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Concept: Ripening

168

Banana finger drop is defined as dislodgement of individual fruits from the hand at the pedicel rupture area. For some banana varieties, this is a major feature of the ripening process, in addition to ethylene production and sugar metabolism. The few studies devoted to assessing the physiological and molecular basis of this process revealed (i) the similarity between this process and softening, (ii) the early onset of related molecular events, between the first and fourth day after ripening induction, and (iii) the putative involvement of ethylene as a regulatory factor. This study was conducted with the aim of identifying, through a candidate gene approach, a quality-related marker that could be used as a tool in breeding programmes. Here we examined the relationship between ripening ethylene biosynthesis (EB) and finger drop in order to gain further insight into the upstream regulatory steps of the banana finger drop process and to identify putative related candidate genes.

Concepts: DNA, Amino acid, Molecular biology, Biology, Fruit, Ethylene, Ripening, Banana

167

Carotenoids represent some of the most important secondary metabolites in the human diet, and tomato (Solanum lycopersicum) is a rich source of these health promoting compounds. In this work, a novel and fruit-related regulator of pigment accumulation in tomato has been identified by Artificial Neural Network Inference Analysis (ANN) and its function validated in transgenic plants. A tomato-fruit gene-regulatory network was generated using ANN and transcription-factor gene-expression profiles (Tfs) derived from fruits sampled at various points during development and ripening. One of the Tfs with a sequence related to an Arabidopsis PSEUDO RESPONSE REGULATOR 2-LIKE gene (APRR2-Like) was up-regulated at the breaker stage in wild type tomato fruits and, when over expressed in transgenic lines, increased plastid number, area and pigment content; enhancing the levels of chlorophyll in immature unripe fruits and carotenoids in red ripe fruits. Analysis of the transcriptome of transgenic lines over expressing the tomato APPR2-Like gene revealed up-regulation of several ripening-related genes in the over-expression lines providing a link between expression of this tomato gene and the ripening process. A putative orthologue of the tomato APPR2-Like gene in sweet pepper was associated with pigment accumulation in fruit tissues. We conclude that the function of this gene is conserved across taxa and that it encodes a protein that has an important role in ripening.

Concepts: Gene expression, Fruit, Tomato, Solanaceae, Vegetable, Eggplant, Ethylene, Ripening

28

Fruits are an important part of a healthy diet. They provide essential vitamins and minerals, and their consumption is associated with a reduced risk of heart disease and certain cancers. These important plant products can, however, be expensive to purchase, may be of disappointing quality and often have a short shelf life. A major challenge for crop improvement in fleshy fruit species is the enhancement of their health-promoting attributes while improving quality and reducing postharvest waste. To achieve these aims, a sound mechanistic understanding of the processes involved in fruit development and ripening is needed. In recent years, substantial insights have been made into the mechanistic basis of ethylene biosynthesis, perception and signalling and the identity of master regulators of ripening that operate upstream of, or in concert with a regulatory pathway mediated by this plant hormone. The role of other plant hormones in the ripening process has, however, remained elusive, and the links between regulators and downstream processes are still poorly understood. In this review, we focus on tomato as a model for fleshy fruit and provide an overview of the molecular circuits known to be involved in ripening, especially those controlling pigment accumulation and texture changes. We then discuss how this information can be used to understand ripening in other fleshy fruit-bearing species. Recent developments in comparative genomics and systems biology approaches are discussed. The potential role of epigenetic changes in generating useful variation is highlighted along with opportunities for enhancing the level of metabolites that have a beneficial effect on human health.

Concepts: Health, Nutrition, Understanding, Fruit, Tomato, Ethylene, Ripening, Banana

28

Ripening of tomato fruits is triggered by the plant hormone ethylene, but its effect is restricted by an unknown developmental cue to mature fruits containing viable seeds. To determine whether this cue involves epigenetic remodeling, we expose tomatoes to the methyltransferase inhibitor 5-azacytidine and find that they ripen prematurely. We performed whole-genome bisulfite sequencing on fruit in four stages of development, from immature to ripe. We identified 52,095 differentially methylated regions (representing 1% of the genome) in the 90% of the genome covered by our analysis. Furthermore, binding sites for RIN, one of the main ripening transcription factors, are frequently localized in the demethylated regions of the promoters of numerous ripening genes, and binding occurs in concert with demethylation. Our data show that the epigenome is not static during development and may have been selected to ensure the fidelity of developmental processes such as ripening. Crop-improvement strategies could benefit by taking into account not only DNA sequence variation among plant lines, but also the information encoded in the epigenome.

Concepts: DNA, Gene expression, Methylation, Fruit, Tomato, Ethylene, Ripening, Banana

28

Low-temperature breakdown (LTB), a disorder inducing quality loss, during and after cold storage of ‘Hayward’ kiwifruit was investigated. Harvested kiwifruits during fruit maturation or after delayed storage (DS) at 20 °C for 0, 1, 2, 3 and 4 weeks and 1 µL L(-1) ethylene treatment for 24 h were stored at - 0.5 °C for 24 weeks and additional ripening at 20 °C for 5 days. Fruit quality indices and LTB incidence and severity were determined before and after treatments.

Concepts: Fruit, Ethylene, Storage, Ripening, Banana, Kiwifruit

27

For the first time, the AuAg-Ag2S heterostructured nanowires consisting of periodic AuAg alloy and Ag2S nanocrytals are synthesized in a simple, one-pot reaction. After the AuAg alloy nanowire with diameter of 2-3 nm is synthesized, it is converted to AuAg-Ag2S heterostructured nanowire by addition of sulfur. The diffusion of Au and Ag in the Ag2S nanocrystals based on the substitutional-interstitial diffusion mechanism and the subsequent Ostwald ripening process are the key reasons for the formation of heterostructured nanowires. The obtained AuAg-Ag2S heter-ostructured nanowires exhibit stability against ripening be-cause of the kinetically prohibited Ostwald ripening process. This new type of hybrid nanostructure undergoes photoinduced charge separation and may have photocatalytic applications.

Concepts: Chemical synthesis, Nanowire, The Key, Silver, One-pot synthesis, Ostwald ripening, Ripening, Wilhelm Ostwald

27

Banana lectin (BanLec) was isolated from slightly overripe bananas (PCI 6-7) by homogenation in NaCl solution, followed by extraction in the presence of glucose, ammonium sulfate precipitation, and affinity chromatography. Yields were approximately 10-fold greater that those of previously published methods using acidic extraction from very overripe fruit (Peel Color Index [PCI] 7+). By dilution of added isotopically labeled recombinant lectin, the content of total exchangeable BalLec was shown to be constant or to slightly decrease with increasing stage of ripeness, even though extractable BanLec increased, followed by rapid decrease in overripened fruit. In the course of this study we observed that recombinant BanLec expressed in Escherichia coli, although chemically and functionally identical to native BanLec, differed slightly in its apparent molecular size on gel filtration, probably due to differences in its native folding.

Concepts: Protein, Ammonia, Molecular biology, Escherichia coli, Fruit, Salt, Ripening, Banana

25

Ethylene is the simplest unsaturated hydrocarbon, yet it has profound effects on plant growth and development, including many agriculturally important phenomena. Analysis of the mechanisms underlying ethylene biosynthesis and signalling have resulted in the elucidation of multistep mechanisms which at first glance appear simple, but in fact represent several levels of control to tightly regulate the level of production and response. Ethylene biosynthesis represents a two-step process that is regulated at both the transcriptional and post-translational levels, thus enabling plants to control the amount of ethylene produced with regard to promotion of responses such as climacteric flower senescence and fruit ripening. Ethylene production subsequently results in activation of the ethylene response, as ethylene accumulation will trigger the ethylene signalling pathway to activate ethylene-dependent transcription for promotion of the response and for resetting the pathway. A more detailed knowledge of the mechanisms underlying biosynthesis and the ethylene response will ultimately enable new approaches to be developed for control of the initiation and progression of ethylene-dependent developmental processes, many of which are of horticultural significance.

Concepts: Plant, Transcription factor, Hydrocarbon, Regulation, Fruit, Ethylene, Ripening, Banana

22

The fruit of Chinese jujube (Ziziphus jujuba Mill.) is immensely popular worldwide, while its fleshy fruit has a very short shelf life and suffers serious postharvest damage. The fruit has been controversially classified as non-climacteric, though the mechanisms underlying its ripening behavior, particularly the role of ethylene, have remained unclear. In this study, low and stable ethylene production was detected during ripening of Z. jujuba ‘Dongzao’ fruit, with production increasing at the full maturity stage. To determine potential ripening behavior, the fruit of five cultivars were harvested at the white mature stage, and all exhibited a first decreasing and then moderately increasing respiration rate without concomitant climacteric-like ethylene production during shelf storage. Treatment with 1.0 μL L-11-methylcyclopropene (1-MCP) inhibited respiration and ethylene production in white mature fruit, though the effects of 100 μL L-1exogenous ethylene were not significant. The transcript levels of genes involved in ethylene biosynthesis, perception, and signal transduction were not elevated during fruit-ripening onset but substantially increased at the full-red ripening stage. Moreover, expression of genes controlling ethylene biosynthesis and perception mainly occurred in an auto-inhibited System-1-like manner, but signaling pathway genes were minimally affected by exogenous ethylene or 1-MCP. These results show that the ripening of Chinese jujube is non-climacteric. The basal level of ethylene likely plays a minor role in ripening regulation but is necessary to maintain normal ripening. This study elucidates the effects of ethylene on jujube fruit ripening, characterizing the ripening of this fruit as non-climacteric, and also provides strategies for the improvement and maintenance of fruit quality and the extension of shelf life during postharvest storage.

Concepts: Gene expression, Fruit, Ethylene, Drupe, Jujube, Ripening, Banana, Ziziphus

22

Brassinosteroids (BRs) are phytohormones that regulate numerous processes including fruit ripening. In this study, persimmon (Diospyros kaki L.) fruit were treated with 24-epibrassinolide (EBR) or brassinazole (Brz, a BR biosynthesis inhibitor) and then stored at ambient temperature. The results show that endogenous BR contents gradually increased during persimmon fruit ripening. EBR treatment significantly increased both the content of water soluble pectin (WSP) and the activities of polygalacturonase (PG), pectate lyase (PL) and endo-1,4-beta-glucanase (EGase), but significantly reduced the content of acid soluble pectin (ASP) and cellulose, resulting in rapid fruit softening. The EBR treatment also promoted ethylene production and respiration rate. In contrast, Brz treatment delayed persimmon fruit ripening. qRT-PCR analysis showed that DkPG1, DkPL1, DkPE2, DkEGase1, DkACO2, DkACS1 and DkACS2 were up-regulated (especially a 38-fold increase in DkEGase1) in the fruit of the EBR-treated group. These results suggest that BRs are involved in persimmon fruit ripening by influencing cell-wall-degrading enzymes and ethylene biosynthesis.

Concepts: Temperature, Solubility, Cell wall, Fruit, Persimmon, Ethylene, Ripening, Banana