Polyphenol Oxidase Gene Expression Research Articles

A comprehensive Study of Juglone's Effect on Polyphenol Oxidase in Cucumber: In Vitro Experiments and Computational Docking and Dynamics Insights.

This study explores the impact of juglone on cucumber (Cucumis sativus cv. Beith Alpha), scrutinizing its effects on seed germination, growth, and the polyphenol oxidase (PPO) enzyme's activity and gene expression. Employing concentrations ranging from 0.01 to 0.5 mM, we found juglone's effects to be concentration-dependent. At lower concentrations (0.01 and 0.1 mM), juglone promoted root and shoot growth along with germination, whereas higher concentrations (0.25 and 0.5 mM) exerted inhibitory effects, delineating a threshold for its allelopathic influence. Notably, PPO activity surged, especially at 0.5 mM in roots, hinting at oxidative stress involvement. Real-time PCR unveiled that juglone modulates PPO gene expression in cotyledons, peaking at 0.1 mM and diminishing at elevated levels. Correlation analyses elucidated a positive link between juglone-induced root growth and cotyledon PPO gene expression but a negative correlation with heightened root enzyme activity. Additionally, germination percentage inversely correlated with root PPO activity, while PPO activities positively associated with dopa and catechol substrates in both roots and cotyledons. Molecular docking studies revealed juglone's selective interactions with PPO's B chain, suggesting regulatory impacts. Protein interaction assessments highlighted juglone's influence on amino acid metabolism, and molecular dynamics indicated juglone's stronger, more stable binding to PPO, inferring potential alterations in enzyme function and stability. Conclusively, our findings elucidate juglone's dose-dependent physiological and biochemical shifts in cucumber plants, offering insights into its role in plant growth, stress response, and metabolic modulation.

Toxicity of bisphenol A and p-nitrophenol on tomato plants: Morpho-physiological, ionomic profile, and antioxidants/defense-related gene expression studies.

Bisphenol A (BPA) and p-nitrophenol (PNP) are emerging contaminants of soils due to their wide presence in agricultural and industrial products. Thus, the present study aimed to integrate morpho-physiological, ionic homeostasis, and defense- and antioxidant-related genes in the response of tomato plants to BPA or PNP stress, an area of research that has been scarcely studied. In this work, increasing the levels of BPA and PNP in the soil intensified their drastic effects on the biomass and photosynthetic pigments of tomato plants. Moreover, BPA and PNP induced osmotic stress on tomato plants by reducing soluble sugars and soluble proteins relative to control. The soil contamination with BPA and PNP treatments caused a decline in the levels of macro- and micro-elements in the foliar tissues of tomatoes while simultaneously increasing the contents of non-essential micronutrients. The Fourier transform infrared analysis of the active components in tomato leaves revealed that BPA influenced the presence of certain functional groups, resulting in the absence of some functional groups, while on PNP treatment, there was a shift observed in certain functional groups compared to the control. At the molecular level, BPA and PNP induced an increase in the gene expression of polyphenol oxidase and peroxidase, with the exception of POD gene expression under BPA stress. The expression of the thaumatin-like protein gene increased at the highest level of PNP and a moderate level of BPA without any significant effect of both pollutants on the expression of the tubulin (TUB) gene. The comprehensive analysis of biochemical responses in tomato plants subjected to BPA and PNP stress illustrates valuable insights into the mechanisms underlying tolerance to these pollutants.

Elicitation of salicylic acid and methyl jasmonate provides molecular and physiological evidence for potato susceptibility to infection by Erwinia carotovora subsp. carotovora

Among the range of severe plant diseases, bacterial soft rot caused by Erwinia carotovora is a significant threat to crops. This study aimed to examine the varying response patterns of distinct potato cultivars to the influence of E. carotovora. Furthermore, it seeks to highlight the potential role of salicylic acid (SA) and methyl jasmonate (MeJA) in stimulating the antioxidant defence system. We collected eight bacterial isolates from diseased and rotted tubers which were morphologically and physiologically identified as E. carotovora subsp. carotovora. We conducted a greenhouse experiment to analyse the antioxidant responses of three different potato cultivars (Diamont, Kara, and Karros) at various time intervals (2, 4, 6, 8, 12, and 24 h) after bacterial infection (hpi). We assessed the extent of disease damage by applying a foliar spray of 0.9 mM salicylic acid (SA) and 70 μM methyl jasmonate (MeJA). Inoculating with Ecc led to an increase in total phenolic levels, as well as the activities and gene expression of phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO) and peroxidase (POX) as time progressed. Additionally, the application of SA and MeJA resulted in a further increase relative to the diseased treatments. The Karros cultivar, unlike the Diamont and Kara cultivars, demonstrated the highest expression levels of PAL, PPO and POX through inoculation, reflecting its higher levels of activity and resistance. Furthermore, the genetic response of potato cultivars to infection at 0 hpi varied depending on their susceptibility. The examination of the rate of PAL activity upregulation following SA or MeJA stimulation clarifies the cultivars' susceptibility over time. In conclusion, the study identified E. carotovora subsp. carotovora as the most virulent isolate causing soft rot disease in potato tubers. It further revealed that the Karros cultivar displayed superior resistance with high activities and gene expression of PAL, PPO and POX, while the cv. Diamont exhibited sensitivity. Additionally, foliar exposure to SA and MeJA induced antioxidant responses, enhancing the potato plants' resistance against Ecc pathogenesis and overall plant defence.

Transcriptome analysis reveals the mechanism of polyethylene atmosphere packaging delaying browning of postharvest Pleurotus eryngii

To elucidate the mechanism of low-density polyethylene(LDPE) atmosphere packaging on delaying browning of Pleurotus eryngii under a postharvest low-temperature storage, this study focused on exploring the effects of two thicknesses (0.02 and 0.05 mm) LDPE packaging on some antioxidant enzymes, and stress markers in P. eryngii. The results showed that LDPE atmosphere packaging maintained the phenolic content and higher superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase (CAT) levels, reduced malondialdehyde (MDA) content, electrolyte leakage rate and polyphenol oxidase (PPO) levels in the mushroom. Also, transcriptome analyses showed that LDPE atmosphere packaging regulated the expression of enzymes PPO, POD, CAT and APX-related genes, inhibited the expression of lipoxygenases (LOX) and phospholipase D (PLD)-related genes and regulated the l-glutathione (GSH) antioxidant system. Overall, LDPE atmosphere packaging delayed the reduction of unsaturated fatty acids and energy loss by maintaining the integrity of cell membranes and regulating important genes involved in membrane lipid metabolism, energy metabolism, and respiratory metabolism of P. eryngii. In addition, it activated the expression of genes related to GSH synthesis and metabolism, thus enhancing the antioxidant capacity of mushroom body, reducing the accumulation of ROS and delaying browning.

Stress response of fresh-cut potatoes to laser irradiation before processing can prevent discoloration and maintain overall quality

Browning is accepted as a main factor affecting quality of fresh cut products. Physical treatments maintaining quality of fresh-cut products have been recently considered by consumer safer than chemical treatments. This study investigated whether preprocess diode laser irradiation can improve the physicochemical quality of fresh-cut potatoes. Laser irradiation at a 450-nm wavelength for 30 min prevented the browning of potato slices during short-term storage at 25 °C. Compared with no treatment, preprocess laser irradiation preserved the appearance and whiteness and inhibited browning and delayed weight loss in potato slices when refrigerated at 4 °C for 96 h. The increases in the total phenol concentration and phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO), and peroxidase (POD) activities of fresh-cut potatoes were diminished after laser irradiation. Moreover, the laser irradiation also enhanced antioxidant activities in the samples. The expression of PAL, PPO and POD genes was suppressed by the laser treatment. Therefore, the laser irradiation at 450 nm wavelength for 30 min effectively eliminates discolouration and improves quality of sliced potatoes during cold storage.

Transcriptional and biochemical profiling of defense enzymes in Citrus sinensis during salicylic acid and cinnamon mediated suppression of green and blue mold.

Green and blue mold of citrus are threatening diseases that continuously inflict economic post-harvest loss. The suppressive effect of salicylic (SA) and Cinnamomum verum (CV) on green and blue mold of sweet oranges was investigated in this study. Among five tested plant extracts methanolic extract of Cinnamon caused the highest colony growth inhibition of P. digitatum and P. italicum in an in vitro antifungal assay.The methanolic extract of Cinnamon in combination with SA showed the lowest disease incidence and severity of green and blue mold on citrus fruit without affecting the fruit quality. Transcriptional profiling of defense enzymes revealed that the polyphenol oxidase (PPO), phenylalanine ammonia-lyase (PAL), and peroxidase (POD) genes were upregulated in fruit treated with CV, SA, and their combination compared to the control. The treatment SA+CV caused the highest upsurge in PPO, POD, and PAL gene expression than the control. Furthermore, the biochemical quantification of PPO, POD and PAL also revealed a similar pattern of activity. The present findings unravel the fact that the escalation in the activity of tested defense enzymes is possibly associated with the reduced incidence of blue and green molds. In conclusion, the study unveils the promising suppressive potential of SA+CV against green and blue mold by regulating the expression of PPO, POD, and PAL genes. Therefore, these treatments can find a role as safer alternatives to chemicals in the management of post-harvest green and blue mold.

Physiological and Molecular Responses of Vitis vinifera cv. Tempranillo Affected by Esca Disease.

Esca is a multi-fungal disease affecting grapevines. The objective of the study was to evaluate the physiological and molecular response of the grapevine cv. Tempranillo to esca disease, carried out in a vineyard under Mediterranean climatic conditions in western Spain. The photosynthetic pigments in the leaves decreased, with a strong decrease in the photosynthetic efficiency. The proline content increased significantly in the early stages of affected leaves, being possibly involved in the maintenance of lipid peroxidation levels in leaves, which do not increase. The phenol, flavonoid, and phenylpropanoid content decreased in esca-affected leaves, as does the total antioxidant capacity (FRAP), while the polyphenol oxidase (PPO) activity suffers a strong increase with the development of the disease. In affected grapes, the lipid peroxidation and the total phenol content decrease, but not the anthocyanin content. The ascorbate pool decreases with the disease and with time. On the other hand, pool GSH + GSSG is lower in affected leaves, but increases with time. These alterations show a clear change in the redox homeostasis. The expression of genes phenylalanine ammonia lyase (PAL), polyphenol oxidase (PPO), superoxide dismutase (SOD), and chalcone synthase (ChaS1 and ChaS3) become considerably higher in response to esca, being even higher when the infection time increases. The alteration of AsA and GSH levels, phenolic compounds, PPO activity, proline content, and FRAP, together with the increase of the PAL, PPO, SOD, ChaS1, and ChaS3 gene expression, are clearly implicated in the esca response in plants. The expression of these genes, similar to the PPO activity, can be used as markers of state in the development of the disease.

Mitigating chilling injury of pomegranate fruit skin

Pomegranate cv. Wonderful fruit are susceptible to chilling injury of the peel (CIp) when stored at 7 °C in modified-atmosphere bags for more than 3 months. The damage, manifested as superficial browning, appears first on the fruit skin, i.e., the outer colored layer of the peel. We previously suggested that susceptibility to CIp is associated with downregulation of PHENYLALANINE AMMONIA-LYASE (PAL) in storage, whereas POLYPHENOL OXIDASE (PPO) expression is upregulated. To further explore the role of PPO and PAL in CIp disorder, we monitored their gene expression and enzyme activity in fruit skin collected at three phases of fruit development: premature fruit 7 days before harvest when the fruit is susceptible to CIp disorder; during commercial harvest when fruit skin is aging; and during the following 4 months of storage. In addition, prestorage washing of the fruit with CaCl2 and citric acid solutions, previously shown to reduce CIp incidence in pomegranate and other fruit and to manipulate the PAL-to-PPO ratio, was applied. We determined total phenol content and antioxidative capacity of fruit skin during cold storage, and analyzed the expression of antioxidation-related genes CATALASE, SUPEROXIDE DISMUTASE, and GLUTATHIONE REDUCTASE, and key anthocyanin-biosynthesis genes CHALCONE SYNTHASE, CHALCONE ISOMERASE, and DIHYDROFLAVONOL 4-REDUCTASE. Results suggested that increased skin susceptibility to CIp is related to high total phenol content, relatively high PAL activity/gene expression, and low anthocyanin levels. Although PPO activity and gene expression increased in cold storage, this pattern was observed in all tissues tested, irrespective of their susceptibility to CIp development. Similarly, no association was evident between antioxidation-related genes’ expression levels and CIp susceptibility. On the other hand, pre-storage treatments with CaCl2 and citric acid mainly decreased CIp incidence associated with increased antioxidation-related gene expression. Hence, the mechanism governing mitigation of CIp development by prestorage washing with CaCl2 and citric acid differs from that observed in native CIp-resistant pomegranate skin.

Application of gamma irradiation on morphological, biochemical, and molecular aspects of wheat (Triticum aestivum L.) under different seed moisture contents

Nuclear technology is currently used as a tool in mutation breeding to improve crops by increasing genetic variation. The ionization of gamma rays produces large amounts of free radicals, simulating stressors in the natural environment. To avoid gamma-ray-induced oxidative stress, plants use antioxidant defense systems. Exposure of plants to irradiation can affect the germination, growth, and production of metabolites. Plants' sensitivity to irradiation depends on genetic and environmental factors such as moisture content. For this purpose, the effects of different gamma irradiation doses [0, 100, 200, 300, and 400 Gray (Gy)] and different seed moisture contents (7, 13, and 19%) on traits such as seed germination, seedling growth, molecular and biochemical alterations in antioxidant enzymes were examined in the current study. Based on the results, the highest seed germination percentage was observed in the interaction effect of seed moisture at 13% with an irradiation dose of 400 Gy (98.89%). Seedling survival percent and seedling length decreased with increasing doses of gamma irradiation at different seed moisture contents. Increasing gamma irradiation doses were reduced root and stem fresh and dry weight, and root and stem length. The highest level of catalase enzyme activity and expression was observed at 200 and 300 Gy irradiation doses at different moisture contents. The peroxidase and polyphenol oxidase gene expression were reduced at all contents of gamma irradiation doses and seed moisture compared to the control. It can be concluded that the dose of 200–300 Gy of gamma irradiation reduced plant growth by 30% in terms of fresh and dry weight and length of plants, as well as enhanced the expression of antioxidant enzymes. The results of this study could help plant breeders select an appropriate dose rate in wheat for further research.

Hydrogen peroxide regulated salicylic acid– and jasmonic acid–dependent systemic defenses in tomato seedlings

To investigate the regulation of hydrogen peroxide (H2O2) in SA- and JA-dependent systemic defense pathways in tomato seedlings damaged by cotton bollworm. The lower two leaves of four-leaf tomato plant seedlings were treated with cotton bollworms. The upper leaves were then removed at various time points and assessed for systemic changes in H2O2 levels, enzyme activity and defense-related genes expression. Wild-type seedlings, def-5 mutant seedlings (deficient in JA accumulation) and transgenic nahG seedlings(deficient in SA content) were constructed at four-leaf stage. Wild-type plants were further treated with dimethylthiourea (DMTU) and 2,5-dihydroxycinnamic acid methyl ester (DHC). Bollworm feeding in the lower leaves induced rapid systemic accumulation of H2O2 in the upper leaves. H2O2 accumulation further increased activities for SA- and JA-dependent enzymes including proteinase inhibitors (PIs), pathogenesis-related gene 1 (PR-1), cathepsin D inhibitor (CDI), β-1,3-glucanse (BGL) and polyphenol oxidase (PPO). Furthermore, H2O2 accumulation enhanced mRNA expression of BGL-2, PR-1, PI, CDI and PPO genes. Suppression of H2O2 accumulation using H2O2 scavengers substantially diminished these effects. mRNA expression of PR-1 was not induced in transgenic nahG plants. SA- and JA-dependent signaling pathways are involved in the tomato systemic defense responses to herbivores, and that H2O2 generation is required for both systemic pathways.

Analysis of differentially expressed genes and differentially abundant metabolites associated with the browning of Meihong red-fleshed apple fruit

Enzymatic browning affects the quality of fresh-cut produce. To elucidate the mechanism regulating the browning of red-fleshed apple, we analyzed the transcriptome and metabolome of freshly cut Meihong apple samples collected at 0, 0.5, and 4 h after cutting. The main differentially abundant metabolites during the browning process were phenols, amino acids and fatty acids. Phenols are substrates for enzymatic browning and most of them are negatively correlated with browning. Amino acids and their derivatives are associated with non-enzymatic browning and most of them are positively correlated with browning. A transmission electron microscopy analysis revealed the degradation of the cell membrane of cut fruit samples, which significantly affected the fatty acid content. The differentially expressed genes were mainly related to plant–pathogen interactions. Polyphenol oxidase (PPO) plays a major role during the early browning stage, but PPO gene expression was not higher than WRKY, AP2 and MYB transcription factor.

Understanding the Browning Reactions in Sweet Basil

Polyphenol oxidase (PPO) is a well-characterized enzyme that is conserved across plant species and is responsible for catalyzing the rapid polymerization of o-quinones that results in tissue browning. As these oxidation reactions result in significant reductions in food crop commercial viability, nutritional content, and general desirability, an opportunity exists to improve a variety of crops via genetic regulation of PPO expression. The main objective of this project was to clone the DNA sequence of genes encoding PPO. Using mRNA as a template to synthesize DNA, two putative PPO sequences were cloned from Ocimum basilicum and Ocimum americanum, two basil species of commercial relevance. A secondary objective of this project was to insert a cloned PPO sequence into an RNAi vector for the purpose of making transgenic basil plants with reduced PPO expression. A hairpin RNAi construct was successfully developed to reduce the expression of both PPO genes as their DNA sequences were highly homologous. The next step for this project will depend on the development of transformation and regeneration protocols for these species, which have proven to be difficult. Once the RNAi PPO constructs are introduced into basil cells and successfully regenerated into plants, reduced browning of basil leaf tissue when it is wounded or cut is expected.

MicroRNA528, a hub regulator modulating ROS homeostasis via targeting of a diverse set of genes encoding copper-containing proteins in monocots.

Plant microRNAs (miRNAs) regulate vital cellular processes, including responses to extreme temperatures with which reactive oxygen species (ROS) are often closely associated. In the present study, it was found that aberrant temperatures caused extensive changes in abundance to numerous miRNAs in banana fruit, especially the copper (Cu)-associated miRNAs. Among them, miR528 was significantly downregulated under cold stress and it was found to target genes encoding polyphenol oxidase (PPO), different from those identified in rice and maize. Expression of PPO genes was upregulated by >100-fold in cold conditions, leading to ROS surge and subsequent peel browning of banana fruit. Extensive comparative genomic analyses revealed that the monocot-specific miR528 can potentially target a large collection of genes encoding Cu-containing proteins. Most of them are actively involved in cellular ROS metabolism, including not only ROS generating oxidases, but also ROS scavenging enzymes. It also was demonstrated that miR528 has evolved a distinct preference of target genes in different monocots, with its target site varying in position among/within gene families, implying a highly dynamic process of target gene diversification. Its broad capacity to target genes encoding Cu-containing protein implicates miR528 as a key regulator for modulating the cellular ROS homeostasis in monocots.

Effect of methyl jasmonate on wound healing and resistance in fresh-cut potato cubes

The aim of this study was to investigate the effects of methyl jasmonate (MeJA) treatment on physiology and quality attributes of fresh-cut potato cubes during storage. The fresh-cut potato cubes treated with MeJA showed occurrence of enzymatic browning, with higher total phenol and flavonoid content than the control cubes during 144 h of storage at room temperature (23 ± 1 °C). The activity of polyphenol oxidase (PPO), peroxidase (POD) and catalase (CAT) in fresh-cut potato cubes have also been increased significantly by MeJA treatment. MeJA induced an increase of activity in phenylalnine ammonialyase (PAL), cinnamate-4-hydroxylase (C4H) and 4-coumarate-CoA ligase (4CL) of fresh-cut potato cubes, compared to the control. RT-qPCR showed that the gene expression of PAL, C4H, 4CL, PPO, POD and CAT was significantly higher in MeJA-treated cube parenchyma tissues than that in control. Our experiments also showed that MeJA treatment could maintain firmness but enhanced the browning of the fresh-cut cubes. In addition, the parenchyma tissues of MeJA-treated fresh-cut cubes also showed higher suberin polyphenolic (SPP) and thicker cell wall than that in the control.

Low-temperature conditioning enhances chilling tolerance and reduces damage in cold-stored eggplant (Solanum melongena L.) fruit

Eggplant fruit are vulnerable to chilling injury (CI) after prolonged storage at temperatures below 12 °C. Here, we tested the effect of low-temperature conditioning (LTC) on chilling tolerance in eggplant fruit, in which fruit were stored at 13 °C for 2 d, and then held at 4 °C. Our results indicated that LTC maintained the appearance of eggplant fruit, and retarded the development of CI; the effect of LTC was characterized by inhibited sepal browning, and reduced loss of anthocyanins, and total phenolic compounds. The activity and relative gene expression of peroxidase (POD) and catalase (CAT) were increased by LTC, whereas the activity and relative gene expression of polyphenol oxidase (PPO) was reduced. In addition, the level of malondialdehyde (MDA) was significantly reduced. Our results suggest that LTC induces a range of physiological and molecular responses that enhance chilling tolerance in eggplant fruit.

Effects of UV-C treatment on browning and the expression of polyphenol oxidase (PPO) genes in different tissues of Agaricus bisporus during cold storage

Polyphenol oxidase (PPO) is the key enzyme leading to browning of Agaricus bisporus (A. bisporus). In order to determine the influence of UV-C treatment on polyphenol oxidase (PPO) of A. bisporus (Cv, 192), the visual quality and color, degree of browning, PPO activity and the expression of PPO genes in different tissues of A. bisporus during cold storage after 1.0 kJ/m2 UV-C treatment were analysed. Results showed that the degree of browning and PPO activity of A. bisporus increased during 21 d of storage at 4 °C. UV-C treatment increased the PPO activity and exacerbated the browning of cap surface of A. bisporus during the beginning period of the storage. However, the degree of browning and the PPO activity in pileus, stipe and gill of A. bisporus were reduced by UV-C treatment. The expression of PPO genes (AbPPO2, AbPPO3 and AbPPO4) in the peel of A. bisporus was stimulated by UV-C treatment during storage at 4 °C, while they were inhibited in pileus, stipe and gill. The patterns of AbPPO2, AbPPO3 and AbPPO4 transcription level in different tissues of A. bisporus were similar to that of the degree of browning and PPO activity. AbPPO1 gene transcription was not in accordance with the degree of browning and PPO activity. UV-C treatment inhibited browning of A. bisporus fruit body (pileus, stipe and gill) and the expression of AbPPO genes.

In vitro response of Colletotrichum to chitosan. Effect on incidence and quality on tropical fruit. Enzymatic expression in mango

Colletotrichum is considered one of the fungal genera with the greatest diversity of species of phytopathogenic fungi and with a wide range of hosts including tropical fruits. In this study, the concentration of chitosan applied was a key factor in the in vitro inhibition of the three Colletotrichum isolates with the most sensitive being the one from banana. Germination was the development stage most affected by the application of this polymer. At the end of 10 days of storage, control of anthracnose in soursop, mango and banana fruit was considerable with the application of 1 % chitosan, with 80 to 100 % inhibition. Likewise, the ripening process of fruit with and without chitosan was generally similar. Activity of the peroxidase (POD) and polyphenol oxidase (PPO) enzymes was observed only in fruit of mango cv. 'Tommy Atkins,' inoculated and non-inoculated with C. gloeosporioides, the activity of both enzymes was higher in the chitosan treatments and the 1.0 % concentration was able to induce gene expression of POD and PPO, occurring until 24 h.

Methyl jasmonate delays postharvest ripening and senescence in the non-climacteric eggplant (Solanum melongena L.) fruit

Physiological disorders in harvested eggplant (Solanum melongena L.) fruit result in quality deterioration, short postharvest life and significant economic losses. In this study, the effects of methyl jasmonate (MeJA) on senescence in eggplant fruit were investigated. The results indicated that, compared to control, MeJA at 5μM depressed ethylene production and effectively delayed fruit senescence. MeJA treatment was beneficial for maintaining fruit quality during storage, inhibiting increase of the calyx browning and weight loss, and retarding decline of sensorial quality, firmness and the anthocyanin content in peel. Moreover, MeJA effectively reduced the total phenolics content and the activity of the polyphenol oxidase (PPO) in calyx and pulp. MeJA also inhibited relative gene expression of PPO in pulp. The activities and gene expression of peroxidase and catalase were significantly induced by MeJA treatment. These findings indicate that the delayed senescence in eggplant fruit by MeJA is related with the decrease of enzymes associated with phenolics metabolism and antioxidant system. Our results suggest that MeJA has potential effect on maintaining the quality and extending postharvest life of eggplant fruit.

Induction of Polyphenol Oxidase in Walnut and Its Relationship to the Pathogenic Response to Bacterial Blight

Plants respond to pathogens with both active and passive defense mechanisms. These defense responses include the induction of defense or defense-related genes such as polyphenol oxidase (PPO) and pathogenesis-related (PR) proteins. The role of PPO in the interaction between bacterial blight [Xanthomonas arboricola pv. juglandis (Xaj)] and walnut (Juglans regia) was studied. JrPPO-1 and P14a genes were identified in two walnut cultivars, Chandler and Serr, using standard polymerase chain reaction (PCR) to understand their inducible ability in response to Xaj. ‘Serr’ and ‘Chandler’ were inoculated with Xaj strain 417. PPO activity in leaves was assayed at 0, 24, 72, 96, 120, and 144 hours after inoculation. Results showed a steady increase in activity commencing within 24 hours of inoculation. Increase in PPO activity was close to 2-fold greater in ‘Chandler’ than in ‘Serr’ at all time points examined. Real-time PCR analysis showed differences between cultivars in PPO gene expression. The JrPPO-1 gene was highly expressed in both cultivars 24 hours after inoculation but expression in ‘Serr’ was much greater than in ‘Chandler’. Significant expression of P14a gene was observed in both cultivars within 24 hours. Expression in ‘Serr’ was strong and maximized with a significant increase at 96 hours. Expression in ‘Chandler’ was far weaker than ‘Serr’ at 24 hours and did not increase further. Our results imply that the walnut–bacterial blight interaction induces the expression of JrPPO-1 and P14a as well as the activity of PPO.

Impacts on the metabolome of down-regulating polyphenol oxidase in potato tubers.

Tubers of potato (Solanum tuberosum L. cv. Estima) genetically modified to reduce polyphenol oxidase (PPO) activity and enzymatic discolouration were assessed for changes in the metabolome using Liquid Chromatography-Mass Spectrometry (LC-MS) and Gas Chromatography (GC)-MS. Metabolome changes induced over a 48 hour (h) period by tuber wounding (sliced transverse sections) were also assessed using two PPO antisense lines (asPPO) and a wild-type (WT) control. Data were analysed using Principal Components Analysis and Analysis of Variance to assess differences between genotypes and temporal changes post-tuber wounding (by slicing). The levels of 15 metabolites (out of a total of 134 that were detected) differed between the WT and asPPO lines in mature tubers at harvest. A considerably higher number (63) of these metabolites changed significantly over a 48 h period following tuber wounding. For individual metabolites the magnitude of the differences between the WT and asPPO lines at harvest were small compared with the impacts of tuber wounding on metabolite levels. Some of the observed metabolite changes are explicable in terms of pathways known to be affected by wound responses. Whilst some statistically significant interactions (11 metabolites) were observed between line and time after wounding, very few profiles were consistent when comparing the WT with both asPPO lines, and the underlying metabolites appeared to be random in terms of the pathways they occupy. Overall, mechanical damage to tubers has a considerably greater impact on the metabolite profile than any potential unintended effects resulting from the down-regulation of PPO gene expression.