invertase in the vacuole - WT wild-type tobacco plant The authors thank A. Großpietsch for her able technical assistance. This work has been supported by the Bundesminister für Forschung und Technologie.     

Reduction in phosphoribulokinase activity by antisense RNA in transgenic tobacco: effect on CO2 assimilation and growth in low irradiance

To quantify the importance of the Calvin cycle enzyme phosphoribulokinase (PRK) in photosynthesis and to perturb photosynthesis without large direct reductions in leaf protein content, tobacco plants (Nicotiana tabacum L.) were transformed with an inverted cDNA encoding tobacco PRK. A population of plants expressing antisense RNA and a range of PRK activities from wild-type to less than 5% of wild-type were obtained. CO₂ assimilation under the growing conditions (330 µmol photons m^?2 sec^?1, 350 µbar CO₂, 25°C) was not inhibited until more than 85% of PRK activity had been removed. With reduction in PRK activity of between 85 and 95%, assimilation rates and amounts of chlorophyll compared with wild-type were reduced by up to half. Decreased absorption of light by leaves with less chlorophyll accounte0d for only a small part of the reduction in assimilation rate. When PRK activity was below 15% of wild-type, amounts of ribulose-5-phosphate, ribose-5-phosphate, ATP and fructose-6-phosphate were 1.5- to fivefold higher and levels of ribulose-1,5-bisphosphate, 3-phosphoglyceric acid and ADP 1.5- to fourfold lower than in wild-type. It is estimated that these changes maintained flux through PRK to realise the assimilation rates observed. A possible shift of control within the Calvin cycle towards fructose-1,6-bisphosphatase in plants with low PRK is discussed. Amounts of hexoses and starch in particular were reduced in plants expressing the lowest PRK activities; amounts of sucrose were little affected. Lower CO₂ assimilation in plants with low PRK activity correlated with reduced relative growth rate of shoots and delayed flowering, but there was no effect on specific leaf area. It is concluded that (i) in wild-type plants grown in constant low light, PRK has a flux-control coefficient for CO₂ assimilation of zero, and that even when amounts of PRK are reduced 20-fold relative to wild-type, altered amounts of metabolites compensate for much of the reduction in PRK protein; (ii) in plants where there is a 95% reduction in amounts of PRK, photosynthesis was reduced twofold without large changes in leaf protein content or leaf geometry.     

Expression of a heterologous SnRK1 in tomato increases carbon assimilation, nitrogen uptake and modifies fruit development

SnRK1 (sucrose non-fermenting-1-related protein kinase 1) plays an important role in plant carbon metabolism and development. To understand the mechanism of carbon and nitrogen metabolism regulated by MhSnRK1 from pingyitiancha (Malus hupehensis Rehd. var. pinyiensis Jiang), two transgenic lines (T2-7 and T2-9) over expressing this gene in tomato were studied. SnRK1 activity in the leaves of 2 transgenic lines was increased by 15-16% compared with that in the wild-type. The leaf photosynthetic rate in transgenic tomatoes was higher than the wild-type. The activity of sucrose synthase breakdown and ADP-glucose pyrophosphorylase was also increased, by approximately 25-36% and 44-48%, respectively, whereas sucrose synthase synthesis and sucrose phosphate synthase activities were unchanged. The content of starch in the leaves and red-ripening fruits was higher than that of the wild-type. The transgenic fruit ripened ～10 days earlier than the wild-type. The nitrate reductase activity (mgplant?1 h?1) shows no significant difference between the transgenic plant and the wild-type, but the N-uptake efficiency and root/shoot ratio in the T2-9 line were 15% and 35% higher than that in the wild-type, respectively. These results suggest that over expressing MhSnRK1 can increase both the carbon and nitrogen assimilation rate of the plant as well as regulate the development of fruit.     

Changes in Nonstructural Carbohydrates in Different Parts of Soybean (Glycine max [L.] Merr.) Plants during a Light/Dark Cycle and in Extended Darkness

Diurnal patterns of nonstructural carbohydrate (starch, sucrose, and hexose sugars) concentration were characterized in different parts (leaves, petioles, stems, and roots) of vegetative soybean (Glycine max [L.] Merr.) plants. Pronounced changes in all carbohydrate pools were observed in all plant parts during the normal photosynthetic period; however, starch accumulation within leaves accounted for more than 80% of the nonstructural carbohydrate accumulated by the plant during the light period. Efficiency of utilization of starch and sucrose during the normal dark period differed among organs, with leaves being most efficient in mobilizing starch reserves and roots being most efficient in utilizing sucrose reserves. The vast majority (about 85%) of the whole plant carbohydrate reserves present at the end of the photosynthetic period were utilized during the normal dark period. Sink leaf expansion ceased in plants transferred to extended darkness and the cessation in leaf expansion corresponded with carbohydrate depletion in the subtending source leaf and the remainder of the plant. Collectively, the results indicated that under the conditions employed, leaves are the whole plant's primary source of carbon at night as well as during the day.     

Sucrose-Phosphate Synthase,Sucrose Synthase,and Invertase in Sugar Beet Leaves

The activity of sucrose-phosphate synthase (SPS) in sugar beet (Beta vulgaris L.) leaves was shown to exceed considerably the synthesizing activity of sucrose synthase (SS). The rise in SPS activity was related to the daylight period; i.e., it was associated with the rate of photosynthesis. The highest SPS activity was characteristic of fully expanded source leaves. In young developing leaves (leaves expanded to less than half of their final size), which represent the sink organs, the SPS activity was 2.5 times lower. At all stages of leaf development, the synthesizing SS activity was rather low. The diurnal change of SS activity was independent of photosynthesis and showed a slight rise from 6:00–8:00 p.m. Under field conditions, the highest SPS activity was found in leaves in the terminal stage of their development (105-day-old plants); the synthesizing activity of SS showed little changes during this period. The activity of soluble acid invertase was characteristic of young leaves. In mature leaves, the activity of this enzyme correlated with the daylight period. These changes occurred on the background of low sucrose content in leaves. The regulation of SPS, SS, and invertase activity is discussed. It is supposed that compartmentation of these enzymes in the photosynthesizing cell is important for transport, metabolism, and the osmotic function of sucrose in leaves.