Specific Transport of Inorganic Phosphate and C3- and C6-Sugar-Phosphates across the Envelope Membranes of Tomato (Lycopersicon esculentum) Leaf-Chloroplasts, Tomato Fruit-Chloroplasts and Fruit-Chromoplasts

Developmental analysis of carbohydrate metabolism in tomato (Lycopersicon esculentum cv. Micro-Tom) fruits

Carbohydrate metabolism during the development of fruits of the tomato cultivar Micro-Tom was studied. The metabolism of the pericarp and placental tissues was found to be different. Starch was degraded more slowly in the placenta in comparison with the pericarp, whereas soluble sugars accumulated to a greater extent in the pericarp. The activities of glycolytic enzymes tended to peak at 40 days after flowering. Two of these, phosphoenolpyruvate phosphatase and pyruvate kinase, showed a dramatic increase in activity just before this peak, possibly indicating a role in up-regulating glycolysis to generate increased ATP that would be used during climacteric respiration. The expression of plastidial transporters was studied. Both the TPT and Glu6P transporter were expressed greatest in green fruits, before declining. The expression of the triose-phosphate transporter was greater than that of the glucose 6-phosphate transporter. The ATP/ADP transporter was expressed to a low level throughout fruit development.

Characterization of two functional phosphoenolpyruvate/phosphate translocator (PPT) genes in Arabidopsis--AtPPT1 may be involved in the provision of signals for correct mesophyll development

The Arabidopsis thaliana chlorophyll a/b-binding protein underexpressed 1 (cue1) mutant shows a reticulate leaf phenotype and is defective in a plastidic phosphoenolpyruvate (PEP)/phosphate translocator (AtPPT1). A functional AtPPT1 providing plastids with PEP for the shikimate pathway is therefore essential for correct leaf development. The Arabidopsis genome contains a second PPT gene, AtPPT2. Both transporters share similar substrate specificities and are therefore able to transport PEP into plastids. The cue1 phenotype could partially be complemented by ectopic expression of AtPPT2 but obviously not by the endogeneous AtPPT2. Both genes are differentially expressed in most tissues: AtPPT1 is mainly expressed in the vasculature of leaves and roots, especially in xylem parenchyma cells, but not in leaf mesophyll cells, whereas AtPPT2 is expressed ubiquitously in leaves, but not in roots. The expression profiles are corroborated by tissue-specific transport data. As AtPPT1 expression is absent in mesophyll cells that are severely affected in the cue1 mutant, we propose that the vasculature-located AtPPT1 is involved in the generation of phenylpropanoid metabolism-derived signal molecules that trigger development in interveinal leaf regions. This signal probably originates from the root vasculature where only AtPPT1, but not AtPPT2, is present.

ADP-glucose pyrophosphorylase is located in the plastid in developing tomato fruit

The subcellular location of activity and protein of ADP-glucose pyrophosphorylase (AGPase) in developing tomato (Lycopersicon esculentum) fruit was determined following a report that the enzyme might be present inside and outside the plastids in this organ. Plastids prepared from crude homogenates of columella and pericarp, the starch-accumulating tissues of developing fruit, contained 8% to 18% of the total activity of enzymes known to be confined to plastids, and 0.2% to 0.5% of the total activity of enzymes known to be confined to the cytosol. The proportion of the total activity of AGPase in the plastids was the same as that of the enzymes known to be confined to the plastid. When samples of plastid and total homogenate fractions were subjected to immunoblotting with an antiserum raised to AGPase, most or all of the protein detected was plastidial. Taken as a whole, these data provide strong evidence that AGPase is confined to the plastids in developing tomato fruit.

NONPHOTOSYNTHETIC METABOLISM IN PLASTIDS

Nonphotosynthetic plastids are important sites for the biosynthesis of starch, fatty acids, and the assimilation of nitrogen into amino acids in a wide range of plant tissues. Unlike chloroplasts, all the metabolites for these processes have to be imported, or generated by oxidative metabolism within the organelle. The aim of this review is to summarize our present understanding of the anabolic pathways involved, the requirement for import of precursors from the cytosol, the provision of energy for biosynthesis, and the interaction between pathways that share common intermediates. We emphasize the temporal and developmental regulation of events, and the variation in mechanisms employed by different species that produce the same end products.

PHOSPHATE TRANSLOCATORS IN PLASTIDS

During photosynthesis, energy from solar radiation is used to convert atmospheric carbon dioxide into intermediates that are used within and outside the chloroplast for a multitude of metabolic pathways. The daily fixed carbon is exported from the chloroplasts as triose phosphates and 3-phosphoglycerate. In contrast, nongreen plastids rely on the import of carbon, mainly hexose phosphates. Most organelles require the import of phosphoenolpyruvate as an immediate substrate for carbon to enter the shikimate pathway, leading to a variety of important secondary compounds. The envelope membrane of plastids contains specific translocators that are involved in these transport processes. Elucidation of the molecular structure of some of these translocators during the past few years has provided new insights in the functioning of particular translocators. This review focuses on the characterization of different classes of phosphate translocators in plastids that mediate the transport of the phosphorylated compounds in exchange with inorganic phosphate.

Photosynthetic and heterotrophic ferredoxin isoproteins are colocalized in fruit plastids of tomato

Fruit tissues of tomato (Lycopersicon esculentum Mill.) contain both photosynthetic and heterotrophic ferredoxin (FdA and FdE, respectively) isoproteins, irrespective of their photosynthetic competence, but we did not previously determine whether these proteins were colocalized in the same plastids. In isolated fruit chloroplasts and chromoplasts, both FdA and FdE were detected by immunoblotting. Colocalization of FdA and FdE in the same plastids was demonstrated using double-staining immunofluorescence microscopy. We also found that FdA and FdE were colocalized in fruit chloroplasts and chloroamyloplasts irrespective of sink status of the plastid. Immunoelectron microscopy demonstrated that FdA and FdE were randomly distributed within the plastid stroma. To investigate the significance of the heterotrophic Fd in fruit plastids, Glucose 6-phosphate dehydrogenase (G6PDH) activity was measured in isolated fruit and leaf plastids. Fruit chloroplasts and chromoplasts showed much higher G6PDH activity than did leaf chloroplasts, suggesting that high G6PDH activity is linked with FdE to maintain nonphotosynthetic production of reducing power. This result suggested that, despite their morphological resemblance, fruit chloroplasts are functionally different from their leaf counterparts.

Metabolite transporters in plastids

Communication between plastids and the surrounding cytosol occurs via the plastidic envelope membrane. Recent findings show that the outer membrane is not as freely permeable to low molecular weight solutes as previously thought, but contains different channel-like proteins that act as selectivity filters. The inner envelope membrane contains a variety of metabolite transporters that mediate the exchange of metabolites between both compartments. Two new classes of phosphate antiporters were recently described that are different in structure and function from the known triose phosphate/phosphate translocator from chloroplasts. In addition, a cDNA coding for an ATP/ADP antiporter from plastids was isolated that shows similarities to a bacterial adenylate translocator.

Evidence for the expression of the triosephosphate translocator gene in green and non-green tissue of tomato and potato

Western blot analysis revealed a cross reaction of an antibody against the spinach triosephosphate translocator with 29 kDa proteins from envelope membranes of plastids from green and red tomato fruits and also of potato tuber amyloplasts. Envelope membranes from potato tubers were isolated from a homogenate of total membranes by isopycnic sucrose density gradient centrifugation. We were able to demonstrate by reverse transcription and sequencing of the PCR product that the mRNA for the triosephosphate translocator in leaves is also present in green and red tomato fruits. The mature protein consists of 330 amino acid residues and is highly homologous to the triosephosphate translocator proteins from potato and tobacco. The PCR product obtained for potato tubers was partly sequenced. It corresponds entirely to the cDNA sequence encoding the potato leaf triosephosphate translocator protein. Evidence for the expression of the triosephosphate translocator gene in various photosynthetic active and inactive tomato tissues (leaf, green fruit, red fruit, root, petal, sepal) and potato tubers was further confirmed by northern blot analysis.

Biochemistry and molecular biology of chromoplast development

Plant cells contain a unique class of organelles, designated the plastids, which distinguish them from animal cells. According to the largely accepted endosymbiotic theory of evolution, plastids are descendants of prokaryotes. This process requires several adaptative changes which involve the maintenance and the expression of part of the plastid genome, as well as the integration of the plastid activity to the cellular metabolism. This is illustrated by the diversity of plastids encountered in plant cells. For instance, in tissues undergoing color changes, i.e., flowers and fruits, the chromoplasts produce and accumulate excess carotenoids. In this paper we attempt to review the basic aspects of chromoplast development.