Interactions between plants and beneficial <Emphasis Type="Italic">Pseudomonas</Emphasis> spp.: exploiting bacterial traits for crop protection

Specific strains of fluorescent Pseudomonas spp. inhabit the environment surrounding plant roots and some even the root interior. Introducing such bacterial strains to plant roots can lead to increased plant growth, usually due to suppression of plant pathogenic microorganisms. We review the modes of action and traits of these beneficial Pseudomonas bacteria involved in disease suppression. The complex regulation of biological control traits in relation to the functioning in the root environment is discussed. Understanding the complexity of the interactions is instrumental in the exploitation of beneficial Pseudomonas spp. in controlling plant diseases.     

Conservation of xcp genes, involved in the two-step protein secretion process, in different Pseudomonas species and other gram-negative bacteria

Summary The two-step protein secretion pathway in Pseudomonas aeruginosa is dependent on the xcp genes. We investigated whether a similar secretion mechanism is present in non-pathogenic Pseudomonas spp. and in other gram-negative bacteria. The plant growth stimulating Pseudomonas strains P. putida WCS358, P. fuorescens WCS374 and Pseudomonas 1310 appeared to secrete proteins into the extracellular medium. Southern hybridization experiments showed the presence of xcp genes in these strains and also in other gram-negative bacteria, including Xanthomonas campestris. Complementation experiments showed that the xcp gene cluster of P. aeruginosa restored protein secretion in an X. campestris secretion mutant. The secretion gene cluster of X. campestris however, restored secretion capacity in P. aeruginosa mutants only to a low degree. Two heterologous proteins were not secreted by P. fuorescens and P. aeruginosa. The results suggest the presence of a similar two-step protein secretion mechanism in different gram-negative bacteria, which however, is not always functional for heterologous proteins.     

Commensal Pseudomonas protect Arabidopsis thaliana from a coexisting pathogen via multiple lineage-dependent mechanisms

Plants are protected from pathogens not only by their own immunity but often also by colonizing commensal microbes. In Arabidopsis thaliana, a group of cryptically pathogenic Pseudomonas strains often dominates local populations. This group coexists in nature with commensal Pseudomonas strains that can blunt the deleterious effects of the pathogens in the laboratory. We have investigated the interaction between one of the Pseudomonas pathogens and 99 naturally co-occurring commensals, finding plant protection to be common among non-pathogenic Pseudomonas. While protective ability is enriched in one specific lineage, there is also a substantial variation for this trait among isolates of this lineage. These functional differences do not align with core-genome phylogenies, suggesting repeated gene inactivation or loss as causal. Using genome-wide association, we discovered that different bacterial genes are linked to plant protection in each lineage. We validated a protective role of several lineage-specific genes by gene inactivation, highlighting iron acquisition and biofilm formation as prominent mechanisms of plant protection in this Pseudomonas lineage. Collectively, our work illustrates the importance of functional redundancy in plant protective traits across an important group of commensal bacteria.Subject terms: Microbial ecology, Plant ecology     

Genetics of naphthalene and phenanthrene degradation by Comamonas testosteroni

Naphthalene and phenanthrene have long been used as model compounds to investigate the ability of bacteria to degrade polycyclic aromatic hydrocarbons. The catabolic pathways have been determined, several of the enzymes have been purified to homogeneity, and genes have been cloned and sequenced. However, the majority of this work has been performed with fast growing Pseudomonas strains related to the archetypal naphthalene-degrading P. putida strains G7 and NCIB 9816-4. Recently Comamonas testosteroni strains able to degrade naphthalene and phenanthrene have been isolated and shown to possess genes for polycyclic aromatic hydrocarbon degradation that are different from the canonical genes found in Pseudomonas species. For instance, C. testosteroni GZ39 has genes for naphthalene and phenanthrene degradation which are not only different from those found in Pseudomonas species but are also arranged in a different configuration. C. testosteroni GZ42, on the other hand, has genes for naphthalene and phenanthrene degradation which are arranged almost the same as those found in Pseudomonas species but show significant divergence in their sequences. Received 10 August 1997/ Accepted in revised form 15 August 1997     

Presence of Quorum-Sensing-Mediated Gene Regulation in Pathogenic Ice-Nucleation-Active (INA) Bacteria

Nine out of a total of 20 pathogenic ice-nucleation-active bacteria, with different levels of inducible INA, were tested and found positive for their ability to synthesize quorum-sensing (QS) signals. The bacteria were isolated from willow plants and belonged to the genera Bacillus, Erwinia, Pseudomonas and Sphingomonas. As reporter bacteria, to detect the homoserine lactone (HSL) autoinducer, Agrobacterium tumefaciens, Chromobacterium violaceum, Pseudomonas aeroginosa, Aeromonas hydrophila and Vibrio fischeri strains were used. We thus provide evidence that pathogenic ice-nucleation bacteria with inducible INA produce QS signals that in other bacteria have been shown to be in the control of genes of importance for pathogenicity.     

The involvement of Pseudomonas bacteria in induced systemic resistance in plants (Review)

This article reviews the most recent results of studies on the mechanism of induced systemic resistance (ISR) elicited in plants by non-pathogenic bacteria of the genus Pseudomonas. Several examples of Pseudomonas strains eliciting resistance against fungal phytopathogens in different species of crop plants are presented. Literature data dealing with bacterial elicitors and the effect of their interaction with plant receptors are quoted. Special focus is focused on the controversial issue of the correlation between the synthesis of pathogenesis-related proteins (PRs) and ISR.     

Pathogenic and mutualistic plant-bacteria interactions: ever increasing similarities

Plant-interacting bacteria can establish either mutualistic or pathogenic interactions that cause beneficial or detrimental effects respectively, to their hosts. In spite of the completely different outcomes, accumulating evidence indicates that similar molecular bases underlie the establishment of these two contrasting plant-bacteria associations. Recent findings observed in the mutualistic nitrogen-fixing Rhizobium-legume symbiosis add new elements to the increasing list of similarities. Amongst these, in this review we describe the role of plant resistance proteins in determining host specificity in the Rhizobium-legume symbiosis that resemble the gene-for-gene resistance of plant-pathogen interactions, and the production of antimicrobial peptides by certain legumes to control rhizobial proliferation within nodules. Amongst common bacterial strategies, cyclic diguanylate (c-di-GMP) appears to be a second messenger used by both pathogenic and mutualistic bacteria to regulate key features for interaction with their plant hosts.     

Flexible exportation mechanisms of arthrofactin in Pseudomonas sp. MIS38

Aims: To obtain further insights into transportation mechanisms of a most effective biosurfactant, arthrofactin in Pseudomonas sp. MIS38. Methods and Results: A cluster genes arfA/B/C encodes an arthrofactin synthetase complex (ArfA/B/C). Downstream of the arfA/B/C lie genes encoding a putative periplasmic protein (ArfD, 362 aa) and a putative ATP‐binding cassette transporter (ArfE, 651 aa), namely arfD and arfE, respectively. The arfA/B/C, arfD, and arfE form an operon suggesting their functional connection. Gene knockout mutants ArfD:Km, ArfE:Km, ArfD:Tc/ArfE:Km, and gene overexpression strains MIS38(pME6032_arfD/E) and ArfE:Km(pME6032_arfD/E) were prepared and analysed for arthrofactin production profiles. It was found that the production levels of arthrofactin were temporally reduced in the mutants or increased in the gene overexpression strains, but they eventually became similar level to that of MIS38. Addition of ABC transporter inhibitors, glibenclamide and sodium ortho‐vanadate dramatically reduced the production levels of arthrofactin. This excludes a possibility that arthrofactin is exported by diffusion with the aid of its own high surfactant activity. Conclusions: ArfD/E is not an exclusive but a primary exporter of arthrofactin during early growth stage. Reduction in the arthrofactin productivity of arfD and arfE knockout mutants was eventually rescued by another ABC transporter system. Effects of arfD and arfE overexpression were evident only for 1‐day cultivation. Multiple ATP dependent active transporter systems are responsible for the production of arthrofactin. Significance and Impact of the Study: Pseudomonas bacteria are characterized to be endued with multiple exporter and efflux systems for secondary metabolites including antibiotics, plant toxins, and biosurfactants. The present work demonstrates exceptionally flexible and highly controlled transportation mechanisms of a most effective lipopeptide biosurfactant, arthrofactin in Pseudomonas sp. MIS38. Because lipopeptide biosurfactants are known to enhance efficacy of bioactive compounds and arfA/B/C/D/E orthologous genes are also found in plant pathogenic P. fluorescens and P. syringae strains, the knowledge would also contribute to develop a technology controlling plant diseases.     

植物病原细菌毒性调控网络的研究进展

植物病原细菌通过复杂和精细的全局性调控网络来协调多个层面的毒性决定因子。在不同的植物病原细菌中,这些全局性的毒性调控网络控制着细菌的侵染策略、存活以及在面临寄主植物防卫系统的互作环境中实现成功侵染的病程。本文详细分析了植物病原细菌4个重要属(假单胞菌属、果胶杆菌属、黄单胞菌属和雷尔氏菌属)的模式病原菌主要的毒性调控系统,包括群体感应系统、双组分调控系统、转录激活调控子以及转录后、翻译后的调控机制。在此基础上,重点评价了一些模式菌株全局性毒性调控机制的异同点,总结了一些最新的研究进展,并绘制了精细的网络调控图。这些分析表明,虽然一些相同的调控系统控制着病原菌的毒性,但是在不同种以及种下的亚种或者致病变种中这些调控机制功能各异,对于病原菌全毒性的贡献也存在着明显的差异。     

Combined use of Alkane-Degrading and Plant Growth-Promoting Bacteria Enhanced Phytoremediation of Diesel Contaminated soil

Inoculation of plants with pollutant-degrading and plant growth-promoting microorganisms is a simple strategy to enhance phytoremediation activity. The objective of this study was to determine the effect of inoculation of different bacterial strains, possessing alkane-degradation and 1-amino-cyclopropane-1-carboxylic acid (ACC) deaminase activity, on plant growth and phytoremediation activity. Carpet grass (Axonopus affinis) was planted in soil spiked with diesel (1% w/w) for 90 days and inoculated with different bacterial strains, Pseudomonas sp. ITRH25, Pantoea sp. BTRH79 and Burkholderia sp. PsJN, individually and in combination. Generally, bacterial application increased total numbers of culturable hydrocarbon-degrading bacteria in the rhizosphere of carpet grass, plant biomass production, hydrocarbon degradation and reduced genotoxicity. Bacterial strains possessing different beneficial traits affect plant growth and phytoremediation activity in different ways. Maximum bacterial population, plant biomass production and hydrocarbon degradation were achieved when carpet grass was inoculated with a consortium of three strains. Enhanced plant biomass production and hydrocarbon degradation were associated with increased numbers of culturable hydrocarbon-degrading bacteria in the rhizosphere of carpet grass. The present study revealed that the combined use of different bacterial strains, exhibiting different beneficial traits, is a highly effective strategy to improve plant growth and phytoremediation activity.     

Diversity of effector genes in plant pathogenic bacteria of genus Xanthomonas

Gram-negative plant pathogenic bacteria are secreting into plant cell a special type of pathogeni city-related proteins called effectors. They are capable of suppressing plant innate immunity or stimulating synthesis and export of metabolites desired by the pathogen. We identified a number of effector-coding genes typical of xanthomonads analyzing 8 completely sequenced genomes of genus Xanthomonas. Using representative collection provided by Russian Research Institute of Phytopathology we identified genetic diversity of effector gene loci in population of Xanthomonas bacteria. Patterns of effector genes were identified for individual strains and statistic linkage between particular genes and race of the pathogen was established. For the first time several untypical effector genes were found in strains of Xanthomonas campestris pv. campestris.     

Dynamics,Diversity and Function of Endophytic Siderophore-Producing Bacteria in Rice

Siderophore production confers to bacteria competitive advantages to colonize plant tissues and to exclude other microorganisms from the same ecological niche. This work shows that the community of endophytic siderophore-producing bacteria (SPB) associated to Oryza sativa cultivated in Uruguayan soils is dynamic and diverse. These bacteria were present in grains, roots, and leaves, and their density fluctuated between log₁₀ 3.44 and log₁₀ 5.52 cfu g⁻¹ fresh weight (fw) during the plant growth. Less than 10% of the heterotrophic bacteria produced siderophores in roots and leaves of young plants, but most of the heterotrophic bacteria were siderophore-producers in mature plants. According to their amplified restriction DNA ribosomal analysis (ARDRA) pattern, 54 of the 109 endophytic SPB isolated from different plant tissues or growth stages from replicate plots, were unique. Bacteria belonging to the genera Sphingomonas, Pseudomonas, Burkholderia, and Enterobacter alternated during plant growth, but the genus Pantoea was predominant in roots at tillering and in leaves at subsequent stages. Pantoea ananatis was the SPB permanently associated to any of the plant tissues, but the genetic diversity within this species—revealed by BOX-PCR fingerprinting- showed that different strains were randomly distributed along time and plant tissue, suggesting that a common trait of the species P. ananatis determined the interaction with the rice plant. Several isolates were stronger IAA producers than Azospirillum brasilense or Herbaspirillum seropedicae. In vitro inhibition assays showed that SPB of the genus Burkholderia were good antagonists of pathogenic fungi and that only one SPB isolate of the genus Pseudomonas was able to inhibit A. brasilense and H. seropedicae. These results denoted that SPB were selected into the rice plant. P. ananatis was the permanent and dominant associated species which was unable to inhibit two of the relevant plant growth-promoting bacteria.     

Pseudomonas as a frequent and important quorum quenching bacterium with biocontrol capability against many phytopathogens

The aim of the present study was to isolate a variety of quorum quenching bacteria (QB) from the rhizosphere and phyllosphere of three agricultural plants using minimal medium (MM)- and non-minimal medium (NM)-based methods. The members of the Pseudomonas genus constituted the most abundant QB genus, particularly in the rhizospheres of all plant samples and showed the highest quorum quenching (QQ) activity according to a screening assay using a biosensor and 3-oxo-C6-HSL (as an important quorum sensing signal in many phytopathogenic bacteria). In addition, QQ-Pseudomonas were recognised as versatile biocontrol agents against non-bacterial and bacterial plant pathogens, such as Pectobacterium carotovorum subsp. carotovorum (Pcc). Three types of quenching activities, including intracellular and extracellular enzymatic and non-enzymatic activities, were observed in QQ-Pseudomonas. Pseudomonas strains, particularly NM-isolated strains with extracellular activity, are the strongest QQ-based biocontrol agents.     

Evaluation of the Potential of five Housekeeping Genes for Identification of Quarantine Pseudomonas syringae

The Pseudomonas syringae species, containing approximately 60 pathovars, causes bacterial plant diseases on numerous crops and results in great economic losses. It is difficult to rapidly and accurately identify and detect P. syringae due to its complicated classification system. It has also been shown that housekeeping genes have great potential in phylogenetic analysis and classification of bacteria. In this study, phylogenetic analyses of five housekeeping genes, including 16S rRNA, rpoD, gapA, gltA and gyrB, were performed for 100 Pseudomonas strains. Our results showed that two housekeeping genes (rpoD and gapA) had the maximum ability in distinguishing the majority of Pseudomonas pathovars, whereas rpoD exhibited the best for precise and efficient detection of five P. syringae strains, which is of quarantine significance to China.     

Induced systemic resistance (ISR) in plants: mechanism of action

Plants possess a range of active defense apparatuses that can be actively expressed in response to biotic stresses (pathogens and parasites) of various scales (ranging from microscopic viruses to phytophagous insect). The timing of this defense response is critical and reflects on the difference between coping and succumbing to such biotic challenge of necrotizing pathogens/parasites. If defense mechanisms are triggered by a stimulus prior to infection by a plant pathogen, disease can be reduced. Induced resistance is a state of enhanced defensive capacity developed by a plant when appropriately stimulated. Systemic acquired resistance (SAR) and induced systemic resistance (ISR) are two forms of induced resistance wherein plant defenses are preconditioned by prior infection or treatment that results in resistance against subsequent challenge by a pathogen or parasite. Selected strains of plant growth-promoting rhizobacteria (PGPR) suppress diseases by antagonism between the bacteria and soil-borne pathogens as well as by inducing a systemic resistance in plant against both root and foliar pathogens. Rhizobacteria mediated ISR resembles that of pathogen induced SAR in that both types of induced resistance render uninfected plant parts more resistant towards a broad spectrum of plant pathogens. Several rhizobacteria trigger the salicylic acid (SA)-dependent SAR pathway by producing SA at the root surface whereas other rhizobacteria trigger different signaling pathway independent of SA. The existence of SA-independent ISR pathway has been studied in Arabidopsis thaliana, which is dependent on jasmonic acid (JA) and ethylene signaling. Specific Pseudomonas strains induce systemic resistance in viz., carnation, cucumber, radish, tobacco, and Arabidopsis, as evidenced by an enhanced defensive capacity upon challenge inoculation. Combination of ISR and SAR can increase protection against pathogens that are resisted through both pathways besides extended protection to a broader spectrum of pathogens than ISR/SAR alone. Beside Pseudomonas strains, ISR is conducted by Bacillus spp. wherein published results show that several specific strains of species B. amyloliquifaciens, B. subtilis, B. pasteurii, B. cereus, B. pumilus, B. mycoides, and B.sphaericus elicit significant reduction in the incidence or severity of various diseases on a diversity of hosts.     

PCR detection of novel non-ribosomal peptide synthetase genes in lipopeptide-producing Pseudomonas

Bacterial lipopeptides (LPs) are a diverse group of secondary metabolites synthesized through one or more non-ribosomal peptide synthetases (NRPSs). In certain genera, such as Pseudomonas and Bacillus, these enzyme systems are often involved in synthesizing biosurfactants or antimicrobial compounds. Several different types of LPs have been reported for non-pathogenic plant-associated Pseudomonas. Focusing on this group of bacteria, we devised and validated a PCR method to detect novel LP-synthesizing NRPS genes by targeting their lipoinitiation and tandem thioesterase domains, thus avoiding amplification of genes for non-LP metabolites, such as the pyoverdine siderophores present in all fluorescent Pseudomonas. This approach enabled detection of as yet unknown NRPS genes in strains producing viscosin, viscosinamide, WLIP, or lokisin. Furthermore, it proved valuable to identify novel candidate LP producers among Pseudomonas rhizosphere isolates. By phylogenetic analysis of these amplicons, several of the corresponding NRPS genes can be tentatively assigned to the viscosin, amphisin, or entolysin biosynthetic groups, while some others may represent novel NRPS systems.     

Evaluation of fluorescent Pseudomonas spp. with single and multiple PGPR traits for plant growth promotion of sorghum in combination with AM fungi

Traits involved in plant growth promotion by bacteria are ambiguously decided as phytohormones, siderophores, HCN, proteases, chitinases, cellulases, ammonia, exopolysaccharide production and phosphate solubilization or antagonistic activity. A total of 40 fluorescent Pseudomonas strains were isolated from diverse soils of various agro-ecosystems of India. Among these 7 strains viz, P1, P10, P13, P18, P21, P28 and P38 were selected depending on their character of possessing single or multiple PGPR traits. These isolates individually and in combination with AM fungi (Glomus fasciculatum and Glomus aggregatum) were used for treating sorghum seeds. 25 days after sowing, plants were analysed for different plant growth promoting parameters. It was observed that strains in presence of mycorrhizae performed well compared to the strains devoid of mycorrhizae. Pseudomonas sp. P1 possessing GA3, EPS production and ‘P’ solubilization performed well. Pseudomonas sp. P38 which was a volatile (HCN) producer but a good phosphate solubilizer did not perform well. Strain P28 with multiple PGPR traits did not show the expected out come. Results varied when mycorrhizae was used in combination. P10 and P13 which were good in IAA, GA3, EPS, siderophore and ‘P’ solubilization performed well with mycorrhizae, with an overall increase in plant biomass, leaf area, total chlorophyll and mycorrhizal infection compared to other combinations. Here, strain P1 in combination with mycorrhizae did not show significant increase in plant growth compared to P10 and P13. Hence, mere possession of multiple PGPR traits does not confer fluorescent Pseudomonas strains as PGPR. Further studies have to be carried out in order to evaluate the other traits which may be involved in plant growth promotion.     

Phylogenetic Characterization lcrD Gene Family: Molecular Evolutionary Aspects of Pathogen-Induced Hypersensitivity in plants

Abstract — Hrp(hypersensitivity response and pathogenicity) genes encode signal-peptide independent transporter molecules that function in the Type III secretion pathway and are present in a number of plant pathogenic bacterial species. These Hrp transporter molecules largely export harpin and other virulence factors across the bacterial membrane and onto theHrploci are part of a largerlcrD family which encode the low calcium response proteins. Members of this family serve to transport a number of diverse virulence factors in a variety of enteric and other purple bacteria species both pathogenichrp-induced pathogenicity by different plant pathogenic bacterial species is the result of a single evolutionary event or evolved independently, cladistic analyses were performedlcrD gene family. The results of these studieslcrD orhrpgeneslcrD homologues which comprised the other twohrptransporter genes do not capture the phylogenetic history of their host bacteriallcrD gene was horizontally introduced into each of four different plant pathogenic species which may have resulted from four independent transfer events. This monophyletic partitioning ofhrpgenes precludes their use as reliable taxonomic markers while further supporting the current notion thathrptransport