Genetic and metabolic effects of ripening mutations and vine detachment on tomato fruit quality.

Tomato (Solanum lycopersicum) fruit ripening is regulated co-operatively by the action of ethylene and a hierarchy of transcription factors, including RIPENING INHIBITOR (RIN) and NON-RIPENING (NOR). Mutations in these two genes have been adopted commercially to delay ripening, and accompanying textural deterioration, as a means to prolong shelf life. However, these mutations also affect desirable traits associated with colour and nutritional value, although the extent of this trade-off has not been assessed in detail. Here, we evaluated changes in tomato fruit pericarp primary metabolite and carotenoid pigment profiles, as well as the dynamics of specific associated transcripts, in the rin and nor mutants during late development and postharvest storage, as well of those of the partially ripening delayed fruit ripening (dfd) tomato genotype. These profiles were compared with those of the wild-type tomato cultivars Ailsa Craig (AC) and M82. We also evaluated the metabolic composition of M82 fruit ripened on or off the vine over a similar period. In general, the dfd mutation resulted in prolonged firmness and maintenance of quality traits without compromising key metabolites (sucrose, glucose/fructose and glucose) and sectors of intermediary metabolism, including tricarboxylic acid cycle intermediates. Our analysis also provided insights into the regulation of carotenoid formation and highlighted the importance of the polyamine, putrescine, in extending fruit shelf life. Finally, the metabolic composition analysis of M82 fruit ripened on or off the vine provided insights into the import into fruit of compounds, such as sucrose, during ripening.

The sensitive to freezing3 mutation of Arabidopsis thaliana is a cold-sensitive allele of homomeric acetyl-CoA carboxylase that results in cold-induced cuticle deficiencies.

The sfr3 mutation causes freezing sensitivity in Arabidopsis thaliana. Mapping, sequencing, and transgenic complementation showed sfr3 to be a missense mutation in ACC1, an essential gene encoding homomeric (multifunctional) acetyl-CoA carboxylase. Cuticle permeability was compromised in the sfr3 mutant when plants were grown in the cold but not in the warm. Wax deposition on the inflorescence stem of cold-grown sfr3 plants was inhibited and the long-chain components of their leaf cuticular wax were reduced compared with wild-type plants. Thus, freezing sensitivity of sfr3 appears, from these results, to be due to cuticular deficiencies that develop during cold acclimation. These observations demonstrated the essential role of the cuticle in tolerance to freezing and drought.

Tomato fruit photosynthesis is seemingly unimportant in primary metabolism and ripening but plays a considerable role in seed development.

Fruit of tomato (Solanum lycopersicum), like those from many species, have been characterized to undergo a shift from partially photosynthetic to truly heterotrophic metabolism. While there is plentiful evidence for functional photosynthesis in young tomato fruit, the rates of carbon assimilation rarely exceed those of carbon dioxide release, raising the question of its role in this tissue. Here, we describe the generation and characterization of lines exhibiting a fruit-specific reduction in the expression of glutamate 1-semialdehyde aminotransferase (GSA). Despite the fact that these plants contained less GSA protein and lowered chlorophyll levels and photosynthetic activity, they were characterized by few other differences. Indeed, they displayed almost no differences in fruit size, weight, or ripening capacity and furthermore displayed few alterations in other primary or intermediary metabolites. Although GSA antisense lines were characterized by significant alterations in the expression of genes associated with photosynthesis, as well as with cell wall and amino acid metabolism, these changes were not manifested at the phenotypic level. One striking feature of the antisense plants was their seed phenotype: the transformants displayed a reduced seed set and altered morphology and metabolism at early stages of fruit development, although these differences did not affect the final seed number or fecundity. Taken together, these results suggest that fruit photosynthesis is, at least under ambient conditions, not necessary for fruit energy metabolism or development but is essential for properly timed seed development and therefore may confer an advantage under conditions of stress.

Antisense suppression of the small chloroplast protein CP12 in tobacco alters carbon partitioning and severely restricts growth.

The thioredoxin-regulated chloroplast protein CP12 forms a multienzyme complex with the Calvin-Benson cycle enzymes phosphoribulokinase (PRK) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). PRK and GAPDH are inactivated when present in this complex, a process shown in vitro to be dependent upon oxidized CP12. The importance of CP12 in vivo in higher plants, however, has not been investigated. Here, antisense suppression of CP12 in tobacco (Nicotiana tabacum) was observed to impact on NAD-induced PRK and GAPDH complex formation but had little effect on enzyme activity. Additionally, only minor changes in photosynthetic carbon fixation were observed. Despite this, antisense plants displayed changes in growth rates and morphology, including dwarfism and reduced apical dominance. The hypothesis that CP12 is essential to separate oxidative pentose phosphate pathway activity from Calvin-Benson cycle activity, as proposed in cyanobacteria, was tested. No evidence was found to support this role in tobacco. Evidence was seen, however, for a restriction to malate valve capacity, with decreases in NADP-malate dehydrogenase activity (but not protein levels) and pyridine nucleotide content. Antisense repression of CP12 also led to significant changes in carbon partitioning, with increased carbon allocation to the cell wall and the organic acids malate and fumarate and decreased allocation to starch and soluble carbohydrates. Severe decreases were also seen in 2-oxoglutarate content, a key indicator of cellular carbon sufficiency. The data presented here indicate that in tobacco, CP12 has a role in redox-mediated regulation of carbon partitioning from the chloroplast and provides strong in vivo evidence that CP12 is required for normal growth and development in plants.

Pleiotropic physiological consequences of feedback-insensitive phenylalanine biosynthesis in Arabidopsis thaliana.

A large proportion of plant carbon flow passes through the shikimate pathway to phenylalanine, which serves as a precursor for numerous secondary metabolites. To identify new regulatory mechanisms affecting phenylalanine metabolism, we isolated Arabidopsis thaliana mutants that are resistant to the phytotoxic amino acid m-tyrosine, a structural analog of phenylalanine. Map-based cloning identified adt2-1D, a dominant point mutation causing a predicted serine to alanine change in the regulatory domain of ADT2 (arogenate dehydratase 2). Relaxed feedback inhibition and increased expression of the mutant enzyme caused up to 160-fold higher accumulation of free phenylalanine in rosette leaves, as well as altered accumulation of several other primary and secondary metabolites. In particular, abundance of 2-phenylethylglucosinolate, which is normally almost undetectable in leaves of the A. thaliana Columbia-0 accession, is increased more than 30-fold. Other observed phenotypes of the adt2-1D mutant include abnormal leaf development, resistance to 5-methyltryptophan, reduced growth of the generalist lepidopteran herbivore Trichoplusia ni (cabbage looper) and increased salt tolerance.

SlCCD7 controls strigolactone biosynthesis, shoot branching and mycorrhiza-induced apocarotenoid formation in tomato.

The regulation of shoot branching is an essential determinant of plant architecture, integrating multiple external and internal signals. One of the signaling pathways regulating branching involves the MAX (more axillary branches) genes. Two of the genes within this pathway, MAX3/CCD7 and MAX4/CCD8, encode carotenoid cleavage enzymes involved in generating a branch-inhibiting hormone, recently identified as strigolactone. Here, we report the cloning of SlCCD7 from tomato. As in other species, SlCCD7 encodes an enzyme capable of cleaving cyclic and acyclic carotenoids. However, the SlCCD7 protein has 30 additional amino acids of unknown function at its C terminus. Tomato plants expressing a SlCCD7 antisense construct display greatly increased branching. To reveal the underlying changes of this strong physiological phenotype, a metabolomic screen was conducted. With the exception of a reduction of stem amino acid content in the transgenic lines, no major changes were observed. In contrast, targeted analysis of the same plants revealed significantly decreased levels of strigolactone. There were no significant changes in root carotenoids, indicating that relatively little substrate is required to produce the bioactive strigolactones. The germination rate of Orobanche ramosa seeds was reduced by up to 90% on application of extract from the SlCCD7 antisense lines, compared with the wild type. Additionally, upon mycorrhizal colonization, C(13) cyclohexenone and C(14) mycorradicin apocarotenoid levels were greatly reduced in the roots of the antisense lines, implicating SlCCD7 in their biosynthesis. This work demonstrates the diverse roles of MAX3/CCD7 in strigolactone production, shoot branching, source-sink interactions and production of arbuscular mycorrhiza-induced apocarotenoids.

Metabolic profiles of sunflower genotypes with contrasting response to Sclerotinia sclerotiorum infection.

We report a comprehensive primary metabolite profiling of sunflower (Helianthus annuus) genotypes displaying contrasting behavior to Sclerotinia sclerotiorum infection. Applying a GC-MS-based metabolite profiling approach, we were able to identify differential patterns involving a total of 63 metabolites including major and minor sugars and sugar alcohols, organic acids, amino acids, fatty acids and few soluble secondary metabolites in the sunflower capitulum, the main target organ of pathogen attack. Metabolic changes and disease incidence of the two contrasting genotypes were determined throughout the main infection period (R5.2-R6). Both point-by-point and non-parametric statistical analyses showed metabolic differences between genotypes as well as interaction effects between genotype and time after inoculation. Network correlation analyses suggested that these metabolic changes were synchronized in a time-dependent manner in response to the pathogen. Concerted differential metabolic changes were detected to a higher extent in the susceptible, rather than the resistant genotype, thereby allowing differentiation of modules composed by intermediates of the same pathway which are highly interconnected in the susceptible line but not in the resistant one. Evaluation of these data also demonstrated a genotype specific regulation of distinct metabolic pathways, suggesting the importance of detection of metabolic patterns rather than specific metabolite changes when looking for metabolic markers differentially responding to pathogen infection. In summary, the GC-MS strategy developed in this study was suitable for detection of differences in carbon primary metabolism in sunflower capitulum, a tissue which is the main entry point for this and other pathogens which cause great detrimental impact on crop yield.

Application of GC-MS for the detection of lipophilic compounds in diverse plant tissues.

BACKGROUND: The concept of metabolite profiling has been around for decades and technical innovations are now enabling it to be carried out on a large scale with respect to the number of both metabolites measured and experiments carried out. However, studies are generally confined to polar compounds alone. Here we describe a simple method for lipophilic compounds analysis in various plant tissues. RESULTS: We choose the same preparative and instrumental platform for lipophilic profiling as that we routinely use for polar metabolites measurements. The method was validated in terms of linearity, carryover, reproducibility and recovery rates, as well as using various plant tissues.As a first case study we present metabolic profiling of Arabidopsis root and shoot tissue of wild type (C24) and mutant (rsr4-1) plants deficient on vitamin B6. We found significant alterations in lipid constituent contents, especially in the roots, which were characterised by dramatic increases in several fatty acids, thus providing further hint for the role of pyridoxine in oxidative stress and lipid peroxidation.The second example is the lipophilic profiling of red and green tomato fruit cuticles of wild type (Alisa Craig) and the DFD (delayed fruit deterioration) mutant, which we compared and contrasted with the more focused wax analysis of these plants reported before. CONCLUSION: We can rapidly and reliably detect and quantify over 40 lipophilic metabolites including fatty acids, fatty alcohols, alkanes, sterols and tocopherols. The method presented here affords a simple and rapid, yet robust complement to previously validated methods of polar metabolite profiling by gas-chromatography mass-spectrometry.

Decreased expression of cytosolic pyruvate kinase in potato tubers leads to a decline in pyruvate resulting in an in vivo repression of the alternative oxidase.

The aim of this work was to investigate the effect of decreased cytosolic pyruvate kinase (PKc) on potato (Solanum tuberosum) tuber metabolism. Transgenic potato plants with strongly reduced levels of PKc were generated by RNA interference gene silencing under the control of a tuber-specific promoter. Metabolite profiling showed that decreased PKc activity led to a decrease in the levels of pyruvate and some other organic acids involved in the tricarboxylic acid cycle. Flux analysis showed that this was accompanied by changes in carbon partitioning, with carbon flux being diverted from glycolysis toward starch synthesis. However, this metabolic shift was relatively small and hence did not result in enhanced starch levels in the tubers. Although total respiration rates and the ATP to ADP ratio were largely unchanged, transgenic tubers showed a strong decrease in the levels of alternative oxidase (AOX) protein and a corresponding decrease in the capacity of the alternative pathway of respiration. External feeding of pyruvate to tuber tissue or isolated mitochondria resulted in activation of the AOX pathway, both in the wild type and the PKc transgenic lines, providing direct evidence for the regulation of AOX by changes in pyruvate levels. Overall, these results provide evidence for a crucial role of PKc in the regulation of pyruvate levels as well as the level of the AOX in heterotrophic plant tissue, and furthermore reveal that these parameters are interlinked in vivo.

A reevaluation of the key factors that influence tomato fruit softening and integrity.

The softening of fleshy fruits, such as tomato (Solanum lycopersicum), during ripening is generally reported to result principally from disassembly of the primary cell wall and middle lamella. However, unsuccessful attempts to prolong fruit firmness by suppressing the expression of a range of wall-modifying proteins in transgenic tomato fruits do not support such a simple model. 'Delayed Fruit Deterioration' (DFD) is a previously unreported tomato cultivar that provides a unique opportunity to assess the contribution of wall metabolism to fruit firmness, since DFD fruits exhibit minimal softening but undergo otherwise normal ripening, unlike all known nonsoftening tomato mutants reported to date. Wall disassembly, reduced intercellular adhesion, and the expression of genes associated with wall degradation were similar in DFD fruit and those of the normally softening 'Ailsa Craig'. However, ripening DFD fruit showed minimal transpirational water loss and substantially elevated cellular turgor. This allowed an evaluation of the relative contribution and timing of wall disassembly and water loss to fruit softening, which suggested that both processes have a critical influence. Biochemical and biomechanical analyses identified several unusual features of DFD cuticles and the data indicate that, as with wall metabolism, changes in cuticle composition and architecture are an integral and regulated part of the ripening program. A model is proposed in which the cuticle affects the softening of intact tomato fruit both directly, by providing a physical support, and indirectly, by regulating water status.

The complex network of non-cellulosic carbohydrate metabolism.

Partitioning of carbon dominates intracellular fluxes in both photosynthetic and heterotrophic plant tissues, and has vast influence on both plant growth and development. Recently, much progress has been made in elucidating the structures of the biosynthetic and degradative pathways that link the major and minor pools of soluble carbohydrates to cellular polymers such as starch, heteroglycans and fructans. In most cases, the regulatory properties of these pathways have been elucidated and the enzymes involved have been investigated using reverse genetics approaches. Although many of the results from these approaches were merely confirmatory, several of them were highly unexpected. The challenge ahead is to achieve better understanding of metabolic regulation at the network level in order to develop more rational strategies for metabolic engineering.

Deficiency of mitochondrial fumarase activity in tomato plants impairs photosynthesis via an effect on stomatal function.

Transgenic tomato (Solanum lycopersicum) plants expressing a fragment of a fumarate hydratase (fumarase) gene in the antisense orientation and exhibiting considerable reductions in the mitochondrial activity of this enzyme show impaired photosynthesis. The rate of the tricarboxylic acid cycle was reduced in the transformants relative to the other major pathways of carbohydrate oxidation and the plants were characterized by a restricted rate of dark respiration. However, biochemical analyses revealed relatively little alteration in leaf metabolism as a consequence of reducing the fumarase activity. That said, in comparison to wild-type plants, CO(2) assimilation was reduced by up to 50% under atmospheric conditions and plants were characterized by a reduced biomass on a whole plant basis. Analysis of further photosynthetic parameters revealed that there was little difference in pigment content in the transformants but that the rate of transpiration and stomatal conductance was markedly reduced. Analysis of the response of the rate of photosynthesis to variation in the concentration of CO(2) confirmed that this restriction was due to a deficiency in stomatal function.

Mitochondrial uncoupling protein is required for efficient photosynthesis.

Uncoupling proteins (UCPs) occur in the inner mitochondrial membrane and dissipate the proton gradient across this membrane that is normally used for ATP synthesis. Although the catalytic function and regulation of plant UCPs have been described, the physiological purpose of UCP in plants has not been established. Here, biochemical and physiological analyses of an insertional knockout of one of the Arabidopsis UCP genes (AtUCP1) are presented that resolve this issue. Absence of UCP1 results in localized oxidative stress but does not impair the ability of the plant to withstand a wide range of abiotic stresses. However, absence of UCP1 results in a photosynthetic phenotype. Specifically there is a restriction in photorespiration with a decrease in the rate of oxidation of photorespiratory glycine in the mitochondrion. This change leads to an associated reduced photosynthetic carbon assimilation rate. Collectively, these results suggest that the main physiological role of UCP1 in Arabidopsis leaves is related to maintaining the redox poise of the mitochondrial electron transport chain to facilitate photosynthetic metabolism.

Analysis of the Arabidopsis rsr4-1/pdx1-3 mutant reveals the critical function of the PDX1 protein family in metabolism, development, and vitamin B6 biosynthesis.

Vitamin B6 represents a highly important group of compounds ubiquitous in all living organisms. It has been demonstrated to alleviate oxidative stress and in its phosphorylated form participates as a cofactor in >100 biochemical reactions. By means of a genetic approach, we have identified a novel mutant, rsr4-1 (for reduced sugar response), with aberrant root and leaf growth that requires supplementation of vitamin B6 for normal development. Cloning of the mutated gene revealed that rsr4-1 carries a point mutation in a member of the PDX1/SOR1/SNZ (for Pyridoxine biosynthesis protein 1/Singlet oxygen resistant 1/Snooze) family that leads to reduced vitamin B6 content. Consequently, metabolism is broadly altered, mainly affecting amino acid, raffinose, and shikimate contents and trichloroacetic acid cycle constituents. Yeast two-hybrid and pull-down analyses showed that Arabidopsis thaliana PDX1 proteins can form oligomers. Interestingly, the mutant form of PDX1 has severely reduced capability to oligomerize, potentially suggesting that oligomerization is important for function. In summary, our results demonstrate the critical function of the PDX1 protein family for metabolism, whole-plant development, and vitamin B6 biosynthesis in higher plants.

The influence of cytosolic phosphorylating glyceraldehyde 3-phosphate dehydrogenase (GAPC) on potato tuber metabolism.

The aim of this work was to investigate the importance of cytosolic phosphorylating glyceraldehyde 3-phosphate dehydrogenase (GAPC) in potato carbohydrate metabolism. For this purpose, the cytosolic isoform of phosphorylating GAPC was cloned and used for an antisense approach to generate transgenic potato plants that exhibited constitutively decreased GAPDH activity. Potato lines with decreased activities of phosphorylating GAPC exhibited no major changes in either whole-plant or tuber morphology. However, the levels of 3-phosphoglycerate were decreased in leaves of the transformants. A broad metabolic phenotyping of tubers from the transformants revealed an increase in sucrose and UDPglucose content, a decrease in the glycolytic intermediates 3-phosphoglycerate and phosphoenolpyruvate but little change in the levels of other metabolites. Moreover, the transformants displayed no differences in cold sweetening with respect to the wild type. Taken together these data suggest that phosphorylating GAPC plays only a minor role in the regulation of potato metabolism. The results presented here are discussed in relation to current models regarding primary metabolism in the potato tuber parenchyma.

Expression of an Escherichia coli phosphoglucomutase in potato (Solanum tuberosum 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.

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.

Enhanced photosynthetic performance and growth as a consequence of decreasing mitochondrial malate dehydrogenase activity in transgenic tomato plants.

Transgenic tomato (Solanum lycopersicum) plants expressing a fragment of the mitochondrial malate dehydrogenase gene in the antisense orientation and exhibiting reduced activity of this isoform of malate dehydrogenase show enhanced photosynthetic activity and aerial growth under atmospheric conditions (360 ppm CO2). In comparison to wild-type plants, carbon dioxide assimilation rates and total plant dry matter were up to 11% and 19% enhanced in the transgenics, when assessed on a whole-plant basis. Accumulation of carbohydrates and redox-related compounds such as ascorbate was also markedly elevated in the transgenics. Also increased in the transgenic plants was the capacity to use L-galactono-lactone, the terminal precursor of ascorbate biosynthesis, as a respiratory substrate. Experiments in which ascorbate was fed to isolated leaf discs also resulted in increased rates of photosynthesis providing strong indication for an ascorbate-mediated link between the energy-generating processes of respiration and photosynthesis. This report thus shows that the repression of this mitochondrially localized enzyme improves both carbon assimilation and aerial growth in a crop species.

Deficiency of a plastidial adenylate kinase in Arabidopsis results in elevated photosynthetic amino acid biosynthesis and enhanced growth.

An Arabidopsis (Arabidopsis thaliana) L. Heynh mutant deficient in an isoform of adenylate kinase (ADK; At2g37250) was isolated by reverse genetics. It contains a T-DNA insertion 377 bp downstream of the start point of transcription. The mutant lacks At2g37250 transcripts and has a mild reduction in total cellular ADK activity. Green fluorescent protein-fusion based cellular localization experiments, carried out with the full-length At2g37250, suggested a plastidial localization for this isoform. In keeping with this observation, organelle isolation experiments revealed that the loss in ADK activity was confined to the inner plastid. This plastid stroma ADK gene was found to be expressed tissue constitutively but at much higher levels in illuminated leaves. Phenotypic and biochemical analyses of the mutant revealed that it exhibited higher amino acid biosynthetic activity in the light and was characterized by an enhanced root growth. When the mutant was subjected to either continuous light or continuous dark, growth phenotypes were also observed in the shoots. While the levels of adenylates were not much altered in the leaves, the pattern of change observed in the roots was consistent with the inhibition of an ATP-consuming reaction. Taken together, these data suggest a role for the plastid stromal ADK in the coordination of metabolism and growth, but imply that the exact importance of this isoform is tissue dependent.

Expression of a yeast acetyl CoA hydrolase in the mitochondrion of tobacco plants inhibits growth and restricts photosynthesis.

Acetyl Coenzyme A (acetyl CoA) is required in the mitochondria to fuel the operation of the Krebs cycle and within the cytosolic, peroxisomal and plastidial compartments wherein it acts as the immediate precursor for a wide range of anabolic functions. Since this metabolite is impermeable to membranes it follows that discrete pathways both for its synthesis and for its utilization must be present in each of these organelles and that the size of the various compartmented pools are independently regulated. To determine the specific role of acetyl CoA in the mitochondria we exploited a transgenic approach to introduce a yeast acetyl CoA hydrolase (EC 3.1.2.1.) into this compartment in tobacco plants. Despite the facts that the introduced enzyme was correctly targeted and that there were marked reductions in the levels of citrate and malate and an increase in the acetate content of the transformants, the transgenic plants surprisingly exhibited increased acetyl CoA levels. The lines were further characterised by a severe growth retardation, abnormal leaf colouration and a dramatic reduction in photosynthetic activity correlated with a marked reduction in the levels of transcripts of photosynthesis and in the content of photosynthetic pigments. The altered rate of photosynthesis in the transgenics was also reflected by a modified carbon partitioning in leaves of these lines, however, further studies revealed that this was most likely caused by a decreased source to sink transport of carbohydrate. In summary these results suggest that the content of acetyl CoA is under tight control and that alterations in the level of this central metabolite have severe metabolic and developmental consequences in tobacco.