Inhibition of the hypersensitive reaction of soybean leaves to incompatible Pseudomonas spp. by blasticidin S, streptomycin or elevated temperature

Discovery of the Hrp Type III Secretion System in Phytopathogenic Bacteria: How Investigation of Hypersensitive Cell Death in Plants Led to a Novel Protein Injector System and a World of Inter-Organismal Molecular Interactions Within Plant Cells

In the early 1960s, Pseudomonas syringae and other host-specific phytopathogenic proteobacteria were discovered to elicit a rapid, resistance-associated death when infiltrated at high inoculum levels into nonhost tobacco leaves. This hypersensitive reaction (or response; HR) was a useful indicator of basic pathogenic ability. Research over the next 20 years failed to identify an elicitor of the HR but revealed that its elicitation required contact between metabolically active bacterial and plant cells. Beginning in the early 1980s, molecular genetic tools were applied to the HR puzzle, revealing the presence in P. syringae of clusters of hrp genes, so named because they are required for the HR and pathogenicity, and of avr genes, so named because their presence confers HR-associated avirulence in resistant cultivars of a host plant species. A series of breakthroughs over the next two decades revealed that (i) hrp gene clusters encode a type III secretion system (T3SS), which injects Avr (now "effector") proteins into plant cells, where their recognition triggers the HR; (ii) T3SSs, which are typically present in pathogenicity islands acquired by horizontal gene transfers, are found in many bacterial pathogens of plants and animals and inject many effector proteins, which are collectively essential for pathogenicity; and (iii) a primary function of phytopathogen effectors is to subvert non-HR defenses resulting from recognition of conserved microbial features presented outside of plant cells. In the 2000s, Hrp system research shifted to extracellular components enabling effector delivery across plant cell walls and plasma membranes, regulation, and tools for studying effectors. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Pepper Plants Harboring L Resistance Alleles Showed Tolerance toward Manifestations of Tomato Brown Rugose Fruit Virus Disease

The tobamovirus tomato brown rugose fruit virus (ToBRFV) infects tomato plants harboring the Tm-2² resistance allele, which corresponds with tobamoviruses’ avirulence (Avr) gene encoding the movement protein to activate a resistance-associated hypersensitive response (HR). ToBRFV has caused severe damage to tomato crops worldwide. Unlike tomato plants, pepper plants harboring the L resistance alleles, which correspond with the tobamovirus Avr gene encoding the coat protein, have shown HR manifestations upon ToBRFV infection. We have found that ToBRFV inoculation of a wide range of undefined pepper plant varieties could cause a “hypersensitive-like cell death” response, which was associated with ToBRFV transient systemic infection dissociated from disease symptom manifestations on fruits. Susceptibility of pepper plants harboring L¹, L³, or L⁴ resistance alleles to ToBRFV infection following HRs was similarly transient and dissociated from disease symptom manifestations on fruits. Interestingly, ToBRFV stable infection of a pepper cultivar not harboring the L gene was also not associated with disease symptoms on fruits, although ToBRFV was localized in the seed epidermis, parenchyma, and endothelium, which borders the endosperm, indicating that a stable infection of maternal origin of these tissues occurred. Pepper plants with systemic ToBRFV infection could constitute an inoculum source for adjacently grown tomato plants.

RPS5-Mediated Disease Resistance: Fundamental Insights and Translational Applications

Focusing on the discovery and characterization of the Arabidopsis disease resistance protein RPS5 and its guardee PBS1, this review discusses work done in the Innes laboratory from the initial identification of the RPS5 gene in 1995 to the recent deployment of the PBS1 decoy system in crops. This is done through discussion of the structure, function, and signaling environment of RPS5 and PBS1, highlighting collaborations and influential ideas along the way. RPS5, a nucleotide-binding leucine-rich repeat (NLR) protein, is activated by the proteolytic cleavage of PBS1. We have shown that the cleavage site within PBS1 can be altered to contain cleavage sites for other proteases, enabling RPS5 activation by these proteases, thereby conferring resistance to different pathogens. This decoy approach has since been translated into crop species using endogenous PBS1 orthologs and holds strong potential for GMO-free development of new genetic resistance against important crop pathogens.

Optimizing the PBS1 Decoy System to Confer Resistance to Potyvirus Infection in Arabidopsis and Soybean

The Arabidopsis resistance protein RPS5 is activated by proteolytic cleavage of the protein kinase PBS1 by the Pseudomonas syringae effector protease AvrPphB. We have previously shown that replacing seven amino acids at the cleavage site of PBS1 with a motif cleaved by the NIa protease of turnip mosaic virus (TuMV) enables RPS5 activation upon TuMV infection. However, this engineered resistance conferred a trailing necrosis phenotype indicative of a cell-death response too slow to contain the virus. We theorized this could result from a positional mismatch within the cell between PBS1TuMV, RPS5, and the NIa protease. To test this, we relocalized PBS1TuMV and RPS5 to cellular sites of NIa accumulation. These experiments revealed that relocation of RPS5 away from the plasma membrane compromised RPS5-dependent cell death in Nicotiana benthamiana, even though PBS1 was efficiently cleaved. As an alternative approach, we tested whether overexpression of plasma membrane-localized PBS1TuMV could enhance RPS5 activation by TuMV. Significantly, overexpressing the PBS1TuMV decoy protein conferred complete resistance to TuMV when delivered by either agrobacterium or by aphid transmission, showing that RPS5-mediated defense responses are effective against bacterial and viral pathogens. Lastly, we have now extended this PBS1 decoy approach to soybean by modifying a soybean PBS1 ortholog to be cleaved by the NIa protease of soybean mosaic virus (SMV). Transgenic overexpression of this soybean PBS1 decoy conferred immunity to SMV, demonstrating that we can use endogenous PBS1 proteins in crop plants to engineer economically relevant disease resistant traits.

Organized Disassembly of Photosynthesis During Programmed Cell Death Mediated By Long Chain Bases

In plants, pathogen triggered programmed cell death (PCD) is frequently mediated by polar lipid molecules referred as long chain bases (LCBs) or ceramides. PCD interceded by LCBs is a well-organized process where several cell organelles play important roles. In fact, light-dependent reactions in the chloroplast have been proposed as major players during PCD, however, the functional aspects of the chloroplast during PCD are largely unknown. For this reason, we investigated events that lead to disassembly of the chloroplast during PCD mediated by LCBs. To do so, LCB elevation was induced with Pseudomonas syringae pv. tomato (a non-host pathogen) or Fumonisin B1 in Phaseolus vulgaris. Then, we performed biochemical tests to detect PCD triggering events (phytosphingosine rises, MPK activation and H2O2 generation) followed by chloroplast structural and functional tests. Observations of the chloroplast, via optical phenotyping methods combined with microscopy, indicated that the loss of photosynthetic linear electron transport coincides with the organized ultrastructure disassembly. In addition, structural changes occurred in parallel with accumulation of H2O2 inside the chloroplast. These features revealed the collapse of chloroplast integrity and function as a mechanism leading to the irreversible execution of the PCD promoted by LCBs.

Durability of Quantitative Resistance in Crops: Greater Than We Know?

Quantitative resistance (QR) to crop diseases has usually been much more durable than major-gene, effector-triggered resistance. It has been observed that the effectiveness of some QR has eroded as pathogens adapt to it, especially when deployment is extensive and epidemics occur regularly, but it generally declines more slowly than effector-triggered resistance. Changes in aggressiveness and specificity of diverse pathogens on cultivars with QR have been recorded, along with experimental data on fitness costs of pathogen adaptation to QR, but there is little information about molecular mechanisms of adaptation. Some QR has correlated or antagonistic effects on multiple diseases. Longitudinal data on cultivars' disease ratings in trials over several years can be used to assess the significance of QR for durable resistance in crops. It is argued that published data likely underreport the durability of QR, owing to publication bias. The implications of research on QR for plant breeding are discussed.

The plant hypersensitive response: concepts, control and consequences

The hypersensitive defence response is found in all higher plants and is characterized by a rapid cell death at the point of pathogen ingress. It is usually associated with pathogen resistance, though, in specific situations, it may have other consequences such as pathogen susceptibility, growth retardation and, over evolutionary timescales, speciation. Due to the potentially severe costs of inappropriate activation, plants employ multiple mechanisms to suppress inappropriate activation of HR and to constrain it after activation. The ubiquity of this response among higher plants despite its costs suggests that it is an extremely effective component of the plant immune system.

The hypersensitive response; the centenary is upon us but how much do we know?

With the centenary of the first descriptions of 'hypersensitiveness' following pathogenic challenge upon us, it is appropriate to assess our current understanding of the hypersensitive response (HR) form of cell death. In recent decades our understanding of the initiation, associated signalling, and some important proteolytic events linked to the HR has dramatically increased. Genetic approaches are increasingly elucidating the function of the HR initiating resistance genes and there have been extensive analyses of death-associated signals, calcium, reactive oxygen species (ROS), nitric oxide, salicylic acid, and now sphingolipids. At the same time, attempts to draw parallels between mammalian apoptosis and the HR have been largely unsuccessful and it may be better to consider the HR to be a distinctive form of plant cell death. We will consider if the HR form of cell death may occur through metabolic dysfunction in which malfunctioning organelles may play a major role. This review will highlight that although our knowledge of parts of the HR is excellent, a comprehensive molecular model is still to be attained.

Noel T. Keen--pioneer leader in molecular plant pathology

Noel T. Keen (1940-2002) made pioneering contributions to molecular plant pathology during a period when the study of disease mechanisms was transformed by the new tools of molecular genetics. His primary contributions involved race-specific elicitors of plant defenses and bacterial pectic enzymes. In collaboration with Brian J. Staskawicz and Frances Jurnak, respectively, Noel cloned the first avirulence gene and determined that pectate lyase C possessed a novel structural motif, known as the parallel beta-helix. Noel received his B.S. and M.S. from Iowa State University in Ames and his Ph.D. from the Department of Plant Pathology at the University of Wisconsin in Madison in 1968. He joined the faculty of the Department of Plant Pathology at the University of California at Riverside the same year and remained there his entire career. He served as Chair of the department from 1983 to 1989 and in 1997 assumed the William and Sue Johnson Endowed Chair in Molecular Plant Pathology. He became a Fellow of the American Phytopathological Society in 1991, a Fellow of the American Association for the Advancement of Science in 1996, a Fellow of the American Academy of Microbiology in 1997, and a member of the National Academy of Sciences in 1997. He was serving as President of the American Phytopathological Society (2001-2002) at the time of his death.

Structural Modifications and Programmed Cell Death of Chili Pepper Fruit Related to Resistance Responses to Colletotrichum gloeosporioides Infection

ABSTRACT Postharvest (detached) and in planta (attached) fruits of pepper plants, Capsicum annuum cv. Jejujaerae (susceptible) and Capsicum baccatum cv. PBC80 (resistant), inoculated with the anthracnose pathogen Colletotrichum gloeosporioides were examined using light, confocal laser scanning, and electron microscopy to compare the cytological differences between the compatible and incompatible interactions. In nonwound inoculation of postharvest pepper fruit, resistant pepper tissues showed a significant increase in the thickness of the cuticle layer compared with that of the susceptible and noninoculated fruit. Cytological features of programmed cell death (PCD) were observed in the resistant pepper fruit with postharvest inoculation, and these were characterized by positive responses to terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling. The oligonucleosomal fragments of DNA were confirmed electrophoretically as DNA laddering. The PCD-positive responses occurred around the inoculation sites early in in planta wound inoculation in the resistant pepper. Nuclear modifications and structural changes of hypersensitivity were also observed in the resistant fruit, including separation of the plasma membrane from the cell wall, dilation of the endoplasmic reticulum, accumulation of electron-dense inclusions in vacuoles, and cytoplasmic vacuolization accompanying fragmentation of the cytoplasm. These structural changes may also implicate PCD-like host responses. In addition, in planta wound inoculation resulted in cell enlargement and cell division during the later stages of infection to form a periderm-like boundary layer around the inoculation site.

The role and regulation of programmed cell death in plant-pathogen interactions

It is commonly known that animal pathogens often target and suppress programmed cell death (pcd) pathway components to manipulate their hosts. In contrast, plant pathogens often trigger pcd. In cases in which plant pcd accompanies disease resistance, an event called the hypersensitive response, the plant surveillance system has learned to detect pathogen-secreted molecules in order to mount a defence response. In plants without genetic disease resistance, these secreted molecules serve as virulence factors that act through largely unknown mechanisms. Recent studies suggest that plant bacterial pathogens also secrete antiapoptotic proteins to promote their virulence. In contrast, a number of fungal pathogens secrete pcd-promoting molecules that are critical virulence factors. Here, we review recent progress in determining the role and regulation of plant pcd responses that accompany both resistance and susceptible interactions. We also review progress in discerning the mechanisms by which plant pcd occurs during these different interactions.

A R2R3-MYB gene, AtMYB30, acts as a positive regulator of the hypersensitive cell death program in plants in response to pathogen attack

Hypersensitive response (HR) is a programmed cell death that is commonly associated with disease resistance in plants. Among the different HR-related early induced genes, the AtMYB30 gene is specifically, rapidly, and transiently expressed during incompatible interactions between Arabidopsis and bacterial pathogens. Its expression was also shown to be deregulated in Arabidopsis mutants affected in the control of cell death initiation. Here, we demonstrate that overexpression in Arabidopsis and tobacco of AtMYB30 (i) accelerates and intensifies the appearance of the HR in response to different avirulent bacterial pathogens, (ii) causes HR-like responses to virulent strains, and (iii) increases resistance against different bacterial pathogens, and a virulent biotrophic fungal pathogen, Cercospora nicotianae. In antisense AtMYB30 Arabidopsis lines, HR cell death is strongly decreased or suppressed in response to avirulent bacterial strains, resistance against different bacterial pathogens decreased, and the expression of HR- and defense-related genes was altered. Taken together, these results strongly suggest that AtMYB30 is a positive regulator of hypersensitive cell death.

Overexpression of Pto induces a salicylate-independent cell death but inhibits necrotic lesions caused by salicylate-deficiency in tomato plants

Tomato plants overexpressing the disease resistance gene Pto (35S::Pto) exhibit spontaneous cell death, accumulation of salicylic acid (SA), elevated expression of pathogenesis-related genes, and enhanced resistance to a broad range of pathogens. Because salicylate plays an important role in the cell death and defense activation in many lesion mimic mutants, we investigated the interaction of SA-mediated processes and the 35S::Pto-mediated defense pathway by introducing the nahG transgene that encodes salicylate hydroxylase. Here, we show that SA is not required for the 35S::Pto-activated microscopic cell death and plays a minor role in defense gene activation and general disease resistance in 35S::Pto plants. In contrast, temperature greatly affects the spontaneous cell death and general resistance in 35S::Pto plants, and high temperature inhibits the cell death. The NahG tomato plants develop spontaneous, unconstrained necrotic lesions on leaves. These lesions also are initiated by the inoculation of a virulent strain of Pseudomonas syringae pv. tomato. However, the NahG-dependent necrotic lesions are inhibited in the NahG/35S::Pto plants. This inhibition is most pronounced under conditions favoring the 35S::Pto-mediated spontaneous cell death development. These results indicate that the signaling pathways activated by Pto overexpression suppress the cellular damage that is caused by SA depletion. We also found that ethylene is dispensable for the 35S::Pto-mediated general defense.

Temperature Sensitivity of the Hypersensitive Response of Bell Pepper to Xanthomonas axonopodis pv. vesicatoria

ABSTRACT When bacterial spot-resistant pepper plants carrying resistance gene Bs2 and infiltrated with incompatible strains of Xanthomonas axonopodis pv. vesicatoria carrying a functional avrBs2 gene (races P1 and P3) were incubated at 32 degrees C, they exhibited an electrolyte leakage and bacterial multiplication pattern in planta similar to that obtained with a compatible strain (race P4) carrying a nonfunctional avrBs2 gene. They also developed disease-like symptoms. Pretreatment of incompatible bacteria at 32 degrees C before infiltration caused a delay in electrolyte leakage less pronounced than that caused by exposing plants to 32 degrees C. Also, plants had to be exposed to 32 degrees C for an hour prior to inoculation to increase symptom expression. These data suggest that the Bs2 gene is temperature sensitive. In other experiments, the avrBs1-Bs1 interaction appeared to be the most heat tolerant and thus the least likely to revert to compatible, whereas the avrBs3-Bs3 interaction had an intermediate sensitivity to elevated temperatures.

A bean cDNA expressed during a hypersensitive reaction encodes a putative calcium-binding protein

The hypersensitive reaction (HR) is an inducible plant response that is associated with disease resistance. It is characterized by rapid, localized cell death at the site of infection and is believed to inhibit the spread of invading pathogens. We have isolated a cDNA clone, designated Hra32 (for hypersensitive reaction associated), corresponding to an RNA transcript that accumulates in bean during an HR. The predicted protein product of the Hra32 cDNA is an approximately 17 kDa protein of 161 amino acids, with four putative EF-hand calcium-binding domains. The temporal pattern of Hra32 transcript accumulation correlated closely with the onset of the HR in bean after inoculation with incompatible Pseudomonas syringae pv. tabaci and pv. tomato and with tobacco necrosis virus. Hra32 transcript also accumulated in bean in response to compatible P. syringae pv. phaseolicola and was correlated with necrotic cell death associated with disease lesion formation. A more transient pattern of Hra32 transcript accumulation occurred in bean in response to general stimuli that did not result in the HR or host cell death. These treatments included infiltration with a P. syringae pv. tabaci Hrp- mutant, P. syringae pv. tabaci cells treated with kanamycin, Escherichia coli, P. fluorescens, or glutathione, and in response to wounding. Thus, there was differential accumulation of the Hra32 transcript in response to specific stimuli resulting in the HR, compared with general stimuli that did not result in cell death. We hypothesize that the Hra32 product may be a component of the pathway that leads to hypersensitive cell death.

Reactive oxygen intermediates as mediators of programmed cell death in plants and animals

Programmed cell death (PCD) is a physiological process occurring during development and in pathological conditions of animals and plants. The cell death program can be subdivided into three functionally different phases: a stimulus-dependent induction phase, an effector phase during which the wide range of death-stimuli are translated to a central coordinator, and a degradation phase during which the alterations commonly considered to define PCD (apoptotic morphology of the nucleus and chromatin fragmentation) become apparent. Recent studies suggest that mitochondrial permeability transition is the central coordinator of PCD and deciding whether or not a cell will die. There is increasing evidence that reactive oxygen intermediates (ROI) serve as direct and indirect mediators of PCD in mammalian and plant cells. Overexpression of genes encoding pro- and antioxidant enzymes in transgenic animals and plants has been informative regarding the function of ROI. Recent data imply a dual role of ROI in the apoptotic process: first, as a facultative signal during the induction phase, and, second, as a common consequence of mitochondrial permeability transition leading to the final destruction of the cell. The present review discusses and compares new insights into the function of ROI during PCD in mammalian cells and in human and plant diseases.

The hypersensitive response. A programmed cell death associated with plant resistance

In plants, the hypersensitive response (HR) is defined as a rapid cell death occurring in response to pathogen attack, and is closely related to active resistance. Initiation of the HR process begins with the recognition of the pathogen by the plant, which is mediated mainly by the pathogen avirulence genes and the plant resistance genes. Then, complex signal transduction pathways intervene, involving changes in protein phosphorylation, production of reactive oxygen species and modification of ion fluxes. Components required for the regulation of the HR cell death are now being identified genetically by the isolation of mutants, in contrast to those involved in the execution of the cell death programme, which are still largely unknown. Further genetic and biochemical analyses will undoubtedly answer the question as to whether this form of programmed cell death (PCD) can be compared with other forms of PCD in plants and with apoptosis in animals.

Current concepts of active defense in plants

A growing body of evidence indicates that elicitation of primary active defense responses results from a recognition event frequently involving protein-protein interactions. Most pathogen avirulence determinants eliciting resistance gene-dependent responses have been shown to be proteins with no apparent enzymic activity. Disruption of the tertiary and quaternary structure of these proteins abolishes their elicitor activity. Critical to their elicitor activity is their display by the pathogen. Resistance genes are proposed to function as receptors for the eliciting proteins. The most consistent feature of resistance gene products is the presence of potential protein binding domains in the form of leucine-rich repeat regions, and there is direct evidence for the physical interaction of elicitor proteins and receptor proteins in several cases. Thus in many but not all cases the primary recognition event eliciting an active defense response during incompatible interactions appears to be a protein-protein interaction occurring between a specific pathogen protein and a strategically placed receptor protein in the host cell. The interaction of elicitor protein with the receptor protein activates a signal transduction pathway leading to programmed cell death and an oxidative burst.

PROGRAMMED CELL DEATH IN PLANT-PATHOGEN INTERACTIONS

Plants cope with pathogen attacks by using mechanisms of resistance that rely both on preformed protective defenses and on inducible defenses. The latter are the most well studied, and progress is being made in determining which induced responses are responsible for limiting pathogen growth. Many plant-pathogen interactions are accompanied by plant cell death. Recent evidence suggests that this cell death is often programmed and results from an active process on the part of the host. The review considers the roles and possible mechanisms of plant cell death in response to pathogens.

Pseudomonas syringae pv. syringae harpinPss: a protein that is secreted via the Hrp pathway and elicits the hypersensitive response in plants

The ability of P. syringae to elicit the hypersensitive response in nonhost plants or pathogenesis in hosts is controlled by hrp genes. The P. syringae pv. syringae 61 hrpZ gene encodes harpinPss, a 34.7 kd extracellular protein that elicits hypersensitive necrosis in tobacco and other plants. HarpinPss is heat stable, glycine rich, dissimilar in amino acid sequence to any known protein, produced only in apoplastic fluid-mimicking minimal media, and secreted in a HrpH-dependent manner. The carboxy-terminal 148 amino acid portion of harpinPss contains two directly repeated sequences of GGGLGTP and QTGT and is sufficient and necessary for elicitor activity. The necrosis elicited by harpinPss is an active response of the plant, which can be inhibited by alpha-amanitin, cycloheximide, lanthanum chloride, or sodium vanadate.

Sequence domains required for the activity of avirulence genes avrB and avrC from Pseudomonas syringae pv. glycinea

avrB and avrC from Pseudomonas syringae pv. glycinea share significant amino acid homology but interact with different soybean resistance genes to elicit the hypersensitive defense reaction. Recombinant genes constructed between avrB and avrC revealed that the central regions were required for avirulence gene activity but the 5' and 3' termini were interchangeable. Recombinants involving the central regions did not yield any detectable avirulence gene activity, and no new avirulence phenotypes were observed from any of the chimeric genes. These results suggest that the protein products of avrB and avrC possess catalytic properties that are required for the avirulence phenotypes.

Influence of Pseudomonas syringae culture conditions on initiation of the hypersensitive response of culture tobacco cells

The inhibitor sensitivity and timing of the ionic response of suspension-cultured tobacco cells were used as a bioassay for the Pseudomonas syringae signal that elicits the hypersensitive response in resistant plants. The ionic response of tobacco cell suspensions inoculated with P. syringae pv. syringae 61 and P. syringae pv. pisi grown in rich media was inhibited by rifampin, tetracycline, and streptomycin during a 2- to 2.5-h induction stage. Coculturing the bacteria with tobacco cells for 3 h or more before inoculating fresh tobacco cells specifically abolished the sensitivity of the ionic response to these inhibitors and reduced the response time of the tobacco cells from 3 to 1 h. The apparent activation of the bacteria during coculture was not dependent on the plant cells and could be achieved by incubating the bacteria in a nitrogen-deficient medium containing a metabolizable carbon source. Addition of proteose peptone and Casamino Acids to this medium suppressed activation of the bacteria. The results suggest that the hypersensitive response-eliciting signal forms late in the induction stage, perhaps as a result of the derepression of some of the P. syringae genes functional in elicitation of the hypersensitive response. The nature of the activated state remains elusive but is consistent with the accumulation of protein(s) whose activity indirectly elicits the ionic response.

Gene expression in Brassica campestris showing a hypersensitive response to the incompatible pathogen Xanthomonas campestris pv. vitians

Xanthomonas campestris pv. vitians, a pathogen of lettuce, elicits a hypersensitive response within 12 hours of inoculation into Brassica leaves, characterized by tissue collapse, loss of membrane integrity, vein blockage and melanin production. In contrast, the compatible pathogen, X. c. pv. campestris, has no visible effects on leaves for 48 hours, after which inoculated areas show chlorosis which eventually spreads, followed by rotting.mRNA was prepared from leaves inoculated with suspensions of both pathovars or with sterile medium up to 24 hours following inoculation. In vitro translation of total and poly A(+) RNA in rabbit reticulocyte lysate in the presence of (35)S methionine followed by separation of the polypeptide products by 2D-PAGE, allowed comparison of the effects of these treatments on plant gene expression. Major changes in gene expression were observed as a consequence of the inoculation technique. In addition, after inoculation with X. c. vitians, up to fifteen additional major polypeptides appeared or greatly increased by four hours. Some of these had disappeared by nine hours and several more had appeared. No major polypeptides disappeared or decreased greatly in intensity following inoculation with X. c. vitians.

Gene cluster of Pseudomonas syringae pv. "phaseolicola" controls pathogenicity of bean plants and hypersensitivity of nonhost plants

Loss of the ability of Pseudomonas syringae pv. "phaseolicola" NPS3121 to elicit a hypersensitive response on tobacco and other nonhost plants was associated with loss of pathogenicity on the susceptible host bean. Eight independent, prototrophic transposon Tn5 insertion mutants which had lost the ability to elicit a hypersensitive response on tobacco plants were identified. Six of these mutants no longer produced disease lesions on primary leaves of the susceptible bean cultivar Red Kidney and failed to elicit a hypersensitive response on the resistant bean cultivar Red Mexican and on the nonhost plants tomato, cowpea, and soybean. The two remaining mutants had reduced pathogenicity on Red Kidney bean and elicited variable hypersensitive responses on the other plants tested. Southern blot analysis indicated that each mutant carried a single independent Tn5 insertion in one of three EcoRI fragments of about 17, 7, and 5 kilobases. Marker exchange mutagenesis further supported the conclusion that the pleiotropic mutant phenotype was not associated with multiple Tn5 insertions. A genomic library of the wild-type strain was constructed in the cosmid vector pLAFR3. A recombinant plasmid, designated pPL6, that carried P. syringae pv. "phaseolicola" genomic sequences was identified by colony hybridization. This plasmid restored the wild-type phenotype to all but one mutant, suggesting that genes affected by the insertions were clustered. Structural analysis of pPL6 and the wild-type genome indicated that the 17- and 5-kilobase EcoRI fragments were contiguous in the strain NPS3121 genome.