Interplay between orfamides, sessilins and phenazines in the control of Rhizoctonia diseases by Pseudomonas sp. CMR12a

Discovery and characterization of the PpqI/R quorum sensing system activated by GacS/A and Hfq in Pseudomonas protegens H78

Pseudomonas protegens can generally produce multiple antibiotics including pyoluteorin (Plt), 2,4-diacetylphloroglucinol (DAPG), and pyrrolnitrin (Prn). In this study, we discovered and characterized a quorum sensing (QS) system, PpqI/R, in P. protegens H78. PpqI/R, encoded by two open reading frames (ORFs) (H78_01960/01961) in P. protegens H78 genome, is a LuxI/R-type QS system. Four long-chain acyl homoserine lactone (AHL) signaling molecules, 3-OH-C10-HSL, 3-OH-C12-HSL, C12-HSL, and 3-OH-C14-HSL, are produced by H78. Biosynthesis of these AHLs is catalyzed by PpqI synthase and activated by the PpqR regulator in H78 and in Escherichia coli when heterologously expressed. PpqR activates ppqI expression by targeting the lux box upstream of the ppqI promoter in cooperation with corresponding AHLs. The four aforementioned AHLs exhibited different capabilities to induce ppqI promoter expression, with 3-OH-C12-HSL showing the highest induction activity. In H78 cells, ppqI/R expression is activated by the two-component system GacS/A and the RNA chaperone Hfq. Differential regulation of the PpqI/R system in secondary metabolism has a negative effect on DAPG biosynthesis and ped operon (involved in volatile organic compound biosynthesis) expression. In contrast, Plt biosynthesis and prn operon expression were positively regulated by PpqI/R. In summary, PpqI/R, the first characterized QS system in P. protegens, is activated by GacS/A and Hfq and controls the expression of secondary metabolites, including antibiotics.

Does regulation hold the key to optimizing lipopeptide production in Pseudomonas for biotechnology?

Lipopeptides (LPs) produced by Pseudomonas spp. are specialized metabolites with diverse structures and functions, including powerful biosurfactant and antimicrobial properties. Despite their enormous potential in environmental and industrial biotechnology, low yield and high production cost limit their practical use. While genome mining and functional genomics have identified a multitude of LP biosynthetic gene clusters, the regulatory mechanisms underlying their biosynthesis remain poorly understood. We propose that regulation holds the key to unlocking LP production in Pseudomonas for biotechnology. In this review, we summarize the structure and function of Pseudomonas-derived LPs and describe the molecular basis for their biosynthesis and regulation. We examine the global and specific regulator-driven mechanisms controlling LP synthesis including the influence of environmental signals. Understanding LP regulation is key to modulating production of these valuable compounds, both quantitatively and qualitatively, for industrial and environmental biotechnology.

Biocontrol Potential of an Endophytic Pseudomonas poae Strain against the Grapevine Trunk Disease Pathogen Neofusicoccum luteum and Its Mechanism of Action

Grapevine trunk diseases (GTDs) impact the sustainability of vineyards worldwide and management options are currently limited. Biological control agents (BCAs) may offer a viable alternative for disease control. With an aim to develop an effective biocontrol strategy against the GTD pathogen Neofusicoccum luteum, this study investigated the following: (1) the efficacy of the strains in suppressing the BD pathogen N. luteum in detached canes and potted vines; (2) the ability of a strain of Pseudomonas poae (BCA17) to colonize and persist within grapevine tissues; and (3) the mode of action of BCA17 to antagonize N. luteum. Co-inoculations of the antagonistic bacterial strains with N. luteum revealed that one strain of P. poae (BCA17) suppressed infection by 100% and 80% in detached canes and potted vines, respectively. Stem inoculations of a laboratory-generated rifampicin-resistant strain of BCA17 in potted vines (cv. Shiraz) indicated the bacterial strain could colonize and persist in the grapevine tissues, potentially providing some protection against GTDs for up to 6 months. The bioactive diffusible compounds secreted by BCA17 significantly reduced the spore germination and fungal biomass of N. luteum and the other representative GTD pathogens. Complementary analysis via MALDI-TOF revealed the presence of an unknown cyclic lipopeptide in the bioactive diffusible compounds, which was absent in a non-antagonistic strain of P. poae (JMN13), suggesting this novel lipopeptide may be responsible for the biocontrol activity of the BCA17. Our study provided evidence that P. poae BCA17 is a potential BCA to combat N. luteum, with a potential novel mode of action.

Plant-associated Bacillus mobilizes its secondary metabolites upon perception of the siderophore pyochelin produced by a Pseudomonas competitor

Bacillus velezensis is considered as model species for plant-associated bacilli providing benefits to its host such as protection against phytopathogens. This is mainly due to the potential to secrete a wide range of secondary metabolites with specific and complementary bioactivities. This metabolite arsenal has been quite well defined genetically and chemically but much remains to be explored regarding how it is expressed under natural conditions and notably how it can be modulated upon interspecies interactions in the competitive rhizosphere niche. Here, we show that B. velezensis can mobilize a substantial part of its metabolome upon the perception of Pseudomonas, as a soil-dwelling competitor. This metabolite response reflects a multimodal defensive strategy as it includes polyketides and the bacteriocin amylocyclicin, with broad antibiotic activity, as well as surfactin lipopeptides, contributing to biofilm formation and enhanced motility. Furthermore, we identified the secondary Pseudomonas siderophore pyochelin as an info-chemical, which triggers this response via a mechanism independent of iron stress. We hypothesize that B. velezensis relies on such chelator sensing to accurately identify competitors, illustrating a new facet of siderophore-mediated interactions beyond the concept of competition for iron and siderophore piracy. This phenomenon may thus represent a new component of the microbial conversations driving the behavior of members of the rhizosphere community.

Altering in vivo membrane sterol composition affects the activity of the cyclic lipopeptides tolaasin and sessilin against Pythium

Cyclic lipopeptides (CLiPs) are secondary metabolites produced by a variety of bacteria. These compounds show a broad range of antimicrobial activities; therefore, they are studied for their potential applications in agriculture and medicine. It is generally assumed that the primary target of the CLiPs is the cellular membrane, where they can permeabilize the lipid bilayer. Model membrane systems are commonly used to investigate the effect of lipid composition on the permeabilizing activity of CLiPs, but these systems do not represent the full complexity of true biological membranes. Here, we introduce a novel method that uses sterol-auxotrophic oomycetes to investigate how the activity of membrane-active compounds is influenced by alterations in membrane sterol composition. More specifically, we investigated how ergosterol, cholesterol, beta-sitosterol and stigmasterol affect the activity of the structurally related Pseudomonas-derived CLiPs tolaasin and sessilin against the oomycete Pythium myriotylum. Both compounds were effective against oomycetes, although tolaasin was considerably more active. Interestingly, tolaasin and sessilin effects were similarly reduced by the presence of sterols, with cholesterol showing the highest reduction of activity.

An Nuclear Magnetic Resonance Fingerprint Matching Approach for the Identification and Structural Re-Evaluation of Pseudomonas Lipopeptides

Cyclic lipopeptides (CLiPs) are secondary metabolites secreted by a range of bacterial phyla. CLiPs from Pseudomonas in particular, display diverse structural variations in terms of the number of amino acid residues, macrocycle size, amino acid identity, and stereochemistry (e.g., d- versus l-amino acids). Reports detailing the discovery of novel or already characterized CLiPs from new sources appear regularly in literature. Increasingly, however, the lack of detailed characterization threatens to cause considerable confusion, especially if configurational heterogeneity is present for one or more amino acids. Using Pseudomonas CLiPs from the Bananamide, Orfamide, and Xantholysin groups as test cases, we demonstrate and validate that the combined 1H and 13C Nuclear Magnetic Resonance (NMR) chemical shifts of CLiPs constitute a spectral fingerprint that is sufficiently sensitive to differentiate between possible diastereomers of a particular sequence even when they only differ in a single d/l configuration. Rapid screening, involving simple matching of the NMR fingerprint of a newly isolated CLiP with that of a reference CLiP of known stereochemistry, can then be applied to resolve dead-ends in configurational characterization and avoid the much more cumbersome chemical characterization protocols. Even when the stereochemistry of a particular reference CLiP remains to be established, its spectral fingerprint allows to quickly verify whether a newly isolated CLiP is novel or already present in the reference collection. We show NMR fingerprinting leads to a simple approach for early on dereplication which should become more effective as more fingerprints are collected. To benefit research involving CLiPs, we have made a publicly available data repository accompanied by a 'knowledge base' at https://www.rhizoclip.be, where we present an overview of published NMR fingerprint data of characterized CLiPs, together with literature data on the originally determined structures. IMPORTANCE Pseudomonas CLiPs are ubiquitous specialized metabolites, impacting the producer's lifestyle and interactions with the (a)biotic environment. Consequently, they generate interest for agricultural and clinical applications. Establishing structure-activity relationships as a premise to their development is hindered because full structural characterization including stereochemical information requires labor-intensive analyses, without guarantee for success. Moreover, increasing use of superficial comparison with previously characterized CLiPs introduces or propagates erroneous attributions, clouding further scientific progress. We provide a generally applicable characterization methodology based on matching NMR spectral fingerprints of newly isolated CLiPs to natural and synthetic reference compounds with (un)known stereochemistry. In addition, NMR fingerprinting is shown to provide a suitable basis for structural dereplication. A publicly available reference compound repository promises to facilitate participation of the lipopeptide research community in structural assessment and dereplication of newly isolated CLiPs, which should also support further developments in genome mining for novel CLiPs.

Pseudomonas Lipopeptide-Mediated Biocontrol: Chemotaxonomy and Biological Activity

Pseudomonas lipopeptides (Ps-LPs) play crucial roles in bacterial physiology, host-microbe interactions and plant disease control. Beneficial LP producers have mainly been isolated from the rhizosphere, phyllosphere and from bulk soils. Despite their wide geographic distribution and host range, emerging evidence suggests that LP-producing pseudomonads and their corresponding molecules display tight specificity and follow a phylogenetic distribution. About a decade ago, biocontrol LPs were mainly reported from the P. fluorescens group, but this has drastically advanced due to increased LP diversity research. On the one hand, the presence of a close-knit relationship between Pseudomonas taxonomy and the molecule produced may provide a startup toolbox for the delineation of unknown LPs into existing (or novel) LP groups. Furthermore, a taxonomy-molecule match may facilitate decisions regarding antimicrobial activity profiling and subsequent agricultural relevance of such LPs. In this review, we highlight and discuss the production of beneficial Ps-LPs by strains situated within unique taxonomic groups and the lineage-specificity and coevolution of this relationship. We also chronicle the antimicrobial activity demonstrated by these biomolecules in limited plant systems compared with multiple in vitro assays. Our review further stresses the need to systematically elucidate the roles of diverse Ps-LP groups in direct plant-pathogen interactions and in the enhancement of plant innate immunity.

Lipopeptide Interplay Mediates Molecular Interactions between Soil Bacilli and Pseudomonads

Some Bacillus species, such as B. velezensis, are important members of the plant-associated microbiome, conferring protection against phytopathogens. However, our knowledge about multitrophic interactions determining the ecological fitness of these biocontrol bacteria in the competitive rhizosphere niche is still limited. Here, we investigated molecular mechanisms underlying interactions between B. velezensis and Pseudomonas as a soil-dwelling competitor. Upon their contact-independent in vitro confrontation, a multifaceted macroscopic outcome was observed and characterized by Bacillus growth inhibition, white line formation in the interaction zone, and enhanced motility. We correlated these phenotypes with the production of bioactive secondary metabolites and identified specific lipopeptides as key compounds involved in the interference interaction and motile response. Bacillus mobilizes its lipopeptide surfactin not only to enhance motility but also to act as a chemical trap to reduce the toxicity of lipopeptides formed by Pseudomonas. We demonstrated the relevance of these unsuspected roles of lipopeptides in the context of competitive tomato root colonization by the two bacterial genera.

IMPORTANCE Plant-associated Bacillus velezensis and Pseudomonas spp. represent excellent model species as strong producers of bioactive metabolites involved in phytopathogen inhibition and the elicitation of plant immunity. However, the ecological role of these metabolites during microbial interspecies interactions and the way their expression may be modulated under naturally competitive soil conditions has been poorly investigated. Through this work, we report various phenotypic outcomes from the interactions between B. velezensis and 10 Pseudomonas strains used as competitors and correlate them with the production of specific metabolites called lipopeptides from both species. More precisely, Bacillus overproduces surfactin to enhance motility, which also, by acting as a chemical trap, reduces the toxicity of other lipopeptides formed by Pseudomonas. Based on data from interspecies competition on plant roots, we assume this would allow Bacillus to gain fitness and persistence in its natural rhizosphere niche. The discovery of new ecological functions for Bacillus and Pseudomonas secondary metabolites is crucial to rationally design compatible consortia, more efficient than single-species inoculants, to promote plant health and growth by fighting economically important pathogens in sustainable agriculture.

Bacterial strains integrated with surfactin molecules of Bacillus subtilis MTCC441 enrich nematocidal activity against Meloidogyne incognita

Bacteria secrete lipopeptide (surfactin) molecules, which are known to act as natural antibiotics. Recently, research on lipopeptide molecules has grown because of their role in increasing resistance to plant pathogens. Isolated surfactin molecules at several concentrations, viz. 35, 25, 15 and 5 ppm, were used in an in vitro study for analysis of egg hatching inhibition and second-stage juvenile (J2) mortality of the nematode Meloidogyne incognita. Cell suspensions (1.2 × 10⁸  cfu·ml^-1 ) of both Bacillus subtilis (MTCC-441) and Pseudomonas putida (MTCC-102) were used in J2 inoculated tomato plants. Root-dip treatment of tomato seedlings with the crude lipopeptide (35 ppm) was also performed to analyse biocontrol potential. MALDI-TOF MS analysis was conducted to determine specific lipopeptide molecules. Data showed nematode egg mortality of 85% in the 35 ppm surfactin concentration 96 h after exposure. The maximum ovicidal activity was 83.97% after incubation with 35 ppm surfactin for the same exposure period. Plant growth attributes and biochemical parameters were significantly improved when bacterial cultures were applied before J2 inoculation of tomato seedlings. We also recorded a reduction in egg masses, nematode population and root galling. The J2 penetration into tomato roots was effective in the root-dip experiments. Surfactin mass peak was determined at m/z 1058 [M+Na]⁺ using MALDI-TOF MS. These results indicate that bacterial cell suspensions can be used as a potent and versatile source to deal with nematode infection and provide a rich source of bioactive compounds with antinematode activity.

Burkholderia in the genomic era: from taxonomy to the discovery of new antimicrobial secondary metabolites

Species of Burkholderia are highly versatile being found not only abundantly in soil, but also as plants and animals' commensals or pathogens. Their complex multireplicon genomes harbour an impressive number of polyketide synthase (PKS) and nonribosomal peptide-synthetase (NRPS) genes coding for the production of antimicrobial secondary metabolites (SMs), which have been successfully deciphered by genome-guided tools. Moreover, genome metrics supported the split of this genus into Burkholderia sensu stricto (s.s.) and five new other genera. Here, we show that the successful antimicrobial SMs producers belong to Burkholderia s.s. Additionally, we reviewed the occurrence, bioactivities, modes of action, structural, and biosynthetic information of thirty-eight Burkholderia antimicrobial SMs shedding light on their diversity, complexity, and uniqueness as well as the importance of genome-guided strategies to facilitate their discovery. Several Burkholderia NRPS and PKS display unusual features, which are reflected in their structural diversity, important bioactivities, and varied modes of action. Up to now, it is possible to observe a general tendency of Burkholderia SMs being more active against fungi. Although the modes of action and biosynthetic gene clusters of many SMs remain unknown, we highlight the potential of Burkholderia SMs as alternatives to fight against new diseases and antibiotic resistance.

Bacillus Cyclic Lipopeptides Iturin and Fengycin Control Rice Blast Caused by Pyricularia oryzae in Potting and Acid Sulfate Soils by Direct Antagonism and Induced Systemic Resistance

Rice monoculture in acid sulfate soils (ASSs) is affected by a wide range of abiotic and biotic constraints, including rice blast caused by Pyricularia oryzae. To progress towards a more sustainable agriculture, our research aimed to screen the biocontrol potential of indigenous Bacillus spp. against blast disease by triggering induced systemic resistance (ISR) via root application and direct antagonism. Strains belonging to the B. altitudinis and B. velezensis group could protect rice against blast disease by ISR. UPLC-MS and marker gene replacement methods were used to detect cyclic lipopeptide (CLiP) production and construct CLiPs deficient mutants of B. velezensis, respectively. Here we show that the CLiPs fengycin and iturin are both needed to elicit ISR against rice blast in potting soil and ASS conditions. The CLiPs surfactin, iturin and fengycin completely suppressed P. oryzae spore germination resulting in disease severity reduction when co-applied on rice leaves. In vitro microscopic assays revealed that iturin and fengycin inhibited the mycelial growth of the fungus P. oryzae, while surfactin had no effect. The capacity of indigenous Bacillus spp. to reduce rice blast by direct and indirect antagonism in ASS conditions provides an opportunity to explore their usage for rice blast control in the field.

Fungicidal Activity of Volatile Organic Compounds Emitted by Burkholderia gladioli Strain BBB-01

A Burkholderia gladioli strain, named BBB-01, was isolated from rice shoots based on the confrontation plate assay activity against several plant pathogenic fungi. The genome of this bacterial strain consists of two circular chromosomes and one plasmid with 8,201,484 base pairs in total. Pangenome analysis of 23 B. gladioli strains suggests that B. gladioli BBB-01 has the closest evolutionary relationship to B. gladioli pv. gladioli and B. gladioli pv. agaricicola. B. gladioli BBB-01 emitted dimethyl disulfide and 2,5-dimethylfuran when it was cultivated in lysogeny broth and potato dextrose broth, respectively. Dimethyl disulfide is a well-known pesticide, while the bioactivity of 2,5-dimethylfuran has not been reported. In this study, the inhibition activity of the vapor of these two compounds was examined against phytopathogenic fungi, including Magnaporthe oryzae, Gibberella fujikuroi, Sarocladium oryzae, Phellinus noxius and Colletotrichumfructicola, and human pathogen Candida albicans. In general, 2,5-dimethylfuran is more potent than dimethyl disulfide in suppressing the growth of the tested fungi, suggesting that 2,5-dimethylfuran is a potential fumigant to control plant fungal disease.

Harnessing Pseudomonas protegens to Control Bacterial Panicle Blight of Rice

Bacterial panicle blight of rice is a seedborne disease caused by the bacterium Burkholderia glumae. This disease has affected rice production worldwide and its effects are likely to become more devastating with the continuous increase in global temperatures, especially during the growing season. The bacterium can cause disease symptoms in different tissues and at different developmental stages. In reproductive stages, the bacterium interferes with grain development in the panicles and, as a result, directly affects rice yield. Currently, there are no methods to control the disease because chemical control is not effective and completely resistant cultivars are not available. Thus, a promising approach is the use of antagonistic microorganisms. In this work, we identified one strain of Pseudomonas protegens and one strain of B. cepacia with antimicrobial activity against B. glumae in vitro and in planta. We further characterized the antimicrobial activity of P. protegens and found that this activity is associated with bacterial secretions. Cell-free secretions from P. protegens inhibited the growth of B. glumae in vitro and also prevented B. glumae from causing disease in rice. Although the specific molecules associated with these activities have not been identified, these findings suggest that the secreted fractions from P. protegens could be harnessed as biopesticides to control bacterial panicle blight of rice.

Phenazine from Pseudomonas aeruginosa UPMP3 induced the host resistance in oil palm (Elaeis guineensis Jacq.)-Ganoderma boninense pathosystem

Pseudomonas aeruginosa developed its biocontrol agent property through the production of antifungal derivatives, with the phenazine among them. In this study, the applications of crude phenazine synthesized by Pseudomonas aeruginosa UPMP3 and hexaconazole were comparatively evaluated for their effectiveness to suppress basal stem rot infection in artificially G. boninense-challenged oil palm seedlings. A glasshouse experiment under the randomized completely block design was set with the following treatments: non-inoculated seedlings, G. boninense inoculated seedlings, G. boninense inoculated seedlings with 1 mg/ml phenazine application, G. boninense inoculated seedlings with 2 mg/ml phenazine application and G. boninense inoculated seedlings with 0.048 mg/ml hexaconazole application. Seedlings were screened for disease parameters and plant vigour traits (plant height, plant fresh weight, root fresh, and dry weight, stem diameter, and total chlorophyll) at 1-to-4 month post-inoculation (mpi). The application of 2 mg/ml phenazine significantly reduced disease severity (DS) at 44% in comparison to fungicide application (DS = 67%). Plant vigour improved from 1 to 4 mpi and the rate of disease reduction in seedlings with phenazine application (2 mg/ml) was twofold greater than hexaconazole. At 4, 6 and 8 wpi, an up-regulation of chitinase and β-1,3 glucanase genes in seedlings treated with phenazine suggests the involvement of induced resistance in G. boninense-oil palm pathosystem.

Biosurfactants in Plant Protection Against Diseases: Rhamnolipids and Lipopeptides Case Study

Biosurfactants are amphiphilic surface-active molecules that are produced by a variety of microorganisms including fungi and bacteria. Pseudomonas, Burkholderia, and Bacillus species are known to secrete rhamnolipids and lipopeptides that are used in a wide range of industrial applications. Recently, these compounds have been studied in a context of plant-microbe interactions. This mini-review describes the direct antimicrobial activities of these compounds against plant pathogens. We also provide the current knowledge on how rhamnolipids and lipopeptides stimulate the plant immune system leading to plant resistance to phytopathogens. Given their low toxicity, high biodegradability and ecological acceptance, we discuss the possible role of these biosurfactants as alternative strategies to reduce or even replace pesticide use in agriculture.

Biosynthesis and Antimicrobial Activity of Pseudodesmin and Viscosinamide Cyclic Lipopeptides Produced by Pseudomonads Associated with the Cocoyam Rhizosphere

Pseudomonas cyclic lipopeptides (CLPs) are encoded non-ribosomally by biosynthetic gene clusters (BGCs) and possess diverse biological activities. In this study, we conducted chemical structure and BGC analyses with antimicrobial activity assays for two CLPs produced by Pseudomonas strains isolated from the cocoyam rhizosphere in Cameroon and Nigeria. LC-MS and NMR analyses showed that the Pseudomonas sp. COR52 and A2W4.9 produce pseudodesmin and viscosinamide, respectively. These CLPs belong to the Viscosin group characterized by a nonapeptidic moiety with a 7-membered macrocycle. Similar to other Viscosin-group CLPs, the initiatory non-ribosomal peptide synthetase (NRPS) gene of the viscosinamide BGC is situated remotely from the other two NRPS genes. In contrast, the pseudodesmin genes are all clustered in a single genomic locus. Nano- to micromolar levels of pseudodesmin and viscosinamide led to the hyphal distortion and/or disintegration of Rhizoctonia solani AG2-2 and Pythium myriotylum CMR1, whereas similar levels of White Line-Inducing Principle (WLIP), another member of the Viscosin group, resulted in complete lysis of both soil-borne phytopathogens. In addition to the identification of the biosynthetic genes of these two CLPs and the demonstration of their interaction with soil-borne pathogens, this study provides further insights regarding evolutionary divergence within the Viscosin group.

Identification of a Strong Quorum Sensing- and Thermo-Regulated Promoter for the Biosynthesis of a New Metabolite Pesticide Phenazine-1-carboxamide in Pseudomonas strain PA1201

Phenazine-1-carboxamide (PCN) produced by multifarious Pseudomonas strains represents a promising candidate as a new metabolite pesticide due to its broad-spectrum antifungal activity and capacity to induce systemic resistance in plants. The rice rhizosphere Pseudomonas strain PA1201 contains two reiterated gene clusters, phz1 and phz2, for phenazine-1-carboxylic acid (PCA) biosynthesis; PCA is further converted into PCN by this strain using a functional phzH-encoding glutamine aminotransferase. However, PCN levels in PA1201 constitute approximately one-fifth of PCA levels and the optimal temperature for PCN synthesis is 28 °C. In this study, the phzH open reading frame (ORF) and promoter region were investigated and reannotated. phzH promoter P_phzH was found to be a weak promoter, and PhzH levels were not sufficient to convert all of the native PCA into PCN. Following RNA Seq and promoter-lacZ fusion analyses, a strong quorum sensing (QS)- and thermo-regulated promoter P_rhlI was identified and characterized. The activity of P_phzH is approximately 1% of P_rhlI in PA1201. After three rounds of promoter editing and swapping by P_rhlI, a new PCN-overproducing strain UP46 was generated. The optimal fermentation temperature for PCN biosynthesis in UP46 was increased from 28 to 37 °C and the PCN fermentation titer increased 179.5-fold, reaching 14.1 g/L, the highest ever reported.

Elicitors of Plant Immunity Triggered by Beneficial Bacteria

The molecular basis of plant immunity triggered by microbial pathogens is being well-characterized as a complex sequential process leading to the activation of defense responses at the infection site, but which may also be systemically expressed in all organs, a phenomenon also known as systemic acquired resistance (SAR). Some plant-associated and beneficial bacteria are also able to stimulate their host to mount defenses against pathogen ingress via the phenotypically similar, induced systemic resistance phenomenon. Induced systemic resistance resembles SAR considering its mechanistic principle as it successively involves recognition at the plant cell surface, stimulation of early cellular immune-related events, systemic signaling via a fine-tuned hormonal cross-talk and activation of defense mechanisms. It thus represents an indirect but efficient mechanism by which beneficial bacteria with biocontrol potential improve the capacity of plants to restrict pathogen invasion. However, according to our current vision, induced systemic resistance is specific considering some molecular aspects underpinning these different steps. Here we overview the chemical diversity of compounds that have been identified as induced systemic resistance elicitors and thereby illustrating the diversity of plants species that are responsive as well as the range of pathogens that can be controlled via this phenomenon. We also point out the need for further investigations allowing better understanding how these elicitors are sensed by the host and the diversity and nature of the stimulated defense mechanisms.

Pseudomonas sp. COW3 Produces New Bananamide-Type Cyclic Lipopeptides with Antimicrobial Activity against Pythium myriotylum and Pyricularia oryzae

Pseudomonas species are metabolically robust, with capacity to produce secondary metabolites including cyclic lipopeptides (CLPs). Herein we conducted a chemical analysis of a crude CLP extract from the cocoyam rhizosphere-derived biocontrol strain Pseudomonas sp. COW3. We performed in silico analyses on its whole genome, and conducted in vitro antagonistic assay using the strain and purified CLPs. Via LC-MS and NMR, we elucidated the structures of four novel members of the bananamide group, named bananamides D-G. Besides variability in fatty acid length, bananamides D-G differ from previously described bananamides A-C and MD-0066 by the presence of a serine and aspartic acid at position 6 and 2, respectively. In addition, bananamide G has valine instead of isoleucine at position 8. Kendrick mass defect (KMD) allowed the assignment of molecular formulae to bananamides D and E. We unraveled a non-ribosomal peptide synthetase cluster banA, banB and banC which encodes the novel bananamide derivatives. Furthermore, COW3 displayed antagonistic activity and mycophagy against Pythium myriotylum, while it mainly showed mycophagy on Pyricularia oryzae. Purified bananamides D-G inhibited the growth of P. myriotylum and P. oryzae and caused hyphal distortion. Our study shows the complementarity of chemical analyses and genome mining in the discovery and elucidation of novel CLPs. In addition, structurally diverse bananamides differ in their antimicrobial activity.

Pseudomonas Cyclic Lipopeptides Suppress the Rice Blast Fungus Magnaporthe oryzae by Induced Resistance and Direct Antagonism

Beneficial Pseudomonas spp. produce an array of antimicrobial secondary metabolites such as cyclic lipopeptides (CLPs). We investigated the capacity of CLP-producing Pseudomonas strains and their crude CLP extracts to control rice blast caused by Magnaporthe oryzae, both in a direct manner and via induced systemic resistance (ISR). In planta biocontrol assays showed that lokisin-, white line inducing principle (WLIP)-, entolysin- and N3-producing strains successfully induced resistance to M. oryzae VT5M1. Furthermore, crude extracts of lokisin, WLIP and entolysin gave similar ISR results when tested in planta. In contrast, a xantholysin-producing strain and crude extracts of N3, xantholysin and orfamide did not induce resistance against the rice blast disease. The role of WLIP in triggering ISR was further confirmed by using WLIP-deficient mutants. The severity of rice blast disease was significantly reduced when M. oryzae spores were pre-treated with crude extracts of N3, lokisin, WLIP, entolysin or orfamide prior to inoculation. In vitro microscopic assays further revealed the capacity of crude N3, lokisin, WLIP, entolysin, xantholysin and orfamide to significantly inhibit appressoria formation by M. oryzae. In addition, the lokisin and WLIP biosynthetic gene clusters in the producing strains are described. In short, our study demonstrates the biological activity of structurally diverse CLPs in the control of the rice blast disease caused by M. oryzae. Furthermore, we provide insight into the non-ribosomal peptide synthetase genes encoding the WLIP and lokisin biosynthetic machineries.

Fitness Features Involved in the Biocontrol Interaction of Pseudomonas chlororaphis With Host Plants: The Case Study of PcPCL1606

The goal of this mini review is to summarize the relevant contribution of some beneficial traits to the behavior of the species Pseudomonas chlororaphis, and using that information, to give a practical point of view using the model biocontrol strain P. chlororaphis PCL1606 (PcPCL1606). Among the group of plant-beneficial rhizobacteria, P. chlororaphis has emerged as a plant- and soil-related bacterium that is mainly known because of its biological control of phytopathogenic fungi. Many traits have been reported to be crucial during the multitrophic interaction involving the plant, the fungal pathogen and the soil environment. To explore the different biocontrol-related traits, the biocontrol rhizobacterium PcPCL1606 has been used as a model in recent studies. This bacterium is antagonistic to many phytopathogenic fungi and displays effective biocontrol against fungal phytopathogens. Antagonistic and biocontrol activities are directly related to the production of the compound 2-hexyl, 5-propyl resorcinol (HPR), despite the production of other antifungal compounds. Furthermore, PcPCL1606 has displayed additional traits regarding its fitness in soil and plant root environments such as soil survival, efficient plant root colonization, cell-to-cell interaction or promotion of plant growth.

Fluorescent Pseudomonas and cyclic lipopeptide diversity in the rhizosphere of cocoyam (Xanthosoma sagittifolium)

Cocoyam (Xanthosoma sagittifolium (L.)), an important tuber crop in the tropics, is severely affected by the cocoyam root rot disease (CRRD) caused by Pythium myriotylum. The white cocoyam genotype is very susceptible while the red cocoyam has some field tolerance to CRRD. Fluorescent Pseudomonas isolates obtained from the rhizosphere of healthy red and white cocoyams from three different fields in Cameroon were taxonomically characterized. The cocoyam rhizosphere was enriched with P. fluorescens complex and P. putida isolates independent of the plant genotype. LC-MS and NMR analyses revealed that 50% of the Pseudomonas isolates produced cyclic lipopeptides (CLPs) including entolysin, lokisin, WLIP, putisolvin and xantholysin together with eight novel CLPs. In general, CLP types were linked to specific taxonomic groups within the fluorescent pseudomonads. Representative CLP-producing bacteria showed effective control against CRRD while purified CLPs caused hyphal branching or hyphal leakage in P. myriotylum. The structure of cocoyamide A, a CLP which is predominantly produced by P. koreensis group isolates within the P. fluorescens complex is described. Compared with the white cocoyam, the red cocoyam rhizosphere appeared to support a more diverse CLP spectrum. It remains to be investigated whether this contributes to the field tolerance displayed by the red cocoyam.

Diversity of phytobeneficial traits revealed by whole-genome analysis of worldwide-isolated phenazine-producing Pseudomonas spp

Plant-beneficial Pseudomonas spp. competitively colonize the rhizosphere and display plant-growth promotion and/or disease-suppression activities. Some strains within the P. fluorescens species complex produce phenazine derivatives, such as phenazine-1-carboxylic acid. These antimicrobial compounds are broadly inhibitory to numerous soil-dwelling plant pathogens and play a role in the ecological competence of phenazine-producing Pseudomonas spp. We assembled a collection encompassing 63 strains representative of the worldwide diversity of plant-beneficial phenazine-producing Pseudomonas spp. In this study, we report the sequencing of 58 complete genomes using PacBio RS II sequencing technology. Distributed among four subgroups within the P. fluorescens species complex, the diversity of our collection is reflected by the large pangenome which accounts for 25 413 protein-coding genes. We identified genes and clusters encoding for numerous phytobeneficial traits, including antibiotics, siderophores and cyclic lipopeptides biosynthesis, some of which were previously unknown in these microorganisms. Finally, we gained insight into the evolutionary history of the phenazine biosynthetic operon. Given its diverse genomic context, it is likely that this operon was relocated several times during Pseudomonas evolution. Our findings acknowledge the tremendous diversity of plant-beneficial phenazine-producing Pseudomonas spp., paving the way for comparative analyses to identify new genetic determinants involved in biocontrol, plant-growth promotion and rhizosphere competence.

Selecting lipopeptide-producing, Fusarium-suppressing Bacillus spp.: Metabolomic and genomic probing of Bacillus velezensis NWUMFkBS10.5

The results of this study indicate that the maize rhizosphere remains a reservoir for microbial strains with unique beneficial properties. The study sought to provide an indigenous Bacillus strain with a bioprotective potential to alleviate maize fusariosis in South Africa. We selected seven Bacillus isolates (MORWBS1.1, MARBS2.7, VERBS5.5, MOREBS6.3, MOLBS8.5, MOLBS8.6, and NWUMFkBS10.5) with biosuppressive effects against two maize fungal pathogens (Fusarium graminearum and Fusarium culmorum) based on 16S rDNA gene characterization and lipopeptide gene analysis. The PCR analysis revealed that lipopeptide genes encoding the synthesis of iturin, surfactin, and fengycin might be responsible for their antifungal activities. Few of the isolates also showed possible biosurfactant capability, and their susceptibility to known antibiotics is indicative of their eco‐friendly attributes. In addition, in silico genomic analysis of our best isolate (Bacillus velezensis NWUMFkBS10.5) and characterization of its active metabolite with FTIR, NMR, and ESI‐Micro‐Tof MS confirmed the presence of valuable genes clusters and metabolic pathways. The versatile genomic potential of our Bacillus isolate emphasizes the continued relevance of Bacillus spp. in biological management of plant diseases.

Cyclic Lipodepsipeptides From Pseudomonas spp. - Biological Swiss-Army Knives

Cyclic lipodepsipeptides produced by Pseudomonas spp. (Ps-CLPs) are biosurfactants that constitute a diverse class of versatile bioactive natural compounds with promising application potential. While chemically diverse, they obey a common structural blue-print, allowing the definition of 14 distinct groups with multiple structurally homologous members. In addition to antibacterial and antifungal properties the reported activity profile of Ps-CLPs includes their effect on bacterial motility, biofilm formation, induced defense responses in plants, their insecticidal activity and anti-proliferation effects on human cancer cell-lines. To further validate their status of potential bioactive substances, we assessed the results of 775 biological tests on 51 Ps-CLPs available from literature. From this, a fragmented view emerges. Taken as a group, Ps-CLPs present a broad activity profile. However, reports on individual Ps-CLPs are often much more limited in the scope of organisms that are challenged or activities that are explored. As a result, our analysis shows that the available data is currently too sparse to allow biological function to be correlated to a particular group of Ps-CLPs. Consequently, certain generalizations that appear in literature with respect to the biological activities of Ps-CLPs should be nuanced. This notwithstanding, the data for the two most extensively studied Ps-CLPs does indicate they can display activities against various biological targets. As the discovery of novel Ps-CLPs accelerates, current challenges to complete and maintain a useful overview of biological activity are discussed.

The cyclic lipopeptide orfamide induces systemic resistance in rice to Cochliobolus miyabeanus but not to Magnaporthe oryzae

The Pseudomonas- derived cyclic lipopeptide orfamide can induce resistance to Cochliobolus miyabeanus but not to Magnaporthe oryzae in rice. Abscisic acid signaling is involved in the induced systemic resistance response triggered by orfamide. Diverse natural products produced by beneficial Pseudomonas species have the potential to trigger induced systemic resistance (ISR) in plants, and thus may contribute to control of diseases in crops. Some beneficial Pseudomonas spp. can produce cyclic lipopeptides (CLPs), amphiphilic molecules composed of a fatty acid tail linked to an oligopeptide which is cyclized. CLPs can have versatile biological functions, but the capacity of Pseudomonas-derived CLPs in triggering ISR responses has barely been studied. Pseudomonas protegens and related species can produce orfamide-type CLPs. Here we show that in rice, orfamides can act as ISR elicitors against the necrotrophic fungus Cochliobolus miyabeanus, the causal agent of brown spot disease, but are not active against the blast fungus Magnaporthe oryzae. Orfamide A can trigger early defensive events and activate transcripts of defense-related genes in rice cell suspension cultures, but does not cause cell death. Further testing in rice cell suspension cultures and rice plants showed that abscisic acid signaling, the transcriptional activator OsWRKY4 and pathogenesis-related protein PR1b are triggered by orfamide A and may play a role in the ISR response against C. miyabeanus.

Coregulation of the cyclic lipopeptides orfamide and sessilin in the biocontrol strain Pseudomonas sp. CMR12a

Cyclic lipopeptides (CLPs) are synthesized by nonribosomal peptide synthetases (NRPS), which are often flanked by LuxR‐type transcriptional regulators. Pseudomonas sp. CMR12a, an effective biocontrol strain, produces two different classes of CLPs namely sessilins and orfamides. The orfamide biosynthesis gene cluster is flanked up‐ and downstream by LuxR‐type regulatory genes designated ofaR1 and ofaR2, respectively, whereas the sessilin biosynthesis gene cluster has one LuxR‐type regulatory gene which is situated upstream of the cluster and is designated sesR. Our study investigated the role of these three regulators in the biosynthesis of orfamides and sessilins. Phylogenetic analyses positioned OfaR1 and OfaR2 with LuxR regulatory proteins of similar orfamide‐producing Pseudomonas strains and the SesR with that of the tolaasin producer, Pseudomonas tolaasii. LC‐ESI‐MS analyses revealed that sessilins and orfamides are coproduced and that production starts in the late exponential phase. However, sessilins are secreted earlier and in large amounts, while orfamides are predominantly retained in the cell. Deletion mutants in ofaR1 and ofaR2 lost the capacity to produce both orfamides and sessilins, whereas the sesR mutant showed no clear phenotype. Additionally, RT‐PCR analysis showed that in the sessilin cluster, a mutation in either ofaR1 or ofaR2 led to weaker transcripts of the biosynthesis genes, sesABC, and putative transporter genes, macA1B1. In the orfamide cluster, mainly the biosynthesis genes ofaBC were affected, while the first biosynthesis gene ofaA and putative macA2B2 transport genes were still transcribed. A mutation in either ofaR1, ofaR2, or sesR genes did not abolish the transcription of any of the other two.

Antimicrobial and Insecticidal: Cyclic Lipopeptides and Hydrogen Cyanide Produced by Plant-Beneficial Pseudomonas Strains CHA0, CMR12a, and PCL1391 Contribute to Insect Killing

Particular groups of plant-beneficial fluorescent pseudomonads are not only root colonizers that provide plant disease suppression, but in addition are able to infect and kill insect larvae. The mechanisms by which the bacteria manage to infest this alternative host, to overcome its immune system, and to ultimately kill the insect are still largely unknown. However, the investigation of the few virulence factors discovered so far, points to a highly multifactorial nature of insecticidal activity. Antimicrobial compounds produced by fluorescent pseudomonads are effective weapons against a vast diversity of organisms such as fungi, oomycetes, nematodes, and protozoa. Here, we investigated whether these compounds also contribute to insecticidal activity. We tested mutants of the highly insecticidal strains Pseudomonas protegens CHA0, Pseudomonas chlororaphis PCL1391, and Pseudomonas sp. CMR12a, defective for individual or multiple antimicrobial compounds, for injectable and oral activity against lepidopteran insect larvae. Moreover, we studied expression of biosynthesis genes for these antimicrobial compounds for the first time in insects. Our survey revealed that hydrogen cyanide and different types of cyclic lipopeptides contribute to insecticidal activity. Hydrogen cyanide was essential to full virulence of CHA0 and PCL1391 directly injected into the hemolymph. The cyclic lipopeptide orfamide produced by CHA0 and CMR12a was mainly important in oral infections. Mutants of CMR12a and PCL1391 impaired in the production of the cyclic lipopeptides sessilin and clp1391, respectively, showed reduced virulence in injection and feeding experiments. Although virulence of mutants lacking one or several of the other antimicrobial compounds, i.e., 2,4-diacetylphloroglucinol, phenazines, pyrrolnitrin, or pyoluteorin, was not reduced, these metabolites might still play a role in an insect background since all investigated biosynthetic genes for antimicrobial compounds of strain CHA0 were expressed at some point during insect infection. In summary, our study identified new factors contributing to insecticidal activity and extends the diverse functions of antimicrobial compounds produced by fluorescent pseudomonads from the plant environment to the insect host.

Role of phenazines and cyclic lipopeptides produced by pseudomonas sp. CMR12a in induced systemic resistance on rice and bean

Pseudomonas sp. CMR12a produces two different classes of cyclic lipopeptides (CLPs) (orfamides and sessilins), which all play a role in direct antagonism against soilborne pathogens. Here we show that Pseudomonas sp. CMR12a is also able to induce systemic resistance to Magnaporthe oryzae on rice and to the web blight pathogen Rhizoctonia solani AG2-2 on bean. Plant assays with biosynthesis mutants of Pseudomonas sp. CMR12a impaired in the production of phenazines and/or CLPs and purified metabolites revealed that distinct bacterial determinants are responsible for inducing systemic resistance in these two pathosystems. In rice, mutants impaired in phenazine production completely lost their ability to induce systemic resistance, while a soil drench with pure phenazine-1-carboxamide (PCN) at a concentration of 0.1 or 1 μM was active in inducing resistance against M. oryzae. In bean, mutants that only produced phenazines, sessilins or orfamides were still able to induce systemic resistance against Rhizoctonia web blight, but a balanced production of these metabolites was needed. This study not only shows that Pseudomonas sp. CMR12a can protect rice to blast disease and bean to web blight disease, but also displays that the determinants involved in induced systemic resistance are plant, pathogen and concentration dependent.

Characterization of Toxin Complex Gene Clusters and Insect Toxicity of Bacteria Representing Four Subgroups of Pseudomonas fluorescens

Ten strains representing four lineages of the Pseudomonas fluorescens group (P. chlororaphis, P. corrugata, P. koreensis, and P. fluorescens subgroups) were evaluated for toxicity to the tobacco hornworm Manduca sexta and the common fruit fly Drosophila melanogaster. The three strains within the P. chlororaphis subgroup exhibited both oral and injectable toxicity to the lepidopteran M. sexta. All three strains have the gene cluster encoding the FitD insect toxin and a ΔfitD mutant of P. protegens strain Pf-5 exhibited diminished oral toxicity compared to the wildtype strain. Only one of the three strains, P. protegens Pf-5, exhibited substantial levels of oral toxicity against the dipteran D. melanogaster. Three strains in the P. fluorescens subgroup, which lack fitD, consistently showed significant levels of injectable toxicity against M. sexta. In contrast, the oral toxicity of these strains against D. melanogaster was variable between experiments, with only one strain, Pseudomonas sp. BG33R, causing significant levels of mortality in repeated experiments. Toxin complex (Tc) gene clusters, which encode insecticidal properties in Photorhabdus luminescens, were identified in the genomes of seven of the ten strains evaluated in this study. Within those seven genomes, six types of Tc gene clusters were identified, distinguished by gene content, organization and genomic location, but no correlation was observed between the presence of Tc genes and insect toxicity of the evaluated strains. Our results demonstrate that members of the P. fluorescens group have the capacity to kill insects by both FitD-dependent and independent mechanisms.

Leuconostoc spp. Associated with Root Rot in Sugar Beet and Their Interaction with Rhizoctonia solani

Rhizoctonia root and crown rot is an important disease problem in sugar beet caused by Rhizoctonia solani and also shown to be associated with Leuconostoc spp. Initial Leuconostoc studies were conducted with only a few isolates and the relationship of Leuconostoc with R. solani is poorly understood; therefore, a more thorough investigation was conducted. In total, 203 Leuconostoc isolates were collected from recently harvested sugar beet roots in southern Idaho and southeastern Oregon during 2010 and 2012: 88 and 85% Leuconostoc mesenteroides, 6 and 15% L. pseudomesenteroides, 2 and 0% L. kimchi, and 4 and 0% unrecognized Leuconostoc spp., respectively. Based on 16S ribosomal RNA sequencing, haplotype 11 (L. mesenteroides isolates) comprised 68 to 70% of the isolates in both years. In pathogenicity field studies with commercial sugar beet 'B-7', all Leuconostoc isolates caused more rot (P < 0.0001; α = 0.05) when combined with R. solani than when inoculated alone in both years. Also, 46 of the 52 combination treatments over the 2 years had significantly more rot (P < 0.0001; α = 0.05) than the fungal check. The data support the conclusion that a synergistic interaction leads to more rot when both Leuconostoc spp. and R. solani are present in sugar beet roots.

Biosynthesis, Chemical Structure, and Structure-Activity Relationship of Orfamide Lipopeptides Produced by Pseudomonas protegens and Related Species

Orfamide-type cyclic lipopeptides (CLPs) are biosurfactants produced by Pseudomonas and involved in lysis of oomycete zoospores, biocontrol of Rhizoctonia and insecticidal activity against aphids. In this study, we compared the biosynthesis, structural diversity, in vitro and in planta activities of orfamides produced by rhizosphere-derived Pseudomonas protegens and related Pseudomonas species. Genetic characterization together with chemical identification revealed that the main orfamide compound produced by the P. protegens group is orfamide A, while the related strains Pseudomonas sp. CMR5c and CMR12a produce orfamide B. Comparison of orfamide fingerprints led to the discovery of two new orfamide homologs (orfamide F and orfamide G) in Pseudomonas sp. CMR5c. The structures of these two CLPs were determined by nuclear magnetic resonance (NMR) and mass spectrometry (MS) analysis. Mutagenesis and complementation showed that orfamides determine the swarming motility of parental Pseudomonas sp. strain CMR5c and their production was regulated by luxR type regulators. Orfamide A and orfamide B differ only in the identity of a single amino acid, while orfamide B and orfamide G share the same amino acid sequence but differ in length of the fatty acid part. The biological activities of orfamide A, orfamide B, and orfamide G were compared in further bioassays. The three compounds were equally active against Magnaporthe oryzae on rice, against Rhizoctonia solani AG 4-HGI in in vitro assays, and caused zoospore lysis of Phytophthora and Pythium. Furthermore, we could show that orfamides decrease blast severity in rice plants by blocking appressorium formation in M. oryzae. Taken all together, our study shows that orfamides produced by P. protegens and related species have potential in biological control of a broad spectrum of fungal plant pathogens.