Barbara Gouble

Tomato fruit ascorbic acid content is linked with monodehydroascorbate reductase activity and tolerance to chilling stress

Quantitative trait loci (QTL) mapping is a step towards the identification of factors regulating traits such as fruit ascorbic acid content. A previously identified QTL controlling variations in tomato fruit ascorbic acid has been fine mapped and reveals that the QTL has a polygenic and epistatic architecture. A monodehydroascorbate reductase (MDHAR) allele is a candidate for a proportion of the increase in fruit ascorbic acid content. The MDHAR enzyme is active in different stages of fruit ripening, shows increased activity in the introgression lines containing the wild-type (Solanum pennellii) allele, and responds to chilling injury in tomato along with the reduced/oxidized ascorbate ratio. Low temperature storage of different tomato introgression lines with all or part of the QTL for ascorbic acid and with or without the wild MDHAR allele shows that enzyme activity explains 84% of the variation in the reduced ascorbic acid levels of tomato fruit following storage at 4 degrees C, compared with 38% at harvest under non-stress conditions. A role is indicated for MDHAR in the maintenance of ascorbate levels in fruit under stress conditions. Furthermore, an increased fruit MDHAR activity and a lower oxidation level of the fruit ascorbate pool are correlated with decreased loss of firmness because of chilling injury.

GDP‐d‐mannose 3,5‐epimerase (GME) plays a key role at the intersection of ascorbate and non‐cellulosic cell‐wall biosynthesis in tomato

The GDP-D-mannose 3,5-epimerase (GME, EC 5.1.3.18), which converts GDP-d-mannose to GDP-l-galactose, is generally considered to be a central enzyme of the major ascorbate biosynthesis pathway in higher plants, but experimental evidence for its role in planta is lacking. Using transgenic tomato lines that were RNAi-silenced for GME, we confirmed that GME does indeed play a key role in the regulation of ascorbate biosynthesis in plants. In addition, the transgenic tomato lines exhibited growth defects affecting both cell division and cell expansion. A further remarkable feature of the transgenic plants was their fragility and loss of fruit firmness. Analysis of the cell-wall composition of leaves and developing fruit revealed that the cell-wall monosaccharide content was altered in the transgenic lines, especially those directly linked to GME activity, such as mannose and galactose. In agreement with this, immunocytochemical analyses showed an increase of mannan labelling in stem and fruit walls and of rhamnogalacturonan labelling in the stem alone. The results of MALDI-TOF fingerprinting of mannanase cleavage products of the cell wall suggested synthesis of specific mannan structures with modified degrees of substitution by acetate in the transgenic lines. When considered together, these findings indicate an intimate linkage between ascorbate and non-cellulosic cell-wall polysaccharide biosynthesis in plants, a fact that helps to explain the common factors in seemingly unrelated traits such as fruit firmness and ascorbate content.

Respiratory parameters and sugar catabolism of mushroom (Agaricus bisporus Lange)

The respiration rate (RR) of mushroom (Agaricus bisporus Lange, strain X25) under air follows Arrhenius’ law with an apparent activation energy of 43 400 J mol−1 corresponding to a Q10 of 2.9 between 10 and 20°C. Oxygen from 20 to 1 kPa did not affect RR at temperatures ranging from 5 to 20°C, confirming previous studies. The initial RR of mushroom (same cultivar, flush, growing conditions) measured 24 h after harvesting can vary from 1 to 2 mmol O2 kg−1 h−1 at 10°C. The apparent Km value was below 0.1 kPa at 10°C, and could not be estimated more accurately due to instrumental limitations. Decrease in dry matter of modified atmosphere-packed mushrooms matched the theoretical carbon consumption calculated from the constant respiration rate. Glucose and glycogen were rapidly catabolized, whereas mannitol consumption began 3 and 8 days after harvest at 20 and 10°C, respectively. It was concluded that no extension of mushroom shelf life was attainable through modified atmosphere packaging. Controlling RH within the package is likely to be more effective.

Two expansin cDNAs from Prunus armeniaca expressed during fruit ripening are differently regulated by ethylene

Abstract Little is known about gene expression during fruit ripening of apricot ( Prunus armeniaca L. cv. Bergeron), especially for enzymes involved in cell wall modifications. A partial cDNA clone encoding a protein homologous to expansin was isolated from a ripe apricot fruit cDNA library. This clone was used to isolate two full-length expansin cDNAs, Pa-Exp1 (accession no. U93167) and Pa-Exp2 (accession no. AF038815) from the same cDNA library. The predicted polypeptides encoded by these two cDNAs are different and belong to the α-expansin family; Pa-Exp1 and Pa-Exp2 are two different members of a multigene family. These two clones are mostly expressed in fruit, during its ripening. Pa-Exp1 mRNA accumulated abundantly at the half-ripe stage of fruit development and decreased thereafter. Pa-Exp2 mRNA level increased from the immature-green stage to the half-ripe stage where it peaked before declining. During the ripening process, Pa-Exp1 and Pa-Exp2 gene expression appeared to be positively correlated with fruit size. Post-harvest treatments by air, ethylene, and 1-methyl cyclopropene led us to conclude that Pa-Exp1 appears to be developmentally down-regulated by ethylene while Pa-Exp2 is not affected. The relationship between Pa-Exp1 , Pa-Exp2 and the softening process is also discussed.

Respiratory parameters of onion bulbs (Allium cepa) during storage. Effects of ionising radiation and temperature

The O 2 and CO 2 respiration rates of untreated and irradiated onion bulbs (Allium cepa )a t 0.15 and 0.30 kGy were measured at 4, 10 and 20 °C. The O 2 respiration rate increased for 24h after treatment from 0.19 mmole kg ˇ1 h ˇ1 at 20 °C for control samples up to 0.26 and 0.39 mmole kg ˇ1 h ˇ1 for 0.15 and 0.3 kGy irradiated onions respectively. Respiratory quotient (RQ) increased with tempera- ture. The Q 10 of the respiration of the control samples (1.61) was lower than that of any other plant tissue, but it increased with storage duration and irradiation dose. The respiration rate of control onions increased steadily over 25 weeks of storage at 4 °C, while that of the irradiated samples decreased during the same period after a peak observed after irradiation treatment. The apparent K m for the Menten-Michaelis equation was determined on a new respirometer and averaged 1.6 kPa at 10 °C and 6.3kPa at 20 °C. However, at this higher temperature (20 °C) apparent K m varied with O 2 partial pressure, proving that the respiration of onion bulbs does not follow a Menten-Michaelis-like process. The Fermentative Index (FI) of onions was measured under anoxic conditions as CO 2 production rates in mmole kg ˇ1 h ˇ1 at 4, 10 and 20 °C. # 2000 Society of Chemical Industry

Pink Discoloration of Canned Pears: Role of Procyanidin Chemical Depolymerization and Procyanidin/Cell Wall Interactions

After canning, pear pieces turn occasionally from whitish-beige to pink. Conditions were set up to obtain this discoloration systematically and investigate its mechanism. Canned pears showed a significantly lower L* coordinate compared with fresh pears, and the L* coordinate of canned pears decreased with decreasing pH. The values of the a* and b* coordinates increased significantly after processing, the increase being greater for the more acidic pH values, with corresponding redder colors. After canning, polyphenol concentrations decreased significantly, mainly due to loss of procyanidins. This supported the hypothesis of conversion of procyanidins to anthocyanin-like compounds. However, no soluble product was detected at 520 nm, the characteristic wavelength of anthocyanins. When purified procyanidins were treated at 95 °C at three different pH values (2.7, 3.3, and 4.0), procyanidin concentrations decreased after treatment, the more so as the pH was lower, and a pinkish color also appeared, attributed to tannin-anthocyanidin pigment. The pink color was bound to cell walls. Extraction of the neoformed pink entities was attempted by successive solvent extractions followed by cell wall degrading enzymes. The pink color persisted in the residues, and canned pears gave significantly higher amounts of residues after solvent and enzyme treatments than fresh pears. Procyanidins were the entities responsible for the appearance of pink discoloration. However, it seems that this pink discoloration also involved the formation of strong, probably covalent, bonds to the cell wall.

Home conservation strategies for tomato (Solanum lycopersicum): storage temperature vs. duration--is there a compromise for better aroma preservation?

Expression of dissatisfaction with tomato aroma prompted us to lead this study on the impact of domestic storage conditions on volatile compounds. Two storage modalities (20 and 4°C) and two cultivars (Levovil and LCx) were used. Volatile compounds were analysed by gas chromatography-mass spectrometry detection after accelerated solvent extraction. Physical characteristics, lipoxygenase activity, hydroperoxide lyase activity; linoleic acid and linolenic acid were monitored. Storing tomatoes at 4°C induced a drastic loss in volatiles, whatever their biosynthetic origin. After 30 days at 4°C, the concentration of volatiles had decreased by 66%. Reconditioning for 24 h at 20°C was able to recover some aroma production after up to 6 days storage at 4°C. Volatile degradation products arising from carotenoids and amino acids increased when tomatoes were kept at 20°C, while lipid degradation products did not vary. Storing tomatoes at fridge temperature, even for short durations, was detrimental for their aroma. This should be taken into account to formulate practical advice for consumers.

Impact of canning and storage on apricot carotenoids and polyphenols

Apricot polyphenols and carotenoids were monitored after industrial and domestic cooking, and after 2months of storage for industrial processing. The main apricot polyphenols were flavan-3-ols, flavan-3-ol monomers and oligomers, with an average degree of polymerization between 4.7 and 10.7 and caffeoylquinic acids. Flavonols and anthocyanins were minor phenolic compounds. Upon processing procyanidins were retained in apricot tissue. Hydroxycinnamic acids, flavan-3-ol monomers, flavonols and anthocyanins leached in the syrup. Flavonol concentrations on per-can basis were significantly increased after processing. Industrial processing effects were higher than domestic cooking probably due to higher temperature and longer duration. After 2months of storage, among polyphenols only hydroxycinnamic acids, flavan-3-ol monomers and anthocyanins were reduced. Whichever the processing method, no significant reductions of total carotenoids were observed after processing. The cis-β-carotene isomer was significantly increased after processing but with a lower extent in domestic cooking. Significant decreased in total carotenoid compounds occurred during storage.

A Systems Biology Study in Tomato Fruit Reveals Correlations between the Ascorbate Pool and Genes Involved in Ribosome Biogenesis, Translation, and the Heat-Shock Response

Changing the balance between ascorbate, monodehydroascorbate, and dehydroascorbate in plant cells by manipulating the activity of enzymes involved in ascorbate synthesis or recycling of oxidized and reduced forms leads to multiple phenotypes. A systems biology approach including network analysis of the transcriptome, proteome and metabolites of RNAi lines for ascorbate oxidase, monodehydroascorbate reductase and galactonolactone dehydrogenase has been carried out in orange fruit pericarp of tomato (Solanum lycopersicum). The transcriptome of the RNAi ascorbate oxidase lines is inversed compared to the monodehydroascorbate reductase and galactonolactone dehydrogenase lines. Differentially expressed genes are involved in ribosome biogenesis and translation. This transcriptome inversion is also seen in response to different stresses in Arabidopsis. The transcriptome response is not well correlated with the proteome which, with the metabolites, are correlated to the activity of the ascorbate redox enzymes—ascorbate oxidase and monodehydroascorbate reductase. Differentially accumulated proteins include metacaspase, protein disulphide isomerase, chaperone DnaK and carbonic anhydrase and the metabolites chlorogenic acid, dehydroascorbate and alanine. The hub genes identified from the network analysis are involved in signaling, the heat-shock response and ribosome biogenesis. The results from this study therefore reveal one or several putative signals from the ascorbate pool which modify the transcriptional response and elements downstream.