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UPDATE ON PLANT ANTIOXIDANTS

Biochemistry and Molecular Biology of Antioxidants in the Rhizobia-Legume Symbiosis^1,2

Departamento de Nutrición Vegetal, Estación Experimental de Aula Dei, Consejo Superior de Investigaciones Científicas, Apartado 202, 50080 Zaragoza, Spain (M.A.M., J.R., M.R.C., M.C.R., M.B.); and Biology Department, Reed College, Portland, Oregon 97202 (D.A.D.)

The complete reduction of molecular oxygen to water requires four electrons and is catalyzed by cytochrome oxidase in aerobic bacteria and mitochondria. However, 1% to 3% of all oxygen consumed by respiration is inevitably reduced to superoxide radicals and hydrogen peroxide (H₂O₂). These and other oxygen-derived molecules with moderate to very high reactivity are known as reactive oxygen species (ROS). The term includes free radicals (molecules with one or more unpaired electrons, such as the superoxide and hydroxyl radicals) and non-free radicals (molecules with no unpaired electrons, such as H₂O₂ and singlet oxygen). The main sources of ROS in plants under physiological conditions are respiration, photosynthesis, and N₂ fixation (Table I). In addition, ROS are produced at high rates when plants are exposed to abiotic, biotic, or xenobiotic stress. Similarly, the term reactive nitrogen species (RNS) refers to nitrogen-derived molecules with variable reactivity and includes free radicals (nitric oxide) and non-free radicals (peroxynitrite). Nitric oxide is involved in many key physiological processes in animals and, as shown in recent years, also in plants (Table I). It reacts with the superoxide radicals to form peroxynitrite and probably with thiol compounds to form nitrosothiols. The investigation of RNS is at present a truly novel and important field in plant biology.

The superoxide radical, H₂O₂, and nitric oxide have moderate reactivity toward biomolecules and, thus, may have some direct detrimental effects in plants. The superoxide radical inactivates dihydroxy-acid dehydratase (required for the synthesis of branched chain amino acids) and aconitase (required for the operation of the Krebs cycle) by oxidizing the iron-sulfur clusters at the active site, and ribonucleotide reductase (required for DNA synthesis) by oxidizing an essential Tyr radical. Also, H₂O₂ can inactivate Calvin cycle enzymes, metalloproteins such as superoxide dismutases (SODs), and hemoproteins such as nodule leghemoglobin (Dalton, 1995; Scandalios et al., 1997). However, the real threat of the superoxide radical and H₂O₂ is their potential to act as precursors of the hydroxyl radical. The hydroyxl radical can readily oxidize amino acid residues of proteins, fatty acids of phospholipids, and deoxy-Rib and bases in DNA (Halliwell and Gutteridge, 1999). Nitric oxide can directly inhibit iron-containing proteins (Neill et al., 2002), but its toxicity stems mainly from its ability to react with the superoxide radical to form peroxynitrite. This compound can induce lipid peroxidation, nitration of Tyr residues of proteins, oxidation of thiols, and nitration or deamination of DNA bases (Halliwell and Gutteridge, 1999).

However, the same three ROS or RNS mentioned above may perform useful roles in plants. This is largely because they show moderate reactivity and are mainly generated by enzymes; hence, their rates and subcellular sites of production may be under metabolic control. The superoxide radical and H₂O₂ are involved in lignification of cell walls, defense against pathogen attack, and sensing of, and subsequent adaptation to, stressful conditions. H₂O₂ can also induce programmed cell death during the plant's hypersensitive response to infection by modulating gene expression (Neill et al., 2002). Nitric oxide also acts as a signal molecule and is involved in the control of gene expression, hypersensitive response, antioxidant defense, organogenesis, and stomatal closure (Neill et al., 2002; Lamattina et al., 2003).

Plant cells contain an impressive array of antioxidant metabolites and enzymes that scavenge or prevent the formation of the most aggressive ROS and RNS, thus protecting cells from oxidative damage. In addition, antioxidant enzymes control the steadystate levels of the moderately reactive ROS and RNS, allowing them to perform important roles at specific sites, environmental conditions, or developmental stages of plants. Although antioxidants have multiple roles in diverse physiological processes in plants, we present here a restricted overview of the role of antioxidants in the rhizobia-legume symbiosis. Readers are referred elsewhere for a more general coverage of antioxidants in plants, in particular the excellent reviews by May et al. (1998) and Mittler (2002).

As a result of the complex and continuous molecular interplay between the bacteria and the plant, large amounts of ROS and possibly RNS are generated during the lifetime of nodules; hence, an important asset of antioxidant enzymes is expected in both symbiotic partners. These and other molecular studies of the symbiosis are greatly facilitated by selecting Medicago truncatula or Lotus japonicus as model legumes, respectively, for indeterminate or determinate nodulation (Udvardi, 2001). Both legume species have a small diploid genome, are autogamous, have a short generation time and large seed production, and are amenable to transformation and mutant screening. In addition, the chloroplastic genome of L. japonicus and the genomes of Sinorhizobium meliloti and Mesorhizobium loti (the bacterial components of the symbioses) have been entirely sequenced, and the nuclear genomes of M. truncatula and L. japonicus are being sequenced at a fast pace.

	ASCORBATE, GLUTATHIONE, AND HOMOGLUTATHIONE ARE MAJOR ANTIOXIDANTS OF NODULES

TOP ASCORBATE, GLUTATHIONE, AND... NODULES CONTAIN THREE TYPES... THE ASCORBATE-GLUTATHIONE... SODs AND CATALASES ARE... NODULES ALSO HAVE IMPORTANT... ROS AND RNS ARE... LITERATURE CITED

 
Ascorbate (vitamin C) is a water-soluble reductant that can be found in nodules at concentrations of 1 to 2 mM. Ascorbate is required for the progression of the cell cycle and for cell elongation. The latter effect has been attributed to its participation as cofactor of prolyl hydroxylase (required for the synthesis of Hyp-rich proteins of the cell wall) and to the ability of apoplastic ascorbate to alter the properties of the plasma membrane or to inhibit the cross-linking of Hyp-rich proteins by phenols (Horemans et al., 2000). However, the best known functions of ascorbate are based on its properties as an antioxidant. Ascorbate regenerates the -tocopherol oxidized by ROS at the membrane-cytosol interface, is a direct scavenger of most ROS, and is the substrate of ascorbate peroxidase (APX). The major pathway for ascorbate synthesis has been elucidated (Wheeler et al., 1998). The last step, catalyzed by L-galactono--lactone dehydrogenase, occurs in the inner membrane of mitochondria (Horemans et al., 2000).

The thiol tripeptide GSH (Glu-Cys-Gly) is also an abundant metabolite of plants, where it performs multiple functions, including transport and storage of sulfur, control of cell redox status, progression of the cell cycle, protection of protein thiol groups, and detoxification of heavy metals and xenobiotics (May et al., 1998). GSH is an important antioxidant in its own right but also as a substrate for glutathione reductase and glutathione peroxidase (GSH-PX). However, in some legumes, homoglutathione (Glu-Cys-Ala) may partially or completely replace GSH. Homoglutathione is the major tripeptide in nodules of soybean (Glycine max), common bean (Phaseolus vulgaris), and mung bean (Vigna radiata), whereas GSH is predominant in nodules of pea (Pisum sativum), alfalfa (Medicago sativa), and cowpea (Vigna unguiculata). In each case, the major thiol is present at concentrations of 0.5 to 1 mM. The synthesis of GSH and homoglutathione proceeds through two ATP-dependent steps catalyzed, respectively, by -glutamylcysteine synthetase and a specific glutathione or homoglutathione synthetase (Fig. 1). The enzymes from pea, mung bean, and tobacco (Nicotiana tabacum) leaves have been partially purified and localized to the cytosol and plastids (Rennenberg, 1997). The biochemical properties of such enzymes, along with the information gained for the nodule enzymes using molecular approaches, are summarized in Table II. Genomic and cDNA clones for all three enzymes have been isolated, and gene structures have been determined. The ecs gene of L. japonicus contains 15 exons with identical size and high sequence homology (78% identity) to that of Arabidopsis (Matamoros et al., 2003). In both M. truncatula and L. japonicus, the gshs and hgshs genes have 12 exons of identical size (except for the first ones, which are very close in size), show high sequence homology (83% identity between the coding sequences of the two genes), and are tandemly arranged (and with the same orientation) in the genome. These observations indicate that the two genes originated by duplication (Frendo et al., 2001; Matamoros et al., 2003). In L. japonicus, the genes are separated by only 8 kb, appear to be present as single copies, and encode proteins with putative plastid signal peptides. The expression patterns of gshs and hgshs are clearly different in the two model legumes. In M. truncatula, hgshs is preferentially expressed in the roots and nodules and gshs in the leaves (Frendo et al., 2001), whereas in L. japonicus, hgshs is expressed in the roots and leaves and gshs in the nodules (Matamoros et al., 2003). Why the hgshs gene was recruited during evolution exclusively in the legume family and is only expressed in some species or organs remain unsolved questions but the differential expression of gshs and hgshs do suggest specific roles for their enzymatic products.

View larger version (18K): [in this window] [in a new window]   Figure 1. Three types of peroxidases that can be found in legume nodules. The scheme depicts gene structures, proteins, and activities catalyzed by representative enzymes of each type: APX of pea leaf cytosol, GPX of horseradish roots, and GSH-PX of L. japonicus nodules. Gene diagrams show exons (except untranslated regions [UTRs]) in red, introns in yellow, and UTRs in blue. Numbers are length in base pairs. Protein diagrams show: a, in APX and GPX, the distal and proximal His residues (H) that bind the heme groups (in red); b, in GPX, the N- and C-terminal signal peptides, the four conserved disulfide bridges, and one of the eight glycosylated Asn residues (N*); and c, in GSH-PX, the plastid signal peptide and some important residues of the three typical domains ("signatures"). Numbers are length in amino acid residues. ASC, ascorbate; MDHA, monodehydroascorbate; RH2, artificial reductant.

Bacteroids also have high GSH concentrations due to their own -glutamylcysteine synthetase and glutathione synthetase (Moran et al., 2000). They lack homoglutathione synthetase, but significant amounts of homoglutathione are found in bean nodule bacteroids as a result of uptake from the host infected cells (Moran et al., 2000). The GSH may be internally consumed by bacteroids in metabolic reactions and maintenance of cellular redox status rather than being exported to the plant (Iturbe-Ormaetxe et al., 2001). Recently, a mutant of Rhizobium tropici has been isolated which is deficient in glutathione synthetase and contains only 3% of the GSH present in the wild-type strain (Riccillo et al., 2000). This mutant is sensitive to weak organic acids and to osmotic and oxidative stress, and the addition of GSH restores the responses to these stresses to wild-type levels. Interestingly, the mutant can form effective nodules on bean, but it is out competed by the wild-type strain, indicating that GSH is important for stress tolerance and the symbiotic process (Riccillo et al., 2000).

	NODULES CONTAIN THREE TYPES OF PEROXIDASES WITH DISTINCT FUNCTIONS

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APXs belong to the class I of hemoperoxidases (intracellular enzymes) and catalyze the reduction of H₂O₂ to water by ascorbate. In nodules, APX activity has been found in the cytosol and mitochondria (Dalton et al., 1993; Iturbe-Ormaetxe et al., 2001), but additional isoforms probably exist in peroxisomes and plastids, as occurs in leaves (Jiménez et al., 1997). Cytosolic APX has been purified from soybean nodules, cDNA clones isolated, and recombinant enzyme and antibody produced. The most important properties of APX are compiled in Table II. The cytosolic enzyme is stable in the absence of ascorbate, contrary to the chloroplastic isoforms, and is also immunologically distinct from them. Its amino acid sequence has little homology with guaiacol peroxidases (GPXs) but significant homology with yeast (Saccharomyces cerevisiae) cytochrome c peroxidase (Mittler and Zilinskas, 1992). The genes for cytosolic APX (apx1) of pea and Arabidopsis have 10 exons and nine introns (Fig. 1). The first intron is located in the 5'-UTR, and this may have an effect on transcription, perhaps enhancing expression level (Mittler and Zilinskas, 1992). This does not occur with the gene (apx2) encoding the two isoforms (stromal and thylakoidal) of chloroplastic APX (Shigeoka et al., 2002).

GPXs (also called "nonspecific" or "classical" peroxidases) are class III peroxidases (secretory enzymes) found in the extracellular spaces and vacuoles. They have been implicated in a wide range of processes, including lignification, suberization, auxin catabolism, defense against pathogens, salt tolerance, and oxidative stress. GPXs use phenolic compounds as substrates and are typically assayed with artificial electron donors. In nodules, they exist as multiple isoforms, but none of them have been characterized. APXs and leghemoglobins also display "GPX" activity but at much lower rates. However, APXs are inactivated by the thiol reagent, p-chloromercuribenzoate, because they contain free Cys residues, whereas archetypal GPXs (e.g. horseradish peroxidases) contain four conserved disulfide bridges. The use of such inhibitors is the basis for an assay to discriminate between GPXs and APXs (Amako et al., 1994). The two types of peroxidases share little homology, with the exception of the heme-binding domain, and the corresponding antibodies do not cross-react. The differences are also important at the gene level. The number and position of introns of GPXs (three or less) are very different from those of APXs (Fig. 1).

GSH-PXs are class I peroxidases that catalyze the reduction of H₂O₂, organic hydroperoxides, and lipid hydroperoxides to water by GSH. Once thought to be present only in animals and bacteria, it now seems that this enzyme is also present in plants. The first evidence came from citrus plants, which were found to contain phospholipid hydroperoxide GSH-PX activity (Beeor-Tzahar et al., 1995). This activity was clearly different from glutathione S-transferases (some isoforms of which may also exhibit GSH-PX activity) and was induced by salt stress. Since then, several cDNA clones encoding homologous enzymes have been isolated in pea and other higher plants (Mullineaux et al., 1998). All of them are predicted to contain Cys (UGU or UGC codons) at their active site instead of the rare seleno-Cys residue (UGA codon) found in mammalian phospholipid hydroperoxide GSH-PXs. Many plant GSH-PXs reported so far are predicted to be located in the plastids; however, putative cytosolic and peroxisomal isoforms were found in barley (Hordeum vulgare; Churin et al., 1999). We also have isolated several cDNA and genomic clones encoding GSH-PXs that are expressed in nodules of L. japonicus (Fig. 1). The deduced proteins contain the three conserved signatures (domains) found in animal and plant GSH-PXs and lack seleno-Cys. Prediction programs of subcellular localization indicate that there are cytosol and plastid isoforms. Two genes have been completely sequenced and found to comprise six exons of almost identical sizes (except for the first exon) but different intron sizes. Therefore, gene structures allow for a clear separation among the three types of nodule peroxidases (Fig. 1).

	THE ASCORBATE-GLUTATHIONE PATHWAY IS CRITICAL FOR NODULE FUNCTIONING

TOP ASCORBATE, GLUTATHIONE, AND... NODULES CONTAIN THREE TYPES... THE ASCORBATE-GLUTATHIONE... SODs AND CATALASES ARE... NODULES ALSO HAVE IMPORTANT... ROS AND RNS ARE... LITERATURE CITED

 
The initial product of APX is monodehydroascorbate (ascorbate free radical), which then disproportionates to ascorbate and dehydroascorbate. Monodehydroascorbate and dehydroascorbate are reduced back to ascorbate by specific reductases using NADH and GSH, respectively. Finally, the GSSG formed by dehydroascorbate reductase is reduced to GSH by glutathione reductase using NADPH. Therefore, the ascorbate-GSH pathway involves four enzymes operating in concert to remove H₂O₂ at the expense, ultimately, of the reducing power of NADH or NADPH (Fig. 2).

View larger version (89K): [in this window] [in a new window]   Figure 2. Antioxidant enzymes of legume nodules. ASC, Ascorbate; CAT, catalase; CuZnSODc, cytosol CuZnSOD; CuZnSODp, plastid CuZnSOD; DHA, dehydroascorbate; DR, dehydroascorbate reductase; EC, Glu-Cys; ECS, -glutamylcysteine synthetase; ETC, electron transport chain; GL, L-galactono--lactone; GLDH, L-galactono-–lactone dehydrogenase; GR, glutathione reductase; (h)GSH, (homo)glutathione, reduced form; (h)GSHS, (homo)glutathione synthetase; (h)GSSG, (homo)glutathione, oxidized form; Lb, leghemoglobin; MDHA, monodehydroascorbate; MR, monodehydroascorbate reductase; Ox met, oxidative metabolism.

APX was described above; hence, we will focus now on the other enzymes of the pathway (Fig. 2). Glutathione reductases are found in the cytosol, mitochondria, and bacteroids, but are probably present also in nodule plastids because the enzyme is abundant in chloroplasts and root plastids (Bielawski and Joy, 1986) and because cDNA clones encoding a putative plastid glutathione reductase have been isolated (Tang and Webb, 1994). It is likely that the enzymes of nodule mitochondria and plastids are coded for by a single gene, as occurs with the enzymes of pea leaves (Creissen et al., 1995). Some nodules, such as those of bean and soybean, synthesize homoglutathione rather than GSH; therefore, the enzyme is functionally a homoglutathione reductase. Monodehydroascorbate reductases are ubiquitous flavoproteins of plants that occur in soluble and membrane-bound isoforms. In nodules, two isoforms have been found, and at least one of them is associated with the cell wall. Only low enzyme levels are detected in the cytosol (Dalton et al., 1993). Thus, it appears that recycling of ascorbate through the ascorbate-GSH pathway in the cytosol is mainly accomplished by dehydroascorbate reductase, whereas monodehydroascorbate reductase may be involved in regeneration of apoplastic ascorbate, synthesis of Hyp-rich proteins, and lignification of cell walls. Very little is known about this enzyme in plants and, in particular, in nodules (Table II). Dehydroascorbate reductase is a monomeric protein with active thiol groups and has been localized to the cytosol and mitochondria of nodule host cells (Dalton et al., 1993; Iturbe-Ormaetxe et al., 2001). The ascorbate-GSH pathway seems to be operative in nodule compartments other than the cytosol. The four enzymes of the pathway have been detected in nodule mitochondria (Dalton et al., 1993; Iturbe-Ormaetxe et al., 2001). A model has been proposed (Iturbe-Ormaetxe et al., 2001) for bean nodule mitochondria, in which H₂O₂ generated in the inner membrane is removed by membrane-bound APX, and the resulting ascorbate oxidation products are regenerated to ascorbate by monodehydroascorbate, dehydroascorbate, and homoglutathione reductases in the matrix or the cytosol (Fig. 2). The enzymes of the ascorbate-GSH pathway have also been found in pea leaf peroxisomes (Jiménez et al., 1997); thus, the pathway is probably functional in nodule peroxisomes.

Several lines of evidence show that the ascorbate-GSH pathway is critical for nodule functioning (Dalton, 1995; Dalton et al., 1998). The activity, protein, and transcript of APX, the key enzyme of the pathway, are very abundant in nodules, particularly in the infected and parenchyma cells (Fig. 3A). In the infected cells, APX protects leghemoglobin and other redox-sensitive proteins from H₂O₂, whereas, in the nodule parenchyma (a few cell layers outside the infected zone), the enzyme may participate in the operation of the oxygen diffusion barrier. This barrier has been proposed to be located, for the most part, in the nodule parenchyma and controls oxygen entry into the infected zone. The parenchyma cells have not only high levels of APX but also of ascorbate and respiratory dehydrogenases (Fig. 3B). Thus, we proposed that the parenchyma cells would regulate oxygen access to the infected region by adjusting their respiratory activity (Dalton et al., 1998). The concentration of the resulting H₂O₂ then would be finely tuned by APX, allowing for H₂O₂ to act as a signal molecule for the "opening" or "closure" of the oxygen diffusion barrier (Minchin, 1997; Dalton et al., 1998).

View larger version (110K): [in this window] [in a new window]   Figure 3. Localization of APX, SODs, and H2O2 in alfalfa nodules. A, Immunofluorescence localization of cytosolic APX. High levels are evident in the central, infected region (arrowhead) and in a ring of cells in the nodule parenchyma (arrow; from Dalton et al., 1998). B, Tetrazolium staining of respiratory dehydrogenase activity. Activity is enhanced in the nodule parenchyma (arrow), indicating increased respiration associated with the oxygen diffusion barrier and probably with the enhanced level of APX protein shown in A (from Dalton et al., 1998). C, In situ hybridization of cytosolic CuZnSOD mRNA. Transcript is most abundant in the nodule apex (arrow), which include the meristem and invasion zones. D, In situ hybridization of MnSOD mRNA. Transcript is most abundant in the infected region, and especially in the infected cells (arrow). E and F, Localization of H2O2. Fresh nodule tissue was perfused with cerium chloride and processed for electron microscopy. The presence of H2O2 is marked by the deposition of cerium perhydroxide precipitates, which can be seen in the walls and matrix of infection threads (arrows in E) and in the cell walls and intercellular spaces of the cortex (arrows in F). Note that H2O2 can be also observed surrounding the bacteria within the threads (arrowhead in E).

There are further indications of the importance of the ascorbate-GSH pathway for N₂ fixation. The activities of all four enzymes are much higher (2- to 36-fold) in nodules than in uninfected roots. The enzyme activities and thiol contents are also substantially higher (1.5- to 5.5-fold) in effective than in ineffective nodules. Also, treatment of plants with fixed nitrogen (urea) inhibits N₂ fixation concomitantly with three enzyme activities of the pathway, indicating that there is a link between N₂ fixation and antioxidant defenses. The most compelling evidence for the connection between antioxidants and N₂ fixation comes from observations that direct infusion of ascorbate into stems of soybean plants leads to an increase in leghemoglobin content, a 4-fold increase in rates of N₂ fixation, and a substantial delay in nodule senescence (Bashor and Dalton, 1999). Inclusion of ascorbate and purified recombinant APX in an in vitro reconstitution system containing leghemoglobin and bacteroids results in improved oxygenation of leghemoglobin and up to a 4.5-fold increase in N₂ fixation (Ross et al., 1999). Collectively, these and other observations have confirmed that antioxidants play an important role in protecting and enhancing N₂ fixation.

	SODs AND CATALASES ARE CRITICAL FOR PROTECTION OF NITROGEN FIXATION AND OCCUR IN BOTH SYMBIOTIC PARTNERS

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SODs are a family of metalloenzymes that catalyze the dismutation of superoxide radicals into molecular oxygen and H₂O₂. Three classes of SODs, differing in their metals at the active site, may coexist in plants, and all of them have been found in the nodule plant fraction. The subcellular localizations and biochemical properties of the CuZnSOD, FeSOD, and MnSOD of nodules are presented in Table II. Recently, the proteins and transcripts of cytosolic CuZnSOD and mitochondrial MnSOD have been localized in alfalfa and pea nodules (M.C. Rubio, E.K. James, M.R. Clemente, B. Bucciarelli, C.P. Vance, and M. Becana, unpublished data). The CuZnSOD is predominant in the nodule apex (Fig. 3C), especially in the infection threads, cytosol adjacent to cell walls, and apoplast; the MnSOD is abundant in the infected zone, especially in the infected cells (Fig. 3D). An additional CuZnSOD isozyme, the plastid CuZnSOD, is localized to the amyloplasts, whereas MnSOD is also found in the bacteroids and bacteria within infection threads. The distinct tissue localizations of "cytosolic" CuZnSOD and MnSOD suggest specific functions for the two enzymes. The CuZnSOD may be associated with cell wall growth in the meristems, infection threads, and apoplast, and with the plant's response to bacterial infection. The MnSOD would play a role related to the protection and functioning of symbiotic tissue in mature nodules.

The structures of the genes encoding cytosolic CuZnSOD (sodCc) and mitochondrial MnSOD (sodA) of L. japonicus have been determined. The sodCc gene consists of eight exons; interestingly, the first intron is in the 5'-UTR, as occurs for the pea apx1 gene. The sodA gene has six exons with no apparent special features. The FeSODs are the most enigmatic class of SODs; in fact, the corresponding gene (sodB) was once thought to be present, or expressed, only in a few families of higher plants. The FeSODs, when present, appear to be localized exclusively in the chloroplast stroma. We have found FeSODs in nodules of most legumes examined and isolated cDNAs for some species. Two types of FeSOD were clearly recognized: the typical FeSOD localized in the plastids of alfalfa and pea nodules and an unusual FeSOD localized in the cytosol of cowpea nodules (Moran et al., 2003).

Bacteroids possess a MnSOD in the cytoplasm and a CuZnSOD in the periplasmic space. These enzymes are encoded by the respective bacterial sodA and sodC genes. The MnSOD of S. meliloti shows high amino acid sequence similarity with bacterial FeSODs and is a "cambialistic" enzyme; in other words, it remains active (though less so) when the Mn is replaced by Fe (Santos et al., 1999). Interestingly, the enzyme is resistant to H₂O₂ regardless of the metal at the active site. The sodA^– mutant of S. meliloti fails to differentiate into bacteroids and nodulates poorly (Santos et al., 2000). The CuZnSOD of S. meliloti is expressed during infection (Ampe et al., 2003), perhaps as a response of the bacteria to the superoxide radicals produced by the plant (see below).

Catalases decompose H₂O₂ to water and molecular oxygen without consuming reductants and, thus, may provide plant cells with an energy-efficient mechanism to remove H₂O₂ (Scandalios et al., 1997). However, catalases have a much lower affinity for H₂O₂ than APXs and are expected to be active only at subcellular sites where H₂O₂ or catalase concentrations are very high, such as the peroxisomes. Catalase from nodule peroxisomes was purified and appears to be similar to other higher plant enzymes. It is unclear if a specific catalase isoform occurs in nodule mitochondria, as was observed for maize (Zea mays) leaves (Scandalios et al., 1997). The production of oxygen by catalase would appear to be unfavorable in the case of bacteroids because nitrogenase and other proteins with high redox potential are readily inactivated by excess oxygen. Therefore, it is interesting that bacteroids do not contain peroxidases but have catalases instead. Free-living S. meliloti and other rhizobia have three catalases: two monofunctional catalases (KatA and KatC) and one bifunctional catalase-peroxidase (KatB). Only KatA is induced by H₂O₂ and highly expressed in bacteroids, whereas KatB and KatC are expressed by the bacteria within the infection threads (Jamet et al., 2003). Double mutants (katAkatC or katBkatC) are severely impaired in N₂ fixation, whereas the single mutants display no effect. Overall, these results demonstrate the essentiality of catalases during the infection process (Jamet et al., 2003).

	NODULES ALSO HAVE IMPORTANT ANTIOXIDANT DEFENSES AGAINST MEMBRANE DAMAGE AND IRON TOXICITY

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Several antioxidant enzymes are bound to plant membranes, such as the APX, monodehydroascorbate reductase, and MnSOD of peroxisomes (Corpas et al., 2001) and the APX of mitochondria (Iturbe-Ormaetxe et al., 2001). These enzymes protect membranes from ROS but may have additional useful roles. Peroxisomal monodehydroascorbate reductase can generate superoxide (a common characteristic with the chloroplastic enzyme), which may then be used by the plant as a signal molecule. Plant membranes are mainly protected against lipid peroxidation and other types of oxidative damage by small lipophilic molecules such as tocopherols, ubiquinol, lipoic acid, and flavonoids. -Tocopherol is found at concentrations of 15 µg g^–1 in both young and old soybean nodules (Evans et al., 1999). Ubiquinol (the reduced form of ubiquinone) and lipoic acid are abundant in membranes of mitochondria and other organelles, where they act as potent inhibitors of lipid peroxidation. However, they have not been quantified in nodules. Flavonoids and other polyphenols are found in nodules at concentrations of 0.4 to 4 mM. Some of these compounds have important antioxidant properties, protecting membranes by neutralizing lipid radicals (Moran et al., 1997). Polyamines are also abundant in nodules and inhibit lipid peroxidation in vitro, probably by their ability to associate with phospholipids and stabilize membranes (Fujihara et al., 1994).

Plant cells also have an adequate protection against iron-mediated toxicity. Iron in the free form or bound to small chelators is potentially toxic because it can catalyze formation of hydroxyl radicals. The exceptions seem to be phytic acid and certain phenolic compounds that are able to chelate iron in a catalytically inactive form and may inhibit oxidative damage of lipids and proteins in vitro (Moran et al., 1997). The supply of free iron must be tightly regulated because plants require a steady, low iron supply for the synthesis of iron-proteins, DNA, and some hormones (Briat and Lobréaux, 1997). This need must be carefully balanced against the potential toxicity of excess iron. The protein ferritin stores up to 4,500 atoms of iron in a form that avoids the deleterious effects of iron while controlling its availability for metabolic purposes. Plant ferritins are composed of a central iron-filled cavity surrounded by a shell of 24 identical subunits. Nodules have an active iron metabolism and abundant ferritin, which is localized in plastids and amyloplasts, much like the ferritin of leaves that is localized exclusively in the chloroplasts (Lucas et al., 1998; Matamoros et al., 1999). The ferritin protein and transcript increase very early in nodulation. Later in nodule development, the ferritin protein (but not its transcript) declines concomitantly with the increase in nitrogenase and leghemoglobin proteins. This suggests that ferritin is a reservoir of iron and supplies it for nitrogenase and leghemoglobin synthesis. Because changes in ferritin protein and transcript are not coordinated, ferritin expression may be posttranscriptionally regulated (Ragland and Theil, 1993). The ferritin content increases during natural and stress-induced senescence of nodules, although variations can be found depending on legume species and nodule tissue. This is most probably due to induction of ferritin expression by the iron released during degradation of leghemoglobin and other iron proteins (Lucas et al., 1998; Matamoros et al., 1999).

	ROS AND RNS ARE INVOLVED IN NODULE FORMATION AND SENESCENCE

TOP ASCORBATE, GLUTATHIONE, AND... NODULES CONTAIN THREE TYPES... THE ASCORBATE-GLUTATHIONE... SODs AND CATALASES ARE... NODULES ALSO HAVE IMPORTANT... ROS AND RNS ARE... LITERATURE CITED

 
Plants respond defensively to pathogen infection with a hypersensitive reaction, an early feature of which is the rapid and transient production of ROS ("oxidative burst") (Lamb and Dixon, 1997). Infection of legume roots by rhizobia also elicits a hypersensitive reaction. After the first nodule primordia have been induced, an increasing proportion of infection threads abort in a few cortical cells in which both rhizobia and host cells undergo necrosis. The hypersensitive reaction may be part of a mechanism whereby the plant controls infection and, thus, regulates nodulation (Vasse et al., 1993). As in the case of attack by pathogens, root cells respond to rhizobial infection with an enhanced production of superoxide and H₂O₂ (Santos et al., 2001; D'Haeze et al., 2002; Ramu et al., 2002). It has not been definitively proven that this is a genuine oxidative burst, but the finding that H₂O₂ accumulation is restricted to the very early stages of nodule formation in Sesbania rostrata (E.K. James, unpublished data) supports this hypothesis. Interestingly, one of the genes more rapidly induced by compatible rhizobia or Nod factors (rip1) seems to encode a peroxidase and has cis-elements in its promoter region that may be responsive to ROS. Because exogenous H₂O₂ is sufficient to activate rip1 transcription in the absence of Nod factors, ROS may act downstream in the signal transduction pathway (Ramu et al., 2002). In this respect, Ramu et al. (2002) and D'Haeze et al. (2002) have concluded that Nod factor-induced nodulation requires H₂O₂.

Most likely, the "early" production of H₂O₂ is part of an oxidative burst, but, in later stages of nodule formation, H₂O₂ accumulation may be more related to cell wall formation and cross-linking of glycoproteins, both of which are required for successful infection. However, an as yet unsolved question is why some rhizobia have success during infection and form functional nodules. It is thought that, during infection, rhizobia may escape or inhibit the defensive response. This inhibition has been attributed to the bacterial exopolysaccarides (González et al., 1996). The enzymes responsible for enhanced ROS formation during infection and nodule organogenesis have not been identified definitively. The superoxide radicals are formed in the infection threads (Santos et al., 2001), possibly by a membrane-bound NADPH-oxidase, much like the superoxide generation during the oxidative burst in activated neutrophils. Possible sources for H₂O₂ are cell wall peroxidases, germin-like oxalate oxidases, and diamine oxidases (Wisniewski et al., 2000). We have found that H₂O₂ accumulates in the walls and lumen of infection threads, surrounding bacteria within the threads, and in the apoplast of the nodule cortex (Fig. 3D). Based mainly on colocalization studies, we propose that CuZnSOD is a potential source of H₂O₂. This peroxide may be important for the cross-linking of cell wall proteins in the apoplast and of the matrix glycoprotein in the infection threads (Wisniewski et al., 2000).

Additional signal molecules may be important for nodule formation. Salicylic acid may be implicated in the early stages of infection because compatible Nod factors inhibit the accumulation of salicylic acid (a defensive response) in the root (Martínez-Abarca et al., 1998). Nitric oxide could be another signal molecule because both nitric oxide synthase-like activity (Cueto et al., 1996) and nitric oxide (Mathieu et al., 1998) have been detected in nodules. Recently, Corpas et al. (2001) have found nitric oxide synthase activity and its product in pea leaf peroxisomes. However, Klessig's group has identified the inducible nitric oxide-producing enzyme as a variant of the P protein of the Gly decarboxylase complex (Chandok et al., 2003). It will be of great interest to determine if nitric oxide is produced in the specialized nodule peroxisomes and to identify its origin. In any case, it is clear that the topic of RNS in nodules is an emerging area of study.

There is a second period in the lifetime of nodules characterized by an enhanced production of ROS and probably RNS. Large amounts of H₂O₂ accumulate in the cells and apoplast in the central zone of senescing soybean nodules (Alesandrini et al., 2003) and surrounding bacteroids in the senescent zone of alfalfa and pea nodules (E.K. James, M.C. Rubio, and M. Becana, unpublished). In the senescing nodule tissue, there is a major decrease in antioxidant defenses, oxidative degradation of leghemoglobin to nonfunctional green pigments, and enhanced autolytic processes (Mellor, 1989; Matamoros et al., 1999). These are all situations conducive to uncontrolled ROS and RNS production. As a consequence, oxidative damage of lipids, proteins, and DNA has been observed in nodules during natural (Evans et al., 1999) and stress-induced (Becana and Klucas, 1992; Matamoros et al., 1999) senescence. Similarly, the structural breakdown of organelles, symbiosomes, and bacteroids in the host cells usually accompanies senescence. All these structural and biochemical alterations may be interpreted in terms of a switch from a reductive to an oxidative state, which may be a general characteristic of plant senescence (Swaraj and Bishnoi, 1996).

	ACKNOWLEDGMENTS

 
We are most grateful to Carmen Pérez-Rontomé for her excellent job with figures and Euan James for helpful comments. Thanks are also due to all collaborators and colleagues who have contributed, directly or indirectly, to the information provided in this Update. We apologize to those colleagues whose work we were unable to cite because of space limitations.

Received April 16, 2003; returned for revision June 9, 2003; accepted July 15, 2003.

	FOOTNOTES

 
Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.103.025619.

¹ This work was supported by the National Science Foundation (grant nos. IBN–9507491 and IBN–9816583 to D.A.D.) and by the Dirección General de Investigación Científica (Spain; grant no. AGL–2002–02876 to M.B.).

² This paper is dedicated to Robert V. Klucas, our friend and mentor, whose wisdom and kind spirit will long remain as an inspiration to those who knew him.

^* Corresponding author; e-mail becana{at}eead.csic.es; fax 34–976–716145.

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TOP ASCORBATE, GLUTATHIONE, AND... NODULES CONTAIN THREE TYPES... THE ASCORBATE-GLUTATHIONE... SODs AND CATALASES ARE... NODULES ALSO HAVE IMPORTANT... ROS AND RNS ARE... LITERATURE CITED

 
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