A metabolome and transcriptome survey to tap the dynamics of fruit prolonged shelf-life and improved quality within Greek tomato germplasm.

Introduction: Tomato is a high economic value crop worldwide with recognized nutritional properties and diverse postharvest potential. Nowadays, there is an emerging awareness about the exploitation and utilization of underutilized traditional germplasm in modern breeding programs. In this context, the existing diversity among Greek accessions in terms of their postharvest life and nutritional value remains largely unexplored. Methods: Herein, a detailed evaluation of 130 tomato Greek accessions for postharvest and nutritional characteristics was performed, using metabolomics and transcriptomics, leading to the selection of accessions with these interesting traits. Results: The results showed remarkable differences among tomato Greek accessions for overall ripening parameters (color, firmness) and weight loss. On the basis of their postharvest performance, a balance between short shelf life (SSL) and long shelf life (LSL) accessions was revealed. Metabolome analysis performed on 14 selected accessions with contrasting shelf-life potential identified a total of 206 phytonutrients and volatile compounds. In turn, transcriptome analysis in fruits from the best SSL and the best LSL accessions revealed remarkable differences in the expression profiles of transcripts involved in key metabolic pathways related to fruit quality and postharvest potential. Discussion: The pathways towards cell wall synthesis, polyamine synthesis, ABA catabolism, and steroidal alkaloids synthesis were mostly induced in the LSL accession, whereas those related to ethylene biosynthesis, cell wall degradation, isoprenoids, phenylpropanoids, ascorbic acid and aroma (TomloxC) were stimulated in the SSL accession. Overall, these data would provide valuable insights into the molecular mechanism towards enhancing shelf-life and improving flavor and aroma of modern tomato cultivars.

LED omics in Rocket Salad (Diplotaxis tenuifolia): Comparative Analysis in Different Light-Emitting Diode (LED) Spectrum and Energy Consumption.

By applying three different LED light treatments, designated as blue (B), red (R)/blue (B), red (R) and white (W) light, as well as the control, the effect on Diplotaxis tenuifolia phenotype (yield and quality), and physiological, biochemical, and molecular status, as well as growing system resource use efficiency, was examined. We observed that basic leaf characteristics, such as leaf area, leaf number, relative chlorophyll content, as well as root characteristics, such as total root length and root architecture, remained unaffected by different LEDs. Yield expressed in fresh weight was slightly lower in LED lights than in the control (1113 g m-2), with R light producing the least (679 g m-2). However, total soluble solids were significantly affected (highest, 5.5° Brix, in R light) and FRAP was improved in all LED lights (highest, 191.8 µg/g FW, in B) in comparison to the control, while the nitrate content was less (lowest, 949.2 µg/g FW, in R). Differential gene expression showed that B LED light affected more genes in comparison to R and R/B lights. Although total phenolic content was improved under all LED lights (highest, 1.05 mg/g FW, in R/B), we did not detect a significant amount of DEGs in the phenylpropanoid pathway. R light positively impacts the expression of the genes encoding for photosynthesis components. On the other hand, the positive impact of R light on SSC was possibly due to the expression of key genes being induced, such as SUS1. In summary, this research is an integrative and innovative study, where the exploration of the effect of different LED lights on rocket growing under protected cultivation, in a closed chamber cultivation system, was performed at multiple levels.

Ascophyllum nodosum and Silicon-Based Biostimulants Differentially Affect the Physiology and Growth of Watermelon Transplants under Abiotic Stress Factors: The Case of Salinity.

Salinization of cultivated soils is a global phenomenon mainly caused by agricultural practices and deteriorates plant production. Biostimulants are products which can be applied exogenously to enhance the plants' defense mechanism and improve their developmental characteristics, also under abiotic stresses. We studied the potential of two biostimulants, Ascophyllum nodosum (Asc) seaweed and a silicon-based (Si), to alleviate the saline conditions endured by watermelon transplants. Three salinity (0 mM, 50 mM, and 100 mM NaCl) treatments were applied in watermelon seedlings transplanted in pots, while the two biostimulants were sprayed in the foliar in the beginning of the experiment. Relative water content was improved by Asc in the high salinity level. The plant area, leaf number, and shoot dry weight deteriorated in relation to the salinity level. However, the root system (total root length and surface area) was enhanced by 50 mM salt, as well as Asc in some cases. The OJIP transient of the photosynthetic apparatus was also evaluated. Some OJIP parameters diminished in the high salinity level after Asc application. It is concluded that after salt stress Asc provoked a positive phenotypic response, while Si did not alleviate the salinity stress of transplanted watermelon.

Over-expression of GGP1 and GPP genes enhances ascorbate content and nutritional quality of tomato.

L-Ascorbic acid (AsA), a strong antioxidant, serves as an enzyme cofactor and redox status marker, modulating a plethora of biological processes. As tomato commercial varieties and hybrids possess relatively low amounts of AsA, the improvement of fruit AsA represents a strategic goal for enhanced human health. Previously, we have suggested that GDP-L-Galactose phosphorylase (GGP) and L-galactose-1-phosphate phosphatase (GPP) can serve as possible targets for AsA manipulation in tomato (Solanum lycopersicon L.) fruit. To this end, we produced and evaluated T3 transgenic tomato plants carrying these two genes under the control of CaMV-35S and two fruit specific promoters, PPC2 and PG-GGPI. The transgenic lines had elevated levels of AsA, with the PG-GGP1 line containing 3-fold more AsA than WT, without affecting fruit characteristics. Following RNA-Seq analysis, 164 and 13 DEGs were up- or down-regulated, respectively, between PG-GGP1 and WT pink fruits. PG-GGP1 fruit had a distinct number of up-regulated transcripts associated with cell wall modification, ethylene biosynthesis and signaling, pollen fertility and carotenoid metabolism. The elevated AsA accumulation resulted in the up regulation of AsA associated transcripts and alternative biosynthetic pathways suggesting that the entire metabolic pathway was influenced, probably via master regulation. We show here that AsA-fortification of tomato ripe fruit via GGP1 overexpression under the action of a fruit specific promoter PG affects fruit development and ripening, reduces ethylene production, and increased the levels of sugars, and carotenoids, supporting a robust database to further explore the role of AsA induced genes for agronomically important traits, breeding programs and precision gene editing approaches.

Atlas of phenotypic, genotypic and geographical diversity present in the European traditional tomato.

The Mediterranean basin countries are considered secondary centres of tomato diversification. However, information on phenotypic and allelic variation of local tomato materials is still limited. Here we report on the evaluation of the largest traditional tomato collection, which includes 1499 accessions from Southern Europe. Analyses of 70 traits revealed a broad range of phenotypic variability with different distributions among countries, with the culinary end use within each country being the main driver of tomato diversification. Furthermore, eight main tomato types (phenoclusters) were defined by integrating phenotypic data, country of origin, and end use. Genome-wide association study (GWAS) meta-analyses identified associations in 211 loci, 159 of which were novel. The multidimensional integration of phenoclusters and the GWAS meta-analysis identified the molecular signatures for each traditional tomato type and indicated that signatures originated from differential combinations of loci, which in some cases converged in the same tomato phenotype. Our results provide a roadmap for studying and exploiting this untapped tomato diversity.

Epigenetic and Physiological Responses to Varying Root-Zone Temperatures in Greenhouse Rocket.

Greenhouse production of baby leaf vegetables grown in hydroponic floating trays has become extremely popular in recent years. Rocket (Eruca sativa Mill.) can grow in temperatures varying between 10 and 20 °C; nevertheless, a root-zone temperature (RZT) range of 18-23 °C is considered optimal for high productivity, photosynthesis, and production of metabolites. Maintaining such temperatures in winter raises production costs and prevents sustainability. In this study, we tested the impact of lower RZT on plants' status and recorded their responses while providing energy for heating using photovoltaic solar panels. We used three hydroponic tanks for cultivation; a non-heated (control) tank (12 °C) and two heated tanks; a solar panel-powered one (16 °C) and a public grid-powered one (22 °C). Methylation-sensitive amplified polymorphisms (MSAP) analysis of global methylation profiles and chlorophyll fluorescence analysis were employed to assess methylation and physiology levels of rocket leaves. We found that there is demethylation at 16 °C RZT in comparison to 22 °C RZT. Reduction of temperature at 12 °C did not reduce methylation levels further but rather increased them. Furthermore, at 16 °C, the effective quantum yield of photosystem II (PSII) photochemistry (ΦPSII) was significantly higher, with a higher PSII electron transport rate (ETR) and a significantly decreased non-regulated energy loss (ΦΝO), suggesting a better light energy use by rocket plants with higher photosynthetic performance. ΦPSII was significantly negatively correlated with DNA methylation levels. Our results show that at 16 °C RZT, where plants grow efficiently without being affected by the cold, DNA methylation and photosynthesis apparatus systems are altered. These findings corroborate previous results where hydroponic production of rocket at RZT of 16 °C is accompanied by sufficient yield showing that rocket can effectively grow in suboptimal yet sustainable root-zone temperatures.

Comparative Transcriptome Analysis in Homo- and Hetero-Grafted Cucurbit Seedlings.

Watermelon (Citrullus lanatus) is a valuable horticultural crop with nutritional benefits grown worldwide. It is almost exclusively cultivated as grafted scions onto interspecific squash rootstock (Cucurbita maxima × Cucurbita moschata) to improve the growth and yield and to address the problems of soilborne diseases and abiotic stress factors. This study aimed to examine the effect of grafting (homo- and hetero-grafting) on the transcriptome level of the seedlings. Therefore, we compared homo-grafted watermelon (WW) with non-grafted watermelon control (W), homo-grafted squash (SS) with non-grafted squash control (S), hetero-grafted watermelon onto squash (WS) with SS, and WS with WW. Different numbers of differentially expressed genes (DEGs) were identified in each comparison. In total, 318 significant DEGs were detected between the transcriptomes of hetero-grafts and homo-grafts at 16 h after grafting. Overall, a significantly higher number of downregulated transcripts was detected among the DEGs. Only one gene showing increased expression related to the cytokinin synthesis was common in three out of four comparisons involving WS, SS, and S. The highest number of differentially expressed (DE) transcripts (433) was detected in the comparison between SS and S, followed by the 127 transcripts between WW and W. The study provides a description of the transcriptomic nature of homo- and hetero-grafted early responses, while the results provide a start point for the elucidation of the molecular mechanisms and candidate genes for the functional analyses of hetero-graft and homo-graft systems in Cucurbitaceae and generally in the plants.

Blue light promotes vascular reconnection, while red light boosts the physiological response and quality of grafted watermelon seedlings.

The wound inflicted during grafting of watermelon seedlings requires rapid and sufficient vascular development which is affected by light quality. Our objective was to investigate the effect of light spectra emitted by light-emitting diodes (LEDs) during healing of grafted watermelon (Citrullus lanatus) seedlings on their vascular development, physiological and phytohormonal profile, and root architecture. Three LEDs emitting red (R), blue (B), and RB with 12% blue (12B) were tested in a healing chamber. During the first three days, the photosynthetic apparatus portrayed by PIABS, φP0, ψE0, and ΔVIP was less damaged and faster repaired in B-treated seedlings. B and 12B promoted vascular reconnection and root development (length, surface area and volume). This was the result of signaling cascade between phytohormones such as indole-3-acetic acid and others. After vascular reconnection the seedlings switched lights for 3 more days and the picture was reversed. Seedlings treated with B for the first 3 days and R for days 4 to 6 had better photosynthetic characteristics, root system development, morphological, shoot and root biomass, and quality (i.e. Dickson's quality index) characteristics. We concluded that blue light is important during the first 3 days of healing, while the presence of red is necessary after vascular reconnection.

Regulation of Vitamin C Accumulation for Improved Tomato Fruit Quality and Alleviation of Abiotic Stress.

Ascorbic acid (AsA) is an essential multifaceted phytonutrient for both the human diet and plant growth. Optimum levels of AsA accumulation combined with balanced redox homeostasis are required for normal plant development and defense response to adverse environmental stimuli. Notwithstanding its moderate AsA levels, tomatoes constitute a good source of vitamin C in the human diet. Therefore, the enhancement of AsA levels in tomato fruit attracts considerable attention, not only to improve its nutritional value but also to stimulate stress tolerance. Genetic regulation of AsA concentrations in plants can be achieved through the fine-tuning of biosynthetic, recycling, and transport mechanisms; it is also linked to changes in the whole fruit metabolism. Emerging evidence suggests that tomato synthesizes AsA mainly through the l-galactose pathway, but alternative pathways through d-galacturonate or myo-inositol, or seemingly unrelated transcription and regulatory factors, can be also relevant in certain developmental stages or in response to abiotic factors. Considering the recent advances in our understanding of AsA regulation in model and other non-model species, this review attempts to link the current consensus with novel technologies to provide a comprehensive strategy for AsA enhancement in tomatoes, without any detrimental effect on plant growth or fruit development.

Comparative Metabolite and Gene Expression Analyses in Combination With Gene Characterization Revealed the Patterns of Flavonoid Accumulation During Cistus creticus subsp. creticus Fruit Development.

Cistus creticus L. subsp. creticus (rockrose) is a shrub widespread in Greece and the Mediterranean basin and has been used in traditional medicine as herb tea for colds, for healing and digestive hitches, for the treatment of maladies, as perfumes, and for other purposes. Compounds from its flavonoid fraction have recently drawn attention due to antiviral action against influenza virus and HIV. Although several bioactive metabolites belonging to this group have been chemically characterized in the leaves, the genes involved in their biosynthesis in Cistus remain largely unknown. Flavonoid metabolism during C. creticus fruit development was studied by adopting comparative metabolomic and transcriptomic approaches. The present study highlights the fruit of C. creticus subsp. creticus as a rich source of flavonols, flavan-3-ols, and proanthocyanidins, all of which displayed a decreasing trend during fruit development. The majority of proanthocyanidins recorded in Cistus fruit are B-type procyanidins and prodelphinidins, while gallocatechin and catechin are the dominant flavan-3-ols. The expression patterns of biosynthetic genes and transcription factors were analyzed in flowers and throughout three fruit development stages. Flavonoid biosynthetic genes were developmentally regulated, showing a decrease in transcript levels during fruit maturation. A high degree of positive correlations between the content of targeted metabolites and the expression of biosynthetic genes indicated the transcriptional regulation of flavonoid biosynthesis during C. creticus fruit development. This is further supported by the high degree of significant positive correlations between the expression of biosynthetic genes and transcription factors. The results suggest that leucoanthocyanidin reductase predominates the biosynthetic pathway in the control of flavan-3-ol formation, which results in catechin and gallocatechin as two of the major building blocks for Cistus proanthocyanidins. Additionally, there is a decline in ethylene production rates during non-climacteric Cistus fruit maturation, which coincides with the downregulation of the majority of flavonoid- and ethylene-related biosynthetic genes and corresponding transcription factors as well as with the decline in flavonoid content. Finally, functional characterization of a Cistus flavonoid hydroxylase (F3'5'H) was performed for the first time.

Influence of Light Spectra from LEDs and Scion × Rootstock Genotype Combinations on the Quality of Grafted Watermelon Seedlings.

Grafting is the main means of propagation for watermelon crops. The aim of the present study was to evaluate whether light quality during graft healing variably affects different scion × rootstock genotype combinations. Two watermelon hybrid scions (Sunny Florida F1 and Celine F1) and two interspecific squash rootstocks (Radik and TZ-148) were used, and four scion × rootstock genotype combinations derived. After grafting, we tested seven light-emitting diodes (LEDs), which provided narrow-band red (R) and blue (B); R-B with 36% (36B), 24% (24B), and 12% (12B) blue; 12B with additional far-red (12B+FR); and white (W), in a healing chamber. In three genotype combinations, shoot length, leaf area, and shoot biomass were mainly enhanced under red-blue LEDs, while stem diameter was greater under R. In contrast, dry weight of roots, Dickson's quality index, and ratio of shoot dry weight/length were variably affected in each genotype combination. From the results, it is concluded that light treatments differentially affected each genotype combination, but some parameters involving biomass production show genotypic dependency.

Controlled root-zone temperature effect on baby leaf vegetables yield and quality in a floating system under mild and extreme weather conditions.

BACKGROUND: A floating system is a suitable low-cost hydroponic method for growing baby leaf vegetables. Among other, an important characteristic of the system is the use of large volume of nutrient solution which is characterized by high heat capacity. The aim of this study was to evaluate the effect of different root-zone temperatures on baby leaves of lettuce and rocket plants grown in a floating system under mild (spring) or extreme environmental conditions (summer and winter). RESULTS: Root-zone temperature was recorded in two tanks, one powered by a photovoltaic system and one where root-zone temperature was not controlled - this was used as a control tank. Photosynthetic parameters, yield, nutritional quality, and mineral composition were determined. In both baby leaf vegetables, during extreme weather conditions, yield was higher in the tanks with controlled root-zone temperature conditions than the control (+18.9% for rocket, and + 31.4% for baby lettuce), while quality parameters and chemical composition were not significantly affected. Stomatal conductance and net photosynthesis values were positively affected only during summer. On the other hand, control of root-zone temperature under mild weather conditions had no significant effect on baby lettuce and rocket. CONCLUSION: Control of the root-zone temperature could be a useful tool to improve productivity for baby lettuce and rocket crops cultivated in floating systems under extreme weather conditions. © 2020 Society of Chemical Industry.

Silencing of ascorbate oxidase results in reduced growth, altered ascorbic acid levels and ripening pattern in melon fruit.

Ascorbate oxidase (AO, EC 1.10.3.3) is a copper-containing enzyme localized at the apoplast, where it catalyzes the oxidation of ascorbic acid (AA) to dehydroascorbic acid (DHA) via monodehydroascorbic acid (MDHA) intermediate. Despite it has been extensively studied, no biological roles have been definitively ascribed. To understand the role of AO in plant metabolism, fruit growth and physiology, we suppressed AO expression in melon (Cucumis melo L.) fruit. Reduction of AO activity increased AA content in melon fruit, which is the result of repression of AA oxidation and simultaneous induction of certain biosynthetic and recycling genes. As a consequence, ascorbate redox state was altered in the apoplast. Interestingly, transgenic melon fruit displayed increased ethylene production rate coincided with elevated levels of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACO, EC 1.14.17.4) activity and gene expression, which might contribute to earlier ripening. Moreover, AO suppressed transgenic melon fruit exhibited a dramatic arrest in fruit growth, due to a simultaneous decrease in fruit cell size and in plasmalemma (PM) ATPase activity. All the above, support for the first time, the in vivo AO participation in the rapid fruit growth of Cucurbitaceae and further suggest an alternative route for AA increase in ripening fruit.

Effects of genetic, pre- and post-harvest factors on phenolic content and antioxidant capacity of white asparagus spears.

The effects of genetic, pre-harvest (season of harvest, spear diameter, spear portion and spear tip color) and post-harvest factors (storage and domestic preparation practices, e.g., peeling and cooking) on total phenolic, flavonoid and ascorbic acid content of white asparagus spears and their correlation with antioxidant capacity (DPPH and FRAP) were studied. Results showed that genetic material was important for the total phenolic content but not season of harvest, spear diameter or storage. Violet spear tips and apical spear portions showed the largest amount of total phenolics. Peeling did not affect total phenolics in fresh asparagus, whereas it reduced their content in stored asparagus, while cooking resulted in an increase in both fresh and stored asparagus. However, the soluble extract of total phenolics and flavonoids were minor and the missing significance of phenolics and flavonoids in antioxidant capacity of white asparagus spears depends on these small amounts.