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BackgroundOne of the main factors that reduce fruit quality and lead to economically important losses is oversoftening. Textural changes during fruit ripening are mainly due to the dissolution of the middle lamella, the reduction of cell-to-cell adhesion and the weakening of parenchyma cell walls as a result of the action of cell wall modifying enzymes. Pectins, major components of fruit cell walls, are extensively modified during ripening. These changes include solubilization, depolymerization and the loss of neutral side chains. Recent evidence in strawberry and apple, fruits with a soft or crisp texture at ripening, suggests that pectin disassembly is a key factor in textural changes. In both these fruits, softening was reduced as result of antisense downregulation of polygalacturonase genes. Changes in pectic polymer size, composition and structure have traditionally been studied by conventional techniques, most of them relying on bulk analysis of a population of polysaccharides, and studies focusing on modifications at the nanostructural level are scarce. Atomic force microscopy (AFM) allows the study of individual polymers at high magnification and with minimal sample preparation; however, AFM has rarely been employed to analyse pectin disassembly during fruit ripening.ScopeIn this review, the main features of the pectin disassembly process during fruit ripening are first discussed, and then the nanostructural characterization of fruit pectins by AFM and its relationship with texture and postharvest fruit shelf life is reviewed. In general, fruit pectins are visualized under AFM as linear chains, a few of which show long branches, and aggregates. Number- and weight-average values obtained from these images are in good agreement with chromatographic analyses. Most AFM studies indicate reductions in the length of individual pectin chains and the frequency of aggregates as the fruits ripen. Pectins extracted with sodium carbonate, supposedly located within the primary cell wall, are the most affected.

Morphogenesis emerges from complex multiscale interactions between genetic and mechanical processes. To understand these processes, the evolution of cell shape, proliferation and gene expression must be quantified. This quantification is usually performed either in full 3D, which is computationally expensive and technically challenging, or on 2D planar projections, which introduces geometrical artifacts on highly curved organs. Here we present MorphoGraphX (www.MorphoGraphX.org), a software that bridges this gap by working directly with curved surface images extracted from 3D data. In addition to traditional 3D image analysis, we have developed algorithms to operate on curved surfaces, such as cell segmentation, lineage tracking and fluorescence signal quantification. The software's modular design makes it easy to include existing libraries, or to implement new algorithms. Cell geometries extracted with MorphoGraphX can be exported and used as templates for simulation models, providing a powerful platform to investigate the interactions between shape, genes and growth.

Biological communication by means of structural color has existed for at least 500 million years. Structural color is commonly observed in the animal kingdom, but has been little studied in plants. We present a striking example of multilayer-based strong iridescent coloration in plants, in the fruit of Pollia condensata . The color is caused by Bragg reflection of helicoidally stacked cellulose microfibrils that form multilayers in the cell walls of the epicarp. We demonstrate that animals and plants have convergently evolved multilayer-based photonic structures to generate colors using entirely distinct materials. The bright blue coloration of this fruit is more intense than that of any previously described biological material. Uniquely in nature, the reflected color differs from cell to cell, as the layer thicknesses in the multilayer stack vary, giving the fruit a striking pixelated or pointillist appearance. Because the multilayers form with both helicoidicities, optical characterization reveals that the reflected light from every epidermal cell is polarized circularly either to the left or to the right, a feature that has never previously been observed in a single tissue.

Background and AimsCondensed tannins (also called proanthocyanidins) are widespread polymers of catechins and are essential for the defence mechanisms of vascular plants (Tracheophyta). A large body of evidence argues for the synthesis of monomeric epicatechin on the cytosolic face of the endoplasmic reticulum and its transport to the vacuole, although the site of its polymerization into tannins remains to be elucidated. The aim of the study was to re-examine the cellular frame of tannin polymerization in various representatives of the Tracheophyta.MethodsLight microscopy epifluorescence, confocal microscopy, transmission electron microscopy (TEM), chemical analysis of tannins following cell fractionation, and immunocytochemistry were used as independent methods on tannin-rich samples from various organs from Cycadophyta, Ginkgophyta, Equisetophyta, Pteridophyta, Coniferophyta and Magnoliophyta. Tissues were fixed in a caffeine–glutaraldehyde mixture and examined by TEM. Other fresh samples were incubated with primary antibodies against proteins from both chloroplastic envelopes and a thylakoidal chlorophyll-carrying protein; they were also incubated with gelatin–Oregon Green, a fluorescent marker of condensed tannins. Coupled spectral analyses of chlorophyll and tannins were carried out by confocal microscopy on fresh tissues and tannin-rich accretions obtained through cell fractionation; chemical analyses of tannins and chlorophylls were also performed on the accretions.Key Results and ConclusionsThe presence of the three different chloroplast membranes inside vacuolar accretions that constitute the typical form of tannin storage in vascular plants was established in fresh tissues as well as in purified organelles, using several independent methods. Tannins are polymerized in a new chloroplast-derived organelle, the tannosome. These are formed by pearling of the thylakoids into 30 nm spheres, which are then encapsulated in a tannosome shuttle formed by budding from the chloroplast and bound by a membrane resulting from the fusion of both chloroplast envelopes. The shuttle conveys numerous tannosomes through the cytoplasm towards the vacuole in which it is then incorporated by invagination of the tonoplast. Finally, shuttles bound by a portion of tonoplast aggregate into tannin accretions which are stored in the vacuole. Polymerization of tannins occurs inside the tannosome regardless of the compartment being crossed. A complete sequence of events apparently valid in all studied Tracheophyta is described.

The epidermis plays a pivotal role in plant development and interaction with the environment. However, it is still poorly understood, especially its outer epidermal wall: a singular wall covered by a cuticle. Changes in the cuticle and cell wall structures are important to fully understand their functions. In this work, an ultrastructure and immunocytochemical approach was taken to identify changes in the cuticle and the main components of the epidermal cell wall during tomato fruit development. A thin and uniform procuticle was already present before fruit set. During cell division, the inner side of the procuticle showed a globular structure with vesicle-like particles in the cell wall close to the cuticle. Transition between cell division and elongation was accompanied by a dramatic increase in cuticle thickness, which represented more than half of the outer epidermal wall, and the lamellate arrangement of the non-cutinized cell wall. Changes in this non-cutinized outer wall during development showed specific features not shared with other cell walls. The coordinated nature of the changes observed in the cuticle and the epidermal cell wall indicate a deep interaction between these two supramolecular structures. Hence, the cuticle should be interpreted within the context of the outer epidermal wall.

Tomato ( Solanum lycopersicum ) is an established model for studying fruit biology; however, most studies of tomato fruit growth and ripening are based on homogenized pericarp, and do not consider the internal tissues, or the expression signatures of individual cell and tissue types. We present a spatiotemporally resolved transcriptome analysis of tomato fruit ontogeny, using laser microdissection (LM) or hand dissection coupled with RNA-Seq analysis. Regulatory and structural gene networks, including families of transcription factors and hormone synthesis and signaling pathways, are defined across tissue and developmental spectra. The ripening program is revealed as comprising gradients of gene expression, initiating in internal tissues then radiating outward, and basipetally along a latitudinal axis. We also identify spatial variations in the patterns of epigenetic control superimposed on ripening gradients. Functional studies elucidate previously masked regulatory phenomena and relationships, including those associated with fruit quality traits, such as texture, color, aroma, and metabolite profiles. Cell-type transcriptome profiling greatly elucidate organismal development. Here, the authors report a spatiotemporally resolved comprehensive transcriptome analysis of tomato fruit ontogeny and suggest a new model of fruit maturation which initiates in internal tissues then radiates outwards.

Antisense-mediated down-regulation of the fruit-specific polygalacturonase (PG) gene FaPG1 in strawberries (Fragaria×ananassa Duch.) has been previously demonstrated to reduce fruit softening and to extend post-harvest shelf life, despite the low PG activity detected in this fruit. The improved fruit traits were suggested to be attributable to a reduced cell wall disassembly due to FaPG1 silencing. This research provides empirical evidence that supports this assumption at the biochemical, cellular, and tissue levels. Cell wall modifications of two independent transgenic antisense lines that demonstrated a >90% reduction in FaPG1 transcript levels were analysed. Sequential extraction of cell wall fractions from control and ripe fruits exhibited a 42% decrease in pectin solubilization in transgenic fruits. A detailed chromatographic analysis of the gel filtration pectin profiles of the different cell wall fractions revealed a diminished depolymerization of the more tightly bound pectins in transgenic fruits, which were solubilized with both a chelating agent and sodium carbonate. The cell wall extracts from antisense FaPG1 fruits also displayed less severe in vitro swelling. A histological analysis revealed more extended cell–cell adhesion areas and an enhanced tissue integrity in transgenic ripe fruits. An immunohistological analysis of fruit sections using the JIM5 antibody against low methyl-esterified pectins demonstrated a higher labelling in transgenic fruit sections, whereas minor differences were observed with JIM7, an antibody that recognizes highly methyl-esterified pectins. These results support that the increased firmness of transgenic antisense FaPG1 strawberry fruits is predominantly due to a decrease in pectin solubilization and depolymerization that correlates with more tightly attached cell wall-bound pectins. This limited disassembly in the transgenic lines indicates that these pectin fractions could play a key role in tissue integrity maintenance that results in firmer ripe fruit.

The fruit trichomes or fruit spines of cucumber, Cucumis sativus L., are highly specialized structures originating from epidermal cells with diverse morphology, which grow perpendicular to the fruit surface. To understand the underlying molecular mechanisms of fruit trichome development, in this study, we conducted morphological characterization and classification of cucumber fruit trichomes and their developmental processes. We examined the fruit trichomes among 200 cucumber varieties, which could be classified into eight morphologically distinct types (I–VIII). Investigation of the organogenesis of the eight types of trichomes revealed two main developmental patterns. The development of glandular trichomes had multiple stages including initiation and expansion of the trichome precursor cell protuberating out of the epidermal surface, followed by periclinal bipartition to two cells (top and bottom) which later formed the head region and the stalk, respectively, through subsequent cell divisions. The non-glandular trichome development started with the expansion of the precursor cell perpendicularly to the epidermal plane followed by cell periclinal division to form a stalk comprising of some rectangle cells and a pointed apex cell. The base cell then started anticlinal bipartition to two cells, which then underwent many cell divisions to form a multicellular spherical structure. In addition, phytohormones as environmental cues were closely related to trichome development. We found that GA and BAP were capable of increasing trichome number per fruit with distinct effects under different concentrations.

As a primary source of lycopene in the human diet, fleshy fruits synthesize this compound both de novo and via chlorophyll metabolism during ripening. SlSGR1 encodes a STAY-GREEN protein that plays a critical role in the regulation of chlorophyll degradation in tomato leaves and fruits. We report that SlSGR1 can regulate tomato (Solanum lycopersicum) lycopene accumulation through direct interaction with a key carotenoid synthetic enzyme SlPSY1, and can inhibit its activity. This interaction with SlSGR1 mediates lycopene accumulation during tomato fruit maturation. We confirmed this inhibitory activity in bacteria engineered to produce lycopene, where the introduction of SlSGR1 reduced dramatically lycopene biosynthesis. The repression of SlSGR1 in transgenic tomato fruits resulted in altered accumulation patterns of phytoene and lycopene, whilst simultaneously elevating SlPSY1 mRNA accumulation and plastid conversion at the early stages of fruit ripening, resulting in increased lycopene and β-carotene (four- and nine-fold, respectively) in red ripe fruits. SlSGR1 influences ethylene signal transduction via the altered expression of ethylene receptor genes and ethylene-induced genes. Fruit shelf-life is extended significantly in SlSGR1-repressed tomatoes. Our results indicate that SlSGR1 plays a pivotal regulatory role in color formation and fruit ripening regulation in tomato, and further suggest that SlSGR1 activity is mediated through direct interaction with PSY1.

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.