The influence of cytosolic phosphoglucomutase on photosynthetic carbohydrate metabolism

The aim of this work was to examine the role of cytosolic phosphoglucomutase (cPGM; EC 5.4.2.2) in photosynthetic carbon partitioning. We have previously described the generation and characterisation of the tuber metabolism of transgenic potato ( Solanum tuberosum cv. Desiree) lines expressing the StcPGM gene in the antisense orientation under the control of the 35S promoter. Here we extend the characterisation of leaf metabolism within these lines, examining properties of gas exchange, carbon partitioning, and the effect of the genetic manipulation on a wide range of metabolites including metabolites of the sucrose-starch transition, glycolysis, the Krebs cycle and amino acid metabolism. The data acquired in the present study surprisingly reveal that the photosynthetic sucrose synthetic capacity of the leaves is largely unaltered but that these plants display a reduced rate of photosynthesis, a dramatic reduction in nucleotide levels, and a general decline of biosynthesis. We conclude that these lines exhibit only moderate changes in sucrose synthesis but more complex changes on a range of diverse metabolic pathways.     

Antisense inhibition of plastidial phosphoglucomutase provides compelling evidence that potato tuber amyloplasts import carbon from the cytosol in the form of glucose-6-phosphate

The aim of this work was to establish whether plastidial phosphoglucomutase is involved in the starch biosynthetic pathway of potato tubers and thereby to determine the form in which carbon is imported into the potato amyloplast. For this purpose, we cloned the plastidial isoform of potato PGM (StpPGM), and using an antisense approach generated transgenic potato plants that exhibited decreased expression of the StpPGM gene and contained significantly reduced total phosphoglucomutase activity. We confirmed that this loss in activity was due specifically to a reduction in plastidial PGM activity. Potato lines with decreased activities of plastidial PGM exhibited no major changes in either whole-plant or tuber morphology. However, tubers from these lines exhibited a dramatic (up to 40%) decrease in the accumulation of starch, and significant increases in the levels of sucrose and hexose phosphates. As tubers from these lines exhibited no changes in the maximal catalytic activities of other key enzymes of carbohydrate metabolism, we conclude that plastidial PGM forms part of the starch biosynthetic pathway of the potato tuber, and that glucose-6-phosphate is the major precursor taken up by amyloplasts in order to support starch synthesis.     

The contribution of plastidial phosphoglucomutase to the control of starch synthesis within the potato tuber

The aim of this work was to evaluate the extent to which plastidial phosphoglucomutase (PGM) activity controls starch synthesis within potato (Solanum tuberosum L. cv. Desirée) tubers. The reduction in the activity of plastidial PGM led to both a correlative reduction in starch accumulation and an increased sucrose accumulation. The control coefficient of plastidial PGM on the accumulation of starch was estimated to approximate 0.24. The fluxes of carbohydrate metabolism were measured by investigating the metabolism of [U-14C]glucose in tuber discs from wild-type and transgenic plants. In tuber discs the control coefficient of plastidial PGM over starch synthesis was estimated as 0.36, indicating that this enzyme exerts considerable control over starch synthesis within the potato tuber.     

Expression of an <Emphasis Type="Italic">Escherichia coli</Emphasis> phosphoglucomutase in potato (<Emphasis Type="Italic">Solanum tuberosum</Emphasis> L.) results in minor changes in tuber metabolism and a considerable delay in tuber sprouting

The aim of this work was to evaluate the influence of elevating the cytosolic activity of phosphoglucomutase (PGM; EC 5.4.2.2) on photosynthesis, growth and heterotrophic metabolism. Here we describe the generation of novel transgenic plants expressing an Escherichia coli phosphoglucomutase (EcPGM) under the control of the 35S promoter. These lines were characterised by an accumulation of leaf sucrose, despite displaying no alterations in photosynthetic carbon partitioning, and a reduced tuber starch content. Determinations of the levels of a wide range of other metabolites revealed dramatic reductions in maltose and other sugars in leaves of the transformants, as well as a modification of the pattern of organic and amino acid content in tubers of these lines. Intriguingly, the transgenics also displayed a dramatically delayed rate of sprouting and significantly enhanced rate of respiration, however, it is important to note that the severity of these traits did not always correlate with the level of transgene expression. These results are discussed in the context of current understanding of the control of respiration and the breaking of tuber dormancy.     

Reduction of the Cytosolic Phosphoglucomutase in Arabidopsis Reveals Impact on Plant Growth,Seed and Root Development,and Carbohydrate Partitioning

Phosphoglucomutase (PGM) catalyses the interconversion of glucose 1-phosphate (G1P) and glucose 6-phosphate (G6P) and exists as plastidial (pPGM) and cytosolic (cPGM) isoforms. The plastidial isoform is essential for transitory starch synthesis in chloroplasts of leaves, whereas the cytosolic counterpart is essential for glucose phosphate partitioning and, therefore, for syntheses of sucrose and cell wall components. In Arabidopsis two cytosolic isoforms (PGM2 and PGM3) exist. Both PGM2 and PGM3 are redundant in function as single mutants reveal only small or no alterations compared to wild type with respect to plant primary metabolism. So far, there are no reports of Arabidopsis plants lacking the entire cPGM or total PGM activity, respectively. Therefore, amiRNA transgenic plants were generated and used for analyses of various parameters such as growth, development, and starch metabolism. The lack of the entire cPGM activity resulted in a strongly reduced growth revealed by decreased rosette fresh weight, shorter roots, and reduced seed production compared to wild type. By contrast content of starch, sucrose, maltose and cell wall components were significantly increased. The lack of both cPGM and pPGM activities in Arabidopsis resulted in dwarf growth, prematurely die off, and inability to develop a functional inflorescence. The combined results are discussed in comparison to potato, the only described mutant with lack of total PGM activity.     

Sucrose synthase controls both intracellular ADP glucose levels and transitory starch biosynthesis in source leaves

The prevailing model on transitory starch biosynthesis in source leaves assumes that the plastidial ADPglucose (ADPG) pyrophosphorylase (AGP) is the sole enzyme catalyzing the synthesis of the starch precursor molecule, ADPG. However, recent investigations have shown that ADPG linked to starch biosynthesis accumulates outside the chloroplast, presumably in the cytosol. This finding is consistent with the occurrence of an 'alternative' gluconeogenic pathway wherein sucrose synthase (SuSy) is involved in the production of ADPG in the cytosol, whereas both plastidial phosphoglucomutase (pPGM) and AGP play a prime role in the scavenging of starch breakdown products. To test this hypothesis, we have compared the ADPG content in both Arabidopsis and potato wild-type (WT) leaves with those of the starch-deficient mutants with reduced pPGM and AGP. These analyses provided evidence against the 'classical' model of starch biosynthesis, since ADPG levels in all the starch-deficient lines were normal compared with WT plants. Whether or not SuSy is involved in the synthesis of ADPG accumulating in leaves was tested by characterizing both SuSy-overexpressing and SuSy-antisensed transgenic leaves. Importantly, SuSy-overexpressing leaves exhibited a large increase of both ADPG and starch levels compared with WT leaves, whereas SuSy-antisensed leaves accumulated low amounts of both ADPG and starch. These findings show that (i) ADPG produced by SuSy is linked to starch biosynthesis; (ii) SuSy exerts a strong control on the starch biosynthetic process; and (iii) SuSy, but not AGP, controls the production of ADPG accumulating in source leaves.     

Alteration of photosynthate partitioning by high-level expression of phosphoglucomutase in tobacco chloroplasts

Plastidial phosphoglucomutase (PGM) plays an important role in starch synthesis and degradation. Nonetheless, the impact of enhanced plastidial PGM activity on metabolism in photosynthetic tissue is yet to be elucidated. In this study, we generated transplastomic tobacco plants overproducing Arabidopsis thaliana plastidial PGM (AtptPGM) in chloroplasts and analyzed the consequent metabolic and physiological parameters in the transplastomic plants. AtptPGM accumulated in the chloroplasts to up to 16% of total soluble protein in the leaves. PGM activity in leaves increased 100-fold relative to that of wild-type plants. The transplastomic plants were phenotypically indistinguishable in their growth rates, photosynthetic activities, and starch synthesis from wild-type plants, but hexose partitioning in the light period was dramatically different. Furthermore, alteration of extracellular invertase activity was observed in the lower leaves of the transplastomic plants. These observations suggest that high-level expression of plastidial PGM alters hexose partitioning in light periods via modification of extracellular invertase activity.     

Antisense Repression of Both ADP-Glucose Pyrophosphorylase and Triose Phosphate Translocator Modifies Carbohydrate Partitioning in Potato Leaves

Previous experiments have shown that carbohydrate partitioning in leaves of potato (Solanum tuberosum L.) plants can be modified by antisense repression of the triose phosphate translocator (TPT), favoring starch accumulation during the light period, or by leaf-specific antisense repression of ADP-glucose pyrophosphorylase (AGPase), reducing leaf starch content. These experiments showed that starch and sucrose synthesis can partially replace each other. To determine how leaf metabolism acclimates to an inhibition of both pathways, transgenic potato (S. tuberosum L. cv Desiree) plants, with a 30% reduction of the TPT achieved by antisense repression, were transformed with an antisense cDNA of the small subunit of AGPase, driven by the leaf-specific ST-LS1 promoter. These double-transformed plants were analyzed with respect to their carbohydrate metabolism, and starch accumulation was reduced in all lines of these plants. In one line with a 50% reduction of AGPase activity, the rate of CO2 assimilation was unaltered. In these plants the stromal level of triose phosphate was increased, enabling a high rate of triose phosphate export in spite of the reduction of the TPT protein by antisense repression. In a second line with a 95% reduction of AGPase activity, the amount of chlorophyll was significantly reduced as a consequence of the lowered triose phosphate utilization capacity.     

Exploiting leaf starch synthesis as a transient sink to elevate photosynthesis, plant productivity and yields

Improvements in plant productivity (biomass) and yield have centered on increasing the efficiency of leaf CO₂ fixation and utilization of products by non-photosynthetic sink organs. We had previously demonstrated a correlation between photosynthetic capacity, plant growth, and the extent of leaf starch synthesis utilizing starch-deficient mutants. This finding suggested that leaf starch is used as a transient photosynthetic sink to recycle inorganic phosphate and, in turn, maximize photosynthesis. To test this hypothesis, Arabidopsis thaliana and rice (Oryza sativa L.) lines were generated with enhanced capacity to make leaf starch with minimal impact on carbon partitioning to sucrose. The Arabidopsis engineered plants exhibited enhanced photosynthetic capacity; this translated into increased growth and biomass. These enhanced phenotypes were displayed by similarly engineered rice lines. Manipulation of leaf starch is a viable alternative strategy to increase photosynthesis and, in turn, the growth and yields of crop and bioenergy plants.     

A comparison of the carbohydrate composition and kinetic properties of sucrose phosphate synthase (SPS) in transgenic tobacco (Nicotiana tabacum) leaves expressing maize SPS protein with untransformed controls

Tobacco plants, transformed with a maize sucrose phosphate synthase (SPS) cDNA clone, had threefold increased SPS activity compared to wild‐type tobacco. Measurement of SPS maximal activity and protein abundance using specific antibodies to the maize protein showed that the specific activity of the maize SPS protein was maintained when expressed in tobacco. Comparison of the kinetic properties of SPS in the transgenic lines compared to either wild‐type maize or tobacco revealed that the heterologously expressed protein had reduced affinity for both substrates (fructose‐6‐phosphate and UDP‐glucose) and reduced sensitivity to allosteric inhibition by inorganic phosphate. Moreover, the extent of light‐induced activation was reduced in the transgenic lines, with smaller changes observed in the K_m for both F6P compared to maize and tobacco wild‐type plants. Increased sucrose concentrations were observed in the transgenic lines at the end of the photoperiod and this was linearly related to SPS activity and associated with a parallel decrease in starch content. This suggests that SPS is a major control point for carbohydrate partitioning between starch and sucrose during photosynthesis.     

Production of high-starch, low-glucose potatoes through over-expression of the metabolic regulator SnRK1

Transgenic potato ( Solanum tuberosum cv. Prairie) lines were produced over-expressing a sucrose non-fermenting-1-related protein kinase-1 gene ( SnRK1 ) under the control of a patatin (tuber-specific) promoter. SnRK1 activity in the tubers of three independent transgenic lines was increased by 55%−167% compared with that in the wild-type. Glucose levels were decreased, at 17%−56% of the levels of the wild-type, and the starch content showed an increase of 23%−30%. Sucrose and fructose levels in the tubers of the transgenic plants did not show a significant change. Northern analyses of genes encoding sucrose synthase and ADP-glucose pyrophosphorylase, two key enzymes involved in the biosynthetic pathway from sucrose to starch, showed that the expression of both was increased in tubers of the transgenic lines compared with the wild-type. In contrast, the expression of genes encoding two other enzymes of carbohydrate metabolism, α-amylase and sucrose phosphate synthase, showed no change. The activity of sucrose synthase and ADP-glucose pyrophosphorylase was also increased, by approximately 20%–60% and three- to five-fold, respectively, whereas the activity of hexokinase was unchanged. The results are consistent with a role for SnRK1 in regulating carbon flux through the storage pathway to starch biosynthesis. They emphasize the importance of SnRK1 in the regulation of carbohydrate metabolism and resource partitioning, and indicate a specific role for SnRK1 in the control of starch accumulation in potato tubers.     

Tuber-specific cytosolic expression of a bacterial phosphoglucomutase in potato (Solanum tuberosum L.) dramatically alters carbon partitioning

Constitutive antisense inhibition of the cytosolic isoform of phosphoglucomutase in the potato (Solanum tuberosum L.) results in restriction of photosynthesis, growth inhibition and modified tuber morphology, and a severe restriction of tuber starch synthesis. Here we describe the consequences of the tuber-specific expression of an Escherichia coli phosphoglucomutase in the cytosol. Analysis of [14C]glucose metabolism by tuber discs isolated from wild type and transformants revealed that the rates of sucrose and starch synthesis were unaltered but that the rate of glycolysis was depressed in the transgenics. The transformant tubers also contained dramatically reduced amino acid content and significantly higher levels of ADP, but were characterized by elevated levels of Krebs cycle intermediates and an unaltered rate of respiration. In addition to these metabolic consequences of the overexpression of the E. coli enzyme, we observed morphological changes in tubers, with the transformants having a smaller number of larger tubers which exhibited delayed rates of sprouting with respect to the wild type. These results are discussed with respect to current models of the regulation of central plant metabolism and tuber dormancy.     

Diurnal periodicity of assimilate transport shapes resource allocation and whole‐plant carbon balance

Whole‐plant carbon balance comprises diurnal fluctuations of photosynthetic carbon gain and respiratory losses, as well as partitioning of assimilates between phototrophic and heterotrophic organs. Because it is difficult to access, the root system is frequently neglected in growth models, or its metabolism is rated based on generalizations from other organs. Here, whole‐plant cuvettes were used for investigating total‐plant carbon exchange with the environment over full diurnal cycles. Dynamics of primary metabolism and diurnally resolved phloem exudation profiles, as proxy of assimilate transport, were combined to obtain a full picture of resource allocation. This uncovered a strong impact of periodicity of inter‐organ transport on the efficiency of carbon gain. While a sinusoidal fluctuation of the transport rate, with minor diel deflections, minimized respiratory losses in Arabidopsis wild‐type plants, triangular or rectangular patterns of transport, found in mutants defective in either starch or sucrose metabolism, increased root respiration at the end or beginning of the day, respectively. Power spectral density and cross‐correlation analysis revealed that only the rate of starch synthesis was strictly correlated to the rate of net photosynthesis in wild‐type, while in a sucrose‐phosphate synthase mutant (spsa1), this applied also to carboxylate synthesis, serving as an alternative carbon pool. In the starchless mutant of plastidial phospho‐gluco mutase (pgm), none of these rates, but concentrations of sucrose and glucose in the root, followed the pattern of photosynthesis, indicating direct transduction of shoot sugar levels to the root. The results demonstrate that starch metabolism alone is insufficient to buffer diurnal fluctuations of carbon exchange.     

A possible role for pyrophosphate in the coordination of cytosolic and plastidial carbon metabolism within the potato tuber

The early stages of tuber development are characterized by cell division, high metabolic activity, and the predominance of invertase as the sucrose (Suc) cleaving activity. However, during the subsequent phase of starch accumulation the cleavage of Suc occurs primarily by the action of Suc synthase. The mechanism that is responsible for this switch in Suc cleaving activities is currently unknown. One striking difference between the invertase and Suc synthase mediated cleavage of Suc is the direct involvement of inorganic pyrophosphate (PPi) in the latter case. There is presently no convincing explanation of how the PPi required to support this process is generated in potato (Solanum tuberosum) tubers. The major site of PPi production in a maturing potato tubers is likely to be the reaction catalyzed by ADP-glucose pyrophosphorylase, the first committed step of starch biosynthesis in amyloplasts. We present data based on the analysis of the PPi levels in various transgenic plants altered in starch and Suc metabolism that support the hypothesis that PPi produced in the plastid is used to support cytosolic Suc breakdown and that PPi is an important coordinator of cytosolic and plastidial metabolism in potato tubers.     

Enzymes of starch and sugar phosphate metabolism in achlorophyllous ribosome-deficient plastids from high-temperature-grown rye leaves

Several enzymes of non–photosynthetic sugar phosphate and starch metabolism were measured in gradient–purified chloroplasts from normal rye leaves ( Secale cereale L. cv. Halo) grown at 22°C and in the non-photosynthetic plastids isolated from 70S ribosome-deficient rye leaves grown at a non–permissive elevated temperature of 32°C. Activities of the enzymes phosphoglycerate kinase (EC 2.7.2.3), hexokinase (EC 2.7.1.1), phosphoglucose isomerase (EC 5.3.1.9), phosphoglucomutase (EC 2.7.5.1), glucose-6-phosphate dehydrogenase (EC 1.1.1.49), 6-phosphogluconate de-hydrogenase (EC 1.1.1.46), ADPglucose pyrophosphorylase (EC 2.7.7.27), starch synthase (EC 2.4.1.21), and phosphorylase (EC 2.4.1.1) were present in ribosome-deficient plastids from 32°C-grown leaves indicating a cytoplasmic origin of the plastid-specific forms of these enzymes. While the photosynthetic marker enzyme NADP⁺-dependent glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13) was considerably diminished, both the specific activities and the total activities per leaf of the plastid-specific forms of hexokinase, phosphoglucose isomerase and phosphoglucomutase were markedly increased in the ribosome–deficient plastids, relative to normal chloroplasts. The results demonstrate that after elimination of functional protein synthesis in the chloroplasts the supply of chloroplast–specific enzymes by the cytoplasm is not generally suppressed as observed for many enzymes and proteins involved in photosynthesis, but may even be increased in accord with changed metabolic demands.     

Heterologous expression of a<Emphasis Type="Italic"> keto</Emphasis>hexokinase 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 ¹⁴CO₂ 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.Abbreviations CaMV Cauliflower mosaic virus - DHAP Dihydroxyacetone phosphate - F1P Fructose 1-phosphate - FBP Fructose 1,6-bisphosphate - KHK Ketohexokinase - NADP-GAPdH NADP-dependent glyceraldehyde-3-phosphate dehydrogenase - PFP Pyrophosphate: fructose 6-phosphate 1-phosphotransferase - 3PGA 3-Phosphoglycerate - PEP Phosphoenolpyruvate - Rubisco Ribulose 1,5-bisphosphate carboxylase/oxygenase - SPS Sucrose phosphate synthase - SuSy Sucrose synthase     

Carbon partitioning in leaves and tubers of transgenic potato plants with reduced activity of fructose-6-phosphate,2-kinase/fructose-2,6-bisphosphatase

The role of fructose-2,6-bisphosphate (Fru-2,6-P₂) in regulation of carbon metabolism was investigated in transgenic potato plants ( Solanum tuberosum L. cv Dianella) transformed with a vector containing a cDNA-sequence encoding fructose-6-phosphate,2-kinase (F6P,2-K, EC 2.7.1.105)/fructose-2,6-bisphosphatase (F26BPase, EC 3.1.3.46) in sense or antisense direction behind a CaMV 35S promoter. The activity of F6P,2-K in leaves was reduced to 5% of wild-type (WT) activity, and the level of Fru-2,6-P₂ was reduced both in leaves (10% of the WT level) and in tubers (40% of the WT level). Analysis of photosynthetic ¹⁴CO₂ metabolism, showed that in plant lines with reduced Fru-2,6-P₂ level the carbon partitioning in the leaves was changed in favour of sucrose biosynthesis, and the soluble sugars-to-starch labelling ratio was doubled. The levels of soluble sugars and hexose phosphates also increased in leaves of the transgenic plants. Most notably, the levels of hexoses were four- to six-fold increased in the transgenic plants. In tubers with reduced levels of Fru-2,6-P₂ only minor effects on carbohydrate levels were observed. Furthermore, carbon assimilation in tuber discs supplied with [U-¹⁴C]-sucrose showed only a moderate increase in labelling of hexoses and a decreased labelling of starch. Similar results were obtained using [U-¹⁴C]-glucose. No differences in growth of the transgenic lines and the WT were observed. Our data provide evidences that Fru-2,6-P₂ is an important factor in the regulation of photosynthetic carbon metabolism in potato leaves, whereas the direct influence of Fru-2,6-P₂ on tuber metabolism was limited.