Plastid changes during the conversion of chloroplasts to chromoplasts in ripening tomatoes

Proteomic analysis of "Oriental Beauty" oolong tea leaves with different degrees of leafhopper infestation

RATIONALE

Oriental Beauty, a type of oolong tea native to Taiwan, is highly prized by connoisseurs for its unique fruity aroma and sweet taste. Leaves of Oriental Beauty vary in appearance, aroma, and taste, depending on the degree of tea green leafhopper (Jacobiasca formosana) infestation. In this study, the aim is to investigate the differential expression of proteins in leaves with low, medium, and high degrees of leafhopper infestation.

METHODS

Proteomic techniques 2DE (two-dimensional electrophoresis) and nanoscale liquid chromatography/tandem mass spectrometry (LC/MS/MS) were used to investigate the differential expression of proteins in tea leaves with different degrees of leafhopper infestation.

RESULTS

A total of 89 proteins were found to exhibit significant differences in expression. In a gene ontology analysis, most of these proteins participated in biosynthesis, carbohydrate metabolism, transport, responses to stress, and amino acid metabolism.

CONCLUSIONS

These results indicated that the unique aroma and taste of the leaves might be influenced by their protein expression profiles, as well as related factors such as defensive responses to tea green leafhopper saliva.

Fractionation of Tomato Fruit Chromoplasts

Chromoplast differentiation involves an active synthesis of carotenoids associated with the remodeling of the preexisting plastid membrane systems to form specialized structures involved in the sequestration and storage of the synthesized carotenoids. These subplastidial structures show remarkable morphological differences and seem to be adapted to the accumulation of particular carotenoids in some plant species and organs. At present, very little is known about chromoplast biogenesis and the role of the different suborganellar structures in the synthesis and storage of carotenoids. The combination of classical fractionation methods with the use of biochemical and -omics techniques represents an attractive approach to unravel novel aspects related with the biochemical and cellular mechanisms underlying the biogenesis of the structures involved in the biosynthesis and storage of carotenoids during chromoplast differentiation. Here we describe a combined protocol for the isolation, lysis and fractionation of tomato fruit chromoplast. The fractions obtained are suitable for metabolomics and proteomics analysis.

Differentiation of chromoplasts and other plastids in plants

Plant cells are characterized by a unique group of interconvertible organelles called plastids, which are descended from prokaryotic endosymbionts. The most studied plastid type is the chloroplast, which carries out the ancestral plastid function of photosynthesis. During the course of evolution, plastid activities were increasingly integrated with cellular metabolism and functions, and plant developmental processes, and this led to the creation of new types of non-photosynthetic plastids. These include the chromoplast, a carotenoid-rich organelle typically found in flowers and fruits. Here, we provide an introduction to non-photosynthetic plastids, and then review the structures and functions of chromoplasts in detail. The role of chromoplast differentiation in fruit ripening in particular is explored, and the factors that govern plastid development are examined, including hormonal regulation, gene expression, and plastid protein import. In the latter process, nucleus-encoded preproteins must pass through two successive protein translocons in the outer and inner envelope membranes of the plastid; these are known as TOC and TIC (translocon at the outer/inner chloroplast envelope), respectively. The discovery of SP1 (suppressor of ppi1 locus1), which encodes a RING-type ubiquitin E3 ligase localized in the plastid outer envelope membrane, revealed that plastid protein import is regulated through the selective targeting of TOC complexes for degradation by the ubiquitin-proteasome system. This suggests the possibility of engineering plastid protein import in novel crop improvement strategies.

Isolation of Chromoplasts and Suborganellar Compartments from Tomato and Bell Pepper Fruit

Tomato is a model for fruit development and ripening. The isolation of intact plastids from this organism is therefore important for metabolic and proteomic analyses. Pepper, a species from the same family, is also of interest since it allows isolation of intact chromoplasts in large amounts. Here, we provide a detailed protocol for the isolation of tomato plastids at three fruit developmental stages, namely, nascent chromoplasts from the mature green stage, chromoplasts from an intermediate stage, and fully differentiated red chromoplasts. The method relies on sucrose density gradient centrifugations. It yields high purity organelles suitable for proteome analyses. Enzymatic and microscopy assays are summarized to assess purity and intactness. A method is also described for subfractionation of pepper chromoplast lipoprotein structures.

Silencing of the SlNAP7 gene influences plastid development and lycopene accumulation in tomato

Ripening is an important stage of fruit development. To screen the genes associated with pigment formation in tomato fruit, a suppression subtractive hybridization (SSH) cDNA library was constructed by using tomato fruit in the green ripe and break ripe stages, and 129 differential genes were obtained. Using redness as a screening marker, virus-induced gene silencing (VIGS) of the differential genes was performed with a sprout vacuum-infiltration system (SVI). The results showed that silencing the SlNAP7 gene affected the chloroplast development of tomato leaves, manifesting as a photo-bleaching phenotype, and silenced fruit significantly affected the accumulation of lycopene, manifested as a yellow phenotype. In our study, we found that silencing the SlNAP7 gene downregulates the expression of the POR and PORA genes and destroys the normal development of the chloroplast. The expression of related genes included in the lycopene biosynthesis pathway was not significantly changed, but lycopene accumulation was significantly reduced in tomato fruit. Perhaps it was caused by the destruction of the chromoplast, which leads to the oxidation of lycopene. The results show that the SlNAP7 gene influences chloroplast development and lycopene accumulation in tomato.

Structural changes in plastids of developing Splachnum ampullaceum sporophytes and relationship to odour production

Many mosses of the family Splachnaceae are entomophilous and rely on flies for spore dispersal. Splachnum ampullaceum produces a yellow- or pink-coloured hypophysis that releases volatile compounds, attracting flies to the mature moss. The biosynthetic sources of the visual and aromatic cues within the hypophysis have not been identified, and may be either symbiotic cyanobacteria or chromoplasts that break down lipids into volatile compounds. Here, we used transmission electron microscopy and gas chromatography-mass spectrometry (GC-MS) to investigate the sources of these attractants, focusing on different tissues and stages of maturation. Microscopy revealed an abundance of plastids within the hypophysis, while no symbiotic bacteria were observed. During plant maturation, plastids differentiated from amyloplasts with large starch granules to photosynthetic chloroplasts and finally to chromoplasts with lipid accumulations. We used GC-MS to identify over 50 volatile organic compounds from mature sporophytes including short-chain oxygenated compounds, unsaturated irregular terpenoids, fatty acid-derived 6- and 8-carbon alcohols and ketones, and the aromatic compounds acetophenone and p-cresol. The hypophysis showed localised production of pungent volatiles, mainly short-chain fermentation compounds and p-cresol. Some of these volatiles have been shown to be produced from lipid oxidase degradation of linolenic acid within chromoplasts. However, other compounds (such as cyclohexanecarboxylic acid esters) may have a microbial origin. Further investigation is necessary to identify the origin of fly attractants in these mosses.

An ATP synthase harboring an atypical γ-subunit is involved in ATP synthesis in tomato fruit chromoplasts

Chromoplasts are non-photosynthetic plastids specialized in the synthesis and accumulation of carotenoids. During fruit ripening, chloroplasts differentiate into photosynthetically inactive chromoplasts in a process characterized by the degradation of the thylakoid membranes, and by the active synthesis and accumulation of carotenoids. This transition renders chromoplasts unable to photochemically synthesize ATP, and therefore these organelles need to obtain the ATP required for anabolic processes through alternative sources. It is widely accepted that the ATP used for biosynthetic processes in non-photosynthetic plastids is imported from the cytosol or is obtained through glycolysis. In this work, however, we show that isolated tomato (Solanum lycopersicum) fruit chromoplasts are able to synthesize ATP de novo through a respiratory pathway using NADPH as an electron donor. We also report the involvement of a plastidial ATP synthase harboring an atypical γ-subunit induced during ripening, which lacks the regulatory dithiol domain present in plant and algae chloroplast γ-subunits. Silencing of this atypical γ-subunit during fruit ripening impairs the capacity of isolated chromoplast to synthesize ATP de novo. We propose that the replacement of the γ-subunit present in tomato leaf and green fruit chloroplasts by the atypical γ-subunit lacking the dithiol domain during fruit ripening reflects evolutionary changes, which allow the operation of chromoplast ATP synthase under the particular physiological conditions found in this organelle.

Precursor uptake assays and metabolic analyses in isolated tomato fruit chromoplasts

BACKGROUND

Carotenoids are the most widespread group of pigments found in nature. In addition to their role in the physiology of the plant, carotenoids also have nutritional relevance as their incorporation in the human diet provides health benefits. In non-photosynthetic tissues, carotenoids are synthesized and stored in specialized plastids called chromoplasts. At present very little is known about the origin of the metabolic precursors and cofactors required to sustain the high rate of carotenoid biosynthesis in these plastids. Recent proteomic data have revealed a number of biochemical and metabolic processes potentially operating in fruit chromoplasts. However, considering that chloroplast to chromoplast differentiation is a very rapid process during fruit ripening, there is the possibility that some of the proteins identified in the proteomic analysis could represent remnants no longer having a functional role in chromoplasts. Therefore, experimental validation is necessary to prove whether these predicted processes are actually operative in chromoplasts.

RESULTS

A method has been established for high-yield purification of tomato fruit chromoplasts suitable for metabolic studies. Radiolabeled precursors were efficiently incorporated and further metabolized in isolated chromoplast. Analysis of labeled lipophilic compounds has revealed that lipid biosynthesis is a very efficient process in chromoplasts, while the relatively low incorporation levels found in carotenoids suggest that lipid production may represent a competing pathway for carotenoid biosynthesis. Malate and pyruvate are efficiently converted into acetyl-CoA, in agreement with the active operation of the malic enzyme and the pyruvate dehydrogenase complex in the chromoplast. Our results have also shown that isolated chromoplasts can actively sustain anabolic processes without the exogenous supply of ATP, thus suggesting that these organelles may generate this energetic cofactor in an autonomous way.

CONCLUSIONS

We have set up a method for high yield purification of intact tomato fruit chromoplasts suitable for precursor uptake assays and metabolic analyses. Using targeted radiolabeled precursors we have been able to unravel novel biochemical and metabolic aspects related with carotenoid and lipid biosynthesis in tomato fruit chromoplasts. The reported chromoplast system could represent a valuable platform to address the validation and characterization of functional processes predicted from recent transcriptomic and proteomic data.

Metabolic and Molecular Events Occurring during Chromoplast Biogenesis

Chromoplasts are nonphotosynthetic plastids that accumulate carotenoids. They derive from other plastid forms, mostly chloroplasts. The biochemical events responsible for the interconversion of one plastid form into another are poorly documented. However, thanks to transcriptomics and proteomics approaches, novel information is now available. Data of proteomic and biochemical analysis revealed the importance of lipid metabolism and carotenoids biosynthetic activities. The loss of photosynthetic activity was associated with the absence of the chlorophyll biosynthesis branch and the presence of proteins involved in chlorophyll degradation. Surprisingly, the entire set of Calvin cycle and of the oxidative pentose phosphate pathway persisted after the transition from chloroplast to chromoplast. The role of plastoglobules in the formation and organisation of carotenoid-containing structures and that of the Or gene in the control of chromoplastogenesis are reviewed. Finally, using transcriptomic data, an overview is given the expression pattern of a number of genes encoding plastid-located proteins during tomato fruit ripening.

Combined pressure-temperature effects on carotenoid retention and bioaccessibility in tomato juice

This study highlights the changes in lycopene and β-carotene retention in tomato juice subjected to combined pressure-temperature (P-T) treatments ((high-pressure processing (HPP; 500-700 MPa, 30 °C), pressure-assisted thermal processing (PATP; 500-700 MPa, 100 °C), and thermal processing (TP; 0.1 MPa, 100 °C)) for up to 10 min. Processing treatments utilized raw (untreated) and hot break (∼93 °C, 60 s) tomato juice as controls. Changes in bioaccessibility of these carotenoids as a result of processing were also studied. Microscopy was applied to better understand processing-induced microscopic changes. TP did not alter the lycopene content of the tomato juice. HPP and PATP treatments resulted in up to 12% increases in lycopene extractability. all-trans-β-Carotene showed significant degradation (p < 0.05) as a function of pressure, temperature, and time. Its retention in processed samples varied between 60 and 95% of levels originally present in the control. Regardless of the processing conditions used, <0.5% lycopene appeared in the form of micelles (<0.5% bioaccessibility). Electron microscopy images showed more prominent lycopene crystals in HPP and PATP processed juice than in thermally processed juice. However, lycopene crystals did appear to be enveloped regardless of the processing conditions used. The processed juice (HPP, PATP, TP) showed significantly higher (p < 0.05) all-trans-β-carotene micellarization as compared to the raw unprocessed juice (control). Interestingly, hot break juice subjected to combined P-T treatments showed 15-30% more all-trans-β-carotene micellarization than the raw juice subjected to combined P-T treatments. This study demonstrates that combined pressure-heat treatments increase lycopene extractability. However, the in vitro bioaccessibility of carotenoids was not significantly different among the treatments (TP, PATP, HPP) investigated.

Carotenoids and their cleavage products: biosynthesis and functions

This review focuses on plant carotenoids, but it also includes progress made on microbial and animal carotenoid metabolism to better understand the functions and the evolution of these structurally diverse compounds with a common backbone. Plants have evolved isogenes for specific key steps of carotenoid biosynthesis with differential expression profiles, whose characteristic features will be compared. Perhaps the most exciting progress has been made in studies of carotenoid cleavage products (apocarotenoids) with an ever-expanding variety of novel functions being discovered. This review therefore covers structural, molecular genetic and functional aspects of carotenoids and apocarotenoids alike. Apocarotenoids are specifically tailored from carotenoids by a family of oxidative cleavage enzymes, but whether there are contributions to their generation from chemical oxidation, photooxidation or other mechanisms is largely unknown. Control of carotenoid homeostasis is discussed in the context of biosynthetic and degradative reactions but also in the context of subcellular environments for deposition and sequestration within and outside of plastids. Other aspects of carotenoid research, including metabolic engineering and synthetic biology approaches, will only be covered briefly.

Chromoplast differentiation: current status and perspectives

Chromoplasts are carotenoid-accumulating plastids conferring color to many flowers and fruits as well as to some tubers and roots. Chromoplast differentiation proceeds from preexisting plastids, most often chloroplasts. One of the most prominent changes is remodeling of the internal membrane system associated with the formation of carotenoid-accumulating structures. During the differentiation process the plastid genome is essentially stable and transcriptional activity is restricted. The buildup of the chromoplast for specific metabolic characteristics is essentially dependent upon the transcriptional activity of the nucleus. Important progress has been made in terms of mediation of the chloroplast-to-chromoplast transition with the discovery of the crucial role of the Or gene. In this article we review recent developments in the structural, biochemical and molecular aspects of chromoplast differentiation and also consider the reverse differentiation of chromoplasts into chloroplast-like structures during the regreening process occurring in some fruit. Future perspectives toward a full understanding of chromoplast differentiation include in-depth knowledge of the changes occurring in the plastidial proteome during chromoplastogenesis, elucidation of the role of hormones and the search for signals that govern the dialog between the nuclear and the chromoplastic genome.

An orange ripening mutant links plastid NAD(P)H dehydrogenase complex activity to central and specialized metabolism during tomato fruit maturation

In higher plants, the plastidial NADH dehydrogenase (Ndh) complex supports nonphotochemical electron fluxes from stromal electron donors to plastoquinones. Ndh functions in chloroplasts are not clearly established; however, its activity was linked to the prevention of the overreduction of stroma, especially under stress conditions. Here, we show by the characterization of Orr(Ds), a dominant transposon-tagged tomato (Solanum lycopersicum) mutant deficient in the NDH-M subunit, that this complex is also essential for the fruit ripening process. Alteration to the NDH complex in fruit changed the climacteric, ripening-associated metabolites and transcripts as well as fruit shelf life. Metabolic processes in chromoplasts of ripening tomato fruit were affected in Orr(Ds), as mutant fruit were yellow-orange and accumulated substantially less total carotenoids, mainly beta-carotene and lutein. The changes in carotenoids were largely influenced by environmental conditions and accompanied by modifications in levels of other fruit antioxidants, namely, flavonoids and tocopherols. In contrast with the pigmentation phenotype in mature mutant fruit, Orr(Ds) leaves and green fruits did not display a visible phenotype but exhibited reduced Ndh complex quantity and activity. This study therefore paves the way for further studies on the role of electron transport and redox reactions in the regulation of fruit ripening and its associated metabolism.

Free amino acid production during tomato fruit ripening: a focus on L-glutamate

In tomato, free amino acids increase dramatically during fruit ripening and their abundance changed differentially. More evident is L-glutamate which gives the characteristic "umami" flavor. Glutamate is the principal free amino acid of ripe fruits of cultivated varieties. In this paper, we examined the capacity of tomato fruits to process endogenous as well as exogenous polypeptides during the ripening transition, in order to analyze their contribution to the free amino acid pool. In addition, the activity of some enzymes involved in glutamate metabolism such as gamma-glutamyl transpeptidase (gamma-GTase), glutamate dehydrogenase (GDH), alpha-ketoglutarate-dependent gamma-aminobutyrate transaminase (GABA-T), alanine and aspartate aminotransferases was evaluated. Results showed that peptidases were very active in ripening fruits, and they were able to release free amino acids from endogenous proteins and glutamate from exogenously added glutamate-containing peptides. In addition, red fruit contained enough gamma-GTase activity to sustain glutamate liberation from endogenous substrates such as glutathione. From all the glutamate metabolizing enzymes, GDH and GABA-T showed the higher increase in activities when the ripening process starts. In summary, tomato fruits increase free amino acid content during ripening, most probably due to the raise of different peptidase activities. However, glutamate level of ripe fruit seems to be mostly related to GDH and GABA-T activities that could contribute to increase L-glutamate level during the ripening transition.

Carotenoid crystal formation in Arabidopsis and carrot roots caused by increased phytoene synthase protein levels

BACKGROUND

As the first pathway-specific enzyme in carotenoid biosynthesis, phytoene synthase (PSY) is a prime regulatory target. This includes a number of biotechnological approaches that have successfully increased the carotenoid content in agronomically relevant non-green plant tissues through tissue-specific PSY overexpression. We investigated the differential effects of constitutive AtPSY overexpression in green and non-green cells of transgenic Arabidopsis lines. This revealed striking similarities to the situation found in orange carrot roots with respect to carotenoid amounts and sequestration mechanism.

METHODOLOGY/PRINCIPAL FINDINGS

In Arabidopsis seedlings, carotenoid content remained unaffected by increased AtPSY levels although the protein was almost quantitatively imported into plastids, as shown by western blot analyses. In contrast, non-photosynthetic calli and roots overexpressing AtPSY accumulated carotenoids 10 and 100-fold above the corresponding wild-type tissues and contained 1800 and 500 microg carotenoids per g dry weight, respectively. This increase coincided with a change of the pattern of accumulated carotenoids, as xanthophylls decreased relative to beta-carotene and carotene intermediates accumulated. As shown by polarization microscopy, carotenoids were found deposited in crystals, similar to crystalline-type chromoplasts of non-green tissues present in several other taxa. In fact, orange-colored carrots showed a similar situation with increased PSY protein as well as carotenoid levels and accumulation patterns whereas wild white-rooted carrots were similar to Arabidopsis wild type roots in this respect. Initiation of carotenoid crystal formation by increased PSY protein amounts was further confirmed by overexpressing crtB, a bacterial PSY gene, in white carrots, resulting in increased carotenoid amounts deposited in crystals.

CONCLUSIONS

The sequestration of carotenoids into crystals can be driven by the functional overexpression of one biosynthetic enzyme in non-green plastids not requiring a chromoplast developmental program as this does not exist in Arabidopsis. Thus, PSY expression plays a major, rate-limiting role in the transition from white to orange-colored carrots.

Plastid transcriptomics and translatomics of tomato fruit development and chloroplast-to-chromoplast differentiation: chromoplast gene expression largely serves the production of a single protein

Plastid genes are expressed at high levels in photosynthetically active chloroplasts but are generally believed to be drastically downregulated in nongreen plastids. The genome-wide changes in the expression patterns of plastid genes during the development of nongreen plastid types as well as the contributions of transcriptional versus translational regulation are largely unknown. We report here a systematic transcriptomics and translatomics analysis of the tomato (Solanum lycopersicum) plastid genome during fruit development and chloroplast-to-chromoplast conversion. At the level of RNA accumulation, most but not all plastid genes are strongly downregulated in fruits compared with leaves. By contrast, chloroplast-to-chromoplast differentiation during fruit ripening is surprisingly not accompanied by large changes in plastid RNA accumulation. However, most plastid genes are translationally downregulated during chromoplast development. Both transcriptional and translational downregulation are more pronounced for photosynthesis-related genes than for genes involved in gene expression, indicating that some low-level plastid gene expression must be sustained in chromoplasts. High-level expression during chromoplast development identifies accD, the only plastid-encoded gene involved in fatty acid biosynthesis, as the target gene for which gene expression activity in chromoplasts is maintained. In addition, we have determined the developmental patterns of plastid RNA polymerase activities, intron splicing, and RNA editing and report specific developmental changes in the splicing and editing patterns of plastid transcripts.

Antisense suppression of deoxyhypusine synthase in tomato delays fruit softening and alters growth and development

The effects of suppressing deoxyhypusine synthase (DHS) have been examined in tomato (Solanum lycopersicum cv UCT5). DHS mediates the first of two sequential enzymatic reactions that activate eukaryotic translation initiation factor-5A (eIF-5A) by converting a conserved Lys to the unusual amino acid, deoxyhypusine. DHS protein levels were suppressed in transgenic plants by expressing the 3'-untranslated region of tomato DHS under regulation of the constitutive cauliflower mosaic virus promoter. Fruit from the transgenic plants ripened normally, but exhibited delayed postharvest softening and senescence that correlated with suppression of DHS protein levels. Northern-blot analysis indicated that all four gene family members of tomato eIF-5A are expressed in fruit, and that three are up-regulated in parallel with enhancement of DHS mRNA as the fruit begin to senesce and soften. Transgenic plants in which DHS was more strongly suppressed were male sterile, did not produce fruit, and had larger, thicker leaves with enhanced levels of chlorophyll. The activity of PSII was 2 to 3 times higher in these transgenic leaves than in corresponding leaves of wild-type plants, and there was also enhanced deposition of starch in the stems. The data collectively indicate that suppression of DHS has pleiotropic effects on growth and development of tomato. This may, in turn, reflect the fact that there is a single DHS gene in tomato and that its cognate protein is involved in the activation of four distinct isoforms of eIF-5A.

Metabolic engineering of ketocarotenoid formation in higher plants

Although higher plants synthesize carotenoids, they do not possess the ability to form ketocarotenoids. In order to generate higher plants capable of synthesizing combinations of ketolated and hydroxylated carotenoids the genes responsible for the carotene 4,4' oxygenase and 3,3' hydroxylase have been transformed into tomato and tobacco. The gene products were produced as a polyprotein. Subsequent cleavage of the polyprotein, targeting of the two enzymes to the plastid and enzyme activities have been shown for both gene products. Metabolite profiling has shown the formation of ketolated carotenoids from beta-carotene and its hydroxylated intermediates in tobacco and tomato leaf. In the nectary tissues of tobacco flowers a quantitative increase (10-fold) as well as compositional changes were evident, including the presence of astaxanthin, canthaxanthin and 4-ketozeaxanthin. Interestingly, in this tissue the newly formed carotenoids resided predominantly as esters. These data are discussed in terms of metabolic engineering of carotenoids and their sequestration in higher plant tissues.

Expression of plastid-encoded photosynthetic genes during chloroplast or chromoplast differentiation in Cucurbitae pepo L. fruits

The objective of the study was to determine the patterns of expression of two photosynthetic genes rbcL and psbA, during chloroplast and chromoplast differentiation in fruit tissues of three Cucurbitae pepo L. cultivars: Early Prolific, Foodhook Zucchini and Bicolor Gourds. In two Early Prolific isogenic lines, YYBB and YYB+B+, the steady-state amounts of rbcL and psbA transcripts increased with fruit development upto 14 days post-pollination. The YYB+B+ line in which chloroplast differentiates into chromoplast at about pollination, did not show significantly higher amounts of both transcripts compared to YYBB, in which chromoplast develops early prior to pollination. In the Bicolor Gourds, in which the chromoplast and chloroplast containing tissues lie in juxtaposition on the same fruit, showed little differences in rbcL and psbA transcripts between the two tissues, if any the chromoplast containing tissue contained more of both transcripts than the chloroplast containing tissue. In Fordhook Zucchini fruits, where the chloroplast containing tissue developed early prior to pollination and was maintained, the steady-state amounts of rbcL transcripts increased to a maximum at 3 days post-pollination and levelled at 14 and 21 days post-pollination. In contrast, in Fordhook Zucchini fruits, the psbA transcript increased gradually up to 21 days post-pollination. In Fordhook Zucchini, the apparent ratios of psbA transcripts versus rbcL transcripts ranged from 2.5 to 3.9, at day 3 to 21 post-pollination, while in Bicolor Gourds were 2.9 and 4.5 at days 14 and 21 post-pollination. The two photosynthetic genes, psbA and rbcL were developmentally regulated and differentially expressed. However, their expression in chloroplast containing fruit tissues was not higher than in the chromoplast containing fruit tissues.

MFP1 is a thylakoid-associated, nucleoid-binding protein with a coiled-coil structure

Plastid DNA, like bacterial and mitochondrial DNA, is organized into protein-DNA complexes called nucleoids. Plastid nucleoids are believed to be associated with the inner envelope in developing plastids and the thylakoid membranes in mature chloroplasts, but the mechanism for this re-localization is unknown. Here, we present the further characterization of the coiled-coil DNA-binding protein MFP1 as a protein associated with nucleoids and with the thylakoid membranes in mature chloroplasts. MFP1 is located in plastids in both suspension culture cells and leaves and is attached to the thylakoid membranes with its C-terminal DNA-binding domain oriented towards the stroma. It has a major DNA-binding activity in mature Arabidopsis chloroplasts and binds to all tested chloroplast DNA fragments without detectable sequence specificity. Its expression is tightly correlated with the accumulation of thylakoid membranes. Importantly, it is associated in vivo with nucleoids, suggesting a function for MFP1 at the interface between chloroplast nucleoids and the developing thylakoid membrane system.

Stable genetic transformation of tomato plastids and expression of a foreign protein in fruit

Transgenic chloroplasts offer unique advantages in plant biotechnology, including high-level foreign protein expression, absence of epigenetic effects, and gene containment due to the lack of transgene transmission through pollen. However, broad application of plastid genome engineering in biotechnology has been largely hampered by both the lack of chloroplast transformation systems for major crop plants and the usually low plastid gene expression levels in nongreen tissues such as fruits, tubers, and other storage organs. Here we describe the development of a plastid transformation system for tomato, Lycopersicon esculentum. This is the first report on the generation of fertile transplastomic plants in a food crop with an edible fruit. We show that chromoplasts in the tomato fruit express the transgene to approximately 50% of the expression levels in leaf chloroplasts. Given the generally very high foreign protein accumulation rates that can be achieved in transgenic chloroplasts (>40% of the total soluble protein), this system paves the way to efficient production of edible vaccines, pharmaceuticals, and antibodies in tomato.

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.

Over-expression of a pepper plastid lipid-associated protein in tobacco leads to changes in plastid ultrastructure and plant development upon stress

Proteins homologous to fibrillin, a pepper plastid lipid-associated protein involved in carotenoid storage in fruit chromoplasts, have been recently identified in leaf chloroplasts from several species and shown to be induced upon environmental stress. To further investigate the role of the protein, transgenic Nicotiana tabacum plants over-expressing fibrillin using a constitutive promoter were generated. Transgenics grown under standard light intensities (300 micromol photons m-2 sec-1) were found to contain substantial amounts of fibrillin in flowers and leaves. In leaves, the protein was immunolocalized within chloroplasts in both stromal and thylakoid subfractions. No change was noticed in thylakoid structures from transgenics, but chloroplasts contained an increased number of plastoglobules organized in clusters. In petals, leucoplasts were also found to contain more agglutinated plastoglobules. The effects of environmental factors on fibrillin gene expression and protein localization were studied in tobacco leaves. Less fibrillin was present in plants grown under low light intensities, which can be explained by the involvement of a light-dependent splicing step in the control of fibrillin gene expression in leaves. Analysis of protein subfractions from plants subjected to drought or high light showed that both stresses resulted in fibrillin association with thylakoids. Whereas no growth difference between wild-type (WT) and transgenic plants was noticed under low light conditions, transgenics exhibit a longer main stem, enhanced development of lateral stems and accelerated floral development under higher light intensities. These data suggest that fibrillin-related proteins fulfil an important function in plant development in relation to environmental constraints.

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.

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.

Chromoplast formation during tomato fruit ripening. No evidence for plastid DNA methylation

Ripening of tomato fruits involves differentiation of chloroplasts into non-photosynthetic chromoplasts. Plastid DNAs isolated either from green leaf chloroplasts or mature red fruit chromoplasts were compared by restriction endonuclease and DNA/DNA hybridization analyses. The same restriction and gene maps were obtained for both types of DNAs, illustrating the lack of major recombinational events during chromoplast formation. Several enzymes were used that discriminate the presence of methylated bases in their target sequences (Pst I, Pvu II, Sal I, Mbo I/Sau 3AI, Msp I/Hpa II, Bst NI/Eco RII). Plastid DNA fragments generated by these enzymes were hybridized against DNA probes encompassing about 85% of the tobacco chloroplast genome. These probes represented genes that follow very different expression behaviors in response to plastid development. Extensive restriction and hybridization analyses failed to reveal any difference between the chloroplast and chromoplast genomes, indicating that no developmentally related DNA methylation was detected by these methods. The results presented here do not support the hypothesis that selective DNA methylation of the chromoplast genome might play a major role in the transcriptional control of gene expression in these non-photosynthetic plastids.

DNA polymerase activity of tomato fruit chromoplasts

DNA polymerase activity was measured in chromoplasts of ripening tomato fruits. Plastids isolated from young leaves or mature red fruits showed similar DNA polymerase activities. The same enzyme species was present in either chloroplasts or chromoplasts as judged by pH and temperature profiles, sensitivities towards different inhibitors and relative molecular mass (Mr 88 kDa). The activities analyzed showed the typical behaviour of plastid-type polymerases. The results presented here suggest that chromoplast maintain their DNA synthesis potential in fruit tissue at chloroplast levels. Consequently, the sharp decrease of the plastid chromosome transcription observed at the onset of fruit ripening could not be due to limitations in the availability of template molecules. Other mechanisms must be involved in the inhibition of chromoplast RNA synthesis.

THE DEGRADATION OF CHLOROPHYLL - A BIOLOGICAL ENIGMA

Some 10⁹ tonnes of chlorophyll are destroyed each year on land and in the oceans. The fate of these chlorophylls is, however, largely unknown. This review describes the developmental stages at which chlorophyll breakdown occurs in aquatic and terrestrial biological systems, and the destruction arising from herbivory, disease, pollution and other physical hazards. At the cellular level, an attempt is made to separate the breakdown of chlorophyll during senescence from the many other events associated with cell destruction and death. A consideration of the more important chemical and biophysical properties of chlorophylls and their derivatives is provided, together with data on their spectral properties. The biosynthetic and biodegradative pathways of chlorophyll metabolism are, so far as is possible, described with some predictions as to the likely fate of the missing tonnes. Two types of degradation are recognized; the first involves up to five defined enzymes concerned with the early stages, the second covers the less well defined enzymic and non-enzymic destruction of the macrocyclic structure. These degradative reactions are compared with the reactions implicated in the breakdown of other porphyrins including haems in plants and animals. A brief description is given of the occurrence of breakdown products of chlorophyll in past biomass, including those of geological significance and those in a more recent archaeological context. Finally, the economic significance of chlorophyll breakdown is considered in the context of agriculture and horticulture, veterinary and medical sciences, food colouring and cosmetic industries, and the multi-million-dollar attraction of autumn leaf fall to tourism. Contents Summary 256 I. Introduction 256 II. Chlorophylls: global production and destruction 259 III. Chlorophylls: nomenclature and chemical characteristics 260 IV. Chlorophyll metabolism 268 V. Chlorophyll degradation during senescence 274 VI. Other degradative conditions 278 VII. Breakdown products in past biomass 287 VIII. Pathways of degradation 289 IX. Economic importance 291 Acknowledgements 294 References 294.