Ozone and nitric oxide induce cGMP-dependent and -independent transcription of defence genes in tobacco

The Arabidopsis thaliana proteome harbors undiscovered multi-domain molecules with functional guanylyl cyclase catalytic centers

BACKGROUND

Second messengers link external cues to complex physiological responses. One such messenger, 3',5'-cyclic guanosine monophosphate (cGMP), has been shown to play a key role in many physiological responses in plants. However, in higher plants, guanylyl cyclases (GCs), enzymes that generate cGMP from guanosine-5'-triphosphate (GTP) have remained elusive until recently. GC search motifs constructed from the alignment of known GCs catalytic centers form vertebrates and lower eukaryotes have led to the identification of a number of plant GCs that have been characterized in vitro and in vivo.Presentation of the hypothesis. Recently characterized GCs in Arabidopsis thaliana contributed to the development of search parameters that can identify novel candidate GCs in plants. We hypothesize that there are still a substantial number (> 40) of multi-domain molecules with potentially functional GC catalytic centers in plants that remain to be discovered and characterized.

TESTING THE HYPOTHESIS

The hypothesis can be tested, firstly, by computational methods constructing 3D models of selected GC candidates using available crystal structures as templates. Homology modeling must include substrate docking that can provide support for the structural feasibility of the GC catalytic centers in those candidates. Secondly, recombinant peptides containing the GC domain need to be tested in in vitro GC assays such as the enzyme-linked immune-sorbent assay (ELISA) and/or in mass spectrometry based cGMP assays. In addition, quantification of in vivo cGMP transients with fluorescent cGMP-reporter assays in wild-type or selected mutants will help to elucidate the biological role of novel GCs. Implications of the hypothesis. If it turns out that plants do harbor a large number of functional GC domains as part of multi-domain enzymes, then major new insights will be gained into the complex signal transduction pathways that link cGMP to fundamental processes such as ion transport and homeostasis, biotic and abiotic stress responses as well as cGMP-dependent responses to hormones.

Interaction between abscisic acid and nitric oxide in PB90-induced catharanthine biosynthesis of catharanthus roseus cell suspension cultures

Elicitations are considered to be an important strategy to improve production of secondary metabolites of plant cell cultures. However, mechanisms responsible for the elicitor-induced production of secondary metabolites of plant cells have not yet been fully elucidated. Here, we report that treatment of Catharanthus roseus cell suspension cultures with PB90, a protein elicitor from Phytophthora boehmeriae, induced rapid increases of abscisic acid (ABA) and nitric oxide (NO), subsequently followed by the enhancement of catharanthine production and up-regulation of Str and Tdc, two important genes in catharanthine biosynthesis. PB90-induced catharanthine production and the gene expression were suppressed by the ABA inhibitor and NO scavenger respectively, showing that ABA and NO are essential for the elicitor-induced catharanthine biosynthesis. The relationship between ABA and NO in mediating catharanthine biosynthesis was further investigated. Treatment of the cells with ABA triggered NO accumulation and induced catharanthine production and up-regulation of Str and Tdc. ABA-induced catharanthine production and gene expressions were suppressed by the NO scavenger. Conversely, exogenous application of NO did not stimulate ABA generation and treatment with ABA inhibitor did not suppress NO-induced catharanthine production and gene expressions. Together, the results showed that both NO and ABA were involved in PB90-induced catharanthine biosynthesis of C. roseus cells. Furthermore, our data demonstrated that ABA acted upstream of NO in the signaling cascade leading to PB90-induced catharanthine biosynthesis of C. roseus cells.

Structural and functional characteristics of cGMP-dependent methionine oxidation in Arabidopsis thaliana proteins

BACKGROUND

Increasing structural and biochemical evidence suggests that post-translational methionine oxidation of proteins is not just a result of cellular damage but may provide the cell with information on the cellular oxidative status. In addition, oxidation of methionine residues in key regulatory proteins, such as calmodulin, does influence cellular homeostasis. Previous findings also indicate that oxidation of methionine residues in signaling molecules may have a role in stress responses since these specific structural modifications can in turn change biological activities of proteins.

FINDINGS

Here we use tandem mass spectrometry-based proteomics to show that treatment of Arabidopsis thaliana cells with a non-oxidative signaling molecule, the cell-permeant second messenger analogue, 8-bromo-3,5-cyclic guanosine monophosphate (8-Br-cGMP), results in a time-dependent increase in the content of oxidised methionine residues. Interestingly, the group of proteins affected by cGMP-dependent methionine oxidation is functionally enriched for stress response proteins. Furthermore, we also noted distinct signatures in the frequency of amino acids flanking oxidised and un-oxidised methionine residues on both the C- and N-terminus.

CONCLUSIONS

Given both a structural and functional bias in methionine oxidation events in response to a signaling molecule, we propose that these are indicative of a specific role of such post-translational modifications in the direct or indirect regulation of cellular responses. The mechanisms that determine the specificity of the modifications remain to be elucidated.

Heme oxygenase-1 is involved in nitric oxide- and cGMP-induced α-Amy2/54 gene expression in GA-treated wheat aleurone layers

Here, α-Amy2/54 gene expression was used as a molecular probe to investigate the interrelationship among nitric oxide (NO), cyclic GMP (cGMP), and heme oxygenase-1 (HO-1) in GA-treated wheat aleurone layers. The inducible expressions of α-Amy2/54 and α-amylase activity were respectively amplified by two NO-releasing compounds, sodium nitroprusside (SNP) and spermine NONOate, in a GA-dependent fashion. Similar responses were observed when an inducer of HO-1, hemin-or one of its catalytic products, carbon monoxide (CO) in aqueous solution-was respectively added. The SNP-induced responses, mimicked by 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP), a cGMP derivative, were NO-dependent. This conclusion was supported by the fact that endogenous NO overproduction was rapidly induced by SNP, and thereafter induction of α-Amy2/54 gene expression and increased α-amylase activity were sensitive to the NO scavenger. We further observed that the above induction triggered by SNP and 8-Br-cGMP was partially prevented by zinc protoporphyrin IX (ZnPPIX), an inhibitor of HO-1. These blocking effects were clearly reversed by CO, confirming that the above response was HO-1-specific. Further analyses showed that both SNP and 8-Br-cGMP rapidly up-regulated HO-1 gene expression and increased HO activity, and SNP responses were sensitive to cPTIO and the guanylate cyclase inhibitor 6-anilino-5,8-quinolinedione (LY83583). Molecular evidence confirmed that GA-induced GAMYB and ABA-triggered PKABA1 transcripts were up-regulated or down-regulated by SNP, 8-Br-cGMP or CO cotreated with GA. Contrasting changes were observed when cPTIO, LY83583, or ZnPPIX was added. Together, our results suggested that HO-1 is involved in NO- and cGMP-induced α-Amy2/54 gene expression in GA-treated aleurone layers.

Computational identification of candidate nucleotide cyclases in higher plants

In higher plants guanylyl cyclases (GCs) and adenylyl cyclases (ACs) cannot be identified using BLAST homology searches based on annotated cyclic nucleotide cyclases (CNCs) of prokaryotes, lower eukaryotes, or animals. The reason is that CNCs are often part of complex multifunctional proteins with different domain organizations and biological functions that are not conserved in higher plants. For this reason, we have developed CNC search strategies based on functionally conserved amino acids in the catalytic center of annotated and/or experimentally confirmed CNCs. Here we detail this method which has led to the identification of >25 novel candidate CNCs in Arabidopsis thaliana, several of which have been experimentally confirmed in vitro and in vivo. We foresee that the application of this method can be used to identify many more members of the growing family of CNCs in higher plants.

Molecular methods for the study of signal transduction in plants

Novel and improved analytical methods have led to a rapid increase in our understanding of the molecular mechanism underlying plant signal transduction. Progress has been made both at the level of single-component analysis and in vivo imaging as well as at the systems level where transcriptomics and particularly phosphoproteomics afford a window into complex biological responses. Here we review the role of the cyclic nucleotides cAMP and cGMP in plant signal transduction as well as the discovery and biochemical and biological characterization of an increasing number of complex multi-domain nucleotide cyclases that catalyze the synthesis of cAMP and cGMP from ATP and GTP, respectively.

Nitric oxide participates in cold-inhibited Camellia sinensis pollen germination and tube growth partly via cGMP in vitro

Nitric oxide (NO) plays essential roles in many biotic and abiotic stresses in plant development procedures, including pollen tube growth. Here, effects of NO on cold stress inhibited pollen germination and tube growth in Camellia sinensis were investigated in vitro. The NO production, NO synthase (NOS)-like activity, cGMP content and proline (Pro) accumulation upon treatment with NO scavenger cPTIO, NOS inhibitor L-NNA, NO donor DEA NONOate, guanylate cyclase (GC) inhibitor ODQ or phosphodiesterase (PDE) inhibitor Viagra at 25°C (control) or 4°C were analyzed. Exposure to 4°C for 2 h reduced pollen germination and tube growth along with increase of NOS-like activity, NO production and cGMP content in pollen tubes. DEA NONOate treatment inhibited pollen germination and tube growth in a dose-dependent manner under control and reinforced the inhibition under cold stress, during which NO production and cGMP content promoted in pollen tubes. L-NNA and cPTIO markedly reduced the generation of NO induced by cold or NO donor along with partly reverse of cold- or NO donor-inhibited pollen germination and tube growth. Furthermore, ODQ reduced the cGMP content under cold stress and NO donor treatment in pollen tubes. Meanwhile, ODQ disrupted the reinforcement of NO donor on the inhibition of pollen germination and tube growth under cold condition. Additionally, Pro accumulation of pollen tubes was reduced by ODQ compared with that receiving NO donor under cold or control condition. Effects of cPTIO and L-NNA in improving cold-treated pollen germination and pollen tube growth could be lowered by Viagra. Moreover, the inhibitory effects of cPTIO and L-NNA on Pro accumulation were partly reversed by Viagra. These data suggest that NO production from NOS-like enzyme reaction decreased the cold-responsive pollen germination, inhibited tube growth and reduced Pro accumulation, partly via cGMP signaling pathway in C. sinensis.

Lead-induced nitric oxide generation plays a critical role in lead uptake by Pogonatherum crinitum root cells

The effects of lead (Pb) on endogenous nitric oxide (NO) generation, the role of NO in Pb uptake and the origin of Pb-induced NO production in Pogonatherum crinitum root cells were evaluated. Pb treatment induced rapid NO generation, showing that Pb exposure triggered endogenous NO signaling of the cells. Pre-treatment of the cells with the NO-specific scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline -1-oxyl-3-oxide (cPTIO) not only abolished the Pb-triggered NO burst but also reduced Pb contents of the cells. Moreover, Pb exposure enhanced nitrate reductase (NR) activity of the cells. The NR inhibitors tungstate and glutamine not only suppressed the Pb-enhanced NR activities but also reduced the Pb-triggered NO generation. Pre-treatment of the cells with tungstate and glutamine suppressed Pb accumulation and the suppression could be restored by application of exogenous NO via its donors sodium nitroprusside (SNP) and S-nitrosoglutathione (GSNO). Together, our results indicated that Pb exposure enhanced NR activity and triggered the NO burst of P. crinitum root cells. Furthermore, the data demonstrated that NR was responsible for the Pb-triggered NO burst and that NR-mediated NO generation played a critical role in Pb uptake by P. crinitum root cells. Thus, our results suggest a potential strategy for controlling Pb uptake by plants by targeting NR as a source of Pb-triggered NO production.

Effects of exogenous nitric oxide in wheat seedlings under chilling stress

The effects of nitric oxide (NO) on chilling tolerance (contents of hydrogen peroxide (H2O2) and superoxide anion (O2–) and lipid peroxidation level (malondialdehyde, MDA)) and the activities of antioxidant enzymes (superoxide dismutase (SOD), peroxidase (POX) and catalase (CAT)) were investigated in the leaves of wheat ( Triticum aestivum L.) exposed to chilling. NO treatment was carried out through spraying of sodium nitroprusside (SNP), which is a donor of NO. To do this, SNP concentrations of 0.1 and 1 mM were applied on the leaves of 11-day plants and the plants were then exposed to chilling conditions (5/2°C) for 3 days. The chilling stress treatment increased both the activities of antioxidant enzymes and the levels of MDA, H2O2 and O2–. Similarly, NO treatment enhanced SOD, POX and CAT activities under chilling stress, whereas it decreased H2O2 and O2.– contents as well as MDA level. The most effective concentration was determined as 0.1 mM SNP. Exogenous SNP application as a donor of NO was found to have an important ameliorative effect on cold tolerance of seedling exposed to chilling stress by stimulating antioxidant enzyme activity.

Involvement of protein kinases and calcium in the NO-signalling cascade for defence-gene induction in ozonated tobacco plants

This study analyses the signalling pathways triggered by nitric oxide (NO) in response to ozone (O(3)) fumigation of tobacco plants, with particular attention to protein kinase cascades and free cytosolic Ca(2+) in defence-gene activation. NO was visualized with the NO probe DAF-FM. Using a pharmacological approach, the effects of different inhibitors on the expression profiles of NO-dependent defence genes were monitored using RT-PCR. The assay of the kinase activity of the immunoprecipitates complexes shows that O(3) stimulates a 48 kDa salicylic acid (SA)-induced protein kinase (SIPK) in an NO-dependent manner. The O(3)-induced alternative oxidase 1a (AOX1a) and phenylalanine ammonia lyase a (PALa) genes are modulated by phosphorylation by protein kinases, and SIPK might have a role in this up-regulation. By contrast, protein dephosphorylation mediates pathogenesis-related protein 1a (PR1a) expression in O(3)-treated tobacco plants. Ca(2+) is essential, but not sufficient, to promote NO accumulation in ozonated tobacco plants. Intracellular Ca(2+) transients are also essential for PALa up-regulation and cGMP-induced PR1a expression. Partial dependence on intracellular Ca(2+) suggests two different pathways of SA accumulation and PR1a induction. A model summarizing the signalling networks involving NO, SA, and the cellular messengers in this O(3)-induced defence gene activation is proposed.

N-3-oxo-decanoyl-L-homoserine-lactone activates auxin-induced adventitious root formation via hydrogen peroxide- and nitric oxide-dependent cyclic GMP signaling in mung bean

N-Acyl-homoserine-lactones (AHLs) are bacterial quorum-sensing signaling molecules that regulate population density. Recent evidence demonstrates their roles in plant defense responses and root development. Hydrogen peroxide (H(2)O(2)), nitric oxide (NO), and cyclic GMP (cGMP) are essential messengers that participate in various plant physiological processes, but how these messengers modulate the plant response to N-acyl-homoserine-lactone signals remains poorly understood. Here, we show that the N-3-oxo-decanoyl-homoserine-lactone (3-O-C10-HL), in contrast to its analog with an unsubstituted branch chain at the C3 position, efficiently stimulated the formation of adventitious roots and the expression of auxin-response genes in explants of mung bean (Vigna radiata) seedlings. This response was mimicked by the exogenous application of auxin, H(2)O(2), NO, or cGMP homologs but suppressed by treatment with scavengers or inhibitors of H(2)O(2), NO, or cGMP metabolism. The 3-O-C10-HL treatment enhanced auxin basipetal transport; this effect could be reversed by treatment with H(2)O(2) or NO scavengers but not by inhibitors of cGMP synthesis. Inhibiting 3-O-C10-HL-induced H(2)O(2) or NO accumulation impaired auxin- or 3-O-C10-HL-induced cGMP synthesis; however, blocking cGMP synthesis did not affect auxin- or 3-O-C10-HL-induced H(2)O(2) or NO generation. Additionally, cGMP partially rescued the inhibitory effect of H(2)O(2) or NO scavengers on 3-O-C10-HL-induced adventitious root development and auxin-response gene expression. These results suggest that 3-O-C10-HL, unlike its analog with an unmodified branch chain at the C3 position, can accelerate auxin-dependent adventitious root formation, possibly via H(2)O(2)- and NO-dependent cGMP signaling in mung bean seedlings.

Ozone affects pollen viability and NAD(P)H oxidase release from Ambrosia artemisiifolia pollen

Air pollution is frequently proposed as a cause of the increased incidence of allergy in industrialised countries. We investigated the impact of ozone (O(3)) on reactive oxygen species (ROS) and allergen content of ragweed pollen (Ambrosia artemisiifolia). Pollen was exposed to acute O(3) fumigation, with analysis of pollen viability, ROS and nitric oxide (NO) content, activity of nicotinamide adenine dinucleotide phosphate (NAD[P]H) oxidase, and expression of major allergens. There was decreased pollen viability after O(3) fumigation, which indicates damage to the pollen membrane system, although the ROS and NO contents were not changed or were only slightly induced, respectively. Ozone exposure induced a significant enhancement of the ROS-generating enzyme NAD(P)H oxidase. The expression of the allergen Amb a 1 was not affected by O(3), determined from the mRNA levels of the major allergens. We conclude that O(3) can increase ragweed pollen allergenicity through stimulation of ROS-generating NAD(P)H oxidase.

Ultraviolet-B-induced flavonoid accumulation in Betula pendula leaves is dependent upon nitrate reductase-mediated nitric oxide signaling

Nitric oxide (NO) is an important signaling molecule involved in many physiological processes in plants. Nitric oxide generation and flavonoid accumulation are two early reactions of plants to ultraviolet-B (UV-B) irradiation. However, the source of UV-B-triggered NO generation and the role of NO in UV-B-induced flavonoid accumulation are not fully understood. In order to evaluate the origin of UV-B-triggered NO generation, we examined the responses of nitrate reductase (NR) activity and the expression levels of NIA1 and NIA2 genes in leaves of Betula pendula Roth (silver birch) seedlings to UV-B irradiation. The data show that UV-B irradiation stimulates NR activity and induces up-regulation of NIA1 but does not affect NIA2 expression during UV-B-triggered NO generation. Pretreatment of the leaves with NR inhibitors tungstate (TUN) and glutamine (Gln) abolishes not only UV-B-triggered NR activities but also UV-B-induced NO generation. Furthermore, application of TUN and Gln suppresses UV-B-induced flavonoid production in the leaves and the suppression of NR inhibitors on UV-B-induced flavonoid production can be reversed by NO via its donor sodium nitroprusside. Together, the data indicate that NIA1 in the leaves of silver birch seedlings is sensitive to UV-B and the UV-B-induced up-regulation of NIA1 may lead to enhancement of NR activity. Furthermore, our results demonstrate that NR is involved in UV-B-triggered NO generation and NR-mediated NO generation is essential for UV-B-induced flavonoid accumulation in silver birch leaves.

Role of nitric oxide in hydrogen peroxide-dependent induction of abiotic stress tolerance by brassinosteroids in cucumber

Brassinosteroids (BRs) can induce plant tolerance to a variety of abiotic stresses by triggering the generation of H(2) O(2) as a signalling molecule in cucumber leaves. Whether nitric oxide (NO) also plays a signalling role and, if so, what is the relationship between NO and H(2) O(2) in BR-induced stress tolerance are unknown. Involvement of NO and H(2) O(2) in BR-induced tolerance was examined. NO accumulation and defence related gene transcripts were monitored by confocal laser-scanning microscopy and qRT-PCR, respectively. NO content was elevated after treatment with 24-epibrassinolide (EBR) and reduced with the inhibition of BR biosynthesis. EBR-induced NO production was blocked by pre-treatment with inhibitor of NADPH oxidase and a reactive oxygen species scavenger. On the other hand, EBR-induced H(2) O(2) generation was not sensitive to NO scavenger or inhibitor of NO production. Scavenging or inhibition of NO production inhibited EBR-induced tolerance to photo-oxidative and cold stress and partly blocked EBR-induced expression and activities of several antioxidant enzymes. Pre-treatment of the exogenous NO precursor, on the other hand, led to both increased stress tolerance and increased expression of antioxidant enzymes. These results strongly suggest that NO plays an important role in H(2) O(2) -dependent induction of plant stress tolerance by BR.

The Arabidopsis wall associated kinase-like 10 gene encodes a functional guanylyl cyclase and is co-expressed with pathogen defense related genes

Background

Second messengers have a key role in linking environmental stimuli to physiological responses. One such messenger, guanosine 3′,5′-cyclic monophosphate (cGMP), has long been known to be an essential signaling molecule in many different physiological processes in higher plants, including biotic stress responses. To date, however, the guanylyl cyclase (GC) enzymes that catalyze the formation of cGMP from GTP have largely remained elusive in higher plants.

Principal Findings

We have identified an Arabidopsis receptor type wall associated kinase–like molecule (AtWAKL10) as a candidate GC and provide experimental evidence to show that the intracellular domain of AtWAKL10_431–700 can generate cGMP in vitro. Further, we also demonstrate that the molecule has kinase activity indicating that AtWAKL10 is a twin-domain catalytic protein. A co-expression and stimulus-specific expression analysis revealed that AtWAKL10 is consistently co-expressed with well characterized pathogen defense related genes and along with these genes is induced early and sharply in response to a range of pathogens and their elicitors.

Conclusions

We demonstrate that AtWAKL10 is a twin-domain, kinase-GC signaling molecule that may function in biotic stress responses that are critically dependent on the second messenger cGMP.

Alternative splicing and gene duplication differentially shaped the regulation of isochorismate synthase in Populus and Arabidopsis

Isochorismate synthase (ICS) converts chorismate to isochorismate for the biosynthesis of phylloquinone, an essential cofactor for photosynthetic electron transport. ICS is also required for salicylic acid (SA) synthesis during Arabidopsis defense. In several other species, including Populus, SA is derived primarily from the phenylpropanoid pathway. We therefore sought to investigate ICS regulation in Populus to learn the extent of ICS involvement in SA synthesis and defense. Arabidopsis harbors duplicated AtICS genes that differ in their exon-intron structure, basal expression, and stress inducibility. In contrast, we found a single ICS gene in Populus and six other sequenced plant genomes, pointing to the AtICS duplication as a lineage-specific event. The Populus ICS encodes a functional plastidic enzyme, and was not responsive to stresses that stimulated phenylpropanoid accumulation. Populus ICS underwent extensive alternative splicing that was rare for the duplicated AtICSs. Sequencing of 184 RT-PCR Populus clones revealed 37 alternative splice variants, with normal transcripts representing approximately 50% of the population. When expressed in Arabidopsis, Populus ICS again underwent alternative splicing, but did not produce normal transcripts to complement AtICS1 function. The splice-site sequences of Populus ICS are unusual, suggesting a causal link between junction sequence, alternative splicing, and ICS function. We propose that gene duplication and alternative splicing of ICS evolved independently in Arabidopsis and Populus in accordance with their distinct defense strategies. AtICS1 represents a divergent isoform for inducible SA synthesis during defense. Populus ICS primarily functions in phylloquinone biosynthesis, a process that can be sustained at low ICS transcript levels.

Gene expression profiling of ozone-treated Arabidopsis abi1td insertional mutant: protein phosphatase 2C ABI1 modulates biosynthesis ratio of ABA and ethylene

We report on the characterization of the interaction between reactive oxygen species signalling and abscisic acid (ABA)-mediated gene network in ozone (O(3)) stress response. To identify the stress-related signalling pathways and possible cross-talk controlled by an ABA-negative regulator, the protein phosphatase 2C abscisic acid insensitive1 (ABI1), we performed a genome-wide transcription profiling of O(3)-treated wild-type and ABI1 knockout (abi1td) plants. In addition, to better understand ABA signalling and the interactions between stress response pathways, we performed a microarray analysis of drought-treated plants. Functional categorization of the identified genes showed that ABI1 is involved in the modulation of several cellular processes including metabolism, transport, development, information pathways and variant splicing. Comparisons with available transcriptome data sets revealed the extent of ABI1 involvement in both ABA-dependent and ABA-independent gene expression. Furthermore, in O(3) stress the ABA hypersensitivity of abi1td resulted in a significant reduction of the ABA level, ethylene (ET) over-production and O(3) tolerance. Moreover, the physical interaction of ABI1 with ACC synthase2 and ACC synthase6 was shown. We provide a model explaining how ABI1 can regulate both ABA and ET biosynthesis. Altogether, our findings indicate that ABI1 plays the role of a general signal transducer linking ABA and ET biosynthesis as well as signalling pathways to O(3) stress tolerance.