bluuebunny/crossref_metadata_2025 huggingface dataset:
Nanogram Amounts of Salicylic Acid Produced by the RhizobacteriumPseudomonas aeruginosa7NSK2 Activate the Systemic Acquired Resistance Pathway in Bean
Authors: De Meyer Geert , Capieau Kristof , Audenaert Kris , Buchala Antony , Métraux Jean-Pierre , Höfte Monica
Published: May 2007

Root colonization by specific nonpathogenic bacteria can induce a systemic resistance in plants to pathogen infections. In bean, this kind of systemic resistance can be induced by the rhizobacterium Pseudomonas aeruginosa 7NSK2 and depends on the production of salicylic acid by this strain. In a model with plants grown in perlite we demonstrated that Pseudomonas aeruginosa 7NSK2-induced resistance is equivalent to the inclusion of 1 nM salicylic acid in the nutrient solution and used the latter treatment to analyze the molecular basis of this phenomenon. Hydroponic feeding of 1 nM salicylic acid solutions induced phenylalanine ammonia-lyase activity in roots and increased free salicylic acid levels in leaves. Because pathogen-induced systemic acquired resistance involves similar changes it was concluded that 7NSK2-induced resistance is mediated by the systemic acquired resistance pathway. This conclusion was validated by analysis of phenylalanine ammonia-lyase activity in roots and of salicylic acid levels in leaves of soil-grown plants treated with Pseudomonas aeruginosa. The induction of systemic acquired resistance by nanogram amounts of salicylic acid is discussed with respect to long-distance signaling in systemic acquired resistance.
The Fungal Gene Avr9 and the Oomycete Gene inf1 Confer Avirulence to Potato Virus X on Tobacco
Authors: Kamoun Sophien , Honée Guy , Weide Rob , Laugé Richard , Kooman-Gersmann Miriam , de Groot Koen , Govers Francine , de Wit Pierre J. G. M.
Published: May 2007

The AVR9 peptide of the fungal pathogen Cladosporium fulvum and the INF1 protein of the oomycete pathogen Phytophthora infestans elicit the hypersensitive response (HR) on Cf9 tomato or Cf-9 transgenic tobacco and on all cultivars of tobacco, respectively. Expression of either the functional Avr9 or inf1 genes from engineered potato virus X (PVX) genomes resulted in localized HR lesions on tobacco plants responsive to the elicitors and inhibited spread of the recombinant virus. In contrast, PVX derivatives producing mutant forms of AVR9 and INF1 with reduced elicitor activity caused systemic necrotic and/or mosaic symptoms, and were unable to inhibit PVX spread. These results demonstrate that HR is a highly versatile defense mechanism active against unrelated pathogens irrespective of the HR-inducing agent, and that resistance to recombinant PVX in tobacco is correlated with the strength of the transgene-encoded elicitor.
Expression of the Ferrioxamine Receptor Gene of Erwinia amylovora CFBP 1430 During Pathogenesis
Authors: Dellagi Alia , Reis Danièle , Vian Brigitte , Expert Dominique
Published: May 2007

Mutants of Erwinia amylovora CFBP 1430 lacking a functional high-affinity iron transport system mediated by desferrioxamine are impaired in their ability to initiate fire blight symptoms (A. Dellagi, M.-N. Brisset, J.-P. Paulin, and D. Expert. Mol. Plant-Microbe Interact. 11:734–742, 1998). In this study, a chromosomal transcriptional lacZ fusion was used to analyze the expression in planta of the E. amylovora ferrioxamine receptor gene foxR. LacZ activity produced by the strain harboring the fusion was highly induced in iron-restricted conditions and in inoculated apple leaf tissues. Microscopic observation revealed differential expression of this gene in relation to the localization and density of bacterial cells within the diseased tissue. Thus, the ability of bacterial cells to express their iron transport system in accordance with environmental conditions is likely important for disease evolution.
During Infection of Its Host, the Plant Pathogen Ralstonia solanacearum Naturally Develops a State of Competence and Exchanges Genetic Material
Authors: Bertolla Franck , Frostegård Åsa , Brito Belen , Nesme Xavier , Simonet Pascal
Published: May 2007

In this work we investigated the ability of the plant pathogen Ralstonia solanacearum to develop a state of competence in planta and to be genetically transformed during the infection process. Tomato plants infected with R. solanacearum GMI1000 were inoculated with plasmid DNA. R. solanacearum clones expressing the marker gene were selected only during the period of time that bacteria were actively multiplying inside the plant vessels. Moreover, experiments in which R. solanacearum strains harboring different marker genes were co-inoculated into the plant demonstrated that infecting bacterial strains exchange genetic information in planta by a mechanism that is likely to be transformation. To our knowledge, these results constitute the first demonstration of soil microorganisms developing a state of competence in planta. The biotechnological implications of this result will be discussed.
Expression Map for Genes Involved in Nitrogen and Carbon Metabolism in Alfalfa Root Nodules
Authors: Trepp Gian B. , Temple Stephen J. , Bucciarelli Bruna , Shi Li Fang , Vance Carroll P.
Published: May 2007

During root nodule development several key genes involved in nitrogen fixation and assimilation exhibit enhanced levels of expression. However, little is known about the temporal and spatial distribution patterns of these transcripts. In a systematic study the transcripts for 13 of the essential enzymes involved in alfalfa (Medicago sativa) root nodule nitrogen and carbon metabolism were localized by in situ hybridization. A serial section approach allowed the construction of a map that reflects the relative distribution of these transcripts. In 33-day-old root nodules, the expression of nifH, NADH-dependent glutamate synthase (NADH-GOGAT; EC 1.4.1.14) and a cytosolic isoform of glutamine synthetase (GS13; GS; EC 6.3.1.2) were localized predominantly in a 5- to 15-cell-wide region in the distal part of the nitrogen-fixing zone. This zone was also the region of high expression for leghemoglobin, a second cytosolic glutamine synthetase isoform (GS100), aspartate aminotransferase-2 (AAT-2; EC 2.6.1.1), asparagine synthetase (AS; 6.3.5.4), phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31), and sucrose synthase (SuSy; EC 2.4.1.13). This suggests that, in 33-day-old alfalfa root nodules, nitrogen fixation is restricted to this 5- to 15-cell-wide area. The continued significant expression of the GS100 subclass of GS and AS in the proximal part of the nitrogen-fixing zone implicates these gene products in nitrogen remobilization. A low constitutive expression of NADH-dependent glutamate dehydrogenase (NADH-GDH; EC 1.4.1.2) was observed throughout the nodule. The transcript distribution map will be used as a navigational tool to assist in developing strategies for the genetic engineering of alfalfa root nodules for enhanced nitrogen assimilation.
The 25-kDa Movement Protein of PVX Elicits Nb-Mediated Hypersensitive Cell Death in Potato
Authors: Malcuit Isabelle , Marano María Rosa , Kavanagh Tony A. , De Jong Walter , Forsyth Alec , Baulcombe David C.
Published: May 2007

The potato gene Nb confers hypersensitive resistance to potato virus X (PVX). To characterize the viral elicitor of this resistance we introduced modifications into the genome of avirulent strains (ROTH1 and CP2) and virulent strains (UK3 and CP4) of PVX. From the analysis of the modified viral genomes, the Nb avirulence determinant was mapped in the PVX 25K gene coding for the 25-kDa movement protein. Furthermore, we showed that the isoleucine residue at position 6 of this protein was required for activation of the Nb response. Transient co-expression of the avirulent 25K gene with the β-glucuronidase (GUS) reporter gene introduced by particle bombardment in resistant and susceptible potato cells confirmed that the elicitor activity provided by the 25-kDa protein did not require other PVX-encoded proteins. To study cellular events associated with the Nb response, the 25-kDa proteins of PVX strains ROTH1 and UK3 were tagged with the green fluorescent protein (GFP) so that the dynamics of the subcellular distribution of the 25KGFP fusion proteins could be followed in living potato epidermal cells by laser scanning confocal microscopy. Using this method, we showed that the Nb-mediated response is associated with degradation of subcellular structures.
Isolation of a cDNA Encoding a β-1,4-endoglucanase in the Root-Knot Nematode Meloidogyne incognita and Expression Analysis During Plant Parasitism
Authors: Rosso Marie-Noëlle , Favery Bruno , Piotte Christine , Arthaud Laury , De Boer Jan M. , Hussey Richard S. , Bakker Jaap , Baum Thomas J. , Abad Pierre
Published: May 2007

A β-1,4-endoglucanase encoding cDNA (EGases, E.C. 3.2.1.4), named Mi-eng-1, was cloned from Meloidogyne incognita second-stage juveniles (J2). The deduced amino acid sequence contains a catalytic domain and a cellulose-binding domain separated by a linker. In M. incognita, the gene is transcribed in the migratory J2, in males, and in the sedentary adult females. In pre-parasitic J2, endoglucanase transcripts are located in the cytoplasm of the subventral esophageal glands. The presence of β-1,4-endoglucanase transcripts in adult females could be related to the expression of the gene in esophageal glands at this stage. However, cellulase activity within the egg matrix of adult females suggests that the endoglucanase may also be synthesized in the rectal glands and involved in the extrusion of the eggs onto the root surface. The maximum identity of the predicted MI-ENG-1 catalytic domain with the recently cloned cyst nematode β-1,4-endoglucanases is 52.5%. In contrast to cyst nematodes, M. incognita pre-parasitic J2 were not found to express a β-1,4-endoglucanase devoid of a cellulose-binding domain.
Activation of the Promoter of the Tnt1 Retrotransposon in Tomato After Inoculation with the Fungal Pathogen Cladosporium fulvum
Authors: Mhiri Corinne , De Wit Pierre J. G. M. , Grandbastien Marie-Angèle
Published: May 2007

The copia-like Tnt1 element of tobacco is one of the few active plant retrotransposons and is transcriptionally activated, in tobacco and in heterologous species, by biotic and abiotic stress factors. In order to establish more precisely the link between Tnt1 activation and plant defense responses, the expression of the Tnt1 promoter was studied in a gene-for-gene pathosystem, the interaction between tomato and the fungal pathogen Cladosporium fulvum. In compatible interactions, Tnt1 expression is highly induced throughout the leaf regions colonized by the fungus, while in incompatible interactions Tnt1 induction is transient and localized in distinct foci. Tnt1 expression after fungal inoculation parallels the differential activation of tomato defense genes. Tnt1 expression is induced by nonspecific factors of plant or fungal origin present in apoplastic fluids of leaf tissues infected by virulent races of C. fulvum, but is also activated by specific factors resulting from the interaction between fungal avirulence peptides and plant resistance genes. Tnt1 activation by apoplastic fluids containing avirulence peptides of C. fulvum is detected soon after elicitation. These results demonstrate that Tnt1 transcriptional activation correlates with biological responses of tomato to infection by C. fulvum and is mediated through signals originating from both race-specific and non-race-specific perception pathways.
A Transient Assay System for the Functional Assessment of Defense-Related Genes in Wheat
Authors: Schweizer Patrick , Pokorny Jana , Abderhalden Olaf , Dudler Robert
Published: May 2007

The generation and characterization of transgenic wheat plants is a tedious and time-consuming process that limits the number of putatively important transgenes that can be tested. We therefore established a transient assay system based on wheat leaves to study the effect of transiently expressed genes on the interaction with the wheat powdery mildew fungus Erysiphe (syn. Blumeria) graminis f. sp. tritici. Young wheat leaves were bombarded with tungsten particles coated with a mixture of plasmids carrying the β-glucuronidase (GUS) reporter gene and a test gene. Leaves were subsequently challenge inoculated with E. graminis and the fungus was allowed to develop for 40 h. After being stained for GUS enzymatic activity as well as for epiphytic fungal structures, the phenotype of transformed epidermal cells was evaluated by bright-field microscopy. The fungus was routinely found to penetrate cells transiently expressing GUS with an efficiency of approximately 35%, which should suffice to detect putative transgene effects. Transgenes encoding a low-molecular-weight cell-wall protein of wheat (WIR1), a thaumatin-like protein, and a glucanase had no effect on fungal penetration of transformed epidermal cells. On the other hand, trans-genes encoding a pathogen-induced wheat protein of unknown function (WCI5), a chitinase, a glucose oxidase, and a putative peroxidase significantly reduced fungal penetration.
Fungal Polygalacturonases Exhibit Different Substrate Degradation Patterns and Differ in Their Susceptibilities to Polygalacturonase-Inhibiting Proteins
Authors: Cook B. J. , Clay R. P. , Bergmann C. W. , Albersheim P. , Darvill A. G.
Published: May 2007

Polygalacturonic acid (PGA) was hydrolyzed by polygalacturonases (PGs) purified from six fungi. The oligogalacturonide products were analyzed by HPAEC-PAD (high performance anion exchange chromatography-pulsed amperimetric detection) to assess their relative amounts and degrees of polymerization. The abilities of the fungal PGs to reduce the viscosity of a solution of PGA were also determined. The potential abilities of four polygalacturonase-inhibiting proteins (PGIPs) from three plant species to inhibit or to modify the hydrolytic activity of the fungal PGs were determined by colorimetric and HPAEC-PAD analyses, respectively. Normalized activities of the different PGs acting upon the same substrate resulted in one of two distinct oligogalacturonide profiles. Viscometric analysis of the effect of PGs on the same substrate also supports two distinct patterns of cleavage. A wide range of susceptibility of the various PGs to inhibition by PGIPs was observed. The four PGs that were inhibited by all PGIPs tested exhibited an endo/exo mode of substrate cleavage, while the three PGs that were resistant to inhibition by one or more of the PGIPs proceed by a classic endo pattern of cleavage.
Regulatory Signals Influencing Expression of the PDA1 Gene of Nectria haematococca MPVI in Culture and During Pathogenesis of Pea
Authors: Khan Rana , Straney David C.
Published: May 2007

The PDA1 gene of the filamentous fungus Nectria haematococca MPVI (anamorph: Fusarium solani) encodes a cytochrome P450 monooxygenase that detoxifies pisatin, the isoflavonoid phytoalexin produced by its host, garden pea (Pisum sativum). PDA1 is regulated by several signals in culture that may control its expression during pathogenesis of pea. It is induced by pisatin and repressed by glucose and amino acids. Deletion analysis was performed on the PDA1 promoter to define regulatory regions, using a β-glucuronidase (GUS) reporter gene fusion. The results identified a region between -287 and -429, relative to the start of transcription, that mediated repression by either glucose or amino acids in culture, independent from pisatin induction. Transformants bearing PDA1 promoter constructs displaying altered regulation in response to the different signals were used to infect pea epicotyls in order to correlate regulation in culture with that observed during pathogenesis of the host. Removal of the nutritional response region did not have a major effect on the induction of the promoter observed during growth in pea. However, induced expression in planta was lacking in a PDA1::GUS construct that lacked pisatin response in culture. These results suggest that the host-specific stimulus, pisatin, is a primary stimulatory signal for PDA1 regulation during pea pathogenesis.
Aberrant Nodulation Response of Vigna umbellata to a Bradyrhizobium japonicum NodZ Mutant and Nodulation Signals
Authors: Cohn Jonathan , Stokkermans Tom , Kolli V. Kumar , Day R. Bradley , Dunlap John , Carlson Russell , Hughes Doug , Peters N. Kent , Stacey Gary
Published: May 2007

The (Brady)rhizobium nodulation gene products synthesize lipo-chitin oligosaccharide (LCO) signal molecules that induce nodule primordia on legume roots. In spot inoculation assays with roots of Vigna umbellata, Bradyrhizobium elkanii LCO and chemically synthesized LCO induced aberrant nodule structures, similar to the activity of these LCOs on Glycine soja (soybean). LCOs containing a pentameric chitin backbone and a reducing-end 2-O-methyl fucosyl moiety were active on V. umbellata. In contrast, the synthetic LCO-IV(C16:0), which has previously been shown to be active on G. soja, was inactive on V. umbellata. A B. japonicum NodZ mutant, which produces LCO without 2-O-methyl fucose at the reducing end, was able to induce nodule structures on both plants. Surprisingly, the individual, purified, LCO molecules produced by this mutant were incapable of inducing nodule formation on V. umbellata roots. However, when applied in combination, the LCOs produced by the NodZ mutant acted cooperatively to produce nodulelike structures on V. umbellata roots.
Rhizobium Nod Factors Induce an Increase in Sub-apical Fine Bundles of Actin Filaments in Vicia sativa Root Hairs within Minutes
Authors: de Ruijter Norbert C. A. , Bisseling Ton , Emons Anne Mie C.
Published: May 2007

We studied the response of the actin cytoskeleton in vetch root hairs after application of host-specific Nod factor. Within 3 to 15 min, the number of sub-apical fine bundles of actin filaments (FB-actin) increased in all developmental stages. Tip growth resumed only in hairs in which the FB-actin density and the length of the region with FB-actin exceeded a minimal value.
Cyst Germination Proteins of the Potato Pathogen Phytophthora infestans Share Homology with Human Mucins
Authors: Görnhardt Birgit , Rouhara Ila , Schmelzer Elmon
Published: May 2007

We have cloned genes of Phytophthora infestans, the causal agent of potato late blight, that are activated shortly before the onset of invasion of the host tissue. The three genes isolated appear to be arranged in a genomic cluster and belong to a small polymorphic gene family. A conspicuous feature of the deduced proteins is an internal octapeptide repeat with the consensus sequence TTYAP TEE. Because of this structural motif, these novel P. infestans proteins were named Car (cyst-germination-specific acidic repeat) proteins. One of the genes, car90, codes for 1,489 amino acids including 120 octapeptide tandem repeats. Car proteins are transiently expressed during germination of cysts and formation of appressoria and are localized at the surface of germlings. The structural motif of tandemly repeated oligopeptides also occurs in a prominent class of proteins, the mucins, from mammals. The P. infestans Car proteins share 51% sequence homology with the tandem repeat region of a subfamily of human mucins. According to the physiological functions ascribed to mucins, we suggest that Car proteins may serve as a mucous cover protecting the germling from desiccation, physical damage, and adverse effects of the plant defense response and may assist in adhesion to the leaf surface.
Functional Expression of the Gene cu, Encoding the Phytotoxic Hydrophobin Cerato-ulmin, Enables Ophiostoma quercus, a Nonpathogen on Elm, to Cause Symptoms of Dutch Elm Disease
Authors: Sorbo Giovanni Del , Scala Felice , Parrella Giuseppe , Lorito Matteo , Comparini Cecilia , Ruocco Michelina , Scala Aniello
Published: May 2007

We studied the involvement of the phytotoxic hydrophobin cerato-ulmin (CU) in pathogenesis and virulence of Dutch elm disease (DED) by expressing its encoding gene (cu) in Ophiostoma quercus, a nonpathogenic species on elm closely related to the DED pathogens O. ulmi and O. novo-ulmi. The production of the toxin was quantitatively determined in culture filtrates and in mycelial extracts of the transformants. Production of CU in vitro was associated with the ability to cause typical DED symptoms, consisting of foliar yellow and wilting and vascular tissue discoloration on a moderately resistant elm genotype. The presence of CU was monitored by enzyme-linked immunosorbent assay in symptomatic leaves of plants inoculated with O. quercus transformants expressing CU and found to be associated with wilt symptoms. In general, the virulence of the cu-expressing transformants, as measured in terms of vascular discoloration and percentage of defoliation, was lower than that of the mildly pathogenic isolate E2 of O. ulmi. However, one transformant (C39) displayed a virulence level intermediate between that of E2 and 182, a highly virulent isolate of O. novo-ulmi. Our results indicate that CU production influences virulence in nonaggressive strains of Ophiostoma fungi.
Analysis of Medicago truncatula Nodule Expressed Sequence Tags
Authors: Györgyey János , Vaubert Danièle , Jiménez-Zurdo José I. , Charon Celine , Troussard Liliane , Kondorosi Ádám , Kondorosi Éva
Published: May 2007

Systematic sequencing of expressed sequence tags (ESTs) can give a global picture of the assembly of genes involved in the development and function of organs. Indeterminate nodules representing different stages of the developmental program are especially suited to the study of organogenesis. With the vector λHybriZAP, a cDNA library was constructed from emerging nodules of Medicago truncatula induced by Sinorhizobium meliloti. The 5′ ends of 389 cDNA clones were sequenced, then these ESTs were analyzed both by sequence homology search and by studying their expression in roots and nodules. Two hundred fifty-six ESTs exhibited significant similarities to characterized data base entries and 40 of them represented 26 nodulin genes, while 133 had no similarity to sequences with known function. Only 60 out of the 389 cDNA clones corresponded to previously submitted M. truncatula EST sequences. For 117 cDNAs, reverse Northern (RNA) hybridization with root and nodule RNA probes revealed enhanced expression in the nodule, 48 clones are likely to code for novel nodulins, 33 cDNAs are clones of already known nodulin genes, and 36 clones exhibit similarity to other characterized genes. Thus, systematic analysis of the EST sequences and their expression patterns is a powerful way to identify nodule-specific and nodulation-related genes.
Transmissible Mitochondrial Hypovirulence in a Natural Population of Cryphonectria parasitica
Authors: Baidyaroy Dipnath , Huber David H. , Fulbright Dennis W. , Bertrand Helmut
Published: May 2007

A cytoplasmically transmissible hypovirulence syndrome has been identified in virus-free strains of the chestnut blight fungus Cryphonectria parasitica isolated from healing cankers on American chestnut trees in southwestern Michigan. The syndrome is associated with symptoms of fungal senescence, including a progressive decline in the growth potential and abundance of conidia, and elevated levels of respiration through the cyanide-insensitive alternative oxidase pathway. Conidia from senescing mycelia exhibited varying degrees of senescence ranging from normal growth to death soon after germination. Cytoplasmic transmission of hypovirulence between mycelia occurred by hyphal contact and coincided with the transfer of a specific restriction fragment length polymorphism from the mitochondrial DNA (mtDNA) of the donor strains into the mtDNA of virulent recipients. The transmission of the senescence phenotype was observed not only among vegetatively compatible strains but also among incompatible strains. Hypovirulence was present in isolates from the same location with different nuclear genotypes as identified by DNA fingerprinting. This study confirms that mitochondrial hypovirulence can occur spontaneously and spread within a natural population of a phytopathogenic fungus.
Stagonospora avenae Secretes Multiple Enzymes that Hydrolyze Oat Leaf Saponins
Authors: Morrissey John P. , Wubben Jos P. , Osbourn Anne E.
Published: May 2007

The phytopathogenic fungus Stagonospora avenae is able to infect oat leaves despite the presence of avenacoside saponins in the leaf tissue. In response to pathogen attack, avenacosides are converted into 26-desglucoavenacosides (26-DGAs), which possess antifungal activity. These molecules are comprised of a steroidal backbone linked to a branched sugar chain consisting of one α-L-rhamnose and two (avenacoside A) or three (avenacoside B) β-D-glucose residues. Isolates of the fungus that are pathogenic to oats are capable of sequential hydrolysis of the sugar residues from the 26-DGAs. Degradation is initiated by removal of the L-rhamnose, which abolishes antifungal activity. The D-glucose residues are then hydrolyzed by β-glucosidase activity. A comprehensive analysis of saponin-hydrolyzing activities was undertaken, and it was established that S. avenae isolate WAC1293 secretes three enzymes, one α-rhamnosidase and two β-glucosidases, that carry out this hydrolysis. The major β-glucosidase was purified and the gene encoding the enzyme cloned. The protein is similar to saponin-hydrolyzing enzymes produced by three other phytopathogenic fungi, Gaeumannomyces graminis, Septoria lycopersici, and Botrytis cinerea, and is a family 3 β-glucosidase. The gene encoding the β-glucosidase is expressed during infection of oat leaves but is not essential for pathogenicity.
The Broad-Spectrum Tospovirus Resistance Gene Sw-5 of Tomato Is a Homolog of the Root-Knot Nematode Resistance Gene Mi
Authors: Brommonschenkel Sergio H. , Frary Amy , Frary Anne , Tanksley Steven D.
Published: May 2007

We used a positional cloning approach to isolate the Sw-5 disease resistance locus of tomato. Complementation experiments with overlapping cosmid clones enabled us to demonstrate that Sw-5 is a single gene locus capable of recognizing several tospovirus isolates and species. Analysis of the predicted Sw-5 protein suggests that it is a cytoplasmic protein, with a potential nucleotide binding site (NBS) domain and a C-terminal end consisting of leucine-rich repeats (LRRs). Based on its structural features, Sw-5 belongs to the class of NBS-LRR resistance genes that includes the tomato Mi, I2, and Prf genes; the Arabidopsis RPM1 gene; and the plant potato virus X resistance gene Rx. The overall similarity between the Sw-5 and Mi proteins of tomato suggests that a shared or comparable signal transduction pathway leads to both virus and nematode resistance in tomato. The similarity also supports the hypothesis that Sw-5 provides resistance via a hypersensitive response. Sw-5 is a member of a loosely clustered gene family in the telomeric region of chromosome 9. Members of this family map to other regions of chromosome 9 and also to chromosome 12, where several fungal, virus, and nematode genes have been mapped, suggesting that paralogs of Sw-5 may have evolved to provide different resistance specificities.
Fructose Utilization and Phytopathogenicity ofSpiroplasma citri
Authors: Gaurivaud Patrice , Danet Jean-Luc , Laigret Frédéric , Garnier Monique , Bové Joseph M.
Published: May 2007

Spiroplasma citri is a plant-pathogenic mollicute. Recently, the so-called nonphytopathogenic S. citri mutant GMT 553 was obtained by insertion of transposon Tn4001 into the first gene of the fructose operon. Additional fructose operon mutants were produced either by gene disruption or selection of spontaneous xylitol-resistant strains. The behavior of these spiroplasma mutants in the periwinkle plants has been studied. Plants infected via leafhoppers with the wild-type strain GII-3 began to show symptoms during the first week following the insect-transmission period, and the symptoms rapidly became severe. With the fructose operon mutants, symptoms appeared only during the fourth week and remained mild, except when reversion to a fructose+phenotype occurred. In this case, the fructose+revertants quickly overtook the fructose¯ mutants and the symptoms soon became severe. When mutant GMT 553 was complemented with the fructose operon genes that restore fructose utilization, severe pathogenicity, similar to that of the wild-type strain, was also restored. Finally, plants infected with the wild-type strain and grown at 23°C instead of 30°C showed late symptoms, but these rapidly became severe. These results are discussed in light of the role of fructose in plants. Fructose utilization by the spiroplasmas could impair sucrose loading into the sieve tubes by the companion cells and result in accumulation of carbohydrates in source leaves and depletion of carbon sources in sink tissues.
Crg, a Gene Required forUr-3-Mediated Rust Resistance in Common Bean, Maps to a Resistance Gene Analog Cluster
Authors: Kalavacharla V. , Stavely J. R. , Myers J. R. , McClean P. E.
Published: May 2007

Race-specific resistance to the bean rust pathogen (Uromyces appendiculatus) is provided by a number of loci in common bean (Phaseolus vulgaris). The Ur-3 locus controls hypersensitive resistance (HR) to 44 of the 89 races curated in the United States. To better understand resistance mediated by this locus, we developed new genetic material for analysis. We developed a population of mutagenized seed of cv. Sierra (genotype = Ur-3 ur-4 ur-6) that was screened with a bean rust race that is normally incompatible (HR response) on Ur-3 genotypes. We discovered two mutants of common bean, crg and ur3-Δ3, in which uredinia formed on leaves (a compatible interaction) following infection. The F1generation from a cross of these two mutants expressed the HR response, and the F2generation segregated in a ratio of 9:7 (HR/uredinia formation). Therefore, the two genes are unlinked. Further genetic analysis determined that the mutation in ur3-Delta;3 was in the Ur-3 locus, and the mutation in crg was in a newly discovered gene given the symbol Crg (Complements resistance gene). Each mutation was inherited in a recessive manner. Unlike ur3-Delta;3, crg expressed reduced compatibility to bean rust races 49 and 47 that are normally fully compatible on genotypes, such as Sierra, that are homozygous recessive at the Ur-4 and Ur-6 loci. This suggests a gene mutated in crg is normally a positive compatibility factor for the bean-bean rust interaction. Polymerase chain reaction analysis of crg with primers to common bean resistance gene analogs (RGA) that contain a nucleotide-binding site sequence similar to those found in a number of plant disease resistance genes revealed that crg is missing the SB1 RGA, but not the linked SB3 and SB5 RGAs. Genetic analyses revealed that Crg cosegregates with the SB1 RGA. These results demonstrate that Crg is located near a RGA cluster in the common bean genome.
Improved gfp and inaZ Broad-Host-Range Promoter-Probe Vectors
Authors: Miller William G. , Leveau Johan H. J. , Lindow Steven E.
Published: May 2007

A new set of broad-host-range promoter-probe vectors has been constructed. One subset contains the pVS1 and p15a replicons and confers resistance to either gentamicin or kanamycin. The other set contains the broad-host-range replicon from pBBR1 and confers resistance to kanamycin, tetracycline, ampicillin, or spectinomycin/streptomycin. Both plasmid sets are highly stable and are maintained without selection for more than 30 generations in several bacterial taxa. Each plasmid contains a promoter-probe cassette that consists of a multicloning site, containing several unique restriction sites, and gfp or inaZ as a reporter gene. The cassette is bound by transcriptional terminators to permit the insertion of strong promoters and to insulate the cassette from external transcription enabling the detection of weak or moderate promoters. The vector suite was augmented with derivatives of the kanamycin-resistant gfp promoter-probe plasmids that encode Gfp variants with different half-life times.
Regulation of hrp Genes and Type III Protein Secretion in Erwinia amylovora by HrpX/HrpY, a Novel Two-Component System, and HrpS
Authors: Wei Zhongmin , Kim Jihyun F. , Beer Steven V.
Published: May 2007

Two novel regulatory components, hrpX and hrpY, of the hrp system of Erwinia amylovora were identified. The hrpXY operon is expressed in rich media, but its transcription is increased threefold by low pH, nutrient, and temperature levels—conditions that mimic the plant apoplast. hrpXY is autoregulated and directs the expression of hrpL; hrpL, in turn, activates transcription of other loci in the hrp gene cluster (Z.-M. Wei and S. V. Beer, J. Bacteriol. 177:6201-6210, 1995). The deduced amino -acid sequences of hrpX and hrpY are similar to bacterial two-component regulators including VsrA/VsrD of Pseudomonas (Ralstonia) solanacearum, DegS/DegU of Bacillus subtilis, and UhpB/UhpA and NarX/NarP, NarL of Escherichia coli. The N-terminal signal-input domain of HrpX contains PAS domain repeats. hrpS, located downstream of hrpXY, encodes a protein with homology to WtsA (HrpS) of Erwinia (Pantoea) stewartii, HrpR and HrpS of Pseudomonas syringae, and other σ54-dependent, enhancer-binding proteins. Transcription of hrpS also is induced under conditions that mimic the plant apoplast. However, hrpS is not autoregulated, and its expression is not affected by hrpXY. When hrpS or hrpL were provided on multicopy plasmids, both hrpX and hrpY mutants recovered the ability to elicit the hypersensitive reaction in tobacco. This confirms that hrpS and hrpL are not epistatic to hrpXY. A model of the regulatory cascades leading to the induction of the E. amylovora type III system is proposed.
Transposon Insertion in the ftsK Gene Impairs In Planta Growth and Lesion-Forming Abilities in Pseudomonas syringae pv. syringae B728a
Authors: Kinscherf Thomas G. , Hirano Susan S. , Willis David K.
Published: May 2007

A Tn5 insertion in the ftsK gene of Pseudomonas syringae pv. syringae B728a impaired brown spot lesion formation on Phaseolus vulgaris, the ability to grow within bean leaves, and swarming ability on semisolid agar. Plasmids containing the ftsK gene were sufficient to complement the original Tn5 mutant for lesion formation and swarming and partially restored in planta growth.
The Pseudomonas syringae avrRpt2 Gene Product Promotes Pathogen Virulence from Inside Plant Cells
Authors: Chen Zhongying , Kloek Andrew P. , Boch Jens , Katagiri Fumiaki , Kunkel Barbara N.
Published: May 2007

Several bacterial avr genes have been shown to contribute to virulence on susceptible plants lacking the corresponding resistance (R) gene. The mechanisms by which avr genes promote parasitism and disease, however, are not well understood. We investigated the role of the Pseudomonas syringae pv. tomato avrRpt2 gene in pathogenesis by studying the interaction of P. syringae pv. tomato strain PstDC3000 expressing avrRpt2 with several Arabidopsis thaliana lines lacking the corresponding R gene, RPS2. We found that PstDC3000 expressing avrRpt2 grew to significantly higher levels and often resulted in the formation of more severe disease symptoms in ecotype No-0 plants carrying a mutant RPS2 allele, as well as in two Col-0 mutant lines, cpr5 rps2 and coi1 rps2, that exhibit enhanced resistance. We also generated transgenic A. thaliana lines expressing avrRpt2 and demonstrated, by using several different assays, that expression of avrRpt2 within the plant also promotes virulence of PstDC3000. Thus, AvrRpt2 appears to promote pathogen virulence from within the plant cell.
Xanthomonas oryzae pv. Oryzae Avirulence Genes Contribute Differently and Specifically to Pathogen Aggressiveness
Authors: Bai Jianfa , Choi Seong-Ho , Ponciano Grisel , Leung Hei , Leach Jan E.
Published: May 2007

Genomic copies of three Xanthomonas oryzae pv. oryzae avirulence (avr) genes, avrXa7, avrXa10, and avrxa5, and four homologous genes, aB3.5, aB3.6, aB4.3, and aB4.5, were mutagenized individually or in combination to study the roles of avr genes in one component of pathogen fitness, i.e., aggressiveness or the amount of disease X. oryzae pv. oryzae causes in susceptible rice lines. These X. oryzae pv. oryzae genes are members of the highly related Xanthomonas avrBs3 gene family. Compared to the wild-type strain, X. oryzae pv. oryzae strains with mutations in avrXa7, avrxa5, and the four homologous genes caused shorter lesions on rice line IR24, which contains no resistance genes relevant to the wild-type strain. The contribution of each gene to lesion length varied, with avrXa7 contributing the most and avrXa10 showing no measurable effect on aggressiveness. The functional, plasmidborne copies of avrXa7, aB4.5, and avrxa5 restored aggressiveness only to strains with mutations in avrXa7, aB4.5, and avrxa5, respectively. Mutations in avrXa7 were not complemented by plasmids carrying any other avr gene family members. These data indicate that some, but not all, avr family members contribute to pathogen aggressiveness and that the contributions are quantitatively different. Furthermore, despite their sequence similarity, the aggressiveness functions of these gene family members are not interchangeable. The results suggest that selection and pyramiding resistance genes can be guided by the degree of fitness penalty that is empirically determined in avr gene mutations.
Rhizobia Can Induce Nodules in White Clover by “Hijacking” Mature Cortical Cells Activated During Lateral Root Development
Authors: Mathesius Ulrike , Weinman Jeremy J. , Rolfe Barry G. , Djordjevic Michael A.
Published: May 2007

We examined a range of responses of root cortical cells to Rhizobium sp. inoculation to investigate why rhizobia preferentially nodulate legume roots in the zone of emerging root hairs, but generally fail to nodulate the mature root. We tested whether the inability to form nodules in the mature root is due to a lack of plant flavonoids to induce the bacterial genes required for nodulation or a failure of mature cortical cells to respond to Rhizobium spp. When rhizobia were inoculated in the zone of emerging root hairs, changes in β-glucuronidase (GUS) expression from an auxin-responsive promoter (GH3), expression from three chalcone synthase promoters, and the accumulation of specific flavonoid compounds occurred in cortical cells prior to nodule formation. Rhizobia failed to induce these responses when inoculated in the mature root, even when co-inoculated with nod gene-inducing flavonoids. However, mature root hairs remained responsive to rhizobia and could support infection thread formation. This suggests that a deficiency in signal transduction is the reason for nodulation failure in the mature root. However, nodules could be initiated in the mature root at sites of lateral root emergence. A comparison between lateral root and nodule formation showed that similar patterns of GH3:gusA expression, chalcone synthase gene expression, and accumulation of a particular flavonoid compound occurred in the cortical cells involved in both processes. The results suggest that rhizobia can“ hijack” cortical cells next to lateral root emergence sites because some of the early responses required for nodule formation have already been activated by the plant in those cells.
An HR-Induced Tobacco Peroxidase Gene Is Responsive to Spermine, but Not to Salicylate, Methyl Jasmonate, and Ethephon
Authors: Hiraga Susumu , Ito Hiroyuki , Yamakawa Hiromoto , Ohtsubo Norihiro , Seo Shigemi , Mitsuhara Ichiro , Matsui Hirokazu , Honma Mamoru , Ohashi Yuko
Published: May 2007

In Tobacco mosaic virus (TMV)-infected tobacco plants carrying the N resistance gene, a hypersensitive reaction or response (HR) occurs to enclose the virus in the infected tissue. Although a contribution of peroxidases to the resistance has been proposed, no evidence has been presented that tobacco peroxidase genes respond to HR. Here, we describe the HR-induced expression of a tobacco peroxidase gene (tpoxC1) whose induction kinetics were slightly different from those of acidic and basic tobacco pathogenesis-related (PR) protein genes. Interestingly, tpoxC1 was insensitive to the inducers of PR genes such as salicylic acid, methyl jasmonate, and ethephon. Spermine activated tpoxC1 gene expression at a low level and both acidic and basic PR gene expression at a considerably higher level. These results indicate that the induced expression of tpoxC1 is regulated differently from that of classical tobacco PR genes in the N gene-mediated self-defense system in tobacco plants.
The purMN Genes of Rhizobium leguminosarum and a Superficial Link with Siderophore Production
Authors: Stevens James B. , de Luca Nicola G. , Beringer John E. , Ringer John P. , Yeoman Kay H. , Johnston Andrew W. B.
Published: May 2007

We isolated a mutant of R. leguminosarum initially on the basis of reduced production of the siderophore vicibactin on chrome azurol sulfonate (CAS)/agar indicator plates. The mutation was in the purMN operon and the mutant was shown to be an adenine auxotroph and defective for nodulation of peas. The siderophore defect appears to be trivial, being due to diminished growth of the auxotroph on agar-based minimal medium, which contains unknown contaminant(s) that allow it grow poorly. Transcriptional fusions showed that purMN was transcribed at relatively high levels in media containing purines. Expression was enhanced, approximately twofold, if purines were omitted.
Heterologous Rhizobial Lipochitin Oligosaccharides and Chitin Oligomers Induce Cortical Cell Divisions in Red Clover Roots, Transformed with the Pea Lectin Gene
Authors: Díaz Clara L. , Spaink Herman P. , Kijne Jan W.
Published: May 2007

Division of cortical cells in roots of leguminous plants is triggered by lipochitin oligosaccharides (LCOs) secreted by the rhizobial microsymbiont. Previously, we have shown that presence of pea lectin in transgenic white clover hairy roots renders these roots susceptible to induction of root nodule formation by pea-specific rhizobia (C. L. Díaz, L. S. Melchers, P. J. J. Hooykaas, B. J. J. Lugtenberg, and J. W. Kijne, Nature 338:579–581, 1989). Here, we report that pea lectin-transformed red clover hairy roots form nodule primordium-like structures after inoculation with pea-, alfalfa-, and Lotus-specific rhizobia, which normally do not nodulate red clover. External application of a broad range of purified LCOs showed all of them to be active in induction of cortical cell divisions and cell expansion in a radial direction, resulting in formation of structures that resemble nodule primordia induced by clover-specific rhizobia. This activity was obvious in about 50% of the red clover plants carrying hairy roots transformed with the pea lectin gene. Also, chitopentaose, chitotetraose, chitotriose, and chitobiose were able to induce cortical cell divisions and cell expansion in a radial direction in transgenic roots, but not in control roots. Sugarbinding activity of pea lectin was essential for its effect. These results show that transformation of red clover roots with pea lectin results in a broadened response of legume root cortical cells to externally applied potentially mitogenic oligochitin signals.
Identification with Proteomics of Novel Proteins Associated with the Peribacteroid Membrane of Soybean Root Nodules
Authors: Panter S. , Thomson R. , de Bruxelles G. , Laver D. , Trevaskis B. , Udvardi M.
Published: May 2007

Soybean peribacteroid membrane (PBM) proteins were isolated from nitrogen-fixing root nodules and subjected to N-terminal sequencing. Sequence data from 17 putative PBM proteins were obtained. Six of these proteins are homologous to proteins of known function. These include three chaperones (HSP60, BiP [HSP70], and PDI) and two proteases (a serine and a thiol protease), all of which are involved in some aspect of protein processing in plants. The PBM homologs of these proteins may play roles in protein translocation, folding, maturation, or degradation in symbiosomes. Two proteins are homologous to known, nodule-specific proteins from soybean, nodulin 53b and nodulin 26B. Although the function of these nodulins is unknown, nodulin 53b has independently been shown to be associated with the PBM. All of the eight proteins with identifiable homologs are likely to be peripheral rather than integral membrane proteins. Possible reasons for this apparent bias are discussed. The identification of homologs of HSP70 and HSP60 associated with the PBM is the first evidence that the molecular machinery for co- or post-translational import of cytoplasmic proteins is present in symbiosomes. This has important implications for the biogenesis of this unique, nitrogen-fixing organelle.
Agroinfiltration Is a Versatile Tool That Facilitates Comparative Analyses ofAvr9/Cf-9-Induced andAvr4/Cf-4-Induced Necrosis
Authors: Van der Hoorn Renier A. L. , Laurent Franck , Roth Ronelle , De Wit Pierre J. G. M.
Published: May 2007

The avirulence genes Avr9 and Avr4 from the fungal tomato pathogen Cladosporium fulvum encode extracellular proteins that elicit a hypersensitive response when injected into leaves of tomato plants carrying the matching resistance genes, Cf-9 and Cf-4, respectively. We successfully expressed both Avr9 and Avr4 genes in tobacco with the Agrobacterium tumefaciens transient transformation assay (agroinfiltration). In addition, we expressed the matching resistance genes, Cf-9 and Cf-4, through agroinfiltration. By combining transient Cf gene expression with either transgenic plants expressing one of the gene partners, Potato virus X (PVX)-mediated Avr gene expression, or elicitor injections, we demonstrated that agroinfiltration is a reliable and versatile tool to study Avr/Cf-mediated recognition. Significantly, agroinfiltration can be used to quantify and compare Avr/Cf-induced responses. Comparison of different Avr/Cf-interactions within one tobacco leaf showed that Avr9/Cf-9-induced necrosis developed slower than necrosis induced by Avr4/Cf-4. Quantitative analysis demonstrated that this temporal difference was due to a difference in Avr gene activities. Transient expression of matching Avr/Cf gene pairs in a number of plant families indicated that the signal transduction pathway required for Avr/Cf-induced responses is conserved within solana-ceous species. Most non-solanaceous species did not develop specific Avr/Cf-induced responses. However, co-expression of the Avr4/Cf-4 gene pair in lettuce resulted in necrosis, providing the first proof that a resistance (R) gene can function in a different plant family.
Symbiotic Induction of Pyruvate Dehydrogenase Genes from Sinorhizobium meliloti
Authors: Cabanes Didier , Boistard Pierre , Batut Jacques
Published: May 2007

Genes coding for components of the pyruvate dehydrogenase (PDH) multienzyme complex (PDHc) from Sinorhizobium meliloti, the alfalfa symbiont, have been isolated on the basis of their high expression in symbiotic bacteria. The E1p component, PDH, is encoded by two genes, pdhAα (1,047 bp) and pdhAβ (1,383 bp), a situation encountered in the α-proteobacteria Rickettsia prowazekii and Zymomonas mobilis as well as in some Gram-positive bacteria and in mitochondria. pdhAα and pdhAβ precede pdhB (1,344 bp), which encodes the E2p component, dihy-drolipoamide acetyltransferase, of the PDHc. No gene encoding the E3 component, lipoamide dehydrogenase, was found in the immediate vicinity of pdhA and pdhB genes. pdhAα, pdhAβ, and pdhB likely constitute an operon. Here, we provide evidence that pdhA expression is induced in the symbiotic stage, compared with free-living conditions. We demonstrate that symbiotic expression of pdhA genes does not depend on the fixLJ regulatory cascade that regulates nitrogen fixation and respiration gene expression in symbiotic S. meliloti cells. Induction of pdhA expression could be obtained under free-living conditions upon the addition of pyruvate to the culture medium. Induction by pyruvate and symbiotic activation of pdh gene expression take place at the same promoter.