Molecular cloning and characterisation of banana fruit polyphenol oxidase

Recent Advances of Polyphenol Oxidases in Plants

Polyphenol oxidase (PPO) is present in most higher plants, but also in animals and fungi. PPO in plants had been summarized several years ago. However, recent advances in studies of PPO in plants are lacking. This review concludes new researches on PPO distribution, structure, molecular weights, optimal temperature, pH, and substrates. And, the transformation of PPO from latent to active state was also discussed. This state shift is a vital reason for elevating PPO activity, but the activation mechanism in plants has not been elucidated. PPO has an important role in plant stress resistance and physiological metabolism. However, the enzymatic browning reaction induced by PPO is a major problem in the production, processing, and storage of fruits and vegetables. Meanwhile, we summarized various new methods that had been invented to decrease enzymatic browning by inhibiting PPO activity. In addition, our manuscript included information on several important biological functions and the transcriptional regulation of PPO in plants. Furthermore, we also prospect some future research areas of PPO and hope they will be useful for future research in plants.

Genome-wide analysis of the polyphenol oxidase gene family reveals that MaPPO1 and MaPPO6 are the main contributors to fruit browning in Musa acuminate

INTRODUCTION

Polyphenol oxidases (PPOs), which are widely present in plants, play an important role in the growth, development, and stress responses. They can catalyze the oxidization of polyphenols and result in the browning of damaged or cut fruit, which seriously affects fruit quality and compromises the sale of fruit. In banana (Musa acuminata, AAA group), 10 PPO genes were determined based on the availability of a high-quality genome sequence, but the role of PPO genes in fruit browning remains unclear.

METHODS

In this study, we analyzed the physicochemical properties, gene structure, conserved structural domains, and evolutionary relationship of the PPO gene family of banana. The expression patterns were analyzed based on omics data and verified by qRT-PCR analysis. Transient expression assay in tobacco leaves was used to identify the subcellular localization of selected MaPPOs, and we analyzed the polyphenol oxidase activity using recombinant MaPPOs and transient expression assay.

RESULTS AND DISCUSSION

We found that more than two-thirds of the MaPPO genes had one intron, and all contained three conserved structural domains of PPO, except MaPPO4. Phylogenetic tree analysis revealed that MaPPO genes were categorized into five groups. MaPPOs did not cluster with Rosaceae and Solanaceae, indicating distant affinities, and MaPPO6/7/8/9/10 clustered into an individual group. Transcriptome, proteome, and expression analyses showed that MaPPO1 exhibits preferential expression in fruit tissue and is highly expressed at respiratory climacteric during fruit ripening. Other examined MaPPO genes were detectable in at least five different tissues. In mature green fruit tissue, MaPPO1 and MaPPO6 were the most abundant. Furthermore, MaPPO1 and MaPPO7 localized in chloroplasts, and MaPPO6 was a chloroplast- and Endoplasmic Reticulum (ER)-localized protein, whereas MaPPO10 only localized in the ER. In addition, the enzyme activity in vivo and in vitro of the selected MaPPO protein showed that MaPPO1 had the highest PPO activity, followed by MaPPO6. These results imply that MaPPO1 and MaPPO6 are the main contributors to banana fruit browning and lay the foundation for the development of banana varieties with low fruit browning.

Effects of short-wave ultraviolet light, ultrasonic and microwave treatments on banana puree during refrigerated storage

Enzymatic browning is a major problem in minimally processed banana puree; it reduces consumeŕs acceptability and affects nutritional quality. The objective of this work was to evaluate the effects ultrasound (40 kHz/10 min), microwave (800 W/ 25 s) and UV-C radiation (1.97 kJ/m²) applied to banana puree. Colour parameters (L*, a*, b*, chroma and hue), browning index (BI), polyphenol oxidase (PPO) and peroxidase (POD) activities, total phenolic compounds (TPC), antioxidant capacity (AOC) and microbiological counts were monitored throughout storage at 4°C. Ultrasound (US) and microwave (MW) treatments achieved a significant (p < 0.05) reduction in PPO activity and BI; moreover, ultrasound effectively retained phenolic compounds content (75% of initial value). The AOC was in coincidence with TPC values. POD activity was partially inhibited by UV-C while MW and US increase its activity. Although UV-C treatment was not effective to control browning development, it was effective to maintain microbiological stability after 20 days of storage (1.48 ± 0.01 log CFU/g). The evaluated treatments have the advantage of being less aggressive than conventional thermal treatments while maintaining fresh characteristics of the product.

Recent advances in polyphenol oxidase-mediated plant stress responses

Plant polyphenol oxidases (PPOs) are ubiquitous copper metalloenzymes with a biochemistry that has been known for more than a century. By the 1990s, biologists began to recognize the importance of PPOs in plant response to the infestation of herbivores and pathogens; ideas concerning a defensive role for PPOs arose to address observed evidence, and several testable hypotheses were suggested. Two pivotal discoveries in tomato (Lycopersicon esculentum Miller) plants, an inverse correlation between PPO levels and insect growth and PPO induction by defence signals, have driven many studies of PPO defence functions in the context of abiotic and biotic stresses. During the past three decades, extensive molecular research in transgenic and non-transgenic systems has partly revealed the sophisticated mechanisms underlying PPO defence against herbivores and pathogens. These understandings, rather than theoretical predictions, have driven the development of new hypotheses and advanced PPO-related studies. Here, we review progress in PPO family features, expression regulation and the defensive role of PPOs in plants. We propose assumptions of an extended range of co- and post-transcriptional processes to the regulation of unexplored PPO expression. In addition, the identification of endogenous PPO substrates and downstream targets of PPO action will be useful for elucidating PPO defensive roles. The potential effects of PPO-mediated oxidative defences on herbivore performance ultimately needs to be further investigated. Therefore, expanding multidisciplinary approaches to unexplored dimensions of PPO defence function should be a future priority.

Physicochemical properties of free and immobilized tyrosinase from different species of yam (Dioscorea spp)

A shortened method of purification and immobilization of tyrosinase from different species of yam (Dioscorea spp) on insoluble supports is described. The enzyme was purified by aqueous two-phase partitioning (ATPS) followed by gel filtration chromatography. The purified enzyme was immobilized on Ca-alginate, polyacrylamide gel or as cross-linked enzyme aggregate (CLEA) to obtain a yield of between 51-64%, 33-46% and 52-65% respectively for all the yam species. The optimum pH obtained for tyrosinase immobilized on polyacrylamide gel and CLEA was equivalent to that of free enzyme (pH 6.5). In contrast, Ca-alginate entrapped tyrosinase exhibited a shift of optimum pH to 7.0. Entrapped Tyrosinase in polyacrylamide gel and Ca-alginate also retained the same optimum temperature as the free enzyme (50 °C). While the optimum temperature of CLEA shifted to 60 °C. When subjected to four repeated use cycles, tyrosinase entrapped in polyacrylamide gel, Ca-alginate and CLEA still retained close to 40, 35 and 45 % of their initial activities respectively after the fourth cycle. The overall result further suggests yam tyrosinase as a promising enzyme for biocatalysis and biotechnological applications.

Regulation of flavonoid metabolism in ginkgo leaves in response to different day-night temperature combinations

Flavonoids are the most important secondary metabolites in ginkgo (Ginkgo biloba L.) leaves that determine its medicinal quality. Studies have suggested that secondary metabolism is strongly affected by temperature in other plant species, but little is known about ginkgo. In this study, we investigated the effects of different day-night temperature combinations (15/10, 25/20, and 35/30 °C (day/night)) on key enzyme activity, growth regulator concentrations, and flavonoid accumulation in ginkgo leaves. We found that phenylalanine ammonia-lyase (PAL) activity was enhanced and inhibited at 15/10 and 35/30 °C, respectively. Cinnamate-4-hydroxylase (C4H) activity was relatively stable under the three temperature conditions, and the p-coumarate CoA ligase (4CL) activity showed different trends under the three temperature conditions. The concentrations of flavonoid constituents (quercetin, kaempferol, and isorhamnetin) were decreased and increased under the 35/30 and 15/10 °C conditions, respectively. Low temperature promoted soluble sugar accumulation, while temperature had a limited impact on the accumulation of soluble protein. The pattern of change in the total flavonoid concentration was not always in agreement with PAL activity due to its complex pathway. Indoleacetic acid (IAA) and gibberellin (GA) changes shared similar patterns and had limited effects on flavonoid accumulation, while abscisic acid (ABA) acted as a promotor of flavonoid accumulation under high-temperature conditions. The total flavonoids achieved the highest content under the 15/10 °C treatment on the 40th day. Therefore, the lower temperature (15/10 °C) is more favorable for flavonoid accumulation and will provide a theoretical basis for further study.

Enzymatic browning in avocado (Persea americana) revisited: History, advances, and future perspectives

Considering nearly 80 years of research regarding one of the enzymes responsible for catalyzing the formation of pigments in higher animals, plants, fungi and bacteria, this review will focus on collecting and categorizing the existing information about polyphenol oxidase (PPO) in fruits, with particular emphasis on the information in relation to avocado, which is one of the hardiest species in terms of inactivation, has documented dual activity (EC 1.14.18.1/EC 1.10.3.1), and represents one of the oldest challenges for food science research and fruit processors. It is expected that this review will contribute to the further development of the field by highlighting the questions that have arisen during the characterization of PPO, the progress that has been made and the questions that remain today, in addition to new methodologies that are being applied to study this system. Holistic methodologies offer unexplored potential for advancing our understanding of the complex phenomena that govern PPO activity in fruits, because these methodologies will enable the characterization of this family of enzymes in all of its complexity. Subsequently, it will be possible to develop better techniques for controlling enzymatic browning in this valuable fruit.

Fully edible biofuel cells

This article describes the first example of edible energy harvesting biofuel cells, based solely on highly biocompatible and ingestible food materials.

Molecular cloning and expression analysis of duplicated polyphenol oxidase genes reveal their functional differentiations in sorghum

Polyphenol oxidase (PPO) is believed to play a role in plant growth, reproduction, and resistance to pathogens and pests. PPO causes browning of grains in cereals. In this study, genetic mapping of sorghum grain for phenol color reaction (PHR) was performed using a recombinant inbred line population. Only one locus was detected between SSR markers SM06072 and Xtxp176 on chromosome 6. Two linked orthologous genes (Sb06PPO1 and Sb06PPO2) within the mapped region were discovered and cloned. Transformation experiments using Nipponbare (a PHR negative rice cultivar) showed that Sb06PPO1 from LTR108 and two Sb06PPO2 alleles from both varieties could complement Nipponbare, whereas Sb06PPO1 from 654 could not. Subsequent quantitative real-time PCR (qPCR) experiments showed that Sb06PPO1 and Sb06PPO2 functioned diversely, Sb06PPO1 was mainly expressed in young panicles before flowering. Sb06PPO2 was strongly expressed in flowering panicles, especially in hulls and branches at filling stage. Moreover, the expression of Sb06PPO1 was found to be significantly up-regulated by exogenous ABA and salt, whereas Sb06PPO2 was not changed significantly, further demonstrating functional differentiation between the two genes.

Polyphenol Oxidases in Crops: Biochemical, Physiological and Genetic Aspects

Enzymatic browning is a colour reaction occurring in plants, including cereals, fruit and horticultural crops, due to oxidation during postharvest processing and storage. This has a negative impact on the colour, flavour, nutritional properties and shelf life of food products. Browning is usually caused by polyphenol oxidases (PPOs), following cell damage caused by senescence, wounding and the attack of pests and pathogens. Several studies indicated that PPOs play a role in plant immunity, and emerging evidence suggested that PPOs might also be involved in other physiological processes. Genomic investigations ultimately led to the isolation of PPO homologs in several crops, which will be possibly characterized at the functional level in the near future. Here, focusing on the botanic families of Poaceae and Solanaceae, we provide an overview on available scientific literature on PPOs, resulting in useful information on biochemical, physiological and genetic aspects.

Cloning and expression analysis of litchi (Litchi Chinensis Sonn.) polyphenol oxidase gene and relationship with postharvest pericarp browning

Polyphenol oxidase (PPO) plays a key role in the postharvest pericarp browning of litchi fruit, but its underlying mechanism remains unclear. In this study, we cloned the litchi PPO gene (LcPPO, JF926153), and described its expression patterns. The LcPPO cDNA sequence was 2120 bps in length with an open reading frame (ORF) of 1800 bps. The ORF encoded a polypeptide with 599 amino acid residues, sharing high similarities with other plant PPO. The DNA sequence of the ORF contained a 215-bp intron. After carrying out quantitative RT-PCR, we proved that the LcPPO expression was tissue-specific, exhibiting the highest level in the flower and leaf. In the pericarp of newly-harvested litchi fruits, the LcPPO expression level was relatively high compared with developing fruits. Regardless of the litchi cultivar and treatment conditions, the LcPPO expression level and the PPO activity in pericarp of postharvest fruits exhibited the similar variations. When the fruits were stored at room temperature without packaging, all the pericarp browning index, PPO activity and the LcPPO expression level of litchi pericarps were reaching the highest in Nandaowuhe (the most rapid browning cultivar), but the lowest in Ziniangxi (the slowest browning cultivar) within 2 d postharvest. Preserving the fruits of Feizixiao in 0.2-μm plastic bag at room temperature would decrease the rate of pericarp water loss, delay the pericarp browning, and also cause the reduction of the pericarp PPO activity and LcPPO expression level within 3 d postharvest. In addition, postharvest storage of Feizixiao fruit stored at 4°C delayed the pericarp browning while decreasing the pericarp PPO activity and LcPPO expression level within 2 d after harvest. Thus, we concluded that the up-regulation of LcPPO expression in pericarp at early stage of postharvest storage likely enhanced the PPO activity and further accelerated the postharvest pericarp browning of litchi fruit.

Isolation of a polyphenol oxidase (PPO) cDNA from artichoke and expression analysis in wounded artichoke heads

The polyphenol oxidase (PPO) enzyme, which can catalyze the oxidation of phenolics to quinones, has been reported to be involved in undesirable browning in many plant foods. This phenomenon is particularly severe in artichoke heads wounded during the manufacturing process. A full-length cDNA encoding for a putative polyphenol oxidase (designated as CsPPO) along with a 1432 bp sequence upstream of the starting ATG codon was characterized for the first time from [Cynara cardunculus var. scolymus (L.) Fiori]. The 1764 bp CsPPO sequence encodes a putative protein of 587 amino acids with a calculated molecular mass of 65,327 Da and an isoelectric point of 5.50. Analysis of the promoter region revealed the presence of cis-acting elements, some of which are putatively involved in the response to light and wounds. Expression analysis of the gene in wounded capitula indicated that CsPPO was significantly induced after 48 h, even though the browning process had started earlier. This suggests that the early browning event observed in artichoke heads was not directly related to de novo mRNA synthesis. Finally, we provide the complete gene sequence encoding for polyphenol oxidase and the upstream regulative region in artichoke.

Polyphenol oxidases in Physcomitrella: functional PPO1 knockout modulates cytokinin-dependent development in the moss Physcomitrella patens

Polyphenol oxidases (PPOs) are copper-binding enzymes of the plant secondary metabolism that oxidize polyphenols to quinones. Although PPOs are nearly ubiquitous in seed plants, knowledge on their evolution and function in other plant groups is missing. This study reports on the PPO gene family in the moss Physcomitrella patens (Hedw.) B.S.G. asan example for an early divergent plant. The P. patens PPO multigene family comprises 13 paralogues. Phylogenetic analyses suggest that plant PPOs evolved with the colonization of land and that PPO duplications within the monophyletic P. patens paralogue clade occurred after the separation of the moss and seed plant lineages. PPO functionality was demonstrated for recombinant PPO6. P. patens was analysed for phenolic compounds and six substances were detected intracellularly by LC-MS analysis: 4-hydroxybenzoic acid, p-cumaric acid, protocatechuic acid, salicylic acid, caffeic acid, and an ester of caffeic acid. Targeted PPO1 knockout (d|ppo1) plants were generated and plants lacking PPO1 exhibited only ~30% of the wild-type PPO activity in the culture medium, thus suggesting extracellular localization of PPO1, which is in contrast to the mostly plastidic PPO localization in seed plants. Further, d|ppo1 lines formed significantly more gametophores with a reduced areal plant size, which could be related to an increase of endogenously produced cytokinins and indicates an impact of PPO1 on plant development. d|ppo1 plants were less tolerant towards applied 4-methylcatechol compared to the wild type, which suggests a role of extracellular PPO1 in establishing appropriate conditions by the removal of inhibitory extracellular phenolic compounds.

Eggplant polyphenol oxidase multigene family: cloning, phylogeny, expression analyses and immunolocalization in response to wounding

Though polyphenol oxidase (PPO) genes from tomato and potato have been extensively studied, information about PPO genes in eggplant (Solanum melongena) is lacking. The main objective of this study is to understand the structural and functional aspects of eggplant PPO genes. Six eggplant PPO genes (SmePPO1-6) cloned by RACE and genome walking were found to be intronless and correspond to eight eggplant unigenes. Comprehensive sequence analyses indicated that the eggplant PPO genes exhibit considerable variation in the transit peptide regions, copper-binding domains and UTRs, and fall into two distinct structural classes. Further, PPO gene members appear to exist in clusters on eggplant chromosome 8 as seen in the case of tomato and potato PPOs. During normal growth and development, SmePPO1 and 2 are expressed in roots, whereas the transcript levels of all the eggplant PPO genes vary considerably in leaves, flowers and fruits. SmePPO1 was expressed in Escherichia coli as a GST fusion protein, and immunoblot using rabbit polyclonal antiserum to GST-SmePPO1 detected a major protein band (~70 kDa) and a minor band (~67 kDa) in eggplant fruit extract. Tissue printing indicated the predominant presence of PPO in the exocarp and the areas surrounding the seeds in the mesocarp of eggplant fruits. Immunolocalization of PPOs in eggplant infested with shoot-and-fruit borer revealed localization of the PPO at the site of infection in tender shoots and fruits, and further inside the mature tissues. The upregulation of eggplant PPO gene transcripts following mechanical injury shows that all the genes except SmePPO2 are induced in the fruit over 6h. On the contrary, the transcripts of SmePPO2 and PPO3 are not detectable in the stem, and expression seems to be prominent over a 2h period for SmePPO1 and SmePPO4-6. Our results show that eggplant PPO genes are structurally different, and are differentially expressed in various tissues of eggplant indicating their functional diversity.

The polyphenol oxidase gene family in poplar: phylogeny, differential expression and identification of a novel, vacuolar isoform

Polyphenol oxidases (PPOs) are oxidative enzymes that convert monophenols and o-diphenols to o-quinones using molecular oxygen. The quinone products are highly reactive following tissue damage and can interact with cellular constituents and cause oxidative browning and cross-linking. The induction of PPO in some plants as a result of wounding, herbivore attack, or pathogen infection has implicated them in defense. However, PPO-like enzymes that act as specific hydroxylases, for example in lignan and pigment biosynthesis, have also been discovered. Here, we present the first genome-enabled analysis of a PPO gene family. The Populus trichocarpa genome was found to contain a minimum of nine complete PPO genes, and seven of these were characterized further. The PPO gene family includes both recently duplicated and divergent sequences that are 36-98% identical at the amino acid level. Gene expression profiling in poplar tissues and organs revealed that the PPO genes are all differentially expressed during normal development, but that only a small subset of PPO genes are significantly upregulated by wounding, methyl jasmonate or pathogen infection. Our studies also identified PtrPPO13, a novel PPO gene that is predicted to encode an N-terminal signal peptide. Transient expression of green fluorescent protein fusions demonstrated its localization to the vacuolar lumen. Together, our findings show that the poplar PPO family is diverse and is likely linked to diverse physiological functions.

Enzyme characterisation, isolation and cDNA cloning of polyphenol oxidase in the hearts of palm of three commercially important species

Heart of palm (palmito) is the edible part of the apical meristem of palms and is considered a gourmet vegetable. Palmitos from the palms Euterpe edulis (Juçara) and Euterpe oleracea (Açaí) oxidise after harvesting, whereas almost no oxidation is observed in palmitos from Bactris gasipaes (Pupunha). Previous investigations showed that oxidation in Juçara and Açaí was mainly attributable to polyphenol oxidase (PPO; EC 1.14.18.1) activity. In this study, we partially purified PPOs from these three palmitos and analysed them for SDS activation, substrate specificity, inhibition by specific inhibitors, thermal stability, optimum pH and temperature conditions, Km and Ki. In addition, the total phenolic content and chlorogenic acid content were determined. Two partial cDNA sequences were isolated and sequenced from Açaí (EoPPO1) and Juçara (EePPO1). Semi-quantitative RT-PCR expression assays showed that Açaí and Juçara PPOs were strongly expressed in palmitos and weakly expressed in leaves. No amplification was observed for Pupunha samples. The lack of oxidation in the palmito Pupunha might be explained by the low PPO expression, low enzyme activity or the phenolic profile, particularly the low content of chlorogenic acid.

Polyphenol oxidase and peroxidase expression in four pineapple varieties (Ananas comosus L.) after a chilling injury

Pineapple internal browning (IB) is a chilling injury that produces enzymatic browning associated with flesh translucency. Pineapple biodiversity allowed the investigation of how polyphenol oxidase (PPO) and peroxidase (POD) activities with their different isoforms are involved in the IB mechanism. Fruits of four varieties that expressed IB symptoms differently, Smooth Cayenne (SCay) and the hybrids MD2, Flhoran 41 (Flh 41), and Flhoran 53 (Flh 53), were stressed by cold. The susceptible varieties showed classical brown spots but different patterns of IB, whereas MD2 and controls showed no IB. Enzymatic activities were measured on fruit protein extracts and PPO and POD isoforms separated on mini-gels (PhastSystem). Only PPO activity was significantly enhanced in the presence of IB. Up to six PPO isoforms were identified in the susceptible varieties. PPO was barely detectable in the nonsusceptible variety MD2 and in controls. The number of PPO isoforms and the total PPO activity after chilling are varietal characteristics.

Independent losses of function in a polyphenol oxidase in rice: differentiation in grain discoloration between subspecies and the role of positive selection under domestication

Asian rice (Oryza sativa) cultivars originated from wild rice and can be divided into two subspecies by several criteria, one of which is the phenol reaction (PHR) phenotype. Grains of indica cultivars turn brown in a phenol solution that accelerates a similar process that occurs during prolonged storage. By contrast, the grains of japonica do not discolor. This distinction may reflect the divergent domestication of these two subspecies. The PHR is controlled by a single gene, Phr1; here, we report the cloning of Phr1, which encodes a polyphenol oxidase. The Phr1 gene is indeed responsible for the PHR phenotype, as transformation with a functional Phr1 can complement a PHR negative cultivar. Phr1 is defective in all japonica lines but functional in nearly all indica and wild strains. Phylogenetic analysis showed that the defects in Phr1 arose independently three times. The multiple recent origins and rapid spread of phr1 in japonica suggest the action of positive selection, which is further supported by several population genetic tests. This case may hence represent an example of artificial selection driving the differentiation among domesticated varieties.

Proteomics of multigenic families from species underrepresented in databases: the case of loquat (Eriobotrya japonica Lindl.) polyphenol oxidases

Here, we approach the problem of obtaining accurate and reliable information about the gene origin of a protein belonging to a multigenic family, polyphenol oxidase, from an underrepresented species, Eriobotrya japonica. De novo sequencing was a key approach to obtain broad sequence coverage. Alignment of peptides on their most similar homologous protein revealed divergent amino acid positions that lead to hypothesize the minimal number of genes encoding for the proteins analyzed.

Proteome analysis of non-model plants: a challenging but powerful approach

Biological research has focused in the past on model organisms and most of the functional genomics studies in the field of plant sciences are still performed on model species or species that are characterized to a great extent. However, numerous non-model plants are essential as food, feed, or energy resource. Some features and processes are unique to these plant species or families and cannot be approached via a model plant. The power of all proteomic and transcriptomic methods, that is, high-throughput identification of candidate gene products, tends to be lost in non-model species due to the lack of genomic information or due to the sequence divergence to a related model organism. Nevertheless, a proteomics approach has a great potential to study non-model species. This work reviews non-model plants from a proteomic angle and provides an outline of the problems encountered when initiating the proteome analysis of a non-model organism. The review tackles problems associated with (i) sample preparation, (ii) the analysis and interpretation of a complex data set, (iii) the protein identification via MS, and (iv) data management and integration. We will illustrate the power of 2DE for non-model plants in combination with multivariate data analysis and MS/MS identification and will evaluate possible alternatives.

Browning in Annona cherimola fruit: role of polyphenol oxidase and characterization of a coding sequence of the enzyme

Cherimoya (Annona cherimola Mill.) fruit is an attractive candidate for food processing applications as fresh cut. However, along with its desirable delicate taste, cherimoya shows a marked susceptibility to browning. This condition is mainly attributed to polyphenol oxidase activity (PPO). A general lack of knowledge regarding PPO and its role in the oxidative loss of quality in processed cherimoya fruit requires a better understanding of the mechanisms involved. The work carried out included the cloning of a full-length cDNA, an analysis of its properties in the deduced amino sequence, and linkage of its mRNA levels with enzyme activity in mature and ripe fruits after wounding. The results showed one gene different at the nucleotide level when compared with previously reported genes, but a well-conserved protein, either in functional and in structural terms. Cherimoya PPO gene (Ac-ppo, GenBank DQ990911) showed to be present apparently in one copy of the genome, and its transcripts could be significantly detected in leaves and less abundantly in flowers and fruits. Analysis of wounded matured and ripened fruits revealed an inductive behavior for mRNA levels in the flesh of mature cherimoya after 16 h. Although the highest enzymatic activity was observed on rind, a consistent PPO activity was detected on flesh samples. A lack of correlation between PPO mRNA level and PPO activity was observed, especially in flesh tissue. This is probably due to the presence of monophenolic substrates inducing a lag period, enzyme inhibitors and/or diphenolic substrates causing suicide inactivation, and proenzyme or latent isoforms of PPO. To our knowledge this is the first report of a complete PPO sequence in cherimoya. Furthermore, the gene is highly divergent from known nucleotide sequences but shows a well conserved protein in terms of its function, deduced structure, and physiological role.

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.

Allelic variation of polyphenol oxidase (PPO) genes located on chromosomes 2A and 2D and development of functional markers for the PPO genes in common wheat

Polyphenol oxidase (PPO) activity is highly related to the undesirable browning of wheat-based end products, especially Asian noodles. Characterization of PPO genes and the development of their functional markers are of great importance for marker-assisted selection in wheat breeding. In the present study, complete genomic DNA sequences of two PPO genes, one each located on chromosomes 2A and 2D and their allelic variants were characterized by means of in silico cloning and experimental validation. Sequences were aligned at both DNA and protein levels. Two haplotypes on chromosome 2D showed 95.2% sequence identity at the DNA level, indicating much more sequence diversity than those on chromosome 2A with 99.6% sequence identity. Both of the PPO genes on chromosomes 2A and 2D contain an open reading frame (ORF) of 1,731 bp, encoding a PPO precursor peptide of 577 amino acids with a predicted molecular mass of approximately 64 kD. Two complementary dominant STS markers, PPO16 and PPO29, were developed based on the PPO gene haplotypes located on chromosome 2D; they amplify a 713-bp fragment in cultivars with low PPO activity and a 490-bp fragment in those with high PPO activity, respectively. The two markers were mapped on chromosome 2DL using a doubled haploid population derived from the cross Zhongyou 9507/CA9632, and a set of nullisomic-tetrasomic lines and ditelosomic line 2DS of Chinese Spring. QTL analysis indicated that the PPO gene co-segregated with the two STS markers and was closely linked to SSR marker Xwmc41 on chromosome 2DL, explaining from 9.6 to 24.4% of the phenotypic variance for PPO activity across three environments. In order to simultaneously detect PPO loci on chromosomes 2A and 2D, a multiplexed marker combination PPO33/PPO16 was developed and yielded distinguishable DNA patterns in a number of cultivars. The STS marker PPO33 for the PPO gene on chromosome 2A was developed from the same gene sequences as PPO18 that we reported previously, and can amplify a 481-bp and a 290-bp fragment from cultivars with low and high PPO activity, respectively. A total of 217 Chinese wheat cultivars and advanced lines were used to validate the association between the polymorphic fragments and grain PPO activity. The results showed that the marker combination PPO33/PPO16 is efficient and reliable for evaluating PPO activity and can be used in wheat breeding programs aimed for noodle and other end product quality improvement.

Polyphenol oxidase (PPO) in wheat and wild relatives: molecular evidence for a multigene family

Wheat polyphenol oxidase (PPO) is the major cause of browning reactions that discolor Asian noodles and other wheat products. It has been hypothesized that genes encoding wheat PPOs may have evolved by gene duplication into a multigene family. Here we characterized PPO genomic sequences from diploid (Triticum monococcum, T. urartu, Aegilops tauschii, and Ae. speltoides), tetraploid (T. turgidum, subspecies dicoccoides and durum) and hexaploid (T. aestivum cultivars Klasic and ID377s) wheat species to gain a better understanding of the structure and organization of PPO genes. DNA fragments were amplified from a highly polymorphic and phylogenetic informative region of the gene. As a result, we obtained highly discriminative sequences. Three distinct PPOs, obtained from the A genome of T. monococcum, provided evidence for gene duplication events (paralogous loci). Furthermore, the number of sequences obtained for bread and durum wheat was higher than the expected number of orthologous loci. Sequence comparison revealed nucleotide and structural diversity, and detected five sequence intron types, all with a common insertion position. This was hypothesized to be homologous to that of intron 2 of previously reported wheat PPOs. A MITE of the Stowaway family accounted for the major difference between the five intervening sequences, and was unique to T. aestivum cv. Klasic. Nucleotide and structural diversity, together with well-resolved phylogenetic trees, provided molecular evidence to support the hypothesis of a PPO multigene family structure and organization.

Extraction and partial characterization of polyphenol oxidase from banana (Musa acuminata Grande naine) roots

Polyphenol oxidase activity (PPO, EC 1.14.18.1, monophenol monooxygenase, and EC 1.10.3.2, o-diphenoloxidase) has been extensively studied in banana fruit for its role in enzymatic browning. Rapid discolouration of leaf, stem and root tissue after injury and strong pigmentation of tissue extracts indicate that PPO and phenolic compounds are ubiquitous in vegetative tissue of banana as well. They hamper biochemical and molecular studies in banana, as cumbersome adaptations of extraction protocols are required. On the other hand, PPO and phenolic compounds could be an important part of the plant's defence system against pests and diseases, including root parasitic nematodes. To facilitate future studies in this area, extraction and assay conditions for PPO from roots of banana (Musa acuminata AAA, Grande naine) were optimized. Highest enzyme activities were obtained in a 0.2 M phosphate buffer at pH 7.0 with 5% insoluble polyvinylpyrrolidone and 0.25% Triton X-100. The lowest K(m) values were obtained for dopamine and D-catechin. Monophenolase activity was shown with p-cresol. Banana root PPO was strongly inhibited by dithiothreitol and sodium metabisulfite. In root sections, oxidation of dopamine strongly co-localized with aerenchyma in the cortex. The experiments revealed indications for the involvement of root PPO and dopamine in resistance of banana against the parasitic nematode Radopholus similis.

Development of polyphenol oxidase activity in the micropylar endosperm of tomato seeds

Polyphenol oxidase (PPO) activity increased markedly in the micropylar region of the endosperm of tomato (Lycopersicon esculentum) seeds after radicle protrusion. Tissue-printing analyses demonstrated that the majority of the activity is localized in the micropylar area. The increase in the activity was due to the increase in the amounts of enzyme. Within the micropylar endosperm region, two PPO isozymes were immunologically detected whose apparent molecular masses were estimated to be approximately 58 and 59 kDa, respectively, by SDS-PAGE. Although PPO activity also developed in the lateral portion of the endosperm, the level of this activity was much lower as compared with that in the micropylar region. Furthermore, the isozyme pattern in the lateral portion differed from that in the micropylar portion. The 58 kDa isozyme that was detected in the latter portion was absent, and only 59 kDa PPO was detectable in the former one. When the endosperm tissues were wounded, an enhancement of the enzyme activity was observed in the wounded region. The wound-induced development of the enzyme activity was associated with the induction of 58 kDa isozyme.

Preparation of protein extracts from recalcitrant plant tissues: an evaluation of different methods for two-dimensional gel electrophoresis analysis

This study focuses on the specific problems of protein extraction from recalcitrant plant tissues and evaluates several methods to bypass them. Sample preparation is a critical step in a two-dimensional gel electrophoresis proteome approach and is absolutely essential for good results. We evaluated four methods: the classical trichloroacetic acid (TCA)/acetone precipitation, TCA/acetone precipitation and fractionation, an alternative based on fractionation and without precipitation, and phenol extraction methanol/ammonium acetate precipitation. We optimized the phenol extraction protocol for small amounts of tissue, which is essential when the study material is limited. The protocol was optimized for banana (Musa spp.) and was subsequently applied to two other plant species: apple (Malus domestica L.) and potato (Solanum tuberosum L.). Banana (Musa spp.) is a good representative of a "difficult" plant species since it contains many interfering metabolites. Only classical TCA/acetone precipitation and phenol extraction methods proved useful as standard methods. Both methods are associated with a minor but reproducible loss of proteins. Every extraction method and the subsequent analytical procedure have their physicochemical limitations; both methods should be investigated before selecting an appropriate protocol. The study, which is presented in this paper, is useful for guiding the experimental setup of many other nonmodel species, containing various interfering elements.

Creation of a BAC resource to study the structure and evolution of the banana (Musa balbisiana) genome

The first bacterial artificial chromosome (BAC) library of the banana species Musa balbisiana 'Pisang Klutuk Wulung' (PKW BAC library) was constructed and characterized. One improved and one novel protocol for nuclei isolation were employed to overcome problems caused by high levels of polyphenols and polysaccharides present in leaf tissues. The use of flow cytometry to purify cell nuclei eliminated contamination with secondary metabolites and plastid DNA. Furthermore, the usefulness of the inducible pCC1BAC vector to obtain a higher amount of BAC DNA was demonstrated. The PKW BAC library represents nine haploid genome equivalents of M. balbisiana and its mean insert size is 135 kb. It consists of two sublibraries, of which the first one (SN sublibrary with 24,960 clones) was prepared according to an improved standard nuclei isolation protocol, whereas the second (FN sublibrary with 11,904 clones) was obtained from flow-sorted nuclei. Screening with 12 RFLP probes, which were genetically anchored to 8 genetic linkage groups of the banana species Musa acuminata, revealed an average of 11 BAC clones per probe, thus confirming the genome coverage estimated based on the insert size, as well as a high level of conservation between the two species of Musa. Localization of selected BAC clones to mitotic chromosomes using FISH indicated that the BAC library represented a useful resource for cytogenetic mapping. As the first step in map-based cloning of a genetic factor that is involved in the activation of integrated pararetroviral sequences of Banana streak virus (BSV), the BSV expressed locus (BEL) was physically delimited. The PKW BAC library represents a publicly available tool, and is currently used to reveal the integration and activation mechanisms of BSV sequences and to study banana genome structure and evolution.

Identification and characterisation of a bryophyte polyphenol oxidase encoding gene from Physcomitrella patens

Polyphenol oxidases (PPO) are enzymes of secondary plant metabolism that catalyse the oxidation of polyphenols to quinones. Because of their ubiquitous appearance in the plant kingdom, an important role is assumed; however, the exact physiological function of PPOs remains unclear. In this work, the identification, cloning, and characterisation of a bryophyte PPO from the moss Physcomitrella patens is presented. PPO activity from protein extracts was determined polarographically after activation by SDS. Four Physcomitrella ESTs with homologies to known plant PPOs were selected from publicly accessible databases, and PCR experiments demonstrated that they belong to the same gene, named Pp_ppo1. The identified cDNA was found to be 2402 bp long, containing a single open reading frame of 1611 bp encoding for a 536 amino acid protein with a molecular mass of 60.1 kDa. Cloning and sequencing of a genomic part of Pp_ppo1 revealed the presence of a 94-bp intron. The time course of Pp_ppo1 gene expression in liquid culture was monitored by real time RT-PCR, revealing increasing transcription levels until the 4th day, a maximum between the 4th and the 8th day, and decreasing transcription until the 12th day. A comparison of the deduced amino acid sequence of Pp_ppo1 with seed plant PPOs revealed similarities such as the presence of two highly conserved copper-binding domains and a similar pattern of hydrophobic regions, but also differences such as a stronger membrane association and a shorter signal sequence, thus reflecting the phylogenetic distance of Physcomitrella from seed plants.

Inducing biochemical changes to simulate after-roast darkening in macadamia kernel

After-roast darkening is a defect in macadamia kernel evident only upon roasting that adversely affects kernel quality. After-roast darkening was artificially induced in 3 cultivars by incubating nut-in-shell of high moisture content (about 22% w/w) at elevated temperatures in either sealed or unsealed polyethylene bags before drying to 1.5% kernel moisture. After oil roasting, darkening was more evident in kernel from nut-in-shell incubated for 24 h in sealed bags at temperatures greater than or equal to 47.5°C. At an incubation temperature of 50°C the critical incubation period was 12 h for nut-in-shell treated in sealed bags. In raw kernel induced to exhibit high after-roast darkening upon roasting, the concentrations of the hexoses, glucose and fructose were elevated and levels of sucrose were reduced compared to non-induced kernel. The change in kernel sugar composition increased with increasing incubation temperature. A loss in cellular viability was also associated with kernel susceptibility to after-roast darkening. These results indicate that after-roast darkening might result from reactions, possibly enzymatic, that change the kernel sugar composition as a result of a loss in membrane integrity.

Grape berry biochemistry revisited upon proteomic analysis of the mesocarp

Major soluble proteins of grapevine ripe berries were extracted from six different cultivars including non vinifera, with trichloroacetic acid acetone and resolved in two-dimensional electrophoresis (2-DE) gels. About three hundred spots were detected on the 2-DE map after colloidal blue staining. From 2-DE map of cv. Gamay mesocarp, 67 proteins were identified (p > 0.95) using matrix-assisted laser desorption/ionization-mass spectrometry analysis. About 34%, 19%, and 13% of identified proteins play, respectively, a role in energy metabolism, defense, and stress response and primary metabolism. 2-DE analysis revealed considerable accumulation of dehydrin, invertase, and a putative transcription factor in the ripe fruit, in addition to pathogenesis-related proteins such as chitinase and thaumatin-like proteins previously documented as prevalent proteins in ripe berries. Actual translation of redundant transcripts of unclear function such as Grip31, Grip32, and Grip61 recently cloned in ripe grape berries was confirmed. The relative abundance of UDP-glucose pyrophosphorylase and vacuolar invertase strongly supported a key role of the apoplastic pathway of sugar loading during ripening. Comparative analysis shows that differences between cultivars were low, but different isoforms of alcohol dehydrogenase and of a transcription factor of hexose transporter were obvious in the six cultivars. Peptide mass fingerprinting suggests that the Adh isoforms would be Adh2/Adh6 or Adh2/Adh7 dimers and unambiguously shows that considerable deletion/insertion inside Adh7 are not cloning artifacts.

Purification and characterization of a latent polyphenol oxidase from beet root (Beta vulgaris L.)

Polyphenol oxidase (PPO) has been extracted from beet root, in both soluble and membrane fractions. In both cases, the enzyme was in its latent state, and it was activated by sodium dodecyl sulfate. PPO was purified to apparent homogeneity. The soluble PPO purification was achieved by hydrophobic interaction chromatography and gel filtration chromatography, with apparent molecular mass of 55 kDa. The membrane PPO purification was achieved by anion exchange chromatography and gel filtration with apparent molecular mass of 54 kDa. A totally denaturing SDS-PAGE indicated the presence of a single polypeptide with an apparent molecular mass of 60 kDa for both fractions, with the band also revealed by Western blot. A partially denaturing SDS-PAGE stained a single active 36 kDa band for both fractions. Under native isoelectric focusing, a major acidic band of pH 5.2 was detected in both fractions. Kinetic characterization of PPO on the natural substrate l-dopa was carried out.

Construction and characterization of a bacterial artificial chromosome library of banana (Musa acuminata Colla)

A bacterial artificial chromosome (BAC) library for banana was constructed from leaves of the wild diploid 'Calcutta 4' clone (Musa acuminata subsp. Burmannicoides 2n = 2 x = 22). 'Calcutta 4' is widely used in breeding programs for its resistance to the current major disease of banana and is being used to build a genetic reference map of banana. As banana leaves are particularly rich in polyphenols and polysaccharides a protocol was adapted to isolate intact nuclei and high-molecular-weight (HMW) DNA. A total of 55,152 clones with an average insert size of 100 kb were picked. The frequency of BAC clones carrying inserts derived from chloroplast and mitochondrial DNA was estimated to be 1.5%. The coverage of the library is equivalent to 9.0-times the haploid genome. The BAC library was screened with 13 RFLP probes belonging to the 8 linkage groups of the consensus molecular map of banana. A total of 135 clones were identified giving an average of 10.38 clones for each locus. This BAC library will be a valuable starting tool for many of the goals of the recently emerged International Musa Genomic Consortium. One of our initial objectives will be to develop a banana physical map by BAC-FISH (fluorescent in situ hybridization) viewing the characterization of translocation break points.

The abundant class III chitinase homolog in young developing banana fruits behaves as a transient vegetative storage protein and most probably serves as an important supply of amino acids for the synthesis of ripening-associated proteins

Analyses of the protein content and composition revealed dramatic changes in gene expression during in situ banana (Musa spp.) fruit formation/ripening. The total banana protein content rapidly increases during the first 60 to 70 d, but remains constant for the rest of fruit formation/ripening. During the phase of rapid protein accumulation, an inactive homolog of class III chitinases accounts for up to 40% (w/v) of the total protein. Concomitant with the arrest of net protein accumulation, the chitinase-related protein (CRP) progressively decreases and several novel proteins appear in the electropherograms. Hence, CRP behaves as a fruit-specific vegetative storage protein that accumulates during early fruit formation and serves as a source of amino acids for the synthesis of ripening-associated proteins. Analyses of individual proteins revealed that a thaumatin-like protein, a beta-1,3-glucanase, a class I chitinase, and a mannose-binding lectin are the most abundant ripening-associated proteins. Because during the ripening of prematurely harvested bananas, similar changes take place as in the in situ ripening bananas, CRP present in immature fruits is a sufficient source of amino acids for a quasi-normal synthesis of ripening-associated proteins. However, it is evident that the conversion of CRP in ripening-associated proteins takes place at an accelerated rate, especially when climacteric ripening is induced by ethylene. The present report also includes a discussion of the accumulation of the major banana allergens and the identification of suitable promoters for the production of vaccines in transgenic bananas.