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Organic acids play critical roles in fruit physiological metabolism and sensory quality. However, the conventional storage of apple fruit at 0 ± 0.1 °C cannot maintain fruit acidity efficiently. This study investigated near-freezing temperature (NFT) storage for 'Golden Delicious' apples, and the quality parameters, organic acid content, and malate metabolism were studied. The results indicate that NFT storage at −1.7 ± 0.1 °C effectively maintained the postharvest quality of apple fruit when compared to traditional storage at 0 ± 0.1 °C. Fruit that underwent NFT storage showed a better appearance and lower respiratory rate, ethylene production, weight loss, and malondialdehyde (MDA) content but higher firmness and soluble solids content. Further, fruit after NFT storage contained higher titratable acid (18.75%), malate (51.61%), citrate (36.59%), and succinate (2.12%) content when compared to the control after 250 days. This was achieved by maintaining higher cytosolic NAD-dependent malate dehydrogenase (cyNAD-MDH), phosphoenolpyruvate carboxylase (PEPC), vacuolar H+-ATPase (V-ATPase), and vacuolar inorganic pyrophosphatase (V-PPase) activities that promote malate biosynthesis and accumulation while inhibiting enzyme activity that is responsible for malate decomposition, including phosphoenolpyruvate carboxylase kinase (PEPCK) as well as the cytosolic NAD phosphate-dependent malic enzyme (cyNADP-ME). Further, storage at NFTs maintained a higher expression of malate biosynthesis-related genes (MdcyNAD-MDH and MdPEPC) and transport-related genes (MdVHA and MdVHP) while suppressing malate consumption-related genes (MdcyME and MdPEPCK). The results demonstrate that NFT storage could be an effective application for apple fruit, which maintains postharvest quality and alleviates organic acid degradation. [ABSTRACT FROM AUTHOR]

In this experiment, we investigated salinity-induced oxidative stress in rice with the involvement of sub1A quantitative trait loci for reactive oxygen species metabolism. We pretreated 15-day-old seedlings of cv. Swarna sub1 (a salt-susceptible variety) and Gosaba 5 (a salt-tolerant variety) using either 10 µM abscisic acid (ABA) or absence. The seedlings were then transferred to hydroponic cultures with either 0 or 100 mM NaCl to induce stress. Oxidative stress in the rice plants was investigated through both in vitro and in vivo detection of superoxide, hydrogen peroxide, and biosynthetic enzymes, including NADP oxidase activity under salinity. cv. Swarna sub1 exhibited enhanced retention of relative water content, elevated K+/Na+ ratio, and membrane stabilization with ABA. ABA also mitigated cell viability loss, though to varying extents among the cultivars, as observed through fluorescence imaging and Evans blue staining. Additionally, ABA treatment reduced cell wall peroxidation, resulting in expanded endodermal lumens and a decrease in malondialdehyde content. Pigment fluorescence and NADP-malic enzyme activity supported oxidative tolerance. A higher ratio of oxidized to reduced states of glutathione and ascorbate indicated improved tolerance when treated with ABA. The biogenesis of ABA was found to correlate with cultivar-specific tolerance, particularly when exogenous ABA was applied. ABA treatment appeared to heighten the sensitivity of enzymatic antioxidant cascades and showed variations in gene polymorphism. The detoxification of methylglyoxal, regulated by ABA, accentuated glyoxalase activities as potential biomarkers for oxidative stress. The study further discusses the roles of ABA metabolism in managing salinity-induced oxidative stress in both sensitive and tolerant rice cultivars.

Organic acids play critical roles in fruit physiological metabolism and sensory quality. However, the conventional storage of apple fruit at 0 ± 0.1 °C cannot maintain fruit acidity efficiently. This study investigated near-freezing temperature (NFT) storage for ‘Golden Delicious’ apples, and the quality parameters, organic acid content, and malate metabolism were studied. The results indicate that NFT storage at −1.7 ± 0.1 °C effectively maintained the postharvest quality of apple fruit when compared to traditional storage at 0 ± 0.1 °C. Fruit that underwent NFT storage showed a better appearance and lower respiratory rate, ethylene production, weight loss, and malondialdehyde (MDA) content but higher firmness and soluble solids content. Further, fruit after NFT storage contained higher titratable acid (18.75%), malate (51.61%), citrate (36.59%), and succinate (2.12%) content when compared to the control after 250 days. This was achieved by maintaining higher cytosolic NAD-dependent malate dehydrogenase (cyNAD-MDH), phosphoenolpyruvate carboxylase (PEPC), vacuolar H+-ATPase (V-ATPase), and vacuolar inorganic pyrophosphatase (V-PPase) activities that promote malate biosynthesis and accumulation while inhibiting enzyme activity that is responsible for malate decomposition, including phosphoenolpyruvate carboxylase kinase (PEPCK) as well as the cytosolic NAD phosphate-dependent malic enzyme (cyNADP-ME). Further, storage at NFTs maintained a higher expression of malate biosynthesis-related genes (MdcyNAD-MDH and MdPEPC) and transport-related genes (MdVHA and MdVHP) while suppressing malate consumption-related genes (MdcyME and MdPEPCK). The results demonstrate that NFT storage could be an effective application for apple fruit, which maintains postharvest quality and alleviates organic acid degradation.

Fruit acidity determines the organoleptic quality and nutritive value of most fruits. In litchi, although the organic acid composition of pulps is known, the molecular mechanisms and genes underlying variation in fruit acidity remain elusive. Herein, developing pulps of two contrasting litchi varieties, Huaizhi (HZ, low-acidity) and Boye_No.8 (B8, high-acidity), were subjected to metabolomics and transcriptomics, and the dynamic metabolome and transcriptional changes were determined. Measurements revealed that the dominant acidity-related organic acid in litchi pulps is malate, followed in low levels by citrate and tartrate. Variation in litchi pulps' acidity is mainly associated with significant differences in malate and citrate metabolisms during fruit development. Malic acid content decreased by 91.43% and 72.28% during fruit ripening in HZ and B8, respectively. The content of citric acid increased significantly in B8, while in HZ it was reduced considerably. Differentially accumulated metabolites and differentially expressed genes analyses unveiled fumarate, succinate, 2-oxoglutarate, GABA (γ-aminobutyric acid), phosphoenolpyruvate, and citrate metabolisms as the key driving pathways of litchi fruits' acidity variation. The drastic malate and citrate degradation in HZ was linked to higher induction of fumarate and GABA biosynthesis, respectively. Thirty candidate genes, including three key genes (LITCHI026501.m2, fumarase; LITCHI020148.m5, glutamate decarboxylase; and LITCHI003343.m3, glutamate dehydrogenase), were identified for functional studies toward genetic modulation of litchi fruit acidity. Our findings provide insights into the molecular basis of acidity variation in litchi and provide valuable resources for fruit quality improvement. [ABSTRACT FROM AUTHOR]

Aims: The work aimed to understand the important changes during glucose metabolism in Saccharomyces cerevisiae under acidified sodium nitrite (ac.NaNO2) mediated nitrosative stress. Methods and Results: Confocal microscopy and fluorescence‐activated cell sorting analysis were performed to investigate the generation of reactive nitrogen and oxygen species, and redox homeostasis under nitrosative stress was also characterized. Quantitative PCR analysis revealed that the expression of ADH genes was upregulated under such condition, whereas the ACO2 gene was downregulated. Some of the enzymes of the tricarboxylic acid cycle were partially inhibited, whereas malate metabolism and alcoholic fermentation were increased under nitrosative stress. Kinetics of ethanol production was also characterized. A network analysis was conducted to validate our findings. In the presence of ac.NaNO2, in vitro protein tyrosine nitration formation was checked by western blotting using pure alcohol dehydrogenase and aconitase. Conclusions: Alcoholic fermentation rate was increased under stress condition and this altered metabolism might be conjoined with the defence machinery to overcome the nitrosative stress. Significance and Impact of the Study: This is the first work of this kind where the role of metabolism under nitrosative stress has been characterized in S. cerevisiae and it will provide a base to develop an alternative method of industrial ethanol production. [ABSTRACT FROM AUTHOR]

Peanut is an important oil and economic crop in China. The rainy season (April–June) in the downstream Yangtze River in China always leads to waterlogging, which seriously affects plant growth and development. Therefore, understanding the metabolic mechanisms under waterlogging stress is important for future waterlogging tolerance breeding in peanut. In this study, waterlogging treatment was carried out in two different peanut cultivars [Zhonghua 4 (ZH4) and Xianghua08 (XH08)] with different waterlogging tolerance. The data-independent acquisition (DIA) technique was used to quantitatively identify the differentially accumulated proteins (DAPs) between two different cultivars. Meanwhile, the functions of DAPs were predicted, and the interactions between the hub DAPs were analyzed. As a result, a total of 6,441 DAPs were identified in ZH4 and its control, of which 49 and 88 DAPs were upregulated and downregulated under waterlogging stress, respectively, while in XH08, a total of 6,285 DAPs were identified, including 123 upregulated and 114 downregulated proteins, respectively. The hub DAPs unique to the waterlogging-tolerant cultivar XH08 were related to malate metabolism and synthesis, and the utilization of the glyoxylic acid cycle, such as L-lactate dehydrogenase, NAD+-dependent malic enzyme, aspartate aminotransferase, and glutamate dehydrogenase. In agreement with the DIA results, the alcohol dehydrogenase and malate dehydrogenase activities in XH08 were more active than ZH4 under waterlogging stress, and lactate dehydrogenase activity in XH08 was prolonged, suggesting that XH08 could better tolerate waterlogging stress by using various carbon sources to obtain energy, such as enhancing the activity of anaerobic respiration enzymes, catalyzing malate metabolism and the glyoxylic acid cycle, and thus alleviating the accumulation of toxic substances. This study provides insight into the mechanisms in response to waterlogging stress in peanuts and lays a foundation for future molecular breeding targeting in the improvement of peanut waterlogging tolerance, especially in rainy area, and will enhance the sustainable development in the entire peanut industry.

Peanut is an important oil and economic crop in China. The rainy season (April–June) in the downstream Yangtze River in China always leads to waterlogging, which seriously affects plant growth and development. Therefore, understanding the metabolic mechanisms under waterlogging stress is important for future waterlogging tolerance breeding in peanut. In this study, waterlogging treatment was carried out in two different peanut cultivars [Zhonghua 4 (ZH4) and Xianghua08 (XH08)] with different waterlogging tolerance. The data-independent acquisition (DIA) technique was used to quantitatively identify the differentially accumulated proteins (DAPs) between two different cultivars. Meanwhile, the functions of DAPs were predicted, and the interactions between the hub DAPs were analyzed. As a result, a total of 6,441 DAPs were identified in ZH4 and its control, of which 49 and 88 DAPs were upregulated and downregulated under waterlogging stress, respectively, while in XH08, a total of 6,285 DAPs were identified, including 123 upregulated and 114 downregulated proteins, respectively. The hub DAPs unique to the waterlogging-tolerant cultivar XH08 were related to malate metabolism and synthesis, and the utilization of the glyoxylic acid cycle, such as L-lactate dehydrogenase, NAD+-dependent malic enzyme, aspartate aminotransferase, and glutamate dehydrogenase. In agreement with the DIA results, the alcohol dehydrogenase and malate dehydrogenase activities in XH08 were more active than ZH4 under waterlogging stress, and lactate dehydrogenase activity in XH08 was prolonged, suggesting that XH08 could better tolerate waterlogging stress by using various carbon sources to obtain energy, such as enhancing the activity of anaerobic respiration enzymes, catalyzing malate metabolism and the glyoxylic acid cycle, and thus alleviating the accumulation of toxic substances. This study provides insight into the mechanisms in response to waterlogging stress in peanuts and lays a foundation for future molecular breeding targeting in the improvement of peanut waterlogging tolerance, especially in rainy area, and will enhance the sustainable development in the entire peanut industry. [ABSTRACT FROM AUTHOR]

Hot air (HA) treatment has been demonstrated to control chilling injury (CI) in peaches effectively, however, little is known about the effects of HA on the changes of sugar and organic acid. In order to explore the relationship among CI susceptibility, soluble sugar and organic acid metabolism, peach fruit (Prunus persica) were treated with HA (40°C, 4 h) and then stored at 1°C for 35 days plus 3 days at 20°C. Results showed that HA avoided or decreased the development of CI symptoms and maintained a better quality of peaches. HA suppressed sucrose degradation by inhibiting expression of invertase as well as enhancing the relative expressions of sucrose synthase and sucrose phosphate synthase. The low degradation of sucrose contributed to low contents of glucose and fructose in the treated groups. Besides, HA significantly increased contents of citric and malic acids during the whole storage by enhancing the expressions of citrate synthase and malate dehydrogenase, but inhibiting malate synthase. These results suggest that HA suppressed chilling injury by regulating sucrose degradation and organic acid metabolism. The high contents of sucrose, citric and malic acids contributed to enhancement of CI tolerance in peaches. [ABSTRACT FROM AUTHOR]

[Display omitted] • mleA was the only gene for malate metabolism in O. oeni and L. plantarum. • MleA of O.oeni shows higher enzyme activity than that of L. plantarum at low pH. • mleA has positive roles in acid stress resistance for O. oeni and L. plantarum. Oenococcus oeni and Lactiplantibacillus plantarum are major wine-associated lactic acid bacteria that positively influence wine by carrying out malolactic fermentation. O. oeni is the most widely used commercial starter in winemaking because of its fast and efficient malate metabolism capacity under harsh wine conditions. To date, very little is known about the specific molecular mechanism underlying the differences in malate metabolism between O. oeni and L. plantarum under harsh wine conditions. Therefore, in this study, the functions of genes encoding malic enzyme (ME) and malolactic enzyme (MLE) under acid stress in O. oeni and L. plantarum , previously described to have the ability to direct malate metabolism, were comparatively verified through genetic manipulation in L. plantarum. Results showed that the MLE was the only enzyme responsible for direct malate metabolism under acid stress in O. oeni and L. plantarum. In addition, the MLEs in O. oeni and L. plantarum were positively related to acid tolerance by metabolizing malate and increasing the medium pH. Furthermore, the MLE in O. oeni exhibited significantly higher malate metabolism activity than that in L. plantarum under acid stress. [ABSTRACT FROM AUTHOR]

During symbiotic nitrogen fixation (SNF), the nodule becomes a strong sink for photosynthetic carbon. Here, it was studied whether nodule dark CO2 fixation could participate in a mechanism for CO2 recycling through C4-type photosynthesis. Differences in the natural δ13C abundance between Lotus japonicus inoculated or not with the N-fixing Mesorhizobium loti were assessed. 13C labelling and gene expression of key enzymes of CO2 metabolism were applied in plants inoculated with wild-type or mutant fix– (deficient in N fixation) strains of M. loti, and in non-inoculated plants. Compared with non-inoculated legumes, inoculated legumes had higher natural δ13C abundance and total C in their hypergeous organs and nodules. In stems, 13C accumulation and expression of genes coding for enzymes of malate metabolism were greater in inoculated compared with non-inoculated plants. Malate-oxidizing activity was localized in stem xylem parenchyma, sieve tubes, and photosynthetic outer cortex parenchyma of inoculated plants. In stems of plants inoculated with fix– M. loti strains, 13C accumulation remained high, while accumulation of transcripts coding for malic enzyme isoforms increased. A potential mechanism is proposed for reducing carbon losses during SNF by the direct reincorporation of CO2 respired by nodules and the transport and metabolism of C-containing metabolites in hypergeous organs. [ABSTRACT FROM PUBLISHER]

This study investigated the respiratory properties and the role of the mitochondria isolated from one phosphoenolpyruvate carboxykinase (PCK)-CAM plant, Hoya carnosa, in malate metabolism during CAM phase III. The mitochondria showed high malate dehydrogenase (mMDH) and aspartate amino transferase (mAST), and a significant amount of malic enzyme (mME) activities. H. carnosa readily oxidized malate via mME and mMDH in the presence of some cofactors such as thiamine pyrophosphate (TPP), coenzyme A (CoA) or NAD+. A high respiration rate of malate oxidation was observed at pH 7.2 with NAD+ and glutamate (Glu). Providing AST and Glu simultaneously into the respiratory medium strongly increased the rates of malate oxidation, and this oxidation was gradually inhibited by an inhibitor of α-ketoglutarate (α-KG) carrier, pyridoxal-5′-phosphate (PLP). The mitochondria readily oxidized aspartate (Asp) or α-KG individually with low rates, while they oxidized Asp and α-KG simultaneously with high rates, and this simultaneous oxidation was also inhibited by PLP. By measuring the capacity of the mitochondrial shuttle, it was found that the OAA produced via mMDH seemed not to be transported outside the mitochondria, but mAST interconverted OAA and Glu to Asp and α-KG, respectively, and exported them out via a malate-aspartate (malate-Asp) shuttle. The data in this research suggest that during phase III of PCK-CAM, H. carnosa mitochondria oxidized malate via both mME and the mMDH systems depending on metabolic requirements. However, malate metabolism by the mMDH system did not operate via a malate-OAA shuttle similarly to Ananas comosus mitochondria, but it operated via a malate-Asp shuttle similarly to Kalanchoë daigremontiana mitochondria. [ABSTRACT FROM PUBLISHER]

In cold‐hardened leaves (CHL) of winter rye (Secale cereale L.) much higher levels of malate were detected by 13C‐NMR than in non‐hardened leaves (NHL). As this was not observed previously, malate metabolism of CHL was studied in more detail by biochemical assays. The activities of several enzymes of malate metabolism, NADP‐malate dehydrogenase, NAD‐malate dehydrogenase, phosphoenolpyruvate carboxylase, and NADP‐malic enzyme, were also increased in CHL. Short exposures to low temperature of 1–3 d did not induce increases in the malate content or in the activities of enzymes of malate metabolism in mature NHL. The malate content and the enzyme activities declined within 1–2 d after a transfer of CHL from their growing temperature of 4 °C to 22 °C. The malate content was further increased when CHL were exposed to a higher light intensity at 4 °C. In CO2‐free air the malate content of CHL strongly declined at 4 °C. Malate may thus serve as an additional carbon sink and as a CO2‐store in CHL. It may further function as a vacuolar osmolyte balancing increased concentrations of soluble sugars previously observed in the cytosol of CHL. Malate was not used as a source of reductants when CHL were exposed to photo‐oxidative stress by treatment with paraquat. However, the activities of enzymes of the oxidative pentose phosphate pathway were markedly increased in CHL and may serve as non‐photosynthetic sources of NADPH and thus contribute to the previously observed superior capacity of CHL of winter rye to maintain their antioxidants in a reduced state in the presence of paraquat. [ABSTRACT FROM PUBLISHER]

The developmental profile of the activities of some enzymes involved in malate metabolism, namely phosphoenolpyruvate carboxylase (PEPC; EC 4. 1. 1. 31), NAD+‐linked (EC 1. 1. 1. 37) and NADP+‐linked (EC 1. 1. 1. 82) malate dehydrosenase (MDH), NAD+linked (EC 1. 1. 1. 39) and NADP+‐linked (EC 1. 1. 1. 40) malic enzyme (ME), has been determined in leaves of peach [Prunus persica (L.) Batsch cv. Maycrest], a woody C3 species. In order to study the role of these enzymes, their activities were related to developmental changes of photosynthesis, respiration, and capacity for N assimilation. Activities of PEPC, NAD(P)+‐MDH and NADP+‐ME were high in young expanding leaves and decreased 2‐ to 3‐fold in mature ones, suggesting that such enzymes play some role during the early stages of leaf expansion. In leaves of peach, such a role did not seem to be linked to C3 photosynthesis or nitrate assimilation, in that photosynthetic O2 evolution and activities of nitrate reductase (EC 1. 6. 6. 1) and glutamine synthetase (EC 6. 3. 1. 2) increased during leaf development. In contrast, leaf respiration strongly decreased with increasing leaf age. We suggest that in expanding leaves of this woody species the enzymes associated with malate metabolism have anaplerotic functions, and that PEPC may also contribute to the recapture of respiratory CO2. [ABSTRACT FROM AUTHOR]

The production of biomass, polysaccharide storage material and H from malate was studied in the wild-type and mutants RdcI, RdcII and RdcI/cII of Rhodobacter capsulatus. The mutants are defective in either copy I, copy II or both copies of the nitrogenase genes nifA and nifB. Stationary phase levels of biomass, polysaccharide and H were determined in phototrophic batch cultures grown with 30 mM of d,l-malate and either 2, 5, or 8 mM of ammonium or 7 mM of glutamate. Calculation of the amounts of malate converted into the three products revealed that, at 8 mM of ammonium and 7 mM of glutamate, malate consumption and product formation were balanced. But with decreasing ammonium concentrations malate not converted into biomass was utilized with decreasing efficiency in polysaccharide and H formation. This suggests formation of unknown products at the lower ammonium concentrations. Under conditions of optimal N supply, 80% of the malate not used for biomass production was converted by the wild-type and strain RdcII to H and CO. Mutant RdcI exhibited slightly decreased H production. The double mutant did not evolve H but accumulated increased amounts of polysaccharide. However, the amounts of polysaccharide were lower than should be expected if all of the spare malate, not utilized by the double mutant for H production, was converted into storage material. This and incomplete conversion of malate into known products at low ammonium supplies suggests that polysaccharide accumulation does not compete with the process of H formation for malate. [ABSTRACT FROM AUTHOR]

Malic enzyme of the phototrophic bacterium Chromatium vinosum strain D that lacks malate dehydrogenase was partially purified yielding a specific activity of 55 units/mg protein. The constitutive enzyme with a molecular weight of 110,000 and a pH optimum of 8.0 was absolutely dependent on the presence of a monovalent cation (NH, K, Cs, or Rb) as well as a divalent cation (Mn, or Mg). The enzyme was inhibited by oxaloacetate, glyoxylate, and NADPH. The K value for L-malate and the inhibition constants for oxaloacetate and glyoxylate are dependent on the concentration of the monovalent cation, whereas the K value for NADP (18 μM) and the K value for NADPH (42 μM) are independent. Throughout all kinetic measurements hyperbolic saturation curves and linear double reciprocal plots were obtained. [ABSTRACT FROM AUTHOR]

Organic acid is an important indicator of fruit quality, and malate is the predominant organic acid in apple fruit. However, the regulation of malate metabolism in postharvest fruit is rarely reported. Here, we found that, compared with a control treatment, a 10 mM γ-aminobutyric acid (GABA) treatment remarkably delayed the loss of tiftratable acidity and malate and increased the succinate and oxalate contents in "Cripps Pink" fruit stored in polyethylene bags at room temperature. The higher malate levels in GABA-treated fruit were accompanied by higher activities of cytosolic nicotinamide adenine dinucleotide-dependent malate dehydrogenase (cyNAD-MDH) and phosphoenolpyruvate carboxylase (PEPC) but lower cytosolic NAD phosphate-dependent malic enzyme (cyNADP-ME) and phosphoenolpyruvate carboxykinase (PEPCK) activities than those seen in control fruit. Notably, ethylene production was significantly reduced by GABA treatment, paralleling the downregulation of MdACS, MdACO, and MdERF expression. Meanwhile, GABA treatment also enhanced the activity of the GABA shunt and promoted the accumulation of GABA. This study provides new insights into the regulation of malate metabolism and reports for the first time the possible interplay between GABA and ethylene signaling pathways in apple fruit during postharvest storage.