Polyphenol oxidase and photosynthesis research

The PPO family in Nicotiana tabacum is an important regulator to participate in pollination

Polyphenol oxidases (PPOs) are type-3 copper enzymes and are involved in many biological processes. However, the potential functions of PPOs in pollination are not fully understood. In this work, we have screened 13 PPO members in Nicotiana. tabacum (named NtPPO1-13, NtPPOs) to explore their characteristics and functions in pollination. The results show that NtPPOs are closely related to PPOs in Solanaceae and share conserved domains except NtPPO4. Generally, NtPPOs are diversely expressed in different tissues and are distributed in pistil and male gametes. Specifically, NtPPO9 and NtPPO10 are highly expressed in the pistil and mature anther. In addition, the expression levels and enzyme activities of NtPPOs are increased after N. tabacum self-pollination. Knockdown of NtPPOs would affect pollen growth after pollination, and the purines and flavonoid compounds are accumulated in self-pollinated pistil. Altogether, our findings demonstrate that NtPPOs potentially play a role in the pollen tube growth after pollination through purines and flavonoid compounds, and will provide new insights into the role of PPOs in plant reproduction.

Effect of Smut Infection on the Photosynthetic Physiological Characteristics and Related Defense Enzymes of Sugarcane

Pathogen infection seriously affects plant development and crop productivity, sometimes causing total crop failure. In this study, artificial stab inoculation was used to inoculate sugarcane smut. The changes in leaf gas exchange, chlorophyll fluorescence variables, and related defense enzyme activities were measured in sugarcane cultivar ROC22 after pathogen infection. The results showed that the net photosynthetic rate (Pn), stomatal conductance (gs), and transpiration rate (Tr) downregulated in the first three days after smut infection and upregulated on the fourth day; intercellular CO₂ concentration (Ci) increased in the first three days of smut infection and reduced on the fourth day. The chlorophyll fluorescence parameters, i.e., Fo, Fm, Fv/Fm, Fs, and Fv′/Fm′ decreased at the initial stage of pathogen infection but increased rapidly up to 3 days after smut infection. It can be seen that sugarcane seedlings showed a positive response to pathogen infection. The correlation coefficient relationship between Pn, gs, and Tr reached above 0.800, showing a significant correlation; Ci was positively correlated with Fv′/Fm′ and ΦPSII, reaching above 0.800 and showing a significant correlation; Fo positively correlated with Fv/Fm, Fs, and ETR; Fv /Fm was positively correlated with Fv′/Fm′; Fs significantly correlated with Fv′/Fm′; and Fv′/Fm′ positively correlated with ΦPSII. After inoculation with smut, the related defense enzymes, i.e., POD, SOD, PPO, and PAL, were increased and upregulated; photosynthetic parameters can be associated with an increase in enzymatic activities. The results of this study will help to further study of the response mechanism to smut in the sugarcane growing period and provide a theoretical reference for sugarcane resistance to smut breeding.

Transcriptional and physiological analyses of reduced density in apple provide insight into the regulation involved in photosynthesis

Different densities have a great influence on the physiological process and growth of orchard plants. Exploring the molecular basis and revealing key candidate genes for different densities management of orchard has great significance for production capacity improvement. In this study, transcriptome sequencing of apple trees was carried out at three different sampling heights to determine gene expression patterns under high density(HD) and low density(LD) and the physiological indices were measured to determine the effect of density change on plants. As a result, physiological indexes showed that the content of Chlorophyll, ACC, RUBP and PEP in the LD was apparently higher than that in control group(high density, HD). While the content of PPO and AO in the LD was noticeably lower than that in the HD. There were 3808 differentially expressed genes (DEGs) were detected between HD and LD, of which 1935, 2390 and 1108 DEGs were found in the three comparisons(middle-upper, lower-outer and lower-inner), respectively. 274 common differentially expressed genes (co-DEGs) were contained in all three comparisons. Functional enrichment and KEGG pathway analysis found these genes were involved in Carbon fixation in photosynthetic organisms, Circadian rhythm, Photosynthesis - antenna proteins, Photosynthesis, chlorophyll metabolism, Porphyrin, sugar metabolism and so on. Among these genes, LHCB family participated in photosynthesis as parts of photosystem II. In addition, SPA1, rbcL, SNRK2, MYC2, BSK, SAUR and PP2C are involved in Circadian rhythm, the expression of genes related to glycometabolism and hormone signaling pathway is also changed. The results revealed that the decrease of plant density changed the photosynthetic efficiency of leaves and the expression of photosynthesis-related genes, which provide a theoretical basis for the actual production regulation of apples.

Correlation Between Polyphenol Oxidase (PPO) Activity and Total Phenolic Contents in Crocus sativus L. Corms During Dormancy and Sprouting Stages

Purification and characterization of polyphenol oxidase (PPO) enzyme and determination of total phenolic contents during dormancy and sprouting stages in Crocus sativus corms were performed. PPO enzyme was purified by ammonium sulfate precipitation and ion-exchange chromatography using DEAE-Sephadex A25 and two isoenzymes were obtained on the SDS-PAGE, which corresponded to molecular weights of 70 and 54 kDa. The Km values of the enzyme were 4.87 and 2.12 mM for l-DOPA in dormancy and waking stages, respectively. Also, enzyme showed higher Vmax values of 0.026 (ΔOD.min-1) in dormancy compared with the value of 0.019 (ΔOD.min-1) in waking corms. Results showed an inverse correlation between phenolic contents and PPO activity. Accordingly, it can be concluded that as plant progressed through sprouting stage, in contrast to polyphenol oxidase activity, there was a significant increase in total amount of phenolic compounds, as determined by Folin-Ciocalteu method and water and aqueous ethanol extractions.

SUMMARY

Purification of polyphenol oxidase enzyme using DEAE-Sephadex A25 in Crocus sativus corms.Characterization of polyphenol oxidase enzyme.Comparison of PPO enzyme characteristics in two different physiologic stages of dormancy and sprouting.Determination of phenolic contents.Correlation between phenolic contents and PPO activity during sprouting and dormancy. Abbreviations used: PPO: Polyphenol Oxidase, DEAE-Sephadex: Diethylaminoethyl Sephadex, SDS-PAGE: Sodium Dodecyl Sulfate- Polyacrylamide Gel Electrophoresis, DOPA: Dihydroxyphenylalanine, PEG: Polyethylene Glycol.

Polyphenol oxidase in leaves: is there any significance to the chloroplastic localization?

Polyphenol oxidase (PPO) catalyses the oxidation of monophenols and/or o-diphenols to o-quinones with the concomitant reduction of oxygen to water which results in protein complexing and the formation of brown melanin pigments. The most frequently suggested role for PPO in plants has been in defence against herbivores and pathogens, based on the physical separation of the chloroplast-localized enzyme from the vacuole-localized substrates. The o-quinone-protein complexes, formed as a consequence of cell damage, may reduce the nutritional value of the tissue and thereby reduce predation but can also participate in the formation of structural barriers against invading pathogens. However, since a sufficient level of compartmentation-based regulation could be accomplished if PPO was targeted to the cytosol, the benefit derived by some plant species in having PPO present in the chloroplast lumen remains an intriguing question. So is there more to the chloroplastic location of PPO? An interaction between PPO activity and photosynthesis has been proposed on more than one occasion but, to date, evidence either for or against direct involvement has been equivocal, and the lack of identified chloroplastic substrates remains an issue. Similarly, PPO has been suggested to have both pro- and anti-oxidant functions. Nevertheless, several independent lines of evidence suggest that PPO responds to environmental conditions and could be involved in the response of plants to abiotic stress. This review highlights our current understanding of the in vivo functions of PPO and considers the potential opportunities it presents for exploitation to increase stress tolerance in food crops.

Modes of action of microbially-produced phytotoxins

Some of the most potent phytotoxins are synthesized by microbes. A few of these share molecular target sites with some synthetic herbicides, but many microbial toxins have unique target sites with potential for exploitation by the herbicide industry. Compounds from both non-pathogenic and pathogenic microbes are discussed. Microbial phytotoxins with modes of action the same as those of commercial herbicides and those with novel modes of action of action are covered. Examples of the compounds discussed are tentoxin, AAL-toxin, auscaulitoxin aglycone, hydantocidin, thaxtomin, and tabtoxin.

Functional analysis of polyphenol oxidases by antisense/sense technology

Polyphenol oxidases (PPOs) catalyze the oxidation of phenolics to quinones, the secondary reactions of which lead to oxidative browning and postharvest losses of many fruits and vegetables. PPOs are ubiquitous in angiosperms, are inducible by both biotic and abiotic stresses, and have been implicated in several physiological processes including plant defense against pathogens and insects, the Mehler reaction, photoreduction of molecular oxygen by PSI, regulation of plastidic oxygen levels, aurone biosynthesis and the phenylpropanoid pathway. Here we review experiments in which the roles of PPO in disease and insect resistance as well as in the Mehler reaction were investigated using transgenic tomato (Lycopersicon esculentum) plants with modified PPO expression levels (suppressed PPO and overexpressing PPO). These transgenic plants showed normal growth, development and reproduction under laboratory, growth chamber and greenhouse conditions. Antisense PPO expression dramatically increased susceptibility while PPO overexpression increased resistance of tomato plants to Pseudomonas syringae. Similarly, PPO-overexpressing transgenic plants showed an increase in resistance to various insects, including common cutworm (Spodoptera litura (F.)), cotton bollworm (Helicoverpa armigera (Hübner)) and beet army worm (Spodoptera exigua (Hübner)), whereas larvae feeding on plants with suppressed PPO activity had higher larval growth rates and consumed more foliage. Similar increases in weight gain, foliage consumption, and survival were also observed with Colorado potato beetles (Leptinotarsa decemlineata (Say)) feeding on antisense PPO transgenic tomatoes. The putative defensive mechanisms conferred by PPO and its interaction with other defense proteins are discussed. In addition, transgenic plants with suppressed PPO exhibited more favorable water relations and decreased photoinhibition compared to nontransformed controls and transgenic plants overexpressing PPO, suggesting that PPO may have a role in the development of plant water stress and potential for photoinhibition and photooxidative damage that may be unrelated to any effects on the Mehler reaction. These results substantiate the defensive role of PPO and suggest that manipulation of PPO activity in specific tissues has the potential to provide broad-spectrum resistance simultaneously to both disease and insect pests, however, effects of PPO on postharvest quality as well as water stress physiology should also be considered. In addition to the functional analysis of tomato PPO, the application of antisense/sense technology to decipher the functions of PPO in other plant species as well as for commercial uses are discussed.

Induction and accumulation of polyphenol oxidase activities as implicated in development of resistance against pearl millet downy mildew disease

Polyphenol oxidase (PPO) activity was analysed in seedlings of resistant and susceptible pearl millet [Pennisetum glaucum (L.) R.Br] cultivars with or without inoculation of the downy mildew pathogen Sclerospora graminicola (Sacc.) Schroet. Seedlings of resistant varieties had greater PPO activity than susceptible seedlings, and inoculated seedlings had significantly higher PPO levels than uninoculated seedlings. Temporal accumulation of PPO showed a maximum activity at 24 h post-inoculation in resistant seedlings, whereas in susceptible seedlings it peaked at 48 h. PPO activity was positively correlated with levels of downy mildew resistance in different pearl millet cultivars under field conditions. Native PAGE staining showed four isoforms of PPO, which were differentially induced in relation to the time of appearance and intensities in the uninoculated seedlings, whereas a fifth PPO isoform appeared after inoculation with S. graminicola. PPO activity was significantly higher in the shoot and leaves of pearl millet than in the root. Tissue printing analysis of the enzyme expression showed that the enzyme is predominantly expressed after pathogen inoculation and is localised in the epidermal and vascular regions. Temporal analysis of transcript accumulation showed that in resistant seedlings PPO mRNAs was expressed earlier and more abundantly than in susceptible seedlings. Our studies demonstrate, for the first time, that PPO is actively involved in plant defence and can be used as a marker of resistance to downy mildew infection in pearl millet.

The studies of FT-IR and CD spectroscopy on catechol oxidase I from tobacco

A novel copper-containing enzyme named COI (catechol oxidase I) has been isolated and purified from tobacco by extracting acetone-emerged powder with phosphate buffer, centrifugation at low temperature, ammonium sulfate fractional precipitation, and column chromatography on DEAE-sephadex (A-50), sephadex (G-75), and DEAE-celluse (DE-52). PAGE, SDS-PAGE were used to detect the enzyme purity, and to determine its molecular weight. Then the secondary structures of COI at different pH, different temperatures and different concentrations of guanidine hydrochloride (GdnHCl) were studied by the FT-IR, Fourier self-deconvolution spectra, and circular dichroism (CD). At pH 2.0, the contents of both alpha-helix and anti-parallel beta-sheet decrease, and that of random coil increases, while beta-turn is unchanged compared with the neutral condition (pH 7.0). At pH 11.0, the results indicate that the contents of alpha-helix, anti-parallel beta-sheet and beta-turn decrease, while random coil structure increases. According to the CD measurements, the relative average fractions of alpha-helix, anti-parallel beta-sheet, beta-turn/parallel beta-sheet, aromatic residues and disulfide bond, and random coil/gamma-turn are 41.7%, 16.7%, 23.5%, 11.3%, and 6.8% at pH 7.0, respectively, while 7.2%, 7.7%, 15.2%, 10.7%, 59.2% at pH 2.0, and 20.6%, 9.5%, 15.2%, 10.5%, 44.2% at pH 11.0. Both alpha-helix and random coil decrease with temperature increasing, and anti-parallel beta-sheet increases at the same time. After incubated in 6 mol/L guanidine hydrochloride for 30 min, the fraction of alpha-helix almost disappears (only 1.1% left), while random coil/gamma-turn increases to 81.8%, which coincides well with the results obtained through enzymatic activity experiment.

Antisense downregulation of polyphenol oxidase results in enhanced disease susceptibility

Polyphenol oxidases (PPOs; EC 1.14.18.1 or EC 1.10.3.2) catalyze the oxidation of phenolics to quinones, highly reactive intermediates whose secondary reactions are responsible for much of the oxidative browning that accompanies plant senescence, wounding, and responses to pathogens. To assess the impact of PPO expression on resistance to Pseudomonas syringae pv. tomato we introduced a chimeric antisense potato PPO cDNA into tomato (Lycopersicon esculentum L.). Oxidation of caffeic acid, the dominant o-diphenolic aglycone of tomato foliage, was decreased ca. 40-fold by antisense expression of PPO. All members of the PPO gene family were downregulated: neither immunoreactive PPO nor PPO-specific mRNA were detectable in the transgenic plants. In addition, the antisense PPO construct suppressed inducible increases in PPO activity. Downregulation of PPO in antisense plants did not affect growth, development, or reproduction of greenhouse-grown plants. However, antisense PPO expression dramatically increased susceptibility to P. syringae expressing the avirulence gene avrPto in both Pto and pto backgrounds. In a compatible (pto) interaction, plants constitutively expressing an antisense PPO construct exhibited a 55-fold increase in bacterial growth, three times larger lesion area, and ten times more lesions cm(-2) than nontransformed plants. In an incompatible (Pto) interaction, antisense PPO plants exhibited 100-fold increases in bacterial growth and ten times more lesions cm(-2) than nontransformed plants. Although it is not clear whether hypersusceptibility of antisense plants is due to low constitutive PPO levels or failure to induce PPO upon infection, these findings suggest a critical role for PPO-catalyzed phenolic oxidation in limiting disease development. As a preliminary effort to understand the role of induced PPO in limiting disease development, we also examined the response of PPO promoter::beta-glucuronidase constructs when plants are challenged with P. syringae in both Pto and pto backgrounds. While PPO B inducibility was the same in both compatible and incompatible interactions, PPO D, E and F were induced to higher levels and with different expression patterns in incompatible interactions.

The purification of polyphenol oxidase from tobacco

A new polyphenol oxidase (PPO) named PPO II was purified from tobacco (Nicotiana tobacum) by using acetone powder, ammonium sulfate precipitation, and column chromatography on DEAE-Sephadex A-50, Sephadex G-75, and CM-Sephadex C-50. It has an active site of a pair of type 3 coppers bridged to phenolate oxygen, which represents a new catalytic mechanism for polyphenol oxidase. PAGE, SDS-PAGE, and matrix-assisted laser desorption/ionization-time of flight mass spectrometry of the purified enzyme demonstrated that the enzyme is a single band with a molecular mass 35,600 Da. Biochemical characteristics include the optimum pH at 6.0, optimum temperature at 40 degrees C, and K(m) of 1.2 mM for catechol as substrate (pH 6.5, 30 degrees C). Substrate specificity studies indicate that the enzyme is of the catechol oxidase family. PPO II inhibits cultures of Escherichia coli and it accumulates on the wounded sites of tobacco leaves indicating that it may act as a defense role in plant defense systems.

Characterization of polyphenol oxidase in coffee

Polyphenol oxidase (PPO) was characterized in partially purified extracts of leaves (PPO-L) and fruit endosperm (PPO-E) of coffee (Coffea arabica L.). PPO activity was higher in early developmental stages of both leaves and endosperm of fruits. Wounding or exposure of coffee leaves to methyl jasmonate increased PPO activity 1.5-4-fold. PPO was not latent and was not activated by protease treatment. PPO activity was stimulated 10-15% with sodium dodecyl sulphate (SDS) at 0.35-1.75 mM, but at higher concentrations activities were similar to the control samples, without detergent. Prolonged incubation of extracts with trypsin or proteinase K inhibited PPO activity but pepsin had no effect. Inhibition of PPO with proteinase K was increased in the presence of SDS. PPO activity from both tissues was optimal at pH 6-7 and at an assay temperature of 30 degrees C. Activity was highest with chlorogenic acid as substrate with a Km of 0.882 mM (PPO-L) and 2.27 mM (PPO-E). Hexadecyl trimethyl-ammonium bromide, polyvinylpyrrolidone 40. cinnamic acid and salicylhydroxamic acid inhibited PPO from both tissues. Both enzymes were inactivated by heat but the activity in endosperm extracts was more heat labile than that from leaves. The apparent Mr determined by gel filtration was 46 (PPO-L) and 50 kDa (PPO-E). Activity-stained SDS polyacrylamide gel electrophoresis (PAGE) gels and western blots probed with PPO antibodies suggested the existence of a 67 kDa PPO which is susceptible to proteolytic cleavage that generates a 45 kDa active form.

Molecular cloning and characterization of apricot fruit polyphenol oxidase

A reverse transcriptase-polymerase chain reaction experiment was done to synthesize a homologous polyphenol oxidase (PPO) probe from apricot (Prunus armeniaca var Bergeron) fruit. This probe was further used to isolate a full-length PPO cDNA, PA-PPO (accession no. AF020786), from an immature-green fruit cDNA library. PA-PPO is 2070 bp long and contains a single open reading frame encoding a PPO precursor peptide of 597 amino acids with a calculated molecular mass of 67.1 kD and an isoelectric point of 6.84. The mature protein has a predicted molecular mass of 56.2 kD and an isoelectric point of 5.84. PA-PPO belongs to a multigene family. The gene is highly expressed in young, immature-green fruit and is turned off early in the ripening process. The ratio of PPO protein to total proteins per fruit apparently remains stable regardless of the stage of development, whereas PPO specific activity peaks at the breaker stage. These results suggest that, in addition to a transcriptional control of PPO expression, other regulation factors such as translational and posttranslational controls also occur.

Isolation and characterization of spinach photosystem II membrane-associated catalase and polyphenol oxidase

Photosystem II (PSII) membranes exhibit catalase and polyphenol oxidase (PPO) activities. Mild heat treatment of PSII membranes for 90 min at 30 degrees C releases most of these enzyme activities into the supernatant, accompanied by a 7-fold activation of PPO. In contrast, mild heat treatment of thylakoid membranes does not release significant amounts of either activity, indicating that both enzymes are bound to the luminal surface of the thylakoid membrane. The heat-released PSII membrane-associated catalase and PPO have been purified and characterized. Catalase activity was correlated with a 63 kDa polypeptide which was purified by batch adsorption to anion-exchange beads followed by gel filtration. The PSII membrane-associated catalase is unstable in solution, probably due to irreversible aggregation. The enzyme was characterized in terms of molecular and subunit size, amino-acid composition, UV-visible absorption, heme content, pH optimum, inhibitor sensitivity, and K(m) value for H2O2. Its properties indicate that the PSII membrane-associated catalase is a luminal thylakoid membrane-bound heme enzyme that has not been identified previously. The residual catalase activity of PSII membranes after mild heat treatment is irreversibly inhibited with 3-amino-1,2,4-triazole, a specific inhibitor of heme catalases, without inhibition of O2-evolution activity. This result indicates that little, if any, of the catalase activity from PSII membranes in the dark is catalyzed by the O2-evolving center of PSII. PPO activity was correlated with a 48 kDa polypeptide. However, the 48 kDa polypeptide and another heat-released polypeptide of 72 kDa have the same N-terminal sequence, which is also identical to that of a known 64 kDa protein [Hind, G., Marshak, D. R., & Coughlan, S. J. (1995) Biochemistry 34, 8157-8164]. During heat treatment of PSII membranes and further manipulations it was found that the 72 kDa polypeptide was largely converted into the 48 kDa polypeptide. Thus, the 72 kDa polypeptide appears to be a latent precursor of the active 48 kDa PPO. The PSII membrane-associated PPO was purified by anion-exchange chromatography and was characterized in terms of substrate specificity, pH optimum, inhibitor sensitivity and native molecular weight. The heat-released PPO appears to be identical to the enzyme previously isolated from spinach thylakoid membranes [Golbeck, J. H., & Cammarata, K. V. (1981) Plant Physiol. 67, 977-984].