Isolation of two cDNA clones from tomato containing two different superoxide dismutase sequences

The AlkB Homolog SlALKBH10B Negatively Affects Drought and Salt Tolerance in Solanum lycopersicum

ALKBH proteins, the homologs of Escherichia coli AlkB dioxygenase, constitute a single-protein repair system that safeguards cellular DNA and RNA against the harmful effects of alkylating agents. ALKBH10B, the first discovered N6-methyladenosine (m6A) demethylase in Arabidopsis (Arabidopsis thaliana), has been shown to regulate plant growth, development, and stress responses. However, until now, the functional role of the plant ALKBH10B has solely been reported in arabidopsis, cotton, and poplar, leaving its functional implications in other plant species shrouded in mystery. In this study, we identified the AlkB homolog SlALKBH10B in tomato (Solanum lycopersicum) through phylogenetic and gene expression analyses. SlALKBH10B exhibited a wide range of expression patterns and was induced by exogenous abscisic acid (ABA) and abiotic stresses. By employing CRISPR/Cas9 gene editing techniques to knock out SlALKBH10B, we observed an increased sensitivity of mutants to ABA treatment and upregulation of gene expression related to ABA synthesis and response. Furthermore, the Slalkbh10b mutants displayed an enhanced tolerance to drought and salt stress, characterized by higher water retention, accumulation of photosynthetic products, proline accumulation, and lower levels of reactive oxygen species and cellular damage. Collectively, these findings provide insights into the negative impact of SlALKBH10B on drought and salt tolerance in tomato plant, expanding our understanding of the biological functionality of SlALKBH10B.

Molecular Evolution and Functional Divergence of Stress-Responsive Cu/Zn Superoxide Dismutases in Plants

Superoxide dismutases (SODs), a family of antioxidant enzymes, are the first line of defense against oxidative damage and are ubiquitous in every cell of all plant types. The Cu/Zn SOD, one of three types of SODs present in plant species, is involved in many of the biological functions of plants in response to abiotic and biotic stresses. Here, we carried out a comprehensive analysis of the Cu/Zn SOD gene family in different plant species, ranging from lower plants to higher plants, and further investigated their organization, sequence features, and expression patterns in response to biotic and abiotic stresses. Our results show that plant Cu/Zn SODs can be divided into two subfamilies (group I and group II). Group II appeared to be conserved only as single- or low-copy genes in all lineages, whereas group I genes underwent at least two duplication events, resulting in multiple gene copies and forming three different subgroups (group Ia, group Ib, and group Ic). We also found that, among these genes, two important events-the loss of introns and the loss of and variation in signal peptides-occurred over the long course of their evolution, indicating that they were involved in shifts in subcellular localization from the chloroplast to cytosol or peroxisome and underwent functional divergence. In addition, expression patterns of Cu/Zn SOD genes from Arabidopsis thaliana and Solanum lycopersicum were tested in different tissues/organs and developmental stages and under different abiotic stresses. The results indicate that the Cu/Zn SOD gene family possesses potential functional divergence and may play vital roles in ROS scavenging in response to various stresses in plants. This study will help establish a foundation for further understanding these genes' function during stress responses.

Site-directed mutagenesis (P61G) of copper, zinc superoxide dismutase enhances its kinetic properties and tolerance to inactivation by H2O2

Superoxide dismutases (SODs) protect the cells by catalyzing the dismutation of harmful superoxide radicals (O₂^•-) into molecular oxygen (O₂) and hydrogen peroxide (H₂O₂). Here, a Cu, Zn SOD (WT) from a high altitude plant (Potentilla atrosanguinea) was engineered by substituting a conserved residue proline to glycine at position 61 (P61G). The computational analysis showed higher structural flexibility and clusters in P61G than WT. The P61G exhibited moderately higher catalytic efficiency (K_m = 0.029 μM, V_max = 1488) than WT protein (K_m = 0.038 μM, V_max = 1290.11). P61G showed higher thermostability as revealed from residual activity (72.25% for P61G than 59.31% for WT after heating at 80 °C for 60 min), differential calorimetry scanning and CD-spectroscopic analysis. Interestingly, the P61G mutation also resulted in enhanced tolerance to H₂O₂ inactivation than WT protein. The finding on enhancing the biophysico-chemical properties by mutating conserved residue could stand as an example to engineer other enzymes. Also, the reported mutant can be exploited in food and pharmaceutical industries.

The SOD Gene Family in Tomato: Identification, Phylogenetic Relationships, and Expression Patterns

Superoxide dismutases (SODs) are critical antioxidant enzymes that protect organisms from reactive oxygen species (ROS) caused by adverse conditions, and have been widely found in the cytoplasm, chloroplasts, and mitochondria of eukaryotic and prokaryotic cells. Tomato (Solanum lycopersicum L.) is an important economic crop and is cultivated worldwide. However, abiotic and biotic stresses severely hinder growth and development of the plant, which affects the production and quality of the crop. To reveal the potential roles of SOD genes under various stresses, we performed a systematic analysis of the tomato SOD gene family and analyzed the expression patterns of SlSOD genes in response to abiotic stresses at the whole-genome level. The characteristics of the SlSOD gene family were determined by analyzing gene structure, conserved motifs, chromosomal distribution, phylogenetic relationships, and expression patterns. We determined that there are at least nine SOD genes in tomato, including four Cu/ZnSODs, three FeSODs, and one MnSOD, and they are unevenly distributed on 12 chromosomes. Phylogenetic analyses of SOD genes from tomato and other plant species were separated into two groups with a high bootstrap value, indicating that these SOD genes were present before the monocot-dicot split. Additionally, many cis-elements that respond to different stresses were found in the promoters of nine SlSOD genes. Gene expression analysis based on RNA-seq data showed that most genes were expressed in all tested tissues, with the exception of SlSOD6 and SlSOD8, which were only expressed in young fruits. Microarray data analysis showed that most members of the SlSOD gene family were altered under salt- and drought-stress conditions. This genome-wide analysis of SlSOD genes helps to clarify the function of SlSOD genes under different stress conditions and provides information to aid in further understanding the evolutionary relationships of SOD genes in plants.

Auxin increases the hydrogen peroxide (H2O2) concentration in tomato (Solanum lycopersicum) root tips while inhibiting root growth

BACKGROUND AND AIMS

The hormone auxin and reactive oxygen species (ROS) regulate root elongation, but the interactions between the two pathways are not well understood. The aim of this study was to investigate how auxin interacts with ROS in regulating root elongation in tomato, Solanum lycopersicum.

METHODS

Wild-type and auxin-resistant mutant, diageotropica (dgt), of tomato (S. lycopersicum 'Ailsa Craig') were characterized in terms of root apical meristem and elongation zone histology, expression of the cell-cycle marker gene Sl-CycB1;1, accumulation of ROS, response to auxin and hydrogen peroxide (H2O2), and expression of ROS-related mRNAs.

KEY RESULTS

The dgt mutant exhibited histological defects in the root apical meristem and elongation zone and displayed a constitutively increased level of hydrogen peroxide (H2O2) in the root tip, part of which was detected in the apoplast. Treatments of wild-type with auxin increased the H2O2 concentration in the root tip in a dose-dependent manner. Auxin and H2O2 elicited similar inhibition of cell elongation while bringing forth differential responses in terms of meristem length and number of cells in the elongation zone. Auxin treatments affected the expression of mRNAs of ROS-scavenging enzymes and less significantly mRNAs related to antioxidant level. The dgt mutation resulted in resistance to both auxin and H2O2 and affected profoundly the expression of mRNAs related to antioxidant level.

CONCLUSIONS

The results indicate that auxin regulates the level of H2O2 in the root tip, so increasing the auxin level triggers accumulation of H2O2 leading to inhibition of root cell elongation and root growth. The dgt mutation affects this pathway by reducing the auxin responsiveness of tissues and by disrupting the H2O2 homeostasis in the root tip.

Intestinal cell targeting of a stable recombinant Cu-Zn SOD from Cucumis melo fused to a gliadin peptide

The mRNA encoding full length chloroplastic Cu-Zn SOD (superoxide dismutase) of Cucumis melo (Cantaloupe melon) was cloned. This sequence was then used to generate a mature recombinant SOD by deleting the first 64 codons expected to encode a chloroplastic peptide signal. A second hybrid SOD was created by inserting ten codons to encode a gliadin peptide at the N-terminal end of the mature SOD. Taking account of codon bias, both recombinant proteins were successfully expressed and produced in Escherichia coli. Both recombinant SODs display an enzymatic activity of ~5000U mg(-1) and were shown to be stable for at least 4h at 37°C in biological fluids mimicking the conditions of intestinal transit. These recombinant proteins were capable in vitro, albeit at different levels, of reducing ROS-induced-apoptosis of human epithelial cells. They also stimulated production and release in a time-dependent manner of an autologous SOD activity from cells located into jejunum biopsies. Nevertheless, the fused gliadin peptide enable the recombinant Cu-Zn SOD to maintain a sufficiently sustained interaction with the intestinal cells membrane in vivo rather than being eliminated with the flow. According to these observations, the new hybrid Cu-Zn SOD should show promise in applications for managing inflammatory bowel diseases.

Differential expression of copper-zinc superoxide dismutase gene of Polygonum sibiricum leaves, stems and underground stems, subjected to high-salt stress

In aerobic organisms, protection against oxidative damage involves the combined action of highly specialized antioxidant enzymes, such as copper-zinc superoxide dismutase. In this work, a cDNA clone which encodes a copper-zinc superoxide dismutase gene, named PS-CuZnSOD, has been identified from P. sibiricum Laxm. by the rapid amplification of cDNA ends method (RACE). Analysis of the nucleotide sequence reveals that the PS-CuZnSOD gene cDNA clone consists of 669 bp, containing 87 bp in the 5′ untranslated region; 459 bp in the open reading frame (ORF) encoding 152 amino acids; and 123 bp in 3′ untranslated region. The gene accession nucleotide sequence number in GenBank is GQ472846. Sequence analysis indicates that the protein, like most plant superoxide dismutases (SOD), includes two conserved ecCuZnSOD signatures that are from the amino acids 43 to 51, and from the amino acids 137 to 148, and it has a signal peptide extension in the front of the N-terminus (1–16 aa). Expression analysis by real-time quantitative PCR reveals that the PS-CuZnSOD gene is expressed in leaves, stems and underground stems. PS-CuZnSOD gene expression can be induced by 3% NaHCO₃. The different mRNA levels’ expression of PS-CuZnSOD show the gene’s different expression modes in leaves, stems and underground stems under the salinity-alkalinity stress.

Molecular cloning and expression of two cytosolic copper-zinc superoxide dismutases genes from Nelumbo nucifera

Two cytosolic copper-zinc superoxide dismutase (cytCuZnSOD) complementary deoxyribonucleic acid were achieved in Nelumbo nucifera (Elian). The active sites and common characteristics of cytCuZnSOD family were showed by homology modeling. The two recombinant proteins expressed by PET-32a vector showed the similar SOD activity (89.94 ± 0.54 U/mg) and could maintain more than 90% activity after incubation at 65°C. The subcellular location by green fluorescent protein revealed that these two isoforms were all located in cytosol and nucleus. The cytCuZnSODs were expressed in various parts of N. nucifera, which were expressed highest in the leafstalks and young leaves and lowest in the roots. The cytCuZnSOD messenger ribonucleic acids isolated from wounded leaves significantly increased at 1.5 h after treatment (HAT) with the highest expression at 3 HAT, after which the level decreased.

Molecular characterization of a cDNA encoding Cu/Zn superoxide dismutase from Deschampsia antarctica and its expression regulated by cold and UV stresses

BACKGROUND

The Copper/Zinc superoxide dismutase (Cu/ZnSOD) gene, SOD gene, was isolated from a Deschampsia antarctica Desv. by cDNA library screening. The expression of SOD gene in the leaves of D. antarctica was determined by RT-PCR and its differential expression of gene transcripts in conditions of cold and UV radiation stresses was revealed by northern blot.

FINDINGS

The molecular characterization shows that SOD cDNA is 709 bp in length, which translates an ORF of 152 amino acids that correspond to a protein of predicted molecular mass of 15 kDa. The assay shows that the expression of SOD gene increases when D. antarctica is acclimatised to 4 degrees C and exposed to UV radiation. These results indicate that the SOD gene of D. antarctica is involved in the antioxidative process triggered by oxidative stress induced by the conditions of environmental change in which they live.

CONCLUSION

The present results allow us to know the characteristics of Cu/ZnSOD gene from D. antarctica and understand that its expression is regulated by cold and UV radiation.

The expression of manganese superoxide dismutase gene from Nelumbo nucifera responds strongly to chilling and oxidative stresses

A manganese superoxide dismutase (Mn-SOD) gene, NnMSD1, was identified from embryonic axes of the sacred lotus (Nelumbo nucifera Gaertn.). The NnMSD1 protein contains all conserved residues of the Mn-SOD protein family, including four consensus metal binding domains and a signal peptide for mitochondrial targeting. Southern blot analysis suggests the existence of two Mn-SOD genes in sacred lotus. NnMSD1 was highly expressed in developing embryonic axes during seed development, but appeared in cotyledons only at the early stage of development and became undetectable in the cotyledons during late embryogenesis. The expression of the NnMSD1 gene in germinating embryonic axes, in response to various stresses such as heat shock, chilling, and exposure to stress-related chemicals, was also studied. Heat shock strongly inhibited the expression of the NnMSD1 gene, whereas the NnMSD1 transcript level increased strongly in chilling stress treatment. An increase in expression was also highly induced by H2O2 in germinating embryonic axes. The results suggest that the expression pattern of the NnMSD1 gene differed between developing axes and cotyledons, and that the NnMSD1 gene expression responds strongly to chilling and oxidative stress.

Over expression of cytosolic copper/zinc superoxide dismutase from a mangrove plant Avicennia marina in indica Rice var Pusa Basmati-1 confers abiotic stress tolerance

Antioxidant enzymes play an important role in conferring abiotic stress tolerance. Superoxide dismutase (SOD) is the first enzyme in the enzymatic antioxidative pathway. Halophytic plants like mangroves have been reported to have a high level of SOD activity, which plays a major role in defending the mangrove species against severe abiotic stresses. We had previously reported the isolation of Sod1, a cDNA encoding a cytosolic copper zinc superoxide dismutase from the mangrove plant Avicennia marina and its mRNA expression pattern during various oxidative and abiotic stresses. The present study is an extension of the previous study in further characterizing the Sod1 cDNA by transforming it into rice and analysing the transgenic plants for abiotic stress tolerance. Southern hybridization of A. marina genomic DNA using Sod1, revealed that this gene in A. marina genome is present as a single copy. The cDNA was cloned into a binary vector (pCAMBIA 1300) and transformed into indica rice var Pusa Basmati-1. Southern hybridization analysis of transgenic rice plants revealed stable integration of the Sod1 transgene in the rice genome. The mRNA transcript of Sod1 was detected by Northern hybridisation in the transgenic rice plants. SOD isozyme assay of the transgenic rice plants revealed the stable expression of the transgenic Sod1 protein. The transgenic plants were more tolerant to methyl viologen mediated oxidative stress in comparison to the untransformed control plants. The transgenic plants also withstood salinity stress of 150 mM of NaCl for a period of eight days while the untransformed control plants wilted at the end of the stress treatment in hydroponics. Pot grown transgenic plants could also tolerate salinity stress better than the untransformed control plants, when irrigated with saline water. The transgenic plants also revealed better tolerance to drought stress in comparison to untransformed control plants.

Superoxide dismutase transgenes in sugarbeets confer resistance to oxidative agents and the fungus C. beticola

Sugarbeets carrying superoxide dismutase transgenes were developed in order to investigate the possibility of enhancing their resistance to oxidative stress. Binary T-DNA vectors carrying the chloroplastic and cytosolic superoxide dismutase genes from tomato, were used for Agrobacterium-mediated transformation of sugarbeet petioles. The transgenic plants were subjected to treatments known to cause oxidative stress, such as the herbicide methyl viologen and a natural photosensitizer toxin produced by the fungus Cercospora beticola, namely cercosporin. The transgenic plants exhibited increased tolerance to methyl viologen, to pure cercosporin, as well as to leaf infection with the fungus C. beticola.

Molecular characterization of a cDNA encoding copper/zinc superoxide dismutase from cultured cells of Manihot esculenta

Superoxide dismutase (SOD) cDNA, mSOD2, encoding cytosolic copper/zinc SOD (CuZnSOD) cDNA was isolated from suspension-cultured cells of cassava (Manihot esculenta Crantz) by cDNA library screening, and its expression was investigated in relation to environmental stress. mSOD2 is 774 bp in length with an open reading frame (ORF) of 152 amino acids, corresponding to a protein of predicted molecular mass 15 kDa and a pI of 5.22. One copy of the mSOD2 gene was found to be present in the cassava genome by Southern analysis using an mSOD2 cDNA-specific probe. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed diverse expression patterns for the mSOD2 gene in various tissues of intact cassava plants, at various stages of the growth in suspension cultures, and in the leaf tissues exposed to different stresses. The mSOD2 gene was highly expressed in suspension-cultured cells and in the stems of intact plants. However, it was expressed at low levels in leaves and roots. During suspension cell growth, the mSOD2 transcript progressively increased during culture. Moreover, the mSOD2 gene in excised cassava leaves responded to various stresses in different ways. In particular, it was highly induced in leaf tissue by several abiotic stresses, including high temperature (37 degrees C), chilling (4 degrees C), methyl viologen (MV) exposure, and wounding treatment. These results indicate that the mSOD2 gene is involved in the antioxidative process triggered by oxidative stress induced by environmental change.

Long Distance Charge Redistribution Upon Cu,Zn-Superoxide Dismutase Reduction

Cu,Zn-superoxide dismutase (Cu,Zn-SOD) is a ubiquitous enzyme with an essential role in antioxidant defense. To better understand structural factors at the origin of the highly efficient superoxide dismutation mechanism, we analyzed the consequence of copper reduction on the electronic properties of the backbone and individual amino acids by using electrochemistry coupled to Fourier transform infrared spectroscopy. Comparison of data recorded with bovine erythrocyte and recombinant chloroplastic Cu,Zn-SOD from Lycopersicon esculentum, expressed as a functional tetramer in Escherichia coli and (14)N- or fully (15)N-labeled, demonstrated that the infrared changes were dominated by reorganizations of peptide bonds and histidine copper ligands. Two main infrared modes of histidine side chain, markers of metal coordination, were identified by using Cu- and Zn-methylimidazole models: the nu(C(4)C(5))at 1605-1594 cm(-1) or approximately 1586 cm(-1) for Ntau or Npi coordination, and the nu(C(5)Ntau) at approximately 1113-1088 cm(-1). These modes, also identified in Cu,Zn-SOD by using (15)N labeling, showed that the electronic properties of the histidine Ntau ligands of copper are mostly affected upon copper reduction. A striking conclusion of this work is that peptide groups from loops and beta-sheet largely participate in charge redistribution upon copper reduction, and in contrast, electronic properties of polar and charged amino acids of the superoxide access channel remain unaffected. This is notably shown for the strictly conserved Arg-143 by site-directed mutagenesis on chloroplastic Cu,Zn-SOD. Charge compensation by the peptide backbone and preserved electronic properties of the superoxide access channel and docking site upon copper reduction may be the determinant factors for the high reaction kinetics of superoxide with both reduced and oxidized Cu,Zn-SOD.

Assessment of cross-reactivity between Japanese cedar (Cryptomeria japonica) pollen and tomato fruit extracts by RAST inhibition and immunoblot inhibition

BACKGROUND

An association between pollinosis and sensitivity to fruits and vegetables has been reported. Although Japanese cedar (Cryptomeria japonica) pollinosis is one of the most widespread diseases in Japan, there have been no reports demonstrating cross-reactivity between Japanese cedar pollen and other plant food.

OBJECTIVE

The aim of this study was to demonstrate cross-reactivity between Japanese cedar pollen and tomato fruit (Lycopersicon esculentum) using RAST inhibition and immunoblot inhibition.

METHODS

The RAST and immunoblot inhibition were performed using sera from patients with oral allergy syndrome (OAS) after ingesting fresh tomatoes. We identified some proteins that took part in cross-reactive IgE by the determination of N-terminal amino acid sequences and a homology search through the SWISS-PROT database.

RESULTS

In the RAST inhibition, the bindings of IgE from the sera from four out of five (4/5) subjects to Japanese cedar pollen discs were inhibited by more than 50% by preincubation of the serum with tomato fruit extracts. Likewise, the IgE bindings to tomato fruit discs were inhibited more than 50% by Japanese cedar pollen extracts in 3/5 sera. In immunoblot inhibition, IgE binding activities of some protein bands on both membranes were decreased by heterologous inhibitors. However, the combinations of these protein bands involved in cross-reactivity were different between patients.

CONCLUSION

We have demonstrated cross-reactivity between Japanese cedar pollen and tomato fruit using RAST inhibition and immunoblot inhibition.

A novel superoxide dismutase with a high isoelectric point in higher plants. expression, regulation, and protein localization

Several isoforms of superoxide dismutase (SOD) with a high isoelectric point (pI) have been identified by isoelectric focusing chromatography in protein extracts from Scots pine (Pinus sylvestris) needles. One of these isoforms, a CuZn-SOD with a pI of about 10 and thus denoted hipI-SOD, has been isolated and purified to apparent homogeneity. A cDNA encoding the hipI-SOD protein was cloned and sequenced. Northern hybridization of mRNA isolated from different organs and tissues showed that hipI-SOD has a markedly different pattern of expression compared with chloroplastic and cytosolic SOD. Furthermore, the transcript levels of hipI-SOD and cytosolic SOD were found to respond differently to mechanical wounding, treatment with oxidized glutathione, paraquat, and ozone. Immunogold electron microscopy localized the hipI-SOD in the plasma membrane of sieve cells and the Golgi apparatus of albuminous cells. Moreover, high protein density was also detected in extracellular spaces such as secondary cell wall thickenings of the xylem and sclerenchyma and in intercellular spaces of parenchyma cells.

Structural organization, regulation, and expression of the chloroplastic superoxide dismutase Sod1 gene in maize

A cDNA and genomic clone encoding maize chloroplastic Cu/Zn superoxide dismutase Sod1 were isolated. Southern blot analysis indicated little homology between the chloroplastic (Sod1) and the cytosolic (Sod2, Sod4, Sod4A) cDNAs. Sequence analysis of the genomic clone revealed a promoter, transit peptide, and partial coding sequence. The promoter contained several response elements (e.g., for light, cold temperature, xenobiotics) that may be involved in the regulation of the Sod1 gene. Sod1 expression during development and in response to physiological and chemical stressors such as temperature, xenobiotics (paraquat), and light were examined.

The influence of alpha-amanitin on the NaCl-induced up-regulation of antioxidant enzyme activity in cotton callus tissue

Liquid suspensions of cotton callus tissue from a NaCl-sensitive cell line and a NaCl-tolerant cell line were subjected to the following treatments: (a) 0 and 150 mM NaCl, respectively (controls); (b) 75 and 250 mM NaCl, respectively; (c) 100 ng ml(-1) alpha-amanitin; or (d) pretreatment for 2 h with 100 ng ml(-1) alpha-amanitin followed by the respective NaCl treatments. The callus tissue was harvested at 0, 0.5, 1, 2, 4, and 8h and analyzed for antioxidant enzyme activity. In the NaCl-tolerant callus, the 250 mM NaCl treatment resulted in transient 2- to 4-fold increases above the control levels in the activities of ascorbate peroxidase, catalase, glutathione reductase, and peroxidase within 1 h after treatment, while superoxide dismutase activity increased 4-fold within 4 h. This rapid increase suggests that the up-regulation of antioxidant capacity is an early response to NaCl stress and perhaps provides protection against oxidative damage until other acclimating mechanisms can be invoked. In the control callus, peroxidase activity remained unchanged, and significant increases in the other enzymes were not observed until 8 h after treatment with 75mM NaCl. Pre-treatment with alpha-amanitin prior to the NaCl treatment completely inhibited the NaCl-induced increase in the activities of all five enzymes in both cell lines. This data supports the conclusion that the NaCl-induced up-regulation of antioxidant enzyme activity in cotton callus tissue is transcriptionally regulated, proceeding via a de novo synthesis of poly(A)+RNA and is not due to the translation of existing transcripts or the mobilization of existing enzyme pools. In addition, the results suggest that it is not only the up-regulation of antioxidant activity that bestows a degree of tolerance to environmental stress, but also the speed with which this response occurs.

Overexpression of iron superoxide dismutase in transformed poplar modifies the regulation of photosynthesis at low CO2 partial pressures or following exposure to the prooxidant herbicide methyl viologen

Chloroplast-targeted overexpression of an Fe superoxide dismutase (SOD) from Arabidopsis thaliana resulted in substantially increased foliar SOD activities. Ascorbate peroxidase, glutathione reductase, and monodehydroascorbate reductase activities were similar in the leaves from all of the lines, but dehydroascorbate reductase activity was increased in the leaves of the FeSOD transformants relative to untransformed controls. Foliar H2O2, ascorbate, and glutathione contents were comparable in all lines of plants. Irradiance-dependent changes in net CO2 assimilation and chlorophyll a fluorescence quenching parameters were similar in all lines both in air (21% O2) and at low (1%) O2. CO2-response curves for photosynthesis showed similar net CO2-exchange characteristics in all lines. In contrast, values of photochemical quenching declined in leaves from untransformed controls at intercellular CO2 (Ci) values below 200 microL L-1 but remained constant with decreasing Ci in leaves of FeSOD transformants. When the O2 concentration was decreased from 21 to 1%, the effect of FeSOD overexpression on photochemical quenching at limiting Ci was abolished. At high light (1000 micromol m-2 s-1) a progressive decrease in the ratio of variable (Fv) to maximal (Fm) fluorescence was observed with decreasing temperature. At 6(o)C the high-light-induced decrease in the Fv/Fm ratio was partially prevented by low O2 but values were comparable in all lines. Methyl viologen caused decreased Fv/Fm ratios, but this was less marked in the FeSOD transformants than in the untransformed controls. These observations suggest that the rate of superoxide dismutation limits flux through the Mehler-peroxidase cycle in certain conditions.

The role of ferredoxin-NADP+ reductase in the concerted cell defense against oxidative damage -- studies using Escherichia coli mutants and cloned plant genes

Ferredoxin-NADP+ reductases (FNR) participate in cellular defense against oxidative damage. Escherichia coli mutants deficient in FNR are abnormally sensitive to methyl viologen and hydrogen peroxide. Tolerance to these oxidants was regained by expression of plant FNR, superoxide dismutase, or catalase genes in the mutant cells. FNR contribution to the concerted defense against viologen toxicity under redox-cycling conditions was similar to that of the two major E. coli superoxide dismutases together, as judged by the phenotypes displayed by relevant mutant strains. However, FNR expression in sodA sodB strains failed to increase their tolerance to viologens, indicating that the FNR target is not the superoxide radical. Sensitivity of FNR-deficient cells to oxidants is related to extensive DNA damage. Incubation of the mutant bacteria with iron chelators or hydroxyl radical scavengers provided significant protection against viologens or peroxide, suggesting that oxidative injury in FNR-deficient cells was mediated by intracellular iron through the formation of hydroxyl radicals in situ. The NADP(H)-dependent activities of the reductase were necessary and sufficient for detoxification, without participation of either ferredoxin or flavodoxin in the process. Possible mechanisms by which FNR may exert its protective role are discussed.

The relationship between yield and the antioxidant defense system in tomatoes grown under heat stress

Four putative heat-tolerant tomato (Lycopersicum esculentum) cultivars (Tamasabro, Heat Wave, LHT-24, and Solar Set) and one putative heat-sensitive tomato cultivar (Floradade) were grown in the field under non-stress (average daily temperature of 26 degrees C) and heat-stress (average daily temperature of 34 degrees C) conditions. At anthesis, approximately five weeks after being transplanted to the field, leaf samples were collected for antioxidant analyses. Yield was determined by harvesting ripe fruit seven weeks after the collection of leaf samples. Heat stress resulted in a 79.1% decrease in yield for the heat-sensitive Floradade, while the fruit yield in the heat-tolerant cultivars Heat Wave, LHT-24, Solar Set, and Tamasabro was reduced 51.5%, 22.1%, 43.8%, and 34.8% respectively. When grown under heat stress, antioxidant activities were also greater in the heat-tolerant cultivars. Superoxide dismutase (SOD) activity increased up to 9-fold in the heat-tolerant cultivars but decreased 83.1% in the heat-sensitive Floradade. Catalase, peroxidase, and ascorbate peroxidase activity increased significantly in all cultivars. Only Heat Wave showed a significant increase in glutathione reductase in response to heat stress but all heat-tolerant cultivars exhibited significantly lower oxidized ascorbate/reduced ascorbate ratios, greater reduced glutathione/oxidized glutathione rations, and greater alpha-tocopherol concentrations compared to the heat-sensitive cultivar Floridade. These data indicate that the more heat-tolerant cultivars had an enhanced capacity for scavenging active oxygen species and a more active ascorbate-glutathione cycle and suggest a strong correlation between the ability to up-regulate the antioxidant defense system and the ability of tomatoes to produce greater yields when grown under heat stress.

The tomato gene for the chloroplastic Cu,Zn superoxide dismutase: regulation of expression imposed in transgenic tobacco plants by a short promoter

The chloroplastic Cu,Zn superoxide dismutase (SOD) has an important role in the defense against damage by oxygen radicals in the chloroplasts. Here, for the first time, we report on the isolation of a genomic DNA clone from tomato that contains all the coding sequence for the chloroplastic Cu,Zn SOD as well as a 442 bp DNA fragment upstream of the translational initiation site. The latter upstream sequence has a putative TATA box and a 285 bp promoter region, 5' of the apparent transcriptional initiation and a 157 bp leader region. The coding sequence is composed of 8 exons that are interspaced by 7 introns; we termed this gene SODCp;Le:1. The 442 bp fragment was cloned into a pBI101 vector, upstream of the uidA (GUS) gene, via transcriptional fusion. Agrobacterium-mediated transformation resulted in transgenic tobacco plants. The progeny (after self-pollination) of 14 transformed plants, which expressed GUS above a threshold of 1 nmol/min per mg protein, were found to fall into two distinct groups. In the seedlings of 10 lines (group A) GUS expression was enhanced by exposure to light. In 4 lines of this group maintenance for 3 days in the dark eliminated GUS activity. The seedlings of group B expressed GUS regardless of the light/dark regime. In plants of group A, GUS expression was also developmentally regulated: high GUS activity in young leaves, low activity in mature leaves and no activity in the roots. The results suggest that this short chloroplastic Cu,Zn SOD promoter contains motifs for developmental (spatial) regulation as well as motifs responsive to light (or to oxygen radicals resulting from light-driven photosynthesis).

Pinus sylvestris L. needles contain extracellular CuZn superoxide dismutase

Four new isoforms of superoxide dismutase (SOD; superoxide: superoxide oxidoreductase, EC 1.15.1.1.) were identified in extracellular washing fluid from Scots pine (Pinus sylvestris L.) needles. The isoforms had an apparent molecular mass of 33 kDa. No neutral carbohydrates were present in the enzymes. The enzymatic activities were inhibited by 3 mM NaCN. One of the putative extracellular SOD isoforms was purified and NH2-terminal-sequenced. The sequence contained the domain KAVAVL. The domains VEG and V(K/S)G, present in chloroplastic and cytosolic CuZn SODs of plants, respectively, were not detected. The enzyme was composed of two subunits of 17.8 kDa each. The isoelectric point was determined to be 6.5. The results suggest the existence of an extracellular SOD in Scots pine.

Cloning and characterization of a cDNA encoding the cytosolic copper/zinc-superoxide dismutase from sweet potato tuberous root

A full-length cDNA clone encoding a putative copper/zinc-superoxide dismutase (SOD) of sweet potato, Ipomoea batatas (L.) Lam. cv Tainong 57, was isolated from a cDNA library constructed in lambda gt10 from tuber root mRNA. Nucleotide sequence analysis of this cDNA clone revealed that it comprises a complete open reading frame coding for 152 amino acid residues. The deduced amino acid sequence showed higher homology (78-86%) with the sequence of the cytosolic SOD than that of the chloroplast SOD from other plant species. The residues required for coordinating copper and zinc are conserved as they are among all reported Cu/Zn-SOD sequences. In addition, it lacks recognizable plastic or mitochondrial targeting sequences. These data suggest that the isolated sweet potato clone encodes a cytosolic Cu/Zn-SOD.

Mutations in Cu/Zn superoxide dismutase gene are associated with familial amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a degenerative disorder of motor neurons in the cortex, brainstem and spinal cord. Its cause is unknown and it is uniformly fatal, typically within five years. About 10% of cases are inherited as an autosomal dominant trait, with high penetrance after the sixth decade. In most instances, sporadic and autosomal dominant familial ALS (FALS) are clinically similar. We have previously shown that in some but not all FALS pedigrees the disease is linked to a genetic defect on chromosome 21q (refs 8, 9). Here we report tight genetic linkage between FALS and a gene that encodes a cytosolic, Cu/Zn-binding superoxide dismutase (SOD1), a homodimeric metalloenzyme that catalyzes the dismutation of the toxic superoxide anion O2.- to O2 and H2O2 (ref. 10). Given this linkage and the potential role of free radical toxicity in other neurodenegerative disorders, we investigated SOD1 as a candidate gene in FALS. We identified 11 different SOD1 missense mutations in 13 different FALS families.

Enhanced oxidative-stress defense in transgenic potato expressing tomato Cu,Zn superoxide dismutases

The two cDNAs coding for the cytosolic (cyt) and the chloroplast-located (chl) Cu,Zn superoxide dismutases (SODs) of tomato (Perl-Treves et al. 1988) were cloned into respective binary vectors and mobilized into Agrobacterium strains. Potato tuber discs were infected with either of the two agrobacterial strains and cultured on selective medium containing kanaymcin. The integration of either of the cyt or the chl SOD transgenes was verified by Southern-blot hybridization. The enzymatic activity of the additional tomato chl Cu,Zn SOD could be distinguished from endogenous SOD activity since the latter isozyme migrated faster on SOD-activity gels. Several transgenic potato lines harboring either the cyt or the chl SOD genes of tomato showed elevated tolerance to the superoxide-generating herbicide paraquat (methyl viologen). After exposure of shoots to paraquat, tolerance was recorded either by scoring symptoms visually or by measurements of photosynthesis using the photoacoustic method. Root cultures from transgenic lines that harbored the additional cyt Cu,Zn SOD gene of tomato were tolerant to methyl viologen up to 10(-5) M; a lower tolerance was recorded in roots of transgenic lines that expressed the additional chl Cu,Zn SOD of tomato.

Metabolism of oxygen radicals in peroxisomes and cellular implications

Peroxisomes are subcellular respiratory organelles which contain catalase and H2O2-producing flavin oxidases as basic enzymatic constituents. These organelles have an essentially oxidative type of metabolism and have the potential to carry out different important metabolic pathways. In recent years the presence of different types of superoxide dismutase (SOD) have been demonstrated in peroxisomes from several plant species, and more recently the occurrence of SOD has been extended to peroxisomes from human and transformed yeast cells. A copper,zinc-containing SOD from plant peroxisomes has been purified and partially characterized. The production of hydroxyl and superoxide radicals has been studied in peroxisomes. There are two sites of O2- production in peroxisomes: (1) in the matrix, the generating system being xanthine oxidase; and (2) in peroxisomal membranes, dependent on reduced nicotinamide adenine dinucleotide (NADH), and the electron transport components of the peroxisomal membrane are possibly responsible. The generation of oxygen radicals in peroxisomes could have important effects on cellular metabolism. Diverse cellular implications of oxyradical metabolism in peroxisomes are discussed in relation to phenomena such as cell injury, peroxisomal genetic diseases, peroxisome proliferation and oxidative stress, metal and salt stress, catabolism of nucleic acids, senescence, and plant pathogenic processes.

The tomato Cu,Zn superoxide dismutase genes are developmentally regulated and respond to light and stress

The expression of the two Cu,Zn superoxide dismutase (SOD) genes of tomato was followed in different organs and plant developmental stages at the transcript and enzymatic activity levels. The cDNA clones used as probes code for the chloroplast Cu,Zn SOD (clone T1) and the cytosolic Cu,Zn SOD (clone P31). The two genes were found to display distinct expression patterns. While the T1 transcript was rare or absent from roots, stems and ripening fruits, the P31 transcript was very abundant in these organs. Shoot tips, flower buds, seedlings and young leaves contained high levels of the two mRNAs. During leaf expansion, the levels of both transcripts diminish markedly. Despite the diminished presence of transcripts, SOD activity levels of the corresponding cytosolic and chloroplast isozymes accumulated and were sustained throughout leaf expansion. In non-photosynthetic organs, the SOD-3 (cytosolic) isozyme contained most of the activity, while in the expanded leaf the SOD-1 (chloroplast) isozyme was more abundant. Light-regulated accumulation of both the P31 transcript (1.7-fold) and the T1 transcript (3-fold) was observed upon light exposure of etiolated seedlings. However, only SOD-1 activity was observed to increase, after a lag of a few hours. The levels of both transcripts increased in response to paraquat and mechanical wounding. The level of the cytosolic transcript and the respective isozyme activity increased dramatically during prolonged drought stress while the chloroplast transcript remained unaffected. The expression of both genes was enhanced by spraying tomato plants with ethephon--a compound that releases ethylene. Our data show that the expression of Cu,Zn SOD genes in tomato is modulated in response to a variety of factors and suggest the importance of oxyradical toxicity as well as the role of SOD in the defence mechanism of plants exposed to stress.

Characterization of the level, target sites and inheritance of cytosine methylation in tomato nuclear DNA

The tomato nuclear genome was determined to have a G + C content of 37% which is among the lowest reported for any plant species. Non-coding regions have a G + C content even lower (32% average) whereas coding regions are considerably richer in G + C (46%). 5-methyl cytosine was the only modified base detected and on average 23% of the cytosine residues are methylated. Immature tissues and protoplasts have significantly lower levels of cytosine methylation (average 20%) than mature tissues (average 25%). Mature pollen has an intermediate level of methylation (22%). Seeds gave the highest value (27%), suggesting de novo methylation after pollination and during seed development. Based on isoschizomer studies we estimate 55% of the CpG target sites (detected by Msp I/Hpa II) and 85% of the CpNpG target sites (detected by Bst NI/Eco RI) are methylated. Unmethylated target sites (both CpG and CpNpG) are not randomly distributed throughout the genome, but frequently occur in clusters. These clusters resemble CpG islands recently reported in maize and tobacco. The low G + C content and high levels of cytosine methylation in tomato may be due to previous transitions of 5mC----T. This is supported by the fact that G + C levels are lowest in non-coding portions of the genome in which selection is relaxed and thus transitions are more likely to be tolerated. This hypothesis is also supported by the general deficiency of methylation target sites in the tomato genome, especially in non-coding regions. Using methylation isoschizomers and RFLP analysis we have also determined that polymorphism between plants, for cytosine methylation at allelic sites, is common in tomato. Comparing DNA from two tomato species, 20% of the polymorphisms detected by Bst NI/Eco RII could be attributed to differential methylation at the CpNpG target sites. With Msp I/Hpa II, 50% of the polymorphisms were attributable to methylation (CpG and CpNpG sites). Moreover, these polymorphisms were demonstrated to be inherited in a mendelian fashion and to co-segregate with the methylation target site and thus do not represent variation for transacting factors that might be involved in methylation of DNA. The potential role of heritable methylation polymorphism in evolution of gene regulation and in RFLP studies is discussed.

Nutritional effect and expression of SODs: induction and gene expression; diagnostics; prospective protection against oxygen toxicity

The effect of micronutrient stress (either deficiency or toxicity) on the expression of different superoxide dismutase isoenzymes in plants is reviewed. The induction of Fe-SOD and Mn-SOD by different metals and the potential use of the metalloenzyme system SOD for the appraisal of the micronutrient status of plants, is examined. At subcellular level, evidence for the participation of peroxisomal SOD in the molecular mechanism of plant tolerance to Cu is presented, and the activated oxygen-dependent toxicity of a xenobiotic (clofibrate) in plant peroxisomes is examined.

Evolutionary aspects of superoxide dismutase: the copper/zinc enzyme

Copper/zinc superoxide dismutase is typically an enzyme of eukaryotes. The presence of the enzyme in the ponyfish symbiont Photobacterium leiognathi and some free living bacteria does not have an immediate explanation. Amino acid sequence alignment of 19 Cu/Zn superoxide dismutases shows 21 invariant residues in key positions related to maintenance of the beta-barrel fold, the active site structure including the electrostatic channel loop, and dimer contacts. Nineteen other residues are invariant in 18 of the 19 sequences. Thirteen of these nearly invariant residues show substitutions in Photobacterium Cu/Zn superoxide dismutase. Copper/zinc superoxide dismutase from the trematode Schistosoma mansoni shows an N-terminal sub-domain with a hydrophobic leader peptide, as in human extracellular superoxide dismutase which is a Cu/Zn enzyme. The latter also has a C-terminal sub-domain with preponderance of hydrophilic and positively charged residues. The amino acid sequence of this superoxide dismutase between the N-terminal and C-terminal regions shares many features of cytosolic Cu/Zn superoxide dismutase, including 20 of the 21 invariant residues found in 19 Cu/Zn enzymes, suggesting a similar type of beta-barrel fold and active site structure for the extracellular enzyme.

The rate of Cu,Zn superoxide dismutase evolution

The rate of amino acid replacement in Cu,Zn SOD greatly departs from the expectations of the molecular clock. We examine 27 Cu,Zn SOD sequences available and conclude that: (1) the SOD enzymes from different mammal families differ from each other by roughly the same number of replacements, which is consistent with a simultaneous mammalian radiation; (2) over the most recent 60 million years (MY) the rate of SOD evolution is fairly high (15 aa/100 aa/100 MYR) and may be considered constant; (3) the rate of accumulation of amino acid replacements since the divergence of fungi, plants and animals to the present is inconstant along different branches of the evolutionary tree; moreover it steadily decreases with time, to the same extent in all lineages; (4) some comparisons exhibit divergences that are in any case greater than expected from a Poisson process on the assumption of a molecular clock; (5) plant chloroplast enzymes display fewer differences from each other than cytoplasmic ones; (6) bacteriocuprein (from Photobacterium leiognathi), fluke and human extracellular SOD are all three extremely remotely related to one another and to the SOD of other eukaryotes. The process of consistent decline of the rate of evolution of Cu, Zn SOD can be described by a number of mathematical functions. We explore simple models that assume constant rates and might be applicable to other proteins or genes that apparently evolve at disparate rates.

Chloroplast and cytosol isozymes of CuZn-superoxide dismutase: their characteristic amino acid sequences

Isozymes of CuZn-superoxide dismutase (SOD) were purified from angiosperms (spinach and rice), fern (horsetail) and green alga (Spirogyra). Occurrence of CuZn-SOD was confirmed by its purification in the group of green algae which shows the phragmoplast type of cell division. Purified CuZn-SODs are divided to chloroplast and cytosol types by their cellular localization and immunological properties. Their amino acid compositions, absorption spectra, CD spectra, and sensitivity to hydrogen peroxide also are distinguished from each other. All organisms including Spirogyra contain both types of isozyme. Thus, the divergence of the two types of CuZn-SOD isozyme occurred immediately after its acquisition by the most evolved green algae. Amino acid sequences of amino-terminal regions of CuZn-SOD isozymes from spinach, rice and horsetail were determined and compared with those of CuZn-SODs from other plants. The chloroplast and cytosol isozymes of CuZn-SOD show each characteristic sequences. Sequence differences among the cytosol CuZn-SODs are greater than those among the chloroplast CuZn-SODs. These observations indicate that each type of isozyme had independently evolved after the acquisition of CuZn-SOD.

Genetic mapping of tomato cDNA clones encoding the chloroplastic and the cytosolic isozymes of superoxide dismutase

The isozyme pattern of superoxide dismutase (SOD) in tomato consists of two Cu,Zn isozymes located, respectively, in the chloroplast and in the cytosol, as well as additional isozymes of the Mn or Fe SOD type. We have shown that SOD-1 is the chloroplastic Cu,Zn SOD and is related to cDNA clone T10. Restriction fragment length polymorphism (RFLP) analysis was performed with two cDNA clones representing tomato Cu,Zn-superoxide dismutases. T10, coding for the chloroplast isozyme, was thus mapped to chromosome 11, between marker TG46 and TG108, while clone P31, coding for the cytosolic Cu,Zn SOD isozyme, was mapped to chromosome 1 between TG24 and TG81. SOD is associated with the response of plants to various environmental stresses; the mapping information presented here would permit the demonstration of this association by genetic analysis.

Transformed plants with elevated levels of chloroplastic SOD are not more resistant to superoxide toxicity

The petunia nuclear gene which encodes the chloroplast isozyme of superoxide dismutase, SOD-1, has been fused with an efficient rbcS promoter fragment and 3' flanking region and introduced into tobacco and tomato cells. Transformed plants carrying this chimeric gene have up to 50-fold the levels of SOD-1 which occur in wild-type plants. However, tobacco plants with 30- to 50-fold the normal SOD-1 activity do not exhibit resistance to the light-activated herbicide paraquat. Similarly, tomato plants with 2- to 4-fold increases in SOD-1 do not exhibit tolerance to photoinhibitory conditions known to increase superoxide levels (high light, low temperatures and low CO2 concentrations). Our data indicate that increasing the chloroplastic SOD level in a plant cell is not sufficient to reduce the toxicity of superoxide.

Characterization of the Bacillus stearothermophilus manganese superoxide dismutase gene and its ability to complement copper/zinc superoxide dismutase deficiency in Saccharomyces cerevisiae

Recombinant clones containing the manganese superoxide dismutase (MnSOD) gene of Bacillus stearothermophilus were isolated with an oligonucleotide probe designed to match a part of the previously determined amino acid sequence. Complementation analyses, performed by introducing each plasmid into a superoxide dismutase-deficient mutant of Escherichia coli, allowed us to define the region of DNA which encodes the MnSOD structural gene and to identify a promoter region immediately upstream from the gene. These data were subsequently confirmed by DNA sequencing. Since MnSOD is normally restricted to the mitochondria in eucaryotes, we were interested (i) in determining whether B. stearothermophilus MnSOD could function in eucaryotic cytosol and (ii) in determining whether MnSOD could replace the structurally unrelated copper/zinc superoxide dismutase (Cu/ZnSOD) which is normally found there. To test this, the sequence encoding bacterial MnSOD was cloned into a yeast expression vector and subsequently introduced into a Cu/ZnSOD-deficient mutant of the yeast Saccharomyces cerevisiae. Functional expression of the protein was demonstrated, and complementation tests revealed that the protein was able to provide tolerance at wild-type levels to conditions which are normally restrictive for this mutant. Thus, in spite of the evolutionary unrelatedness of these two enzymes, Cu/ZnSOD can be functionally replaced by MnSOD in yeast cytosol.