Flavohaemoglobin HmpX from Erwinia chrysanthemi confers nitrosative stress tolerance and affects the plant hypersensitive reaction by intercepting nitric oxide produced by the host

Insight into metabolic sensors of nitrosative stress protection in Phytophthora infestans

Phytophthora infestans, a representative of phytopathogenic oomycetes, have been proven to cope with redundant sources of internal and host-derived reactive nitrogen species (RNS). To gain insight into its nitrosative stress resistance mechanisms, metabolic sensors activated in response to nitrosative challenge during both in vitro growth and colonization of the host plant were investigated. The conducted analyses of gene expression, protein accumulation, and enzyme activity reveal for the first time that P. infestans (avirulent MP946 and virulent MP977 toward potato cv. Sarpo Mira) withstands nitrosative challenge and has an efficient system of RNS elimination. The obtained data indicate that the system protecting P. infestans against nitric oxide (NO) involved the expression of the nitric oxide dioxygenase (Pi-NOD1) gene belonging to the globin family. The maintenance of RNS homeostasis was also supported by an elevated S-nitrosoglutathione reductase activity and upregulation of peroxiredoxin 2 at the transcript and protein levels; however, the virulence pattern determined the expression abundance. Based on the experiments, it can be concluded that P. infestans possesses a multifarious system of metabolic sensors controlling RNS balance via detoxification, allowing the oomycete to exist in different micro-environments flexibly.

NorA, HmpX, and NorB Cooperate to Reduce NO Toxicity during Denitrification and Plant Pathogenesis in Ralstonia solanacearum

Ralstonia solanacearum, which causes bacterial wilt disease of many crops, requires denitrifying respiration to survive in its plant host. In the hypoxic environment of plant xylem vessels, this pathogen confronts toxic oxidative radicals like nitric oxide (NO), which is generated by both bacterial denitrification and host defenses. R. solanacearum has multiple distinct mechanisms that could mitigate this stress, including putative NO-binding protein (NorA), nitric oxide reductase (NorB), and flavohaemoglobin (HmpX). During denitrification and tomato pathogenesis and in response to exogenous NO, R. solanacearum upregulated norA, norB, and hmpX. Single mutants lacking ΔnorB, ΔnorA, or ΔhmpX increased expression of many iron and sulfur metabolism genes, suggesting that the loss of even one NO detoxification system demands metabolic compensation. Single mutants suffered only moderate fitness reductions in host plants, possibly because they upregulated their remaining protective genes. However, ΔnorA/norB, ΔnorB/hmpX, and ΔnorA/hmpX double mutants grew poorly in denitrifying culture and in planta. It is likely that the loss of norA, norB, and hmpX is lethal, since the methods used to construct the double mutants could not generate a triple mutant. Functional aconitase activity assays showed that NorA, HmpX, and especially NorB are important for maintaining iron-sulfur cluster proteins. Additionally, plant defense genes were upregulated in tomatoes infected with the NO-overproducing ΔnorB mutant, suggesting that bacterial detoxification of NO reduces the ability of the plant host to perceive the presence of the pathogen. Thus, R. solanacearum's three NO detoxification systems each contribute to and are collectively essential for overcoming metabolic nitrosative stress during denitrification, for virulence and growth in the tomato, and for evading host plant defenses. IMPORTANCE The soilborne plant pathogen Ralstonia solanacearum (Rs) causes bacterial wilt, a serious and widespread threat to global food security. Rs is metabolically adapted to low-oxygen conditions, using denitrifying respiration to survive in the host and cause disease. However, bacterial denitrification and host defenses generate nitric oxide (NO), which is toxic and also alters signaling pathways in both the pathogen and its plant hosts. Rs mitigates NO with a trio of mechanistically distinct proteins: NO-reductase (NorB), predicted iron-binding (NorA), and oxidoreductase (HmpX). This redundancy, together with analysis of mutants and in-planta dual transcriptomes, indicates that maintaining low NO levels is integral to Rs fitness in tomatoes (because NO damages iron-cluster proteins) and to evading host recognition (because bacterially produced NO can trigger plant defenses).

Bacterial nitric oxide metabolism: Recent insights in rhizobia

Nitric oxide (NO) is a reactive gaseous molecule that has several functions in biological systems depending on its concentration. At low concentrations, NO acts as a signaling molecule, while at high concentrations, it becomes very toxic due to its ability to react with multiple cellular targets. Soil bacteria, commonly known as rhizobia, have the capacity to establish a N₂-fixing symbiosis with legumes inducing the formation of nodules in their roots. Several reports have shown NO production in the nodules where this gas acts either as a signaling molecule which regulates gene expression, or as a potent inhibitor of nitrogenase and other plant and bacteria enzymes. A better understanding of the sinks and sources of NO in rhizobia is essential to protect symbiotic nitrogen fixation from nitrosative stress. In nodules, both the plant and the microsymbiont contribute to the production of NO. From the bacterial perspective, the main source of NO reported in rhizobia is the denitrification pathway that varies significantly depending on the species. In addition to denitrification, nitrate assimilation is emerging as a new source of NO in rhizobia. To control NO accumulation in the nodules, in addition to plant haemoglobins, bacteroids also contribute to NO detoxification through the expression of a NorBC-type nitric oxide reductase as well as rhizobial haemoglobins. In the present review, updated knowledge about the NO metabolism in legume-associated endosymbiotic bacteria is summarized.

Flavohaemoglobin: the pre-eminent nitric oxide-detoxifying machine of microorganisms

Flavohaemoglobins were first described in yeast as early as the 1970s but their functions were unclear. The surge in interest in nitric oxide biology and both serendipitous and hypothesis-driven discoveries in bacterial systems have transformed our understanding of this unusual two-domain globin into a comprehensive, yet undoubtedly incomplete, appreciation of its pre-eminent role in nitric oxide detoxification. Here, I focus on research on the flavohaemoglobins of microorganisms, especially of bacteria, and update several earlier and more comprehensive reviews, emphasising advances over the past 5 to 10 years and some controversies that have arisen. Inevitably, in light of space restrictions, details of nitric oxide metabolism and globins in higher organisms are brief.

The Nitrate Assimilatory Pathway in Sinorhizobium meliloti: Contribution to NO Production

The interaction between rhizobia and their legume host plants culminates in the formation of specialized root organs called nodules in which differentiated endosymbiotic bacteria (bacteroids) fix atmospheric nitrogen to the benefit of the plant. Interestingly, nitric oxide (NO) has been detected at various steps of the rhizobium-legume symbiosis where it has been shown to play multifaceted roles. It is recognized that both bacterial and plant partners of the Sinorhizobium meliloti–Medicago truncatula symbiosis are involved in NO synthesis in nodules. S. meliloti can also produce NO from nitrate when living as free cells in the soil. S. meliloti does not possess any NO synthase gene in its genome. Instead, the denitrification pathway is often described as the main driver of NO production with nitrate as substrate. This pathway includes the periplasmic nitrate reductase (Nap) which reduces nitrate into nitrite, and the nitrite reductase (Nir) which reduces nitrite into NO. However, additional genes encoding putative nitrate and nitrite reductases (called narB and nirB, respectively) have been identified in the S. meliloti genome. Here we examined the conditions where these genes are expressed, investigated their involvement in nitrate assimilation and NO synthesis in culture and their potential role in planta. We found that narB and nirB are expressed under aerobic conditions in absence of ammonium in the medium and most likely belong to the nitrate assimilatory pathway. Even though these genes are clearly expressed in the fixation zone of legume root nodule, they do not play a crucial role in symbiosis. Our results support the hypothesis that in S. meliloti, denitrification remains the main enzymatic way to produce NO while the assimilatory pathway involving NarB and NirB participates indirectly to NO synthesis by cooperating with the denitrification pathway.

Volatile compounds other than CO2 emitted by different microorganisms promote distinct posttranscriptionally regulated responses in plants

A "box-in-box" cocultivation system was used to investigate plant responses to microbial volatile compounds (VCs) and to evaluate the contributions of organic and inorganic VCs (VOCs and VICs, respectively) to these responses. Arabidopsis plants were exposed to VCs emitted by adjacent Alternaria alternata and Penicillium aurantiogriseum cultures, with and without charcoal filtration. No VOCs were detected in the headspace of growth chambers containing fungal cultures with charcoal filters. However, these growth chambers exhibited elevated CO₂ and bioactive CO and NO headspace concentrations. Independently of charcoal filtration, VCs from both fungal phytopathogens promoted growth and distinct developmental changes. Plants cultured at CO₂ levels observed in growth boxes containing fungal cultures were identical to those cultured at ambient CO₂ . Plants exposed to charcoal-filtered fungal VCs, nonfiltered VCs, or superelevated CO₂ levels exhibited transcriptional changes resembling those induced by increased irradiance. Thus, in the "box-in-box" system, (a) fungal VICs other than CO₂ and/or VOCs not detected by our analytical systems strongly influence the plants' responses to fungal VCs, (b) different microorganisms release VCs with distinct action potentials, (c) transcriptional changes in VC-exposed plants are mainly due to enhanced photosynthesis signaling, and (d) regulation of some plant responses to fungal VCs is primarily posttranscriptional.

Intracellular Ca2+ is important for flagellin-triggered defense in Arabidopsis and involves inositol polyphosphate signaling

Cytosolic Ca2+ increase is a crucial and early step of plant immunity evoked by pathogen-associated molecular patterns (PAMPs) such as flagellin (flg). Components responsible for this increase are still not uncovered, although current models of plant immune signaling portray extracellular Ca2+ influx as paramount to flg activation of defense pathways. Work presented here provides new insights into cytosolic Ca2+ increase associated with flg-induced defense responses. We show that extracellular Ca2+ contributes more to immune responses evoked by plant elicitor peptide (Pep3) than that evoked by flg, indicating an intracellular Ca2+ source responsible for immune responses evoked by flg. Genetic impairment of the inositol polyphosphate (InsP) and G-protein signal associated with flg perception reduced flg-dependent immune responses. Previous work indicates that prior exposure of Arabidopsis plants to flg leads to an immune response reflected by less vigorous growth of a pathogenic microbe. We found that this immune response to flg was compromised in mutants lacking the ability to generate an InsP or G-protein signal. We conclude that the recruitment of intracellular Ca2+ stores by flg may involve InsP and G-protein signaling. We also found a notable difference in contribution of intracellular stores of Ca2+ to the immune signaling evoked by another PAMP, elf18 peptide, which had a very different response profile to impairment of InsP signaling. Although Ca2+ signaling is at the core of the innate immune as well as hypersensitive response to plant pathogens, it appears that the molecular mechanisms generating the Ca2+ signal in response to different PAMPs are different.

ABA Suppresses Botrytis cinerea Elicited NO Production in Tomato to Influence H2O2 Generation and Increase Host Susceptibility

Abscisic acid (ABA) production has emerged a susceptibility factor in plant-pathogen interactions. This work examined the interaction of ABA with nitric oxide (NO) in tomato following challenge with the ABA-synthesizing pathogen, Botrytis cinerea. Trace gas detection using a quantum cascade laser detected NO production within minutes of challenge with B. cinerea whilst photoacoustic laser detection detected ethylene production - an established mediator of defense against this pathogen - occurring after 6 h. Application of the NO generation inhibitor N-Nitro-L-arginine methyl ester (L-NAME) suppressed both NO and ethylene production and resistance against B. cinerea. The tomato mutant sitiens fails to accumulate ABA, shows increased resistance to B. cinerea and we noted exhibited elevated NO and ethylene production. Exogenous application of L-NAME or ABA reduced NO production in sitiens and reduced resistance to B. cinerea. Increased resistance to B. cinerea in sitiens have previously been linked to increased reactive oxygen species (ROS) generation but this was reduced in both L-NAME and ABA-treated sitiens. Taken together, our data suggests that ABA can decreases resistance to B. cinerea via reduction of NO production which also suppresses both ROS and ethylene production.

Nitric Oxide in the Offensive Strategy of Fungal and Oomycete Plant Pathogens

In the course of evolutionary changes pathogens have developed many invasion strategies, to which the host organisms responded with a broad range of defense reactions involving endogenous signaling molecules, such as nitric oxide (NO). There is evidence that pathogenic microorganisms, including two most important groups of eukaryotic plant pathogens, also acquired the ability to synthesize NO via non-unequivocally defined oxidative and/or reductive routes. Although the both kingdoms Chromista and Fungi are remarkably diverse, the experimental data clearly indicate that pathogen-derived NO is an important regulatory molecule controlling not only developmental processes, but also pathogen virulence and its survival in the host. An active control of mitigation or aggravation of nitrosative stress within host cells seems to be a key determinant for the successful invasion of plant pathogens representing different lifestyles and an effective mode of dispersion in various environmental niches.

Ralstonia solanacearum uses inorganic nitrogen metabolism for virulence, ATP production, and detoxification in the oxygen-limited host xylem environment

Genomic data predict that, in addition to oxygen, the bacterial plant pathogen Ralstonia solanacearum can use nitrate (NO₃⁻), nitrite (NO₂⁻), nitric oxide (NO), and nitrous oxide (N₂O) as terminal electron acceptors (TEAs). Genes encoding inorganic nitrogen reduction were highly expressed during tomato bacterial wilt disease, when the pathogen grows in xylem vessels. Direct measurements found that tomato xylem fluid was low in oxygen, especially in plants infected by R. solanacearum. Xylem fluid contained ~25 mM NO₃⁻, corresponding to R. solanacearum’s optimal NO₃⁻ concentration for anaerobic growth in vitro. We tested the hypothesis that R. solanacearum uses inorganic nitrogen species to respire and grow during pathogenesis by making deletion mutants that each lacked a step in nitrate respiration (ΔnarG), denitrification (ΔaniA, ΔnorB, and ΔnosZ), or NO detoxification (ΔhmpX). The ΔnarG, ΔaniA, and ΔnorB mutants grew poorly on NO₃⁻ compared to the wild type, and they had reduced adenylate energy charge levels under anaerobiosis. While NarG-dependent NO₃⁻ respiration directly enhanced growth, AniA-dependent NO₂⁻ reduction did not. NO₂⁻ and NO inhibited growth in culture, and their removal depended on denitrification and NO detoxification. Thus, NO₃⁻ acts as a TEA, but the resulting NO₂⁻ and NO likely do not. None of the mutants grew as well as the wild type in planta, and strains lacking AniA (NO₂⁻ reductase) or HmpX (NO detoxification) had reduced virulence on tomato. Thus, R. solanacearum exploits host NO₃⁻ to respire, grow, and cause disease. Degradation of NO₂⁻ and NO is also important for successful infection and depends on denitrification and NO detoxification systems.

The plant-pathogenic bacterium Ralstonia solanacearum causes bacterial wilt, one of the world’s most destructive crop diseases. This pathogen’s explosive growth in plant vascular xylem is poorly understood. We used biochemical and genetic approaches to show that R. solanacearum rapidly depletes oxygen in host xylem but can then respire using host nitrate as a terminal electron acceptor. The microbe uses its denitrification pathway to detoxify the reactive nitrogen species nitrite (a product of nitrate respiration) and nitric oxide (a plant defense signal). Detoxification may play synergistic roles in bacterial wilt virulence by converting the host’s chemical weapon into an energy source. Mutant bacterial strains lacking elements of the denitrification pathway could not grow as well as the wild type in tomato plants, and some mutants were also reduced in virulence. Our results show how a pathogen’s metabolic activity can alter the host environment in ways that increase pathogen success.

Methanol extract of mycelia from Phytophthora infestans-induced resistance in potato

Plants recognize certain microbial compounds as elicitors in their active defence mechanisms. It has been shown that a series of defence reactions are induced in potato plant cells after treatment with water-soluble hyphal wall components prepared from Phytophthora infestans. In this study, a methanol extract from mycelia of P. infestans (MEM), which contains lipophilic compounds, was used as another elicitor for the induction of the defence reactions in potato. MEM elicitor induced reactive oxygen species (ROS), especially O2(-) and H2O2 production, and nitric oxide (NO) generation in potato leaves and suspension-cultured cells. Hypersensitive cell death was detected in potato leaves within 6-8 h after MEM elicitor treatment. The accumulation of phytoalexins was detected by MEM elicitor treatment in potato tubers. In potato suspension-cultured cells, several defence-related genes were induced by MEM elicitors, namely Strboh, Sthsr203J, StPVS3, StPR1, and StNR5, which regulate various defence-related functions. Enhanced resistance against P. infestans was found in MEM-treated potato plants. These results suggested that MEM elicitor is recognized by host and enhances defence activities to produce substances inhibitory to pathogens.

Nitrogen metabolism meets phytopathology

Nitrogen (N) is essential for life and is a major limiting factor of plant growth. Because soils frequently lack sufficient N, large quantities of inorganic N fertilizers are added to soils for crop production. However, nitrate, urea, and ammonium are a major source of global pollution, because much of the N that is not taken up by plants enters streams, groundwater, and lakes, where it affects algal production and causes an imbalance in aquatic food webs. Many agronomical data indicate that the higher use of N fertilizers during the green revolution had an impact on the incidence of crop diseases. In contrast, examples in which a decrease in N fertilization increases disease severity are also reported, indicating that there is a complex relationship linking N uptake and metabolism and the disease infection processes. Thus, although it is clear that N availability affects disease, the underlying mechanisms remain unclear. The aim of this review is to describe current knowledge of the mechanisms that link plant N status to the plant's response to pathogen infection and to the virulence and nutritional status of phytopathogens.

Nitric oxide: an effective weapon of the plant or the pathogen?

An explosion of research in plant nitric oxide (NO) biology during the last two decades has revealed that NO is a key signal involved in plant development, abiotic stress responses and plant immunity. During the course of evolutionary changes, microorganisms parasitizing plants have developed highly effective offensive strategies, in which NO also seems to be implicated. NO production has been demonstrated in several plant pathogens, including fungi, but the origin of NO seems to be as puzzling as in plants. So far, published studies have been spread over multiple species of pathogenic microorganisms in various developmental stages; however, the data clearly indicate that pathogen-derived NO is an important regulatory molecule involved not only in developmental processes, but also in pathogen virulence and its survival in the host. This review also focuses on the search for potential mechanisms by which pathogens convert NO messages into a physiological response or detoxify both endo- and exogenous NO. Finally, taking into account the data available from model bacteria and yeast, a basic draft for the mode of NO action in phytopathogenic microorganisms is proposed.

Hydrogen Peroxide- and Nitric Oxide-mediated Disease Control of Bacterial Wilt in Tomato Plants

Reactive oxygen species (ROS) generation in tomato plants by Ralstonia solanacearum infection and the role of hydrogen peroxide (H2O2) and nitric oxide in tomato bacterial wilt control were demonstrated. During disease development of tomato bacterial wilt, accumulation of superoxide anion (O2 (-)) and H2O2 was observed and lipid peroxidation also occurred in the tomato leaf tissues. High doses of H2O2and sodium nitroprusside (SNP) nitric oxide donor showed phytotoxicity to detached tomato leaves 1 day after petiole feeding showing reduced fresh weight. Both H2O2and SNP have in vitro antibacterial activities against R. solanacearum in a dose-dependent manner, as well as plant protection in detached tomato leaves against bacterial wilt by 10(6) and 10(7) cfu/ml of R. solanacearum. H2O2- and SNP-mediated protection was also evaluated in pots using soil-drench treatment with the bacterial inoculation, and relative 'area under the disease progressive curve (AUDPC)' was calculated to compare disease protection by H2O2 and/or SNP with untreated control. Neither H2O2 nor SNP protect the tomato seedlings from the bacterial wilt, but H2O2+ SNP mixture significantly decreased disease severity with reduced relative AUDPC. These results suggest that H2O2 and SNP could be used together to control bacterial wilt in tomato plants as bactericidal agents.

Bis-aryl methanone compound is a candidate of nitric oxide producing elicitor and induces resistance in Nicotiana benthamiana against Phytophthora infestans

Nitric oxide (NO) is important in some physiological responses of plants and plays a crucial role in the regulation of both defense responses and inducing resistance to fungal pathogens. NUBS-4190, a new bis-aryl-methanone compound elicited NO production and defense responses in Nicotiana benthamiana against Phytophthora infestans. NUBS-4190 induced resistance in N. benthamiana to P. infestans, without association of reactive oxygen generation and hypersensitive cell death. Callose induction was reduced in NUBS-4190-treated N. benthamiana leaves after challenge inoculation of P. infestans indicating the penetration resistance. Involvement of pathogenesis-related 1a (NbPR1a) and nitric oxide associated 1 (NbNOA1) genes in the induced resistance to N. benthamiana against P. infestans was found to be associated with resistance. Increased susceptibility in NbPR1a- and NbNOA1-silenced plants correlated with the constitutive accumulation of PR1a transcripts and NO associated salicylic acid. Moreover, reduced NO generation in NOA1 silenced N. benthamiana plants treated with NUBS-4190 indicated that NbNOA1 is involved in NUBS-4190-mediated NO production and is required for defense responses.

Microbial eukaryote globins

A bioinformatics survey of about 120 protist and 240 fungal genomes and transcriptomes revealed a broad array of globins, representing five of the eight subfamilies identified in bacteria. Most conspicuous is the absence of protoglobins and globin-coupled sensors, except for a two-domain globin in Leishmanias, that comprises a nucleotidyl cyclase domain, and the virtual absence of truncated group 3 globins. In contrast to bacteria, co-occurrence of more than two globin subfamilies appears to be rare in protists. Although globins were lacking in the Apicomplexa and the Microsporidia intracellular pathogens, they occurred in the pathogenic Trypanosomatidae, Stramenopiles and certain fungi. Flavohaemoglobins (FHbs) and related single-domain globins occur across the protist groups. Fungi are unique in having FHbs co-occurring with sensor single-domain globins (SSDgbs). Obligately biotrophic fungi covered in our analysis lack globins. Furthermore, SSDgbs occur only in a heterolobosean amoeba, Naegleria and the stramenopile Hyphochytrium. Of the three subfamilies of truncated Mb-fold globins, TrHb1s appear to be the most widespread, occurring as multiple copies in chlorophyte and ciliophora genomes, many as multidomain proteins. Although the ciliates appear to have only TrHb1s, the chlorophytes have Mb-like globins and TrHb2s, both closely related to the corresponding plant globins. The presently available number of protist genomes is inadequate to provide a definitive census of their globins. Bayesian molecular analyses of single-domain 3/3 Mb-fold globins suggest a close relationship of chlorophyte and haptophyte globins, including choanoflagellate and Capsaspora globins to land plant symbiotic and non-symbiotic haemoglobins and to vertebrate neuroglobins.

Integrating nitric oxide into salicylic acid and jasmonic acid/ ethylene plant defense pathways

Plant defense against pests and pathogens is known to be conferred by either salicylic acid (SA) or jasmonic acid (JA)/ethylene (ET) pathways, depending on infection or herbivore-grazing strategy. It is well attested that SA and JA/ET pathways are mutually antagonistic allowing defense responses to be tailored to particular biotic stresses. Nitric oxide (NO) has emerged as a major signal influencing resistance mediated by both signaling pathways but no attempt has been made to integrate NO into established SA/JA/ET interactions. NO has been shown to act as an inducer or suppressor of signaling along each pathway. NO will initiate SA biosynthesis and nitrosylate key cysteines on TGA-class transcription factors to aid in the initiation of SA-dependent gene expression. Against this, S-nitrosylation of NONEXPRESSOR OF PATHOGENESIS-RELATED PROTEINS1 (NPR1) will promote the NPR1 oligomerization within the cytoplasm to reduce TGA activation. In JA biosynthesis, NO will initiate the expression of JA biosynthetic enzymes, presumably to over-come any antagonistic effects of SA on JA-mediated transcription. NO will also initiate the expression of ET biosynthetic genes but a suppressive role is also observed in the S-nitrosylation and inhibition of S-adenosylmethionine transferases which provides methyl groups for ET production. Based on these data a model for NO action is proposed but we have also highlighted the need to understand when and how inductive and suppressive steps are used.

Nitric oxide in plants: an assessment of the current state of knowledge

BACKGROUND AND AIMS

After a series of seminal works during the last decade of the 20th century, nitric oxide (NO) is now firmly placed in the pantheon of plant signals. Nitric oxide acts in plant-microbe interactions, responses to abiotic stress, stomatal regulation and a range of developmental processes. By considering the recent advances in plant NO biology, this review will highlight certain key aspects that require further attention.

SCOPE AND CONCLUSIONS

The following questions will be considered. While cytosolic nitrate reductase is an important source of NO, the contributions of other mechanisms, including a poorly defined arginine oxidizing activity, need to be characterized at the molecular level. Other oxidative pathways utilizing polyamine and hydroxylamine also need further attention. Nitric oxide action is dependent on its concentration and spatial generation patterns. However, no single technology currently available is able to provide accurate in planta measurements of spatio-temporal patterns of NO production. It is also the case that pharmaceutical NO donors are used in studies, sometimes with little consideration of the kinetics of NO production. We here include in planta assessments of NO production from diethylamine nitric oxide, S-nitrosoglutathione and sodium nitroprusside following infiltration of tobacco leaves, which could aid workers in their experiments. Further, based on current data it is difficult to define a bespoke plant NO signalling pathway, but rather NO appears to act as a modifier of other signalling pathways. Thus, early reports that NO signalling involves cGMP-as in animal systems-require revisiting. Finally, as plants are exposed to NO from a number of external sources, investigations into the control of NO scavenging by such as non-symbiotic haemoglobins and other sinks for NO should feature more highly. By crystallizing these questions the authors encourage their resolution through the concerted efforts of the plant NO community.

Genomic basis of broad host range and environmental adaptability of Rhizobium tropici CIAT 899 and Rhizobium sp. PRF 81 which are used in inoculants for common bean (Phaseolus vulgaris L.)

BACKGROUND

Rhizobium tropici CIAT 899 and Rhizobium sp. PRF 81 are α-Proteobacteria that establish nitrogen-fixing symbioses with a range of legume hosts. These strains are broadly used in commercial inoculants for application to common bean (Phaseolus vulgaris) in South America and Africa. Both strains display intrinsic resistance to several abiotic stressful conditions such as low soil pH and high temperatures, which are common in tropical environments, and to several antimicrobials, including pesticides. The genetic determinants of these interesting characteristics remain largely unknown.

RESULTS

Genome sequencing revealed that CIAT 899 and PRF 81 share a highly-conserved symbiotic plasmid (pSym) that is present also in Rhizobium leucaenae CFN 299, a rhizobium displaying a similar host range. This pSym seems to have arisen by a co-integration event between two replicons. Remarkably, three distinct nodA genes were found in the pSym, a characteristic that may contribute to the broad host range of these rhizobia. Genes for biosynthesis and modulation of plant-hormone levels were also identified in the pSym. Analysis of genes involved in stress response showed that CIAT 899 and PRF 81 are well equipped to cope with low pH, high temperatures and also with oxidative and osmotic stresses. Interestingly, the genomes of CIAT 899 and PRF 81 had large numbers of genes encoding drug-efflux systems, which may explain their high resistance to antimicrobials. Genome analysis also revealed a wide array of traits that may allow these strains to be successful rhizosphere colonizers, including surface polysaccharides, uptake transporters and catabolic enzymes for nutrients, diverse iron-acquisition systems, cell wall-degrading enzymes, type I and IV pili, and novel T1SS and T5SS secreted adhesins.

CONCLUSIONS

Availability of the complete genome sequences of CIAT 899 and PRF 81 may be exploited in further efforts to understand the interaction of tropical rhizobia with common bean and other legume hosts.

Hemoglobin: a nitric-oxide dioxygenase

Members of the hemoglobin superfamily efficiently catalyze nitric-oxide dioxygenation, and when paired with native electron donors, function as NO dioxygenases (NODs). Indeed, the NOD function has emerged as a more common and ancient function than the well-known role in O2 transport-storage. Novel hemoglobins possessing a NOD function continue to be discovered in diverse life forms. Unique hemoglobin structures evolved, in part, for catalysis with different electron donors. The mechanism of NOD catalysis by representative single domain hemoglobins and multidomain flavohemoglobin occurs through a multistep mechanism involving O2 migration to the heme pocket, O2 binding-reduction, NO migration, radical-radical coupling, O-atom rearrangement, nitrate release, and heme iron re-reduction. Unraveling the physiological functions of multiple NODs with varying expression in organisms and the complexity of NO as both a poison and signaling molecule remain grand challenges for the NO field. NOD knockout organisms and cells expressing recombinant NODs are helping to advance our understanding of NO actions in microbial infection, plant senescence, cancer, mitochondrial function, iron metabolism, and tissue O2 homeostasis. NOD inhibitors are being pursued for therapeutic applications as antibiotics and antitumor agents. Transgenic NOD-expressing plants, fish, algae, and microbes are being developed for agriculture, aquaculture, and industry.

Haemoglobin modulates salicylate and jasmonate/ethylene-mediated resistance mechanisms against pathogens

Nitric oxide (NO) plays a role in defence against hemibiotrophic pathogens mediated by salicylate (SA) and also necrotrophic pathogens influenced by jasmonate/ethylene (JA/Et). This study examined how NO-oxidizing haemoglobins (Hb) encoded by GLB1, GLB2, and GLB3 in Arabidopsis could influence both defence pathways. The impact of Hb on responses to the hemibiotrophic Pseudomonas syringae pathovar tomato (Pst) AvrRpm1 and the necrotrophic Botrytis cinerea were investigated using glb1, glb2, and glb3 mutant lines and also CaMV 35S GLB1 and GLB2 overexpression lines. In glb1, but not glb2 and glb3, increased resistance was observed to both pathogens but was compromised in the 35S-GLB1. A quantum cascade laser-based sensor measured elevated NO production in glb1 infected with Pst AvrRpm1 and B. cinerea, which was reduced in 35S-GLB1 compared to Col-0. SA accumulation was increased in glb1 and reduced in 35S-GLB1 compared to controls following attack by Pst AvrRpm1. Similarly, JA and Et levels were increased in glb1 but decreased in 35S-GLB1 in response to attack by B. cinerea. Quantitative PCR assays indicated reduced GLB1 expression during challenge with either pathogen, thus this may elevate NO concentration and promote a wide-ranging defence against pathogens.

Protection from nitrosative stress: a central role for microbial flavohemoglobin

Nitric oxide (NO) is an inevitable product of life in an oxygen- and nitrogen-rich environment. This reactive diatomic molecule exhibits microbial cytotoxicity, in large part by facilitating nitrosative stress and inhibiting heme-containing proteins within the aerobic respiratory chain. Metabolism of NO is therefore essential for microbial life. In many bacteria, fungi, and protozoa, the evolutionarily ancient flavohemoglobin (flavoHb) converts NO and O(2) to inert nitrate (NO(3)(-)) and undergoes catalytic regeneration via flavin-dependent reduction. Since its identification, widespread efforts have characterized roles for flavoHb in microbial nitrosative stress protection. Subsequent genomic studies focused on flavoHb have elucidated the transcriptional machinery necessary for inducible NO protection, such as NsrR in Escherichia coli, as well as additional proteins that constitute a nitrosative stress protection program. As an alternative strategy, flavoHb has been heterologously employed in higher eukaryotic organisms such as plants and human tumors to probe the function(s) of endogenous NO signaling. Such an approach may also provide a therapeutic route to in vivo NO depletion. Here we focus on the molecular features of flavoHb, the hitherto characterized NO-sensitive transcriptional machinery responsible for its induction, the roles of flavoHb in resisting mammalian host defense systems, and heterologous applications of flavoHb in plant/mammalian systems (including human tumors), as well as unresolved questions surrounding this paradigmatic NO-consuming enzyme.

The diversity of microbial responses to nitric oxide and agents of nitrosative stress close cousins but not identical twins

Nitric oxide and related nitrogen species (reactive nitrogen species) now occupy a central position in contemporary medicine, physiology, biochemistry, and microbiology. In particular, NO plays important antimicrobial defenses in innate immunity but microbes have evolved intricate NO-sensing and defense mechanisms that are the subjects of a vast literature. Unfortunately, the burgeoning NO literature has not always been accompanied by an understanding of the intricacies and complexities of this radical and other reactive nitrogen species so that there exists confusion and vagueness about which one or more species exert the reported biological effects. The biological chemistry of NO and derived/related molecules is complex, due to multiple species that can be generated from NO in biological milieu and numerous possible reaction targets. Moreover, the fate and disposition of NO is always a function of its biological environment, which can vary significantly even within a single cell. In this review, we consider newer aspects of the literature but, most importantly, consider the underlying chemistry and draw attention to the distinctiveness of NO and its chemical cousins, nitrosonium (NO(+)), nitroxyl (NO(-), HNO), peroxynitrite (ONOO(-)), nitrite (NO(2)(-)), and nitrogen dioxide (NO(2)). All these species are reported to be generated in biological systems from initial formation of NO (from nitrite, NO synthases, or other sources) or its provision in biological experiments (typically from NO gas, S-nitrosothiols, or NO donor compounds). The major targets of NO and nitrosative damage (metal centers, thiols, and others) are reviewed and emphasis is given to newer "-omic" methods of unraveling the complex repercussions of NO and nitrogen oxide assaults. Microbial defense mechanisms, many of which are critical for pathogenicity, include the activities of hemoglobins that enzymically detoxify NO (to nitrate) and NO reductases and repair mechanisms (e.g., those that reverse S-nitrosothiol formation). Microbial resistance to these stresses is generally inducible and many diverse transcriptional regulators are involved-some that are secondary sensors (such as Fnr) and those that are "dedicated" (such as NorR, NsrR, NssR) in that their physiological function appears to be detecting primarily NO and then regulating expression of genes that encode enzymes with NO as a substrate. Although generally harmful, evidence is accumulating that NO may have beneficial effects, as in the case of the squid-Vibrio light-organ symbiosis, where NO serves as a signal, antioxidant, and specificity determinant. Progress in this area will require a thorough understanding not only of the biology but also of the underlying chemical principles.

Nitric oxide in legume-rhizobium symbiosis

Nitric oxide (NO) is a gaseous signaling molecule with a broad spectrum of regulatory functions in plant growth and development. NO has been found to be involved in various pathogenic or symbiotic plant-microbe interactions. During the last decade, increasing evidence of the occurrence of NO during legume-rhizobium symbioses has been reported, from early steps of plant-bacteria interaction, to the nitrogen-fixing step in mature nodules. This review focuses on recent advances on NO production and function in nitrogen-fixing symbiosis. First, the potential plant and bacterial sources of NO, including NO synthase-like, nitrate reductase or electron transfer chains of both partners, are presented. Then responses of plant and bacterial cells to the presence of NO are presented in the context of the N(2)-fixing symbiosis. Finally, the roles of NO as either a regulatory signal of development, or a toxic compound with inhibitory effects on nitrogen fixation, or an intermediate involved in energy metabolism, during symbiosis establishment and nodule functioning are discussed.

Methods of nitric oxide detection in plants: a commentary

Over the last decade nitric oxide (NO) has been shown to influence a range of processes in plants. However, when, where and even if NO production occurs is controversial in several physiological scenarios in plants. This arises from a series of causes: (a) doubts have arisen over the specificity of widely used 4,5-diaminofluorescein diacetate (DAF-2DA)/4-amino-5-methylamino-2,7-difluorofluorescein (DAF-FM) dyes for NO, (b) no plant nitric oxide synthase (NOS) has been cloned, so that the validity of using mammalian NOS inhibitors to demonstrate that NO is being measured is debatable, (c) the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-l-oxyl-3-oxide (cPTIO) needs to be used with caution, and (d) some discrepancies between assays for in planta measurements and another based on sampling NO from the gas phase have been reported. This review will outline some commonly used methods to determine NO, attempt to reconcile differing results obtained by different laboratories and suggest appropriate approaches to unequivocally demonstrate the production of NO.

Nitric oxide is required for an optimal establishment of the Medicago truncatula-Sinorhizobium meliloti symbiosis

Nitric oxide (NO) is a gaseous molecule that participates in numerous plant signalling pathways. It is involved in plant responses to pathogens and development processes such as seed germination, flowering and stomatal closure. Using a permeable NO-specific fluorescent probe and a bacterial reporter strain expressing the lacZ gene under the control of a NO-responsive promoter, we detected NO production in the first steps, during infection threads growth, of the Medicago truncatula-Sinorhizobium meliloti symbiotic interaction. Nitric oxide was also detected, by confocal microscopy, in nodule primordia. Depletion of NO caused by cPTIO (2-(4-carboxyphenyl)-4,4,5,5-tetramethyl imidazoline-1-oxyl-3-oxide), an NO scavenger, resulted in a significant delay in nodule appearance. The overexpression of a bacterial hmp gene, encoding a flavohaemoglobin able to scavenge NO, under the control of a nodule-specific promoter (pENOD20) in transgenic roots, led to the same phenotype. The NO scavenging resulting from these approaches provoked the downregulation of plant genes involved in nodule development, such as MtCRE1 and MtCCS52A. Furthermore, an Hmp-overexpressing S. meliloti mutant strain was found to be less competitive than the wild type in the nodulation process. Taken together, these results indicate that NO is required for an optimal establishment of the M. truncatula-S. meliloti symbiotic interaction.

The language of nitric oxide signalling

Nitric oxide (NO) has recently joined the select circle of the ubiquitous molecules of plant signalling networks. Indeed, the last decade has produced a tremendous amount of data that evidence the diversity of physiological situations in which NO is involved in plants and the complexity of NO biology. These data also underline our difficulties in providing simple answers to the cardinal questions of where NO comes from and how the NO message is converted into a physiological response. The identification of NO primary targets and NO-regulated genes provides new opportunities to connect NO biochemistry and NO biology. This review summarises our current understanding of NO signalling, from the generation of the NO message to its execution into a cellular response. The review particularly considers whether and how NO may be responsible for specific signalling in different physiological processes.

Vibrio fischeri flavohaemoglobin protects against nitric oxide during initiation of the squid-Vibrio symbiosis

Nitric oxide (NO) is implicated in a wide range of biological processes, including innate immunity against pathogens, signal transduction and protection against oxidative stress. However, its possible roles in beneficial host-microbe associations are less well recognized. During the early stages of the squid-vibrio symbiosis, the bacterial symbiont Vibrio fischeri encounters host-derived NO, which has been hypothesized to serve as a specificity determinant. We demonstrate here that the flavohaemoglobin, Hmp, of V. fischeri protects against NO, both in culture and during colonization of the squid host. Transcriptional analyses indicate that hmp expression is highly responsive to NO, principally through the repressor, NsrR. Hmp protects V. fischeri from NO inhibition of aerobic respiration, and removes NO under both oxic and anoxic conditions. A Δhmp mutant of V. fischeri initiates squid colonization less effectively than wild type, but is rescued by the presence of an NO synthase inhibitor. The hmp promoter is activated during the initial stage of colonization, during which the Δhmp strain fails to form normal-sized aggregates of colonizing cells. Taken together, these results suggest that the sensing of host-derived NO by NsrR, and the subsequent removal of NO by Hmp, influence aggregate size and, thereby, V. fischeri colonization efficiency.

The response to nitric oxide of the nitrogen-fixing symbiont Sinorhizobium meliloti

Nitric oxide (NO) is crucial in animal- and plant-pathogen interactions, during which it participates in host defense response and resistance. Indications for the presence of NO during the symbiotic interaction between the model legume Medicago truncatula and its symbiont Sinorhizobium meliloti have been reported but the role of NO in symbiosis is far from being elucidated. Our objective was to understand the role or roles played by NO in symbiosis. As a first step toward this goal, we analyzed the bacterial response to NO in culture, using a transcriptomic approach. We identified approximately 100 bacterial genes whose expression is upregulated in the presence of NO. Surprisingly, most of these genes are regulated by the two-component system FixLJ, known to control the majority of rhizobial genes expressed in planta in mature nodules, or the NO-dedicated regulator NnrR. Among the genes responding to NO is hmp, encoding a putative flavohemoglobin. We report that an hmp mutant displays a higher sensitivity toward NO in culture and leads to a reduced nitrogen fixation efficiency in planta. Because flavohemoglobins are known to detoxify NO in numerous bacterial species, this result is the first indication of the importance of the bacterial NO response in symbiosis.

Molecular cloning, functional analysis and localization of a novel gene encoding polygalacturonase-inhibiting protein in Chorispora bungeana

Polygalacturonase-inhibiting proteins (PGIPs) are plant defense proteins. To date, no spatial distribution of PGIPs and interaction between PGIPs and nitric oxide (NO) in plant were described. Here, we first reported the full-length cDNA sequence of PGIP of Chorispora bungeana (CbPGIP1). Notably, immunofluorescence localization showed that the CbPGIP was evenly distributed in leaves but it was mainly localized in epidermis and vascular bundle in stems and roots. Further studies indicated that CbPGIP had higher abundance in roots than in stems and leaves. Conversely, the bulk PGIP of C. bungeana showed a higher activity in leaves than in stems and roots. In addition, quantitative real-time polymerase chain reaction demonstrated that CbPGIP1 expression was induced by Stemphylium solani, salicylic acid (SA), 4, -4 degrees C and NO. This is a first report attempting to predict if NO can induce the PGIP expression. Taken together, these findings showed that the gene was spatially regulated and NO and SA might take part in CbPGIP1 expression induced by biotic and abiotic stresses. This study highlighted the potential importance of CbPGIP1 and NO in plant resistance.

Nitric oxide as a partner of reactive oxygen species participates in disease resistance to nectrotophic pathogen Botryis cinerea in Nicotiana benthamiana

Nitric oxide (NO) is an essential regulatory molecule in plant immunity in synergy with reactive oxygen species (ROS). However, little is known about the role of NO in disease resistance to necrotrophic pathogens. NO and oxidative bursts were induced during necrotrophic fungal pathogen Botrytis cinerea and Nicotiana benthamiana compatible interaction. Histochemical analyses showed that both NO and ROS were produced in adjacent cells of invaded areas in N. benthamiana leaves. Activation of salicylic acid-induced protein kinase, which regulates the radical burst, and several defense-related genes were induced after inoculation of B. cinerea. Loss-of-function analyses using inhibitors and virus-induced gene silencing were done to investigate the role of the radical burst in pathogenesis. We showed that NO plays a pivotal role in basal defense against B. cinerea and PR-1 gene expression in N. benthamiana. By contrast, ROS function has a negative role in resistance or has a positive role in expansion of disease lesions during B. cinerea-N. benthamiana interaction.

Nitric oxide synthesis and signalling in plants

As with all organisms, plants must respond to a plethora of external environmental cues. Individual plant cells must also perceive and respond to a wide range of internal signals. It is now well-accepted that nitric oxide (NO) is a component of the repertoire of signals that a plant uses to both thrive and survive. Recent experimental data have shown, or at least implicated, the involvement of NO in reproductive processes, control of development and in the regulation of physiological responses such as stomatal closure. However, although studies concerning NO synthesis and signalling in animals are well-advanced, in plants there are still fundamental questions concerning how NO is produced and used that need to be answered. For example, there is a range of potential NO-generating enzymes in plants, but no obvious plant nitric oxide synthase (NOS) homolog has yet been identified. Some studies have shown the importance of NOS-like enzymes in mediating NO responses in plants, while other studies suggest that the enzyme nitrate reductase (NR) is more important. Still, more published work suggests the involvement of completely different enzymes in plant NO synthesis. Similarly, it is not always clear how NO mediates its responses. Although it appears that in plants, as in animals, NO can lead to an increase in the signal cGMP which leads to altered ion channel activity and gene expression, it is not understood how this actually occurs. NO is a relatively reactive compound, and it is not always easy to study. Furthermore, its biological activity needs to be considered in conjunction with that of other compounds such as reactive oxygen species (ROS) which can have a profound effect on both its accumulation and function. In this paper, we will review the present understanding of how NO is produced in plants, how it is removed when its signal is no longer required and how it may be both perceived and acted upon.

The Response regulator HrpY of Dickeya dadantii 3937 regulates virulence genes not linked to the hrp cluster

HrpX/Y is a putative two-component system (TCS) encoded within the type III secretion system (T3SS) gene cluster of Dickeya dadantii. A linear regulatory cascade initiated by HrpX/Y that leads to activation of the downstream T3SS genes via HrpS and HrpL was described previously. Therefore, in D. dadantii, HrpX/Y plays an important role in regulation of genes involved in bacteria-plant interactions and bacterial aggregation via the T3SS. HrpX/Y is the only TCS shared among the plant-pathogenic enterobacteria that is not also present in animal-associated enterobacteria. To date, the genes known to be regulated by HrpY are restricted to the hrp and hrc genes and no signal has been identified that triggers HrpY-dependent gene expression. We demonstrated that HrpY interacts with the hrpS promoter in vitro. We then used a transposon-based system to isolate previously unidentified HrpY-dependent genes, including genes previously shown to affect virulence, including kdgM and acsC. HrpY is a dual regulator, positively regulating at least 10 genes in addition to those in the hrp gene cluster and negatively regulating at least 5 genes. The regulatory effect on one gene depended on the culture medium used. Of the 16 HrpY-regulated genes identified in this screen, 14 are not present in Pectobacterium atrosepticum, the nearest relative of D. dadantii with a sequenced genome. None of the newly identified HrpY-regulated genes were required for bacterial aggregation; thus, neither acyl-homoserine lactone-mediated quorum sensing nor the Rcs signal transduction system which regulates colanic acid, a molecule that plays an important role in biofilm formation in other enterobacteria, are required for D. dadantii aggregation.

Biogenesis of Fe/S proteins and pathogenicity: IscR plays a key role in allowing Erwinia chrysanthemi to adapt to hostile conditions

The Erwinia chrysanthemi genome is predicted to encode three systems, Nif, Isc and Suf, known to assist Fe/S cluster biogenesis and the CsdAE cysteine desulphurase. Single iscU, hscA and fdx mutants were found sensitive to paraquat and exhibited reduced virulence on both chicory leaves and Arabidopsis thaliana. Depletion of the whole Isc system led to a pleiotropic phenotype, including sensitivity to both paraquat and 2,2'-dipyridyl, auxotrophies for branched-chain amino acids, thiamine, nicotinic acid, and drastic alteration in virulence. IscR was able to suppress all of the phenotypes listed above in a sufC-dependent manner while depletion of the Isc system led to IscR-dependent activation of the suf operon. No virulence defects were found associated with csdA or nifS mutations. Surprisingly, we found that the sufC mutant was virulent against A. thaliana, whereas its virulence had been found altered in Saintpaulia. Collectively, these results lead us to propose that E. chrysanthemi possess the Fe/S biogenesis strategy suited to the physico-chemical conditions encountered in its host upon infection. In this view, the IscR regulator, which controls both Isc and Suf, is predicted to play a major role in the ability of E. chrysanthemi to colonize a wide array of different plants.

CTA4 transcription factor mediates induction of nitrosative stress response in Candida albicans

This work has identified regulatory elements in the major fungal pathogen Candida albicans that enable response to nitrosative stress. Nitric oxide (NO) is generated by macrophages of the host immune system and commensal bacteria, and the ability to resist its toxicity is one adaptation that promotes survival of C. albicans inside the human body. Exposing C. albicans to NO induces upregulation of the flavohemoglobin Yhb1p. This protein confers protection by enzymatically converting NO to harmless nitrate, but it is unknown how C. albicans is able to detect NO in its environment and thus initiate this defense only as needed. We analyzed this problem by incrementally mutating the YHB1 regulatory region to identify a nitric oxide-responsive element (NORE) that is required for NO sensitivity. Five transcription factor candidates of the Zn(II)2-Cys6 family were then isolated from crude whole-cell extracts by using magnetic beads coated with this DNA element. Of the five, only deletion of the CTA4 gene prevented induction of YHB1 transcription during nitrosative stress and caused growth sensitivity to the NO donor dipropylenetriamine NONOate; Cta4p associates in vivo with NORE DNA from the YHB1 regulatory region. Deletion of CTA4 caused a small but significant decrease in virulence. A CTA4-dependent putative sulfite transporter encoded by SSU1 is also implicated in NO response, but C. albicans ssu1 mutants were not sensitive to NO, in contrast to findings in Saccharomyces cerevisiae. Cta4p is the first protein found to be necessary for initiating NO response in C. albicans.

The hypersensitive response; the centenary is upon us but how much do we know?

With the centenary of the first descriptions of 'hypersensitiveness' following pathogenic challenge upon us, it is appropriate to assess our current understanding of the hypersensitive response (HR) form of cell death. In recent decades our understanding of the initiation, associated signalling, and some important proteolytic events linked to the HR has dramatically increased. Genetic approaches are increasingly elucidating the function of the HR initiating resistance genes and there have been extensive analyses of death-associated signals, calcium, reactive oxygen species (ROS), nitric oxide, salicylic acid, and now sphingolipids. At the same time, attempts to draw parallels between mammalian apoptosis and the HR have been largely unsuccessful and it may be better to consider the HR to be a distinctive form of plant cell death. We will consider if the HR form of cell death may occur through metabolic dysfunction in which malfunctioning organelles may play a major role. This review will highlight that although our knowledge of parts of the HR is excellent, a comprehensive molecular model is still to be attained.

The hmp gene encoding the NO-inducible flavohaemoglobin in Escherichia coli confers a protective advantage in resisting killing within macrophages, but not in vitro: Links with swarming motility

Escherichia coli flavohaemoglobin (Hmp) is the best-understood nitric oxide (NO) detoxifying protein and exhibits a robust dioxygenase activity, converting NO to nitrate ion with oxygen as co-substrate. Synthesis of Hmp via transcriptional regulation of hmp gene expression is an adaptive response to NO and related nitrosative stresses since Hmp levels are greatly elevated on exposure in vitro to these agents. Here we show that expression of hmp is greatly enhanced by NO but not by other haem ligands (azide, cyanide and carbon monoxide). Flavohaemoglobins of other pathogenic bacteria have been implicated in conferring resistance to NO in vitro and in macrophage-like cells but the role of the E. coli flavohaemoglobin has not been studied in macrophages. We therefore compared survival of wild-type K-12 E. coli cells and an isogenic hmp mutant after internalisation by human macrophages. Wild-type bacteria survived significantly better than the hmp mutant after incubation with macrophages, despite binding and internalisation rates being similar for both strains. Unexpectedly, however, when grown in MOPS minimal medium, in mixed cultures, more hmp mutant cells were recovered than wild-type. Significantly, an hmp mutant failed to exhibit swarming motility on soft agar and this phenotype was rescued by a plasmid-borne copy of the wild-type hmp(+) gene. Thus, although Hmp constitutes an important mechanism of protection from NO-mediated killing by human macrophages in the model E. coli strain K-12, and probably contributes to the survival of enteropathogenic E. coli during the intestinal inflammatory response, synthesis of Hmp in vitro may represent a selective disadvantage. The lack of swarming motility of the hmp mutant and its aflagellate state suggest that Hmp synthesis is a metabolic burden in the absence of NO-related stresses.

AtPTR3, a wound-induced peptide transporter needed for defence against virulent bacterial pathogens in Arabidopsis

Mutation in the wound-induced peptide transporter gene AtPTR3 (At5g46050) of Arabidopsis thaliana has been shown to affect germination on media containing a high salt concentration. The heterologous expression in yeast was utilized to verify that the AtPTR3 protein transports di-and tripeptides. The T-DNA insert in the Atptr3-1 mutant in the Arabidopsis ecotype C24 revealed two T-DNA copies, the whole vector sequence, and the gus marker gene inserted in the second intron of the AtPTR3 gene. An almost identical insertion site was found in the Atptr3-2 mutant of the Col-0 ecotype. The AtPTR3 expression was shown to be regulated by several signalling compounds, most clearly by salicylic acid (SA), but also methyl jasmonate (MeJA) and abscisic acid. Real-time PCR experiments suggested that the wound-induction of the AtPTR3 gene was abolished in the SA and JA signalling mutants. The Atptr3 mutant plants had increased susceptibility to virulent pathogenic bacteria Erwinia carotovora subsp. carotovora and Pseudomonas syringae pv. tomato, and produced more reactive oxygen species when grown on media containing paraquat or rose bengal. Public microarray data suggest that the AtPTR3 expression was induced by Pseudomonas elicitors and by avirulent P. syringae pathovars and type III secretion mutants. This was verified experimentally for the hrpA mutant with real-time PCR. These results suggest that AtPTR3 is one of the defence-related genes whose expression is reduced by virulent bacterium by type III dependent fashion. Our results suggest that AtPTR3 protects the plant against biotic and abiotic stresses.

Efflux pump gene expression in Erwinia chrysanthemi is induced by exposure to phenolic acids

Salicylic acid (SA) is an important signaling molecule in local and systemic plant resistance. Following infection by microbial pathogens and the initial oxidative burst in plants, SA accumulation functions in the amplification of defense gene expression. Production of pathogenesis-related proteins and toxic antimicrobial chemicals serves to protect the plant from infection. Successful microbial pathogens utilize a variety of mechanisms to rid themselves of toxic antimicrobial compounds. Important among these mechanisms are multidrug-resistance pumps that bring about the active efflux of toxic compounds from microbial cells. Here, we show that a combination SA and its precursors, t-cinnamic acid and benzoic acid, can activate expression of specific multidrug efflux pump-encoding genes in the plant pathogen Erwinia chrysanthemi and enhance survival of the bacterium in the presence of model as well as plant-derived antimicrobial chemicals. This ability of plant-pathogenic bacteria to co-opt plant defense-signaling molecules to activate multidrug efflux pumps may have evolved to ensure bacterial survival in susceptible host plants.

Ectopic Expression of the Cotton Non-symbiotic Hemoglobin Gene GhHbd1 Triggers Defense Responses and Increases Disease Tolerance in Arabidopsis

Plant non-symbiotic hemoglobins (nsHbs) play important roles in a variety of cellular processes. Previous evidence from this laboratory indicates that the expression of a class 1 nsHb gene (GhHb1) from cotton is induced in cotton roots challenged with the Verticillium wilt fungus. The present study examined further the expression patterns of the GhHb1 gene in cotton plants and characterized its in vivo function through ectopic overexpression of the gene in Arabidopsis thaliana. Expression of GhHb1 in cotton plants was induced by exogenously applied salicylic acid, methyl jasmonic acid, ethylene, hydrogen peroxide (H(2)O(2)) and nitric oxide (NO). Ectopic overproduction of GhHb1 in Arabidopsis led to constitutive expression of the defense genes PR-1 and PDF1.2, and conferred enhanced disease resistance to Pseudomonas syringae and tolerance to V. dahliae. GhHb1-transgenic Arabidopsis seedlings were more tolerant to exogenous NO and contained lower levels of cellular NO than the wild-type control. Moreover, transgenic plants with relatively high levels of expression of the GhHb1 gene developed spontaneous hypersensitive lesions on the leaves in the absence of pathogen inoculation. Our results indicate that GhHb1 proteins play a role in the defense responses against pathogen invasions, possibly by modulating the NO level and the ratio of H(2)O(2)/NO in the defense process.

Hemoglobins dioxygenate nitric oxide with high fidelity

Distantly related members of the hemoglobin (Hb) superfamily including red blood cell Hb, muscle myoglobin (Mb) and the microbial flavohemoglobin (flavoHb) dioxygenate nitric oxide (.NO). The reaction serves important roles in .NO metabolism and detoxification throughout the aerobic biosphere. Analysis of the stoichiometric product nitrate shows greater than 99% double O-atom incorporation from Hb(18)O(2), Mb(18)O(2) and flavoHb(18)O(2) demonstrating a conserved high fidelity .NO dioxygenation mechanism. Whereas, reactions of .NO with the structurally unrelated Turbo cornutus MbO(2) or free superoxide radical (-O.(2)) yield sub-stoichiometric nitrate showing low fidelity O-atom incorporation. These and other results support a .NO dioxygenation mechanism involving (1) rapid reaction of .NO with a Fe(III-)O.(2) intermediate to form Fe(III-)OONO and (2) rapid isomerization of the Fe(III-)OONO intermediate to form nitrate. A sub-microsecond isomerization event is hypothesized in which the O-O bond homolyzes to form a protein caged [Fe(IV)O .NO(2)] intermediate and ferryl oxygen attacks .NO(2) to form nitrate. Hb functions as a .NO dioxygenase by controlling O(2) binding and electrochemistry, guiding .NO diffusion and reaction, and shielding highly reactive intermediates from solvent water and biomolecules.

NO way to live; the various roles of nitric oxide in plant-pathogen interactions

Nitric oxide has attracted considerable interest from plant pathologists due its established role in regulating mammalian anti-microbial defences, particularly via programmed cell death (PCD). Although NO plays a major role in plant PCD elicited in response to certain types of pathogenic challenge, the race-specific hypersensitive response (HR), it is now evident that NO also acts in the regulation of non-specific, papilla-based resistance to penetration by plant cells that survive attack and, possibly, in systemic acquired resistance. Equally, the potential roles of NO signalling/scavenging within the pathogen are being recognized. This review will consider key defensive roles played by NO in living cells during plant-pathogen interactions, as well as in those undergoing PCD.