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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. ROS Accumulation during Plant-microorganism Interactions
  5. Scavenging and ROS Producing Systems during Plant-microorganism Interactions
  6. Conclusions
  7. Acknowledgments
  8. References

Reactive Oxygen Species (ROS) are continuously produced as a result of aerobic metabolism or in response to biotic and abiotic stresses. ROS are not only toxic by-products of aerobic metabolism, but are also signalling molecules involved in several developmental processes in all organisms. Previous studies have clearly shown that an oxidative burst often takes place at the site of attempted invasion during the early stages of most plant-pathogen interactions. Moreover, a second ROS production can be observed during certain types of plant-pathogen interactions, which triggers hypersensitive cell death (HR). This second ROS wave seems absent during symbiotic interactions. This difference between these two responses is thought to play an important signalling role leading to the establishment of plant defense. In order to cope with the deleterious effects of ROS, plants are fitted with a large panel of enzymatic and non-enzymatic antioxidant mechanisms. Thus, increasing numbers of publications report the characterisation of ROS producing and scavenging systems from plants and from microorganisms during interactions. In this review, we present the current knowledge on the ROS signals and their role during plant-microorganism interactions.

inline imageChristophe Dunand (Corresponding author)

Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. ROS Accumulation during Plant-microorganism Interactions
  5. Scavenging and ROS Producing Systems during Plant-microorganism Interactions
  6. Conclusions
  7. Acknowledgments
  8. References

Reactive Oxygen Species (ROS), such as superoxide anion (O2•−) and hydrogen peroxide (H2O2), are by-products constantly produced during normal metabolic processes, such as photosynthesis or glycolysis. ROS produced at high levels have first been described as lethal for the cell integrity. However, high ROS production is also necessary for plant defense (oxidative burst, necrosis). Currently, their involvement as signal molecule during cellular growth, control of stomata closing (Pei et al. 2000), plant-pathogen interactions (Apel and Hirt 2004), programmed cell death (Gechev and Hille 2005) and stress responses have been demonstrated in plants (Laloi et al. 2007; Miller et al. 2007). The regulation and the involvement of ROS in Legume –Rhizobia symbiotic relations (Santos et al. 2001; Ramu et al. 2002; Shaw and Long 2003; Rubio et al. 2004) and during the establishment of both endo- and ectomycorrhiza have also been described (Fester and Hause 2005; Baptista et al. 2007).

In both symbiotic and pathogenic relations, the transient production of ROS is detected in the early events of plant-microorganism interactions and appears as the only common feature of the plant responses. This production called oxidative burst could be considered as a specific signal during the interaction process.

Prevention of ROS toxicity and control of ROS signalling require a large gene network, the so called “ROS gene network” is composed of at least 150 genes in Arabidopsis (Mittler et al. 2004). A smaller network is also detected in microorganisms (Passardi et al. 2007). Several families of proteins from plants and microorganisms are associated with the regulation of ROS levels (Table 1). Among them, catalases (Kat), detected in all kingdoms, and catalase-peroxidases (CP), present in some fungi and in the majority of bacteria, can both reduce H2O2 (Passardi et al. 2007). Other proteins detected in all kingdoms can generate ROS such as NADPH oxidases (NOx/RBOH), or scavenge ROS such as glutathione peroxidases (GPx) (Margis et al. 2008), peroxiredoxins (Rouhier and Jacquot 2002) and thioredoxins (Alkhalfioui et al. 2008). The plant specific Class III peroxidases (Prx) are also members of the “ROS gene network” and have the interesting capacity to both scavenge and produce ROS (Passardi et al. 2004).

Table 1. ROS scavenging and producing protein family distribution across the major kingdoms
 ProkaryotesPlantsFungiOther eukaryotes
  1. The presence or the absence of the different families in the various taxonomic groups has been identified. * presence due to lateral gene transfer.

Animal peroxidase (12 subfamilies)
Catalase (Kat)
Di-haem peroxidase superfamily   
Class I peroxidase:
 Ascorbate peroxidase (APx)  
 Catalase peroxidase (CP) **
 Cytochrome C peroxidase (CcP)  
Class III peroxidase   
Thioredoxin (TRX)-like superfamily    
 Glutathione peroxidase (GPx)
 Peroxiredoxin
 Other Thioredoxin (AhpC/TSA…)
NADPH oxidase 
Organic hydroperoxide resistance (ohr) – thiol-dependent peroxidase   
Superoxide Dismutase (SOD)

This review describes the involvement of ROS and highlights the different ROS producing and ROS scavenging enzymatic systems characterized during plant biotic interactions. Special attention will be paid to plant symbiotic interactions. In the meantime, there are several recent specialised reviews where the reader can find more information about the role of ROS during plant abiotic stresses (Miller et al. 2008; Torres and Dangl 2005).

ROS Accumulation during Plant-microorganism Interactions

  1. Top of page
  2. Abstract
  3. Introduction
  4. ROS Accumulation during Plant-microorganism Interactions
  5. Scavenging and ROS Producing Systems during Plant-microorganism Interactions
  6. Conclusions
  7. Acknowledgments
  8. References

During the first minutes of interaction between plants and microorganisms, a molecular dialogue involving several signal molecules, takes place in the rhizosphere and at the cell surface, leading to physical interaction. For example, in the case of the Legume –Rhizobia symbiotic interaction, flavonoids from the plant root exudates induce the synthesis of Nodulation Factor (NF) from Rhizobia. Both compounds are responsible for the setup of the early interaction steps and for the establishment of the new root organ, the nodule (for review, Oldroyd and Downie 2008). A similar dialogue is observed during mycorrhizal fungus and plant interaction leading to the production of plant strigolactons (Akiyama et al. 2005) and putative Myc factor by the fungus (Kosuta et al. 2003).

Plant-pathogen interactions

One of the most rapid defense reactions to pathogen attack is the so-called oxidative burst, which constitutes to the ROS production, primarily superoxide (O2•−) and hydrogen peroxide (H2O2), at the site of attempted invasion (Apel and Hirt 2004). This response is involved in pathogenic as well as in symbiotic interactions. Doke first reported the oxidative burst (Doke 1983), demonstrating that potato tuber tissue generated O2•− that is rapidly transformed into H2O2 following inoculation with an avirulent oomyceta Phytopthera infestans. Similar H2O2 production is also observed during avirulent interaction between the bacteria Pseudomonas syringae strain DC3000 and Arabidopsis (Alvarez et al. 1998). A virulent race of the same pathogen failed to induce O2•− production. Subsequently, O2•− generation has been identified in a wide range of plant-pathogen interactions involving avirulent bacteria, fungi, and viruses (Low and Merida 1996). Since then, further research has shown that avirulent pathogens induce a biphasic ROS production in plants, consisting of a low amplitude first phase, followed by a much higher and sustained accumulation during the second phase (Lamb and Dixon 1997; Torres et al. 2006). However, only the first phase has been detected during interactions with virulent pathogens (Bolwell et al. 2002). Furthermore, in the case of symbiotic interactions, ROS have also been observed but a suppression of the second wave of ROS seems to take place (Shaw and Long 2003; Lohar et al. 2007). This second response or lack of response is thought to play an important signalling role in the activation of plant defense. ROS have therefore been proposed to play a key role in the establishment of plant defense responses (Levine et al. 1994). In fact, the important ROS accumulation during the second phase has been reported to precede the hypersensitive response (HR) cell death that often accompanies successful pathogen recognition leading to the incompatible interaction (Mehdy 1994; Levine et al. 1996).

These events establish the involvement of ROS signalling in the activation or deactivation of the plants defense processes during different plant-microorganism interactions.

Legume – Rhizobia symbiotic interactions

The symbiosis between legumes and compatible Rhizobia takes place in a nitrogen-limited environment thanks to a molecular dialogue between the two actors. Rhizobia secret Nod Factors (NFs) in response to plant root exudates containing flavonoids. The perception of these NFs by the plant triggers several responses such as ion changes, cytoplasmic alkanisation, calcium oscillations and gene expression leading to the formation of an infection thread and a new organ, the root nodule, containing the nitrogen fixing rhizobia bacteroid (Cardenas et al. 2000; Oldroyd and Downie 2004). The plant provides bacteria with energy and a micro-aerobic environment compatible with nitrogenase activity. In exchange, bacteria provide the plant with a nitrogen supply. Nodules represent therefore a unique model for the study of developmental processes, plant-microorgansim and carbon/nitrogen/oxygen metabolism interactions.

The involvement of ROS during the Legume –Rhizobium symbiosis has been highlighted during this last decade (Pauly et al. 2006). For example, during the establishment of the symbiotic interaction, by using Nitroblue tetrazolium (NBT) that forms a dark blue precipitate with O2•− (Bielski et al. 1980), can be detected in infection threads, indicating that O2•− is produced during the infection process and could have a role in the control of the bacteria development (Santos et al. 2001; Ramu et al. 2002). ROS production in the infection threads is dependent on the NFs production because no ROS was observed when Medicago truncatula plants were inoculated with bacteria unable to produce NFs (Ramu et al. 2002). Recently, Jamet et al. (2007) showed that an S. meliloti mutant impaired in H2O2 steady state is affected in its ability to establish an optimal symbiosis. This clearly indicates the role of H2O2 in the early steps of the interaction (Jamet et al. 2007). Moreover, the generation of ROS in the cortical cells of M. truncatula roots after inoculation with Sinorhizobium meliloti was observed in vivo, using a ROS fluorescent probe (Peleg-Grossman et al. 2007). Moreover, a transient increase in intracellular ROS level at the tip of growing Phaseolus vulgaris root hairs has been shown within a few minutes after treatment with NFs (Cardenas et al. 2008). However, the extracellular ROS situation may be different in the very early steps of the symbiotic interaction, where the production of H2O2 appears to be inhibited by NFs treatment (Shaw and Long 2003; Lohar et al. 2007) or at least not induced. In the same way, a S. meliloti nodC mutant, defective in NFs biosynthesis, triggers an important increase in H2O2 accumulation in Medicago sativa roots after inoculation (Bueno et al. 2001). Moreover, the compatible interaction between M. sativa and S. meliloti is linked, at least in part, with an increase of the antioxidant defense (particularly catalase and lipoxygenase) during the preinfection period (Bueno et al. 2001).

More recently, the role of ROS in root hair deformation in the M. truncatula – S. meliloti symbiosis has been highlighted (Lohar et al. 2007). Exogenous application of ROS as well as the inhibition of ROS production using diphenylene iodonium (DPI), a commonly used NADPH oxidase inhibitor, both prevented root hair swelling and branching normally induced following treatment with NFs. However, transient treatment of M. truncatula roots with DPI, mimicked NFs treatment and resulted in root hair branching in the absence of NFs. Interestingly, the same transient DPI treatment on non-legumes such as Arabidopsis thaliana and Lycopersicon esculentum did not induce root hair branching. These results suggest a role for the transient reduction of ROS accumulation in governing NF-induced root hair deformation in legumes (Lohar et al. 2007).

The transient decrease of intracellular ROS accumulation in legume root hairs, in response to rhizobial secretion of NFs, seems to play a key role in a compatible Legume-Rhizobium interaction by actively promoting the root infection by bacteria. However, without recognition of the NFs or by using non host NFs, the plant seems to consider the bacteria as a pathogen and mobilizes its defense mechanisms.

Plant-fungus symbiotic interactions

Most land plants can form mutualistic symbiosis with mycorrhizal fungi which can be divided into two categories: ectomycorrhizae (EM) with extracellular hyphal structures and endomycorrhizae or arbuscular mycorrhizae (AM) with intracellular hyphal structures (Bonfante and Anca 2009). In AM symbioses, fungal hyphae form appressoria at the root surface, before intercellular invasion of epidermal and root cortical cells (Harrison 2005). Intensive nutrient exchange takes place across membrane interfaces between the fungus and the plant during symbiosis. The fungus provides the plant with nutrients like phosphorus, which the plant can have difficulty extracting from the soil. In return, the plant delivers carbon and lipids to the fungal symbionts.

As observed during the other plant-microorganism interactions, ROS have also been evidenced in mycorrhizal symbiosis: in the M. truncatula – Glomus intraradices interaction, H2O2 accumulation observed in plant cells was hypothesized to be a consequence of activation of a plant plasma membrane NADPH oxidase (NOx) in response to the fungus (Salzer et al. 1999), analogous to what occurs during the HR. In this case, H2O2 accumulation is mostly observed in arbuscule-containing cells, more precisely surrounding the arbuscular structures. This suggests that ROS play a role in the control of fungal proliferation within the plant (Fester and Hause 2005). However, the involvement of ROS during the early plant-AM fungus interactions has yet to be studied. Recently, H2O2 production was also reported during the symbiosis between Gigaspora margarita and two legume species including M. truncatula and Lotus japonicus, mainly located in the intraradical fungal structures (Lanfranco et al. 2005).

Evidence for the participation of ROS and antioxidant systems in ectomycorrhizal symbiosis has been found between the fungus Pisolithus tinctorius and the plant Castanea sativa (Baptista et al. 2007). During the early stages of this symbiosis, three peaks of H2O2 production were detected in C. sativa 2 h, 5 h and 11 h post inoculation, the first two coinciding with O2•− bursts. It is noteworthy that no O2•− was detected by NBT staining in P. tinctorius hyphae. The first phase of production of O2•− seems to be extracellular which suggests that the early EM fungus-plant interaction takes place at the cell wall and plasma membrane surfaces (Baptista et al. 2007). This first phase is similar to pathogenic attack responses, where the main sources of ROS production have been identified as membrane-bound NOx (Wojtaszek 1997). During the second phase of ROS production, O2•− accumulates in microdomains within cells. This could result from activation of the ROS-producing systems and down-regulation of the ROS-scavenging ones. Furthermore, superoxide dismutases (SOD) appear to be up-regulated and catalases (Kat) to be down-regulated during these early stages. This could explain the H2O2 accumulation, as SOD convert O2•− to H2O2 and Kat convert H2O2 to water. The combined action of these two enzymes seems play an important role during plant-EM fungus interactions (Baptista et al. 2007).

Once again, ROS seem to play an important role in the different symbiotic interactions. These results suggest a main role in the control of the fungus proliferation in the plant. However, knowledge about ROS during early symbiotic interactions between plants and fungi is very limited, due to the asynchronous nature of the mycorrhization process.

Scavenging and ROS Producing Systems during Plant-microorganism Interactions

  1. Top of page
  2. Abstract
  3. Introduction
  4. ROS Accumulation during Plant-microorganism Interactions
  5. Scavenging and ROS Producing Systems during Plant-microorganism Interactions
  6. Conclusions
  7. Acknowledgments
  8. References

ROS are known to play major roles in various plant and microorganism developmental processes, such as cell elongation (root hairs, pollen tube or appressoria growth) and during biotic interactions (Figure 1). In order to avoid ROS accumulation leading to cell death (Mittler et al. 2004) organisms have evolved enzymatic and non-enzymatic antioxidant mechanisms constantly generating and deteriorating ROS (Figure 1). In plants, ROS are unavoidable by-products of biochemical pathways, such as glycolysis and photosynthesis. As a result, plants have evolved enzymatic and non-enzymatic antioxidant mechanisms to eliminate ROS and avoid oxidative destruction (Apel and Hirt 2004). On the other hand, ROS production is necessary for cell elongation (root hairs, appresoria growth) and plant-microorganism interactions. It is therefore necessary for the plant to possess very complex and well-tuned ROS producing and scavenging systems capable of maintaining ROS homeostasis in the cells.

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Figure 1. Schematic representation of dual ROS level regulation (production and scavenging) and the main ROS functions related to plant-microorganism interactions.

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We now pay special attention to ROS regulating systems during plant-microorganism interactions.

Enzymatic ROS scavenging mechanisms involved in plant-microorganism interactions

The large battery of ROS scavenging enzymes included in the “ROS gene network” contains catalases (Kat), superoxide dismutases (SOD), ascorbate peroxidases (APx, detected in plants) cytochrome C peroxidases (CcP, detected in fungi). They are present in several intracellular compartments as well as in the apoplast in order to regulate both intracellular and extracellular ROS accumulation (Mittler et al. 2004). Considering the extracellular region, both plant and microorganism are capable of regulating the ROS level in this area during the early steps of the interaction (Figure 2).

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Figure 2. ROS regulation during development and plant-microorganism interactions. Constitutive ROS production (photosynthesis and respiration) have also been included in the cartoon. T-bars and arrows correspond to scavenging and production of ROS respectively. Pink lines stand for ROS involved in plant-microorganism interactions and blue lines for ROS involved in the development

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The symbiotic Rhizobia appears to have an efficient antioxidant defense. Indeed, although ROS were present in the infection threads they weren't detected inside the bacteria progressing within the infection thread (Santos et al. 2001; Rubio et al. 2004). Indeed, S. meliloti possesses two SOD that convert O2•− to O2 and H2O2 (Santos et al. 2000; Hérouart et al. 2002) and three heme b containing catalases, which are able to scavenge H2O2 (Hérouart et al. 1996; Ardissone et al. 2004). Bacterial catalases appear to play an important role in the nodule formation process as the double katB/katC and katA/katC mutants of S. meliloti are strongly impaired in nodule formation (Jamet et al. 2003). In addition to the catalases, the S. meliloti genome contains three thiol peroxidase encoding genes: the alkyl hydroperoxide reductase ahpC-like and two organic hydroperoxide resistance ohr-like genes. Both types of enzyme display biochemically equivalent functions and catalyze the reduction of organic peroxides to the corresponding less toxic organic alcohols.

In the case of a pathogenic interaction between rice and Magnaporthe griseae, the causal agent of rice blast disease, the fungus has to overcome the plants innate immunity in order to infect it. The massive production of ROS during the early stages of interaction is part of the plants innate immunity response. To overcome this line of defense, M. griseae must be able to counter the oxidative burst by producing ROS scavenging enzymes. A novel gene related to pathogenecity has recently been isolated in M. griseae: Defense Suppressor 1 (DES1) (Chi et al. 2009) . Δdes1 deficient mutants were hypersensitive to exogenous oxidative stress and the transcription of extracellular enzymes such as peroxidases and laccases were severely reduced. In interaction with a susceptible rice cultivar, the mutants displayed an important reduction of infectious hyphal extension, leading to a decrease in pathogenicity. Interestingly, the Δdes1 deficient mutants recovered their normal infectious growth when interacting with DPI treated plant tissue. These results strongly support the possibility that ROS play a major role in the first line of plant defenses at the cell surface both as toxic molecules as well as signalling actors.

Mechanisms to generate ROS

Several different enzymes have been implicated in the generation of ROS. Among these, NADPH oxidases (NOx) correspond to one of the most studied systems that play an important role in the production of superoxide radicals during the oxidative burst to defend cells from invasion. NOx are integral membrane proteins capable of oxidizing NADPH in the cell as well as reducing molecular oxygen into superoxide radicals in the apoplast (Sumimoto 2008), which is quickly dismutated into H2O2 either spontaneously or by SOD enzymes. ROS produced by the NADPH oxidases function in defense, development and redox-dependent signalling. They share common structural features and are evolutionarily of ancient origin and thus ubiquitous in multicellular eukaryotes (Bedard and Krause 2007; Bedard et al. 2007). In plants, NADPH oxidases form a small multigenic family and are involved in diverse events including innate immunity development. Due to the fact that ROS are toxic and in many cases short-lived, the activity of these oxidases is tightly regulated both temporally and spatially.

The use of DPI, that inhibits flavoproteins such as NOx, and abolished ROS production, strongly supports the possible involvement of M. truncatula NOx homologues in ROS production. Moreover, a DPI treatment during the early stages of M. truncatula – S. meliloti interaction not only abolished ROS production but also suppressed root hair curling and infection thread formation (Peleg-Grossman et al. 2007; Cardenas et al. 2008). These results emphasize the involvement of M. truncatula NADPH oxidase homologues in the early steps of Rhizobium infection.

The involvement of plant NOx in plant-microorganism interactions have clearly been shown (see earlier reviews Apel and Hirt 2004; Torres and Dangl 2005). NOx are present in all the fungi forming fruit bodies where they seem to participate in sexual reproduction. The inactivation of Aspergillus nidulans NoxA gene resulted in a decrease of ROS production, inhibition of the formation of cleistothecia at early stages of development, stimulation of mycelium growth and suppression of asexual reproduction (Lara-Ortiz et al. 2003). In addition NOx from fungi are also important during the infection process. Moreover in the symbiotic interaction between the fungus Epistle fistulae and the plant Folium perenne, a NOXA deficient fungus mutant is unable to undergo symbiosis and induces plant death. This shows that fungus produced ROS that also play a major role in the establishment of this symbiosis (Takemoto et al. 2006; Tanaka et al. 2006).

More recently, several reports from the microbe side indicate a major role of these genes in the pathogenicity process (Egan et al. 2007; Giesbert et al. 2008) and thus, play a positive role for the pathogen. Accordingly, during plant infection, NOx from M. grisea generate ROS. This oxidative burst is associated with the development of specialised infection structures called appressoria. Pharmacological scavenging of these oxygen radicals significantly delayed the development of appressoria and affected their morphology. Using a genetic approach targeting two NOx genes, Egan et al. (2007) showed that these genes are independently required for pathogenicity of M. grisea (inability to initiate appressorium-mediated cuticle penetration for the mutants) and are involved in ROS production (Egan et al. 2007). In a similar approach, the deletion of a putative NOx from the ergot fungus of ryegrass, Claviceps purpurea, has an impact on germination of conidia and pathogenicity, although its involvement in focusing ROS production has not been shown (Giesbert et al. 2008).

Other proteins, such as class III peroxidases, regulate for ROS homeostasis. Class III peroxidases are only detected in Viridiplantae and are present as large multigenic families in all land plants (Bakalovic et al. 2006). Released from the cell surface into the apoplast, peroxidases are an important class of enzymes responsible for the stress-induced formation and degradation of ROS (Bolwell et al. 2002; Bindschedler et al. 2006; Fecht-Christoffers et al. 2006). Apart from their indirect role in H2O2 detoxification through its peroxidative activities, some apoplastic class III peroxidases can also generate O2•− or H2O2 at physiological pHs via its oxidative cycle (Minibayeva et al. 2009). The cell wall has an enormous capacity to retain proteins in normal growth conditions, most of the peroxidases for instance, which may be released following abiotic stress. The involvement of class III peroxidases during the symbiotic process has already been observed. For example, Rip1, encoding a peroxidase from Medicago is rapidly and transiently induced by Rhizobium meliloti or after NFs treatment (Cook et al. 1995). Moreover, this gene is induced by H2O2 (Ramu et al. 2002).

More recently, a class III peroxidase (Srprx1) has been shown to be crucial for the bacterial invasion of the tropical Legume, Sesbania rostrata (Den Herder et al. 2007). The expression of Srprx1 is strictly dependent on bacterial nodulation factors (NFs) and could be modulated by H2O2, a downstream signal for crack-entry invasion. Its expression was not induced after wounding or pathogen attack, indicating that the peroxidase is a symbiosis-specific isoform. More interestingly, lack of Srprx1 gene expression could cause an aberrant structure of the infection threads (Den Herder et al. 2007).

OsPrx53, encoding a peroxidase from rice, is the strongest gene induced after Glomus infection (Guimil et al. 2005). Peroxidases seem important for the initiation of symbiosis but no direct evidence has demonstrated their implication for ROS production in the early steps of interaction and the development of the infection (Guimil et al. 2005).

Furthermore, the involvement of peroxidases in H2O2 synthesis during plant-pathogen interactions has been recently highlighted (Choi et al. 2007). Thus, H2O2 production is also compromised after inoculation of Capsicum annum, silenced for a peroxidase, by avirulent Xanthomonas campestris bacteria (Choi et al. 2007). This clearly demonstrate the peroxidase involvement in ROS production (Choi et al. 2007).

Although NOx and peroxidases represent the main characterized plant ROS producing systems at the plant cell surface, one should note that several oxidative and reductive systems are present in the plant plasmalemma (Vuletic et al. 2005). More interestingly, special attention has been recently paid to the plasma membrane microdomains. Recently, a proteomic approach based on the purification of lipid-rafts in plasma membrane from M. truncatula identified several putative ROS producing systems, including peroxidase (Furt et al. 2007; Lefebvre et al. 2007). This approach does not allow the identification of NOx although their presence has been previously shown in elicitor treated tobacco cells (Mongrand et al. 2004).

Other possible sources for H2O2 in the Legume –Rhizobium symbiosis are germin-like oxalate oxidases or diamine oxidases (Wisniewski et al. 2000). Indeed, a germin-like oxidase from Pisum sativum has been characterized (PsGER1). This protein has a superoxide dismutase activity, and is associated with nodules (Gucciardo et al. 2007).

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. ROS Accumulation during Plant-microorganism Interactions
  5. Scavenging and ROS Producing Systems during Plant-microorganism Interactions
  6. Conclusions
  7. Acknowledgments
  8. References

ROS are produced by all living organisms, either constitutively as by-products of several metabolic processes or in a more controlled manner during developmental processes as well as at the early stages of plant-microorganism interactions. The oxidative burst, that often takes place at the very first stage of the interaction, acts as the first line of plant defense. The microbe must therefore produce ROS scavenging enzymes in order to successfully infect the plant or down-regulate the plant ROS producing systems. This massive ROS production and scavenging takes place in the apoplast between the cell surfaces of the two organisms or in the rhizosphere. However, the oxidative burst seems to differ in intensity and length between plant-pathogen and plant-symbiote interactions. This difference could act as a specific signal predefining the host's response to the microbe.

Moreover, the plant NOx have been found to play an important role in the oxidative burst during plant-pathogen interactions. However, no functional evidence of NOx involvement in ROS production during Legume –Rhizobium interactions has been found. This further supports the idea that different ROS regulating systems could be activated by interactions with a pathogen or a symbiote. In this line, class III peroxidases could be a good candidate for ROS regulation.

Whatever the system involved during the different symbiotic interactions, it would be of interest to analyze the consequences of modifying plant ROS-producing activities on the symbiotic capacities. This could allow us to better understand the signalling role of ROS molecules and it's consequences on the establishment of symbiosis.

(Co-Editor: Kurt V. Fagerstedt)

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. ROS Accumulation during Plant-microorganism Interactions
  5. Scavenging and ROS Producing Systems during Plant-microorganism Interactions
  6. Conclusions
  7. Acknowledgments
  8. References