Silencing of ascorbate oxidase results in reduced growth, altered ascorbic acid levels and ripening pattern in melon fruit

Ascorbate oxidase (AO, EC 1.10.3.3) is a copper-containing enzyme localized at the apoplast, where it catalyzes the oxidation of ascorbic acid (AA) to dehydroascorbic acid (DHA) via monodehydroascorbic acid (MDHA) intermediate. Despite it has been extensively studied, no biological roles have been definitively ascribed. To understand the role of AO in plant metabolism, fruit growth and physiology, we suppressed AO expression in melon (Cucumis melo L.) fruit. Reduction of AO activity increased AA content in melon fruit, which is the result of repression of AA oxidation and simultaneous induction of certain biosynthetic and recycling genes. As a consequence, ascorbate redox state was altered in the apoplast. Interestingly, transgenic melon fruit displayed increased ethylene production rate coincided with elevated levels of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACO, EC 1.14.17.4) activity and gene expression, which might contribute to earlier ripening. Moreover, AO suppressed transgenic melon fruit exhibited a dramatic arrest in fruit growth, due to a simultaneous decrease in fruit cell size and in plasmalemma (PM) ATPase activity. All the above, support for the first time, the in vivo AO participation in the rapid fruit growth of Cucurbitaceae and further suggest an alternative route for AA increase in ripening fruit.

Efficient protein extraction for shotgun proteomics from hydrated and desiccated leaves of resurrection Ramonda serbica plants

Resurrection plant Ramonda serbica is a suitable model to investigate vegetative desiccation tolerance. However, the detailed study of these mechanisms at the protein level is hampered by the severe tissue water loss, high amount of phenolics and polysaccharide, and possible protein modifications and aggregations during the extraction and purification steps. When applied to R. serbica leaves, widely used protein extraction protocols containing polyvinylpolypyrrolidone and ascorbate, as well as the phenol/SDS/buffer-based protocol recommended for recalcitrant plant tissues failed to eliminate persistent contamination and ensure high protein quality. Here we compared three protein extraction approaches aiming to establish the optimal one for both hydrated and desiccated R. serbica leaves. To evaluate the efficacy of these protocols by shotgun proteomics, we also created the first R. serbica annotated transcriptome database, available at http://www.biomed.unipd.it/filearrigoni/Trinity_Sample_RT2.fasta . The detergent-free phenol-based extraction combined with dodecyl-β-D-maltoside-assisted extraction enabled high-yield and high-purity protein extracts. The phenol-based protocol improved the protein-band resolution, band number, and intensity upon electrophoresis, and increased the protein yield and the number of identified peptides and protein groups by LC-MS/MS. Additionally, dodecyl-β-D-maltoside enabled solubilisation and identification of more membrane-associated proteins. The presented study paves the way for investigating the desiccation tolerance in R. serbica, and we recommend this protocol for similar recalcitrant plant material.

Resource allocation in response to herbivory and gall formation in Linaria vulgaris

Trehalose and its precursor, trehalose 6-phosphate (T6P), are essential regulators of plant response to abiotic and biotic stress. Here we used the specific host-insect interaction between Linaria vulgaris (Plantaginaceae) and stem-galling weevil, Rhinusa pilosa (Mecinini, Curculionidae) with the aim to distinguish carbohydrate allocation patterns in response to herbivory, gall formation (G1, 24 h after oviposition), and gall development (G2, 7 days after oviposition) under controlled conditions. The hypothesis is that herbivory and galling induce distinct responses in both leaves and stems, and that shifts in carbon allocations are regulated by signaling sugars. Systemic response to herbivory was accumulation of T6P and maltose. The main feature of G1 in the stems was accumulation of trehalose, accompanied by increased T6P, turanose and glucose content, oppositely to the leaves. In G2, galls had 3-folds higher weight than controls, with further accumulation of fructose, glucose, turanose, and total water-insoluble carbohydrates (TIC), while the sucrose/hexose ratio decreased. Analysis of fast chlorophyll fluorescence kinetic (OJIP) transients in G2 showed a slight decrease in quantum yield of electron transport flux from Q_A to Q_B, and towards photosystem I acceptor side, correlated with the decreased content of photosynthetic pigments and hexoses accumulation. Redistribution of photosynthates, and accumulation of T6P were induced in response to herbivory, indicating its signaling role. The results support the hypothesis that R. pilosa can induce plant reprogramming towards the accumulation of beneficial carbohydrates in developing gall by mechanisms which include both T6P and trehalose.

Biomonitoring of Urban Pollution Using Silicon-Accumulating Species, Phyllostachys aureosulcata 'Aureocaulis'

We investigated metal accumulation in bamboo leaves during three seasons at three urban locations differing in pollution levels. The higher content of Cu, Pb, and Zn in the leaves was in correlation with the highest bioavailable content of these elements in the soil at the most polluted location. The content of leaf trace elements was higher in summer and autumn compared to spring. Scanning electron microscopy with energy dispersive X-ray spectroscopy showed that Si accumulation in bamboo leaves was the highest in epidermis and vascular tissue, and was co-localized with trace metals. Analysis of phytoliths showed co-deposition of Al, C, and Si, implying the involvement of Si in metal detoxification. Compared to a common urban tree, linden, bamboo showed better capacity to maintain cellular redox homeostasis under deteriorated environmental conditions. The results suggest that bamboo can be efficiently used for biomonitoring of air and soil metal pollution and remediation in urban areas.

Physiological basis of differential zinc and copper tolerance of Verbascum populations from metal-contaminated and uncontaminated areas

Metal contamination represents a strong selective pressure favoring tolerant genotypes and leading to differentiation between plant populations. We investigated the adaptive capacity of early-colonizer species of Verbascum recently exposed to Zn- and Cu-contaminated soils (10-20 years). Two Verbascum thapsus L. populations from uncontaminated sites (NMET1, NMET2), one V. thapsus from a zinc-contaminated site (MET1), and a Verbascum lychnitis population from an open-cast copper mine (MET2) were exposed to elevated Zn or Cu in hydroponic culture under glasshouse conditions. MET populations showed considerably higher tolerance to both Zn and Cu than NMET populations as assessed by measurements of growth and net photosynthesis, yet they accumulated higher tissue Zn concentrations in the shoot. Abscisic acid (ABA) concentration increased with Zn and Cu treatment in the NMET populations, which was correlated to stomatal closure, decrease of net photosynthesis, and nutritional imbalance, indicative of interference with xylem loading and divalent-cation homeostasis. At the cellular level, the sensitivity of NMET2 to Zn and Cu was reflected in significant metal-induced ROS accumulation and ion leakage from roots as well as strong induction of peroxidase activity (POD, EC 1.11.1.7), while Zn had no significant effect on ABA concentration and POD activity in MET1. Interestingly, MET2 had constitutively higher root ABA concentration and POD activity. We propose that ABA distribution between shoots and roots could represent an adaptive mechanism for maintaining low ABA levels and unaffected stomatal conductance. The results show that metal tolerance can occur in Verbascum populations after relatively short time of exposure to metal-contaminated soil, indicating their potential use for phytostabilization.

The effects of manganese and copper in vitro and in vivo on peroxidase catalytic cycles

Here we present the results of in vitro and in vivo studies of the influence of Mn²+ and Cu²+ on the peroxidative and oxidative catalytic functions of class III peroxidase. Complex peroxidase catalysis by intermediates generated in the reaction was analyzed by utilizing the activating effect of Mn²+ and the inhibitory effect of Cu²+ on the oxidative reaction in vitro. p-Coumaric acid was used as an enzyme substrate in the peroxidative reaction and as a cofactor in the oxidative reaction. In order to correlate the observed in vitro effects with the in vivo situation, we exposed maize plants to excess concentrations of Mn²+ and Cu²+ in the hydroponic solutions. Copper severely arrested plant growth, while manganese exerted no significant effect. The effects on peroxidase activity and isoforms profile of root soluble and cell wall bound fractions were studied. Inhibition of the peroxidase oxidative function by copper was reversible, localized in the cell wall, and accompanied by disappearance of some and appearance of new cationic isoforms. Copper-mediated changes were suppressed by the presence of manganese, although Mn²+ treatment per se did not affect the activity of the peroxidase enzyme. The results on the peroxidase activity in maize roots grown with excess Mn²+ and Cu²+ point to the coupling between the oxidative cycle, root growth and different peroxidase isoforms.

Characterisation of phenol oxidase and peroxidase from maize silk

Silk of some maize genotypes contains a high level of phenolics that undergo enzymatic oxidation to form quinones, which condense among themselves or with proteins to form brown pigments. Two phenolic oxidizing enzymes, peroxidase (POD; EC 1.11.1.7) and polyphenol oxidase (PPO; EC 1.10.3.1), from maize (Zea mays L.) silk were characterised with respect to their preferred substrate, different isoforms and specific effectors. One browning silk sample with high, and two non-browning samples with low phenolic content were investigated. Although POD oxidizes a wide range of phenolic substrates in vitro, its activity rate was independent of silk phenolic content. PPO activity, detected with o-diphenolic substrates, was abundant only in browning silk, and low or absent in non-browning silk. Pollination increased POD but not PPO activity. Isoelectric-focusing (IEF) and specific staining for POD and PPO showed a high degree of polymorphism that varied with silk origin. The IEF pattern of POD revealed a number of anionic and several cationic isoenzymes, with the most pronounced having neutral pI 7 and a basic isoform with pI 10. Detected isoforms of PPO were anionic, except for one neutral form found only in browning silk, and occupied positions different from those of POD. Different inhibitory effects of NaN(3), EDTA, KCN, and L-cysteine, as well as different impacts of a variety of cations on the oxidation of chlorogenic acid, mediated by PPO or POD, were detected. The findings are discussed in terms of a possible roles of these enzymes in defence and pollination.

Induction of peroxidases and superoxide dismutases in transformed embryogenic calli of alfalfa (Medicago sativa L.)

Peroxidase (POD) and superoxide dismutase (SOD) enzyme activities were analyzed in non-regenerative transformed embryogenic lines of alfalfa (Medicago sativa L.) carrying wound-inducible oryzacystatin I (OC-I), wound-inducible oryzacystatin I antisense (OC-Ias), or hygromycin phosphotransferase (hpt) genes. All of the transformed lines analyzed had elevated levels of all POD isoforms. Three POD isoforms with pI values of approximately 4.5, 4.8, and 8.4, and one additional pair of isoforms with a pI value of approximately 8.8 were separated from tissue extracts of all transgenic lines. Isoelectrofocusing patterns revealed the induction of one isoform of SOD with a pI of about 5.6 in all transgenic lines compared with non-transformed embryogenic tissue. These results indicate that the process of transformation may disrupt redox homeostasis in alfalfa tissues.

Effectiveness of phenoxyl radicals generated by peroxidase/H2O2-catalyzed oxidation of caffeate, ferulate, and p-coumarate in cooxidation of ascorbate and NADH

The rate of ascorbate and nicotinamide adenine dinucleotide plus hydrogen (NADH) cooxidation (i.e., their nonenzymic oxidation by peroxidase/H2O2-generated phenoxyl radicals of three hydroxycinnamates: caffeate, ferulate and p-coumarate) was studied in vitro. The reactions initiated by different sources of peroxidase (EC 1.11.1.7) [isolates from soybean (Glycine max L.) seed coat, maize (Zea mays L.) root-cell wall, and commercial horseradish peroxidase] were monitored. Native electrophoresis of samples and specific staining for peroxidase activity revealed various isoforms in each of the three enzyme sources. The peroxidase sources differed both in the rate of H2O2-dependent hydroxycinnamate oxidation and in the order of affinity for the phenolic substrates. The three hydroxycinnamates did not differ in their ability to cooxidize ascorbate, whereas NADH cooxidation was affected by substitution of the phenolic ring. Thus, p-coumarate was more efficient than caffeate in NADH cooxidation, with ferulate not being effective at all. Metal ions (Zn2+ and Al3+) inhibited the reaction of peroxidase with p-coumarate and affected the cooxidation rate of ascorbate and the peroxidase reaction in the same manner with all substrates used. However, inhibition of p-coumarate oxidation by metal ions did not affect NADH cooxidation rate. We propose that both the ascorbate and NADH cooxidation systems can function as mechanisms to scavenge H2O2 and regenerate phenolics in different cellular compartments, thus contributing to protection from oxidative damage.

Characterization of NAD-dependent malate dehydrogenases from spinach leaves

Spinach leaves were used to extract isoforms of NAD-dependent malate dehydrogenase (NAD-MDH) (EC 1.1.1.37), either soluble or bound to microsomal, plasma, or chloroplast envelope membranes. All fractions were subjected to isoelectric focusing analysis, which showed that purified chloroplast envelopes contain an NAD-MDH isoform tightly bound to the membranes, since treatment with 0.5 or 1% Triton X-100 was not able to release the enzyme from the envelopes. In contrast, plasma membranes released an isoform with a pI of 3.5 following treatment with 0.5% Triton X-100. The most abundant soluble leaf isoform had a pI of 9, while the chloroplast stroma contained an isoform with a pI of 5.3. Kinetic analysis of oxaloacetate (OAA)-dependent NADH oxidation in different fractions gave different Km values for both substrates, the envelope- and plasma membrane-bound NAD-MDH exhibiting the highest affinities for OAA. Leaf plasma membrane-bound MDH exhibited a high capacity for both reaction directions (malate oxidation and OAA reduction), while the two chloroplast isoforms (stromal and envelope-bound) preferentially reduced OAA. Our results indicate that the chloroplast envelope contains a specifically attached NAD-MDH isoform that could provide direct coupling between chloroplast and cytosol adenylate pools.

Ascorbic acid and the oxidative processes in pea root cell wall isolates: characterization by fluorescence and EPR spectroscopy

A comparative fluorescence and oxygen radical-sensitive spin trap EPR spectroscopic study of isolated cell walls (with proteins or deproteinated), in the presence and absence of ascorbate and H(2)O(2) is presented. Fluorescence spectra indicate the presence of at least two fluorophores, one degraded and the other synthesized after reduction or oxidation, indicating phenol di/polymerization. DEPMPO spin trap measurements show that isolated cell walls are capable of oxygen-dependent hydroxyl radical generation in the absence of NADH or other reductants, ascorbate addition, or deproteination of the cell wall abolishing the signal due to hydroxyl radicals.

Induction of oscillations in chlorophyll fluorescence by re-illumination of intact isolated pea chloroplasts

Oscillations in chlorophyll fluorescence yield were observed upon re-illumination of intact isolated pea (Pisum sativumL.) chloroplasts that had attained their maximal rate of photosynthesis and had spent a short period in darkness. The oscillations depended on the length of the previous dark period, the length of previous illumination, and the reaction temperature. This finding confirms the presence of an "oscillatory center" in the chloroplasts temselves.