Phytoene synthase 2 in tomato fruits remains functional and contributes to abscisic acid formation

In ripening tomato fruits, the leaf-specific carotenoids biosynthesis mediated by phytoene synthase 2 (PSY2) is replaced by a fruit-specific pathway by the expression of two chromoplast-specific genes: phytoene synthase 1 (PSY1) and lycopene-β-cyclase (CYCB). Though both PSY1 and PSY2 genes express in tomato fruits, the functional role of PSY2 is not known. To decipher whether PSY2-mediated carotenogenesis operates in ripening fruits, we blocked the in vivo activity of lycopene-β-cyclases in fruits of several carotenoids and ripening mutants by CPTA (2-(4-Chlorophenylthio)triethylamine hydrochloride), an inhibitor of lycopene-β-cyclases. The CPTA-treatment induced accumulation of lycopene in leaves, immature-green and ripening fruits. Even in psy1 mutants V7 and r that are deficient in fruit-specific carotenoid biosynthesis, CPTA triggered lycopene accumulation but lowered the abscisic acid level. Differing from fruit-specific carotenogenesis, CPTA-treated V7 and r mutant fruits accumulated lycopene but not phytoene and phytofluene. The lack of phytoene and phytofluene accumulation was reminiscent of PSY2-mediated leaf-like carotenogenesis, where phytoene and phytofluene accumulation is never seen. The lycopene accumulation was associated with the partial transformation of chloroplasts to chromoplasts bearing thread-like structures. Our study uncovers the operation of a parallel carotenogenesis pathway mediated by PSY2 that provides precursors for abscisic acid biosynthesis in ripening tomato fruits.

The esterification of xanthophylls in Solanum lycopersicum (tomato) chromoplasts; the role of a non-specific acyltransferase

The esterification of carotenoids has been associated with high-level accumulation, greater stability and potentially improved dietary bioavailability. Engineering the formation of ketocarotenoids into tomato fruit has resulted in the esterification of these non-endogenous metabolites. A genotype of tomato was created that contains; (i) the mutant pale yellow petal (pyp)1-1 allele, which is responsible for the absence of carotenoid esters in tomato flowers and (ii) the heterologous enzymes for ketocarotenoid formation. Analysis of the resulting progeny showed altered quantitative and qualitative differences in esterified carotenoids. For example, in ripe fruit tissues, in the presence of the pyp mutant allele, non-endogenous ketocarotenoid esters were absent while their free forms accumulated. These data demonstrate the involvement of the PYP gene product in the esterification of diverse xanthophylls.

Light Microscopy and Raman Imaging of Carotenoids in Plant Cells In Situ and in Released Carotene Crystals

Light microscopy with a bright field mode offers an easy and fast examination of plant specimen for carotenoid presence in its cells. Using basic techniques such as hand sectioned or squashed preparations, carotenoid-rich chromoplasts can be identified without applying any staining procedure and their localization within the cell, their shape and number can be assessed. More detailed information can be obtained by using Raman spectroscopy which is suitable for the analysis of carotenoids due to their unique Raman spectra and allows semiquantification of their contents. Raman imaging (mapping) can be additionally used to show the distribution of carotenoids within the sample. Raman spectra can be taken from extracted carotenoids but can be also obtained directly from plant tissues or cells as Raman measurements are nondestructive for the sample. Here we describe preparations of intact tissue samples, monolayer cell samples, isolated protoplasts as well as carotene crystals released from chromoplasts that are suitable for subsequent observations using light microscopy and for analysis using Raman spectroscopy.

Influence of high hydrostatic pressure pretreatment on properties of vacuum-freeze dried strawberry slices

Effects of high hydrostatic pressure (HHP) (50, 100, 150, 200 and 250 MPa) pretreatment on water mobility and distribution, drying duration, microstructure, color, cell wall fraction and tissue structure of strawberry slices were investigated. HHP significantly increased water mobility of the strawberry slices, resulting in the reduction of drying duration by 9-24%. As the pretreatment pressure was increased, redness value and anthocyanin content continuously increased, soluble pectin (SBP) content increased and then decreased, while the contents of protopectin (PTP) and cellulose decreased. After the HHP pretreatment, chromoplasts and moisture was distributed more uniformly in the strawberry slices. Microscopy images showed the formation of microscopic holes or channels in the matrix and the breakdown of tissue structure by HHP. Results suggested HHP pretreatment disrupted the integrity of the fresh strawberry which enhanced the drying efficiency and migration of the chromoplasts during the vacuum-freeze drying process.

The effect of low ascorbic acid content on tomato fruit ripening

The oxidant/antioxidant balance affects the ripening time of tomato fruit. Ripening of tomato fruit is associated with several modifications such as loss of cell wall firmness and transformation of chloroplasts to chromoplasts. Besides a peak in H₂O₂, reactive oxygen species (ROS) are observed at the transition stage. However, the role of different components of oxidative stress metabolism in fruit ripening has been scarcely addressed. Two GDP-L-galactose phosphorylase (GGP) Solanum lycopersicum L. cv Micro-Tom mutants which have fruit with low ascorbic acid content (30% of wild type) were used in this work to unravel the participation of ascorbic acid and H₂O₂ in fruit maturation. Both GGP mutants show delayed fruit maturation with no peak of H₂O₂; treatment with ascorbic acid increases its own concentration and accelerates ripening only in mutants to become like wild type plants. Unexpectedly, the treatment with ascorbic acid increases H₂O₂ synthesis in both mutants resembling what is observed in wild type fruit. Exogenous supplementation with H₂O₂ decreases its own synthesis delaying fruit maturation in plants with low ascorbic acid content. The site of ROS production is localized in the chloroplasts of fruit of all genotypes as determined by confocal microscopy analysis. The results presented here demonstrate that both ascorbic acid and H₂O₂ actively participate in tomato fruit ripening.

Different colored Chrysanthemum × morifolium cultivars represent distinct plastid transformation and carotenoid deposit patterns

Carotenoids are the most important pigments determining the color of C. × morifolium; however, it is still unknown whether the changes of plastid ultrastructure affect carotenoids accumulation. In this study, we compared the change of carotenoid composition, content, and the plastid ultrastructures in the different developmental stages of capitulum among fourteen C. × morifolium cultivars from seven color groups. We found that the carotenoids and plastids detected at the early stage of capitulum development were similar in all cultivars, including violaxanthin, lutein, and β-carotene, which were present in proplastids and immature chloroplasts. Immature chloroplasts were degraded completely, forming loosely broken plastids during the development of the capitulum in white and pink cultivars. Meanwhile, a number of lipid vesicles appeared at proplastids, which resulted in only trace amounts of carotenoid accumulation in these cultivars. For yellow, orange, red, and brown cultivars, a great number of chromoplasts were found, which contained diverse ultrastructures, such as plastoglobules, tubules, and lipid droplets; these chromoplasts were derived from proplastids or chloroplasts. Compared with the early stage of capitulum development, these cultivars accumulated large amounts of carotenoids, primarily including lutein, lutein derivatives, and their isomers. In green cultivars, proplastids and immature chloroplasts were completely transformed into mature chloroplasts. These chloroplasts mainly contained violaxanthin, lutein, β-carotene, and two new components, (9Z)-lutein and (9'Z)-lutein, compared with carotenoid components presented in proplastids and immature chloroplasts. This research will be helpful for understanding the mechanisms of carotenoid metabolism of C. × morifolium. Furthermore, we found that two different chromoplast transformation patterns could be present in the same tissue cell, which contributed to the research on plastid differentiation and development in higher plants.

Comparative transcriptomics and weighted gene co-expression correlation network analysis (WGCNA) reveal potential regulation mechanism of carotenoid accumulation in Chrysanthemum × morifolium

The variation of flower color of chrysanthemum (Chrysanthemum×morifolium) is extremely rich, and carotenoids, which are mainly stored in the plastid, are important pigments that determine the color of chrysanthemum. However, the genetic regulation of the carotenoid metabolism pathway in this species still remains unclear. In this study, a pink chrysanthemum cultivar, 'Jianliuxiang Pink', and its three bud sport mutants (including white, yellow and red color mutants, 'Jianliuxiang White', 'Jianliuxiang Yellow' and 'Jianliuxiang Red', respectively) were used as experimental materials to analyze the dynamic changes of carotenoid components and plastid ultrastructure at different developmental stages of ray florets. We found that the carotenoid components and plastid ultrastructure of the four color cultivars in the early developmental stage of the chrysanthemum capitulum (S1) were almost identical, and the carotenoids mainly included violaxanthin, lutein and β-carotene, which exist in proplastids and immature chloroplasts. With the development of capitulum, the chloroplasts in 'Jianliuxiang White' and 'Jianliuxiang Pink' were degraded, and the protoplasts did not transform but rather formed vesicles that accumulated trace amounts of carotenoids. The proplastids and chloroplasts in 'Jianliuxiang Yellow' and 'Jianliuxiang Red' were all transformed into chromoplasts and consist of lutein as well as lutein's isomer and derivatives. Using comparative transcriptomics combined with gene expression analysis, we found that CmPg-1, CmPAP10, and CmPAP13, which were involved in chromoplast transformation, CmLCYE, which was involved in carotenoid biosynthesis, and CmCCD4a-2, which was involved in carotenoid degradation, were differentially expressed between four cultivars, and these key genes therefore should affect the accumulation of carotenoids in chrysanthemum. In addition, six transcription factors, CmMYB305, CmMYB29, CmRAD3, CmbZIP61, CmAGL24, CmNAC1, were screened using weighted gene co-expression correlation network analysis (WGCNA) combined with correlative analysis to determine whether they play an important role in carotenoid accumulation by regulating structural genes related to the carotenoid metabolism pathway and plastid development. This study analyzed dynamic changes of carotenoid components and plastid ultrastructure of the four bud mutation cultivars of chrysanthemum and identified structural genes and transcription factors that may be involved in carotenoid accumulation. The above results laid a solid foundation for further analysis of the regulatory mechanism of the carotenoid biosynthesis pathway in chrysanthemum.

Microstructural and histochemical characteristics of Lycium barbarum L. fruits used in folk herbal medicine and as functional food

Lycium barbarum L. fruits, referred to as functional food, have long been used in traditional and folk herbal medicine due to their therapeutic properties. The fruit microstructure was analysed using light, scanning and transmission electron microscopes. The distribution of bioactive compounds in drupe tissues was assessed with histochemical and fluorescence assays. The analysis of the microstructure has shown that the fruit is covered by a skin with an amorphous cuticle and a layer of amorphous epicuticular waxes on the surface. The skin is composed of a single-layered epidermis with thickened walls and one layer of hypodermis with slightly thickened periclinal walls. The pericarp cells contain different types of chromoplasts, which most often contained exhibited reticulotubules/fibrils of carotenoid pigments and phytoferritine deposits. The results of the histochemical assays demonstrated that the secondary metabolites with high phytotherapeutic importance were located in all layers of the pericarp and seeds and, specifically, in the drupe exocarp and endocarp. The phytochemicals were represented by polysaccharides (LBP), lipid compounds (carotenoids, essential oils, sesquiterpenes, steroids), polyphenols (tannins and flavonoids), and alkaloids. This study, which is the first report of the microstructure and localisation of bioactive compounds in wolfberries, is a valuable complement of phytochemical analyses and can be helpful for enhancement of the therapeutic effect of the fruit as well as preliminary assessment of the medicinal potential in the search for new pharmaceuticals. Detailed anatomical studies are crucial for exploration of determinants of fruit quality and useful for identification of diagnostic taxonomic traits.

Study of commercial quality parameters, sugars, phenolics, carotenoids and plastids in different tomato varieties

The aim of this study was to assess commercial quality parameters, sugars, phenolics, carotenoids and plastid in diverse and little studied tomato varieties to gain insight into their commercial and functional quality and reveal possible noticeable differences. Five cherry tomato varieties and six common (i.e., non-cherry) tomatoes were evaluated. The highest levels of lycopene were detected in 'Tigerella' and 'Byelsa', and those of phytoene in 'Orange', those of phenolics in 'Green Zebra', all of them common tomatoes. The levels of sugars in both groups of tomatoes were comparable. Interesting differences in plastid carotenoid-accumulating sub-structures as a function of the carotenoid profile were observed. Given the importance of chromoplasts in the deposition of carotenoids in plants and their release during digestion, this information can be valuable in investigations on the regulation of the biosynthesis and the bioavailability of tomato carotenoids.

Raman, AFM and SNOM high resolution imaging of carotene crystals in a model carrot cell system

Three non-destructive and complementary techniques, Raman imaging, Atomic Force Microscopy and Scanning Near-field Optical Microscopy were used simultaneously to show for the first time chemical and structural differences of carotenoid crystals. Spectroscopic and microscopic scanning probe measurements were applied to the released crystals or to crystals accumulated in a unique, carotenoids rich callus tissue growing in vitro that is considered as a new model system for plant carotenoid research. Three distinct morphological crystal types of various carotenoid composition were identified, a needle-like, rhomboidal and helical. Raman imaging using 532 and 488 nm excitation lines provided evidence that the needle-like and rhomboidal crystals had similar carotenoid composition and that they were composed mainly of β-carotene accompanied by α-carotene. However, the presence of α-carotene was not identified in the helical crystals, which had the characteristic spatial structure. AFM measurements of crystals identified by Raman imaging revealed the crystal topography and showed the needle-like and rhomboidal crystals were planar but they differed in all three dimensions. Combining SNOM and Raman imaging enabled indication of carotenoid rich structures and visualised their distribution in the cell. The morphology of identified subcellular structures was characteristic for crystalline, membraneous and tubular chromoplasts that are plant organelles responsible for carotenoid accumulation in cells.

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.

Carotenoids from gac fruit aril (Momordica cochinchinensis [Lour.] Spreng.) are more bioaccessible than those from carrot root and tomato fruit

Using a simulated digestion procedure in vitro, liberation and bioaccessibility of β-carotene (29.5±1.7% and 22.6±0.9%, respectively) and lycopene (51.3±2.6% and 33.2±3.1%, respectively) from gac fruit aril were found to be significantly higher than from carrot root (β-carotene, 5.2±0.5% and 0.5±0.2%, respectively) and tomato fruit (lycopene, 15.9±2.8% and 1.8±0.5%, respectively). Gac fruit aril naturally contained significantly more lipids (11% on fresh weight base) than carrot root and tomato fruit (<1%). However, when test meals were supplemented with an O/W emulsion to match the content of gac fruit aril, carotenoid bioaccessibility was still considerably lower than that from genuine gac fruit aril. Carotenoids in gac fruit aril were found to be stored in small, round-shaped chromoplasts. Despite the high lipid content, these carotenoids are unlikely to occur in a lipid-dissolved state according to simple solubility estimations, instead being possibly deposited as submicroscopic crystallites. In contrast, carotenoids of carrot root and tomato fruit were stored in large, needle-like crystallous chromoplasts. Consequently, we hypothesized the natural deposition form to be majorly responsible for the observed differences in bioaccessibility. A favorable surface-to-volume ratio of the deposition form in gac fruit aril might have allowed a more rapid micellization during digestion, and thus, an enhanced bioaccessibility. Irrespective of the ultimate reason, gac fruit aril provided a highly bioaccessible form of both lycopene and provitamin A (β-carotene), thus offering a most valuable dietary source of both carotenoids. Currently, gac is majorly grown in Southeast Asia, where its consumption might help to diminish the 'hidden hunger' namely the insufficient supply with vitamin A. Ultimately, gac fruit might thus contribute to alleviating most severe health implications of vitamin A deficiency, such as anaemia and xerophthalmia, the prevailing cause of preventable childhood blindness, as well as mortality from infectious diseases.

Plastids and Carotenoid Accumulation

Plastids are ubiquitously present in plants and are the organelles for carotenoid biosynthesis and storage. Based on their morphology and function, plastids are classified into various types, i.e. proplastids, etioplasts, chloroplasts, amyloplasts, and chromoplasts. All plastids, except proplastids, can synthesize carotenoids. However, plastid types have a profound effect on carotenoid accumulation and stability. In this chapter, we discuss carotenoid biosynthesis and regulation in various plastids with a focus on carotenoids in chromoplasts. Plastid transition related to carotenoid biosynthesis and the different capacity of various plastids to sequester carotenoids and the associated effect on carotenoid stability are described in light of carotenoid accumulation in plants.

Regulation of Carotenoid Biosynthesis During Fruit Development

Carotenoids are recognized as the main pigments in most fruit crops, providing colours that range from yellow and pink to deep orange and red. Moreover, the edible portion of widely consumed fruits or their derived products represent a major dietary source of carotenoids for animals and humans. Therefore, these pigments are crucial compounds contributing to fruit aesthetic and nutritional quality but may also have protecting and ecophysiological functions in coloured fruits. Among plant organs, fruits display one of the most heterogeneous carotenoids patterns in terms of diversity and abundance. In this chapter a comprehensive list of the carotenoid content and profile in the most commonly cultivated fleshy fruits is reported. The proposed fruit classification systems attending to carotenoid composition are revised and discussed. The regulation of carotenoids in fruits can be rather complex due to the dramatic changes in content and composition during ripening, which are also dependent on the fruit tissue and the developmental stage. In addition, carotenoid accumulation is a dynamic process, associated with the development of chromoplasts during ripening. As a general rule, carotenoid accumulation is highly controlled at the transcriptional level of the structural and accessory proteins of the biosynthetic and degradation pathways, but other mechanisms such as post-transcriptional modifications or the development of sink structures have been recently revealed as crucial factors in determining the levels and stability of these pigments. In this chapter common key metabolic reactions regulating carotenoid composition in fruit tissues are described in addition to others that are restricted to certain species and generate unique carotenoids patterns. The existence of fruit-specific isoforms for key steps such as the phytoene synthase, lycopene β-cyclases or catabolic carotenoid cleavage dioxygenases has allowed an independent regulation of the pathway in fruit tissues and a source of variability to create novel activities or different catalytic properties. Besides key genes of the carotenoid pathway, changes in carotenoid accumulation could be also directly influenced by differences in gene expression or protein activity in the pathway of carotenoid precursors and some relevant examples are discussed. The objective of this chapter is to provide an updated review of the main carotenoid profiles in fleshy fruits, their pattern of changes during ripening and our current understanding of the different regulatory levels responsible for the diversity of carotenoid accumulation in fruit tissues.

WHEN IS A CHROMOPLAST?

Chromoplasts are heterogeneous organelles and their carotenoids can be associated with several different structural elements including globules, tubules and membranes. In the flower petals of some species, carotenoids are absent or present only in trace amounts. Nevertheless, the plastids in such flowers can closely resemble or even be indistinguishable in structure from chromoplasts.