Plant Pathology Commons^™

Genome Sequences Of Agropyron Mosaic Virus And Hordeum Mosaic Virus Support Reciprocal Monophyly Of The Genera Potyvirus And Rymovirus In The Family Potyviridae, Roy C. French, D.C. Stenger

Department of Plant Pathology: Faculty Publications

Assignment of mite-transmitted species to the genus Rymovirus (family Potyviridae) has changed several times, and the status of the genus has been questioned. To address this issue, complete genome sequences of the rymoviruses Agropyron mosaic virus (AgMV) and Hordeum mosaic virus (HoMV) were determined. AgMV (9540 nucleotides) and HoMV (9463 nucleotides) each encode a single polyprotein with proteinase cleavage sites demarcating protein products characteristic of monopartite species of the family Potyviridae. Of the described species of Potyviridae, AgMV and HoMV are most closely related to each other (68.5% nucleotide and 71.6% amino acid sequence identity) and equidistant …

Expression Of The Tobacco Β- 1,3-Glucanase Gene, Pr-2d, Following Induction Of Sar With Peronospora Tabacina, Deanna L. Funnell, Christopher B. Lawrence, Jeffrey F. Pedersen, Christopher L. Schardl

Department of Plant Pathology: Faculty Publications

Systemic acquired resistance (SAR) is induced following inoculation of Peronospora tabacina sporangia into the stems of Nicotiana tabacum plants highly susceptible to the pathogen. Previous results have shown that accumulation of acidic P-1,3-glucanases (PR-2's) following induction of SAR by P. tabacina may contribute to resistance to P. tabacina. We showed that up-regulation of the PR-2 gene, PR-2d, following stem inoculation with P. tabacina, is associated with SAR. Studies using plants transformed with GUS constructs containing the full length promoter from PR-2d or promoter deletions, provided evidence that a previously characterized regulatory element that is involved in response to …

Necrotic Lesion Resistance Induced By Peronospora Tabacina On Leaves Of Nicotiana Obtusifolia, E.P. Heist, D. Zaitlin, Deanna L. Funnell, W.C. Nesmith, C.L. Schardl

Department of Plant Pathology: Faculty Publications

Infection of Nicotiana obtusifolia plant introduction (PI) #555573 by the downy mildew pathogen Peronospora tabacina resulted in a compatible interaction, in which P. tabacina penetrated and freely colonized host leaf tissue. This interaction became incompatible 5 to 6 days later, with the appearance of necrotic lesions (NLs) and inhibition of pathogen growth and subsequent sporulation. NL development depended upon the presence of P. tabacina in host tissue, was not due to the effects of other microbes, and occurred co-incident in time with the pathogen’s ability to produce asexual sporangia on a susceptible N. obtusifolia genotype. Inhibition of the necrotic response …

Disabling Surveillance: Bacterial Type Iii Secretion System Effectors That Suppress Innate Immunity, Avelina Espinosa, James R. Alfano

Center for Plant Science Innovation: Faculty Publications

Many Gram-negative bacterial pathogens of plants and animals are dependent on a type III protein secretion system (TTSS). TTSSs translocate effector proteins into host cells and are capable of modifying signal transduction pathways. The innate immune system of eukaryotes detects the presence of pathogens using specific pathogen recognition receptors (PRRs). Plant PRRs include the FLS2 receptor kinase and resistance proteins. Animal PRRs include Tolllike receptors and nucleotide-binding oligomerization domain proteins. PRRs initiate signal transduction pathways that include mitogen-activated protein kinase (MAPK) cascades that activate defencerelated transcription factors. This results in induction of proinflammatory cytokines in animals, and hallmarks of defence …

Mesa, A Novel Fungal Protein Required For The Stabilization Of Polarity Axes In Aspergillus Nidulans, Claire L. Pearson, Kaimei Xu, Kathryn E. Sharpless, Steven D. Harris

Department of Plant Pathology: Faculty Publications

The Aspergillus nidulans proteome possesses a single formin, SepA, which is required for actin ring formation at septation sites and also plays a role in polarized morphogenesis. Previous observations imply that complex regulatory mechanisms control the function of SepA and ensure its correct localization within hyphal tip cells. To characterize these mechanisms, we undertook a screen for mutations that enhance sepA defects. Of the mutants recovered, mesA1 causes the most dramatic defect in polarity establishment when SepA function is compromised. In a wild-type background, mesA1 mutants undergo aberrant hyphal morphogenesis, whereas septum formation remains unaffected. Molecular characterization revealed that MesA …

Wheat Cultivar-Specific Selection Of 2,4-Diacetylphloroglucinol-Producing Fluorescent Pseudomonas Species From Resident Soil Populations, M. Mazzola, Deanna L. Funnell, J.M. Raaijmakers

Department of Plant Pathology: Faculty Publications

An emerging body of evidence indicates a role for plant genotype as a determinant of the species and genetic composition of the saprophytic microbial community resident to the rhizosphere. In this study, experiments were conducted to determine the capacity of five different wheat cultivars to enhance resident populations and support introduced strains of 2,4-diacetylphloroglucinol (2,4-DAPG)-producing fluorescent pseudomonads, a group of bacteria known to provide biological control of several soilborne diseases. When soils were cropped with three successive 28-day growth cycles of wheat, the 2,4- DAPG-producing strains were consistently recovered from the rhizosphere of the cultivar Lewjain, and commonly were present …

Spinach Curly Top Virus: A Newly Described Curtovirus Species From Southwest Texas With Incongruent Gene Phylogenies, Surendranath Baliji, Mark C. Black, Roy C. French, Drake C. Stenger, Garry Sunter

Department of Plant Pathology: Faculty Publications

A curtovirus associated with a disease of spinach was isolated in southwest Texas during 1996. Disease symptoms included severe stunting and chlorosis, with younger leaves curled, distorted, and dwarfed. Viral DNA was purified and an infectious clone obtained. Agroinoculation using a construct bearing full-length tandem repeats of the cloned viral genome resulted in systemic infection of species in six of seven plant families tested, indicating that the virus has a wide host range. Symptoms produced in spinach agroinoculated with cloned viral DNA were similar to those observed in the field. Viral single-stranded and double-stranded DNA forms typical of curtovirus infection …

Promoters From Chlorella Virus Genes Providing For Expression Of Genes In Prokaryotic And Eukaryotic Hosts, Amitava Mitra, James L. Van Etten

Department of Plant Pathology: Faculty Publications

The invention is directed to novel promoters or mutants thereof from Chlorella virus DNA methyltansferase genes. A Chlorella Virus gene promoter is operably linked to a first and/or Second DNA sequence encoding a gene that is different from the Chlorella virus to form an expression cassette. An expression cassette can be introduced into prokaryotic and/or eukaryotic cells and can provide for a high level of expression of the gene encoded by the first and/or Second DNA sequence. The invention also provides a method for Screening other Chlorella virus genes for promoters that can function to express a heterologous gene in …

The Pseudomonas Syringae Hopptov Protein Is Secreted In Culture And Translocated Into Plant Cells Via The Type Iii Protein Secretion System In A Manner Dependent On The Shcv Type Iii Chaperone, Misty D. Wehling, Ming Guo, Zheng Qing Fu, James R. Alfano

Center for Plant Science Innovation: Faculty Publications

The bacterial plant pathogen Pseudomonas syringae depends on a type III protein secretion system and the effector proteins that it translocates into plant cells to cause disease and to elicit the defense-associated hypersensitive response on resistant plants. The availability of the P. syringae pv. tomato DC3000 genome sequence has resulted in the identification of many novel effectors. We identified the hopPtoV effector gene on the basis of its location next to a candidate type III chaperone (TTC) gene, shcV, and within a pathogenicity island in the DC3000 chromosome. A DC3000 mutant lacking ShcV was unable to secrete detectable amounts …

Review Of Bacterial Disease Resistance In Plants: Molecular Biology And Biotechnological Applications By P. Vidhyasekaran, Marty Dickman

Department of Plant Pathology: Faculty Publications

The molecular biology of plant-bacterial interactions is a field that continues to provide a wealth of information in the context of infectious diseases of plants. Thus, it is curious that few textbooks on this topic are available. Admittedly, active areas of research change rapidly, and book content can be rendered obsolete prior to publication. Even given this scenario, a textbook that summarizes the development (both conceptually and experimentally) of research strategies, the status of model systems, and future issues is needed. Thus, the timing and need for this broad-based volume, which covers the molecular phytobacteriology, is long overdue. An important …

Rapid Iodine Staining Techniques For Identifying The Waxy Phenotype In Sorghum Grain And Waxy Genotype In Sorghum Pollen, Jeffrey F. Pedersen, S.R. Bean, Deanna L. Funnell, Robert A. Graybosch

Department of Plant Pathology: Faculty Publications

Visual classification of sorghum [Sorghum bicolor (L.) Moench] grain for the waxy phenotype is subjective and can be confounded by genetic background, maturity, environment, and experience of the classifier. Rapid iodine staining methods for identifying the waxy phenotype in sorghum grain and waxy genotypes in sorghum pollen were developed. Mature single sorghum seeds were placed in 48-well micro-plates and crushed. Water was added and the mixture heated to 95°C for 1 h to gelatinize the starch. After cooling, iodine solution was added and color scored after 10 to 60 s allowing for very high sample throughput. Sorghum pollen was …

Proceedings Of The 31st Annual Meeting, Southern Soybean Disease Workers (February 15-16, 2004, St. Louis, Missouri), Boyd Padgett, Patrick Fenn, Ray Schneider, Jason Bond, Stephen R. Koenning

Southern Soybean Disease Workers: Conference Proceedings

Contents

Southern United States Soybean Disease Loss Estimate for 2003. Compiled by Stephen R. Koenning

Shifts in Soybean Disease Incidence and Severity Following Adaptation of the ESPS (Early Soybean Productions system) in Mississippi. GL Sciumbato and DH Poston

Impact of Foliar Fungicides on Soybean Yield and Net Returns. BL Spinks, DH Poston, GL Sciumbato, and MA Blaine

Inheritance of Resistance to Phompsis Seed Decay in Soybean PI 80837. EW Jackson, P Fenn, P Chen, and PK Miller

Inheritance of Resistance to Purple Seed Stain in Soybean PI 80837. EW Jackson, P Fenn, P Chen, PK Miller

Population Structure of Cercospora …

Complete Nucleotide Sequence Of Oat Necrotic Mottle Virus: A Distinct Tritimovirus Species (Family Potyviridae) Most, D.C. Stenger, Roy C. French

Department of Plant Pathology: Faculty Publications

The RNA genome (9346 nucleotides) of Oat necrotic mottle virus (ONMV) was cloned and sequenced. Complete genome comparisons indicated that ONMV, currently classified as a rymovirus, was most closely related (~73% nucleotide, ~79% amino acid identity) to the tritimovirus Wheat streak mosaic virus (WSMV). ONMV encoded a single polyprotein, with proteinase cleavage sites very similar to those ofWSMV. Pairwise comparison of ONMV andWSMV cistrons revealed that P3 was most conserved (~79% nucleotide, ~86% amino acid), whereas HC-Pro was most divergent (~67% nucleotide, ~69% amino acid). In contrast, the ONMV sequence was distantly related (~40% nt, ~26% amino acid identity) to …

Closterovirus Bipolar Virion: Evidence For Initiation Of Assembly By Minor Coat Protein And Its Restriction To The Genomic Rna 5' Region, Satyanarayana Tatineni, Siddarame Gowda, Maria A. Ayllon, William O. Dawson

Department of Plant Pathology: Faculty Publications

The long flexuous virions of the Closteroviridae have a unique bipolar architecture incorporating two coat proteins, with most of the helical nucleocapsid encapsidated by the major coat protein (CP) and a small portion of one end encapsidated by the minor coat protein (CPm). It is not known whether CPm encapsidates the genomic RNA and, if so, which end and what effects transition between the two coat proteins. Two other virus-encoded proteins, an HSP70 homolog (HSP70h) and an = 61-kDa protein, are required to augment virion assembly. In this work, we examine the in vivo encapsidation of Citrus tristeza virus by …

Phenotypic And Genotypic Characterization Of Uromyces Appendiculatus From Phaseolus Vulgaris In The Americas, C. M. Araya, A. T. Alleyne, J. R. Steadman, Kent M. Eskridge, D. P. Coyne

Department of Plant Pathology: Faculty Publications

Populations of 90 Uromyces appendiculatus isolates were collected from throughout the Americas and evaluated for virulence on 19 standard bean rust differentials, and also on 12 landraces of Phaseolus vulgaris from South and Central America. The landrace differentials represented geographical centers of bean domestication. Three groups were observed. Two groups were isolates from centers of bean domestication and a third heterogeneous group comprised isolates from countries in South and Central America. Molecular analysis using random amplified polymorphic DNA–polymerase chain reaction (RAPD-PCR) was also conducted on these isolates. Cluster analysis of the molecular profiles showed three groups that corresponded to those …

Pseudomonas Syringae Type Iii Secretion System Targeting Signals And Novel Effectors Studied With A Cya Translocation Reporter, Lisa M. Schechter, Kathy A. Roberts, Yashitola Jamir, James R. Alfano, Alan Collmer

Center for Plant Science Innovation: Faculty Publications

Pseudomonas syringae pv. tomato strain DC3000 is a pathogen of tomato and Arabidopsis. The hrp-hrcencoded type III secretion system (TTSS), which injects bacterial effector proteins (primarily called Hop or Avr proteins) into plant cells, is required for pathogenicity. In addition to being regulated by the HrpL alternative sigma factor, most avr or hop genes encode proteins with N termini that have several characteristic features, including (i) a high percentage of Ser residues, (ii) an aliphatic amino acid (Ile, Leu, or Val) or Pro at the third or fourth position, and (iii) a lack of negatively charged …

Molecular Differentiation Of Two Sibling Species Of The Black Fly Simulium Vittatum (Diptera: Simuliidae) Based On Random Amplified Polymorphic Dna, G. A. Duncan, P. H. Adler, K. P. Pruess, Thomas O. Powers

Department of Plant Pathology: Faculty Publications

Larvae of the black fly morphospecies Simulium vittatum from Colorado, Montana, Nebraska, and New Hampshire were cytologically identified as either the IS-7 or the IIIL-1 cytospecies. DNA was PCR amplified from cytotyped larvae using eight 10-mer primers, labeled with ₃₃P, and run on polyacrylamide gels. The entire data set of 96 amplicons produced incomplete separation of the two cytospecies when subjected to neighbor-joining and maximum parsimony analyses. However, when analyzed within geographical regions, separate species status was supported. Boot- strap support for distinctness of the two cytospecies was best in Colorado where they were collected in true sympatry. The …

Glycosyltransferases Encoded By Viruses, Nicolas Markine-Goriaynoff, Laurent Gillet, James L. Van Etten, Haralambos Korres, Naresh Verma, Alain Vanderplasschen

James Van Etten Publications

Studies of cellular biology in recent decades have highlighted the crucial roles of glycans in numerous important biological processes, raising the concept of glycomics that is now considered as important as genomics, transcriptomics and proteomics. For millions of years, viruses have been co-evolving with their hosts. Consequently, during this co-evolution process, viruses have acquired mechanisms to mimic, hijack or sabotage host processes that favour their replication, including mechanisms to modify the glycome. The importance of the glycome in the regulation of host–virus interactions has recently led to a new concept called ‘glycovirology’. One fascinating aspect of glycovirology is the study …

Morphological And Molecular Characterization Of Meloidogyne Mayaguensis Isolates From Florida, J. Brito, Thomas O. Powers, P. G. Mullin, Florida Department Of Agriculture And Consumer Services N., D. W. Dickson

Department of Plant Pathology: Faculty Publications

The discovery of Meloidogyne mayaguensis is confirmed in Florida; this is the first report for the continental United States. Meloidogyne mayaguensis is a virulent species that can reproduce on host cultivars bred for nematode resistance. The perineal patterns of M. mayaguensis isolates from Florida show morphological variability and often are similar to M. incognita. Useful morphological characters for the separation of M. mayaguensis from M. incognita from Florida are the male stylet length values (smaller for M. mayaguensis than M. incognita) and J2 tail length values (greater for M. mayaguensis than M. incognita). Meloidogyne mayaguensis values for …

The Hopptof Locus Of Pseudomonas Syringae Pv. Tomato Dc3000 Encodes A Type Iii Chaperone And A Cognate Effector, Libo Shan, Hye-Sook Oh, Jianfu Chen, Ming Guo, Jianmin Zhou, James R. Alfano, Alan Collmer, Xu Jia, Xiaoyan Tang

Department of Plant Pathology: Faculty Publications

Type III secretion systems are highly conserved among gram-negative plant and animal pathogenic bacteria. Through the type III secretion system, bacteria inject a number of virulence proteins into the host cells. Analysis of the whole genome sequence of Pseudomonas syringae pv. tomato DC3000 strain identified a locus, named HopPtoF, that is homologous to the avirulence gene locus avrPphF in P. syringae pv. phaseolicola. The HopPtoF locus harbors two genes, ShcF_Pto and HopF_Pto, that are preceded by a single hrp box promoter. We present evidence here to show that ShcF_Pto and HopF_Pto encode a type …

Effects Of Choline, Betaine, And Wheat Floral Extracts On Growth Of Fusarium Graminearum, Jessica S. Engle, Patrick E. Lipps, Terry L. Graham, Michael J. Boehm

Department of Plant Pathology: Faculty Publications

Fusarium head blight has been more severe when infection occurs during anthesis, indicating that floral organs may be important infection courts. Choline acetate and glycinebetaine have been extracted from wheat and reported to be growth stimulants of Fusarium graminearum. They are hypothesized to enhance infection and tissue colonization. Growth of F. graminearum was examined on media amended with extracts from floral parts of nine wheat genotypes with various Fusarium head blight resistance levels. Results indicated no significant effect of anther, palea, or lemma extracts on radial growth when compared with unamended controls. Effects on spore germination and hyphal growth …

Nematode Molecular Diagnostics: From Bands To Barcodes, Tom Powers

Department of Plant Pathology: Faculty Publications

Nematodes are considered among the most difficult animals to identify. DNA-based diagnostic methods have already gained acceptance in applications ranging from quarantine determinations to assessments of biodiversity. Researchers are currently in an information-gathering mode, with intensive efforts applied to accumulating nucleotide sequence of 18S and 28S ribosomal genes, internally transcribed spacer regions, and mitochondrial genes. Important linkages with collateral data such as digitized images, video clips and specimen voucher web pages are being established on GenBank and NemATOL, the nematode-specific Tree of Life database. The growing DNA taxonomy of nematodes has lead to their use in testing specific short sequences …

Type Iii Secretion System Effector Proteins: Double Agents In Bacterial Disease And Plant Defense, James R. Alfano, Alan Collmer

Department of Plant Pathology: Faculty Publications

Many phytopathogenic bacteria inject virulence effector proteins into plant cells via a Hrp type III secretion system (TTSS). Without the TTSS, these pathogens cannot defeat basal defenses, grow in plants, produce disease lesions in hosts, or elicit the hypersensitive response (HR) in nonhosts. Pathogen genome projects employing bioinformatic methods to identify TTSS Hrp regulon promoters and TTSS pathway targeting signals suggest that phytopathogenic Pseudomonas, Xanthomonas, and Ralstonia spp. harbor large arsenals of effectors. The Hrp TTSS employs cus- tomized cytoplasmic chaperones, conserved export components in the bacterial en- velope (also used by the TTSS of animal pathogens), and …

Small Potassium Ion Channel Proteins Encoded By Chlorella Viruses, Ming Kang, Anna Moroni, Sabrina Gazzarrini, Dario Difrancesco, Gerhard Thiel, Maria Severino, James L. Van Etten

Department of Plant Pathology: Faculty Publications

Kcv, a 94-aa protein encoded by Paramecium bursaria chlorella virus 1, is the smallest known protein to form a functional potassium ion channel and basically corresponds to the ‘‘pore module’’ of potassium channels. Both viral replication and channel activity are inhibited by the ion channel blockers barium and amantadine but not by cesium. Genes encoding Kcv-like proteins were isolated from 40 additional chlorella viruses. Differences in 16 of the 94 amino acids were detected, producing six Kcv-like proteins with amino acid substitutions occurring in most of the functional domains of the protein (N terminus, transmembrane 1, pore helix, selectivity filter, …

The Accidental Plant Pathologist, Anne K. Vidaver

Department of Plant Pathology: Faculty Publications

This article presents the experiences of a woman in academic plant pathology from the 1950s to today. Topics include the social climate for women in science, personal and professional developments and research discoveries, public policy issues in agriculture and biotechnology affecting plant pathology, and projections for the future of plant pathology.

Biography Of James L. Van Etten, Christen Brownlee, James L. Van Etten

Department of Plant Pathology: Faculty Publications

Green algae, in surface layers of almost every lake or stream, are some of the most common aquatic creatures. However, unbeknownst to researchers until recently, viruses that infect algae are almost as widespread. Entire ecosystems of algal hosts and their corresponding viruses lay hidden until the 1980s, when James L. Van Etten, a professor of plant pathology at the University of Nebraska (Lincoln), and his colleague Russ Meints discovered and began to characterize the first member of what is now a rapidly expanding family of algal viruses. Van Etten and his colleagues have continued to study these intriguing viruses, focusing …

Deletion Of The Δ12-Oleic Acid Desaturase Gene Of A Nonaflatoxigenic Aspergillus Parasiticus Field Isolate Affects Conidiation And Sclerotial Development, Perng Kuang Chang, Richard A. Wilson, Nancy P. Keller, Thomas E. Cleveland

Fungal Molecular Plant-Microbe Interactions

Aims: To investigate how linoleic acid affects conidial production and sclerotial development in a strictly mitotic Aspergillus parasiticus field isolate as related to improving biocompetitivity of atoxigenic Aspergillus species.

Methods and Results: We disrupted A. parasiticus Δ12-oleic acid desaturase gene (odeA) responsible for the conversion of oleic acid to linoleic acid. We examined conidiation and sclerotial development of SRRC 2043 and three isogenic mutant strains deleted for the odeA gene (DodeA), either with or without supplementing linoleic acid, on one complex potato dextrose agar (PDA) medium and on two defined media: nitrate-containing Czapek agar (CZ) and Cove’s ammonium …

Characterization Of The Aspergillus Parasiticus Δ12-Desaturase Gene: A Role For Lipid Metabolism In The Aspergillus–Seed Interaction, Richard A. Wilson, Ana M. Calvo, Perng-Kuang Chang, Nancy P. Keller

Fungal Molecular Plant-Microbe Interactions

In the mycotoxigenic oilseed pathogens Aspergillus flavus and Aspergillus parasiticus and the model filamentous fungus Aspergillus nidulans, unsaturated fatty acids and their derivatives act as important developmental signals that affect asexual conidiospore, sexual ascospore and/or sclerotial development. To dissect the relationship between lipid metabolism and fungal development, an A. parasiticus Δ¹²-desaturase mutant that was unable to convert oleic acid to linoleic acid and was thus impaired in polyunsaturated fatty acid biosynthesis was generated. The Δ¹²-desaturase mutant demonstrates delayed spore germination, a twofold reduction in growth, a reduced level of conidiation and complete loss of sclerotial …

Two Δ9-Stearic Acid Desaturases Are Required For Aspergillus Nidulans Growth And Development, Richard A. Wilson, Perng-Kuang Chang, Agnieszka Dobrzyn, James B. Ntambi, Robert Zarnowski, Nancy P. Keller

Fungal Molecular Plant-Microbe Interactions

Unsaturated fatty acids are important constituents of all cell membranes and are required for normal growth. In the filamentous fungus Aspergillus nidulans, unsaturated fatty acids and their derivatives also influence asexual (conidial) and sexual (ascospore) sporulation processes. To investigate the relationship between fatty acid metabolism and fungal development, we disrupted the A. nidulans sdeA and sdeB genes, both encoding Δ9-stearic acid desaturases responsible for the conversion of palmitic acid (16:0) and stearic acid (18:0) to palmitoleic acid (16:1) and oleic acid (18:1). The effects of sdeA deletion on development were profound, such that growth, conidial and ascospore production were …

Gaca, The Response Regulator Of A Two-Component System, Acts As A Master Regulator In Pseudomonas Syringae Pv. Tomato Dc3000 By Controlling Regulatory Rna, Transcriptional Activators, And Alternate Sigma Factors, Asita Chatterjee, Yaya Cui, Hailian Yang, Alan Collmer, James R. Alfano, Arun K. Chatterjee

Department of Plant Pathology: Faculty Publications

Concerted investigations of factors affecting host-pathogen interactions are now possible with the model plant Arabidopsis thaliana and its model pathogen Pseudomonas syringae pv. tomato DC3000, as their whole genome sequences have become available. As a prelude to analysis of the regulatory genes and their targets, we have focused on GacA, the response regulator of a two-component system. The DC3000 gene was cloned by testing for the reversal of phenotypes of an Erwinia GacA– mutant. A GacA– mutant of DC3000 constructed by marker exchange produces much-reduced levels of transcripts of three alternate sigma factors: HrpL, required for the production of effector …