Salinity Stress Affects Assimilate Metabolism at the Gene-expression Level during Fruit Development and Improves Fruit Quality in Tomato (Solanum lycopersicum L.)

ABIOTIC STRESS GENE 1 mediates aroma volatiles accumulation by activating MdLOX1a in apple

Fruit aroma is an important organoleptic quality, which influences consumer preference and market competitiveness. Aroma compound synthesis pathways in plants have been widely identified, among the lipoxygenase pathway is crucial for fatty acid catabolism to form esters in apple. However, the regulatory mechanism of this pathway remains elusive. In this study, linear regression analysis and transgene verification revealed that the lipoxygenase MdLOX1a is involved in ester biosynthesis. Yeast one-hybrid library screening indicated that a protein, MdASG1 (ABIOTIC STRESS GENE 1), was a positive regulator of MdLOX1a and ester production based on yeast one-hybrid and dual-luciferase assays, as well as correlation analysis among eight different apple cultivars. Overexpression of MdASG1 in apple and tomato stimulated the lipoxygenase pathway and increased the fatty acid-derived volatile content, whereas the latter was decreased by MdASG1 silencing and CRISPR/Cas9 knockout. Furthermore, MdASG1 overexpression enhanced the salt-stress tolerance of tomato and apple 'Orin' calli accompanied by a higher content of fatty acid-derived volatiles compared to that of non-stressed transgenic tomato fruit, while MdASG1-Cas9 knockdown calli do not respond to salt stress and promote the biosynthesis of fatty acid-derived volatiles. Collectively, these findings indicate that MdASG1 activates MdLOX1a expression and participates in the lipoxygenase pathway, subsequently increasing the accumulation of aroma compounds, especially under moderate salt stress treatment. The results also provide insight into the theory for improving fruit aroma quality in adversity.

Optimizing sweet potato production: insights into the interplay of plant sanitation, virus influence, and cooking techniques for enhanced crop quality and food security

This study investigates the impact of sweet potato plant sanitation on the yield and external and internal quality root storage exploring the nutritional content affected by various cooking methods (raw, boiled, and oven-cooked). The presence of viruses, and concretely of the sweet potato leaf curl virus (SPLCV), in sweet potato propagation material is shown to significantly reduce yield and modify storage root quality. Notably, the research reveals a substantial improvement in crop yield and external quality, reinforcing the efficacy of plant sanitation methods, specifically apical meristem culture, in preserving the overall productivity of sweet potato crops. Furthermore, the investigation identifies a noteworthy decrease in starch content, suggesting a dynamic interaction between plant sanitation and starch metabolism in response to viral diseases. The study also delves into the alteration of mineral absorption patterns, shedding light on how plant sanitation influences the uptake of essential minerals in sweet potato storage roots. While the health status of the plants only slightly affected magnesium (Mg) and manganese (Mn) accumulation, indicating a potential resilience of mineral balance under virus-infected conditions. Moreover, the research identifies significant modifications in antioxidant levels, emphasizing the role of plant sanitation in enhancing the nutritional quality of sweet potatoes. Heat-treated storage roots, subjected to various cooking methods such as boiling and oven-cooking, exhibit notable differences in internal quality parameters. These differences include increased concentrations of total soluble solids (SS) and heightened levels of antioxidant compounds, particularly phenolic and flavonoid compounds. The observed increase in antioxidant capacity underscores the potential health-promoting benefits associated with plant sanitation practices. Overall, the study underscores the critical importance of plant sanitation in enhancing sweet potato production sustainability, contributing to food security, and supporting local agricultural economies. The results emphasize the need for further research to optimize plant sanitation methods and promote their widespread adoption globally, providing valuable insights into the complex relationships in food quality.

Tolerance and adaptation mechanism of Solanaceous crops under salinity stress

Solanaceous crops act as a source of food, nutrition and medicine for humans. Soil salinity is a damaging environmental stress, causing significant reductions in cultivated land area, crop productivity and quality, especially under climate change. Solanaceous crops are extremely vulnerable to salinity stress due to high water requirements during the reproductive stage and the succulent nature of fruits and tubers. Salinity stress impedes morphological and anatomical development, which ultimately affect the production and productivity of the economic part of these crops. The morpho-physiological parameters such as root-to-shoot ratio, leaf area, biomass production, photosynthesis, hormonal balance, leaf water content are disturbed under salinity stress in Solanaceous crops. Moreover, the synthesis and signalling of reactive oxygen species, reactive nitrogen species, accumulation of compatible solutes, and osmoprotectant are significant under salinity stress which might be responsible for providing tolerance in these crops. The regulation at the molecular level is mediated by different genes, transcription factors, and proteins, which are vital in the tolerance mechanism. The present review aims to redraw the attention of the researchers to explore the mechanistic understanding and potential mitigation strategies against salinity stress in Solanaceous crops, which is an often-neglected commodity.

Ameliorating effect of nanoparticles and seeds' heat pre-treatment on soybean plants exposed to sea water salinity

Impairing plant growth and reducing crop production, salinity is considered as major problem in modern agriculture. The current study aimed to investigate the role of seeds' heat pretreatment at 45 °C as well as application of two different nanoparticles nanosilica (N1) and nanoselenium (N2) in reducing salinity stress in three genotypes of Egyptian commercial soybeans (Glycine max L.). Two levels of salt stress using diluted sea water (1/12 and 1/6) were tested either alone or in combination with protective treatments. Obtained results revealed that salinity caused a significant reduction in all tested physiological parameters such as germination rate and membrane stability in soybean plants. A significant reduction in mitotic index and arrest in metaphase were recorded under both tested levels of salinity. It was also revealed that chromosomal abnormalities in soybean plants were positively correlated with the applied salinity concentrations. The fragmentation effect of salinity on the nuclear DNA was investigated and confirmed using Comet assay analysis. Seeds heat pre-treatment (45 °C) and both types of nanoparticles' treatments yielded positive effects on both the salt-stressed and unstressed plants. Quantitative real-time reverse transcription PCR (qRT-PCR) analysis for salt stress responsive marker genes revealed that most studied genes (CAT, APX, DHN2, CAB3, GMPIPL6 and GMSALT3) responded favorably to protective treatments. The modulation in gene expression pattern was associated with improving growth vigor and salinity tolerance in soybean plants. Our results suggest that seeds' heat pretreatment and nanoparticle applications support the recovery against oxidative stresses and represent a promising strategy for alleviating salt stress in soybean genotypes.

Integrating multiple regulations on enzyme activity: the case of phosphoenolpyruvate carboxykinases

Data on protein post-translational modifications (PTMs) increased exponentially in the last years due to the refinement of mass spectrometry techniques and the development of databases to store and share datasets. Nevertheless, these data per se do not create comprehensive biochemical knowledge. Complementary studies on protein biochemistry are necessary to fully understand the function of these PTMs at the molecular level and beyond, for example, designing rational metabolic engineering strategies to improve crops. Phosphoenolpyruvate carboxykinases (PEPCKs) are critical enzymes for plant metabolism with diverse roles in plant development and growth. Multiple lines of evidence showed the complex regulation of PEPCKs, including PTMs. Herein, we present PEPCKs as an example of the integration of combined mechanisms modulating enzyme activity and metabolic pathways. PEPCK studies strongly advanced after the production of the recombinant enzyme and the establishment of standardized biochemical assays. Finally, we discuss emerging open questions for future research and the challenges in integrating all available data into functional biochemical models.

Stress induced production of plant secondary metabolites in vegetables: Functional approach for designing next generation super foods

Plant secondary metabolites are vital for human health leading to the gain the access to natural products. The quality of crops is the result of the interaction of different biotic and abiotic factors. Abiotic stresses during plant growth may reduce the crop performance and quality of the produce. However, abiotic stresses can result in numerous physiological, biochemical, and molecular responses in plants, aiming to deal with these conditions. Abiotic stresses are also elicitors of the biosynthesis of plant secondary metabolites in plants which possess plant defense mechanisms as well as human health benefits such as anti-inflammatory, antioxidative properties etc. Plants either synthesize new compounds or alter the concentration of bioactive compounds. Due to increasing attention towards the production of bioactive compounds, the understanding of crop responses to abiotic stresses in relation to the biosynthesis of bioactive compounds is critical. Plants alter their metabolism at the genetic level in response to different abiotic stresses resulting the changes in secondary metabolite production. Transcriptional factors regulate genes responsible for secondary metabolite biosynthesis in several plants under stress conditions. Understanding the signaling pathways involved in the secondary metabolite biosynthesis has become easy with the use of molecular biology. Therefore, aim of writing the review is to focus on secondary metabolite production in vegetable crops, their health benefits and transcription regulation under various abiotic stresses.

Physiological, transcriptomic, and metabolic analyses reveal that mild salinity improves the growth, nutrition, and flavor properties of hydroponic Chinese chive (Allium tuberosum Rottler ex Spr)

Environmental stressors such as salinity have pronounced impacts on the growth, productivity, nutrition, and flavor of horticultural crops, though yield loss sometimes is inevitable. In this study, the salinity influences were evaluated using hydroponic Chinese chive (Allium tuberosum) treated with different concentrations of sodium chloride. The results demonstrated that lower salinity could stimulate plant growth and yield. Accordingly, the contents of soluble sugar, ascorbic acid, and soluble protein in leaf tissues increased, following the decrease of the nitrate content, under mild salinity (6.25 or 12.5 mM NaCl). However, a higher level of salinity (25 or 50 mM NaCl) resulted in growth inhibition, yield reduction, and leaf quality deterioration of hydroponic chive plants. Intriguingly, the chive flavor was boosted by the salinity, as evidenced by pungency analysis of salinity-treated leaf tissues. UPLC-MS/MS analysis reveals that mild salinity promoted the accumulation of glutamic acid, serine, glycine, and proline in leaf tissues, and thereby enhanced the umami and sweet flavors of Chinese chive upon salinity stress. Considering the balance between yield and flavor, mild salinity could conduce to hydroponic Chinese chive cultivation. Transcriptome analysis revealed that enhanced pungency could be ascribed to a salt stress-inducible gene, AtuFMO1, associated with the biosynthesis of S-alk(en)yl cysteine sulphoxides (CSOs). Furthermore, correlation analysis suggested that two transcription factors, AtubHLH and AtuB3, were potential regulators of AtuFMO1 expressions under salinity. Thus, these results revealed the molecular mechanism underlying mild salinity-induced CSO biosynthesis, as well as a practical possibility for producing high-quality Chinese chive hydroponically.

Effects of light quality, photoperiod, CO2 concentration, and air temperature on chlorogenic acid and rutin accumulation in young lettuce plants

Environmental stimuli modulate plant metabolite accumulation, facilitating adaptation to stressful conditions. In this study, the effects of blue and red light, photoperiod, CO2 concentration, and air temperature on the chlorogenic acid (CGA) and rutin contents of lettuce (Lactuca sativa L.) were evaluated. Under continuous blue light and a high CO2 concentration (1000 ppm), the CGA level increased. The increased expression of phenylalanine ammonia-lyase (PAL) and activity of its product were correlated with high expression of cinnamate 4-hydroxylase (C4H) and coumarate 3-hydroxylase (C3H). Furthermore, changes in PAL activity altered the CGA content in lettuce exposed to the three environmental factors, blue light, continuous lighting and high CO2 concentration. In addition, the expression levels of genes related to flavonoid biosynthesis increased in accordance with the promotion of CGA accumulation by the environmental factors. Under continuous blue light, 400 ppm CO2 promoted rutin accumulation to a greater degree compared to 1000 ppm CO2, by downregulating DFR expression. Low air temperature induced CGA accumulation in lettuce grown under continuous blue light and 1000 ppm CO2. Therefore, light quality, photoperiod, CO2 concentration, and air temperature exert synergistic effects on the CGA and rutin contents of lettuce by modulating activity in the corresponding biosynthesis pathways.

The Efficiency of Different Priming Agents for Improving Germination and Early Seedling Growth of Local Tunisian Barley under Salinity Stress

The current work aimed to investigate the effect of seed priming with different agents (CaCl₂, KCl, and KNO₃) on germination and seedling establishment in seeds of the barley species of both Hordeum vulgare (L. Manel) and Hordeum maritimum germinated with three salt concentrations (0, 100, and 200 mM NaCl). The results showed that under unprimed conditions, salt stress significantly reduced the final germination rate, the mean daily germination, and the seedling length and dry weight. It led to a decrease in the essential nutrient content (iron, calcium, magnesium, and potassium) against an increase in sodium level in both of the barley species. Moreover, this environmental constraint provoked a membrane injury caused by a considerable increase in electrolyte leakage and the malondialdehyde content (MDA). Data analysis proved that seed priming with CaCl₂, KCl, and KNO₃ was an effective method for alleviating barley seed germination caused by salt stress to varying degrees. Different priming treatments clearly stimulated germination parameters and the essential nutrient concentration, in addition to increasing the seedling growth rate. The application of seed priming reduced the accumulation of sodium ions and mitigated the oxidative stress of seeds caused by salt. This mitigation was traduced by the maintenance of low levels of MDA and electrolyte leakage. We conclude that the priming agents can be classed into three ranges based on their efficacy on the different parameters analyzed; CaCl₂ was placed in the first range, followed closely by KNO₃, while the least effective was KCl, which placed in the third range.

Moderate Salinity Stress Affects Expression of Main Sugar Metabolism and Transport Genes and Soluble Carbohydrate Content in Ripe Fig Fruits (Ficus carica L. cv. Dottato)

Fig trees (Ficus carica L.) are commonly grown in the Mediterranean area, where salinity is an increasing problem in coastal areas. Young, fruiting plants of cv. Dottato were subjected to moderate salt stress (100 mM NaCl added to irrigation water) for 48 days before fruit sampling. To clarify the effect of salinity stress, we investigated changes in the transcription of the main sugar metabolism-related genes involved in the synthesis, accumulation and transport of soluble carbohydrates in ripe fruits by quantitative real-time PCR as well as the content of soluble sugars by quantitative ¹H nuclear magnetic resonance spectroscopy. A general increase in the transcript levels of genes involved in the transport of soluble carbohydrates was observed. Alkaline-neutral and Acid Invertases transcripts, related to the synthesis of glucose and fructose, were up-regulated in ripe fruits of NaCl-stressed plants without a change in the content of D-glucose and D-fructose. The increases in sucrose and D-sorbitol contents were likely the result of the up-regulation of the transcription of Sucrose-Synthase- and Sorbitol-Dehydrogenase-encoding genes.

Proteolytic cleavage of Arabidopsis thaliana phosphoenolpyruvate carboxykinase-1 modifies its allosteric regulation

Phosphoenolpyruvate carboxykinase (PEPCK) plays a crucial role in gluconeogenesis. In this work, we analyze the proteolysis of Arabidopsis thaliana PEPCK1 (AthPEPCK1) in germinating seedlings. We found that the amount of AthPEPCK1 protein peaks at 24-48 h post-imbibition. Concomitantly, we observed shorter versions of AthPEPCK1, putatively generated by metacaspase-9 (AthMC9). To study the impact of AthMC9 cleavage on the kinetic and regulatory properties of AthPEPCK1, we produced truncated mutants based on the reported AthMC9 cleavage sites. The Δ19 and Δ101 truncated mutants of AthPEPCK1 showed similar kinetic parameters and the same quaternary structure as the wild type. However, activation by malate and inhibition by glucose 6-phosphate were abolished in the Δ101 mutant. We propose that proteolysis of AthPEPCK1 in germinating seedlings operates as a mechanism to adapt the sensitivity to allosteric regulation during the sink-to-source transition.

Interactive effects of salinity and silicon application on Solanum lycopersicum growth, physiology and shelf-life of fruit produced hydroponically

BACKGROUND

Using water with high salinity for plant fertigation may have detrimental effects on plant development and total yield and on the quality of the crop produced. As a possible means to alleviate the negative effects of salinity, silicon (Si) can be incorporated in the nutrient solution supplied to plants. In the present study, hydroponically grown tomato (Solanum lycopersicum Mill.) plants were subjected to two different salinity levels (0 and 50 mmol L^-1 NaCl) with and without the application of Si (0 and 2 mmol L^-1 K₂ SiO₃ ) in order to evaluate its possible positive impact on mitigation of salinity stress-induced symptoms. An additional experiment was implemented with postharvest Si application (sodium silicate) to investigate effects on the shelf-life of tomato fruit.

RESULTS

Salinity (50 mmol L^-1 NaCl) decreased plant size, total yield and fresh fruit weight while a high percentage of blossom end rot symptoms of tomato fruit was also observed. The application of Si in the nutrient solution counteracted these detrimental effects, generating a higher yield and healthier fruit (lower blossom end rot incidence) compared to the untreated plants (no application of Si). Salinity improved several quality-related traits in tomato fruit, resulting in higher marketability, whereas the addition of Si (pre- and postharvest) maintained fruit firmness following storage thereby increasing the shelf-life of tomato fruit.

CONCLUSIONS

These findings indicate that Si application (pre- and postharvest) could provide an effective means of alleviating the unfavorable effects of using low-quality water in plant fertigation on tomato plant development, fruit yield and post-harvest quality, through increased fruit firmness. © 2019 Society of Chemical Industry.

The assessment of tomato fruit quality parameters under different sound waves

Sound stress is an abiotic stress factor wherein the sound wave form affects the growth and development of plants as an alternative mechanical stress. To explore this, 10-week-old tomato (Solanum lycopersicum) plants were used in this experiment. The tomato plants were exposed to three different frequency values consecutively: 600 Hz in the first week, 1240 Hz in the second week and 1600 Hz in the third week. The decibel (dB) value was adjusted to 90 dB in the sound amplifier. At the end of the experiment, lycopene, vitamin C, total sugar, total acid and total phenol levels were analysed and pH and ⁰Brix were measured in tomato fruits. As a result, it was determined that as the sound frequency intensity level increased, the concentration of fruit parameters also increased: lycopene, vitamin C, total sugar, total acid and total phenol. The total phenol content, lycopene content and ascorbic acid of the tomato plants that were exposed to sound waves at different frequencies increased at a rate of 70%, 20% and 14%, respectively. According to the results of all measured parameters in tomato fruits, 1600 Hz has been determined the best of sound wave frequency value.

Influence of the proline metabolism and glycine betaine on tolerance to salt stress in tomato (Solanum lycopersicum L.) commercial genotypes

Tomato is the crop with the greatest economic importance in the world and salinity stress causes a reduction in the quantity and quality of crop production. The objective of this work is to verify if the accumulation of proline and glycine betaine (GB) and their metabolisms improve tolerance to salt stress. Two commercial genotypes of Solanum Lycopersicum L., Grand Brix and Marmande RAF were used for this work. The analyzed parameters were growth parameters, proline concentration and its metabolism, GB and its above betaine aldehyde dehydrogenase (BADH) synthesis and some related amino acids. Saline stress reduced biomass and relative growth rate (RGR) in both genotypes, this effect being greater in Marmande RAF. These results, together with the proline accumulation indicate that Grand Brix is more tolerant to saline stress. The proline increase in Grand Brix came by the ornithine pathway, leaving the glutamate pathway repressed. On the other hand, it was found in both genotypes a BADH and GB decreases as a salinity tolerance mechanism. We propose that, unlike proline, GB synthesis can produce H₂O₂ thereby, GB not act as compatible solute and salt tolerance does not improve.

Regulation of phytochemicals in fruits and berries by environmental variation-Sugars and organic acids

Sugars and organic acids are important phytochemicals contributing to the nutrition and sensory properties of fruits and berries. Their contents are closely correlated to the genetic background of plants as well as to the environmental conditions during growth. This review focuses on the recent researches on the metabolism of these compounds in fruits and berries in response to the variation of environmental conditions, including temperature, radiation, and water supply. A great deal of investigations indicates that the influence of environmental factors on the composition of fruits/berries depended largely on the genetic background. Moreover, the metabolic regulation in response to environmental changes also varies between different plant developmental stages. Nevertheless, some general trends, like the positive correlation between light intensity and sugar content, were observed in most investigations. In grapes (Vitis vinifera L.), the content of malic acid always decreases as light intensity increases, and as the water supply decreases. PRACTICAL APPLICATIONS: The contents of sugars and organic acids, and especially their relative ratio, are important indicators determining the taste and quality of fruits and fruit products. In this review, we summarized the investigations carried out on the regulation of these sensory contributing primary metabolites in fruits and berries in relation to the variation of environmental conditions. It was indicated that various factors, such as plant genotype, growing period, and interaction between environmental factors, might contribute to the impact of environmental changes on the composition of fruits/berries. The article not only provides comprehensive knowledges in food chemistry and plant physiology but also provide important background knowledge for berry cultivation and breeding, as well as useful guidelines for utilization of fruits and berries in food industry.

Study of phytohormone profile and oxidative metabolism as key process to identification of salinity response in tomato commercial genotypes

Climatic change, intensive agriculture, and worsening water quality induce abiotic stress conditions for plants. Among these factors, salinity stress is a limit factor for plant growth. Therefore, the purpose of this study was to analyze the phytohormones role and oxidative metabolism in response to salt stress of two genotypes of tomato cv. Grand Brix and cv. Marmande RAF, the crops were carried out in a growth chamber. Salinity stress reduces biomass and relative growth rate (RGR) in both genotypes, this effect being greater in cv. Marmande RAF. These results, together with main stress indicator response, the O₂^.-, indicate that cv. Marmande RAF is more sensitive to Saline stress. Grand Brix showed less oxidative stress, because it presented greater detoxification of the O₂^-, due to SOD enzyme activity induction and greater antioxidant capacity. Furthermore, Grand Brix has a better hormonal profile adapted to salt stress resistance, the accumulation of IAA, GA4 and CKs and their beneficial role against oxidative stress could make the difference between resistance and sensitivity to salt stress. On the other hand, a lower ACC concentration, ethylene precursor, combined with a greater O₂^.- detoxification in the cv. Grand Brix could play a fundamental role in tolerance to saline stress. Besides, an increase in ABA levels promotes better stomatal closure, better photosynthesis control and a lower rate of water loss. This data could be essential to select plants with greater resistance to saline stress.

Phosphoenolpyruvate carboxykinase, pyruvate orthophosphate dikinase and isocitrate lyase in both tomato fruits and leaves, and in the flesh of peach and some other fruits

In this study the occurrence of a number of enzymes involved in gluconeogenesis was investigated in both tomato fruits and leaves during their development and senescence and in some other fruits. The enzymes studied were phosphoenolpyruvate carboxykinase (PEPCK), pyruvate orthophosphate dikinase (PPDK) and glyoxysomal isocitrate lyase (ICL). PPDK was detected in the ripe flesh of tomato, and much smaller amounts were detected in the flesh of both peach and pepper, whereas it was not detected (not present or at very low abundance) in the other fruits which were investigated (apricot, aubergine, blackberry, blueberry, cherry, grape, plum, raspberry and red current). By contrast PEPCK was present in the flesh of all the fruits investigated. Very small amounts of ICL were detected in ripe tomato flesh. PEPCK was present in the skin, flesh, locular gel and columella of tomato fruit, and in these its abundance increased greatly during ripening. PPDK showed a similar distribution, however, its abundance did not increase during ripening. PEPCK was not detected in tomato leaves at any stage of their development or senescence. The content of PPDK g(-1) fresh weight (FW) increased in tomato leaves as they matured, however, it declined during their senescence. In tomato leaves the content of ICL g(-1) FW increased until the mid-stage of development, then decreased as the leaf matured, and then increased during the latter stages of senescence. In the flesh of tomato fruits the contents of PPDK and PEPCK g(-1) FW decreased during senescence. The results suggest that in fruits other than tomato the bulk of any gluconeogenic flux proceeds via PEPCK, whereas in tomato both PEPCK and PPDK could potentially be utilised. Further, the results indicate that the conversion of pyruvate/acetyl-CoA to malate by the glyoxylate cycle, for which ICL is necessary, is not a major pathway utilised by gluconeogenesis in fruits under normal conditions of growth. Finally, the results contribute to our understanding of the role of several enzymes in the senescence of both leaves and fruits.

Leaf Senescence by Magnesium Deficiency

Magnesium ions (Mg(2+)) are the second most abundant cations in living plant cells, and they are involved in various functions, including photosynthesis, enzyme catalysis, and nucleic acid synthesis. Low availability of Mg(2+) in an agricultural field leads to a decrease in yield, which follows the appearance of Mg-deficient symptoms such as chlorosis, necrotic spots on the leaves, and droop. During the last decade, a variety of physiological and molecular responses to Mg(2+) deficiency that potentially link to leaf senescence have been recognized, allowing us to reconsider the mechanisms of Mg(2+) deficiency. This review focuses on the current knowledge about the physiological responses to Mg(2+) deficiency including a decline in transpiration, accumulation of sugars and starch in source leaves, change in redox states, increased oxidative stress, metabolite alterations, and a decline in photosynthetic activity. In addition, we refer to the molecular responses that are thought to be related to leaf senescence. With these current data, we give an overview of leaf senescence induced by Mg deficiency.

Phosphoenolpyruvate carboxykinase (PEPCK) deficiency affects the germination, growth and fruit sugar content in tomato (Solanum lycopersicum L.)

Phosphoenolpyruvate carboxykinase (PEPCK) is a key regulatory enzyme and is utilized in the gluconeogenesis pathway in plants. Although, its catalytic and regulatory properties are quite well understood, there are uncertainties regarding its physiological role in many plants tissues such as the flesh of developing fruits. To further understand the function of PEPCK in fruits and other tissues, RNAi transgenic tomato plants in which SlPEPCK transcription was down-regulated by either CaMV 35S constitutive promoter or the fruit-specific E8 promoter were generated and characterized on the basis of their phenotypic and metabolic aspects. In the PEPCK-deficient lines, prominent growth suppression of germinated seedlings was observed and other vegetative suppression appeared during the early stage of plant growth in the 35S promoter-driven lines. In particular, root elongation was most obviously suppressed in the germinated seedlings, indicating that the gluconeogenesis pathway is involved in the root growth of seedlings. Regarding the primary metabolism in fruit, the soluble sugar content tended to decrease, whereas the malate content tended to increase in ripening fruits of the RNAi lines compared with the wild type. These results indicate that activation of the gluconeogenesis pathway from organic acids to sugars occurs during ripening but is suppressed by the knocking down of the PEPCK gene, suggesting that PEPCK participates in determining the sugar/acid ratio in ripening fruit.

Reference gene selection for quantitative real-time PCR normalization in Reaumuria soongorica

Despite its superiority for evaluating gene expression, real-time quantitative polymerase chain reaction (qPCR) results can be significantly biased by the use of inappropriate reference genes under different experimental conditions. Reaumuria soongorica is a dominant species of desert ecosystems in arid central Asia. Given the increasing interest in ecological engineering and potential genetic resources for arid agronomy, it is important to analyze gene function. However, systematic evaluation of stable reference genes should be performed prior to such analyses. In this study, the stabilities of 10 candidate reference genes were analyzed under 4 kinds of abiotic stresses (drought, salt, dark, and heat) within 4 accessions (HG010, HG020, XGG030, and XGG040) from 2 different habitats using 3 algorithms (geNorm, NormFinder, and BestKeeper). After validation of the ribulose-1,5-bisphosphate carboxylase/oxygenase large unite (rbcL) expression pattern, our data suggested that histone H2A (H2A) and eukaryotic initiation factor 4A-2 (EIF4A2) were the most stable reference genes, cyclophilin (CYCL) was moderate, and elongation factor 1α (EF1α) was the worst choice. This first systematic analysis for stably expressed genes will facilitate future functional analyses and deep mining of genetic resources in R. soongorica and other species of the Reaumuria genus.

Modulation of organic acids and sugar content in tomato fruits by an abscisic acid-regulated transcription factor

Growing evidence suggests that the phytohormone abscisic acid (ABA) plays a role in fruit development. ABA signaling components of developmental programs and responses to stress conditions include the group of basic leucine zipper transcriptional activators known as ABA-response element binding factors (AREBs/ABFs). AREB transcription factors mediate ABA-regulated gene expression involved in desiccation tolerance and are expressed mainly in seeds and in vegetative tissues under stress; however, they are also expressed in some fruits such as tomato. In order to get an insight into the role of ABA signaling in fruit development, the expression of two AREB-like factors were investigated during different developmental stages. In addition, tomato transgenic lines that overexpress and downregulate one AREB-like transcription factor, SlAREB1, were used to determine its effect on the levels of some metabolites determining fruit quality. Higher levels of citric acid, malic acid, glutamic acid, glucose and fructose were observed in SlAREB1-overexpressing lines compared with those in antisense suppression lines in red mature fruit pericarp. The higher hexose concentration correlated with increased expression of genes encoding a vacuolar invertase (EC 3.2.1.26) and a sucrose synthase (EC 2.4.1.13). No significant changes were found in ethylene content which agrees with the normal ripening phenotype observed in transgenic fruits. These results suggest that an AREB-mediated ABA signal affects the metabolism of these compounds during the fruit developmental program.

Enhanced polyamine accumulation alters carotenoid metabolism at the transcriptional level in tomato fruit over-expressing spermidine synthase

Polyamines are involved in crucial plant physiological events, but their roles in fruit development remain unclear. We generated transgenic tomato plants that show a 1.5- to 2-fold increase in polyamine content by over-expressing the spermidine synthase gene, which encodes a key enzyme for polyamine biosynthesis. Pericarp-columella and placental tissue from transgenic tomato fruits were subjected to (1)H-nuclear magnetic resonance (NMR) for untargeted metabolic profiling and high-performance liquid chromatography-diode array detection for carotenoid profiling to determine the effects of high levels of polyamine accumulation on tomato fruit metabolism. A principal component analysis of the quantitative (1)H NMR data from immature green to red ripe fruit showed a clear discrimination between developmental stages, especially during ripening. Quantification of 37 metabolites in pericarp-columella and 41 metabolites in placenta tissues revealed distinct metabolic profiles between the wild type and transgenic lines, particularly at the late ripening stages. Notably, the transgenic tomato fruits also showed an increase in carotenoid accumulation, especially in lycopene (1.3- to 2.2-fold), and increased ethylene production (1.2- to 1.6-fold) compared to wild-type fruits. Genes responsible for lycopene biosynthesis, including phytoene synthase, phytoene desaturase, and deoxy-d-xylulose 5-phosphate synthase, were significantly up-regulated in ripe transgenic fruits, whereas genes involved in lycopene degradation, including lycopene-epsilon cyclase and lycopene beta cyclase, were down-regulated in the transgenic fruits compared to the wild type. These results suggest that a high level of accumulation of polyamines in the tomato regulates the steady-state level of transcription of genes responsible for the lycopene metabolic pathway, which results in a higher accumulation of lycopene in the fruit.

Metabolic alterations in organic acids and gamma-aminobutyric acid in developing tomato (Solanum lycopersicum L.) fruits

Salt stress improves the quality of tomato fruits. To clarify the mechanism(s) underlying this phenomenon, we investigated metabolic alterations in tomato fruits exposed to 160 mM salt, focusing on metabolism of organic acids related to the tricarboxylic acid (TCA) cycle and gamma-aminobutyric acid (GABA). Quantitative analyses revealed that most amino acids increased in response to salt stress throughout fruit development, and the effect of the stress was greater in the pericarp than in the columella, whereas organic acids did not show a remarkable tendency to salt stress. The transcript levels of 20 genes encoding enzymes of the TCA cycle and peripheral pathways were also analyzed in salt-stressed fruit. Genes responsive to salt stress could be categorized into two types, which were expressed during early development or ripening stages. During fruit development, phosphoenolpyruvate carboxylase 2 and phosphoenolpyruvate carboxykinase displayed contrasting expression patterns between early development and ripening, suggesting a switch of carbohydrate metabolism after the turning stage. Our results revealed a new metabolic pathway for GABA during the development of tomato fruits. At the start of ripening, GABA is first converted to malate via succinate semialdehyde, and it passes into a shunt through pyruvate. Then, it flows back to the TCA cycle and is stored as citrate, which contributes as a substrate for respiration during fruit maturation.

Salinity induces carbohydrate accumulation and sugar-regulated starch biosynthetic genes in tomato (Solanum lycopersicum L. cv. 'Micro-Tom') fruits in an ABA- and osmotic stress-independent manner

Salinity stress enhances sugar accumulation in tomato (Solanum lycopersicum) fruits. To elucidate the mechanisms underlying this phenomenon, the transport of carbohydrates into tomato fruits and the regulation of starch synthesis during fruit development in tomato plants cv. 'Micro-Tom' exposed to high levels of salinity stress were examined. Growth with 160 mM NaCl doubled starch accumulation in tomato fruits compared to control plants during the early stages of development, and soluble sugars increased as the fruit matured. Tracer analysis with (13)C confirmed that elevated carbohydrate accumulation in fruits exposed to salinity stress was confined to the early development stages and did not occur after ripening. Salinity stress also up-regulated sucrose transporter expression in source leaves and increased activity of ADP-glucose pyrophosphorylase (AGPase) in fruits during the early development stages. The results indicate that salinity stress enhanced carbohydrate accumulation as starch during the early development stages and it is responsible for the increase in soluble sugars in ripe fruit. Quantitative RT-PCR analyses of salinity-stressed plants showed that the AGPase-encoding genes, AgpL1 and AgpS1 were up-regulated in developing fruits, and AgpL1 was obviously up-regulated by sugar at the transcriptional level but not by abscisic acid and osmotic stress. These results indicate AgpL1 and AgpS1 are involved in the promotion of starch biosynthesis under the salinity stress in ABA- and osmotic stress-independent manners. These two genes are differentially regulated at the transcriptional level, and AgpL1 is suggested to play a regulatory role in this event.

Regulation of malate metabolism in grape berry and other developing fruits

Organic acids are present in all plants, supporting numerous and varied facets of cellular metabolism. The type of organic acid found, and the levels to which they accumulate are extremely variable between species, developmental stages and tissue types. Acidity plays important roles in the organoleptic properties of plant tissues, where examples of both enhanced and reduced palatability can be ascribed to the presence of specific organic acids. In fruits, sourness is generally attributed to proton release from acids such as citric, malic, oxalic, quinic, succinic and tartaric, while the anion forms each contribute a distinct taste. Acidity imposes a strong influence on crop quality, and is an important factor in deciding the harvest date, particularly for fruits where acidity is important for further processing, as in wine grapes. In the grape, as for many other fruits, malate is one of the most prevalent acids, and is an important participant in numerous cellular functions. The accumulation of malate is thought to be due in large part to de novo synthesis in fruits such as the grape, through metabolism of assimilates translocated from leaf tissues, as well as photosynthetic activity within the fruit itself. During ripening, the processes through which malate is catabolised are of interest for advancing metabolic understanding, as well as for potential crop enhancement through agricultural or molecular practices. A body of literature describes research that has begun to unravel the regulatory mechanisms of enzymes involved in malate metabolism during fruit development, through exploration of protein and gene transcript levels. Datasets derived from a series of recent microarray experiments comparing transcript levels at several stages of grape berry development have been revisited, and are presented here with a focus on transcripts associated with malate metabolism. Developmental transcript patterns for enzymes potentially involved in grape malate metabolism have shown that some flux may occur through pathways that are less commonly regarded in ripening fruit, such as aerobic ethanol production. The data also suggest pyruvate as an important intermediate during malate catabolism in fruit. This review will combine an analysis of microarray data with information available on protein and enzyme activity patterns in grapes and other fruits, to explore pathways through which malate is conditionally metabolised, and how these may be controlled in response to developmental and climatic changes. Currently, an insufficient understanding of the complex pathways through which malate is degraded, and how these are regulated, prevents targeted genetic manipulation aimed at modifying fruit malate metabolism in response to environmental conditions.