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1.
Systemic acquired resistance (SAR) develops in response to local microbial leaf inoculation and renders the whole plant more resistant to subsequent pathogen infection. Accumulation of salicylic acid (SA) in noninfected plant parts is required for SAR, and methyl salicylate (MeSA) and jasmonate (JA) are proposed to have critical roles during SAR long-distance signaling from inoculated to distant leaves. Here, we address the significance of MeSA and JA during SAR development in Arabidopsis thaliana. MeSA production increases in leaves inoculated with the SAR-inducing bacterial pathogen Pseudomonas syringae; however, most MeSA is emitted into the atmosphere, and only small amounts are retained. We show that in several Arabidopsis defense mutants, the abilities to produce MeSA and to establish SAR do not coincide. T-DNA insertion lines defective in expression of a pathogen-responsive SA methyltransferase gene are completely devoid of induced MeSA production but increase systemic SA levels and develop SAR upon local P. syringae inoculation. Therefore, MeSA is dispensable for SAR in Arabidopsis, and SA accumulation in distant leaves appears to occur by de novo synthesis via isochorismate synthase. We show that MeSA production induced by P. syringae depends on the JA pathway but that JA biosynthesis or downstream signaling is not required for SAR. In compatible interactions, MeSA production depends on the P. syringae virulence factor coronatine, suggesting that the phytopathogen uses coronatine-mediated volatilization of MeSA from leaves to attenuate the SA-based defense pathway.  相似文献   

2.
Extracellular polysaccharides (EPS) produced by Pseudomonas syringae pv. phaseolicola are obviously composed of two main components: the long known levan consisting of fructose, and a mannuronan consisting mainly of mannuronic acid (manA), thus resembling alginic acid (alginate). The identification of manA was established by TLC utilizing different developing systems, and by cellulose acetate electrophoresis in different buffers. References were authentic uronic acids and hydrolyzed authentic alginate. A rough quantification of the “alginate” present in crude EPS was achieved with a selective colour reaction which largely excluded compounds other than uronic acids. Levan was only synthesized with sucrose as primary carbon source. When grown on several other sugars and related compounds “alginate” was the predominant component of the EPS. Additionally, rhamnose, fucose, glucose and amino sugars were found in some instances in hydrolysates of crude EPS, suggesting the release of lipopolysaccharides (LPS) from the bacterial cell walls during culture. Growth on carbon sources not related to sugars resulted in these “LPS” as the main constituent of EPS. After cultivation with sucrose, the “alginate” was restricted to the “slime” fraction of the EPS. In the “capsular” fraction, levan was predominating. A screening program revealed the capacity to synthesize the “alginate” in six additional P. syringae pathovars: pisi, lachrymans, aptata, tomato, syringae, and glycinea. All of the strains tested so far produced levan from sucrose, however, the “alginate” was formed not by all of them. There was a tendency that fresh isolates produced more “alginate” than strains subcultured for an extended time in vitro. This was also true for the total amount of EPS.  相似文献   

3.
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.  相似文献   

4.
The fire blight pathogen Erwinia amylovora was assayed for survival under unfavourable conditions such as on nitrocellulose filters, in non‐host plants as well as in inoculated mature apples and in infested apple stem sections. In a sterile dry environment, an E. amylovora EPS (exopolysaccharide) mutant, and to a lesser extent its parental wild‐type strain decreased within 3 weeks to a low titre. However, under moist conditions the decrease of viable cells occurred only partially for both strains. Very low cell titres were recovered after application of E. amylovora onto the surface of tobacco leaves, whereas infiltration into the leaves produced lesions (hypersensitive response, HR), in which the bacteria survived in significant amounts. A similar effect was found for the necrotic zones of HR in tobacco leaves caused by E. pyrifoliae, by Pseudomonas syringae pathovars and HR‐deficient E. amylovora mutants or mutants deficient in EPS synthesis and disease‐specific genes. During 7 years of storage, the viability of E. amylovora in wood sections from fire blight‐infested apple trees declined to a low titre. In tissue of mature apples, E. amylovora cells slowly dispersed and could still be recovered after several weeks of storage at room temperature. A minimal risk of accidental dissemination of E. amylovora apart from infested host plants can experimentally not be excluded, but other data confirm a very low incidence of any long distance distribution.  相似文献   

5.
Cucumber (Cucumis sativa) leaves infiltrated with Pseudomonas syringae pv. syringae cells produced a mobile signal for systemic acquired resistance between 3 and 6 h after inoculation. The production of a mobile signal by inoculated leaves was followed by a transient increase in phenylalanine ammonia-lyase (PAL) activity in the petioles of inoculated leaves and in stems above inoculated leaves; with peaks in activity at 9 and 12 h, respectively, after inoculation. In contrast, PAL activity in inoculated leaves continued to rise slowly for at least 18 h. No increases in PAL activity were detected in healthy leaves of inoculated plants. Two benzoic acid derivatives, salicylic acid (SA) and 4-hydroxybenzoic acid (4HBA), began to accumulate in phloem fluids at about the time PAL activity began to increase, reaching maximum concentrations 15 h after inoculation. The accumulation of SA and 4HBA in phloem fluids was unaffected by the removal of all leaves 6 h after inoculation, and seedlings excised from roots prior to inoculation still accumulated high levels of SA and 4HBA. These results suggest that SA and 4HBA are synthesized de novo in stems and petioles in response to a mobile signal from the inoculated leaf.  相似文献   

6.
7.
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.  相似文献   

8.
A simple method for the extraction of extracellular polysaccharides (EPS) from plant tissue was developed. The polysaccharides of bacterial and plant origin present in the crude leaf extracts were separated by column chromatography on DEAE-fractogel, and the bacterial polymers were identified by IR spectroscopy. In extracts from infected leaves as well as in exudates (ooze) from leaf axils, alginate (an acetylated mannuronan) and levan (β-2,6-fructofuranan) were detected as the major components amounting up to 80% of the crude extracts. A race-1 isolate of P. phaseolicola synthesized both levan and alginate in about equal amounts in planta, whereas a race-2 strain produced EPS composed almost solely of alginate. Extraction of healthy leaves yielded low amounts of complex polysaccharides. These consisted mainly of galactose, arabinose, and galacturonic acid. Neither fructose nor mannuronic acid were detected. Kinetic studies indicated that the main production of bacterial EPS in planta was correlated with the appearance of the water-soaked symptom in leaves. However, before water-soaking became apparent, alginate was detected in infected leaves (1, day after inoculation). The high amount of extractable material (ca. 50 mg levan plus alginate per g of dry weight of diseased tissue) suggests that the bacterial EPS is responsible for the typical water-soaked appearance of lesions after bacterial infection. Since alginate was predominantly synthesized by the more virulent race-2 isolate, this component of bacterial EPS was suspected to be a decisive factor of virulence of P. phaseolicola. A possible function of alginate during pathogenesis is discussed.  相似文献   

9.
The phytotoxic exopolysaccharides produced by Pseudomonas syringae pv. actinidiae, the causal agent of bacterial canker of kiwifruit, were isolated and partially identified. Their phytotoxic activity was evaluated on host and non-host plants and their role in the complex mechanisms of host-pathogen interaction was also discussed. The phytotoxic exopolysaccharides, which are natural antigens, were used to arise specific antibodies by rat immunization. The antibodies were used to develop a rapid and specific method to unambiguously detect P.s. pv. actinidiae exopolysaccharides isolated from bacterial culture and infected plant samples. Indeed, the antibodies recognized the exopolysaccharides produced by other two strains of P. s. pv. actinidiae but did not cross reacted with those isolated from P. s. pv. syringae and Pseudomonas viridiflava culture filtrates. Finally, the same antibodies significantly recognized the exopolysaccharides extracted from infected kiwi leaves.  相似文献   

10.
The invasion and exclusion abilities of coexisting Pseudomonas syringae strains were quantified on leaves. Twenty-nine P. syringae strains were inoculated onto plants in 107 pairwise combinations. All pairs were duplicated so that each strain was inoculated both first as an antagonist strain (day 0) and second as a challenge strain (day 3). The population size of each strain in a mixture was quantified on day 6 following incubation under moist conditions. For P. syringae strains, the presence of an established population often significantly reduced the growth of subsequently arriving challenge strains on the leaf surface. Invasion and exclusion abilities, quantified by contrasting population sizes of challenge strains in the presence and in the absence of another strain, varied significantly among P. syringae strains and were partly a function of the particular strain pair. The population size of a strain when present alone on a leaf was not predictive of invasion or exclusion ability. Successful invaders were significantly less likely to exclude challenge populations than were nonsuccessful invaders. Population sizes of successful excluders were negatively correlated with population sizes of coexisting challenge strains, while population sizes of successful invaders were positively correlated with those of coexisting antagonist strains. The patterns of interaction among coexisting strains suggest mechanisms for successful invasion and exclusion among P. syringae strains on leaves.  相似文献   

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