Nitric oxide and salicylic acid signaling in plant defense

Transcript profiling of antioxidant genes during biotic and abiotic stresses in Panax ginseng C. A. Meyer

The regulation of reactive oxygen scavengers against biotic and abiotic conditions were investigated in the seedling of Panax ginseng C. A. Meyer. From the EST library we selected the antioxidant marker genes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione peroxidase (GPX), and glutathione synthase (GS). The abiotic chilling, heat, osmotic, oxidative, and wounding stresses and biotic stresses with fungal pathogens were tested against 3-week-grown seedlings. The expression patterns of the genes were analyzed by means of real-time quantitative RT-PCR. The transcriptome result under abiotic stresses showed differential expression and elevated up-regulation of PgSOD, PgGPX, PgGS, and PgAPX, thus it may prove the generation of ROS in ginseng. Whereas, in biotic stress the up-regulation of transcript level merely based on the incompatible interactions. But PgAPX and PgCAT showed no significant change or slight down-regulation of transcript level during pathogen interaction. Thus it may suggest that in ginseng, plant-pathogen interaction triggers defense-related gene transcription via salicylic acid mediated signaling mechanism, and also possess crosstalk signaling networks between abiotic and biotic stress responses.

Deciphering the structural and biological properties of the lipid A moiety of lipopolysaccharides from Burkholderia cepacia strain ASP B 2D, in Arabidopsis thaliana

Lipopolysaccharides (LPSs) are major, indispensable cell surface components of Gram-negative bacteria that have diverse roles in bacterial pathogenesis of plants. Environmental strains of Burkholderia cepacia have been described as phytopathogens, growth promotors, biocontrol agents and bioremediation agents. We have previously shown that LPSs from B. cepacia can be recognized as microbe-associated molecular pattern molecules, to elicit defense responses in plants. Recent findings suggest that the lipid A moiety might be partially responsible for LPSs perception. These studies were extended by analysis of the structure and biological activity of the lipid A moiety of LPSs of B. cepacia(.) The full structure was determined by a combination of negative/positive-ion matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS) on intact and partially degraded substrates. B. cepacia lipid A was found to contain a tetra- or penta-acylated, 1,4'-diphosphorylated, β-(1-6)-linked D-GlcN disaccharide and further substituted by L-Ara4N in position 4'. As primary fatty acids, R-configurated 16:0(3-OH) (amide-linked in 2 and 2') and 14:0(3-OH) (ester-linked in 3 and 3', nonstoichiometric) were identified. A secondary 14:0 was located at position 2'. Its biological activity to elicit defense-related responses was subsequently investigated by monitoring the changes in the transcriptome of Arabidopsis thaliana seedlings. Genes found to be upregulated code for proteins involved in signal perception and transduction, transcriptional regulation, defense and stress responses. Furthermore, genes encoding proteins involved in chaperoning, protein interactions and protein degradation were differentially expressed as part of the metabolic reprogramming of cellular activities in support of immunity and defense.

Redox regulation of the NPR1-TGA1 system of Arabidopsis thaliana by nitric oxide

The role of reactive oxygen and nitrogen species in local and systemic defense reactions is well documented. NPR1 and TGA1 are key redox-controlled regulators of systemic acquired resistance in plants. NPR1 monomers interact with the reduced form of TGA1, which targets the activation sequence-1 (as-1) element of the promoter region of defense proteins. Here, we report the effect of the physiological nitric oxide donor S-nitrosoglutathione on the NPR1/TGA1 regulation system in Arabidopsis thaliana. Using the biotin switch method, we demonstrate that both NPR1 and TGA1 are S-nitrosylated after treatment with S-nitrosoglutathione. Mass spectrometry analyses revealed that the Cys residues 260 and 266 of TGA1 are S-nitrosylated and S-glutathionylated even at GSNO concentrations in the low micromolar range. Furthermore, we showed that S-nitrosoglutathione protects TGA1 from oxygen-mediated modifications and enhances the DNA binding activity of TGA1 to the as-1 element in the presence of NPR1. In addition, we observed that the translocation of NPR1 into the nucleus is promoted by nitric oxide. Taken together, our results suggest that nitric oxide is a redox regulator of the NPR1/TGA1 system and that they underline the importance of nitric oxide in the plant defense response.

Regulation of Nicotiana tabacum osmotic stress-activated protein kinase and its cellular partner GAPDH by nitric oxide in response to salinity

Several studies focusing on elucidating the mechanism of NO (nitric oxide) signalling in plant cells have highlighted that its biological effects are partly mediated by protein kinases. The identity of these kinases and details of how NO modulates their activities, however, remain poorly investigated. In the present study, we have attempted to clarify the mechanisms underlying NO action in the regulation of NtOSAK (Nicotiana tabacum osmotic stress-activated protein kinase), a member of the SNF1 (sucrose non-fermenting 1)-related protein kinase 2 family. We found that in tobacco BY-2 (bright-yellow 2) cells exposed to salt stress, NtOSAK is rapidly activated, partly through a NO-dependent process. This activation, as well as the one observed following treatment of BY-2 cells with the NO donor DEA/NO (diethylamine-NONOate), involved the phosphorylation of two residues located in the kinase activation loop, one being identified as Ser158. Our results indicate that NtOSAK does not undergo the direct chemical modifications of its cysteine residues by S-nitrosylation. Using a co-immunoprecipitation-based strategy, we identified several proteins present in immunocomplex with NtOSAK in salt-treated cells including the glycolytic enzyme GAPDH (glyceraldehyde-3-phosphate dehydrogenase). Our results indicate that NtOSAK directly interacts with GAPDH in planta. Furthermore, in response to salt, GAPDH showed a transient increase in its S-nitrosylation level which was correlated with the time course of NtOSAK activation. However, GADPH S-nitrosylation did not influence its interaction with NtOSAK and did not have an impact on the activity of the protein kinase. Taken together, the results support the hypothesis that NtOSAK and GAPDH form a cellular complex and that both proteins are regulated directly or indirectly by NO.

Role of nitric oxide and reactive oxygen [corrected] species in disease resistance to necrotrophic pathogens

Nitric oxide (NO) and reactive oxygen species (ROS) are important signaling molecules in plant immunity. However, roles of NO and ROS in disease resistance to necrotrophic pathogens are not fully understood. We have recently demonstrated that NO plays a pivotal role in basal defense against Botrytis cinerea and the expression of the salicylic acid (SA) -responsive gene PR-1in Nicotiana benthamiana. By contrast, ROS function negatively in resistance or positively in expansion of disease lesions during B. cinerea-N. benthamiana interaction. Here, analysis in NahG-transgenic N. benthamiana showed that SA signaling is not involved in resistance to B. cinerea in N. benthamiana. We discuss how NO and ROS participate in disease resistance to necrotrophic pathogens on the basis of recent reports.

Effects of exogenous salicylic acid pre-treatment on cadmium toxicity and leaf lipid content in Linum usitatissimum L

The effects of salicylic acid (SA) on cadmium (Cd) toxicity in flax plants were studied by investigating plant growth, lipid peroxidation and fatty acid composition. Cadmium inhibited biomass production as well as the absorption of K, Ca, Mg and Fe. Furthermore, it dramatically increased Cd accumulation in both roots and shoots. The pre-soaking of dry flax grains in SA-containing solutions partially protected seedlings from Cd toxicity during the following growth period. SA treatment decreased the uptake and the transport of Cd, alleviated the Cd-induced inhibition of Ca, Mg and Fe absorption and promoted plant growth. At leaf level, Cd significantly decreased both total lipid (TL) and chlorophyll (Chl) content and enhanced electrolyte leakage and lipid peroxidation as indicated by malondialdehyde (MDA) accumulation. Concomitantly, Cd caused a shift in fatty acid composition, resulting in a lower degree of their unsaturation. SA pre-soaking ameliorated the increased electrolyte leakage as well as Chl, MDA and TL content. SA particularly increased the percentage of linolenic acid and lowered that of palmitic acid by the same proportion. These results suggest that SA could be used as a potential growth regulator and a stabilizer of membrane integrity to improve plant resistance to Cd stress.

Salicylic acid reduces napropamide toxicity by preventing its accumulation in rapeseed (Brassica napus L.)

Napropamide is a widely used herbicide for controlling weeds in crop production. However, extensive use of the herbicide has led to its accumulation in ecosystems, thus causing toxicity to crops and reducing crop production and quality. Salicylic acid (SA) plays multiple roles in regulating plant adaptive responses to biotic and environmental stresses. However, whether SA regulates plant response to herbicides (or pesticides) was unknown. In this study, we investigated the effect of SA on herbicide napropamide accumulation and biological processes in rapeseed (Brassica napus). Plants exposed to 8 mg kg(-1) napropamide showed growth stunt and oxidative damage. Treatment with 0.1 mM SA improved growth and reduced napropamide levels in plants. Treatment with SA also decreased the abundance of O (2) (-.) and H(2)O(2) as well as activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), and increased activities of guaiacol peroxidase (POD) and glutathione-S-transferase (GST) in napropamide-exposed plants. Analysis of SOD, CAT, and POD activities using nondenaturing polyacrylamide gel electrophoresis (PAGE) confirmed the results. These results may help to understand how SA regulates plant response to organic contaminants and provide a basis to control herbicide/pesticide contamination in crop production.

Infection processes and involvement of defense-related genes in the expression of resistance in cultivars of subterranean clover (Trifolium subterraneum) to Phytophthora clandestina

Studies on infection processes and gene expression were done to determine differential responses of cultivars of Trifolium subterraneum resistant and susceptible to infection by races of Phytophthora clandestina. In the infection process study, one race was inoculated onto the roots of T. subterraneum cvs. Woogenellup and Junee (compatible or incompatible interactions, respectively). There were no differences in relation to the processes of cyst attachment, germination, and hyphal penetration. There were, however, major differences in infection progression observed post-penetration between compatible and incompatible interactions. In susceptible cv. Woogenellup, hyphae grew into the vascular bundles and produced intercellular antheridia and oogonia in the cortex and stele by 4 days postinoculation (dpi), oospores in the cortex and stele by 8 dpi, when sporangia were evident on the surface of the root. Infected taproots were discolored. Early destruction of taproots prevented emergence of lateral roots. Roots of resistant cv. Junee showed no oospores or sporangia and no disease at 8 dpi. In the gene expression studies, two races of P. clandestina were inoculated onto three cultivars of T. subterraneum. Results showed that three genes known to be associated with plant defense against plant pathogens were differentially expressed in the roots during compatible and incompatible interactions. Phenylalanine ammonia lyase and chalcone synthase genes were activated 4 h postinoculation (hpi) and cytochrome P450 trans-cinnamic acid 4-monooxygenase gene was activated 8 hpi in the incompatible interactions in cvs. Denmark and Junee following inoculation with Race 177. In contrast, in compatible interactions in cv. Woogenellup, there were no significant changes in the activation of these three genes following inoculation, indicating that these three genes were associated with the expression of resistance to Race 177 of the pathogen by the host. To confirm this result, in the second test, cv. Woogenellup was challenged by Race 000 of P. clandestina. In this incompatible interaction, cv. Woogenellup was resistant and expressed highly all three genes in the manner similar to the incompatible interactions observed in the first test.

Production of nitric oxide in host-virus interaction: a case study with a compatible Begomovirus-Kenaf host-pathosystem

Nitric oxide (NO) plays a key role in plant diseases resistance. Here we have first time demonstrated that begomovirus infection in susceptible H. cannabinus plants, results in elevated NO and reactive nitrogen species production during early infection stage not only in infected leaf but also in root and shoot. Production of NO was further confirmed by oxyhemoglobin assay. Furthermore, we used Phenyl alanine ammonia lyase as marker of pathogenesis related enzyme. In addition evidence for protein tyrosine nitration during the early stage of viral infection clearly showed the involvement of nitrosative stress.

The expression, function and regulation of mitochondrial alternative oxidase under biotic stresses

To survive, plants possess elaborate defence mechanisms to protect themselves against virus or pathogen invasion. Recent studies have suggested that plant mitochondria may play an important role in host defence responses to biotic stresses. In contrast with animal mitochondria, plant mitochondria possess a unique respiratory pathway, the cyanide-insensitive alternative pathway, which is catalysed by the alternative oxidase (AOX). Much work has revealed that the genes encoding AOX, AOX protein and the alternative respiratory pathway are frequently induced during plant-pathogen (or virus) interaction. This raises the possibility that AOX is involved in host defence responses to biotic stresses. Thus, a key to the understanding of the role of mitochondrial respiration under biotic stresses is to learn the function and regulation of AOX. In this article, we focus on the theoretical and experimental progress made in the current understanding of the function and regulation of AOX under biotic stresses. We also address some speculative aspects to aid further research in this area.

Proteomic analysis of salicylate-induced proteins of pea (Pisum sativum L.) leaves

The effect of 50 microM salicylic acid on soluble proteins of pea (Pisum sativum L.) leaves was studied by proteomic analysis. Thirty-two salicylate-induced proteins were found, and 13 of these were identified using MALDI TOF MS. Salicylate-induced increased content was shown for the first time for the family 18 glycoside hydrolase, alpha-amylase, 33 kDa protein of photosystem II, lipid-desaturase-like protein, and glutamine amidotransferase. Increased content of protective proteins of direct antipathogenic action such as chitinase and beta-1,3-glucanases was also noted.

ROS in biotic interactions

Production of reactive oxygen species (ROS) is a hallmark of successful recognition of infection and activation of plant defenses. ROS play multifaceted signaling functions mediating the establishment of multiple responses and can act as local toxins. Controversy surrounds the origin of these ROS. Several enzymatic mechanisms, among them a plasma membrane NADPH oxidase and cell wall peroxidases, can be responsible for the ROS detected in the apoplast. However, high levels of ROS from metabolic origins and/or from downregulation of ROS-scavenging systems can also accumulate in different compartments of the plant cell. This compartmentalization could contribute to the specific functions attributed to ROS. Additionally, ROS interact with other signals and phytohormones, which could explain the variety of different scenarios where ROS signaling plays an important part. Interestingly, pathogens have developed ways to alter ROS accumulation or signaling to modify plant defenses. Although ROS have been mainly associated with pathogen attack, ROS are also detected in other biotic interactions including beneficial symbiotic interactions with bacteria or mycorrhiza, suggesting that ROS production is a common feature of different biotic interactions. Here, we present a comprehensive review describing the newer views in ROS signaling and function during biotic stress.

Inhibition of HIV-1 replication by small interfering RNAs directed against glioma pathogenesis related protein (GliPR) expression

BACKGROUND

Previously, we showed that glioma pathogenesis related protein (GliPR) is induced in CEM T cells upon HIV-1 infection in vitro. To examine whether GliPR plays a role as HIV dependency factor (HDF), we tested the effect of GliPR suppression by siRNA on HIV-1 replication.

RESULTS

Induction of GliPR expression by HIV-1 was confirmed in P4-CCR5 cells. When GliPR was suppressed by siRNA, HIV-1 replication was significantly reduced as measured by HIV-1 transcript levels, HIV-1 p24 protein levels, and HIV-1 LTR-driven reporter gene expression, suggesting that GliPR is a cellular co-factor of HIV-1. Microarray analysis of uninfected HeLa cells following knockdown of GliPR revealed, among a multitude of gene expression alterations, a down-regulation of syndecan-1, syndecan-2, protein kinase C alpha (PRKCA), the catalytic subunit beta of cAMP-dependent protein kinase (PRKACB), nuclear receptor co-activator 3 (NCOA3), and cell surface protein CD59 (protectin), all genes having relevance for HIV-1 pathology.

CONCLUSIONS

The up-regulation of GliPR by HIV-1 and the early significant inhibition of HIV-1 replication mediated by knockdown of GliPR reveal GliPR as an important HIV-1 dependency factor (HDF), which may be exploited for HIV-1 inhibition.

Nitric oxide in fruit ripening: trends and opportunities

Monitoring ethylene is crucial in regulating post-harvest life of fruits. The concept of nitric oxide (NO) involvement in antagonizing ethylene is new. NO mediated physiologies casted through regulation of plant hormones are widely reported during developmental and stress chemistry having no direct link with ripening. Research in NO biology and understanding its interplay with other signal molecules in ripening fruits suggest ways of achieving greater synergies with NO applications. Experiments focused at convincingly demonstrating the involvement of NO in altering ripening-related ethylene profile of fruits, would help develop new processes for shelf life extension. This issue being the central theme of this review, the putative mechanisms of NO intricacies with other primary and secondary signals are hypothesized. The advantage of eliciting NO endogenously may open up various biotechnological opportunities for its precise delivery into the target tissues.

Transcriptome analysis of Escherichia coli O157:H7 exposed to lysates of lettuce leaves

Harvesting and processing of leafy greens inherently cause plant tissue damage, creating niches on leaves that human pathogens can exploit. We previously demonstrated that Escherichia coli O157:H7 (EcO157) multiplies more rapidly on shredded leaves than on intact leaves (M. T. Brandl, Appl. Environ. Microbiol. 74:5285-5289, 2008). To investigate how EcO157 cells adapt to physicochemical conditions in injured lettuce tissue, we used microarray-based whole-genome transcriptional profiling to characterize gene expression patterns in EcO157 after 15- and 30-min exposures to romaine lettuce lysates. Multiple carbohydrate transport systems that have a role in the utilization of substrates known to be prevalent in plant cells were activated in EcO157. This indicates the availability to the human pathogen of a variety of carbohydrates released from injured plant cells that may promote its extensive growth in leaf lysates and, thus, in wounded leaf tissue. In addition, microarray analysis revealed the upregulation of numerous genes associated with EcO157 attachment and virulence, with oxidative stress and antimicrobial resistance (including the OxyR and Mar regulons), with detoxification of noxious compounds, and with DNA repair. Upregulation of oxidative stress and antimicrobial resistance genes in EcO157 was confirmed on shredded lettuce by quantitative reverse transcription-PCR. We further demonstrate that this adaptation to stress conditions imparts the pathogen with increased resistance to hydrogen peroxide and calcium hypochlorite. This enhanced resistance to chlorinated sanitizers combined with increased expression of virulence determinants and multiplication at sites of injury on the leaves may help explain the association of processed leafy greens with outbreaks of EcO157.

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.

Effect of nitrate supply and mycorrhizal inoculation on characteristics of tobacco root plasma membrane vesicles

Plant plasma membrane (pm) vesicles from mycorrhizal tobacco (Nicotiana tabacum cv. Samsun) roots were isolated with negligible fungal contamination by the aqueous two-phase partitioning technique as proven by fatty acid analysis. Palmitvaccenic acid became apparent as an appropriate indicator for fungal membranes in root pm preparations. The pm vesicles had a low specific activity of the vanadate-sensitive ATPase and probably originated from non-infected root cells. In a phosphate-limited tobacco culture system, root colonisation by the vesicular arbuscular mycorrhizal fungus, Glomus mosseae, is inhibited by external nitrate in a dose-dependent way. However, detrimental high concentrations of 25 mM nitrate lead to the highest colonisation rate observed, indicating that the defence system of the plant is impaired. Nitric oxide formation by the pm-bound nitrite:NO reductase increased in parallel with external nitrate supply in mycorrhizal roots in comparison to the control plants, but decreased under excess nitrate. Mycorrhizal pm vesicles had roughly a twofold higher specific activity as the non-infected control plants when supplied with 10-15 mM nitrate.

Systemic induction and role of mitochondrial alternative oxidase and nitric oxide in a compatible tomato-Tobacco mosaic virus interaction

The role of mitochondrial alternative oxidase (AOX) and the relationship between AOX and nitric oxide (NO) in virus-induced systemic defense to Tobacco mosaic virus (TMV) were investigated in susceptible tomato (Solanum lycopersicum) plants. TMV inoculation to the lower leaves induced a rapid NO synthesis and AOX activation in upper uninoculated leaves as early as 0.5 day postinoculation. Application of exogenous potassium cyanide (KCN, a cytochrome pathway inhibitor) at nonlethal concentrations and NO donor diethylamine NONOate (DEA/NO) to the upper uninoculated leaves greatly induced accumulation of AOX transcript, reduced TMV viral RNA accumulation, and increased the leaf photochemical quantum yield at photosystem II. Pretreatment with NO scavenger almost completely blocked TMV-induced AOX induction and substantially increased TMV susceptibility. Salicylhydroxamic acid (SHAM, an AOX inhibitor) pretreatment reduced the DEA/NO-induced cyanide-resistant respiration and partially compromised induced resistance to TMV. Conversely, KCN and SHAM pretreatment had very little effect on generation of NO, and pretreatment with NO scavenger did not affect KCN-induced AOX induction and TMV resistance. These results suggest that TMV-induced NO generation acts upstream and mediates AOX induction which, in turn, induces mitochondrial alternative electron transport and triggers systemic basal defense against the viral pathogen.

Structural studies of soybean calmodulin isoform 4 bound to the calmodulin-binding domain of tobacco mitogen-activated protein kinase phosphatase-1 provide insights into a sequential target binding mode

The calcium regulatory protein calmodulin (CaM) binds in a calcium-dependent manner to numerous target proteins. The calmodulin-binding domain (CaMBD) region of Nicotiana tabacum MAPK phosphatase has an amino acid sequence that does not resemble the CaMBD of any other known Ca(2+)-CaM-binding proteins. Using a unique fusion protein strategy, we have been able to obtain a high resolution solution structure of the complex of soybean Ca(2+)-CaM4 (SCaM4) and this CaMBD. Complete isotope labeling of both parts of the complex in the fusion protein greatly facilitated the structure determination by NMR. The 12-residue CaMBD region was found to bind exclusively to the C-lobe of SCaM4. A specific Trp and Leu side chain are utilized to facilitate strong binding through a novel "double anchor" motif. Moreover, the orientation of the helical peptide on the surface of Ca(2+)-SCaM4 is distinct from other known complexes. The N-lobe of Ca(2+)-SCaM4 in the complex remains free for additional interactions and could possibly act as a calcium-dependent adapter protein. Signaling through the MAPK pathway and increases in intracellular Ca(2+) are both hallmarks of the plant stress response, and our data support the notion that coordination of these responses may occur through the formation of a unique CaM-MAPK phosphatase multiprotein complex.

Exogenous nitric oxide enhances cadmium tolerance of rice by increasing pectin and hemicellulose contents in root cell wall

To study the mechanisms of exogenous NO contribution to alleviate the cadmium (Cd) toxicity in rice (Oryza sativa), rice plantlets subjected to 0.2-mM CdCl(2) exposure were treated with different concentrations of sodium nitroprusside (SNP, a NO donor), and Cd toxicity was evaluated by the decreases in plant length, biomass production and chlorophyll content. The results indicated that 0.1 mM SNP alleviated Cd toxicity most obviously. Atomic absorption spectrometry and fluorescence localization showed that treatment with 0.1 mM SNP decreased Cd accumulation in both cell walls and soluble fraction of leaves, although treatment with 0.1 mM SNP increased Cd accumulation in the cell wall of rice roots obviously. Treatment with 0.1 mM SNP in nutrient solution had little effect on the transpiration rate of rice leaves, but this treatment increased pectin and hemicellulose content and decreased cellulose content significantly in the cell walls of rice roots. Based on these results, we conclude that decreased distribution of Cd in the soluble fraction of leaves and roots and increased distribution of Cd in the cell walls of roots are responsible for the NO-induced increase of Cd tolerance in rice. It seems that exogenous NO enhances Cd tolerance of rice by increasing pectin and hemicellulose content in the cell wall of roots, increasing Cd accumulation in root cell wall and decreasing Cd accumulation in soluble fraction of leaves.

Proteome of plant peroxisomes: new perspectives on the role of these organelles in cell biology

Peroxisomes are cell organelles bounded by a single membrane with a basically oxidative metabolism. Peroxisomes house catalase and H(2)O(2)-producing flavin-oxidases as the main protein constituents. However, since their discovery in early fifties, a number of new enzymes and metabolic pathways have been reported to be also confined to these organelles. Thus, the presence of exo- and endo-peptidases, superoxide dismutases, the enzymes of the plant ascorbate-glutathione cycle plus ascorbate and glutathione, several NADP-dehydrogenases, and also L-arginine-dependent nitric oxide synthase activity has evidenced the relevant role of these organelles in cell physiology. In recent years, the study of new functions of peroxisomes has become a field of intensive research in cell biology, and these organelles have been proposed to be a source of important signal molecules for different transduction pathways. In plants, peroxisomes participate in seed germination, leaf senescence, fruit maturation, response to abiotic and biotic stress, photomorphogenesis, biosynthesis of the plant hormones jasmonic acid and auxin, and in cell signaling by reactive oxygen and nitrogen species (ROS and RNS, respectively). In order to decipher the nature and specific role of the peroxisomal proteins in these processes, several approaches including in vivo and in vitro import assays and generation of mutants have been used. In the last decade, the development of genomics and the report of the first plant genomes provided plant biologists a powerful tool to assign to peroxisomes those proteins which harbored any of the two peroxisomal targeting signals (PTS, either PTS1 or PTS2) described so far. Unfortunately, those molecular approaches could not give any response to those proteins previously localized in plant peroxisomes by classical biochemical and cell biology methods that did not contain any PTS. However, more recently, proteomic studies of highly purified organelles have provided evidence of the presence in peroxisomes of new proteins not previously reported. Thus, the contribution of proteomic approaches to the biology of peroxisomes is essential, not only for elucidation of the mechanisms involved in the import of the PTS1- and PTS2-independent proteins, but also to the understanding of the role of these organelles in the cell physiology of plant growth and development.

Purification of novel protein elicitor from Botrytis cinerea that induces disease resistance and drought tolerance in plants

PebC1, a novel protein elicitor was isolated and purified from the mycelium of gray mold fungus, Botrytis cinerea strain BC-4-2-2-1. The protein was eluted through HiTrap DEAE FF and RESOURCE Q anion exchange chromatography and displayed as a single band with an apparent molecular weight of 36 kDa on silver staining sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The pI of the purified protein PebC1 was determined by 2-DE and was 4.85. Three peptide segments were obtained by MALDI-TOF. Similarity, the homology matching using protein BLAST search found that two proteins, viz. XP_001593856 and XP_001551609 were having high score and covered sequence of the three peptides. Protein XP_001551609, a deduced protein nascent polypeptide-associated complex alpha-polypeptide, was more authentic because it was from Botryotinia fuckeliana that is better known as its anamorph, B.cinerea and showed 95% homology with the three polypeptides. The full cDNA sequence encoding for pebC1 (Genbank accession number FJ748868) was amplified from B. cinerea and consists of 639bp, which is same as a registered gene of XM_001551559, a nascent polypeptide-associated complex alpha-polypeptide partial mRNA. The gene encode a hypothetical protein speculated from an annotated genomic sequence from B. fuckeliana B05.10 (NW_001814507) and there is no publication about the gene. The PebC1 protein significantly promoted wheat seedling growth with an optimum protein concentration of 5 microg/mL. Root systemic activity of wheat with 4-5 leaves increased by 1.29 fold, and the wheat seedling drought resistance integrated index increased from 36.53 to 57.08 under two cycles of drought stress after treatment of PebC1. PebC1 protein at the optimum concentration of 10 microg/mL induced 69.19% disease resistance against gray mold fungus in tomato. Furthermore, phenylalanine ammonia-lyase (PAL), peroxides (POD), and polyphenol oxidase (PPO) related to plant resistance metabolism were also increased considerably after PebC1 treatment. PAL activity was increased by 46.84% at 24h post-treatment, while POD and PPO activity increased by 109.5% and 111.0% at 72 h, respectively over the control.

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.

The synthetic elicitor 3,5-dichloroanthranilic acid induces NPR1-dependent and NPR1-independent mechanisms of disease resistance in Arabidopsis

Immune responses of Arabidopsis (Arabidopsis thaliana) are at least partially mediated by coordinated transcriptional up-regulation of plant defense genes, such as the Late/sustained Up-regulation in Response to Hyaloperonospora parasitica (LURP) cluster. We found a defined region in the promoter of the LURP member CaBP22 to be important for this response. Using a CaBP22 promoter-reporter fusion, we have established a robust and specific high-throughput screening system for synthetic defense elicitors that can be used to trigger defined subsets of plant immune responses. Screening a collection of 42,000 diversity-oriented molecules, we identified 114 candidate LURP inducers. One representative, 3,5-dichloroanthranilic acid (DCA), efficiently induced defense reactions to the phytopathogens H. parasitica and Pseudomonas syringae. In contrast to known salicylic acid analogs, such as 2,6-dichloroisonicotinic acid (INA), which exhibit a long-lasting defense-inducing activity and are fully dependent on the transcriptional cofactor NPR1 (for Nonexpresser of Pathogenesis-Related genes1), DCA acts transiently and is only partially dependent on NPR1. Microarray analyses revealed a cluster of 142 DCA- and INA-responsive genes that show a pattern of differential expression coinciding with the kinetics of DCA-mediated disease resistance. These ACID genes (for Associated with Chemically Induced Defense) constitute a core gene set associated with chemically induced disease resistance, many of which appear to encode components of the natural immune system of Arabidopsis.

Cyclic nucleotide gated channels and related signaling components in plant innate immunity

Although plants lack the mobile sentry cells present in animal innate immune systems, plants have developed complex innate immune reactions triggering basal resistance and the hypersensitive response (HR). Cytosolic Ca(2+) elevation is considered to be an important early event in this pathogen response signal transduction cascade. Plasma membrane (PM)-localized cyclic nucleotide gated channels (CNGCs) contribute to the cytosolic Ca(2+) rise upon pathogen perception. Recent work suggests that some PM-localized leucine-rich-repeat receptor-like kinases (LRR-RLKs) may be involved in the perception of pathogen associated molecular pattern molecules and triggering some pathogen responses in plants, some of these LRR-RLKs might have cyclic nucleotide cyclase activity. The recognition of pathogens may be connected to cyclic nucleotide generation and the activation of CNGCs, followed by cytosolic Ca(2+) increase and downstream signaling events (possibly involving nitric oxide, reactive oxygen species (ROS), calmodulin (CaM), CaM-like protein (CML) and protein kinases). Notably, CaM or CML could be the crucial sensor downstream from the early Ca(2+) signal leading to nitric oxide (NO) production during plant innate immune responses.

Deciphering cGMP signatures and cGMP-dependent pathways in plant defence

The second messenger, 3′,5′-cyclic monophosphate (cGMP), is a critical component of many different processes in plants while guanylyl cyclases that catalyse the formation of cGMP from GTP have remained somewhat elusive in higher plants. Consequently, two major aims are the discovery of novel GCs and the identification of cGMP mediated processes. Recently, we have reported temporal signatures of ozone (O3)-induced hydrogen peroxide (H2O2) and nitric oxide (NO) generation, their effect on cGMP generation, and consequent transcriptional changes of genes diagnostic for stress responses in tobacco. We demonstrated that O3 and NO induced early transcriptional activation of the scavenger encoding proteins, alternative oxidase (AOX1a ), glutathione peroxidase (GPX ) and the induction of ethylene production through aminocyclopropancarboxylic acid synthase (ACS2 ) are cGMP-independent. By contrast, the early response of the phenylalanine ammonia lyase gene (PALa ) and the late response of the gene encoding the pathogenesis-related protein (PR1a ) show critical dependence on cGMP. Here we show differential cGMP responses to virulent and avirulent Pseudomonas syringae strains and propose that host-pathogen recognition and/or down-stream processes are transduced by complex cGMP signatures. This is in accordance with the identification of a growing number of multi-domain molecules in Arabidopsis that are reported to contain putative functional GC catalytic centers.

Role of geranylgeranyl reductase gene in organ development and stress response in olive (Olea europaea) plants

The NADPH-dependent geranylgeranyl reductase gene (OeCHLP) was characterised in olive (Olea europaea L.). OeCHLP catalyses the formation of carbon double bonds in the phytolic side chain of chlorophyll, tocopherols and plastoquinones and, therefore, is involved in metabolic pathways related to plant productivity and stress response, besides to nutritional value of its products. The nuclear OeCHLP encodes a deduced product of 51 kDa, which harbours a transit peptide for cytoplasm-to-chloroplast transport and a nicotinamide binding domain. Two estimated identical copies of gene are harboured per haploid genome of the cv. 'Carolea' used in the present study. Levels and cytological pattern of OeCHLP transcription were investigated by quantitative RT-PCR and in situ hybridisation. In line with the presence of ubiquitous tocopherols and/or chlorophyll, OeCHLP transcripts were present in various organs of plants. In leaves and fruits at different developmental stages, OeCHLP was differentially expressed in relation to their morpho-physiological features. An early and transient enhancement of gene transcription was detected in leaves of different age exposed to cold treatment (4°C), as well as in fruits mechanically wounded. Moreover, OeCHLP transcripts locally increased in specific cell domains of fruits severely damaged by the pathogen Bactrocera olea. Combined, these data show that OeCHLP expression early responds to biotic and abiotic stressful factors. Levels of tocopherols also increased in leaves exposed to cold conditions and fruits severely damaged by pathogen. We suggest that gene activity under stress condition could be related to tocopherol action.

A possible role of class 1 plant hemoglobin at the early stage of legume-rhizobium symbiosis

Leguminous plants form root nodules, in which symbiotic rhizobia fix atmospheric nitrogen and supply the fixation products to their host plants as a nitrogen source. On the process of establishing the symbiosis, rhizobia induce genes involved in the defense system of their host plants. However, the host defense responses will be cancelled by unknown mechanism. We focused on nitric oxide (NO) as a key molecule of plant defense system and class 1 plant hemoglobin (Hb) as a scavenger of NO. The inoculation of a symbiotic rhizobium, Mesorhizobium loti MAFF303099, induced transiently NO production and expression of a class 1 Hb gene LjHb1 in the roots of a model legume Lotus japonicus. In this addendum, we show that the lipopolysaccharide of M. loti induces NO production and expression of LjHb1 in L. japonicus, and we propose the role of NO and Hb at the early stage of symbiosis.

A horizontal gene transfer at the origin of phenylpropanoid metabolism: a key adaptation of plants to land

BACKGROUND

The pioneering ancestor of land plants that conquered terrestrial habitats around 500 million years ago had to face dramatic stresses including UV radiation, desiccation, and microbial attack. This drove a number of adaptations, among which the emergence of the phenylpropanoid pathway was crucial, leading to essential compounds such as flavonoids and lignin. However, the origin of this specific land plant secondary metabolism has not been clarified.

RESULTS

We have performed an extensive analysis of the taxonomic distribution and phylogeny of Phenylalanine Ammonia Lyase (PAL), which catalyses the first and essential step of the general phenylpropanoid pathway, leading from phenylalanine to p-Coumaric acid and p-Coumaroyl-CoA, the entry points of the flavonoids and lignin routes. We obtained robust evidence that the ancestor of land plants acquired a PAL via horizontal gene transfer (HGT) during symbioses with soil bacteria and fungi that are known to have established very early during the first steps of land colonization. This horizontally acquired PAL represented then the basis for further development of the phenylpropanoid pathway and plant radiation on terrestrial environments.

CONCLUSION

Our results highlight a possible crucial role of HGT from soil bacteria in the path leading to land colonization by plants and their subsequent evolution. The few functional characterizations of sediment/soil bacterial PAL (production of secondary metabolites with powerful antimicrobial activity or production of pigments) suggest that the initial advantage of this horizontally acquired PAL in the ancestor of land plants might have been either defense against an already developed microbial community and/or protection against UV.

Activation of a nuclear-localized SIPK in tobacco cells challenged by cryptogein, an elicitor of plant defence reactions

When a plant cell is challenged by a well-defined stimulus, complex signal transduction pathways are activated to promote the modulation of specific sets of genes and eventually to develop adaptive responses. In this context, protein phosphorylation plays a fundamental role through the activation of multiple protein kinase families. Although the involvement of protein kinases at the plasma membrane and cytosolic levels are now well-documented, their nuclear counterparts are still poorly investigated. In the field of plant defence reactions, no known study has yet reported the activation of a nuclear protein kinase and/or its nuclear activity in plant cells, although some protein kinases, e.g. MAPK (mitogen-activated protein kinase), are known to be translocated into the nucleus. In the present study, we investigated the ability of cryptogein, a proteinaceous elicitor of tobacco defence reactions, to induce different nuclear protein kinase activities. We found that at least four nuclear protein kinases are activated in response to cryptogein treatment in a time-dependent manner, some of them exhibiting Ca(2+)-dependent activity. The present study focused on one 47 kDa protein kinase with a Ca(2+)-independent activity, closely related to the MAPK family. After purification and microsequencing, this protein kinase was formally identified as SIPK (salicyclic acid-induced protein kinase), a biotic and abiotic stress-activated MAPK of tobacco. We also showed that cytosolic activation of SIPK is not sufficient to promote a nuclear SIPK activity, the latter being correlated with cell death. In that way, the present study provides evidence of a functional nuclear MAPK activity involved in response to an elicitor treatment.

FORCA, a promoter element that responds to crosstalk between defense and light signaling

BACKGROUND

Recognition of pathogenic microorganisms triggers in plants comprehensive transcriptional reprogramming. In order to identify transcriptome-level control elements required for plant immune responses we are examining several sets of genes found by microarray experiments to be co-activated in Arabidopsis thaliana (Arabidopsis) seedlings infected with the oomycete Hyaloperonospora parasitica. Promoter motifs conserved in clusters of co-expressed genes may be involved in mediating coordinated gene activity patterns. Although numerous studies identified such conserved promoter motifs in co-expressed gene sets, reports confirming their function as regulatory elements are rare.

RESULTS

FORCA is a hexameric promoter motif that is conserved in clusters of Arabidopsis genes co-expressed in response to fungal or oomycete pathogens as well as defined light treatments. FORCA is generally more frequently present in Arabidopsis promoter regions than statistically expected. It constitutively interacts in a DNA-sequence specific manner with nuclear Arabidopsis proteins. These interactions are suppressed by defense-related stimuli and enhanced by prolonged exposure to constant light. Furthermore FORCA mediates constitutive reporter gene expression in transiently transformed Nicotiana benthamiana leaves as well as in stably transformed Arabidopsis plants. Its responsiveness to defense-stimuli is modulated by the duration of light exposure. In plants grown under normal light conditions or constant darkness defense-related stimuli result in suppression of FORCA-mediated reporter gene expression, while in plants grown under constant light exposure, defense-induction results in enhanced FORCA-mediated expression. In addition, we found plants subjected to constant light exposure to exhibit reduced susceptibility to virulent H. parasitica.

CONCLUSION

We propose that FORCA is a regulatory cis-element that is present in a wide variety of Arabidopsis promoters. It integrates light- and defense-related signals and participates in adjusting the transcriptome to changes in environmental conditions.

Nitric oxide and oxygen regulate truncated hemoglobin gene expression in Frankia strain CcI3

The Frankia genome contains two truncated hemoglobin genes (hboN and hboO) whose functions remain to be determined. Nitric oxide (NO) generated by the addition of 400 microM SNAP (S-nitroso-N-acetylpenicillamine) caused a 10-fold increase in hboN gene expression but had no effect on hboO expression. The addition of the NO scavenger, carboxy-PT10, reduced the effect of SNAP. hboO gene expression increased under low-oxygen conditions, while hboN expression was unaffected. These results suggest that HboN may function in protection from nitrosative stress and that HboO may act as an oxygen transport molecule for increased respiration in hypoxic environments.

Root-secreted malic acid recruits beneficial soil bacteria

Beneficial soil bacteria confer immunity against a wide range of foliar diseases by activating plant defenses, thereby reducing a plant's susceptibility to pathogen attack. Although bacterial signals have been identified that activate these plant defenses, plant metabolites that elicit rhizobacterial responses have not been demonstrated. Here, we provide biochemical evidence that the tricarboxylic acid cycle intermediate L-malic acid (MA) secreted from roots of Arabidopsis (Arabidopsis thaliana) selectively signals and recruits the beneficial rhizobacterium Bacillus subtilis FB17 in a dose-dependent manner. Root secretions of L-MA are induced by the foliar pathogen Pseudomonas syringae pv tomato (Pst DC3000) and elevated levels of L-MA promote binding and biofilm formation of FB17 on Arabidopsis roots. The demonstration that roots selectively secrete L-MA and effectively signal beneficial rhizobacteria establishes a regulatory role of root metabolites in recruitment of beneficial microbes, as well as underscores the breadth and sophistication of plant-microbial interactions.

Root-secreted malic acid recruits beneficial soil bacteria

Beneficial soil bacteria confer immunity against a wide range of foliar diseases by activating plant defenses, thereby reducing a plant's susceptibility to pathogen attack. Although bacterial signals have been identified that activate these plant defenses, plant metabolites that elicit rhizobacterial responses have not been demonstrated. Here, we provide biochemical evidence that the tricarboxylic acid cycle intermediate L-malic acid (MA) secreted from roots of Arabidopsis (Arabidopsis thaliana) selectively signals and recruits the beneficial rhizobacterium Bacillus subtilis FB17 in a dose-dependent manner. Root secretions of L-MA are induced by the foliar pathogen Pseudomonas syringae pv tomato (Pst DC3000) and elevated levels of L-MA promote binding and biofilm formation of FB17 on Arabidopsis roots. The demonstration that roots selectively secrete L-MA and effectively signal beneficial rhizobacteria establishes a regulatory role of root metabolites in recruitment of beneficial microbes, as well as underscores the breadth and sophistication of plant-microbial interactions.

Stimulus-induced downregulation of root water transport involves reactive oxygen species-activated cell signalling and plasma membrane intrinsic protein internalization

The water uptake capacity of plant roots (i.e. their hydraulic conductivity, Lp(r)) is determined in large part by aquaporins of the plasma membrane intrinsic protein (PIP) subfamily. In the present work, we investigated two stimuli, salicylic acid (SA) and salt, because of their ability to induce an accumulation of reactive oxygen species (ROS) and an inhibition of Lp(r) concomitantly in the roots of Arabidopsis plants. The inhibition of Lp(r) by SA was partially counteracted by preventing the accumulation of hydrogen peroxide (H(2)O(2)) with exogenous catalase. In addition, exogenous H(2)O(2) was able to reduce Lp(r) by up to 90% in <15 min. Based on the lack of effects of H(2)O(2) on the activity of individual aquaporins in Xenopus oocytes, and on a pharmacological dissection of the action of H(2)O(2) on Lp(r), we propose that ROS do not gate Arabidopsis root aquaporins through a direct oxidative mechanism, but rather act through cell signalling mechanisms. Expression in transgenic roots of PIP-GFP fusions and immunogold labelling indicated that external H(2)O(2) enhanced, in <15 min, the accumulation of PIPs in intracellular structures tentatively identified as vesicles and small vacuoles. Exposure of roots to SA or salt also induced an intracellular accumulation of the PIP-GFP fusion proteins, and these effects were fully counteracted by co-treatment with exogenous catalase. In conclusion, the present work identifies SA as a novel regulator of aquaporins, and delineates an ROS-dependent signalling pathway in the roots of Arabidopsis. Several abiotic and biotic stress-related stimuli potentially share this path, which involves an H(2)O(2)-induced internalization of PIPs, to downregulate root water transport.

The oomycete response gene LURP1 is required for defense against Hyaloperonospora parasitica in Arabidopsis thaliana

LURP1 is a member of the LURP cluster and the PR1 regulon, two previously defined sets of co-expressed Arabidopsis thaliana genes that share a pronounced upregulation in response to infections by the pathogenic oomycete Hyaloperonospora parasitica. LURP1 shows the most extreme transcriptional inducibility by H. parasitica of all LURP and PR1 regulon genes. Using insertion mutants we found that LURP1 is required for full basal defense to H. parasitica and resistance to this pathogen mediated by the R-proteins RPP4 and RPP5. The LURP1 protein shows no obvious similarity to proteins of known molecular function. We identified a 39-bp region of the LURP1 promoter that mediates reporter gene expression in response to H. parasitica and salicylic acid. This promoter region contains a W box motif, W(LURP1), that interacts in vitro with nuclear factors producing two separate DNA-binding patterns. W(LURP1)-related sequences are strongly enriched in the promoters of the PR1 regulon, suggesting a role for this motif in the coordinated expression of genes inducible by H. parasitica and related defense conditions.

Transcriptome profiling identified novel genes associated with aluminum toxicity, resistance and tolerance in Medicago truncatula

Oligonucleotide microarrays corresponding to over 16,000 genes were used to analyze changes in transcript accumulation in root tips of the Al-sensitive Medicago truncatula cultivar Jemalong genotype A17 in response to Al treatment. Out of 2,782 genes with significant changes in transcript accumulation, 324 genes were up-regulated and 267 genes were down-regulated at least twofold by Al. Up-regulated genes were enriched in transcripts involved in cell-wall modification and abiotic and biotic stress responses while down-regulated genes were enriched in transcripts involved in primary metabolism, secondary metabolism, protein synthesis and processing, and the cell cycle. Known markers of Al-induced gene expression including genes associated with oxidative stress and cell wall stiffening were differentially regulated in this study. Transcript profiling identified novel genes associated with processes involved in Al toxicity including cell wall modification, cell cycle arrest and ethylene production. Novel genes potentially associated with Al resistance and tolerance in M. truncatula including organic acid transporters, cell wall loosening enzymes, Ca(2+) homeostasis maintaining genes, and Al-binding were also identified. In addition, expression analysis of nine genes in the mature regions of the root revealed that Al-induced gene expression in these regions may play a role in Al tolerance. Finally, interfering RNA-induced silencing of two Al-induced genes, pectin acetylesterase and annexin, in A17 hairy roots slightly increased the sensitivity of A17 to Al suggesting that these genes may play a role in Al resistance.

The solution structures of two soybean calmodulin isoforms provide a structural basis for their selective target activation properties

The intracellular calcium ion is one of the most important secondary messengers in eukaryotic cells. Ca(2+) signals are translated into physiological responses by EF-hand calcium-binding proteins such as calmodulin (CaM). Multiple CaM isoforms occur in plant cells, whereas only a single CaM protein is found in animals. Soybean CaM isoform 1 (sCaM1) shares 90% amino acid sequence identity with animal CaM (aCaM), whereas sCaM4 is only 78% identical. These two sCaM isoforms have distinct target-enzyme activation properties and physiological functions. sCaM4 is highly expressed during the self-defense reaction of the plant and activates the enzyme nitric-oxide synthase (NOS), whereas sCaM1 is incapable of activating NOS. The mechanism of selective target activation by plant CaM isoforms is poorly understood. We have determined high resolution NMR solution structures of Ca(2+)-sCaM1 and -sCaM4. These were compared with previously determined Ca(2+)-aCaM structures. For the N-lobe of the protein, the solution structures of Ca(2+)-sCaM1, -sCaM4, and -aCaM all closely resemble each other. However, despite the high sequence identity with aCaM, the C-lobe of Ca(2+)-sCaM1 has a more open conformation and consequently a larger hydrophobic target-protein binding pocket than Ca(2+)-aCaM or -sCaM4, the presence of which was further confirmed through biophysical measurements. The single Val-144 --> Met substitution in the C-lobe of Ca(2+)-sCaM1, which restores its ability to activate NOS, alters the structure of the C-lobe to a more closed conformation resembling Ca(2+)-aCaM and -sCaM4. The relationships between the structural differences in the two Ca(2+)-sCaM isoforms and their selective target activation properties are discussed.

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.

Measuring NO production by plant tissues and suspension cultured cells

We describe an inexpensive and reliable detector for measuring NO emitted in the gas phase from plants. The method relies on the use of a strong oxidizer to convert NO to NO2 and subsequent capture of NO2 by a Griess reagent trap. The set-up approaches the sensitivity for NO comparable to that of instruments based on chemiluminescence and photoacoustic detectors. We demonstrate the utility of our set-up by measuring NO produced by a variety of well established plant sources. NO produced by nitrate reductase (NR) in tobacco leaves and barley aleurone was readily detected, as was the production of NO from nitrite by the incubation medium of barley aleurone. Arabidopsis mutants that overproduce NO or lack NO-synthase (AtNOS1) also displayed the expected NO synthesis phenotype when assayed by our set-up. We could also measure NO production from elicitor-treated suspension cultured cells using this set-up. Further, we have focused on the detection of NO by a widely used fluorescent probe 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM). Our work points to the pitfalls that must be avoided when using DAF-FM to detect the production of NO by plant tissues. In addition to the dramatic effects that pH can have on fluorescence from DAF-FM, the widely used NO scavengers 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) can produce anomalous and unexpected results. Perhaps the most serious drawback of DAF-FM is its ability to bind to dead cells and remain NO-sensitive.

Nitric oxide in plants: production and cross-talk with Ca2+ signaling

Nitric oxide (NO) is a diatomic gas that performs crucial functions in a wide array of physiological processes in animals. The past several years have revealed much about its roles in plants. It is well established that NO is synthesized from nitrite by nitrate reductase (NR) and via chemical pathways. There is increasing evidence for the occurrence of an alternative pathway in which NO production is catalysed from L-arginine by a so far non-identified enzyme. Contradictory results have been reported regarding the respective involvement of these enzymes in specific physiological conditions. Although much remains to be proved, we assume that these inconsistencies can be accounted for by the limited specificity of the pharmacological agents used to suppress NO synthesis but also by the reduced content of L-arginine as well as the inactivity of nitrate-permeable anion channels in nitrate reductase- and/or nitrate/nitrite-deficient plants. Another unresolved issue concerns the molecular mechanisms underlying NO effects in plants. Here, we provide evidence that the second messenger Ca2+, as well as protein kinases including MAPK and SnRK2, are very plausible mediators of the NO signals. These findings open new perspectives about NO-based signaling in plants.

A guanylyl cyclase-like gene is associated with Gibberella ear rot resistance in maize (Zea mays L.)

Gibberella ear rot, caused by the fungal pathogen Fusarium graminearum Schwabe, is a serious disease of maize (Zea mays L.) grown in northern climates. The infected maize grain contains toxins that are very harmful to livestock and humans. A maize gene that encodes a putative 267-amino acid guanylyl cyclase-like protein (ZmGC1) was characterized and shown to be associated with resistance to this disease. The putative ZmGC1 amino acid sequence is 53% identical and 65% similar to AtGC1, an Arabidopsis guanylyl cyclase. The Zmgc1 coding sequence is nearly identical in a Gibberella ear rot-resistant line (CO387) and a susceptible line (CG62) but several nucleotide sequence differences were observed in the UTRs and introns of the two alleles. Using a 463 bp probe derived from the CG62 allele of Zmgc1 and a recombinant inbred (RI) mapping population developed from a CG62 x CO387 cross, six Zmgc1 restriction fragment length polymorphism (RFLP) fragments (ER1_1, ER1_2, ER1_3, ER1_4, ER1_5, and ER5_1) were mapped on maize chromosomes 2, 3, 7, and 8. Markers ER1_1 and ER5_1 on chromosomes 7 and 8, respectively, were significantly associated with Gibberella ear rot resistance, each in three different environments. The amount of Zmgc1 transcript in ear tissues increased more quickly and to a greater extent in the resistant genotype compared to the susceptible genotype after inoculation with F. graminearum. Zmgc1 is the first guanylyl cyclase gene characterized in maize and the first gene found to be associated with Gibberella ear rot resistance in this plant.