bluuebunny/crossref_metadata_2025 huggingface dataset:
Relationship Between Avirulence Gene (avrA) Diversity in Ralstonia solanacearum and Bacterial Wilt Incidence
Authors: Robertson Alison E. , Wechter W. Patrick , Denny Timothy P. , Fortnum Bruce A. , Kluepfel Daniel A.
Published: May 2007

Bacterial wilt, caused by Ralstonia solanacearum, is a serious disease of tobacco in North and South Carolina. In contrast, the disease rarely occurs on tobacco in Georgia and Florida, although bacterial wilt is a common problem on tomato. We investigated whether this difference in disease incidence could be explained by qualitative characteristics of avirulence gene avrA in the R. solanacearum population in the southeastern United States. Sequence analysis established that wild-type avrA has a 792-bp open reading frame. Polymerase chain reaction (PCR) amplification of avrA from 139 R. solanacearum strains generated either 792-bp or ≈960-bp DNA fragments. Strains that elicited a hypersensitive reaction (HR) on tobacco contained the 792-bp allele, and were pathogenic on tomato and avirulent on tobacco. All HR-negative strains generated a ≈960-bp DNA fragment, and wilted both tomato and tobacco. The DNA sequence of avrA in six HR-negative strains revealed the presence of one of two putative miniature inverted-repeat transposable elements (MITEs): a 152-bp MITE between nucleotides 542 and 543, or a 170-bp MITE between nucleotides 461 and 462 or 574 and 575. Southern analysis suggested that the 170-bp MITE is unique to strains from the southeastern United States and the Caribbean. Mutated avrA alleles were present in strains from 96 and 75% of North and South Carolina sites, respectively, and only in 13 and 0% of the sites in Georgia and Florida, respectively. Introduction of the wild-type allele on a plasmid into four HR-negative strains reduced their virulence on both tobacco and tomato. Inactivation of avrA in an HR-positive, avirulent strain, resulted in a mutant that was weakly virulent on tobacco. Thus, the incidence of bacterial wilt of tobacco in the southeastern United States is partially explained by which avrA allele dominates the local R. solanacearum population.
Analysis of Erwinia chrysanthemi EC16 pelE∷uidA, pelL∷uidA, and hrpN∷uidA Mutants Reveals Strain-Specific Atypical Regulation of the Hrp Type III Secretion System
Authors: Ham Jong Hyun , Cui Yaya , Alfano James R. , Rodríguez-Palenzuela Pablo , Rojas Clemencia M. , Chatterjee Arun K. , Collmer Alan
Published: May 2007

The plant pathogen Erwinia chrysanthemi produces a variety of factors that have been implicated in its ability to cause soft-rot diseases in various hosts. These include HrpN, a harpin secreted by the Hrp type III secretion system; PelE, one of several major pectate lyase isozymes secreted by the type II system; and PelL, one of several secondary Pels secreted by the type II system. We investigated these factors in E. chrysanthemi EC16 with respect to the effects of medium composition and growth phase on gene expression (as determined with uidA fusions and Northern analyses) and effects on virulence. pelE was induced by polygalacturonic acid, but pelL was not, and hrpN was expressed unexpectedly in nutrient-rich King's medium B and in minimal salts medium at neutral pH. In contrast, the effect of medium composition on hrp expression in E. chrysanthemi CUCPB1237 and 3937 was like that of many other phytopathogenic bacteria in being repressed in complex media and induced in acidic pH minimal medium. Northern blot analysis of hrpN and hrpL expression by the wild-type and hrpL∷ΩCmr and hrpS∷ΩCmr mutants revealed that hrpN expression was dependent on the HrpL alternative sigma factor, whose expression, in turn, was dependent on the HrpS putative σ54 enhancer binding protein. The expression of pelE and hrpN increased strongly in late logarithmic growth phase. To test the possible role of quorum sensing in this expression pattern, the expI/expR locus was cloned in Escherichia coli on the basis of its ability to direct production of acyl-homoserine lactone and then used to construct expI mutations in pelE∷uidA, pelL∷uidA, and hrpN∷uidA Erwinia chrysanthemi strains. Mutation of expI had no apparent effect on the growth-phase-dependent expression of hrpN and pelE, or on the virulence of E. chrysanthemi in witloof chicory leaves. Overexpression of hrpN in E. chrysanthemi resulted in approximately 50% reduction of lesion size on chicory leaves without an effect on infection initiation.
Transcriptional Responses of Paxillus involutus and Betula pendula During Formation of Ectomycorrhizal Root Tissue
Authors: Johansson Tomas , Le Quéré Antoine , Ahren Dag , Söderström Bengt , Erlandsson Rikard , Lundeberg Joakim , Uhlén Mathias , Tunlid Anders
Published: May 2007

In order to obtain information on genes specifically expressed in the ectomycorrhizal symbiosis, 3,555 expressed sequence tags (ESTs) were analyzed from a cDNA library constructed from ectomycorrhiza formed between the basidiomycete Paxillus involutus and birch (Betula pendula). cDNA libraries from saprophytically growing fungus (3,964 ESTs) and from axenic plants (2,532 ESTs) were analyzed in parallel. By clustering all the EST obtained, a nonredundant set of 2,284 unique transcripts of either fungal or plant origin were identified. The expression pattern of these genes was analyzed using cDNA microarrays. The analyses showed that the plant and fungus responded to the symbiosis by altering the expression levels of a number of enzymes involved in carbon metabolism. Several plant transcripts with sequence similarities to genes encoding enzymes in the tricarboxylic cycle and electron transport chain were down regulated as compared with the levels in free-living roots. In the fungal partner, a number of genes encoding enzymes in the lipid and secondary metabolism were down regulated in mycorrhiza as compared with the saprophytically growing mycelium. A substantial number of the ESTs analyzed displayed significant sequence similarities to proteins involved in biotic stress responses, but only a few of them showed differential expression in the mycorrhizal tissue, including plant and fungal metal-lothioneins and a plant defensin homologue. Several of the genes that were differentially expressed in the mycorrhizal root tissue displayed sequence similarity to genes that are known to regulate growth and development of plant roots and fungal hyphae, including transcription factors and Rho-like GTPases.
Cell Surface Interactions of Rhizobium Bacteroids and Other Bacterial Strains with Symbiosomal and Peribacteroid Membrane Components from Pea Nodules
Authors: Bolaños Luis , Redondo-Nieto Miguel , Rivilla Rafael , Brewin Nicholas J. , Bonilla Ildefonso
Published: May 2007

Samples of Rhizobium bacteroids isolated from pea nodule symbiosomes reacted positively with a monoclonal antibody recognizing N-linked glycan epitopes on plant glycoproteins associated with the peribacteroid membrane and peribacteroid fluid. An antiserum recognizing the symbiosomal lectin-like glycoprotein PsNLEC-1 also reacted positively. Samples of isolated bacteroids also reacted with an antibody recognizing a glycolipid component of the peribacteroid membrane and plasma membrane. Bacterial cells derived from free-living cultures then were immobilized on nitrocellulose sheets and tested for their ability to associate with components of plant extracts derived from nodule fractionation. A positive antibody-staining reaction indicated that both PsNLEC-1 and membrane glycolipid had become associated with the bacterial surface. A range of rhizobial strains with mutants affecting cell surface polysaccharides all showed similar interactions with PsNLEC-1 and associated plant membranes, with the exception of strain B659 (a deep-rough lipopolysaccharide mutant of Rhizobium leguminosarum). However, the presence of a capsule of extracellular polysaccharide apparently prevented interactions between rhizobial cells and these plant components. The importance of a close association between peribacteroid membranes, PsNLEC-1, and the bacterial surface is discussed in the context of symbiosome development.
Satellite Panicum Mosaic Virus Capsid Protein Elicits Symptoms on a Nonhost Plant and Interferes with a Suppressor of Virus-Induced Gene Silencing
Authors: Qiu Wenping , Scholthof Karen-Beth G.
Published: May 2007

The capsid protein (CP) of satellite panicum mosaic virus (SPMV) has been implicated as a pathogenicity factor, inducing severe chlorosis on millet plants co-infected with SPMV and its helper virus, Panicum mosaic virus (PMV). In this study, we tested the effects of SPMV CP on Nicotiana benthamiana, a plant that does not support PMV+SPMV infections. SPMV CP expressed from a Potato virus X (PVX) gene vector elicited necrotic lesions on N. benthamiana. Pathogenicity factors often have the additional feature of acting as suppressors of gene silencing; therefore, several assays were developed to test if SPMV CP could act in such a capacity. The results showed that SPMV CP failed to act as a suppressor of posttranscriptional gene silencing when such tests were performed with transgenic N. benthamiana plants silenced for green fluorescent protein (GFP) expression by agroinfiltration or plant virus vectors. However SPMV CP expressed from the PVX gene vector did interfere with suppressor activity associated with PVX p25. This included a rebounded level of GFP silencing along the vascular tissues, including the veins on upper noninoculated leaves. Therefore, the roles of the SPMV CP now include encapsidation of the SPMV RNA, activity as a pathogenicity factor in both host and nonhost plants, and the enigmatic feature of interfering with suppression of gene silencing.
Superoxide and Hydrogen Peroxide Play Different Roles in the Nonhost Interaction of Barley and Wheat with Inappropriate formae speciales of Blumeria graminis
Authors: Trujillo Marco , Kogel Karl-Heinz , Hückelhoven Ralph
Published: May 2007

Nonhost resistance of cereals to inappropriate formae speciales of Blumeria graminis is little understood. However, on the microscopic level, nonhost defense to B. graminis is reminiscent of host defense preventing fungal development by penetration resistance and the hypersensitive cell death response (HR). We analyzed histochemically the accumulation of superoxide anion radicals (O2•¯) and hydrogen peroxide (H2O2) at sites of B. graminis attack in nonhost barley and wheat. Superoxide visualized by subcellular reduction of nitroblue tetrazolium accumulated in association with successful fungal penetration in attacked cells and in cells neighboring HR. In contrast, H2O2 accumulated in cell wall appositions beneath fungal penetration attempts or in the entire epidermal cell during HR. The data provide evidence for different roles and sources of superoxide and H2O2 in the nonhost interaction of cereals with inappropriate formae speciales of B. graminis.
Regulation of AHL Production and Its Contribution to Epiphytic Fitness in Pseudomonas syringae
Authors: Quiñones Beatriz , Pujol Catherine J. , Lindow Steven E.
Published: May 2007

Pseudomonas syringae forms large cell aggregates that are more stress tolerant than solitary cells during epiphytic growth on plants. The differential survival of aggregates on leaves suggests that epiphytic fitness traits may be controlled in a density-dependent manner via cell-cell signaling. We investigated this hypothesis in P. syringae B728a. Synthesis of N-acyl-homoserine lactone (AHL), 3-oxo-hexanoyl homo-serine lactone, and the expression of the gene encoding AHL synthase ahlI were maximal at high cell concentrations. The expression of the AHL regulator ahlR, in contrast, was similar at all cell concentrations. A screen of Tn5 mutants revealed that P. syringae B728a requires a novel transcriptional activator for AHL production. This regulator, which belongs to the TetR family, was also required for epiphytic fitness and has been designated AefR (for AHL and epiphytic fitness regulator). The expression of ahlI was greatly reduced in both aefR¯ and gacA¯ mutants and was completely restored in either mutant after addition of exogenous AHL. In contrast, the expression of aefR was not reduced in either gacS¯ or gacA¯ mutants. Thus, AefR appears to positively regulate AHL production independently of the regulators GacS/GacA and also controls traits in P. syringae B728a that are required for epiphytic colonization.
The Tobacco mosaic virus 126-kDa Protein Associated with Virus Replication and Movement Suppresses RNA Silencing
Authors: Ding Xin Shun , Liu Jianzhong , Cheng Ning-Hui , Folimonov Alexey , Hou Yu-Ming , Bao Yiming , Katagi Chika , Carter Shelly A. , Nelson Richard S.
Published: May 2007

Systemic symptoms induced on Nicotiana tabacum cv. Xanthi by Tobacco mosaic virus (TMV) are modulated by one or both amino-coterminal viral 126- and 183-kDa proteins: proteins involved in virus replication and cell-to-cell movement. Here we compare the systemic accumulation and gene silencing characteristics of TMV strains and mutants that express altered 126- and 183-kDa proteins and induce varying intensities of systemic symptoms on N. tabacum. Through grafting experiments, it was determined that MIC1,3, a mutant of the masked strain of TMV that accumulated locally and induced no systemic symptoms, moved through vascular tissue but failed to accumulate to high levels in systemic leaves. The lack of MIC1,3 accumulation in systemic leaves was correlated with RNA silencing activity in this tissue through the appearance of virus-specific, approximately 25-nucleotide RNAs and the loss of fluorescence from leaves of transgenic plants expressing the 126-kDa protein fused with green fluorescent protein (GFP). The ability of TMV strains and mutants altered in the 126-kDa protein open reading frame to cause systemic symptoms was positively correlated with their ability to transiently extend expression of the 126-kDa protein:GFP fusion and transiently suppress the silencing of free GFP in transgenic N. tabacum and transgenic N. benthamiana, respectively. Suppression of GFP silencing in N. benthamiana occurred only where virus accumulated to high levels. Using agro-infiltration assays, it was determined that the 126-kDa protein alone could delay GFP silencing. Based on these results and the known synergies between TMV and other viruses, the mechanism of suppression by the 126-kDa protein is compared with those utilized by other originally characterized suppressors of RNA silencing.
Bacterial Genes Involved in Type I Secretion and Sulfation Are Required to Elicit the Rice Xa21-Mediated Innate Immune Response
Authors: da Silva Francisco Goes , Shen Yuwei , Dardick Christopher , Burdman Saul , Yadav Ram C. , de Leon Alfredo Lopez , Ronald Pamela C.
Published: May 2007

Innate immunity to microorganisms relies on the specific sensing of pathogen-associated molecules by host recognition receptors. Whereas studies in animals have largely focused on the recognition of extracellular pathogen-associated molecules by the TLR (toll-like receptor) superfamily, few studies have been carried out in plants, and it is not understood how these molecules are secreted or modified. The rice Xa21 gene encodes a receptor-like kinase that provides immunity against strains of the bacterial pathogen Xanthomonas oryzae pv. oryzae carrying AvrXa21 activity. We identified four X. oryzae pv. oryzae genes that are required for AvrXa21 activity. raxA, raxB, and raxC encode proteins with similarity to a membrane fusion protein, an ATP-binding cassette transporter, and an outer membrane protein, respectively, of bacterial type I secretion systems. The fourth gene, raxST, encodes a sulfotransferase-like protein. Sequence analysis of three naturally occurring X. oryzae pv. oryzae strains no longer recognized by Xa21 revealed alterations in the raxST and raxA genes. The raxC gene complemented an Escherichia coli tolC mutant for secretion of a double glycine-leader peptide confirming the function of raxC in type I secretion. These results indicate that bacterial type I secretion is necessary for Xa21-mediated recognition and immunity and further suggest that type I secretion and modification of pathogen-associated molecules play an important role in triggering the innate immune response in rice.
The Arabidopsis TIR-NB-LRR Gene RAC1 Confers Resistance to Albugo candida (White Rust) and Is Dependent on EDS1 but not PAD4
Authors: Borhan Mohammad H. , Holub Eric B. , Beynon Jim L. , Rozwadowski Kevin , Rimmer S. Roger
Published: May 2007

Resistance to Albugo candida isolate Acem1 is conferred by a dominant gene, RAC1, in accession Ksk-1 of Arabidopsis thaliana. This gene was isolated by positional cloning and is a member of the Drosophila toll and mammalian interleukin-1 receptor (TIR) nucleotide-binding site leucine-rich repeat (NB-LRR) class of plant resistance genes. Strong identity of the TIR and NB domains was observed between the predicted proteins encoded by the Ksk-1 allele and the allele from an Acem1-susceptible accession Columbia (Col) (99 and 98%, respectively). However, major differences between the two predicted proteins occur within the LRR domain and mainly are confined to the β-strand/β-turn structure of the LRR. Both proteins contain 14 imperfect repeats. RAC1-mediated resistance was analyzed further using mutations in defense regulation, including: pad4-1, eds1-1, and NahG, in the presence of the RAC1 allele from Ksk-1. White rust resistance was completely abolished by eds1-1 but was not affected by either pad4-1 or NahG.
Accumulation of Lipochitin Oligosaccharides and NodD-Activating Compounds in an Efficient Plant-Rhizobium Nodulation Assay
Authors: Tak Teun , van Spronsen Paulina C. , Kijne Jan W. , van Brussel Anton A. N. , Boot Kees J. M.
Published: May 2007

During legume plant-Rhizobium spp. interactions, leading to the formation of nitrogen-fixing root nodules, the two ma-or determinants of host plant-specificity are plant-produced nod gene inducers (NodD protein activating compounds) and bacterial lipochitin oligosaccharides (LCOs or Nod factors). In a time course, we describe the accumulation of LCOs in an efficient nodulation assay with Vicia sativa subsp. nigra and Rhizobium leguminosarum, in connection with the presence of NodD-activating compounds in the exudate of V. sativa roots. Relatively small amounts of both LCOs and NodD-activating compounds were found to be required for initiation of nodulation during the first days after inoculation. A strong increase in the amount of NodRlv-V[18:4, Ac] LCOs preceded root infection and nodule primordium formation. In contrast to the situation with non-nodulating rhizobia and nonmitogenic LCOs, the amount of NodD-activating compounds in the culture medium remained small after addition of nodulating rhizobia or mitogenic LCOs. Furthermore, addition of nodulating rhizobia or mitogenic LCOs resulted in nearly complete inhibition of root hair formation and elongation, whereas nonmitogenic LCOs stimulated root hair growth. Retention of NodD-activating compounds in the root may inhibit root hair growth.
The Genome Sequence of the Gram-Positive Sugarcane Pathogen Leifsonia xyli subsp. xyli
Authors: Monteiro-Vitorello Claudia B. , Camargo Luis E. A. , Van Sluys Marie A. , Kitajima João P. , Truffi Daniela , do Amaral Alexandre M. , Harakava Ricardo , de Oliveira Julio C. F. , Wood Derek , de Oliveira Mariana C. , Miyaki Cristina , Takita Marco A. , da Silva Ana C. R. , Furlan Luis R. , Carraro Dirce M. , Camarotte Giovana , Almeida Nalvo F. , Carrer Helaine , Coutinho Luiz L. , El-Dorry Hamza A. , Ferro Maria I. T. , Gagliardi Paulo R. , Giglioti Eder , Goldman Maria H. S. , Goldman Gustavo H. , Kimura Edna T. , Ferro Emer S. , Kuramae Eiko E. , Lemos Eliana G. M. , Lemos Manoel V. F. , Mauro Sonia M. Z. , Machado Marcos A. , Marino Celso L. , Menck Carlos F. , Nunes Luiz R. , Oliveira Regina C. , Pereira Gonçalo G. , Siqueira Walter , de Souza Alessandra A. , Tsai Siu M. , Zanca A. S. , Simpson Andrew J. G. , Brumbley Stevens M. , Setúbal João C.
Published: May 2007

The genome sequence of Leifsonia xyli subsp. xyli, which causes ratoon stunting disease and affects sugarcane worldwide, was determined. The single circular chromosome of Leifsonia xyli subsp. xyli CTCB07 was 2.6 Mb in length with a GC content of 68% and 2,044 predicted open reading frames. The analysis also revealed 307 predicted pseudogenes, which is more than any bacterial plant pathogen sequenced to date. Many of these pseudogenes, if functional, would likely be involved in the degradation of plant heteropolysaccharides, uptake of free sugars, and synthesis of amino acids. Although L. xyli subsp. xyli has only been identified colonizing the xylem vessels of sugarcane, the numbers of predicted regulatory genes and sugar transporters are similar to those in free-living organisms. Some of the predicted pathogenicity genes appear to have been acquired by lateral transfer and include genes for cellulase, pectinase, wilt-inducing protein, lysozyme, and desaturase. The presence of the latter may contribute to stunting, since it is likely involved in the synthesis of abscisic acid, a hormone that arrests growth. Our findings are consistent with the nutritionally fastidious behavior exhibited by L. xyli subsp. xyli and suggest an ongoing adaptation to the restricted ecological niche it inhabits.
Polygalacturonase-Inhibiting Proteins Can Function as Activators of Polygalacturonase
Authors: Kemp G. , Stanton L. , Bergmann C. W. , Clay R. P. , Albersheim P. , Darvill A.
Published: May 2007

The interaction between fungal endopolygalacturonases (EPGs) and polygalacturonase-inhibiting proteins (PGIPs) found in plant cell walls has been well established. The typical EPG/PGIP interaction is characterized by high affinity, reversibility, and a 1:1 stoichiometry that results in lowering the catalytic rate of a particular endopolygalacturonase by up to 99.7%. Various EPG and PGIP isoforms and glycoforms have been isolated and characterized, and combinations of EPGs and PGIPs demonstrate a range of enzyme inhibition. EPG/PGIP interactions have prompted many researchers to suspect the involvement of these proteins in the production of specific signals (oligosaccharins) during plant pathogenesis. We have recently reported on initial studies in our laboratory indicating that, for certain EPG/PGIP combinations, the specific activity of EPG is increased beyond that characteristic of the enzyme alone. In this paper, we rpesent a detailed analysis of the product of the interaction of native Phaseolus vulgaris PGIP-2 with five EPGs from Aspergillus niger, namely PGI, PGII, PGA, PGB, and PGC in the presence of homogalacturonan. We demonstrate that for PGA and PGC, the interaction with PGIP-2 may result in either inhibition or activation in a manner that is pH dependent. This data suggests the need for a reevaluation of the conventional description applied to PGIPs; suggestions include polygalacturonase-binding protein and polygalacturonase-modulating protein.
Distribution and Sequence Analysis of a Family of Type III-Dependent Effectors Correlate with the Phylogeny of Ralstonia solanacearum Strains
Authors: Lavie Muriel , Seunes Benjamin , Prior Philippe , Boucher Christian
Published: May 2007

In Ralstonia solanacearum, we previously have reported on the characterization of popP1 and popP2 genes. These genes encode type III-dependent pathogenicity effectors related to the large family of AvrRxv/YopJ cysteine prote-ases that are shared among pathogens of plants and animals. In this study, we identify a third gene, named popP3, that is inactivated in the genome sequence of strain GMI1000 by insertion of a copy of the insertion sequence ISRso13. The three popP genes are localized on two large chromosomal pathogenicity islands, with popP1 and popP2 being present on the same island. Phylogenic analysis demonstrated that the PopP2 and PopP3 proteins are clearly distinct from other effectors of this family previously characterized in plant and animal pathogens. Analysis of the distribution and allelic variations of the three genes in 30 strains representative of the biodiversity of R. solanacearum established that popP genes are distributed widely among strains from two of the three phyla previously defined on the basis of the structure of the core genome. Sequencing of the popP genes from the different strains revealed limited allelic variations at the three loci but did not show evidence of recombination between the popP genes. Limited allelic variation together with occurrence of insertion sequences within or in the close vicinity of popP genes and the presence of gene duplications in these pathogenicity islands suggest that genomic rearrangements might be a major evolutionary driving force controlling evolution of the genes encoded in these regions. The implications of these observations in terms of bacterial evolution, gene acquisition, and horizontal gene transfers are discussed.
Stimulation of the Lipoxygenase Pathway Is Associated with Systemic Resistance Induced in Bean by a Nonpathogenic Pseudomonas Strain
Authors: Ongena Marc , Duby Francéline , Rossignol Fanny , Fauconnier Marie-Laure , Dommes Jacques , Thonart Philippe
Published: May 2007

Systemic defense reactions induced in bean by the nonpathogenic Pseudomonas putida BTP1 strain reduced disease caused by Botrytis cinerea. Phenylalanine ammonialyase activity and the level of endogenous free salicylic acid were compared in plant growth-promoting rhizobacteria-treated versus control plants, but no significant differences were detected. Furthermore, no enhanced fungitoxicity was detected in methanolic leaf extracts, suggesting that accumulation of bean phytoalexins was not part of the stimulated defense mechanisms. However, BTP1-inoculated plants showed increased levels of both linoleic and linolenic acids. On this basis, we further investigated whether the lipoxygenase pathway, leading to antifungal phytooxylipins, could have been stimulated. Two key enzymatic activities of this metabolic route, namely lipoxygenase and hydroperoxide lyase, were significantly stimulated during the first four days after challenging BTP1-treated plants with the pathogen. This was observed in parallel with a more rapid consumption of the respective substrates of these enzymes, as revealed by measurements of endogenous concentrations of linolenic acid and their hydroperoxide derivatives. Moreover, headspace-gas chromatography analyses showed significantly higher concentrations of the fungitoxic final product Z-3-hexenal in leaves from BTP1-inoculated beans as compared with control plants. Taken together, these results strongly suggest that the oxylipin pathway can be associated with enhanced disease resistance induced in bean plants by nonpathogenic rhizobacteria.
TtsI, a Key Regulator of Rhizobium Species NGR234 Is Required for Type III-Dependent Protein Secretion and Synthesis of Rhamnose-Rich Polysaccharides
Authors: Marie Corinne , Deakin William J. , Ojanen-Reuhs Tuula , Diallo Ericka , Reuhs Brad , Broughton William J. , Perret Xavier
Published: May 2007

Formation of nitrogen-fixing nodules on legume roots by Rhizobium sp. NGR234 requires an array of bacterial factors, including nodulation outer proteins (Nops) secreted through a type III secretion system (TTSS). Secretion of Nops is abolished upon inactivation of ttsI (formerly y4xI), a protein with characteristics of two-component response regulators that was predicted to activate transcription of TTSS-related genes. During the symbiotic interaction, the phenotype of NGRΩttsI differs from that of a mutant with a nonfunctional secretion machine, however. This indicated that TtsI regulates the synthesis of other symbiotic factors as well. Conserved sequences, called tts boxes, proposed to act as binding sites for TtsI, were identified not only within the TTSS cluster but also in the promoter regions of i) genes predicted to encode homologs of virulence factors secreted by pathogenic bacteria, ii) loci involved in the synthesis of a rhamnose-rich component (rhamnan) of the lipopolysaccharides (LPS), and iii) open reading frames that play roles in plasmid partitioning. Transcription studies showed that TtsI and tts boxes are required for the activation of TTSS-related genes and those involved in rhamnose synthesis. Furthermore, extraction of polysaccharides revealed that inactivation of ttsI abolishes the synthesis of the rhamnan component of the LPS. The phenotypes of mutants impaired in TTSS-dependent protein secretion, rhamnan synthesis, or in both functions were compared to assess the roles of some of the TtsI-controlled factors during symbiosis.
Prevalence and Severity of Nattrassia mangiferae Root and Stem Rot Pathogen of Cassava in Bénin
Authors: Msikita W. , Bissang B. , James B. D. , Baimey H. , Wilkinson H. T. , Ahounou M. , Fagbemissi R.
Published: May 2007

Root rot pathogens were found through diagnostic surveys in all departments (regions) of Bénin, West Africa, to affect 86 to 100% and 96 to 100% of cassava fields during the dry and rainy seasons, respectively. Disease incidence in individual fields ranged between 0 and 53%, and averaged 16 to 27% per department. Nattrassia mangiferae was consistently the most frequently isolated root rot pathogen (56% in the dry season and 22 to 52% in the rainy season). Pathogenicity of N. mangiferae was confirmed on four cultivars of cassava using stem cuttings and storage roots. For all four cultivars, N. mangiferae significantly reduced the number of roots. Lesions (3 to 15 cm long) formed on the lower stem portion of all inoculated plants, whereas control plants remained symptom free. On storage roots, the disease profile was similar to that formed on stem cuttings. Other root rot pathogens detected during the dry season were Macrophomina phaseolina (14.2%), Fusarium spp. (11.8%), Botryodiplodia theobromae (7.7%), and Pythium spp. (2.9%). During the rainy season, Fusarium spp. were the second most commonly isolated root rot pathogens in three departments (Atlantique, Borgou, and Mono). In Oueme and Zou, B. theobromae was the second most isolated root rot pathogen (ranging between 24 and 28%) during the rainy season. During the same season, Pythium spp. were pronounced in Borgou (18%), followed by Mono (11%), Atlantique (9%), Atacora (8%), Oueme (5%), and Zou (6%). Results of the study are discussed with a view to creating awareness of the destructive power of N. mangiferae, a hitherto poorly recognized root rot pathogen of cassava in Bénin and West Africa in general.
Diagnosis and Population Analysis of Pythium Species Using AFLP Fingerprinting
Authors: Garzón Carla D. , Geiser David M. , Moorman Gary W.
Published: May 2007

Accurate identification of Pythium species, the causal agents of Pythium root rot and dampingoff of seedlings, and characterization of populations within the species would greatly assist in selecting and implementing control strategies for these pathogens. Several molecular techniques offer methods for accurate and rapid identification of species, but provide little information about their populations. In this study, amplified fragment length polymorphism (AFLP) fingerprinting was used to characterize plant-pathogenic Pythium species and intraspecific populations. Species-diagnostic AFLP fingerprints for Pythium aphanidermatum, P. irregulare, and P. ultimum, and tentative fingerprints for six other species, were identified. Intraspecific distance analyses of P. aphanidermatum, P. ultimum, and P. irregulare revealed distinct patterns of intraspecific variation among the three species. P. aphanidermatum showed the smallest mean distance among isolates (15%), followed by P. ultimum (37%). P. irregulare had the largest mean distance among isolates (64%), which were divided into two populations with great genetic differentiation (FST = 0.2), suggesting the presence of a cryptic species boundary within this species.
Analyses of the Relationships Between Lettuce Downy Mildew and Weather Variables Using Geographic Information System Techniques
Authors: Wu B. M. , Subbarao K. V. , van Bruggen A. H. C.
Published: May 2007

Previous studies in coastal California suggested that morning leaf wetness duration and temperature immediately after the prolonged leaf wetness period affect infection of lettuce by the downy mildew pathogen, Bremia lactucae. In this study, spatial analysis tools in a geographic information system were used to interpolate disease assessment data and then relate them to weather variables measured in 1995 and 1997 at weather stations in the Salinas Valley. Among the variables monitored at these weather stations, midday temperature (10:00 A.M. to 2:00 P.M.) was related most strongly to the interpolated downy mildew incidence in a circular area (radius = 5 km) around each station (r = 0.52, P < 0.0001); the higher the midday temperature, the lower the disease incidence. High humidity and prolonged morning leaf wetness duration also were associated with high downy mildew incidence. Cluster analysis resulted in distinct regions with different midday temperatures, which overlapped well (92.2% of the total area) with regions distinguished in previous cluster analyses of downy mildew incidence. Clusters of morning relative humidity showed similar patterns, although they overlapped less well with clusters of disease incidence. These results confirmed that midday temperature is an important determining factor for lettuce downy mildew, and its effects should be incorporated into a disease warning system for coastal California. Cluster analyses based on the effects of temperature have great potential for use in regional downy mildew risk assessment.
First Report of Leaf Spots on Laurus nobilis Caused by Phytophthora citricola in Belgium
Authors: Crepel C. , Inghelbrecht S. , Baeyen S. , Maes M. , Bobev S. G.
Published: May 2007

Laurus nobilis (laurel tree) is used as an herbal and ornamental tree in gardens in Belgium. During the summer of 2001, a serious outbreak of leaf spots was observed in some Belgian nurseries. Symptoms were large, irregular, brown leaf spots located primarily on the distal half of leaves and delimited by a black margin. As the disease progressed, the spots enlarged, coalesced, and finally led to leaf withering. The isolated fungus had morphological characteristics typical of Phytophthora citricola. On potato dextrose agar (PDA) it formed white, entire rosette colonies. Within 1 week, semipapillate sporangia were found abundantly in water at room temperature. They were mostly ovoid and highly variable in size (20 to 82 × 12 to 48 µm). The sporangial apex was broadly papillate. Oogonia were spherical (18 to 38 µm in diameter), and the antheridia were paragynous. Pathogenicity of the isolated fungus was confirmed by inoculating two visibly healthy L. nobilis plants. Five leaves per plant were wounded by a scalpel, subsequently inoculated with 5-mm-diameter mycelial plugs on PDA, and sealed with Parafilm. As a control, noncolonized agar plugs were placed on wounded leaves from a third L. nobilis plant. The inoculated plants and the control plant were kept for 1 day under a plastic cover (approximately 95% relative humidity) on the laboratory bench. Within 1 week, all inoculated leaves developed symptoms, whereas the control leaves remained symptomless. P. citricola was sucessfully re-isolated, satisfying Koch's postulates. The pathogen was also characterized using molecular tools. The ribosomal DNA regions, ITS1 and 5.8S rDNA-ITS2, were sequenced and highest similarity scores were obtained with corresponding Phytophthora citricola sequence regions (>99% identity for both sequences). The new GenBank Accession Nos. are AY525786 (ITS1) and AY525787 (5.8S rDNA-ITS2). On the basis of the symptoms, cultural and morphological characteristics, and positive results in pathogenicity and PCR tests, the isolate was considered to be P. citricola. To our knowledge, this is the first record of P. citricola on leaves of L. nobilis in Belgium.
Prevalence and Distribution of Common Barberry, the Alternate Host of Puccinia graminis, in Minnesota
Authors: Peterson P. D. , Leonard K. J. , Miller J. D. , Laudon R. J. , Sutton T. B.
Published: May 2007

A federal and state program operated from 1918 until the 1980s to eradicate common barberry (Berberis vulgaris), the alternate host of Puccinia graminis, from the major areas of cereal production in the United States. Over 500 million bushes were destroyed nationally during the program, approximately 1 million in Minnesota. Some sites in Minnesota where barberry bushes were destroyed remained in the “active” class when eradication was phased out in the 1980s. Active sites were defined as those on which there was still a possibility of emergence of barberry seedlings or sprouts arising from the parent bush. In the present study, from 1998 to 2002, 72 of the approximately 1,200 active sites in Minnesota were surveyed. Areas within 90 m of mapped locations of previously destroyed bushes were searched carefully at each site. Reemerged barberry plants were found on 32 sites. The reproductive status and GPS coordinates were recorded for each reemerged bush. More than 90% of the barberry bushes were found in counties with less than 400 ha of wheat per county, mostly in southeastern Minnesota, but one bush was found in a major wheat-producing county in northwestern Minnesota. Reemergence of barberry may serve as a source of new wheat stem rust races in future epidemics.
Teleomorph Formation of Phaeoacremonium aleophilum, Cause of Esca and Grapevine Decline in California
Authors: Rooney-Latham S. , Eskalen A. , Gubler W. D.
Published: May 2007

Phaeoacremonium is a recently described genus of the hyphomycetes and includes species associated with grapevine (Vitis vinifera) declines worldwide. Spores of Phaeoacremonium spp. have been trapped in infested vineyards, but neither asexual nor sexual fruiting structures have been observed in the field. Mating studies were carried out to determine if California P. aleophilum isolates are capable of forming a teleomorph in vitro. Sterilized grapevine shavings were placed on the surface of water agar plates with pairs of different California isolates of P. aleophilum, an isolate from the holotype of P. aleophilum, plus other related Phaeoacremonium spp. After approximately 28 to 35 days, perithecia were seen forming on wood chips and agar of many pairings. Upon maturation, fertile perithecia had gelatinous droplets of ascospores oozing from their ostioles. Successful crosses, resulting in mature perithecia, corresponded to a heterothallic mating type system. When F1 progeny were backcrossed with their parents, heterothallism was confirmed. Molecular analyses of the internal transcribed spacer (ITS) region of the nuclear ribosomal DNA from ascospore progeny and perithecia confirmed that these perithecia were the teleomorph of P. aleophilum, Togninia minima. Furthermore, 4 months after moist incubating grapevine pieces from naturally infected vineyards, mature perithecia of T. minima could be seen forming on the xylem and pith tissues, suggesting both mating types occur on the same vine.
Identification, Mefenoxam Sensitivity, and Compatibility Type of Phytophthora spp. Attacking Floriculture Crops in North Carolina
Authors: Hwang J. , Benson D. M.
Published: May 2007

Phytophthora isolates were collected from floriculture crops grown in commercial greenhouses in North Carolina for species identification, compatibility type determination, and mefenoxam sensitivity tests. Isolation from 41 symptomatic plant species at 29 production locations resulted in 483 isolates from eight crops at seven locations. Phytophthora cryptogea (184 isolates) was recovered from dusty miller and gerbera daisy. All isolates of P. cryptogea were insensitive or intermediate in sensitivity to mefenoxam at 1 μg a.i./ml and were A1 compatibility type. P. nicotianae (273 isolates) was isolated from African violet, lavender, pansy, petunia, and vinca. Of these isolates, 21% were insensitive to mefenoxam at either 1 or 100 μg a.i./ml. Isolates of P. nicotianae from five locations were A2 compatibility type, whereas isolates on pansy at one location were A1 compatibility type. English ivy grown at two locations was infected with P. palmivora. All 26 isolates of P. palmivora were sensitive to mefenoxam and were A1 compatibility type. Mating type and mefenoxam sensitivity were uniform among isolates of P. nicotianae and P. palmivora from a given crop at a given location, suggesting that epidemics within a location may have originated from a single source of inoculum.
First Report of Root Rot of Bean and Soybean Caused by Rhizoctonia zeae in Turkey
Authors: Erper I. , Karaca G. , Özkoç I.
Published: May 2007

To determine the species of Rhizoctonia on bean and soybean plants grown in Samsun (Turkey), field surveys were performed at 104 locations during 2001 and 2002. Rhizoctonia spp. were obtained from isolations from the necrotic lesions on the hypocotyl and rhizosphere soils. Species were identified according to Ogoshi (3) on the basis of hyphal and colony morphology and anastomosis reaction with known tester isolates (provided by M. Hyakumachi, Gifu University, Japan). Fifty Rhizoctonia spp. isolates obtained from these locations were identified as Rhizoctonia zeae (teleomorph Waitea circinata var. zeae). Nine of the 27 bean isolates and 8 of the 23 soybean isolates were recovered from plant tissues. These isolates had optimum temperature (32°C) for growth. Colonies were orange when young, becoming salmon colored with age. Sclerotia formed both on the agar surface or submerged in the medium. Superficial sclerotia were more uniform and nearly spherical, mostly 0.2 to 0.5 mm in diameter, and they were first orange and then turned brown. Pathogenicity was tested with three R. zeae isolates grown on sterile oat seeds at 25°C for 10 days. Bean and soybean seedlings grown in 1-liter plastic pots containing sterile potting mix (field soil/composted manure/sand 2:2:1 [v/v]) at true-leaf stage were inoculated by placing five infested oat seeds adjacent to the roots. Sterile oat seeds were used for controls. After 3 to 4 weeks of incubation at 17 to 25°C in a glasshouse, roots of the plants were cleaned with tap water and evaluated for disease severity. Four replicate pots were used for each isolate/plant combination. All isolates produced superficial brown lesions on roots and hypocotyls similar to those observed on plants used for isolations and root growth declined. R. zeae was reisolated from the lesions on all bean and soybean plants used for the pathogenicity test. While R. zeae was previously reported from Johnsongrass roots (1) and corn kernels (2), to our knowledge, this is the first report of R. zeae isolated from bean and soybean plants and rhizosphere soils in Turkey.

References: (1) E. Demirci, and C. Eken. Plant Dis. 83:200, 1999. (2) E. Demirci and S. Kordali. Plant Dis. 83:879, 1999. (3) A. Ogoshi. Rev. Plant. Prot. Res. 8:93, 1975.
First Report of Botryosphaeria ribis as a Branch Dieback Pathogen of Olive Trees in Spain
Authors: Romero M. A. , Sánchez M. E. , Trapero A.
Published: May 2007

Botryosphaeria spp. are known to produce cankers and dieback in several woody hosts. Since 2001, a severe disease resulting in the dieback of branches was observed in olive (Olea europaea) orchards in Andalucía, southern Spain, where the only cultivar affected was Gordal de Sevilla. The affected trees showed abundant dead twigs, and wilted leaves remained attached. These symptoms were similar to those caused by Resseliella oleisuga, a bark borer insect common in the region. Closer inspection showed no evidence of insects, but revealed reddish brown bark lesions that girdled the affected branches. When the outer bark was removed, the affected tissue appeared dark brown, in contrast to the yellowish green of healthy inner bark. A Fusicoccum sp. was consistently isolated from the necrotic bark. Conidia produced in black pycnidia were hyaline, aseptate, fusoid, with a truncate base averaging 19.9 × 6.7 μm, becoming pale brown with all cells the same color, and developing one or two septa when germinated. These characteristics conform to Fusicoccum ribis (1), the anamorph of B. ribis. Pathogenicity of two isolates from symptomatic branches was determined by stem inoculations of 4-year-old cv. Gordal de Sevilla grafted on wild olive trees growing in plastic pots containing a sand/lime/peat soil mixture. Both isolates were inoculated by three techniques. In the first, 7-mm-diameter agar plugs bearing actively growing mycelium were applied to 7-mm-diameter bark wounds made with a cork borer on the middle point of stems (averaging 10 mm in diameter). In the second technique, the same type of inoculum was applied directly to the bark without any wounding. For the third technique, 100 μl of conidial suspension in water (2 × 105 conidia per ml) was applied to bark wounds. The inoculated stem sections were wrapped in Parafilm to retain moisture. All inoculated and control plants (only sterile agar medium or sterile water added) were kept in a greenhouse and watered as needed. There were three replicate plants per isolate and inoculation technique. After 3 months, all plants inoculated with mycelium applied to wounds showed girdling and apical death with browning and wilting of leaves above the inoculation point. In the treatments where mycelium was applied to unwounded bark or where conidial suspensions were applied to wounds, small bark cankers developed to 56 mm long, but neither girdling nor foliar wilting were observed. No symptoms developed in the control plants. There were no significant differences in pathogenicity between the two F. ribis isolates. F. ribis was recovered from necrotic bark from every inoculated plant. These results demonstrate that B. ribis is pathogenic on cv. Gordal de Sevilla olives, and they indicate that bark wounding favors lesion development but is not needed for infection.

Reference: (1) B. Slippers et al. Mycologia 96:83, 2004.
First Report of Rust Caused by Puccinia carthami on Safflower in Oman
Authors: Deadman M. L. , Al Sadi A. M. , Al Jahdhami S. , Aime M. C.
Published: May 2007

Safflower (Carthamus tinctorius L.) is a minor, but culturally important crop in Oman; the dried flowers produce a pigment used in facial ornamentation. Although Oman is not a commercial producer of safflower, the region is a center of diversity and a source of genetic material for breeding programs. Production of oil from safflower has potential in Oman, where plant growth is prolific. In April 2004, leaf samples showing rust symptoms were collected from Mudhaibi, 100 km south of Muscat. Chestnut brown pustules covered both sides of the leaves, but not the stems, and yielded urediniospores and teliospores typical of the pathogen. Urediniospores were globose, 25 μm in diameter with three germ pores. Two-celled teliospores were chestnut brown, minutely verrucose, with a single, depressed germ pore in each cell. The pathogen was identified as Puccinia carthami Corda (voucher specimen deposited in the U.S. National Fungus Collections, BPI863557; nuclear ribosomal large subunit DNA voucher sequence deposited in GenBank, Accession No. AY787782). On the basis of phylogenetic analyses, the rust from Oman belongs to a complex of closely related Puccinia spp. that infects members of the Cardueae. Elsewhere, in addition to leaf infections, P. carthami causes foot and root disease of safflower (1) with teliospores surviving in the soil and on seed to initiate new infections. Germplasm is now being collected and will be screened for variation in response to rust infection.

Reference: (1) M. L. Schuster et al. Phytopathology 42:211, 1952.