Oligogalacturonides: novel signaling molecules in Rhizobium-legume communications

Pectate lyase-like lubricates the male gametophyte's path toward its mating partner

The pollen tube is an extension of the male gametophyte in plants and mediates sexual reproduction by delivering the sperm cells to the female gametophyte. To accomplish this task, the elongating pollen tube must break through the thick wall of the pollen grain and penetrate multiple pistillar tissues. Both processes require the loosening of cell wall material-that of the pollen intine and that of the apoplast of the transmitting tract. The enzymatic toolbox for these cell wall modifying processes employed by the invading male gametophyte is elusive. We investigated the role of the pectin-digesting pectate lyase-like (PLL) by combining mutant analysis with microscopy observations, fluorescence recovery after photo-bleaching experiments, and immuno-detection. We show that in Arabidopsis (Arabidopsis thaliana), PLLs are required for intine loosening during the first steps of pollen tube germination. We provide evidence that during pollen tube elongation, PLLs are released by the pollen tube into the extracellular space, suggesting that they may be employed to soften the apoplast of the transmitting tissue. The synergistic enzymatic action of PLLs in the pollen grain, the pollen tube, and the transmitting track contribute to an effective fertilization process.

Transcriptome Dynamics Underlying Planticine®-Induced Defense Responses of Tomato (Solanum lycopersicum L.) to Biotic Stresses

The induction of natural defense mechanisms in plants is considered to be one of the most important strategies used in integrated pest management (IPM). Plant immune inducers could reduce the use of chemicals for plant protection and their harmful impacts on the environment. Planticine® is a natural plant defense biostimulant based on oligomers of α(1→4)-linked D-galacturonic acids, which are biodegradable and nontoxic. The aim of this study was to define the molecular basis of Planticine’s biological activity and the efficacy of its use as a natural plant resistance inducer in greenhouse conditions. Three independent experiments with foliar application of Planticine® were carried out. The first experiment in a climatic chamber (control environment, no pest pressure) subjected the leaves to RNA-seq analysis, and the second and third experiments in greenhouse conditions focused on efficacy after a pest infestation. The result was the RNA sequencing of six transcriptome libraries of tomatoes treated with Planticine® and untreated plants; a total of 3089 genes were found to be differentially expressed genes (DEGs); among them, 1760 and 1329 were up-regulated and down-regulated, respectively. DEG analysis indicated its involvement in such metabolic pathways and processes as plant-pathogen interaction, plant hormone signal transduction, MAPK signaling pathway, photosynthesis, and regulation of transcription. We detected up-regulated gene-encoded elicitor and effector recognition receptors (ELRR and ERR), mitogen-activated protein kinase (MAPKs) genes, and transcription factors (TFs), i.e., WRKY, ERF, MYB, NAC, bZIP, pathogenesis-related proteins (PRPs), and resistance-related metabolite (RRMs) genes. In the greenhouse trials, foliar application of Planticine® proved to be effective in reducing the infestation of tomato leaves by the biotrophic pathogen powdery mildew and in reducing feeding by thrips, which are insect herbivores. Prophylactic and intervention use of Planticine® at low infestation levels allows the activation of plant defense mechanisms.

Effect of Oligogalacturonides on Seed Germination and Disease Resistance of Sugar Beet Seedling and Root

Oligogalacturonides (OGs) are a bioactive carbohydrate derived from homogalacturonan. The OGs synthesized in this study significantly inhibited the mycelial growth of Rhizoctonia solani AG-4HGI in vitro, even at a low concentration (10 mg/L). The seed vigor test demonstrated that the application of 50 mg/L OGs to sugar beet seeds significantly increased average germination percentage, germination energy, germination index, and seedling vigor index. The same concentration of OGs also improved the seedling emergence percentage of sugar beet when seeds were sown in soil inoculated with D2 and D31 isolates, respectively. The lesion diameter on mature sugar beet roots caused by R. solani AG-4HGI isolates D2 and D31 also decreased by 40.60% and 39.86%, respectively, in sugar beets roots first treated with 50 mg/mL OGs in the wound site, relative to lesion size in untreated/pathogen inoculated wounds. Sugar beet roots treated with 50 mg/mL OGs prior to inoculation with the D2 isolate exhibited up-regulation of the defense-related genes glutathione peroxidase (GPX) and superoxide dismutase (SOD) by 2.4- and 1.6-fold, respectively, relative to control roots. Sugar beet roots treated with 50 mg/mL OGs prior to inoculation with D31 exhibited a 2.0- and 1.6-fold up-regulation of GPX and SOD, respectively, relative to the control. Our results indicate that OGs have the potential to be used for the protection of sugar beet against R. solani AG-4HGI.

Plant-Microbe Symbiosis: What Has Proteomics Taught Us?

Beneficial microbes have a positive impact on the productivity and fitness of the host plant. A better understanding of the biological impacts and underlying mechanisms by which the host derives these benefits will help to address concerns around global food production and security. The recent development of omics-based technologies has broadened our understanding of the molecular aspects of beneficial plant-microbe symbiosis. Specifically, proteomics has led to the identification and characterization of several novel symbiosis-specific and symbiosis-related proteins and post-translational modifications that play a critical role in mediating symbiotic plant-microbe interactions and have helped assess the underlying molecular aspects of the symbiotic relationship. Integration of proteomic data with other "omics" data can provide valuable information to assess hypotheses regarding the underlying mechanism of symbiosis and help define the factors affecting the outcome of symbiosis. Herein, an update is provided on the current and potential applications of symbiosis-based "omic" approaches to dissect different aspects of symbiotic plant interactions. The application of proteomics, metaproteomics, and secretomics as enabling approaches for the functional analysis of plant-associated microbial communities is also discussed.

Optimization of cell suspension culture of transformed and untransformed lettuce for the enhanced production of secondary metabolites and their pharmaceutical evaluation

In vitro suspension culture techniques are cost effective for large-scale production of secondary metabolites. In the present study, firstly, suspension cultures of untransformed Lactuca sativa were prepared using different hormonal combinations and were subjected to different pH, temperature and salt concentrations. Maximum biomass was obtained for suspensions supplemented with 1.5 mg/L BAP and 0.1 mg/L NAA, at pH 5.8, temperature 28 °C and 0 mM NaCl concentration. Using these parameters, suspensions were produced for rol ABC- and rol C-transformed lines of L. sativa. All the transgenic lines showed prominent increase in fresh weight (FW) and dry weight (DW) with maximum values for rol ABC2 line producing 169.8 mg/mL FW and 25.3 mg/mL DW. The exudates of transformed and untransformed plants were tested for the antioxidant activity and in vivo assays on rats. Maximum phenolic content (261 μg/mL) and flavonoid content (637.6 μg/mL) were obtained for rol C1 transgenic line. Total antioxidant capacity was found maximum (1451.7 μg/mL) for untransformed lettuce, whereas rol C1 showed maximum total reducing power activity (637.6 μg/mL). In DPPH assay, maximum activity (104.7 μg/mL) was shown by rol ABC3 line. In rats analgesic assay, maximum activity (74.9%) was shown by rol C2. Line rol C1 showed maximum anti-inflammatory activity (69.2%) and maximum antidepressant activity (minimum immobility time of 55 s). Maximum anticoagulant activity was observed for rol ABC2 with maximum clotting time of 130 s. The present study could help in using lettuce suspension culture as platform for the enhanced production of important metabolites.

Jasmonic acid and ethylene are involved in the accumulation of osmotin in germinating tomato seeds

The expression of SlNP24 encoding osmotin was studied in germinating tomato seeds Solanum lycopersicum L. cv. Moneymaker. The results show that the accumulation of the transcripts of SlNP24 and its potential upstream regulator TERF1 encoding an ethylene response factor was induced by ethylene and methyl jasmonate in germinating tomato seeds. There was no effect of gibberellins on the expression of the genes studied. The expression of SlNP24 was localized in the micropylar region of the endosperm of tomato seeds. The promoter of tomato osmotin was active in the endosperm cells of transgenic Arabidopsis thaliana seeds, which contain reporter genes under control of SlNP24 promoter. The activity of SlNP24 promoter in A. thaliana reporter line seeds was visible when the expression of its ortholog gene in A. thaliana (AtOMS34) was observed. The mechanism of induction and a possible role of NP24 in germinating tomato seeds are discussed.

The Role of OmpR in the Expression of Genes of the KdgR Regulon Involved in the Uptake and Depolymerization of Oligogalacturonides in Yersinia enterocolitica

Oligogalacturonide (OGA)-specific porins of the KdgM family have previously been identified and characterized in enterobacterial plant pathogens. We found that deletion of the gene encoding response regulator OmpR causes the porin KdgM2 to become one of the most abundant proteins in the outer membrane of the human enteropathogen Yersinia enterocolitica. Reporter gene fusion and real-time PCR analysis confirmed that the expression of kdgM2 is repressed by OmpR. We also found that kdgM2 expression is subject to negative regulation by KdgR, a specific repressor of genes involved in the uptake and metabolism of pectin derivatives in plant pathogens. The additive effect of kdgR and ompR mutations suggested that KdgR and OmpR regulate kdgM2 expression independently. We confirmed that kdgM2 occurs in an operon with the pelP gene, encoding the periplasmic pectate lyase PelP. A pectinolytic assay showed strong upregulation of PelP production/activity in a Y. enterocolitica strain lacking OmpR and KdgR, which corroborates the repression exerted by these regulators on kdgM2. In addition, our data showed that OmpR is responsible for up regulation of the kdgM1 gene encoding the second specific oligogalacturonide porin KdgM1. This indicates the involvement of OmpR in the reciprocal regulation of both KdgM1 and KdgM2. Moreover, we demonstrated the negative impact of OmpR on kdgR transcription, which might positively affect the expression of genes of the KdgR regulon. Binding of OmpR to the promoter regions of the kdgM2-pelP-sghX operon, and kdgM1 and kdgR genes was confirmed using the electrophoretic mobility shift assay, suggesting that OmpR can directly regulate their transcription. We also found that the overexpression of porin KdgM2 increases outer membrane permeability. Thus, OmpR-mediated regulation of the KdgM porins may contribute to the fitness of Y. enterocolitica in particular local environments.

Dissecting stimulus-specific Ca2+ signals in amyloplasts and chloroplasts of Arabidopsis thaliana cell suspension cultures

Calcium is used by plants as an intracellular messenger in the detection of and response to a plethora of environmental stimuli and contributes to a fine-tuned internal regulation. Interest in the role of different subcellular compartments in Ca(2+) homeostasis and signalling has been growing in recent years. This work has evaluated the potential participation of non-green plastids and chloroplasts in the plant Ca(2+) signalling network using heterotrophic and autotrophic cell suspension cultures from Arabidopsis thaliana plant lines stably expressing the bioluminescent Ca(2+) reporter aequorin targeted to the plastid stroma. Our results indicate that both amyloplasts and chloroplasts are involved in transient Ca(2+) increases in the plastid stroma induced by several environmental stimuli, suggesting that these two functional types of plastids are endowed with similar mechanisms for handling Ca(2+) A comparison of the Ca(2+) trace kinetics recorded in parallel in the plastid stroma, the surface of the outer membrane of the plastid envelope, and the cytosol indicated that plastids play an essential role in switching off different cytosolic Ca(2+) signals. Interestingly, a transient stromal Ca(2+) signal in response to the light-to-dark transition was observed in chloroplasts, but not amyloplasts. Moreover, significant differences in the amplitude of specific plastidial Ca(2+) changes emerged when the photosynthetic metabolism of chloroplasts was reactivated by light. In summary, our work highlights differences between non-green plastids and chloroplasts in terms of Ca(2+) dynamics in response to environmental stimuli.

Nice to meet you: genetic, epigenetic and metabolic controls of plant perception of beneficial associative and endophytic diazotrophic bacteria in non-leguminous plants

A wide range of rhizosphere diazotrophic bacteria are able to establish beneficial associations with plants, being able to associate to root surfaces or even endophytically colonize plant tissues. In common, both associative and endophytic types of colonization can result in beneficial outcomes to the plant leading to plant growth promotion, as well as increase in tolerance against biotic and abiotic stresses. An intriguing question in such associations is how plant cell surface perceives signals from other living organisms, thus sorting pathogens from beneficial ones, to transduce this information and activate proper responses that will finally culminate in plant adaptations to optimize their growth rates. This review focuses on the recent advances in the understanding of genetic and epigenetic controls of plant-bacteria signaling and recognition during beneficial associations with associative and endophytic diazotrophic bacteria. Finally, we propose that "soil-rhizosphere-rhizoplane-endophytes-plant" could be considered as a single coordinated unit with dynamic components that integrate the plant with the environment to generate adaptive responses in plants to improve growth. The homeostasis of the whole system should recruit different levels of regulation, and recognition between the parties in a given environment might be one of the crucial factors coordinating these adaptive plant responses.

Induction of host defences by Rhizobium during ineffective nodulation of pea (Pisum sativum L.) carrying symbiotically defective mutations sym40 (PsEFD), sym33 (PsIPD3/PsCYCLOPS) and sym42

Rhizobia are able to establish a beneficial interaction with legumes by forming a new organ, called the symbiotic root nodule, which is a unique ecological niche for rhizobial nitrogen fixation. Rhizobial infection has many similarities with pathogenic infection and induction of defence responses accompanies both interactions, but defence responses are induced to a lesser extent during rhizobial infection. However, strong defence responses may result from incompatible interactions between legumes and rhizobia due to a mutation in either macro- or microsymbiont. The aim of this research was to analyse different plant defence reactions in response to Rhizobium infection for several pea (Pisum sativum) mutants that result in ineffective symbiosis. Pea mutants were examined by histochemical and immunocytochemical analyses, light, fluorescence and transmission electron microscopy and quantitative real-time PCR gene expression analysis. It was observed that mutations in pea symbiotic genes sym33 (PsIPD3/PsCYCLOPS encoding a transcriptional factor) and sym40 (PsEFD encoding a putative negative regulator of the cytokinin response) led to suberin depositions in ineffective nodules, and in the sym42 there were callose depositions in infection thread (IT) and host cell walls. The increase in deposition of unesterified pectin in IT walls was observed for mutants in the sym33 and sym42; for mutant in the sym42, unesterified pectin was also found around degrading bacteroids. In mutants in the genes sym33 and sym40, an increase in the expression level of a gene encoding peroxidase was observed. In the genes sym40 and sym42, an increase in the expression levels of genes encoding a marker of hypersensitive reaction and PR10 protein was demonstrated. Thus, a range of plant defence responses like suberisation, callose and unesterified pectin deposition as well as activation of defence genes can be triggered by different pea single mutations that cause perception of an otherwise beneficial strain of Rhizobium as a pathogen.

Identification of ferredoxin II as a major calcium binding protein in the nitrogen-fixing symbiotic bacterium Mesorhizobium loti

BACKGROUND

Legumes establish with rhizobial bacteria a nitrogen-fixing symbiosis which is of the utmost importance for both plant nutrition and a sustainable agriculture. Calcium is known to act as a key intracellular messenger in the perception of symbiotic signals by both the host plant and the microbial partner. Regulation of intracellular free Ca(2+) concentration, which is a fundamental prerequisite for any Ca(2+)-based signalling system, is accomplished by complex mechanisms including Ca(2+) binding proteins acting as Ca(2+) buffers. In this work we investigated the occurrence of Ca(2+) binding proteins in Mesorhizobium loti, the specific symbiotic partner of the model legume Lotus japonicus.

RESULTS

A soluble, low molecular weight protein was found to share several biochemical features with the eukaryotic Ca(2+)-binding proteins calsequestrin and calreticulin, such as Stains-all blue staining on SDS-PAGE, an acidic isoelectric point and a Ca(2+)-dependent shift of electrophoretic mobility. The protein was purified to homogeneity by an ammonium sulfate precipitation procedure followed by anion-exchange chromatography on DEAE-Cellulose and electroendosmotic preparative electrophoresis. The Ca(2+) binding ability of the M. loti protein was demonstrated by (45)Ca(2+)-overlay assays. ESI-Q-TOF MS/MS analyses of the peptides generated after digestion with either trypsin or endoproteinase AspN identified the rhizobial protein as ferredoxin II and confirmed the presence of Ca(2+) adducts.

CONCLUSIONS

The present data indicate that ferredoxin II is a major Ca(2+) binding protein in M. loti that may participate in Ca(2+) homeostasis and suggest an evolutionarily ancient origin for protein-based Ca(2+) regulatory systems.

Bacterial pectate lyases, structural and functional diversity

Pectate lyases are enzymes involved in plant cell wall degradation. They cleave pectin using a β-elimination mechanism, specific for acidic polysaccharides. They are mainly produced by plant pathogens and plant-associated organisms, and only rarely by animals. Pectate lyases are also commonly produced in the bacterial world, either by bacteria living in close proximity with plants or by gut bacteria that find plant material in the digestive tract of their hosts. The role of pectate lyases is essential for plant pathogens, such as Dickeya dadantii, that use a set of pectate lyases as their main virulence factor. Symbiotic bacteria produce their own pectate lyases, but they also induce plant pectate lyases to initiate the symbiosis. Pectin degradation products may act as signals affecting the plant–bacteria interactions. Bacterial pectate lyases are also essential for using the pectin of dead or living plants as a carbon source for growth. In the animal gut, Bacteroides pectate lyases degrade the pectin of ingested food, and this is particularly important for herbivores that depend on their microflora for the digestion of pectin. Some human pathogens, such as Yersinia enterocolitica, produce a few intracellular pectate lyases that can facilitate their growth in the presence of highly pectinolytic bacteria, at the plant surface, in the soil or in the animal gut.

Calcium-dependent regulation of genes for plant nodulation in Rhizobium leguminosarum detected by iTRAQ quantitative proteomic analysis

Rhizobia, the nitrogen-fixing bacterial symbionts of legumes, represent an agricultural application of primary relevance and a model of plant-microbe molecular dialogues. We recently described rhizobium proteome alterations induced by plant flavonoids using iTRAQ. Herein, we further extend that experimentation, proving that the transient elevation in cytosolic calcium is a key signaling event necessary for the expression of the nodulation (nod) genes. Ca(2+) involvement in nodulation is a novel issue that we recently flagged with genetic and physiological approaches and that hereby we demonstrate also by proteomics. Exploiting the multiple combinations of 4-plex iTRAQ, we analyzed Rhizobium leguminosarum cultures grown with or without the nod gene-inducing plant flavonoid naringenin and in the presence or absence of the extracellular Ca(2+) chelator EGTA. We quantified over a thousand proteins, 189 of which significantly altered upon naringenin and/or EGTA stimulation. The expression of NodA, highly induced by naringenin, is strongly reduced when calcium availability is limited by EGTA. This confirms, from a proteomic perspective, that a Ca(2+) influx is a necessary early step in flavonoid-mediated legume nodulation by rhizobia. We also observed other proteins affected by the different treatments, whose identities and roles in nodulation and rhizobium physiology are likewise discussed.

Pathogenicity of and plant immunity to soft rot pectobacteria

Soft rot pectobacteria are broad host range enterobacterial pathogens that cause disease on a variety of plant species including the major crop potato. Pectobacteria are aggressive necrotrophs that harbor a large arsenal of plant cell wall-degrading enzymes as their primary virulence determinants. These enzymes together with additional virulence factors are employed to macerate the host tissue and promote host cell death to provide nutrients for the pathogens. In contrast to (hemi)biotrophs such as Pseudomonas, type III secretion systems (T3SS) and T3 effectors do not appear central to pathogenesis of pectobacteria. Indeed, recent genomic analysis of several Pectobacterium species including the emerging pathogen Pectobacterium wasabiae has shown that many strains lack the entire T3SS as well as the T3 effectors. Instead, this analysis has indicated the presence of novel virulence determinants. Resistance to broad host range pectobacteria is complex and does not appear to involve single resistance genes. Instead, activation of plant innate immunity systems including both SA (salicylic acid) and JA (jasmonic acid)/ET (ethylene)-mediated defenses appears to play a central role in attenuation of Pectobacterium virulence. These defenses are triggered by detection of pathogen-associated molecular patterns (PAMPs) or recognition of modified-self such as damage-associated molecular patterns (DAMPs) and result in enhancement of basal immunity (PAMP/DAMP-triggered immunity or pattern-triggered immunity, PTI). In particular plant cell wall fragments released by the action of the degradative enzymes secreted by pectobacteria are major players in enhanced immunity toward these pathogens. Most notably bacterial pectin-degrading enzymes release oligogalacturonide (OG) fragments recognized as DAMPs activating innate immune responses. Recent progress in understanding OG recognition and signaling allows novel genetic screens for OG-insensitive mutants and will provide new insights into plant defense strategies against necrotrophs such as pectobacteria.

Quantitative analysis of the naringenin-inducible proteome in Rhizobium leguminosarum by isobaric tagging and mass spectrometry

The rhizobium-legume interaction is a critical cornerstone of crop productivity and environmental sustainability. Its potential improvement relies on elucidation of the complex molecular dialogue between its two partners. In the present study, the proteomic patterns of gnotobiotic cultures of Rhizobium leguminosarum bv. viciae 3841 grown for 6 h in presence or absence of the nod gene-inducing plant flavonoid naringenin (10 μM) were analyzed using the iTRAQ approach. A total of 1334 proteins were identified corresponding to 18.67% of the protein-coding genes annotated in the sequenced genome of bv. viciae 3841. The abundance levels of 47 proteins were increased upon naringenin treatment showing fold change ratios ranging from 1.5 to 25 in two biological replicates. Besides the nod units, naringenin enhanced the expression of a number of other genes, many of which organized in operons, including β(1-2) glucan production and secretion, succinoglycan export, the RopA outer membrane protein with homology to an oligogalacturonide-specific porin motif, other enzymes for carbohydrate and amino acid metabolism, and proteins involved in the translation machinery. Data were validated at the transcriptional and phenotypic levels by RT-PCR and an assay of secreted sugars in culture supernatants, respectively. The current approach provides not only a high-resolution analysis of the prokaryotic proteome but also unravels the rhizobium molecular dialogue with legumes by detecting the enhanced expression of several symbiosis-associated proteins, whose flavonoid-dependency had not yet been reported.