Phytotoxin production by Pseudomonas syringae pv. syringae: Syringopeptin production by syr mutants defective in biosynthesis or secretion of syringomycin

Abstract Syringomycin and syringopeptin are lipodepsipeptide phytotoxins produced by Pseudomonas syringae pv. syringae . Four syr genes were identified previously and hypothesized to be involved in the regulation ( syrA ), biosynthesis ( syrB and syrC ), or export ( syrD ) of syringomycin. This study determines the influence of syr mutations on the composition of phytotoxic metabolites produced by P. syringae pv. syringae strain B301D-R. Levels of syringomycin and syringopeptin produced in liquid cultures were estimated by reverse phase HPLC analyses and differential antimicrobial assays. Significant quantities of syringopeptin were produced by both syrB and syrC mutants despite their inability to produce syringomycin. Only trace quantities of both lipodepsipeptides were produced by syrA and syrD mutants of P. syringae pv. syringae . These results indicate that syringomycin and syringopeptin are synthesized by separate pathways, but may share common mechanisms for secretion and regulation.     

The phytotoxin coronatine is a multifunctional component of the virulence armament of Pseudomonas syringae

Plant pathogens deploy an array of virulence factors to suppress host defense and promote pathogenicity. Numerous strains of Pseudomonas syringae produce the phytotoxin coronatine (COR). A major aspect of COR function is its ability to mimic a bioactive jasmonic acid (JA) conjugate and thus target the JA-receptor COR-insensitive 1 (COI1). Biological activities of COR include stimulation of JA-signaling and consequent suppression of SA-dependent defense through antagonistic crosstalk, antagonism of stomatal closure to allow bacterial entry into the interior of plant leaves, contribution to chlorotic symptoms in infected plants, and suppression of plant cell wall defense through perturbation of secondary metabolism. Here, we review the virulence function of COR, including updates on these established activities as well as more recent findings revealing COI1-independent activity of COR and shedding light on cooperative or redundant defense suppression between COR and type III effector proteins.     

Structure of the Lipopolysaccharide Side-chain of Pseudomonas syringae pv. syringae Strain S29

Using ¹H‐ and ¹³C‐nuclear magnetic resonance spectroscopy, the repeat unit of the lipopolysaccharide side‐chain from Pseudomonas syringae pv. syringae strain S29 was shown to have the following structure: This structure is identical with that of the side‐chain of Pseudomonas syringae pv. mori CFPB 1656. a     

丁香假单胞菌冰核基因inaQ变速箱启动子结构与活性分析

序列比对与结构预测显示丁香假单胞菌(Pseudomonas syringae)野生型菌株MB03的冰核基因inaQ启动子为一种在细菌中罕见的变速箱型启动子。通过克隆长度为522bp的inaQ基因启动子区(P522)并与绿色荧光蛋白基因gfp构建融合基因P522gfp后,在恶臭假单胞菌AB92019菌株中进行表达分析。结果表明,包含结构模块A-Box和B-Box的P522在该菌株中具有启动子活性,且在寡营养条件和较低温度下具有更高的活性,是一种可调控启动子。     

Host specificity,pathogenicity and the presence of virulence genes in Iranian strains of Pseudomonas syringae pv. syringae from different hosts

Pseudomonas syringae pv. syringae (Pss) strains were isolated from almond, apricot, peach, pear, sweet cheery and wheat in Kohgiluye and Boyer-Ahmad, Kordestan, Fras and Chaharmahal and Bakhtiari provinces of Iran. The strains were examined for host specificity, the presence of virulence genes and pathogenicity on different hosts. After inoculation of isolates, in compatible reactions bacterial populations increased within six days of inoculation and final cell numbers increased several-fold over initial inoculum levels, but in incompatible reactions, bacterial populations declined within four days of inoculation. Almond, sweet cherry and wheat isolates induced progressive necrotic symptoms on almond leaves and stems. Apricot, peach and sweet cherry isolates induced necrotic lesions when inoculated on apricot leaves. On pear leaves and stems, only the pear isolate incited pathogenic reaction and isolates from other hosts did not. The syrB gene was detected in all of the tested isolates. Almond and pear isolates did not have the syrD gene. The sypA gene was detected in the almond, peach, pear and sweet cherry isolates while the sypB gene was detected in the apricot, peach, sweet cherry and wheat isolates. Almond, apricot, pear and wheat isolates gave negative results for the detection of nit gene. The gene Ach, was detected only in the peach isolate and gene hrmA, was detected only in the wheat isolate. This study indicates that host specificity exists among different Pss strains, and genes responsible for syringomycin and syringopeptin production contribute to the virulence of Pss strains.     

In vitro Antibiosis by Pseudomonas syringae Pss22d,Acting Against the Bacterial Blight Pathogen of Soybean Plants,Does Not Influence In planta Biocontrol

The epiphyte Pseudomonas syringae pv. syringae 22d / 93 (Pss22d), isolated from soybean leaves, had been characterized as a promising and species‐specific biocontrol strain in vitro and in planta against the plant pathogen P. syringae pv. glycinea (Psg), which causes bacterial blight of soybean. Three toxins are known to be produced by Pss22d: syringomycin, syringopeptin and 3‐methylarginine (MeArg). In contrast to syringopeptin and syringomycin, MeArg inhibited the growth of Psg in vitro. To examine if the toxins produced by Pss22d are responsible for antagonistic effects in planta, the pathogen Psg was co‐inoculated with either Pss22d wild‐type, a syringopeptin/syringomycin‐negative double mutant (Pss22d.ΔsypA/syrE), or a MeArg‐negative mutant (Pss22d.1) into wounds of pin‐pricked leaves of greenhouse‐grown soybean plants, respectively. In all three cases, the wild‐type Pss22d and its toxin‐deficient mutants prevented development of disease symptoms normally caused by Psg. These results indicated that neither syringopeptin, nor syringomycin, nor MeArg was required for Pss22d’s antagonistic activity in planta. Consequently, factors other than the three toxins may contribute to the intra‐species antagonism in planta.     

Elicitation of the hypersensitive response by Pseudomonas syringae

The hypersensitive response (HR), a rapid local necrosis accompanied by the production of antimicrobial compounds, is a manifestation of the primary defense mechanisms through which plants prevent colonization by potentially pathogenic microorganisms. The pathogen factors that trigger HR-associated defense mechanisms have yet to be established. Application of transposon mutagenesis and molecular cloning techniques have provided new insights into the bacterial elicitation of the HR. This review focuses on the interaction of Pseudomonas syringae with non-host plants as a model system for the bacterial induction of the HR and discusses (a) the genetics of Pseudomonas syringae pathogenicity and HR elicitation, (b) the environmental conditions controlling expression of Pseudomonas syringae pathogenicity/HR determinants, (c) the nature of the HR elicitor and (d) early plant responses to the pathogen.     

丁香假单胞菌环式脂肽毒素的生理和分子生物学研究

综合评述了近10年来在丁香假单胞菌脂肽毒素生理和分子生物学研究上的发现。这些毒素依肽部AA数目可分两组。丁香假单胞霉素组(syringomycuns)已报告4个成员,肽部有9个AA;丁香假单胞肽毒素组有2个成员,肽部分别有22个和25个AA。肽部C端羧基与分子内羟基氨基酸残基(AA)的羟基酯化闭合成环,再由羟基脂肪酸酰化。两组毒素都诱导植物电解质渗漏、人和动物红血球溶解,其机制在于在细胞膜上形成二价阳离子可通过的寡体通道。对酵母菌的抑制作用受固醇的种类影响,以胆固醇的保护作用最强。丁香假单胞霉素的合成涉及一个多酶系统,有些负责肽合成,有些负责运输或调节,除受内源调节蛋白调节外,也受外源信号分子调节,尤其是受植物酚糖苷诱导。这些毒素具有抗真菌活性,对人和动物的一些病原霉菌有明显效果,在试验剂量无副作用,在医药上应用的前景良好。     

A revised model for the role of GacS/GacA in regulating type III secretion by Pseudomonas syringae pv. tomato DC3000

GacS/GacA is a conserved two-component system that functions as a master regulator of virulence-associated traits in many bacterial pathogens, including Pseudomonas spp., that collectively infect both plant and animal hosts. Among many GacS/GacA-regulated traits, type III secretion of effector proteins into host cells plays a critical role in bacterial virulence. In the opportunistic plant and animal pathogen Pseudomonas aeruginosa, GacS/GacA negatively regulates the expression of type III secretion system (T3SS)-encoding genes. However, in the plant pathogenic bacterium Pseudomonas syringae, strain-to-strain variation exists in the requirement of GacS/GacA for T3SS deployment, and this variability has limited the development of predictive models of how GacS/GacA functions in this species. In this work we re-evaluated the function of GacA in P. syringae pv. tomato DC3000. Contrary to previous reports, we discovered that GacA negatively regulates the expression of T3SS genes in DC3000, and that GacA is not required for DC3000 virulence inside Arabidopsis leaf tissue. However, our results show that GacA is required for full virulence of leaf surface-inoculated bacteria. These data significantly revise current understanding of GacS/GacA in regulating P. syringae virulence.     

Extracellular protease and phospholipase C are controlled by the global regulatory gene gacA in the biocontrol strain Pseudomonas fluorescens CHA0

Abstract Pseudomonas fluorescens strain CHA0 protects plants from various root diseases. Antibiotic metabolites synthesized by this strain play an important role in disease suppression; their production is mediated by the g lobal ac tivator gene gacA . Here we show by complementation that the gacA gene is also essential for the expression of two extracellular enzymes in P. fluorescens CHA0: phospholipase C and a 47-kDa metalloprotease. In contrast, the production of another exoenzyme, lipase, is not regulated by the gacA gene. Protease, phospholipase and antibiotics of P. fluorescens are all known to be optimally produced at the end of exponential growth; thus the gacA gene appears to be a general stationary-phase regulator.     

First Report of Bacterial Leaf Spot of Parsley Caused by Pseudomonas syringae pv. apii in Turkey

Since March, 2011, typical leaf spot symptoms were observed on parsley in several fields inspected in Hatay and Adana provinces of Turkey. Incidence of the disease was 5–15% in the regions. Symptoms were characterized as angular to irregular, initially water soaked later brown to dark black spots. Spots often limited by veins which were visible from both adaxial and abaxial sides of leaves but were not present on stems. Fluorescent bacterial colonies were consistently isolated from typical leaf spots. Biochemical tests, fatty acid methyl ester (FAME) analysis, molecular, pathogenicity tests and sequence of 16S ribosomal DNA of bacterial isolates were performed to identify possible causal disease agent. The causal disease agent was identified as Pseudomonas syringae pv. apii based on symptoms, biochemical, molecular, pathogenicity tests and sequencing. To our knowledge, this is the first report of bacterial leaf spot on parsley caused by Pseudomonas syringae pv. apii in Turkey.     

Evolution,genomics and epidemiology of Pseudomonas syringae

A remarkable shift in our understanding of plant‐pathogenic bacteria is underway. Until recently, nearly all research on phytopathogenic bacteria was focused on a small number of model strains, which provided a deep, but narrow, perspective on plant–microbe interactions. Advances in genome sequencing technologies have changed this by enabling the incorporation of much greater diversity into comparative and functional research. We are now moving beyond a typological understanding of a select collection of strains to a more generalized appreciation of the breadth and scope of plant–microbe interactions. The study of natural populations and evolution has particularly benefited from the expansion of genomic data. We are beginning to have a much deeper understanding of the natural genetic diversity, niche breadth, ecological constraints and defining characteristics of phytopathogenic species. Given this expanding genomic and ecological knowledge, we believe the time is ripe to evaluate what we know about the evolutionary dynamics of plant pathogens.