Microbiota Associated with Sclerotia of Soilborne Fungal Pathogens – A Novel Source of Biocontrol Agents Producing Bioactive Volatiles
Abstract and Figures
Soilborne plant pathogens are an increasing problem in modern agriculture, and their ability to survive long periods in soil as persistent sclerotia makes control and treatment particularly challenging. To develop new control strategies, we explored bacteria associated with sclerotia of Sclerotinia sclerotiorum and Rhizoctonia solani, two soilborne fungi causing high yield losses. We combined different methodological approaches to get insights into the indigenous microbiota of sclerotia, to compare it to bacterial communities of the surrounding environment, and to identify novel biocontrol agents and antifungal volatiles. Analysis of 16S rRNA gene fragment amplicons revealed significant compositional differences in the bacterial microbiomes of Rhizoctonia sclerotia, the unaffected tuber surface and surrounding soil. Moreover, distinctive bacterial lineages were associated with specific sample types. Flavobacteriaceae and Caulobacteraceae were primarily found in unaffected areas, while Phyllobacteriaceae and Bradyrhizobiaceae were associated with sclerotia of R. solani. In parallel, we studied a strain collection isolated from sclerotia of the pathogens for emission of bioactive volatile compounds. Isolates of Bacillus, Pseudomonas, and Buttiauxella exhibited high antagonistic activity toward both soilborne pathogens and were shown to produce novel, not yet described volatiles. Differential imaging showed that volatiles emitted by the antagonists altered the melanized sclerotia surface of S. sclerotiorum. Interestingly, combinations of bacterial antagonists increased inhibition of mycelial growth up to 60% when compared with single isolates. Our study showed that fungal survival structures are associated with a specific microbiome, which is also a reservoir for new biocontrol agents.
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... also release an array of antifungal diffusible compounds (Liu et al., 2019), hydrolytic enzymes (Liu et al., 2019;Tahtamouni et al., 2006), and VOCs (Gebily et al., 2021;Wan et al., 2008) that prevent S. sclerotiorum from growing and producing sclerotia. Other bacterial species reported to secrete antifungal VOCs to suppress S. sclerotiorum mycelia growth and sclerotia germination include Xenorhabdus szentirmaii (Chacon-Orozco et al., 2020), P. brassicacearum, P. putida (Giorgio et al., 2015), Buttiauxella warmboldiae, and P. helmanticensis (Mülner et al., 2019). Morphological analyses of sclerotia that were exposed to antifungal VOCs reveal discoloration of the inner sclerotial tissue (Mülner et al., 2019). ...
... Other bacterial species reported to secrete antifungal VOCs to suppress S. sclerotiorum mycelia growth and sclerotia germination include Xenorhabdus szentirmaii (Chacon-Orozco et al., 2020), P. brassicacearum, P. putida (Giorgio et al., 2015), Buttiauxella warmboldiae, and P. helmanticensis (Mülner et al., 2019). Morphological analyses of sclerotia that were exposed to antifungal VOCs reveal discoloration of the inner sclerotial tissue (Mülner et al., 2019). In addition to suppressing mycelial growth and apothecial formation, VOCs produced by Pseudomonas spp. ...
Biological control using antagonistic microorganisms as biological control agents (BCAs) has been considered a potential environmentally-friendly alternative or supplement for the control of soil-borne plant pathogens. This may be part of an integrated disease management system, thus contributing to a reduction in the use of chemical products and the protection of the environment. This review aims to provide an updated overview and insight into the underlying antagonistic mechanisms of BCAs to directly or indirectly suppress Sclerotinia sclerotiorum, an economically devastating soil-borne fungal pathogen of a broad range of plants. Further, the application of organic amendments (OAs) has demonstrated the potential to suppress soil-borne plant pathogens through the introduction of BCAs to the soil and/or enhancing the activity of the existing soil microbiome and BCAs which improves general plant health. Therefore, the manipulation and exploitation of soil microbial communities, through the application of OAs, has the potential as part of a sustainable integrated disease management strategy for the control of soil-borne plant pathogens, particularly S. sclerotiorum. In this review, we highlight gaps in the current understanding of the interaction of BCAs, OAs, and S. sclerotiorum and suggest future directions for research in this space, including improving our understanding of the microbiomes of soils and OAs suppressive to S. sclerotiorum.
... It is worth noting that, in some cases, plant defense can be more effective and efficient when induced by microbial consortia, compared with the application of a single microbial inoculant. Therefore, considering the ability of some microbial consortiums to trigger induced systemic resistance and biologically control and rebalance the microbial community, there are many opportunities to exploit PGP traits of microbial consortiums not only to effectively increase yield, but also to resist phytopathogens in plant cultivation [120][121][122][123][124]. A number of studies indicate the possibility of controlling fungal phytopathogens, such as Rhizoctonia solani, as well as members of the genera Fusarium and Sclerotium. ...
... cereus EPP5, B. amyloliquefaciens EPP62, B. subtilis EPP65) showed strong inhibitory effects on phytopathogenic fungi (including Rhizoctonia solani, Sclerotium rolfsii, and Fusarium solani) and also improved pearl millet (Pennisetum glaucum) plant biomass and its shoot and root length. In another study, the synergistic action of consortium members B. cereus Rs-MS53 and Pseudomonas helmanticensis Sc-B94 resulted in an increased efficacy against the pathogenic fungi Rhizoctonia solani and Sclerotinia sclerotiorum [124]. The production of microbial volatile organic compounds (mVOCs) with combinations of these strains has been identified as the main mechanism of antagonism. ...
Plant growth-promoting bacteria (PGPB) appear to be a sensible competitor to conventional fertilization, including mineral fertilizers and chemical plant protection products. Undoubtedly, one of the most interesting bacteria exhibiting plant-stimulating traits is, more widely known as a pathogen, Bacillus cereus. To date, several environmentally safe strains of B. cereus have been isolated and described, including B. cereus WSE01, MEN8, YL6, SA1, ALT1, ERBP, GGBSTD1, AK1, AR156, C1L, and T4S. These strains have been studied under growth chamber, greenhouse, and field conditions and have shown many significant traits, including indole-3-acetic acid (IAA) and aminocyclopropane-1-carboxylic acid (ACC) deaminase production or phosphate solubilization, which allows direct plant growth promotion. It includes an increase in biometrics traits, chemical element content (e.g., N, P, and K), and biologically active substances content or activity, e.g., antioxidant enzymes and total soluble sugar. Hence, B. cereus has supported the growth of plant species such as soybean, maize, rice, and wheat. Importantly, some B. cereus strains can also promote plant growth under abiotic stresses, including drought, salinity, and heavy metal pollution. In addition, B. cereus strains produced extracellular enzymes and antibiotic lipopeptides or triggered induced systemic resistance, which allows indirect stimulation of plant growth. As far as biocontrol is concerned, these PGPB can suppress the development of agriculturally important phytopathogens, including bacterial phytopathogens (e.g., Pseudomonas syringae, Pectobacterium carotovorum, and Ralstonia solanacearum), fungal phytopathogens (e.g., Fusarium oxysporum, Botrytis cinerea, and Rhizoctonia solani), and other phytopathogenic organisms (e.g., Meloidogyne incognita (Nematoda) and Plasmodiophora brassicae (Protozoa)). In conclusion, it should be noted that there are still few studies on the effectiveness of B. cereus under field conditions, particularly, there is a lack of comprehensive analyses comparing the PGP effects of B. cereus and mineral fertilizers, which should be reduced in favor of decreasing the use of mineral fertilizers. It is also worth mentioning that there are still very few studies on the impact of B. cereus on the indigenous microbiota and its persistence after application to soil. Further studies would help to understand the interactions between B. cereus and indigenous microbiota, subsequently contributing to increasing its effectiveness in promoting plant growth.
... These "cry for help" cases also offer opportunities for screening biocontrol agents. Mülner et al. (2019) isolated Bacillus agents from the sclerotia of S. sclerotiorum and R. solani, which exhibited high antagonistic activity towards both pathogens by producing unidentified novel volatiles. ...
Species of the genus Bacillus have been widely used for the biocontrol of plant diseases in the demand for sustainable agricultural development. New mechanisms underlying Bacillus biocontrol activity have been revealed with the development of microbiome and microbe‐plant interaction research. In this review, we first briefly introduce the typical Bacillus biocontrol mechanisms, such as the production of antimicrobial compounds, competition for niches/nutrients, and induction of systemic resistance. Then, we discussed in detail the new mechanisms of pathogen quorum sensing interference and reshaping of the soil microbiota. The “cry for help” mechanism was also introduced, in which plants can release specific signals under pathogen attack to recruit biocontrol Bacillus for root colonization against invasion. Finally, two emerging strategies for enhancing the biocontrol efficacy of Bacillus agents, including the construction of synthetic microbial consortia and the application of rhizosphere‐derived prebiotics, were proposed.
i>Gilbertella persicaria is a fungus causing dragon fruit rot discovered recently and there is still no effective measure to control this fungus. Several studies demonstrated that microorganisms were applied for controlling G. persicaria on other crops. The purpose of this study was to evaluate the potential application of rhizobacteria in the in vitro control of G. persicaria . Eighty-nine bacterial isolates were collected from eleven rhizosphere soil samples. Four isolates, including LA2.9, LA3.2, LA4.5, and LA6.1, were screened based on inhibitory zone diameters and the ratio between the diameter of the inhibitory zone and the diameter of the bacterial growth zone. All of these four isolates were identified as belonging to the Bacillus genus and were compatible with each other. Random combinations of the selected strains could increase the in vitro growth inhibition of Gilbertella persicaria GTC2.3.1 in some cases. These results once again showed that the individual or multiple applications of the antagonistic bacterial strains was a promising approach to control G. persicaria . Therefore, it is necessary to isolate and collect more bacterial antagonists to develop targeted fungicidal formulations.
Root-knot nematodes (RKN) are one of the most harmful soil-borne plant pathogens in the world. Actinobacteria are known phytopathogen control agents. The aim of this study was to select soil actinobacteria with control potential against the RKN (Meloidogyne javanica) in tomato plants and to determine mechanisms of action. Ten isolates were tested and a significant reduction was observed in the number of M. javanica eggs, and galls 46 days after infestation with the nematode. The results could be explained by the combination of different mechanisms including parasitism and induction of plant defense response. The M. javanica eggs were parasited by all isolates tested. Some isolates reduced the penetration of juveniles into the roots. Other isolates using the split-root method were able to induce systemic defenses in tomato plants. The 4L isolate was selected for analysis of the expression of the plant defense genes TomLoxA, ACCO, PR1, and RBOH1. In plants treated with 4L isolate and M. javanica, there was a significant increase in the number of TomLoxA and ACCO gene transcripts. In plants treated only with M. javanica, only the expression of the RBOH1 and PR1 genes was induced in the first hours after infection. The isolates were identified using 16S rRNA gene sequencing as Streptomyces sp. (1A, 3F, 4L, 6O, 8S, 9T, and 10U), Kribbella sp. (5N), Kitasatospora sp. (2AE), and Lentzea sp. (7P). The efficacy of isolates from the Kitasatospora, Kribbella, and Lentzea genera was reported for the first time, and the efficacy of Streptomyces genus isolates for controlling M. javanica was confirmed. All the isolates tested in this study were efficient against RKN. This study provides the opportunity to investigate bacterial genera that have not yet been explored in the control of M. javanica in tomatoes and other crops.
By ecologically interacting with various biotic and abiotic agents acting in soil ecosystems, highly diverse soil microorganisms establish complex and stable assemblages and survive in a community context in natural settings. Besides facilitating soil microbiome to maintain great levels of population homeostasis, such microbial interactions drive soil microbes to function as the major engine of terrestrial biogeochemical cycling. It is verified that the regulative effect of microbe-microbe interplay plays an instrumental role in microbial-mediated promotion of soil health, including bioremediation of soil pollutants and biocontrol of soil-borne phytopathogens, which is considered an environmentally friendly strategy for ensuring the healthy condition of soils. Specifically, in microbial consortia, it has been proven that microorganism-microorganism interactions are involved in enhancing the soil health-promoting effectiveness (i.e., efficacies of pollution reduction and disease inhibition) of the beneficial microbes, here defined as soil health-promoting agents. These microbial interactions can positively regulate the soil health-enhancing effect by supporting those soil health-promoting agents utilized in combination, as multi-strain soil health-promoting agents, to overcome three main obstacles: inadequate soil colonization, insufficient soil contaminant eradication and inefficient soil-borne pathogen suppression, all of which can restrict their probiotic functionality. Yet the mechanisms underlying such beneficial interaction-related adjustments and how to efficiently assemble soil health-enhancing consortia with the guidance of microbe-microbe communications remain incompletely understood. In this review, we focus on bacterial and fungal soil health-promoting agents to summarize current research progress on the utilization of multi-strain soil health-promoting agents in the control of soil pollution and soil-borne plant diseases. We discuss potential microbial interaction-relevant mechanisms deployed by the probiotic microorganisms to upgrade their functions in managing soil health. We emphasize the interplay-related factors that should be taken into account when building soil health-promoting consortia, and propose a workflow for assembling them by employing a reductionist synthetic community approach.
The Magnaporthe oryzae Triticum (MoT) pathotype is the causal agent of wheat blast, which has caused significant economic losses and threatens wheat production in South America, Asia, and Africa. Three bacterial strains from rice and wheat seeds (B. subtilis BTS-3, B. velezensis BTS-4, and B. velezensis BTLK6A) were used to explore the antifungal effects of volatile organic compounds (VOCs)
of Bacillus spp. as a potential biocontrol mechanism against MoT. All bacterial treatments significantly inhibited both the mycelial growth and sporulation of MoT in vitro. We found that this inhibition was caused by Bacillus VOCs in a dose-dependent manner. In addition, biocontrol assays using detached wheat leaves infected with MoT showed reduced leaf lesions and sporulation compared
to the untreated control. VOCs from B. velezensis BTS-4 alone or a consortium (mixture of B. subtilis BTS-3, B. velezensis BTS-4, and B. velezensis BTLK6A) of treatments consistently suppressed MoT in vitro and in vivo. Compared to the untreated control, VOCs from BTS-4 and the Bacillus consortium reduced MoT lesions in vivo by 85% and 81.25%, respectively. A total of thirty-nine VOCs (from nine different VOC groups) from four Bacillus treatments were identified by gas chromatography–mass spectrometry (GC–MS), of which 11 were produced in all Bacillus treatments. Alcohols, fatty acids, ketones, aldehydes, and S-containing compounds were detected in all four bacterial treatments. In vitro assays using pure VOCs revealed that hexanoic acid, 2-methylbutanoic acid, and phenylethyl alcohol are potential VOCs emitted by Bacillus spp. that are suppressive for MoT. The minimum inhibitory concentrations for MoT sporulation were 250 mM for phenylethyl alcohol and 500 mM for 2-methylbutanoic acid and hexanoic acid. Therefore, our results indicate that VOCs from Bacillus spp. are effective compounds to suppress the growth and sporulation of MoT. Understanding the MoT sporulation reduction mechanisms exerted by Bacillus VOCs may provide novel options to manage the further spread of wheat blast by spores.
Magnaporthe oryzae Triticum (MoT) pathotype is the causal agent of wheat blast, which has caused significant economic losses and threatens wheat production in South America, Asia and Africa. Three bacterial strains from rice and wheat seeds (B. subtilis BTS-3, B. velezensis BTS-4, and B. velezensis BTLK6A) were used to explore the antifungal effects of volatile organic compounds (VOCs) of Bacillus spp. as a potential biocontrol mechanism against MoT. All bacterial treatments significantly inhibited both mycelial growth and sporulation of MoT in vitro. We found that this inhibition was caused by Bacillus VOCs in a dose-dependent manner. In addition, biocontrol assays using detached wheat leaves infected with MoT showed reduced leaf lesions and sporulation compared to the untreated control. VOCs from B. velezensis BTS-4 alone or a consortium (mixture of B. subtilis BTS-3, B. velezensis BTS-4, and B. velezensis BTLK6A) treatments consistently suppressed MoT in vitro and in vivo. Compared to the untreated control, VOCs from BTS-4 and Bacillus consortium reduced MoT lesions in vivo by 85% and 81.25%, respectively. A total of thirty-nine VOCs (from 9 different VOC groups) from 4 Bacillus treatments were identified by gas chromatography-mass spectrometry (GC-MS), of which 11 were produced in all Bacillus treatments. Alcohols, fatty acids, ketones, aldehydes, and S-containing compounds were detected in all four bacterial treatments. In vitro assays using pure VOCs revealed that hexanoic acid, 2-methylbutanoic acid, and phenylethyl alcohol are potential VOCs emitted by Bacillus spp. that are suppressive for MoT. The minimum inhibitory concentrations for MoT sporulation were 250 mM for phenylethyl alcohol and 500 mM for 2-methylbutanoic acid and hexanoic acid. Therefore, our results indicate that VOCs from Bacillus spp. are effective compounds to suppress the growth and sporulation of MoT. Understanding the MoT sporulation reduction mechanisms by Bacillus VOCs may provide novel options to manage the further spread of wheat blast by spores.
Despite the numerous benefits plants receive from probiotics, maintaining consistent results across applications is still a challenge. Cultivation-independent methods associated with reduced sequencing costs have considerably improved the overall understanding of microbial ecology in the plant environment. As a result, now it is possible to engineer a consortium of microbes aiming for improved plant health. Such synthetic microbial communities (SynComs) contain carefully chosen microbial species to produce the desired microbiome function. Microbial biofilm formation, production of secondary metabolites and ability to induce plant resistance are some of the microbial traits to take into consideration when designing SynComs. Plant-associated microbial communities are not assembled randomly. Ecological theories suggest that these communities have a defined phylogenetic organization structured by general community assembly rules. Using machine learning, we can study these rules and target microbial functions that generate desired plant phenotypes. Well-structured assemblages are more likely to lead to a stable SynCom that thrives under environmental stressors, as compared to the classical selection of single microbial activities or taxonomy. However, ensuring microbial colonization and long-term plant phenotype stability are still some of the challenges to overcome with SynComs, as the synthetic community may change over time with microbial horizontal gene transfer and retained mutations. Here, we explored the advances made in SynCom research regarding plant health focusing on bacteria, as they are the most dominant microbial form compared with other members of the microbiome and the most commonly found in SynCom studies.
Metabolic capabilities of microorganisms include the production of secondary metabolites (e.g. antibiotics). The analysis of microbial volatile organic compounds (mVOCs) is an emerging research field with huge impact on medical, agricultural and biotechnical applied and basic science. The mVOC database (v1) has grown with microbiome research and integrated species information with data on emitted volatiles. Here, we present the mVOC 2.0 database with about 2000 compounds from almost 1000 species and new features to work with the database. The extended collection of compounds was augmented with data regarding mVOC-mediated effects on plants, fungi, bacteria and (in-)vertebrates. The mVOC database 2.0 now features a mass spectrum finder, which allows a quick mass spectrum comparison for compound identification and the generation of species-specific VOC signatures. Automatic updates, useful links and search for mVOC literature are also included. The mVOC database aggregates and refines available information regarding microbial volatiles, with the ultimate aim to provide a comprehensive and informative platform for scientists working in this research field. To address this need, we maintain a publicly available mVOC database at: http://bioinformatics.charite.de/mvoc.
Recent studies indicated that the production of secondary metabolites by soil bacteria can be triggered by interspecific interactions. However, little is known to date about interspecific interactions between Gram-positive and Gram-negative bacteria. In this study, we aimed to understand how the interspecific interaction between the Gram-positive Paenibacillus sp. AD87 and the Gram-negative Burkholderia sp. AD24 affects the fitness, gene expression and the production of soluble and volatile secondary metabolites of both bacteria. To obtain better insight into this interaction, transcriptome and metabolome analyses were performed. Our results revealed that the interaction between the two bacteria affected their fitness, gene expression and the production of secondary metabolites. During interaction, the growth of Paenibacillus was not affected, whereas the growth of Burkholderia was inhibited at 48 and 72 h. Transcriptome analysis revealed that the interaction between Burkholderia and Paenibacillus caused significant transcriptional changes in both bacteria as compared to the monocultures. The metabolomic analysis revealed that the interaction increased the production of specific volatile and soluble antimicrobial compounds such as 2,5-bis(1-methylethyl)-pyrazine and an unknown Pederin-like compound. The pyrazine volatile compound produced by Paenibacillus was subjected to bioassays and showed strong inhibitory activity against Burkholderia and a range of plant and human pathogens. Moreover, strong additive antimicrobial effects were observed when soluble extracts from the interacting bacteria were combined with the pure 2,5-bis(1-methylethyl)-pyrazine. The results obtained in this study highlight the importance to explore bacterial interspecific interactions to discover novel secondary metabolites and to perform simultaneously metabolomics of both, soluble and volatile compounds.
Remote effects (occurring without physical contact) of two plant growth-promoting bacteria (PGPB)
Azospirillum brasilense Cd and Bacilus pumilus ES4 on growth of the green microalga Chlorella
sorokiniana UTEX 2714 were studied. The two PGPB remotely enhanced the growth of the microalga,
up to six-fold, and its cell volume by about three-fold. In addition to phenotypic changes, both bacteria
remotely induced increases in the amounts of total lipids, total carbohydrates, and chlorophyll a in the
cells of the microalga, indicating an alteration of the microalga’s physiology. The two bacteria produced
large amounts of volatile compounds, including CO2, and the known plant growth-promoting volatile
2,3-butanediol and acetoin. Several other volatiles having biological functions in other organisms, as
well as numerous volatile compounds with undefined biological roles, were detected. Together, these
bacteria-derived volatiles can positively affect growth and metabolic parameters in green microalgae
without physical attachment of the bacteria to the microalgae. This is a new paradigm on how
PGPB promote growth of microalgae which may serve to improve performance of Chlorella spp. for
biotechnological applications.
The ubiquitous soilborne and plant-pathogenic basidiomycete Rhizoctonia solani, although classified as a single species, is a complex of anastomosis groups (AGs) that cause disease in a broad range of higher plants. Here, we investigated the persistent co-isolation of bacteria with R. solani from brown patch-infected, cool-season turfgrasses, and report the presence of endo-hyphal bacteria, related to members in the genus Enterobacter, in an isolate of R. solani AG 2-2IIIB. The intracellular localization of the bacteria was corroborated by fluorescence, confocal and electron microscopy, and DNA analysis. Furthermore, the Enterobacter sp., which is rod-shaped in the free-living form, exists as an L-form (spheroid) within the fungus, a phenomenon not previously reported in endosymbionts. Our findings also indicate that the bacterium is required for full virulence of R. solani on creeping bentgrass and production of wild type levels of the toxin phenylacetic acid in fungal cultures. The possible presence of ...
Certain bacterial species produce antimicrobial compounds only in the presence of a competing species. However little is known on the frequency of interaction-mediated induction of antibiotic compound production in natural communities of soil bacteria. Here we developed a high-throughput method to screen for the production of antimicrobial activity by monocultures and pair-wise combinations of 146 phylogenetically different bacteria isolated from similar soil habitats. Growth responses of two human pathogenic model organisms, Escherichia coli WA321 and Staphylococcus aureus 533R4, were used to monitor antimicrobial activity. From all isolates, 33% showed antimicrobial activity only in monoculture and 42% showed activity only when tested in interactions. More bacterial isolates were active against S. aureus than against E. coli. The frequency of interaction-mediated induction of antimicrobial activity was 6% (154 interactions out of 2798) indicating that only a limited set of species combinations showed such activity. The screening revealed also interaction-mediated suppression of antimicrobial activity for 22% of all combinations tested. Whereas all patterns of antimicrobial activity (non-induced production, induced production and suppression) were seen for various bacterial classes, interaction-mediated induction of antimicrobial activity was more frequent for combinations of Flavobacteria and alpha- Proteobacteria. The results of our study give a first indication on the frequency of interference competitive interactions in natural soil bacterial communities which may forms a basis for selection of bacterial groups that are promising for the discovery of novel, cryptic antibiotics.
There is increasing evidence that volatile organic compounds (VOCs) play an important role in the interactions between fungi and bacteria, two major groups of soil inhabiting microorganisms. Yet, most of the research has been focused on effects of bacterial volatiles on suppression of plant pathogenic fungi whereas little is known about the responses of bacteria to fungal volatiles. In the current study we performed a metabolomics analysis of volatiles emitted by several fungal and oomycetal soil strains under different nutrient conditions and growth stages. The metabolomics analysis of the tested fungal and oomycetal strains revealed different volatile profiles dependent on the age of the strains and nutrient conditions. Furthermore, we screened the phenotypic responses of soil bacterial strains to volatiles emitted by fungi. Two bacteria, Collimonas pratensis Ter291 and Serratia plymuthica PRI-2C, showed significant changes in their motility, in particular to volatiles emitted by Fusarium culmorum. This fungus produced a unique volatile blend, including several terpenes. Four of these terpenes were selected for further tests to investigate if they influence bacterial motility. Indeed, these terpenes induced or reduced swimming and swarming motility of S. plymuthica PRI-2C and swarming motility of C. pratensis Ter291, partly in a concentration-dependent manner. Overall the results of this work revealed that bacteria are able to sense and respond to fungal volatiles giving further evidence to the suggested importance of volatiles as signaling molecules in fungal–bacterial interactions.
Plants release a wide set of secondary metabolites including volatile organic compounds (VOCs). Many of those compounds are considered to function as defense against herbivory, pests, and pathogens. However, little knowledge exists about the role of belowground plant VOCs for attracting beneficial soil microorganisms. We developed an olfactometer system to test the attraction of soil bacteria by VOCs emitted by Carex arenaria roots. Moreover, we tested whether infection of C. arenaria with the fungal pathogen Fusarium culmorum modifies the VOCs profile and bacterial attraction. The results revealed that migration of distant bacteria in soil towards roots can be stimulated by plant VOCs. Upon fungal infection, the blend of root VOCs changed and specific bacteria with antifungal properties were attracted. Tests with various pure VOCs indicated that those compounds can diffuse over long distance but with different diffusion abilities. Overall, this work highlights the importance of plant VOCs in belowground long-distance plant-microbe interactions.
Volatile organic compounds (VOCs) from microbial origin are relevant in biological interactions and are considered promising environmentally safer fumigant agents to control postharvest diseases of fruits. The antagonist yeast Saccharomyces cerevisiae produces VOCs able to inhibit the development of plant pathogens, including the filamentous fungus Phyllosticta citricarpa, causal agent of citrus black spot. Thus, it was evaluated the effectiveness of VOCs produced by S. cerevisiae to control P. citricarpa in orange fruits stored in sealed glass containers. The exposure of P. citricarpa growing on potato-dextrose-agar to the synthetic mixture of VOCs, originally identified from S. cerevisiae, affected negatively the phytopathogen. Individually, 3-methyl-1-butanol and 2-methyl-1-butanol were the most effective VOCs inhibiting completely the mycelial growth and the germination and appressorium formation by conidia. Seven-day fumigation of orange fruits carrying quiescent infections of P. citricarpa, employing the VOC-producing S. cerevisiae or 3-methyl-1-butanol at 0.33 μl ml⁻¹ of air space, controlled the development of new lesions close to 90%, even after removing the fruits from the VOC influence. Therefore, the biological fumigation of citrus fruits with S. cerevisiae or the use of formulations based on their VOCs are promising ecofriendly approaches to control citrus black spot during storage and shipment.
Biotic interactions through volatile organic compounds (VOC) are frequent in nature. This investigation aimed to study the role ofBacillusVOC for the beneficial effects on plants observed as improved growth and pathogen control. FourBacillus amyloliquefacienssubsp.plantarumstrains were screened for VOC effects onArabidopsis thalianaCol-0 seedlings andBrassicafungal phytopathogens. VOC from all fourBacillusstrains could promote growth ofArabidopsisplants resulting in increased shoot biomass but the effects were dependent on the growth medium. Dose response studies with UCMB5113 on MS agar with or without root exudates showed significant plant growth promotion even at low levels of bacteria.BacillusVOC antagonized growth of several fungal pathogensin vitro However, the plant growth promotion efficacy and fungal inhibition potency varied among theBacillusstrains. VOC inhibition of several phytopathogens indicated efficient microbial antagonism supporting high rhizosphere competence of theBacillusstrains. GC-MS analysis identified several VOC structures where the profiles differed depending on the growth medium. The ability ofBacillusstrains to produce both volatile and soluble compounds for plant growth promotion and disease biocontrol provides examples of rhizosphere microbes as an important ecosystem service with high potential to support sustainable crop production.