bluuebunny/crossref_metadata_2025 huggingface dataset:
The Melanin Biosynthesis Genes of Alternaria alternata Can Restore Pathogenicity of the Melanin-Deficient Mutants of Magnaporthe grisea
Authors: Kawamura Chie , Moriwaki Jouji , Kimura Norio , Fujita Yasunari , Fuji Shin-ichi , Hirano Tetsuji , Koizumi Shinzo , Tsuge Takashi
Published: May 2007

The phytopathogenic fungi Magnaporthe grisea and Alternaria alternata produce melanin via the polyketide biosynthesis, and both fungi form melanized colonies. However, the site of melanin deposition and the role of melanin in pathogenicity differ between these two fungi. M. grisea accumulates melanin in appressoria, and their melanization is essential for host penetration. On the other hand, A. alternata produces colorless appressoria, and melanin is not relevant to host penetration. We examined whether the melanin biosynthesis genes of A. alternata could complement the melanin-deficient mutations of M. grisea. Melanin-deficient, nonpathogenic mutants of M. grisea, albino (Alb¯), rosy (Rsy¯), and buff (Buf¯), were successfully transformed with a cosmid clone pMBR1 that carries melanin biosynthesis genes ALM, BRM1, and BRM2 of A. alternata. This transformation restored the melanin synthesis of the Alb¯ and Buf¯ mutants, but not that of the Rsy¯ mutant. The melanin-restored transformants regained mycelial melanization, appressorium melanization, and pathogenicity to rice. Further, transformation of Alb¯ and Buf¯ mutants with subcloned ALM and BRM2 genes, respectively, also produced melanin-restored transformants. These results indicate that the Alternaria genes ALM and BRM2 can restore pathogenicity to the mutants Alb¯ and Buf¯, respectively, due to their function during appressorium development in M. grisea.
hrpF of Xanthomonas campestris pv. vesicatoria Encodes an 87-kDa Protein with Homology to NolX of Rhizobium fredii
Authors: Huguet Elisabeth , Bonas Ulla
Published: May 2007

The gram-negative bacterium Xanthomonas campestris pv. vesicatoria is the causal agent of bacterial spot disease on pepper and tomato plants. The main hrp (hypersensitive reaction and pathogenicity) gene cluster in X. campestris pv. vesicatoria spans a 23-kb chromosomal region, comprising six complementation groups designated hrpA to hrpF. Analysis of the hrpF locus revealed a single open reading frame encoding HrpF (86.4 kDa). HrpF is predominantly hydrophilic, and contains two hydrophobic domains in the C terminus. An interesting feature is the presence of two imperfect direct repeats in the N-terminal region. Deletion studies showed that one repeat is sufficient for function. Epitope tagging of HrpF allowed detection of the protein in X. campestris pv. vesicatoria. Subcellular localization studies suggest that HrpF is both in the soluble fraction and in the inner membrane. Interestingly, HrpF is 48% identical (67% similar) to the Rhizobium fredii NolX protein that is part of the host specificity locus. Since several Hrp proteins are believed to be components of the type III Hrp protein secretion apparatus, allowing export of proteins essential for the interaction with the plant, the possible role of hrpF and nolX in secretion is discussed.
A Single Amino Acid Substitution in the Antimicrobial Defense Protein Cecropin B Is Associated with Diminished Degradation by Leaf Intercellular Fluid
Authors: Owens Lowell D. , Heutte Thomas M.
Published: May 2007

Degradation is one of several factors that may affect the level of accumulation of transgene products in plants. In plants engineered to secrete antimicrobial proteins to the intercellular compartment of leaves, the degradative activity of proteases residing in leaf intercellular fluid (IF) could be critical to achieving the expected transgene function. We synthesized a structural analogue (MB39) of the antibacterial protein cecropin B and compared the susceptibility of both proteins to degradation in vitro by IF extracted from leaves of various crops. The half-life of the two proteins in the various IF extracts ranged from 3 min to 25.5 h, with the analogue MB39 displaying the longer half-life in IF from nine of 10 species. Overall, the half-life of MB39 averaged 2.9 times greater than that of cecropin B. Analysis of the peptides produced by endopeptidase activity in potato IF indicated that the 5.7-fold lower degradation rate of MB39 was associated with the substitution of valine for methionine at residue 11 of cecropin B. These findings point to the possibility of tailoring antimicrobial protein genes to reduce the rate of protein degradation in a particular target crop.
Identification and Characterization of a Gene on Rhizobium meliloti pSyma, syrB, That Negatively Affects syrM Expression
Authors: Barnett Melanie J. , Long Sharon R.
Published: May 2007

The Rhizobium meliloti SyrM protein activates transcription of nodD3 and syrA. Regulation of syrM is complex and may involve as yet undiscovered genes. Here we report the isolation of insertion mutants showing increased expression of a syrM-gusA gene fusion. Characterization of one mutant strain, designated SYR-B, revealed a mutation consisting of a transposon insertion linked to a large deletion. The corresponding wild-type DNA was cloned as a 5.3-kb BamHI fragment. Genetic and physical analysis of this DNA demonstrated that an open reading frame (ORF) near one end of the fragment, encoding the 16.5-kDa SyrB protein, is responsible for the repression of syrM activity. Results of complementation experiments with the 5.3-kb BamHI DNA led us to hypothesize that other genes within this DNA fragment interfere with the expression or activity of SyrB. Our analysis showed that the region upstream of syrB contains three ORFs. One ORF is similar to the Ros repressor of Agrobacterium tumefaciens and the MucR repressor of R. meliloti.
High Expression of Truncated Viral Rep Protein Confers Resistance to Tomato Yellow Leaf Curl Virus in Transgenic Tomato Plants
Authors: Brunetti A. , Tavazza M. , Noris E. , Tavazza R. , Caciagli P. , Ancora G. , Crespi S. , Accotto G. P.
Published: May 2007

A truncated version of the C1 gene of tomato yellow leaf curl geminivirus (TYLCV), encoding the first 210 amino acids of the multifunctional Rep protein, was introduced by Agrobacterium transformation into Lycopersicon esculentum cv. Moneymaker plants under the transcriptional control of an enhanced cauliflower mosaic virus 35S promoter. One R0 plant (line 47) carrying the C1 gene in two loci (A and B) and accumulating the truncated Rep protein (T-Rep), was crossed with either a wild-type plant, or a C1 antisense plant (line 10). The wild type (wt) × 47 progeny were phenotypically homogeneous, contained either A or B locus, expressed high levels of T-Rep protein, had a “curled” phenotype, and were resistant to TYLCV when challenged either by agroinfection or by the vector Bemisia tabaci. In the 10 × 47 progeny, plants carrying only the sense gene behaved like the wt × 47 progeny, while those containing both sense and antisense transgenes did not accumulate the T-Rep protein, showed a normal phenotype, and were not resistant, showing that accumulation of T-Rep protein is required to confer TYLCV resistance. Plants accumulating T-Rep were susceptible to a distinct geminivirus, tomato leaf curl virus (ToLCV-Au).
Evolutionary Diversity of Symbiotically Induced Nodule MADS Box Genes: Characterization of nmhC5, a Member of a Novel Subfamily
Authors: Heard Jacqueline , Caspi Michal , Dunn Kathleen
Published: May 2007

Unique organs called nodules form on legume roots in response to intracellular infection by soil bacteria in the genus Rhizobium. This study describes a new MADS box gene, nmhC5, which along with nmh7 (J. Heard and K. Dunn, Proc. Natl. Acad. Sci. USA 92:5273-5277, 1995), is expressed in alfalfa (Medicago sativa) root nodules. Together, these genes represent the first putative transcription factors identified in nodules. Expression in a root-derived structure supports the growing sentiment that MADS box proteins have diverse roles in plant development. Comparison of the putative translation product of nmhC5 with those of other reported members of the MADS box family suggests that the overall structure of nmhC5 is conserved. Evolutionary analysis among the MADS box family showed that nmhC5 is orthologous to a root-expressed clone in Arabidopsis thaliana, agl17, and that nmh7 is similar to the floral subfamily with AP3 (DefA)/PI (Glo). Consistent with a prediction of homodimer formation, NMHC5 was shown to bind a CArG consensus sequence in vitro. In contrast, NMH7, which shows structural similarity to MADS box proteins that form heterodimers, did not bind the CArG element in an in vitro DNA-binding assay, suggesting the existence of an unknown dimer partner. The root-derived MADS box genes constitute a novel subfamily of vegetatively expressed MADS box genes. The evolutionary diversity between nmh7 and nmhC5 could represent an overall mechanistic conservation between plant developmental processes or could mean that nmh7 and nmhC5 make fundamentally different contributions to the development of the nodule.
Nod Factors of Azorhizobium caulinodans Strain ORS571 Can Be Glycosylated with an Arabinosyl Group, a Fucosyl Group, or Both
Authors: Mergaert Peter , Ferro Myriam , D'Haeze Wim , Van Montagu Marc , Holsters Marcelle , Promé Jean-Claude
Published: May 2007

In addition to the previously described arabinosylated Nod factors, Azorhizobium caulinodans can also produce fucosylated Nod factors and Nod factors that are both arabinosylated and fucosylated. The presence of a plasmid carrying extra copies of a subset of nod genes as well as bacterial growth conditions influence the relative proportion of carbamoylated, fucosylated, and arabinosylated Nod factors. By using a root hair formation assay, we demonstrate that the Nod factor glycosylations are important for biological activity on Sesbania rostrata roots.
Differential Induction of Systemic Resistance in Arabidopsis by Biocontrol Bacteria
Authors: Van Wees Saskia C. M. , Pieterse Corné M. J. , Trijssenaar Annemiek , Van 't Westende Yvonne A. M. , Hartog Femke , Van Loon Leendert C.
Published: May 2007

Selected nonpathogenic, root-colonizing bacteria are able to elicit induced systemic resistance (ISR) in plants. To elucidate the molecular mechanisms underlying this type of systemic resistance, an Arabidopsis-based model system was developed in which Pseudomonas syringae pv. tomato and Fusarium oxysporum f. sp. raphani were used as challenging pathogens. In Arabidopsis thaliana ecotypes Columbia and Landsberg erecta, colonization of the rhizosphere by P. fluorescens strain WCS417r induced systemic resistance against both pathogens. In contrast, ecotype RLD did not respond to WCS417r treatment, whereas all three ecotypes expressed systemic acquired resistance upon treatment with salicylic acid (SA). P. fluorescens strain WCS374r, previously shown to induce ISR in radish, did not elicit ISR in Arabidopsis. The opposite was found for P. putida strain WCS358r, which induced ISR in Arabidopsis but not in radish. These results demonstrate that rhizosphere pseudomonads are differentially active in eliciting ISR in related plant species. The outer membrane lipopolysaccharide (LPS) of WCS417r is the main ISR-inducing determinant in radish and carnation, and LPS-containing cell walls also elicit ISR in Arabidopsis. However, mutant WCS417rOA¯, lacking the O-antigenic side chain of the LPS, induced levels of protection similar to those induced by wild-type WCS417r. This indicates that ISR-inducing bacteria produce more than a single factor that trigger ISR in Arabidopsis. Furthermore, WCS417r and WCS358r induced protection in both wildtype Arabidopsis and SA-nonaccumulating NahG plants without activating pathogenesis-related gene expression. This suggests that elicitation of an SA-independent signaling pathway is a characteristic feature of ISR-inducing biocontrol bacteria.
Evidence for Different Signaling Pathways Activated by Inducers of Acquired Resistance in Wheat
Authors: Schaffrath Ulrich , Freydl Ernst , Dudler Robert
Published: May 2007

Acquired resistance (AR) of wheat (Triticum aestivum) to the powdery mildew fungus Erysiphe graminis f. sp. tritici can be induced either by inoculation with the nonhost pathogen E. graminis f. sp. hordei or by treatment with chemical substances such as benzo(1,2,3)thiodiazole-7-carbothioic acid S-methyl ester (BTH). In the dicotyledonous plants tobacco and Arabidopsis, induction of AR by pathogens and BTH is accompanied by the expression of a characteristic set of genes. Here we report that in wheat, BTH treatment failed to activate genes whose transcripts accumulate after AR induction by nonhost pathogens, whereas BTH-inducible genes were not activated by an appropriate pathogen inoculation. This suggests that at least two different pathways exist for the induction of AR in monocots.
Differential Induction of a Peroxidase Gene Family During Infection of Rice by Xanthomonas oryzae pv. oryzae
Authors: Chittoor Jaishree M. , Leach Jan E. , White Frank F.
Published: May 2007

Induction of peroxidase has been correlated with resistant interactions between rice and Xanthomonas oryzae pv. oryzae. To assist in analysis of the role of rice peroxidases in plant defense against the bacterial pathogen, three peroxidase genes, POX22.3, POX8.1, and POX5.1, were identified from a rice cDNA library that was constructed from leaves of plants undergoing a resistant reaction. These genes were highly similar in nucleic acid and amino acid sequences and belonged to a gene family. The three genes showed differential expression in infiltrated rice leaves during pathogen interactions and mechanical stress. Only two peroxidase genes, POX8.1 and POX22.3, were predominantly expressed during resistant interactions. These two genes also were expressed during susceptible interactions, but induction was delayed compared with resistant interactions. POXgX9, a fourth peroxidase gene that was isolated from a genomic library, is adjacent to POX22.3 in the rice genome and has greater than 90% similarity in nucleotide and amino acid sequence identity to POX22.3. Interestingly, POXgX9 was expressed only in the roots of rice plants. While POX22.3 was expressed in both leaves and roots, POX8.1 and POX5.1 were not detected in roots but were induced in leaves by mechanical wounding at different times after treatment. POX22.3, POX8.1, and POX5.1 were estimated to be present in single copies in rice haploid genome. These results indicate that different members of the rice peroxidase gene family are distinctly regulated in response to various environmental cues.
Nod Factors from Sinorhizobium saheli and S. teranga bv. sesbaniae Are Both Arabinosylated and Fucosylated, a Structural Feature Specific to Sesbania rostrata Symbionts
Authors: Lorquin Jean , Lortet Gilles , Ferro Myriam , Méar Nathalie , Dreyfus Bernard , Promé Jean-Claude , Boivin Catherine
Published: May 2007

Sesbania spp. can establish symbiotic interactions with rhizobia from diverse taxonomical origins, including the newly described Sinorhizobium saheli and S. teranga bv. sesbaniae, in addition to the Sesbania rostrata-specific symbiont Azorhizobium caulinodans. These different rhizobia exhibit a narrow host range, which is limited mainly to Sesbania spp. Nod factors from overproducing strains of S. saheli ORS611 were purified and their structures determined. Remarkably, the terminal reducing glucosamine of most compounds was found to bear both an arabinosyl group on C-3 and a fucosyl substitution on C-6. Other structural features are as follows: Nod factors are mainly chitopentameric compounds, N-methylated, O-carbamoylated and N-substituted either by a C18:1 or a C16:0 acyl chain at their nonreducing end. Nod factors from an overproducing strain of S. teranga bv. sesbaniae ORS604 were found to be identical to those of S. saheli on the basis of high-pressure liquid chromatography separation and liquid secondary ion mass spectrometry analysis. The Nod factor double glycosylation, i.e., arabinosylation and fucosylation, which appears to be unique to Sesbania symbionts, probably reflects a high selection pressure from Sesbania host plants.
Resistance to Tobacco Etch Virus in Capsicum annuum: Inhibition of Virus RNA Accumulation
Authors: Deom C. Michael , Murphy John F. , Paguio Onofre R.
Published: May 2007

Resistance to tobacco etch potyvirus (TEV) in Capsicum annuum cv. Dempsey gene eta . When Dempsey pepper plants were inoculated with either of two TEV isolates, virus did not accumulate in inoculated or noninoculated leaves. In contrast, each TEV isolate replicated to high levels in C. annuum cv. Jupiter, a susceptible parent of Dempsey pepper. To determine the basis of the resistance, Dempsey and Jupiter pepper plants were inoculated with a TEV isolate (TEV-HAT) engineered to express the β-glucuronidase gene (TEV-GUS). TEV-GUS moved cell-to-cell and systemically in infected Jupiter pepper plants. In contrast, there was no evidence of any single-cell infection foci induced by TEV-GUS in Dempsey pepper plants. Northern blot analysis of protoplasts inoculated with TEV-HAT RNA indicated that virus RNA did not accumulate in Dempsey pepper protoplasts, whereas TEV-HAT did replicate and accumulate in protoplasts from Jupiter pepper plants. Thus, the resistance to TEV conferred by the eta gene is due to interference with virus RNA accumulation.
A Biotin-Regulated Locus, bioS, in a Possible Survival Operon of Rhizobium meliloti
Authors: Streit Wolfgang R. , Philips Donald A.
Published: May 2007

A novel biotin-inducible DNA locus, bioS, was found in Rhizobium meliloti 1021 by mutation with a transposable promoter probe. The promoterless nptII reporter responded to 40 nM biotin with a sixfold increase in NPTII protein. Stationary-phase mutant cells accumulated 50% more 14C-biotin than corresponding wild-type cells. The prototrophic mutant competed equally with the wild type in biotin-free medium but poorly when 40 nM biotin was present. The 588-bp bioS is located among three genes showing homology to survival operons of other bacteria, and it may be part of a system that R. meliloti uses to respond to plant biotin signals.
A Transgenic Mutant ofLactuca sativa(Lettuce) with a T-DNA Tightly Linked to Loss of Downy Mildew Resistance
Authors: Okubara Patricia A. , Arroyo-Garcia Rosa , Shen Katherine A. , Mazier Marianne , Meyers Blake C. , Ochoa Oswaldo E. , Kim Shinje , Yang Chang-Hsien , Michelmore Richard W.
Published: May 2007

One hundred and ninety-two independent primary transformants of lettuce cv. Diana were obtained by co-cultivation with Agrobacterium tumefaciens carrying constructs containing maize Ac transposase and Ds. R2 families were screened for mutations at four genes (Dm) for resistance to downy mildew. One family, designated dm3t524, had lost resistance to an isolate of Bremia lactucae expressing the avirulence gene Avr3. Loss of resistance segregated as a single recessive allele of Dm3. The mutation was not due to a large deletion as all molecular markers flanking Dm3 were present. Loss of Dm3 activity co-segregated with a T-DNA from which Ds had excised. Genomic DNA flanking the right border of this T-DNA was isolated by inverse polymerase chain reaction. This genomic sequence was present in four to five copies in wild-type cv. Diana. One copy was missing in all eight deletion mutants of Dm3 and altered in dm3t524, indicating tight physical linkage to Dm3. Three open reading frames (ORFs) occurred in a 6.6-kb region flanking the insertion site; however, expression of these ORFs was not detected. No similarities were detected between these ORFs and resistance genes cloned from other species. Transgenic complementation with 11-to 27-kb genomic fragments of Diana spanning the insertion site failed to restore Dm3 function to two ethyl methanesulfonate (EMS)-induced mutants of Dm3 or to cv. Cobham Green, which naturally lacks Dm3 activity. Therefore, either the T-DNA inserted extremely close to, but not within, Dm3 and the mutation may have been caused by secondary movement of Ds, or Dm3 activity is encoded by a gene extending beyond the fragments used for complementation.
Phenotypic Variation in Transgenic Tobacco Expressing Mutated Geminivirus Movement/Pathogenicity (BC1) Proteins
Authors: Duan Yong-Ping , Powell Charles A. , Purcifull Dan E. , Broglio Peter , Hiebert Ernest
Published: May 2007

Tobacco plants were transformed with the movement protein (pathogenicity) gene (BC1) from tomato mottle gem-inivirus (TMoV), using Agrobacterium-mediated transformation. Different transgenic tobacco lines that expressed high levels of the BC1 protein had phenotypes ranging from plants with severe stunting and leaf mottling (resembling geminivirus symptoms) to plants with no visible symptoms. The sequence data for the BC1 transgene from the transgenic plants with the different phenotypes indicated an association of spontaneously mutated forms of the BC1 gene in the transformed tobacco with phenotype variations. One mutated transgene associated with an asymptomatic phenotype had a major deletion at the C terminus of 119 amino acid residues with a recombination resulting in the addition of 26 amino acid residues of unidentified origin. This asymptomatic, mutated BC1 attenuated the phenotypic expression of the symptomatic BC1 in a tobacco line containing both copies of the BC1 gene. Another mutated form of the BC1 gene amplified from an asymptomatic, multicopy transgenic tobacco plant did not induce symptoms when transiently expressed in tobacco via a virus vector. The symptom attenuation in the transgenic tobacco by the asymptomatic BC1 may involve trans-dominant negative interference.
A Two-Component System Plays an Important Role in the Root-Colonizing Ability of Pseudomonas fluorescens Strain WCS365
Authors: Dekkers Linda C. , Bloemendaal Cees Jan P. , de Weger Letty A. , Wijffelman Carel A. , Spaink Herman P. , Lugtenberg Ben J. J.
Published: May 2007

We describe the characterization of a novel Tn5lacZ colonization mutant of the efficiently colonizing Pseudomonas fluorescens strain WCS365, mutant strain PCL1210, which is at least 300- to 1,000-fold impaired in colonization of the potato root tip after co-inoculation of potato stem cuttings with a 1:1 mixture of mutant and parental cells. Similarly, the mutant is also impaired in colonization of tomato, wheat, and radish, indicating that the gene involved plays a role in the ability of P. fluorescens WCS365 to colonize a wide range of plant species. A 3.1-kb DNA fragment was found to be able to complement the observed mutation. The nucleotide sequence of the region around the Tn5lacZ insertion showed three open reading frames (ORFs). The transcriptional start site was determined. The operon is preceded by an integration host factor (IHF) binding site consensus sequence whereas no clear −10 and −35 sequences are present. The deduced amino acid sequences of the first two genes of the operon, designated as colR and colS, show strong similarity with known members of two-component regulatory systems. ColR has homology with the response regulators of the OmpR-PhoB subclass whereas ColS, the product of the gene in which the mutation resides, shows similarity to the sensor kinase members of these two-component systems. Hydrophobicity plots show that this hypothetical sensor kinase has two transmembrane domains, as is also known for other sensor kinases. The product of the third ORF, Orf222, shows no homology with known proteins. Only part of the orf222 gene is present in the colonization-complementing, 3.1-kb region, and it therefore does not play a role in complementation. No experimental evidence for a role of the ColR/ ColS two-component system in the suspected colonization traits chemotaxis and transport of exudate compounds could be obtained. The function of this novel two-component system therefore remains to be elucidated. We conclude that colonization is an active process in which an environmental stimulus, through this two-component system, activates a so far unknown trait that is crucial for colonization.
The Earlyenod2 and the Leghemoglobin (lbc3) Genes Segregate Independently from Other Known Soybean Symbiotic Genes
Authors: Ghassemi Farshid , Gresshoff Peter M.
Published: May 2007

Recombinant inbred lines (RILs) as well as an F2segregating population of soybean Glycine max facilitated the mapping of two expressed sequence tags involved in early nodulation and subsequent nitrogen fixation in soybean. For the early nodulin gene enod2, the parents of RILs, Minsoy and Noir1, showed a polymorphism (5.5 vs 5.9 kb) after EcoRV digestion. Restriction fragment length polymorphism (RFLP) patterns of 42 RILs were analyzed with the MAPMAKER program, linking enod2 to the seed coat color gene, I, with a distance of 11.1 cM on linkage group U3 of RIL map. enod2 and I are located close to Rhg4, a soybean cyst nematode resistance gene, and a locus for seed coat hardness. The molecular marker pA-110 and seed coat color were used to integrate enod2 on an F2segregating population (72 plants) generated from a cross between cv. Bragg and G. soja PI468.397. enod2 was mapped in the same order as on the RIL map but 18.5 cM from the I locus on the TN map. A microsatellite from the 5′ region of enod2b was mapped in the same position, demonstrating that enod2b and not enod2a was mapped. An RFLP for lbc3 (leghemoglobin) segregated independently from enod2 and the nts-1 supernodulating locus suggesting that in soybean symbiotically significant loci (including rj1, Rj2, and rj6) are not clustered.
Cell Defense Responses Associated with Localized and Systemic Resistance to Phytophthora parasitica Induced in Tomato by an Arbuscular Mycorrhizal Fungus
Authors: Cordier C. , Pozo M. J. , Barea J. M. , Gianinazzi S. , Gianinazzi-Pearson V.
Published: May 2007

The arbuscular mycorrhizal fungus Glomus mosseae is able to confer bioprotection against Phytophthora parasitica in tomato roots. Localized and induced systemic resistance (ISR) have been demonstrated to be involved in pathogen control in mycorrhizal and nonmycorrhizal roots with a split root experimental system. Decreased pathogen development in mycorrhizal and nonmycorrhizal parts of mycorrhizal root systems is associated with accumulation of phenolics and plant cell defense responses. G. mosseae-containing cortical cells in the mycorrhizal tissues are immune to the pathogen and exhibit a localized resistance response with the formation of cell wall appositions reinforced by callose adjacent to intercellular hyphae. The systemically induced resistance in nonmycorrhizal root parts is characterized by elicitation of host wall thickenings containing non-esterified pectins and PR-1a protein in reaction to intercellular pathogen hyphae, and by the formation of callose-rich encasement material around P. parasitica hyphae that are penetrating root cells. PR-la protein is detected in the pathogen wall only in these tissues. None of these cell reactions are observed in nonmycorrhizal pathogen-infected root systems, where disease development leads to host cell death. The cellular and molecular basis of bioprotection by an arbuscular mycorrhizal fungus is discussed in relation to that induced by other nonpathogenic microorganisms.
nec1, a Gene Conferring a Necrogenic Phenotype, Is Conserved in Plant-Pathogenic Streptomyces spp. and Linked to a Transposase Pseudogene
Authors: Bukhalid Raghida A. , Chung Soo Young , Loria Rosemary
Published: May 2007

We are investigating the genetic basis for, and evolution of, plant pathogenicity in Streptomyces spp. The plant-pathogenic species S. scabies, S. acidiscabies, and S. turgidiscabies cause the scab disease of potato and produce the phytotoxins, thaxtomins. Forty-three Streptomyces strains representing the three species were evaluated; all thaxtomin A-producing Streptomyces strains were pathogenic on potato tubers and all but one hybridized to nec1 and ORFtnp, two genes previously cloned from S. scabies ATCC 41973. nec1 confers a pathogenic phenotype on S. lividans TK24, a nonpathogen, and ORFtnp is a transposase pseudogene located 5′ to nec1. The eight nonpathogenic strains tested neither produced thaxtomin A nor hybridized to nec1. ORFtnp and nec1 occurred on a single PvuII restriction fragment in all thaxtomin A-producing Streptomyces strains. The nucleotide sequences of the homologs of nec1 and ORFtnp from two pathogenic strains each of S. scabies, S. acidiscabies, and S. turgidiscabies were identical; oligonucleotide primers specific to this gene amplified homologs from all strains that hybridized to nec1. We propose that nec1 and ORFtnp have been horizontally mobilized from S. scabies to S. acidiscabies and S. turgidiscabies, and that nec1 is involved in pathogenicity and physically linked to the thaxtomin A biosynthetic genes.
Roles for Azorhizobial Nod Factors and Surface Polysaccharides in Intercellular Invasion and Nodule Penetration, Respectively
Authors: D'Haeze Wim , Gao Mengsheng , De Rycke Riet , Van Montagu Marc , Engler Gilbert , Holsters Marcelle
Published: May 2007

In the symbiotic interaction between Azorhizobium caulinodans and Sesbania rostrata root and stem-borne nodules are formed. The bacteria enter the host via intercellular spaces at lateral or adventitious root bases and form infection pockets in outer cortical layers. Infection threads guide the bacteria to nodule primordia where plant cells are invaded. To identify bacterial functions that are required for this infection process, two mutants defective in nodulation were studied; one produced no Nod factors (nodA mutant), the other had altered surface polysaccharides (SPS) and induced the formation of pseudo-nodules. Bacteria were visualized with the help of a nodA-uidA reporter fusion that was functional during nodule development and in bacteroids. In contrast to the SPS mutant, nodA mutants were unable to colonize outer cortical regions. In mixed inoculations with both mutants, functional nodules were formed, the central tissue of which was occupied by the nodA mutant. These observations suggest that SPS play a role in deeper invasion and that Nod factors are necessary for entry. Simultaneous application of purified Nod factors and nodA mutant bacteria restored the formation of outer cortical infection pockets leading to the conclusion that intercellular infection is an active process that is dependent on bacterial Nod factor signaling.
Conservation of noIR in the Sinorhizobium and Rhizobium Genera of the Rhizobiaceae Family
Authors: Kiss Ernö , Mergaert Peter , Olàh Boglàrka , Kereszt Attila , Staehelin Christian , Davies Andrea E. , Downie J. Allan , Kondorosi Adam , Kondorosi Eva
Published: May 2007

In Sinorhizobium meliloti the NolR repressor displays differential negative regulation of nodulation genes and is required for optimal nodulation. Here, we demonstrate that the NolR function is not unique to S. meliloti but is also present in other species of the Rhizobiaceae family. DNA hybridization indicates the presence of nolR homologous sequences in species belonging to the Rhizobium and Sinorhizobium genera while no hybridization signal was detected in species from the Mesorhizobium, Bradyrhizo-bium, Azorhizobium, and Agrobacterium genera. We isolated the nolR gene from the Rhizobium leguminosarum bv. viciae strain TOM and showed that the TOM nolR gene acts similarly to S. meliloti nolR by repressing the expression of both the nodABCIJ and the nodD genes, resulting in decreased Nod factor production. The presence of a functional nolR gene in R. leguminosarum is correlated with an increased rate and extent of nodulation of pea. The conserved primary structure, the location of the DNA-binding domain, and the similar size of NolR proteins, compared with a family of small bacterial regulatory proteins including HlyU, SmtB, and the ArsR-type regulators, revealed that NolR belongs to this family.
Differential Interactions of Bamboo Mosaic Potexvirus Satellite RNAs, Helper Virus, and Host Plants
Authors: Hsu Yau-Heiu , Lee Yun-Shien , Liu Jih-Shiou , Lin Na-Sheng
Published: May 2007

Bamboo mosaic potexvirus (BaMV) supports the replication and encapsidation of a single-stranded satellite RNA (satBaMV). Full-length infectious cDNA clones of sat-BaMV variants with a 0.7 to 7.5% difference in nucleotide sequence were constructed, and in vitro transcripts were co-inoculated with BaMV RNA to barley protoplasts. Co-inoculation with BSL6 satBaMV transcripts resulted in a greatly reduced accumulation of genomic and subgenomic RNAs whereas co-inoculation with the prototype BSF4 or other variants did not. satBaMV variant BSL6-mediated interference occurred at the genomic RNA, subgenomic RNA, and capsid protein levels whereas the synthesis of minus strand was not significantly affected. Attenuation of the BaMV symptoms also was observed in Chenopodium quinoa and Nicotiana benthamiana plants co-inoculated with BSL6 satBaMV. In protoplasts of barley and N. benthamiana, BSF4 and BSL6 variants replicated at a similar level whereas in the inoculated leaves of C. quinoa and N. benthamiana BSL6 accumulated to a much lower level than BSF4, and was encapsidated. BSL6 satBaMV was rarely detected in the systemic leaves of N. benthami-ana co-inoculated with BaMV RNA, indicating that BSL6 satBaMV may be a movement-inefficient variant. It is suggested that this variant may provide a potential source for both resistance against BaMV and insights into the interactions of satellite RNA, helper virus, and symptom expression.
Role of Exopolysaccharides of Rhizobium leguminosarum bv. viciae as Host Plant-Specific Molecules Required for Infection Thread Formation During Nodulation of Vicia sativa
Authors: van Workum Wilbert A. T. , van Slageren Sophie , van Brussel Anton A. N. , Kijne Jan W.
Published: May 2007

Mutants of Rhizobium leguminosarum bv. viciae bacteria that are affected in the biosynthesis of exopolysaccharides (EPS) are unable to effectively nodulate their host plants. By studying defined mutants, we show that R. legumi-nosarum bv. viciae strains require EPS for formation of infection threads in Vicia sativa (vetch) as well as for efficient induction of tight root hair curling. Results of coinoculation experiments with the EPS-deficient pssD111 mutant of R. leguminosarum bv. viciae in combination with heterologous EPS-producing strains indicated that vetch has certain structural requirements for rhizobial EPS to function in symbiosis. We hypothesize that EPS accelerates root hair curling and infection to such an extent that rhizobial root penetration precedes a plant defense response.
The Bundle Sheath-Phloem Interface of Cucumis sativus Is a Boundary to Systemic Infection by Tomato Aspermy Virus
Authors: Thompson Jeremy R. , García-Arenal Fernando
Published: May 2007

The progress of infection of two cucumoviruses in cucumber plants was analyzed immunohistochemically. Strain Fny of cucumber mosaic virus (CMV, FFF) was found to infect cucumber tissues systemically by 6 days postinoculation (dpi), while a reassortant virus with RNAs 1+2 of Fny-CMV plus RNA3 of strain 1 of tomato aspermy virus (FFT) was unable to move long distance and infect cucumber plants systemically. FFF infection of the vasculature was detected 6 dpi in the phloem of a low percentage of both minor (order VII–VI) and major (order V–IV) veins. At 9 dpi, infection was detected in phloem cells of about 50% of both minor and major veins. FFT colonization of inoculated cotyledons followed a pathway similar to that of FFF, but virus accumulation was never detected in vascular tissues. In minor or major veins, FFT infection was arrested at the bundle sheath (BS), and at 9 dpi was not detected in intermediary or other phloem cells. Thus, our data indicate that the BS-phloem interface is a boundary for the systemic movement of these viruses in cucumber, and provide evidence of a functional difference between the plasmodesmata connecting mesophyll cells, as well as mesophyll and BS cells, which allow the movement of both FFF and FFT, from the plasmodesmata connecting BS and phloem, which allow movement of FFF but not of FFT.
Potyvirus Genome-Linked Protein (VPg) Determines Pea Seed-Borne Mosaic Virus Pathotype-Specific Virulence in Pisum sativum
Authors: Keller Karen E. , Johansen Elisabeth , Martin Robert R. , Hampton R. O.
Published: May 2007

The mechanism of Pisum sativum pathotype-specific resistance to pea seed-borne mosaic potyvirus (PSbMV) was investigated and the coding region determinant of PSbMV virulence was defined. Homozygous recessive sbm-1 peas are unable to support replication of PSbMV pathotype 1 (P-1), whereas biochemically and serologically related pathotype 4 (P-4) is fully infectious in the sbm-1/sbm-1 genotype. We were unable to detect viral coat protein or RNA with double antibody sandwich-enzyme-linked im-munosorbent assay and reverse transcription-polymerase chain reaction in sbm-1/sbm-1 P-1-inoculated protoplasts and plants. Lack of viral coat protein or RNA in P-1 trans-fected sbm-1/sbm-1 protoplasts suggests that sbm-1 resistance is occurring at the cellular level and that inhibition of cell-to-cell virus movement is not the operating form of resistance. In addition, because virus products were not detected at any time post-inoculation, resistance must either be constitutive or expressed very early in the virus infection process. P-1-resistant peas challenged with full-length, infectious P-1/P-4 recombinant clones demonstrated that a specific P-4 coding region, the 21-kDa, genome-linked protein (VPg), was capable of overcoming sbm-1 resistance, whereas clones containing the P-1 VPg coding region were noninfectious to sbm-1/sbm-1 peas. VPg is believed to be involved in potyvirus replication and its identification as the PSbMV determinant of infectivity in sbm-1/sbm-1 peas is consistent with disruption of an early P-1 replication event.
Natural Plant Protection by 2,4-Diacetylphloroglucinol-Producing Pseudomonas spp. in Take-All Decline Soils
Authors: Raaijmakers Jos M. , Weller David M.
Published: May 2007

Take-all decline (TAD) is a natural biological control of the wheat root disease “take-all” that develops in response to the disease during extended monoculture of wheat. The research to date on TAD has been mostly descriptive and no particular occurrence is yet fully understood. We demonstrate that root-associated fluorescent Pseudomonas spp. producing the antibiotic 2,4-diacetylphloroglucinol (Phl) are key components of the natural biological control that operates in TAD soils in Washington State (U.S.A.). Phl-producing Pseudomonas spp. were present on roots of wheat grown in TAD soils at or above the threshold population density required for significant suppression of take-all of wheat. The specific suppression that operates in TAD soils was lost when Phl-producing fluorescent Pseudomonas spp. were eliminated, and conducive soils gained suppressiveness to take-all when Phl-producing Pseudomonas strains were introduced via mixing in small amounts of TAD soil. Introduction of selected Phl-producing strains into take-all conducive soils provided control of take-all of wheat to a level similar to that obtained in the complementary TAD soils.
Host Adaptation and Replication Properties of Two Bipartite Geminiviruses and Their Pseudorecombinants
Authors: Hou Yu-Ming , Paplomatas Epaminondas J. , Gilbertson Robert L.
Published: May 2007

To investigate factors involved in host adaptation and specificity of bipartite geminiviruses, the infectivity of bean dwarf mosaic (BDMV) and tomato mottle (ToMoV) geminiviruses and the BDMV/ToMoV pseudorecombinants [BDMV DNA-A + ToMoV DNA-B (BA+TB) and ToMoV DNA-A + BDMV DNA-B (TA+BB)] in Phaseolus vulgaris, Lycopersicon esculentum, Nicotiana benthamiana, and N. tabacum plants was determined. Additionally, replication of these viruses was examined in protoplasts prepared from N. tabacum BY2 and Xanthi-nc cells. In adapted hosts and the permissive experimental host, N. benthamiana, BDMV and ToMoV infected nearly 100% of inoculated plants, induced severe symptoms, and had high levels of both DNA components. In nonadapted hosts, BDMV and ToMoV infected approximately 40% of inoculated plants, induced no symptoms, and had reduced levels of both DNA components. For the pseudorecombinants, symptoms were observed only in TA+BB-infected N. benthamiana and P. vulgaris plants. In the other pseudorecombinant/host combinations, symptomless infections were detected and some plants were infected with the DNA-A component only. Symptom development and/or higher infection rates for the pseudorecombinants were correlated with the host-adapted DNA-B component, and pseudorecombinant-infected plants had reduced levels of DNA-B. Protoplast replication assays revealed inefficient DNA-B replication for the pseudorecombinants, and differences in viral replication properties in the two N. tabacum cell lines.
The Early Nodulin GeneENOD2Shows Different Expression Patterns DuringSesbania rostrataStem Nodule Development
Authors: Goormachtig Sofie , Van Montagu Marc , Holsters Marcelle
Published: May 2007

During nodule development on stems of Sesbania rostrata, the ENOD2 gene was expressed in the nodule parenchyma and in outer cortical cells. The latter; novel expression pattern was low in uninfected nodulation sites, strongly enhanced 1 day after infection, and correlated with young peridermal cells at later stages. The induction of both ENOD2 transcript accumulation patterns was dependent on Nod factor-producing bacteria.
Genetic Mapping of Ph-2, a Single Locus Controlling Partial Resistance to Phytophthora infestans in Tomato
Authors: Moreau Philippe , Thoquet Philippe , Olivier Jocelyne , Laterrot Henri , Grimsley Nigel
Published: May 2007

Late blight caused by the fungal pathogen Phytophthora infestans is one of the most important diseases of potato (Solanum tuberosum) and tomato (Lycopersicon esculentum). Genetic analysis of the resistance to this pathogen was performed on an F2 progeny of 322 plants derived from a cross between the tomato line L. esculentum var. Hawaii7996 susceptible to late blight and the resistant wild relative L. pimpinellifolium WVa700. Qualitative and quantitative analyses of the resistance with restriction fragment length polymorphism markers spanning the genome showed that this resistance was controlled by a single, incompletely dominant allele, Ph-2, present on the distal part of the long arm of chromosome 10 in an interval of 8.4 cM flanked by markers CP105 and TG233. Genetic analysis of F2 progeny from a second cross between an L. esculentum introgression line IL10-3 carrying a homozygous L. pennellii segment spanning the distal part of the long arm of chromosome 10 and WVa700 confirmed the map location, but high suppression of recombination was observed in this cross in the introgressed fragment. A high-resolution genetic linkage map of the chromosomal region surrounding Ph-2 was initiated to permit future map-based cloning of this gene. Amplified fragment length polymorphism markers closely linked to Ph-2 were screened by bulked segregant analysis.
Dynamics of Bean Dwarf Mosaic Geminivirus Cell-to-Cell and Long-Distance Movement in Phaseolus vulgaris Revealed, Using the Green Fluorescent Protein
Authors: Sudarshana M. R. , Wang H. L. , Lucas W. J. , Gilbertson R. L.
Published: May 2007

The cell-to-cell and long-distance movement of the bipartite geminivirus, bean dwarf mosaic (BDMV), in Phaseolus vulgaris plants was examined with the noninvasive reporter, the green fluorescent protein (GFP). A modified GFP gene (mGFP4) was inserted into the BDMV DNA-A component in place of the coat protein gene (BDMVA-mGFP4), and particle bombardment was used to introduce viral DNA into bean seedlings (radicle and hypocotyl tissues). Fluorescence analysis of GFP expressed from BDMVA-mGFP4 established that particle bombardment introduced viral DNA only into epidermal cells, and the requirement for the DNA-B-encoded proteins (BV1 and BC1) in the cell-to-cell movement of BDMVA-mGFP4. This GFP reporter system was used to follow the viral infection process from the seedling stage throughout the entire plant life cycle. In inoculated hypocotyls, BDMV moved from cell to cell through the cortex and showed a striking phloem tropism. Upon entry into phloem tissues, BDMV moved rapidly toward the root via the long-distance transport system, and toward the shoot apex by a combination of cell-to-cell and long-distance movement. Analysis of GFP distribution in systemically infected tissues revealed that BDMV was restricted to phloem cells in both roots and stems. In systemically infected primary and trifoliolate leaves, BDMV infected phloem cells associated with all vein orders (first through fifth), and the capacity of BDMV to exit from phloem tissue into nonphloem cells was correlated with the stage of plant development. Finally, fluorescence analysis of GFP in reproductive tissues established that BDMV infected flower, pod, and seed-coat tissues, but was excluded from the embryo.
Tissue-Specific Superoxide Generation at Interaction Sites in Resistant and Susceptible Near-Isogenic Barley Lines Attacked by the Powdery Mildew Fungus (Erysiphe graminis f. sp. hordei)
Authors: Hückelhoven Ralph , Kogel Karl-Heinz
Published: May 2007

The pathogenesis-related, azide-insensitive generation of superoxide anions (O2 -) was comparatively analyzed in near-isogenic barley (Hordeum vulgare cv. Pallas) lines carrying the powdery mildew (Erysiphe graminis f. sp. hordei) resistance genes Mla12, Mlg, and mlo5, respectively, by the microscopic detection of nitroblue tetra-zolium (NBT) reduction to dark blue formazan dyes. These genes govern fungal arrest at different stages of the interaction: (i) at the penetration stage within cell wall appositions (papillae) leaving the attacked cell alive (mlo); (ii) within papillae of cells that subsequently undergo a hypersensitive cell death (HR) (Mlg); or (iii) after penetration by a subsequent HR (Mla12). The susceptible parent line Pallas showed a transient O2 - generation in penetrated epidermal cells at 18 h after inoculation (hai), whereas epidermal cells of the resistant BCPMla12 produced O2 - over a longer time range (by 18 to 36 hai) preceding cell death. No oxidative burst was detected in association with penetration resistance due to effective papillae (BCPMlg and BCPmlo5) although Mlg specified an HR subsequent to fungal arrest. Hence, O2 - generation in attacked epidermal cells was a result of fungal penetration of the host cell walls and subsequent contact with the host plasma membrane, and not a requirement for HR elicitation. O2 - generation in the mesophyll tissue beneath attacked cells was associated with the response mediated by the genes Mla12 and Mlg. However, only BCPMla12 showed mesophyll cell death. The data indicate that, in barley, O2 - accumulation is not a single key determinant of HR in response to a powdery mildew attack.
Maackiain Detoxification Contributes to the Virulence of Nectria haematococca MP VI on Chickpea
Authors: Enkerli Jürg , Bhatt Garima , Covert Sarah F.
Published: May 2007

Nectria haematococca mating population (MP) VI isolates that contain the MAK1 gene are able to degrade maackiain, a chickpea (Cicer arietinum) phytoalexin, to a less toxic compound. To test the contribution of MAK1 to the virulence of N. haematococca MP VI on chickpea, the MAK1 gene was disrupted in a highly virulent Mak+ isolate or added to a weakly virulent Mak- isolate via transformation. The disruption of MAK1 decreased virulence to a moderate level, while addition of multiple copies of MAK1 increased virulence to either a moderate or a high level. These data demonstrate that maackiain detoxification is a determinant, but not the only determinant, of virulence in N. haematococca MP VI isolates capable of causing disease on chickpea. MAK1 is located on a 1.6-Mb conditionally dispensable chromosome. To ascertain if there are additional genes influencing virulence toward chickpea stems on the MAK1 chromosome, the loss of this chromosome was chemically induced in an isolate containing the disrupted MAK1 gene. Loss of the MAK1 chromosome did not reduce virulence toward chickpea stems further, thus indicating that no additional genes for virulence on this part of the host plant are located on the MAK1 chromosome.