Pseudomonas syringae pv. tomato infection of tomato plants is mediated by GABA and l‐Pro chemoperception

Foliar bacterial pathogens have to penetrate the plant tissue and access the interior of the apoplast in order to initiate the pathogenic phase. The entry process is driven by chemotaxis towards plant‐derived compounds in order to locate plant openings. However, information on plant signals recognized by bacterial chemoreceptors is scarce. Here, we show that the perception of GABA and l‐Pro, two abundant components of the tomato apoplast, through the PsPto‐PscC chemoreceptor drives the entry of Pseudomonas syringae pv. tomato into the tomato apoplast. The recognition of both compounds by PsPto‐PscC caused chemoattraction to both amino acids and participated in the regulation of GABA catabolism. Mutation of the PsPto‐PscC chemoreceptor caused a reduced chemotactic response towards these compounds which in turn impaired entry and reduced virulence in tomato plants. Interestingly, GABA and l‐Pro levels significantly increase in tomato plants upon pathogen infection and are involved in the regulation of the plant defence response. This is an example illustrating how bacteria respond to plant signals produced during the interaction as cues to access the plant apoplast and to ensure efficient infection.     

Construction and Use of a Nonradioactive DNA Hybridization Probe for Detection of Pseudomonas syringae pv. Tomato on Tomato Plants

Pseudomonas syringae pv. tomato, the causal agent for bacterial speck of tomato, produces the phytotoxin coronatine. A 5.3-kilobase XhoI fragment from the chromosomal region controlling toxin production was cloned into the plasmid pGB2, and the resulting recombinant plasmid, pTPR1, was tested for its ability to serve as a diagnostic probe for P. syringae pv. tomato. In a survey of 75 plant-associated bacteria, pTPR1 hybridized exclusively to those strains that produced coronatine. The detection limit for this probe, which was labeled with the Chemiprobe nonradioactive reporter system, was approximately 4 × 10³ CFU of lesion bacteria. During the 1989 growing season, a total of 258 leaf and fruit lesions from nine tomato fields were screened for P. syringae pv. tomato by using pTPR1 and the culture method of detection. The best agreement between the two methods, 90%, occurred early in the season with samples taken from relatively young (5-week-old) plants. Young plants also had a higher percentage of P. syringae pv. tomato-positive lesions. P. syringae pv. tomato was the only coronatine producer recovered from the nine tomato fields. All 244 P. syringae pv. tomato strains isolated during this study reacted strongly with the probe. The P. syringae pv. tomato population of healthy field tomato leaves was determined by a pTPR1 colony hybridization procedure. Every probe-positive colony that was isolated and characterized was identified as P. syringae pv. tomato. The pTPR1 probe should expedite disease diagnosis and facilitate epidemiological studies of this pathogen. It also should aid in screening transplant seedlings for bacterial speck infestation.     

Generation and Characterization of Tn5 Insertion Mutations in Pseudomonas syringae pv. tomato

Tn5-induced insertion mutations were generated in the Pseudomonas syringae pv. tomato genome by mating this plant pathogen with an Escherichia coli strain carrying the suicide plasmid vector for Tn5, pGS9. Km^r transconjugants occurred at frequencies ranging from 2 × 10⁻⁷ to 9 × 10⁻⁶; approximately 5.5% of these transconjugants were also Cm^r, indicating the presence of additional pGS9 DNA sequences. Approximately 1% of the Km^r Cm^s mutants were auxotrophic. Southern blot analysis revealed that the Tn5 element had inserted into one unique site on the chromosome for each Km^r Cm^s transconjugant examined. Physical and genetic tests of Tn5-induced auxotrophs showed that Tn5 mutations in P. syringae pv. tomato were very stable and that secondary transposition of Tn5 or its insertion sequence IS50 was a rare event. Nine of 920 Km^r Cm^s transconjugants screened on tomato seedlings either were avirulent or produced very mild symptoms. Each of the virulence mutants was the result of a unique single-site Tn5 insertion. Five mutants also failed to induce a hypersensitivity reaction on tobacco.     

Tomato 14-3-3 Protein 7 Positively Regulates Immunity-Associated Programmed Cell Death by Enhancing Protein Abundance and Signaling Ability of MAPKKK α

Programmed cell death (PCD) is triggered when Pto, a Ser-Thr protein kinase, recognizes either the AvrPto or AvrPtoB effector from Pseudomonas syringae pv tomato. This PCD requires mitogen-activated protein kinase kinase kinase (MAPKKK α ) as a positive regulator in tomato (Solanum lycopersicum) and Nicotiana benthamiana. To examine how PCD-eliciting activity of the tomato MAPKKK α protein is regulated, we screened for MAPKKK α -interacting proteins in tomato and identified a 14-3-3 protein, TFT7. Virus-induced gene silencing using the TFT7 gene in N. benthamiana compromised both Pto- and MAPKKK α -mediated PCD, and coexpression of TFT7 with tomato MAPKKK α enhanced MAPKKK α -mediated PCD. TFT7 was also required for PCD associated with several other disease resistance proteins and contributed to resistance against P. syringae pv tomato. Coexpression of TFT7 with MAPKKK α in vivo caused increased accumulation of the kinase and enhanced phosphorylation of two MAP kinases. TFT7 protein contains a phosphopeptide binding motif that is present in human 14-3-3 ϵ, and substitutions in this motif abolished interaction with MAPKKK α in vivo and also the PCD-enhancing activity of TFT7. A 14-3-3 binding motif, including a putative phosphorylated Ser-535, is present in the C-terminal region of MAPKKK α. An S535A substitution in MAPKKK α reduced interaction with TFT7 and both PCD-eliciting ability and stability of MAPKKK α. Our results provide new insights into a role for 14-3-3 proteins in regulating immunity-associated PCD pathways in plants.     

Effects of Tabtoxinine-beta-Lactam on Nitrogen Metabolism in Avena sativa L. Roots

The effects of tabtoxinine-β-lactam (T-β-L) on nitrate uptake and glutamine synthetase (GS) and nitrate reductase (NR) activities in roots of Avena sativa seedlings were determined. Seven-day-old oat seedlings placed in a 10 mm KNO₃ and 0.5 mm T-β-L solution for 24 hours took up T-β-L and lost approximately 90% of their root GS activity. [³H]-T-β-L taken up by roots of seven-day-old oat seedlings was associated with GS immunoprecipitated from the extract of these roots. Total nitrate uptake and in vivo NR activity were decreased approximately 50% in the T-β-L treated roots. However, T-β-L uptake did not affect the induction phases of nitrate uptake or reduction, nor did it inhibit in vitro NR activity. Thus, the decrease in nitrate uptake and reduction is a secondary effect of T-β-L action. Roots of seven-day-old oat seedlings were inoculated with Pseudomonas syringae pv tabaci (Tox+) and the pathogen population in the rhizosphere was estimated by dilution plate count; 6 × 10¹³ bacteria were recovered after 3 days, as compared to the original inoculation with 7 × 10⁹ bacteria, indicating a significant growth of the pathogen in the rhizosphere. The bacteria recovered from the rhizosphere caused chlorosis in tobacco leaves and produced T-β-L in culture; 1 × 10¹⁴ bacteria were recovered from roots of seedlings inoculated with P. syringae pv tabaci (Tox−) using the same inoculation and assay procedure as for the pv tabaci (Tox+). Extracts of surface-sterilized roots previously inoculated with P. syringae pv tabaci (Tox+) did not produce viable bacterial cultures when plated out on a complete medium. Oat seedlings growing in sand culture and inoculated with P. syringae pv tabaci (Tox+) had developed chlorosis, and root GS activity had declined to less than 10% of controls after 3 days. Conversely, seedlings inoculated with P. syringae pv tabaci (Tox−) never developed chlorosis and maintained normal levels of GS activity. All oat plants inoculated with P. syringae pv tabaci (Tox+) died within 7 days after inoculation as compared to the plants inoculated with P. syringae pv tabaci (Tox−) which grew to maturity.     

Morphology of Leaf Surfaces of Tomato Cultivars in Relation to Possible Invasion into the Leaf by Pseudomonas syringae pv. tomato

The role of morphological characteristics of tomato leaves inthe infection process of the pathogen Pseudomonas syringae pv.tomato was studied in 15 cultivars of varying susceptibility.Natural openings such as stomata, broken trichomes and cuticularcracks were counted. It was found that susceptibility increasedin proportion to the number of these potential penetration sites.However, their role is probably limited because even in thehighly resistant cultivars there are enough natural openingsto enable successful bacterial penetration. Bacterial speck of tomato, Lycopersicon esculentum, phytopathogenic bacteria, plant morphology, Pseudomonas syringae pv. tomato, tomato     

Differences Between Lipopolysaccharide Compositions of Plant Pathogenic and Saprophytic Pseudomonas Species

Lipopolysaccharides (LPS) were obtained by washing cells of plant pathogenic and saprophytic Pseudomonas species with saline (fraction 1) and then with saline-EDTA (fraction 2). The cells subsequently were extracted with phenol to yield a third aqueous preparation (fraction 3). Each fraction type contained the LPS components, lipid A, heptose, 2-keto-3-deoxy sugar, and neutral and amino sugars. The neutral sugar compositions of fractions 1, 2, and 3, although similar within a species, differed between the Pseudomonas species. The LPS of two pathovars (pv.) of Pseudomonas syringae had glucose and rhamnose as major components: 13 (±3)% glucose and 87 (±3)% rhamnose for P. syringae pv. pisi and 18 (±5)% glucose and 76 (±2)% rhamnose for P. syringae pv. syringae. Fucose was present in addition to glucose and rhamnose for P. syringae pv. phaseolicola (68 [±8]% rhamnose, 14 [±1]% fucose, and 14 [±5]% glucose) and P. syringae pv. tabaci (24 [±2]% rhamnose, 54 [±3]% fucose, and 17 [±1]% glucose). The LPS from different races of P. syringae pv. pisi and P. syringae pv. phaseolicola could not be distinguished by neutral sugar composition. Three saprophytic species, P. aeruginosa, P. fluorescens, and P. putida, also produced LPS which had different proportions of rhamnose, fucose, and glucose. The LPS from three isolates of P. putida were distinct in possessing a high proportion of amino sugar and containing glucose as the major neutral sugar component (86 to 100%). The LPS fractions from plant pathogenic and saprophytic Pseudomonas species did not elicit browning or phytoalexin production in treated dark red kidney bean cotyledons or red Mexican bean leaves. Rather, chlorosis of the LPS-treated leaf tissue was observed.     

Polysaccharides of <Emphasis Type="Italic">Pseudomonas</Emphasis> Pathovar Strains That Infect Pea,Tomato, and Soya Bean

In order to understand the mode of action of taxonomically related Pseudomonas syringae pathovar strains that infect pea, tomato, and soya bean, we examined their extracellular polysaccharides (EPS). Maximum production of polysaccharide in shake culture of these pathogens was observed between 24 and 60 h. P. syringae pv. pisi 519, the bacterial blight pathogen of pea, produced a higher amount of polysaccharide (34.87 g/mL) at 60 h compared with 32.67 g/mL produced by P. syringae pv. glycinea NCPPB 1783, the bacterial blight pathogen of soya bean, and 30.03 g/mL produced by P. syringae pv. tomato NCPPB 269, the bacterial speck pathogen of tomato. EPS produced by P. syringae pv. pisi 519, P. syringae pv. tomato NCPPB 269, and P. syringae pv. glycinea NCPPB 1783 was characterized with infrared (FTIR), nuclear magnetic resonance (NMR), high performance thin layer chromatography, (HPTLC), and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. HPTLC profiles revealed the presence of glucose and glucuronic acid in all bacteria and mannose only in P. syringae pv. tomato. Molecular mass of EPS of P. syringae pv. pisi (m/z 933.8), P. syringae pv. tomato (m/z 950.4), and P. syringae pv. glycinea (m/z 933.5) was confirmed by MALDI-TOF mass spectrometry.     

Nutritional Similarity between Leaf-Associated Nonpathogenic Bacteria and the Pathogen Is Not Predictive of Efficacy in Biological Control of Bacterial Spot of Tomato

It has been demonstrated that for a nonpathogenic, leaf-associated bacterium, effectiveness in the control of bacterial speck of tomato is correlated with the similarity in the nutritional needs of the nonpathogenic bacterium and the pathogen Pseudomonas syringae pv. tomato. This relationship was investigated further in this study by using the pathogen Xanthomonas campestris pv. vesicatoria, the causal agent of bacterial spot of tomato, and a collection of nonpathogenic bacteria isolated from tomato foliage. The effects of inoculation of tomato plants with one of 34 nonpathogenic bacteria prior to inoculation with the pathogen X. campestris pv. vesicatoria were quantified by determining (i) the reduction in disease severity (number of lesions per square centimeter) in greenhouse assays and (ii) the reduction in leaf surface pathogen population size (log₁₀ of the number of CFU per leaflet) in growth chamber assays. Nutritional similarity between the nonpathogenic bacteria and X. campestris pv. vesicatoria was quantified by using either niche overlap indices (NOI) or relatedness in cluster analyses based upon in vitro utilization of carbon or nitrogen sources reported to be present in tomato tissues or in Biolog GN plates. In contrast to studies with P. syringae pv. tomato, nutritional similarity between the nonpathogenic bacteria and the pathogen X. campestris pv. vesicatoria was not correlated with reductions in disease severity. Nutritional similarity was also not correlated with reductions in pathogen population size. Further, the percentage of reduction in leaf surface pathogen population size was not correlated with the percentage of reduction in disease severity, suggesting that the epiphytic population size of X. campestris pv. vesicatoria is not related to disease severity and that X. campestris pv. vesicatoria exhibits behavior in the phyllosphere prior to lesion formation that is different from that of P. syringae pv. tomato.     

A Lipoxygenase from Leaves of Tomato (Lycopersicon esculentum Mill.) Is Induced in Response to Plant Pathogenic Pseudomonads

Lipoxygenase (LOX) mRNA, enzyme protein, and enzyme activity were found to be induced in leaves of tomato (Lycopersicon esculentum Mill. cv Moneymaker) on inoculation with plant pathogenic bacteria. The rate of enzyme activity with linoleic or linolenic acid as substrate was approximately 10 times greater than that with arachidonic acid. Optimum activity was at pH 7.0. In the incompatible interaction, which was associated with a hypersensitive reaction (HR), a single band with relative molecular weight approximately 100,000 was revealed by probing western blots of enzyme extracts with antiserum raised against a pea lipoxygenase. Changes in the intensity of this band reflected the changes observed in LOX enzyme activity after bacterial inoculations. In the hypersensitive reaction, i.e. after inoculation with Pseudomonas syringae pv syringae, LOX mRNA was induced by 3 hours and enzyme activity began to increase between 6 and 12 hours and had reached maximum levels by 24 to 48 hours. In tomato leaves inoculated with P. syringae pv tomato (compatible interaction), LOX mRNA was induced later and enzyme activity changed only marginally in the first 24 hours, then increased steadily up to 72 hours, reaching the levels seen in the HR.     

Protection of Tomato Seedlings against Infection by Pseudomonas syringae pv. Tomato by Using the Plant Growth-Promoting Bacterium Azospirillum brasilense

Pseudomonas syringae pv. tomato, the causal agent of bacterial speck of tomato, and the plant growth-promoting bacterium Azospirillum brasilense were inoculated onto tomato plants, either alone, as a mixed culture, or consecutively. The population dynamics in the rhizosphere and foliage, the development of bacterial speck disease, and their effects on plant growth were monitored. When inoculated onto separate plants, the A. brasilense population in the rhizosphere of tomato plants was 2 orders of magnitude greater than the population of P. syringae pv. tomato (10⁷ versus 10⁵ CFU/g [dry weight] of root). Under mist chamber conditions, the leaf population of P. syringae pv. tomato was 1 order of magnitude greater than that of A. brasilense (10⁷ versus 10⁶ CFU/g [dry weight] of leaf). Inoculation of seeds with a mixed culture of the two bacterial strains resulted in a reduction of the pathogen population in the rhizosphere, an increase in the A. brasilense population, the prevention of bacterial speck disease development, and improved plant growth. Inoculation of leaves with the mixed bacterial culture under mist conditions significantly reduced the P. syringae pv. tomato population and significantly decreased disease severity. Challenge with P. syringae pv. tomato after A. brasilense was established in the leaves further reduced both the population of P. syringae pv. tomato and disease severity and significantly enhanced plant development. Both bacteria maintained a large population in the rhizosphere for 45 days when each was inoculated separately onto tomato seeds (10⁵ to 10⁶ CFU/g [dry weight] of root). However, P. syringae pv. tomato did not survive in the rhizosphere in the presence of A. brasilense. Foliar inoculation of A. brasilense after P. syringae pv. tomato was established on the leaves did not alleviate bacterial speck disease, and A. brasilense did not survive well in the phyllosphere under these conditions, even in a mist chamber. Several applications of a low concentration of buffered malic acid significantly enhanced the leaf population of A. brasilense (>10⁸ CFU/g [dry weight] of leaf), decreased the population of P. syringae pv. tomato to almost undetectable levels, almost eliminated disease development, and improved plant growth to the level of uninoculated healthy control plants. Based on our results, we propose that A. brasilense be used in prevention programs to combat the foliar bacterial speck disease caused by P. syringae pv. tomato.     

The Pseudomonas syringae pv. tomato HrpW Protein Has Domains Similar to Harpins and Pectate Lyases and Can Elicit the Plant Hypersensitive Response and Bind to Pectate

The host-specific plant pathogen Pseudomonas syringae elicits the hypersensitive response (HR) in nonhost plants and secretes the HrpZ harpin in culture via the Hrp (type III) secretion system. Previous genetic evidence suggested the existence of another harpin gene in the P. syringae genome. hrpW was found in a region adjacent to the hrp cluster in P. syringae pv. tomato DC3000. hrpW encodes a 42.9-kDa protein with domains resembling harpins and pectate lyases (Pels), respectively. HrpW has key properties of harpins. It is heat stable and glycine rich, lacks cysteine, is secreted by the Hrp system, and is able to elicit the HR when infiltrated into tobacco leaf tissue. The harpin domain (amino acids 1 to 186) has six glycine-rich repeats of a repeated sequence found in HrpZ, and a purified HrpW harpin domain fragment possessed HR elicitor activity. In contrast, the HrpW Pel domain (amino acids 187 to 425) is similar to Pels from Nectria haematococca, Erwinia carotovora, Erwinia chrysanthemi, and Bacillus subtilis, and a purified Pel domain fragment did not elicit the HR. Neither this fragment nor the full-length HrpW showed Pel activity in A₂₃₀ assays under a variety of reaction conditions, but the Pel fragment bound to calcium pectate, a major constituent of the plant cell wall. The DNA sequence of the P. syringae pv. syringae B728a hrpW was also determined. The Pel domains of the two predicted HrpW proteins were 85% identical, whereas the harpin domains were only 53% identical. Sequences hybridizing at high stringency with the P. syringae pv. tomato hrpW were found in other P. syringae pathovars, Pseudomonas viridiflava, Ralstonia (Pseudomonas) solanacearum, and Xanthomonas campestris. ΔhrpZ::nptII or hrpW::ΩSp^r P. syringae pv. tomato mutants were little reduced in HR elicitation activity in tobacco, whereas this activity was significantly reduced in a hrpZ hrpW double mutant. These features of hrpW and its product suggest that P. syringae produces multiple harpins and that the target of these proteins is in the plant cell wall.     

Ribosomal protein QM/RPL10 positively regulates defence and protein translation mechanisms during nonhost disease resistance

Ribosomes play an integral part in plant growth, development, and defence responses. We report here the role of ribosomal protein large (RPL) subunit QM/RPL10 in nonhost disease resistance. The RPL10-silenced Nicotiana benthamiana plants showed compromised disease resistance against nonhost pathogen Pseudomonas syringae pv. tomato T1. The RNA-sequencing analysis revealed that many genes involved in defence and protein translation mechanisms were differentially affected due to silencing of NbRPL10. Arabidopsis AtRPL10 RNAi and rpl10 mutant lines showed compromised nonhost disease resistance to P. syringae pv. tomato T1 and P. syringae pv. tabaci. Overexpression of AtRPL10A in Arabidopsis resulted in reduced susceptibility against host pathogen P. syringae pv. tomato DC3000. RPL10 interacts with the RNA recognition motif protein and ribosomal proteins RPL30, RPL23, and RPS30 in the yeast two-hybrid assay. Silencing or mutants of genes encoding these RPL10-interacting proteins in N. benthamiana or Arabidopsis, respectively, also showed compromised disease resistance to nonhost pathogens. These results suggest that QM/RPL10 positively regulates the defence and translation-associated genes during nonhost pathogen infection.     

The PTI1-like kinase ZmPti1a from maize (Zea mays L.) co-localizes with callose at the plasma membrane of pollen and facilitates a competitive advantage to the male gametophyte

Background  

The tomato kinase Pto confers resistance to bacterial speck disease caused by Pseudomonas syringae pv. tomato in a gene for gene manner. Upon recognition of specific avirulence factors the Pto kinase activates multiple signal transduction pathways culminating in induction of pathogen defense. The soluble cytoplasmic serine/threonine kinase Pti1 is one target of Pto phosphorylation and is involved in the hypersensitive response (HR) reaction. However, a clear role of Pti1 in plant pathogen resistance is uncertain. So far, no Pti1 homologues from monocotyledonous species have been studied.     

Expression of a bacterial ice nucleation gene in plants

We have introduced an ice nucleation gene (inaZ) from Pseudomonas syringae pv. syringae into Nicotiana tabacum, a freezing-sensitive species, and Solanum commersonii, a freezing-tolerant species. Transformants of both species showed increased ice nucleation activity over untransformed controls. The concentration of ice nuclei detected at −10.5°C in 15 different primary transformants of S. commersonii varied by over 1000-fold, and the most active transformant contained over 100 ice nuclei/mg of tissue. The temperature of the warmest freezing event in plant samples of small mass was increased from approximately −12°C in the untransformed controls to −4°C in inaZ-expressing transformants. The threshold nucleation temperature of samples from transformed plants did not increase appreciably with the mass of the sample. The most abundant protein detected in transgenic plants using immunological probes specific to the inaZ protein exhibited a higher mobility on sodium dodecyl sulfate polyacrylamide gels than the inaZ protein from bacterial sources. However, some protein with a similar mobility to the inaZ protein could be detected. Although the warmest ice nucleation temperature detected in transgenic plants is lower than that conferred by this gene in P. syringae (−2°C), our results demonstrate that the ice nucleation gene of P. syringae can be expressed in plant cells to produce functional ice nuclei.     

Studies on the Infection,Colonization, and Movement of Pseudomonas syringae pv. actinidiae in Kiwifruit Tissues Using a GFPuv-Labeled Strain

Kiwifruit bacterial canker, an economically important disease caused by Pseudomonas syringae pv. actinidiae (Psa), has caused severe losses in all major areas of kiwifruit cultivation. Using a GFPuv-labeled strain of Psa, we monitored the invasion, colonization, and movement of the pathogen in kiwifruit twigs, leaves and veins. The pathogen can invade twigs through both wounds and natural openings; the highest number of Psa is obtained in cut tissues. We determined that, following spray inoculation, Psa-GFPuv could infect leaves and cause lesions in the presence and absence of wounds. Light and transmission electron microscopic observations showed that bacterial cells colonize both phloem and xylem vessels. Bacterial infection resulted in marked alterations of host tissues including the disintegration of organelles and degeneration of protoplasts and cell walls. Furthermore, low temperature was conducive to colonization and movement of Psa-GFPuv in kiwifruit tissues. Indeed, the pathogen migrated faster at 4°C than at 16°C or 25°C in twigs. However, the optimum temperature for colonization and movement of Psa in leaf veins was 16°C. Our results, revealing a better understanding of the Psa infection process, might contribute to develop more efficacious disease management strategies.