Bacillus megaterium strain XTBG34 promotes plant growth by producing 2-pentylfuran

Ecotoxicological assessment of biomass-derived furan platform chemicals using aquatic and terrestrial bioassays

An environmental toxicological assessment of fourteen furanic compounds serving as valuable building blocks produced from biomass was performed. The molecules selected included well studied compounds serving as control examples to compare the toxicity exerted against a variety of highly novel furans which have been additionally targeted as potential or current alternatives to biofuels, building blocks and polymer monomers. The impact of the furan platform chemicals targeted on widely applied ecotoxicity model organisms was determined employing the marine bioluminescent bacterium Aliivibrio fischeri and the freshwater green microalgae Raphidocelis subcapitata, while their ecotoxicity effects on plants were assessed using dicotyledonous plants Sinapis alba and Lepidium sativum. Regarding the specific endpoints evaluated, the furans tested were slightly toxic or practically nontoxic for A. fischeri following 5 and 15 min of exposure. Moreover, most of the building blocks did not affect the growth of L. sativum and S. alba at 150 mg L-1 for 72 h of exposure. Specifically, 9 and 11 out of the 14 furan platform chemicals tested were non-effective or stimulant for L. sativum and S. alba respectively. Given that furans comprise common inhibitors in biorefinery fermentations, the growth inhibition of the specific building blocks was studied using the industrial workhorse yeast Saccharomyces cerevisiae, demonstrating insignificant inhibition on eukaryotic cell growth following 6, 12 and 16 h of exposure at a concentration of 500 mg L-1. The study provides baseline information to unravel the ecotoxic effects and to confirm the green aspects of a range of versatile biobased platform molecules.

Important soil microbiota's effects on plants and soils: a comprehensive 30-year systematic literature review

Clarifying the relationship between soil microorganisms and the plant-soil system is crucial for encouraging the sustainable development of ecosystems, as soil microorganisms serve a variety of functional roles in the plant-soil system. In this work, the influence mechanisms of significant soil microbial groups on the plant-soil system and their applications in environmental remediation over the previous 30 years were reviewed using a systematic literature review (SLR) methodology. The findings demonstrated that: (1) There has been a general upward trend in the number of publications on significant microorganisms, including bacteria, fungi, and archaea. (2) Bacteria and fungi influence soil development and plant growth through organic matter decomposition, nitrogen, phosphorus, and potassium element dissolution, symbiotic relationships, plant growth hormone production, pathogen inhibition, and plant resistance induction. Archaea aid in the growth of plants by breaking down low-molecular-weight organic matter, participating in element cycles, producing plant growth hormones, and suppressing infections. (3) Microorganism principles are utilized in soil remediation, biofertilizer production, denitrification, and phosphorus removal, effectively reducing environmental pollution, preventing soil pathogen invasion, protecting vegetation health, and promoting plant growth. The three important microbial groups collectively regulate the plant-soil ecosystem and help maintain its relative stability. This work systematically summarizes the principles of important microbial groups influence plant-soil systems, providing a theoretical reference for how to control soil microbes in order to restore damaged ecosystems and enhance ecosystem resilience in the future.

The Complex GH32 Enzyme Orchestra from Priestia megaterium Holds the Key to Better Discriminate Sucrose-6-phosphate Hydrolases from Other β-Fructofuranosidases in Bacteria

Inulin is widely used as a prebiotic and emerging as a priming compound to counteract plant diseases. We isolated inulin-degrading strains from the lettuce phyllosphere, identified as Bacillus subtilis and Priestia megaterium, species hosting well-known biocontrol organisms. To better understand their varying inulin degradation strategies, three intracellular β-fructofuranosidases from P. megaterium NBRC15308 were characterized after expression in Escherichia coli: a predicted sucrose-6-phosphate (Suc6P) hydrolase (SacAP1, supported by molecular docking), an exofructanase (SacAP2), and an invertase (SacAP3). Based on protein multiple sequence and structure alignments of bacterial glycoside hydrolase family 32 enzymes, we identified conserved residues predicted to be involved in binding phosphorylated (Suc6P hydrolases) or nonphosphorylated substrates (invertases and fructanases). Suc6P hydrolases feature positively charged residues near the structural catalytic pocket (histidine, arginine, or lysine), whereas other β-fructofuranosidases contain tryptophans. This correlates with our phylogenetic tree, grouping all predicted Suc6P hydrolases in a clan associated with genomic regions coding for transporters involved in substrate phosphorylation. These results will help to discriminate between Suc6P hydrolases and other β-fructofuranosidases in future studies and to better understand the interaction of B. subtilis and P. megaterium endophytes with sucrose and/or fructans, sugars naturally present in plants or exogenously applied in the context of defense priming.

Advances in microbial analysis: based on volatile organic compounds of microorganisms in food

In recent years, microbial volatile organic compounds (mVOCs) produced by microbial metabolism have attracted more and more attention because they can be used to detect food early contamination and flaws. So far, many analytical methods have been reported for the determination of mVOCs in food, but few integrated review articles discussing these methods are published. Consequently, mVOCs as indicators of food microbiological contamination and their generation mechanism including carbohydrate, amino acid, and fatty acid metabolism are introduced. Meanwhile, a detailed summary of the mVOCs sampling methods such as headspace, purge trap, solid phase microextraction, and needle trap is presented, and a systematic and critical review of the analytical methods (ion mobility spectrometry, electronic nose, biosensor, and so on) of mVOCs and their application in the detection of food microbial contamination is highlighted. Finally, the future concepts that can help improve the detection of food mVOCs are prospected.

Variation with In Vitro Analysis of Volatile Profiles among Aspergillus flavus Strains from Louisiana

Volatile organic compounds (VOCs) produced by A. flavus strains were first captured and identified to discern between non-aflatoxigenic and toxigenic phenotypes, and more recently to help with detecting fungal infection, but not with the goal of using VOCs produced by non-aflatoxigenic strains to inhibit growth and/or production of one or more mycotoxins (e.g., aflatoxin and cyclopiazonic acid) by toxigenic aspergilli. In this study, four Aspergillus strains from Louisiana (one non-aflatoxigenic and three toxigenic) were grown on various substrates and had their headspaces captured and analyzed by solid-phase microextraction/gas chromatography/mass spectroscopy (SPME/GC/MS), to find biocontrol and biomarker compounds. Here, we present a collection of nearly 100 fungus-related VOCs, many of which were substrate dependent. Thirty-one were produced across multiple replicates and the rest were observed in a single replicate. At least three VOCs unique to non-aflatoxigenic strain LA1 can be tested for biocontrol properties (e.g., euparone, 4-nonyne), and at least four VOCs unique to toxigenic strains LA2-LA4 can be explored as biomarkers (e.g., 2-heptanone, glycocyamidine) to detect their presence while infecting crops in the field or in storage.

Bacterial volatile organic compounds (VOCs) promote growth and induce metabolic changes in rice

Plant growth-promoting bacteria (PGPB) represent an eco-friendly alternative to reduce the use of chemical products while increasing the productivity of economically important crops. The emission of small gaseous signaling molecules from PGPB named volatile organic compounds (VOCs) has emerged as a promising biotechnological tool to promote biomass accumulation in model plants (especially Arabidopsis thaliana) and a few crops, such as tomato, lettuce, and cucumber. Rice (Oryza sativa) is the most essential food crop for more than half of the world's population. However, the use of VOCs to improve this crop performance has not yet been investigated. Here, we evaluated the composition and effects of bacterial VOCs on the growth and metabolism of rice. First, we selected bacterial isolates (IAT P4F9 and E.1b) that increased rice dry shoot biomass by up to 83% in co-cultivation assays performed with different durations of time (7 and 12 days). Metabolic profiles of the plants co-cultivated with these isolates and controls (without bacteria and non-promoter bacteria-1003-S-C1) were investigated via ¹H nuclear magnetic resonance. The analysis identified metabolites (e.g., amino acids, sugars, and others) with differential abundance between treatments that might play a role in metabolic pathways, such as protein synthesis, signaling, photosynthesis, energy metabolism, and nitrogen assimilation, involved in rice growth promotion. Interestingly, VOCs from IAT P4F9 displayed a more consistent promotion activity and were also able to increase rice dry shoot biomass in vivo. Molecular identification by sequencing the 16S rRNA gene of the isolates IAT P4F9 and E.1b showed a higher identity with Serratia and Achromobacter species, respectively. Lastly, volatilomes of these and two other non-promoter bacteria (1003-S-C1 and Escherichia coli DH5α) were evaluated through headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry. Compounds belonging to different chemical classes, such as benzenoids, ketones, alcohols, sulfide, alkanes, and pyrazines, were identified. One of these VOCs, nonan-2-one, was validated in vitro as a bioactive compound capable of promoting rice growth. Although further analyses are necessary to properly elucidate the molecular mechanisms, our results suggest that these two bacterial isolates are potential candidates as sources for bioproducts, contributing to a more sustainable agriculture.

Microbial Volatile Organic Compounds: An Alternative for Chemical Fertilizers in Sustainable Agriculture Development

Microorganisms are exceptional at producing several volatile substances called microbial volatile organic compounds (mVOCs). The mVOCs allow the microorganism to communicate with other organisms via both inter and intracellular signaling pathways. Recent investigation has revealed that mVOCs are chemically very diverse and play vital roles in plant interactions and microbial communication. The mVOCs can also modify the plant's physiological and hormonal pathways to augment plant growth and production. Moreover, mVOCs have been affirmed for effective alleviation of stresses, and also act as an elicitor of plant immunity. Thus, mVOCs act as an effective alternative to various chemical fertilizers and pesticides. The present review summarizes the recent findings about mVOCs and their roles in inter and intra-kingdoms interactions. Prospects for improving soil fertility, food safety, and security are affirmed for mVOCs application for sustainable agriculture.

Effect of Plant Preservative MixtureTM on Endophytic Bacteria Eradication from In Vitro-Grown Apple Shoots

Endophytic contaminants are a common problem for the in vitro propagation of woody plants and have significant economic repercussions for the conservation of plant genetic resources and commercial micropropagation. In this study, first, the microbial contamination that appeared around the base of in vitro-grown apple shoots was identified as Bacillus megaterium. Then, plant preservative mixture (PPM^TM) was used as a bactericidal agent in plant tissue culture. Its efficacy for eradicating endophytic B. megaterium in in vitro cultures of apple was tested. In vitro-contaminated shoots were grown in tissue culture medium supplemented with 0.2% v/v PPM^TM for 12 weeks and then transferred to medium without any PPM^TM and cultured for 24 weeks. This study showed that PPM^TM is an effective agent for controlling the growth of B. megaterium. Our results highlight the species-specific response of apple shoots to PPM^TM. PPM^TM was effective in controlling endogenous microbial contaminations from apple varieties 'Golden Delicious', 'Landsberger Renette', 'Suislepper', and 'Aport krovavo-krasnyi'; meanwhile, in 'KG 7' and 'Gold Rush', all the plants grown in the absence of PPM^TM were still bacterially contaminated, even though they were pre-treated for 12 weeks in PPM^TM-supplemented medium. These results therefore suggest the essentiality of further testing of extended incubation of PPM^TM in these cultivars that had outbreaks of bacterial contamination.

Plant Growth-Promoting Rhizobacteria Eliminate the Effect of Drought Stress in Plants: A Review

Plants evolve diverse mechanisms to eliminate the drastic effect of biotic and abiotic stresses. Drought is the most hazardous abiotic stress causing huge losses to crop yield worldwide. Osmotic stress decreases relative water and chlorophyll content and increases the accumulation of osmolytes, epicuticular wax content, antioxidant enzymatic activities, reactive oxygen species, secondary metabolites, membrane lipid peroxidation, and abscisic acid. Plant growth-promoting rhizobacteria (PGPR) eliminate the effect of drought stress by altering root morphology, regulating the stress-responsive genes, producing phytohormones, osmolytes, siderophores, volatile organic compounds, and exopolysaccharides, and improving the 1-aminocyclopropane-1-carboxylate deaminase activities. The use of PGPR is an alternative approach to traditional breeding and biotechnology for enhancing crop productivity. Hence, that can promote drought tolerance in important agricultural crops and could be used to minimize crop losses under limited water conditions. This review deals with recent progress on the use of PGPR to eliminate the harmful effects of drought stress in traditional agriculture crops.

Plant growth-promoting and non-promoting rhizobacteria from avocado trees differentially emit volatiles that influence growth of Arabidopsis thaliana

Microbial volatile organic compounds (mVOCs) play important roles in inter- and intra-kingdom interactions, and they are also important as signal molecules in physiological processes acting either as plant growth-promoting or negatively modulating plant development. We investigated the effects of mVOCs emitted by PGPR vs non-PGPR from avocado trees (Persea americana) on growth of Arabidopsis thaliana seedlings. Chemical diversity of mVOCs was determined by SPME-GC-MS; selected compounds were screened in dose-response experiments in A. thaliana transgenic lines. We found that plant growth parameters were affected depending on inoculum concentration. Twenty-six compounds were identified in PGPR and non-PGPR with eight of them not previously reported. The VOCs signatures were differential between those groups. 4-methyl-2-pentanone, 1-nonanol, 2-phenyl-2-propanol and ethyl isovalerate modified primary root architecture influencing the expression of auxin- and JA-responsive genes, and cell division. Lateral root formation was regulated by 4-methyl-2-pentanone, 3-methyl-1-butanol, 1-nonanol and ethyl isovalerate suggesting a participation via JA signalling. Our study revealed the differential emission of volatiles by PGPR vs non-PGPR from avocado trees and provides a general view about the mechanisms by which those volatiles influence plant growth and development. Rhizobacteria strains and mVOCs here reported are promising for improvement the growth and productivity of avocado crop.

Mitigation of CaCO3 Influence on Ipomoea batatas Plants Using Bacillus megaterium DSM 2894

The application of PGPB is considered a surrogate approach to reducing the amounts of phosphorus fertilizers applied in addition to its role in improving nutrient availability under stress conditions. The objective of this study was to evaluate five levels of calcium superphosphate (CSP); ultimately, CSP was applied in five levels: CSP20, CSP40, CSP60, CSP80, and CSP100 were applied at 69, 138, 207, 276, and 345 kg ha−1, respectively, and two treatments of Bacillus megaterium DSM 2894 (with and without) were applied on sweet potato (Beauregard cv.) plants grown in calcareous soils in the 2019 and 2020 seasons in Egypt. Some macro- and micronutrient (i.e., nitrogen (N), phosphorus (P), calcium (Ca), iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu)) uptake, antiradical power (ARP), and protein and total root yields (TRYs) were determined. The plants inoculated with B. megaterium DSM 2894 had increased leaf N, P, and Mn contents in both seasons; in addition, Ca was increased in the second season. Furthermore, all of the root nutrient contents (except N) as well as the ARP and TRY were increased in both seasons as compared with those of the noninoculated plants. On the other hand, the maximum values of the leaf Ca, Fe, and Cu contents and the root Ca, Fe, and Zn contents were recorded with the CSP20 treatment in both seasons. CSP60 was the superior treatment for N (in the leaves), Mn (in the roots), ARP, protein contents, and TRY in both seasons and for the leaf Zn content in the 2019 season. The application of the CSP100 treatment gave the highest values for the leaf and root P contents and the root Cu contents in both seasons as well as for the leaf Mn content in the first season and the root N content in the 2020 growth season. Thus, it was concluded that the application of CSP20, CSP60, and CSP100 treatments with the B. megaterium DSM2894 mixture gave the best values compared to the use of CSP or DSM2894 individually to attenuate CaCO3-induced damage.

Identification of Volatile Organic Compounds Produced by Xenorhabdus indica Strain AB and Investigation of Their Antifungal Activities

Xenorhabdus spp. are symbiotic bacteria associated with entomopathogenic nematodes to form a model complex that is used for the biological control of insect pests. These bacteria also produce secondary metabolites that have commercial potential in the pharmaceutical and agroforestry industries. Volatile organic compounds (VOCs) produced by the Xenorhabdus indica "strain AB" have been shown to have significant antifungal activity against Fusarium oxysporum f. sp. cucumerinum. Using gas chromatography-mass spectrometry, we identified 61 volatiles in the mixture of VOCs emitted by strain AB compared to a control strain, 6 of which were investigated for their antifungal activities. Of these, methyl anthranilate exhibited the highest mycelial growth suppression toward F. oxysporum, with a minimum inhibitory volume (MIV) of 50 μL/plate. Fluorescence assays, scanning electron microscopy, and measurements of the leakage of intracellular components revealed that the use of methyl anthranilate changed cell wall and cell membrane integrity as well as the permeability of the plasma membrane. Furthermore, methyl anthranilate treatment upregulated the transcription level of target genes related to redox reactions and the cell wall integrity pathway. The results suggest a novel mechanism used by Xenorhabdus spp. to overcome competitors during its life cycle and open up a new approach to using these bacteria in biological control. IMPORTANCE Fungal phytopathogens, particularly Fusarium oxysporum, are a major problem worldwide, especially in the postharvest of vital economic crops. Concerns about negative effects on the environment and human health have led to increasing restrictions on the use of chemical fungicides, and therefore, biological control agents are now being considered alternatives. It is in this context that we investigated the antifungal activity of VOCs produced by X. indica strain AB against F. oxysporum. We found that AB VOCs have a strong effect on the growth of the fungal phytopathogen. In addition, 85% of the identified volatile compounds were determined to be new compounds, opening up new lines of research to discover their properties, effects, and potential for pharmaceutical and agricultural applications. Antifungal assays proved that four of the six compounds with a high concentration in the GC-MS profile had a significant inhibitory effect on pathogen growth. Accordingly, this study opens up a new approach for the use of these bacteria in biocontrol.

Pangenome analyses of Bacillus pumilus, Bacillus safensis, and Priestia megaterium exploring the plant-associated features of bacilli strains isolated from canola

Previous genome mining of the strains Bacillus pumilus 7PB, Bacillus safensis 1TAz, 8Taz, and 32PB, and Priestia megaterium 16PB isolated from canola revealed differences in the profile of antimicrobial biosynthetic genes when compared to the species type strains. To evaluate not only the similarities among B. pumilus, B. safensis, and P. megaterium genomes but also the specificities found in the canola bacilli, we performed comparative genomic analyses through the pangenome evaluation of each species. Besides that, other genome features were explored, especially focusing on plant-associated and biotechnological characteristics. The combination of the genome metrics Average Nucleotide Identity and digital DNA-DNA hybridization formulas 1 and 3 adopting the universal thresholds of 95 and 70%, respectively, was suitable to verify the identification of strains from these groups. On average, core genes corresponded to 45%, 52%, and 34% of B. pumilus, B. safensis, and P. megaterium open pangenomes, respectively. Many genes related to adaptations to plant-associated lifestyles were predicted, especially in the Bacillus genomes. These included genes for acetoin production, polyamines utilization, root exudate chemoreceptors, biofilm formation, and plant cell-wall degrading enzymes. Overall, we could observe that strains of these species exhibit many features in common, whereas most of their variable genome portions have features yet to be uncovered. The observed antifungal activity of canola bacilli might be a result of the synergistic action of secondary metabolites, siderophores, and chitinases. Genome analysis confirmed that these species and strains have biotechnological potential to be used both as agricultural inoculants or hydrolases producers. Up to our knowledge, this is the first work that evaluates the pangenome features of P. megaterium.

Biocontrol properties from phyllospheric bacteria isolated from Solanum lycopersicum and Lactuca sativa and genome mining of antimicrobial gene clusters

BACKGROUND

Biocontrol agents are sustainable eco-friendly alternatives for chemical pesticides that cause adverse effects in the environment and toxicity in animals including humans. An improved understanding of the phyllosphere microbiology is of vital importance for biocontrol development. Most studies have been directed towards beneficial plant-microbe interactions and ignore the pathogens that might affect humans when consuming vegetables. In this study we extended this perspective and investigated potential biocontrol strains isolated from tomato and lettuce phyllosphere that can promote plant growth and potentially antagonize human pathogens as well as plant pathogens. Subsequently, we mined into their genomes for discovery of antimicrobial biosynthetic gene clusters (BGCs), that will be further characterized.

RESULTS

The antimicrobial activity of 69 newly isolated strains from a healthy tomato and lettuce phyllosphere against several plant and human pathogens was screened. Three strains with the highest antimicrobial activity were selected and characterized (Bacillus subtilis STRP31, Bacillus velezensis SPL51, and Paenibacillus sp. PL91). All three strains showed a plant growth promotion effect on tomato and lettuce. In addition, genome mining of the selected isolates showed the presence of a large variety of biosynthetic gene clusters. A total of 35 BGCs were identified, of which several are already known, but also some putative novel ones were identified. Further analysis revealed that among the novel BGCs, one previously unidentified NRPS and two bacteriocins are encoded, the gene clusters of which were analyzed in more depth.

CONCLUSIONS

Three recently isolated strains of the Bacillus genus were identified that have high antagonistic activity against lettuce and tomato plant pathogens. Known and unknown antimicrobial BGCs were identified in these antagonistic bacterial isolates, indicating their potential to be used as biocontrol agents. Our study serves as a strong incentive for subsequent purification and characterization of novel antimicrobial compounds that are important for biocontrol.

Unraveling Microbial Volatile Elicitors Using a Transparent Methodology for Induction of Systemic Resistance and Regulation of Antioxidant Genes at Expression Levels in Chili against Bacterial Wilt Disease

Microbial volatiles benefit the agricultural ecological system by promoting plant growth and systemic resistance against diseases without harming the environment. To explore the plant growth-promoting efficiency of VOCs produced by Pseudomonas fluorescens PDS1 and Bacillus subtilis KA9 in terms of chili plant growth and its biocontrol efficiency against Ralstonia solanacearum, experiments were conducted both in vitro and in vivo. A closure assembly was designed using a half-inverted plastic bottle to demonstrate plant–microbial interactions via volatile compounds. The most common volatile organic compounds were identified and reported; they promoted plant development and induced systemic resistance (ISR) against wilt pathogen R. solanacearum. The PDS1 and KA9 VOCs significantly increased defensive enzyme activity and overexpressed the antioxidant genes PAL, POD, SOD, WRKYa, PAL1, DEF-1, CAT-2, WRKY40, HSFC1, LOX2, and NPR1 related to plant defense. The overall gene expression was greater in root tissue as compared to leaf tissue in chili plant. Our findings shed light on the relationship among rhizobacteria, pathogen, and host plants, resulting in plant growth promotion, disease suppression, systemic resistance-inducing potential, and antioxidant response with related gene expression in the leaf and root tissue of chili.

Plant Growth-Promoting Rhizobacteria as a Green Alternative for Sustainable Agriculture

Environmental stress is a major challenge for sustainable food production as it reduces yield by generating reactive oxygen species (ROS) which pose a threat to cell organelles and biomolecules such as proteins, DNA, enzymes, and others, leading to apoptosis. Plant growth-promoting rhizobacteria (PGPR) offers an eco-friendly and green alternative to synthetic agrochemicals and conventional agricultural practices in accomplishing sustainable agriculture by boosting growth and stress tolerance in plants. PGPR inhabit the rhizosphere of soil and exhibit positive interaction with plant roots. These organisms render multifaceted benefits to plants by several mechanisms such as the release of phytohormones, nitrogen fixation, solubilization of mineral phosphates, siderophore production for iron sequestration, protection against various pathogens, and stress. PGPR has the potential to curb the adverse effects of various stresses such as salinity, drought, heavy metals, floods, and other stresses on plants by inducing the production of antioxidant enzymes such as catalase, peroxidase, and superoxide dismutase. Genetically engineered PGPR strains play significant roles to alleviate the abiotic stress to improve crop productivity. Thus, the present review will focus on the impact of PGPR on stress resistance, plant growth promotion, and induction of antioxidant systems in plants.

The hidden effects of agrochemicals on plant metabolism and root-associated microorganisms

Agrochemicals are commonly used in agriculture to protect crops and ensure yields. Several of them are mobile within the plant and, being perceived as xenobiotics regardless of their protective/curative roles, they induce a reprogramming of secondary metabolism linked to the detoxification processes even in the absence of phenotype symptoms. Moreover, it is well documented that plants are able to shape the microbial population at the rhizosphere and to significantly affect the processes occurring therein thanks to the root exudation of different metabolites. Here we show that plant metabolic response to foliarly-applied pesticides is much broader than what previously thought and includes diverse and compound-specific hidden processes. Among others, stress-related metabolism and phytohormones profile underwent a considerable reorganization. Moreover, a distinctive microbial rearrangement of the rhizosphere was recorded following foliar application of pesticides. Such effects have unavoidably energetic and metabolic costs for the plant paving the way to both positive and negative aspects. The understanding of these effects is crucial for an increasingly sustainable use of pesticides in agriculture.

The "beauty in the beast"-the multiple uses of Priestia megaterium in biotechnology

Abstract

Over 30 years, the Gram-positive bacterium Priestia megaterium (previously known as Bacillus megaterium) was systematically developed for biotechnological applications ranging from the production of small molecules like vitamin B₁₂, over polymers like polyhydroxybutyrate (PHB) up to the in vivo and in vitro synthesis of multiple proteins and finally whole-cell applications. Here we describe the use of the natural vitamin B₁₂ (cobalamin) producer P. megaterium for the elucidation of the biosynthetic pathway and the subsequent systematic knowledge-based development for production purposes. The formation of PHB, a natural product of P. megaterium and potential petro-plastic substitute, is covered and discussed. Further important biotechnological characteristics of P. megaterium for recombinant protein production including high protein secretion capacity and simple cultivation on value-added carbon sources are outlined. This includes the advanced system with almost 30 commercially available expression vectors for the intracellular and extracellular production of recombinant proteins at the g/L scale. We also revealed a novel P. megaterium transcription-translation system as a complementary and versatile biotechnological tool kit. As an impressive biotechnology application, the formation of various cytochrome P450 is also critically highlighted. Finally, whole cellular applications in plant protection are completing the overall picture of P. megaterium as a versatile giant cell factory.

Key points

• The use of Priestia megaterium for the biosynthesis of small molecules and recombinant proteins through to whole-cell applications is reviewed.

• P. megaterium can act as a promising alternative host in biotechnological production processes.

Bacillus cereus MH778713 elicits tomato plant protection against Fusarium oxysporum

AIM

The genus Fusarium comprises plant pathogenic species with agricultural relevance. Fusarium oxysporum causes tomato wilt disease with significant production losses. The use of agrochemicals to control the Fusarium wilt of tomato is not environmentally friendly. Bacillus species, as biocontrol agents, provide a safe and sustainable means to control Fusarium-induced plant diseases. In this study, the ability of Bacillus cereus MH778713, a strain isolated from root nodules of Prosopis laevigata, to protect tomato plants against Fusarium wilt was evaluated.

METHODS AND RESULTS

Bacillus cereus MH778713 and its volatiles inhibited the radial growth of F. oxysporum and stimulated tomato seedling growth in in vitro and in vivo tests. When tomato plants growing in the greenhouse were inoculated with B. cereus MH778713, the percentage of wilted plants decreased from 96% to 12%, indicating an effective crop protection against Fusarium wilt. Among the metabolites produced by B. cereus MH778713, hentriacontane and 2,4-di-tert-butylphenol promoted tomato seedling growth and showed antifungal activity against the target pathogen.

CONCLUSION

The inoculation of B. cereus MH778713 on tomato seedlings helped plants to manage Fusarium wilt, suggesting the potential of B. cereus MH778713 as a biocontrol agent.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results complement our previous studies on chromium tolerance and bioremediation traits of B. cereus MH778713 by highlighting the potential of this metal-resistant micro-organism to boost crop growth and disease resistance.

Volatile organic compounds produced by Pseudomonas pseudoalcaligenes alleviated drought stress by modulating defense system in maize (Zea mays L.)

Research on plant growth-promoting bacteria (PGPR) revealed an effective role of bacterial volatile organic compounds (VOCs) in stress alleviation. Out of 15 PGPR strains, infection with VOCs from Pseudomonas pseudoalcaligenes' resulted in maximum germination, growth promotion, and drought tolerance in maize plants. The VOCs of P. pseudoalcaligenes caused induced systemic tolerance in maize plants during 7 days of drought stress. The VOCs exposed plants displayed resistance to drought stress by reducing electrolyte leakage and malondialdehyde content and increasing the synthesis of photosynthetic pigments, proline, and phytohormones contents. Maize plants revealed enhanced resistance by showing higher activities of antioxidant defense enzymes both in shoots and roots under drought stress. Activities of antioxidant enzymes were more pronounced in shoots than roots. Gas chromatography and mass spectrophotometric (GC-MS) analysis comparing VOCs produced by the most efficient P. pseudoalcaligenes strain and inefficient strains of Pseudomonas sp. grown in culture media revealed nine compounds that they had in common. However, dimethyl disulfide, 2,3-butanediol, and 2-pentylfuran were detected only in P. pseudoalcaligenes, indicating these compounds are potential candidates for drought stress induction. Further studies are needed to unravel the molecular mechanisms of VOCs-mediated systemic drought tolerance in plants related to each identified VOC.

Nematicidal Volatiles from Bacillus atrophaeus GBSC56 Promote Growth and Stimulate Induced Systemic Resistance in Tomato against Meloidogyne incognita

Bacillus volatiles to control plant nematodes is a topic of great interest among researchers due to its safe and environmentally friendly nature. Bacillus strain GBSC56 isolated from the Tibet region of China showed high nematicidal activity against M. incognita, with 90% mortality as compared with control in a partition plate experiment. Pure volatiles produced by GBSC56 were identified through gas chromatography and mass spectrometry (GC-MS). Among 10 volatile organic compounds (VOCs), 3 volatiles, i.e., dimethyl disulfide (DMDS), methyl isovalerate (MIV), and 2-undecanone (2-UD) showed strong nematicidal activity with a mortality rate of 87%, 83%, and 80%, respectively, against M. incognita. The VOCs induced severe oxidative stress in nematodes, which caused rapid death. Moreover, in the presence of volatiles, the activity of antioxidant enzymes, i.e., SOD, CAT, POD, and APX, was observed to be enhanced in M. incognita-infested roots, which might reduce the adverse effect of oxidative stress-induced after infection. Moreover, genes responsible for plant growth promotion SlCKX1, SlIAA1, and Exp18 showed an upsurge in expression, while AC01 was downregulated in infested plants. Furthermore, the defense-related genes (PR1, PR5, and SlLOX1) in infested tomato plants were upregulated after treatment with MIV and 2-UD. These findings suggest that GBSC56 possesses excellent biocontrol potential against M. incognita. Furthermore, the study provides new insight into the mechanism by which GBSC56 nematicidal volatiles regulate antioxidant enzymes, the key genes involved in plant growth promotion, and the defense mechanism M. incognita-infested tomato plants use to efficiently manage root-knot disease.

A sustainable way to reuse Cr(VI) into an efficient biological nanometer electrocatalyst by Bacillus megaterium

The remediation of heavy metal is facing the great challenge of failing to achieve valuable transformation. Therefore, the development of a sustainable technology for heavy metal recycling and reuse is essential. The present study proposed a new way to convert Cr(VI) into value-added biological Cr₂O₃ nanoparticles (bio-Cr₂O₃ NPs) with B. megaterium-secreted tryptophan residues proteins (TPN). In this process, Cr(VI) was reduced extracellularly to Cr(III) by B. megaterium without additional reductant and electron donors. This study overcomes the difficulty of separation of NPs and biomass, and realizes the recovery of bio-Cr₂O₃ NPS from biomass. The conversing efficiency of bio-Cr₂O₃ NPs reached the highest level (96.56%) at the concentration of 10 ppm Cr(VI). In particular, bio-Cr₂O₃ NPs exhibited excellent catalytic activity for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1 M KOH, outperforming chemically synthesized Cr-base catalysts. Three-dimensional matrix fluorescence (EEM), verification of tryptophan reduction and computation chemistry fully confirmed that TPN was responsible for the bio-Cr₂O₃ NPs formation. This comprehensive approach to bioremediation, synthesis NPs and recovery, as well as application will open a window for sustainable energy development and heavy metal pollution remediation.

Volatile compounds from beneficial rhizobacteria Bacillus spp. promote periodic lateral root development in Arabidopsis

Lateral root formation is coordinated by both endogenous and external factors. As biotic factors, plant growth-promoting rhizobacteria can affect lateral root formation, while the regulation mechanism is unclear. In this study, by applying various marker lines, we found that volatile compounds (VCs) from Bacillus amyloliquefaciens SQR9 induced higher frequency of DR5 oscillation and prebranch site formation, accelerated the development and emergence of the lateral root primordia and thus promoted lateral root development in Arabidopsis. We demonstrated a critical role of auxin on B. amyloliquefaciens VCs-induced lateral root formation via respective mutants and pharmacological experiments. Our results showed that auxin biosynthesis, polar transport and signalling pathway are involved in B. amyloliquefaciens VCs-induced lateral roots formation. We further showed that acetoin, a major component of B. amyloliquefaciens VCs, is less active in promoting root development compared to VC blends from B. amyloliquefaciens, indicating the presence of yet uncharacterized/unknown VCs might contribute to B. amyloliquefaciens effect on lateral root formation. In summary, our study revealed an auxin-dependent mechanism of B. amyloliquefaciens VCs in regulating lateral root branching in a non-contact manner, and further efforts will explore useful VCs to promote plant root development.

Delineation of mechanistic approaches employed by plant growth promoting microorganisms for improving drought stress tolerance in plants

Drought stress is expected to increase in intensity, frequency, and duration in many parts of the world, with potential negative impacts on plant growth and productivity. The plants have evolved complex physiological and biochemical mechanisms to respond and adjust to water-deficient environments. The physiological and biochemical mechanisms associated with water-stress tolerance and water-use efficiency have been extensively studied. Besides these adaptive and mitigating strategies, the plant growth-promoting rhizobacteria (PGPR) play a significant role in alleviating plant drought stress. These beneficial microorganisms colonize the endo-rhizosphere/rhizosphere of plants and enhance drought tolerance. The common mechanism by which these microorganisms improve drought tolerance included the production of volatile compounds, phytohormones, siderophores, exopolysaccharides, 1-aminocyclopropane-1-carboxylate deaminase (ACC deaminase), accumulation of antioxidant, stress-induced metabolites such as osmotic solutes proline, alternation in leaf and root morphology and regulation of the stress-responsive genes. The PGPR is an easy and efficient alternative approach to genetic manipulation and crop enhancement practices because plant breeding and genetic modification are time-consuming and expensive processes for obtaining stress-tolerant varieties. In this review, we will elaborate on PGPR's mechanistic approaches in enhancing the plant stress tolerance to cope with the drought stress.

2-Pentylfuran: a novel repellent of Drosophila suzukii

BACKGROUND

Drosophila suzukii (Matsumura), spotted-wing drosophila (SWD), is a major invasive pest of soft-skinned fruits in North America and Europe. Although insecticides are currently the primary method of SWD control, it is imperative to develop alternative management approaches, such as behavioral control through the use of repellents and attractants. This study explores the repellent properties of 2-pentylfuran as an oviposition deterrent on raspberries.

RESULTS

2-Pentylfuran was found to be aversive to SWD in laboratory multiple-choice tests. When co-released from a vial (loaded as neat compound) with a synthetic SWD lure, 2-pentylfuran reduced SWD attraction to the SWD lure by 98% and the effect appeared 17% stronger compared to 1-octen-3-ol, a known SWD repellent. Releasing 50% 2-pentylfuran mixed with mineral oil from a vial located near ripe raspberries resulted in 30% reduction in SWD oviposition in the field. In laboratory no-choice assays, 2-pentylfuran reduced SWD oviposition on raspberries above 2.5 mg h^-1 with greater repellency achieved at higher release rates. A release rate of 10 mg h^-1 from a polyethylene sachet reduced egg-laying on raspberries by 60% in a semifield cage choice experiment. In a field experiment using fruiting raspberry clusters, 14 mg h^-1 release rate of 2-pentylfuran was effective at reducing SWD infestations by 56% compared to untreated plots.

CONCLUSION

2-Pentylfuran acts as a repellent for SWD and can significantly reduce fruit infestations under field conditions and high SWD pressure. Given that 2-pentylfuran is a registered food additive and generally regarded as safe, 2-pentylfuran has a potential use in behavioral control strategies against SWD. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.

Current advances in plant-microbe communication via volatile organic compounds as an innovative strategy to improve plant growth

Volatile organic compounds (VOCs) emitted by microorganisms have demonstrated an important role to improve growth and tolerance against abiotic stress on plants. Most studies have used Arabidopsis thaliana as a model plant, extending to other plants of commercial interest in the last years. Interestingly, the microbial VOCs are characterized by its biodegradable structure, quick action, absence of toxic substances, and acts at lower concentration to regulate plant physiological changes. These compounds modulate plant physiological processes such as phytohormone pathways, photosynthesis, nutrient acquisition, and metabolisms. Besides, the regulation of gene expression associated with cell components, biological processes, and molecular function are triggered by microbial VOCs. Otherwise, few studies have reported the important role of VOCs for confer plant tolerance to abiotic stress, such as drought and salinity. Although VOCs have shown an efficient action to enhance the plant growth under controlled conditions, there are still great challenges for their greenhouse or field application. Therefore, in this review, we summarize the current knowledge about the technical procedures, study cases, and physiological mechanisms triggered by microbial VOCs to finally discuss the challenges of its application in agriculture.

Role of Volatiles from the Endophytic Fungus Trichoderma asperelloides PSU-P1 in Biocontrol Potential and in Promoting the Plant Growth of Arabidopsis thaliana

Fungal volatile organic compounds (VOCs) emitted by Trichoderma species interact with a plant host and display multifaceted mechanisms. In this study, we investigated the antifungal activity of VOCs emitted by Trichoderma asperelloides PSU-P1 against fungal pathogens, as well as the ability of VOCs to activate defense responses and to promote plant growth in Arabidopsis thaliana. The strain’s VOCs had remarkable antifungal activity against fungal pathogens, with an inhibition range of 15.92–84.95% in a volatile antifungal bioassay. The VOCs of T. asperelloides PSU-P1 promoted the plant growth of A. thaliana, thereby increasing the fresh weight, root length, and chlorophyll content in the VOC-treated A. thaliana relative to those of the control. High expression levels of the chitinase (CHI) and β-1,3-glucanase (GLU) genes were found in the VOC-treated A. thaliana by quantitative reverse transcription polymerase chain reaction (RT-PCR). The VOC-treated A. thaliana had higher defense-related enzyme (peroxidase (POD)) and cell wall-degrading enzyme (chitinase and β-1,3-glucanase) activity than in the control. The headspace VOCs produced by PSU-P1, trapped with solid phase microextraction, and tentatively identified by gas chromatography–mass spectrometry, included 2-methyl-1-butanol, 2-pentylfuran, acetic acid, and 6-pentyl-2H-pyran-2-one (6-PP). The results suggest that T. asperelloides PSU-P1 emits VOCs responsible for antifungal activity, for promoting plant growth, and for inducing defense responses in A. thaliana.

Induction of Salt Tolerance in Arabidopsis thaliana by Volatiles From Bacillus amyloliquefaciens FZB42 via the Jasmonic Acid Signaling Pathway

Previously, we showed that Bacillus amyloliquefaciens FZB42 can confer salt tolerance in plants by root inoculation under salt stress condition, and the FZB42 volatile organic compounds (VOCs) promoted plant growth and development under non-salt stress condition. In the present study, we investigated the mechanism that allows FZB42 VOCs to confer salt tolerance in Arabidopsis without colonization of plant roots. We found that FZB42 VOCs significantly increased the biomass of Arabidopsis and also maintained the leaf chlorophyll content under salt stress condition. Physiological tests showed that the plant anti-oxidation system was activated by FZB42 VOCs, where higher peroxidase (POD), catalase (CAT), and superoxide dismutase (SOD) activities were detected in plants exposed to FZB42 VOCs compared with non-exposed plants. In addition, FZB42 VOCs increased the leaf total soluble sugars (TSS) content but decreased the proline content compared with the non-exposed plants. Moreover, FZB42 VOCs significantly decreased the Na⁺ contents of the whole plants and induced the expression of genes (NHX1; Na⁺/H⁺ exchanger 1 and HKT1; high-affinity K⁺ transporter 1) that function to alleviate Na⁺ toxicity. Furthermore, analysis of mutants with defects in specific hormone pathways showed that FZB42 VOCs induced salt tolerance in plants by modulating jasmonic acid (JA) signaling, which was confirmed by the up-regulation of JA synthesis, defense-related genes, and JA biosynthesis inhibitor tests. The results of this study provide new insights into the molecular mechanism related to the interactions between plant growth-promoting rhizobacteria and plants under salt stress condition.

Harnessing microbial volatiles to replace pesticides and fertilizers

Global agricultural systems are under increasing pressure to deliver sufficient, healthy food for a growing population. Seasonal inputs, including synthetic pesticides and fertilizers, are applied to crops to reduce losses by pathogens, and enhance crop biomass, although their production and application can also incur several economic and environmental penalties. New solutions are therefore urgently required to enhance crop yield whilst reducing dependence on these seasonal inputs. Volatile Organic Compounds (VOCs) produced by soil microorganisms may provide alternative, sustainable solutions, due to their ability to inhibit plant pathogens, induce plant resistance against pathogens and enhance plant growth promotion. This review will highlight recent advances in our understanding of the biological activities of microbial VOCs (mVOCs), providing perspectives on research required to develop them into viable alternatives to current unsustainable seasonal inputs. This can identify potential new avenues for mVOC research and stimulate discussion across the academic community and agri-business sector.

Mechanisms of Soybean Host-Plant Resistance Against Megacopta cribraria (Hemiptera: Plataspidae)

A number of soybean varieties traditionally bred for resistance to various soybean arthropod pests have been identified as resistant to Megacopta cribraria (F.) (Hemiptera: Plataspidae). However, the mechanisms of host-plant resistance (HPR) in this system are not understood. The goal of this study was to identify the mechanisms of resistance by examining the role of plant volatile organic compounds (VOCs) and free amino acids (FAAs) among 16 soybean varieties. Choice and no-choice cage experiments identified several soybean varieties that demonstrated antixenosis as well as antibiosis. However, resistance varied over time in certain soybean varieties, such as N02-7002 and PI567352B. Mean nymph number from choice experiments had positive correlations with the FAAs asparagine, tryptophan, alanine, phenylanaline, and serine; negative correlation with leucine and threonine. Four plant volatiles, hexanal, 2-pentylfuran, beta-cyclocitral, and cis-9-hexadecenal, were positively correlated with subsequent nymph development, whereas n-hexadecenoic acid was negatively correlated with nymph number only, in adult choice cage experiments. This study contributes to understanding the mechanisms of HPR through associations with plant VOCs and FAAs in relation to M. cribraria development and provides useful knowledge for developing soybean varieties for M. cribraria management.

Antifungal Effects of Volatiles Produced by Bacillus subtilis Against Alternaria solani in Potato

Antifungal activities of plant-beneficial Bacillus have been widely studied in recent years. Numerous studies have studied the antifungal mechanisms of soluble non-volatile bioactive compounds such as lipopeptides and proteins produced by Bacillus against soil-borne diseases. However, the antagonistic mechanisms of volatile organic compounds (VOCs) from Bacillus against airborne phytopathogens are still largely unknown, and the function of Alternaria solani pathogenic genes has not been well identified. Here, we first isolated a Bacillus strain with strong antifungal activity and finally identified it as B. subtilis ZD01. Then, the antagonistic mechanisms of VOCs produced by strain ZD01, against A. solani, an airborne fungal pathogen that can cause early blight diseases of potato, were studied. We showed that VOCs produced by strain ZD01 can reduce the colony size and mycelial penetration and can cause serious morphological changes of A. solani. Scanning electron microscope (SEM) observation showed that VOCs released by ZD01 could cause more flaccid and gapped hyphae of A. solani. Also, we found that VOCs produced by ZD01 can inhibit the conidia germination and reduce the lesion areas and number of A. solani in vivo significantly. Meanwhile, based on gas chromatography/mass spectrometry (GC/MS) analysis, 29 volatile compounds produced by strain ZD01 were identified. Out of 29 identified VOCs, 9 VOCs showed complete growth inhibition activities against A. solani. Moreover, we identified two virulence-associated genes (slt2 and sod) in A. solani. slt2 is a key gene that regulates the mycelial growth, penetration, sporulation, and virulence in vivo in A. solani. In addition, sod plays a significant role in the SOD synthetic pathway in A. solani. Results from qRT-PCR showed that the transcriptional expression of these two genes was down-regulated after being treated by VOCs produced by ZD01. These results are useful for a better understanding of the biocontrol mechanism of Bacillus and offer a potential method for potato early blight disease control.

Stereoisomers of Nonvolatile Acetylbutanediol Metabolites Produced by Bacillus velezensis WRN031 Improved Root Elongation of Maize and Rice

Inoculation of crop plants with strains of beneficial bacteria can result in promotion of plant growth. In our study, we demonstrated that Bacillus velezensis WRN031 as a plant-growth-promoting rhizobacteria (PGPR) improved the maize seedling growth following inoculation with WRN031. Fluorescence microscopy visualization indicated that GFP-labeled B. velezensis WRN031 had accumulated on the maturation zones of both primary and lateral roots of maize. Two metabolites were detected in the rhizosphere soil of maize root inoculation with WRN031 using HPLC-DAD analyses. Through guided isolation from an ethyl acetate extract of B. velezensis WRN031, these two nonvolatile meso stereoisomers 3S,4R-acetylbutanediol (3S,4R-ABD, 1) and 3R,4R-acetylbutanediol (3R,4R-ABD, 2) were identified and found to occur at a ratio of 1:2 (v/v) in maize rhizosphere soil. Bioactivity screening indicated that compounds 1 and 2, as well as a v/v = 1:2 mixture of both 1 and 2, significantly improved the root elongation of both maize and rice, with the effective enhancement concentration related to their concentration in rhizosphere soil. These results suggested that 3S,4R-ABD and 3R,4R-ABD produced by B. velezensis WRN031 might improve the growth of their host plants and provides evidence that nonvolatiles accumulating in the root maturation zone may regulate the relationship between roots and beneficial bacteria.

Unearthing the Plant Growth-Promoting Traits of Bacillus megaterium RmBm31, an Endophytic Bacterium Isolated From Root Nodules of Retama monosperma

Plants live in association with complex populations of microorganisms, including Plant Growth-Promoting Rhizobacteria (PGPR) that confer to plants an improved growth and enhanced stress tolerance. This large and diverse group includes endophytic bacteria that are able to colonize the internal tissues of plants. In the present study, we have isolated a nonrhizobial species from surface sterilized root nodules of Retama monosperma, a perennial leguminous species growing in poor and high salinity soils. Sequencing of its genome reveals this endophytic bacterium is a Bacillus megaterium strain (RmBm31) that possesses a wide range of genomic features linked to plant growth promotion. Furthermore, we show that RmBm31 is able to increase the biomass and positively modify the root architecture of seedlings of the model plant species Arabidopsis thaliana both in physical contact with its roots and via the production of volatile organic compounds. Lastly, we investigated the molecular mechanisms implicated in RmBm31 plant beneficial effects by carrying out a transcriptional analysis on a comprehensive set of phytohormone-responsive marker genes. Altogether, our results demonstrate that RmBm31 displays plant growth-promoting traits of potential interest for agricultural applications.

Metagenomic Analysis of the Bacterial and Fungal Community Associated to the Rhizosphere of Tabebuia chrysantha and T. billbergii

The rhizosphere of plants contains a diversity of microorganisms, some of which play an important role in the growth and development of the host plant. In this work, the diversity of fungi and bacteria associated to the rhizosphere of Tabebuia chrysantha and T. billbergii plants was analyzed. The molecular identification was performed by sequencing the ITS and 16S rDNA for fungi and bacteria, respectively. The analysis of the rDNA sequences of the rhizosphere of T. billergii showed that for domain Eukaria, the most abundant phyla were Glomeromycota (56%) and Ascomycota (39%), and for domain Bacteria, the phylum Firmicutes (19.17%) was the most abundant followed by Actinobacteria (14.90%) and Proteobacteria (8.94%). In the rhizosphere of T. chrysantha the most abundant phylum of Eukaria was Ascomycota (98%), and for Bacteria the most representative phyla were Proteobacteria (18.61%) and Actinobacteria (11.93%). A diversity of genera and species of fungi and bacteria was observed, to be more significant in T. chrysantha than T. billbergii. The taxonomic assignment of metagenomic sequences revealed a homology associated with genomic sequences of 546 bacteria and 147 fungi in T. chrysantha and 154 bacteria and 122 fungi in T. billbergii.

Volatile compounds other than CO2 emitted by different microorganisms promote distinct posttranscriptionally regulated responses in plants

A "box-in-box" cocultivation system was used to investigate plant responses to microbial volatile compounds (VCs) and to evaluate the contributions of organic and inorganic VCs (VOCs and VICs, respectively) to these responses. Arabidopsis plants were exposed to VCs emitted by adjacent Alternaria alternata and Penicillium aurantiogriseum cultures, with and without charcoal filtration. No VOCs were detected in the headspace of growth chambers containing fungal cultures with charcoal filters. However, these growth chambers exhibited elevated CO₂ and bioactive CO and NO headspace concentrations. Independently of charcoal filtration, VCs from both fungal phytopathogens promoted growth and distinct developmental changes. Plants cultured at CO₂ levels observed in growth boxes containing fungal cultures were identical to those cultured at ambient CO₂ . Plants exposed to charcoal-filtered fungal VCs, nonfiltered VCs, or superelevated CO₂ levels exhibited transcriptional changes resembling those induced by increased irradiance. Thus, in the "box-in-box" system, (a) fungal VICs other than CO₂ and/or VOCs not detected by our analytical systems strongly influence the plants' responses to fungal VCs, (b) different microorganisms release VCs with distinct action potentials, (c) transcriptional changes in VC-exposed plants are mainly due to enhanced photosynthesis signaling, and (d) regulation of some plant responses to fungal VCs is primarily posttranscriptional.

Smells from the desert: Microbial volatiles that affect plant growth and development of native and non-native plant species

The plant microbiota can affect host fitness via the emission of microbial volatile organic compounds (mVOCs) that influence growth and development. However, evidence of these molecules and their effects in plants from arid ecosystems is limited. We screened the mVOCs produced by 40 core and representative members of the microbiome of agaves and cacti in their interaction with Arabidopsis thaliana and Nicotiana benthamiana. We used SPME-GC-MS to characterize the chemical diversity of mVOCs and tested the effects of selected compounds on growth and development of model and host plants. Our study revealed that approximately 90% of the bacterial strains promoted plant growth both in A. thaliana and N. benthamiana. Bacterial VOCs were mainly composed of esters, alcohols, and S-containing compounds with 25% of them not previously characterized. Remarkably, ethyl isovalerate, isoamyl acetate, 3-methyl-1-butanol, benzyl alcohol, 2-phenylethyl alcohol, and 3-(methylthio)-1-propanol, and some of their mixtures, displayed beneficial effects in A. thaliana and also improved growth and development of Agave tequilana and Agave salmiana in just 60 days. Volatiles produced by bacteria isolated from agaves and cacti are promising molecules for the sustainable production of crops in arid and semi-arid regions.

Antifungal and plant growth promotion activity of volatile organic compounds produced by Bacillus amyloliquefaciens

Fusarium wilt of watermelon, caused by F. oxysporum f.sp. niveum (FON), is a devastating disease that causes extensive losses throughout the world. Five bacterial strains (L3, h, β, b, and L) isolated from the watermelon rhizosphere showed antagonistic activity against FON during in vitro tests. Strain L3 produced diffusible and volatile organic compounds (VOCs) which showed the strongest antifungal activity. Arabidopsis thaliana plantlets exposed to VOCs produced by strain L3 showed a 2.39‐fold increase in biomass, 1.40‐fold increase in primary root length, and 5.05‐fold increase in number of lateral roots. Confocal laser scanning microscope showed that the GFP‐labeled strain L3 could colonize along the elongation and differentiation zones of watermelon roots. In greenhouse pot experiments, the biocontrol efficiency of strain L3 against fusarium wilt of watermelon was up to 68.4% in comparison with the control treatment. In addition, inoculation of the strain L3 resulted in a 23.4% increase in plant fresh weight. Based on 16S rDNA sequence analysis, the strain L3 was identified as Bacillus amyloliquefaciens L3. Fourteen VOCs produced by strain L3 were identified through GC‐MS analysis. Of nine VOCs tested, 2‐nonanone and 2‐heptanone were proved to have strong antifungal properties. Acetoin and 2,3‐butanediol were found to promote plant growth. The results suggested B. amyloliquefaciens L3 was a potential biocontrol agent, and that VOCs produced by B. amyloliquefaciens L3 play important roles in the process of biocontrol and plant growth promotion.

Physiological response of Lactuca sativa exposed to 2-nonanone emitted by Bacillus sp. BCT9

Volatile organic compounds (VOCs) released from bacterial species have been reported as plant growth inducers. In this sense, Lactuca sativa was used as model vegetable to prospect the effects of 2-nonanone released by Bacillus sp. BCT9 at cellular and organ structure level, so we present preliminary results about the physiological effects. In this study, 2-day-old L. sativa were exposed to 2-nonanone for 10 days under two delivery systems: 1) 2-nonanone (abrupt delivery) and 2) 2-nonanone + lanolin (controlled delivery). The X-ray elemental microanalysis, scanning electron and confocal laser microscopies techniques were used to evaluate physiological changes "in vivo" conditions. The results indicated that 2-nonanone increased root and shoot length independently of 2-nonanone delivery system after 7 days of exposition. Additionally, 2-nonanone elicited the increase of anthocyanin and not affects chlorophyll content and electrolyte leakage percentage. The abrupt delivery elicited the increase of both length and density of root hair without causing changes in size of cell epidermis, while controlled delivery induced stomatal opening. Besides, 2-nonanone exposition did not modify the composition and distribution of carbon, nitrogen, phosphorus, potassium, and chlorine in the surface of plant tissue. The results suggested that 2-nonanone acts as a bacterial signal molecule to elicit changes related to root development without damaging the external morphology while epidermal cells at leaf level are not affected, suggesting that 2-nonanone can be an important tool to apply to vegetables.

Exposure in vitro to an Environmentally Isolated Strain TC09 of Cladosporium sphaerospermum Triggers Plant Growth Promotion, Early Flowering, and Fruit Yield Increase

A growing number of bacteria and fungi have been found to promote plant growth through mutualistic interactions involving elements such as volatile organic compounds (VOCs). Here, we report the identification of an environmentally isolated strain of Cladosporium sphaerospermum (herein named TC09), that substantially enhances plant growth after exposure in vitro beyond what has previously been reported. When cultured on Murashige and Skoog (MS) medium under in vitro conditions, tobacco seedlings (Nicotiana tabacum) exposed to TC09 cultures for 20 days increased stem height and whole plant biomass up to 25- and 15-fold, respectively, over controls without exposure. TC09-mediated growth promotion required >5 g/L sucrose in the plant culture medium and was influenced by the duration of exposure ranging from one to 10 days, beyond which no differences were detected. When transplanted to soil under greenhouse conditions, TC09-exposed tobacco plants retained higher rates of growth. Comparative transcriptome analyses using tobacco seedlings exposed to TC09 for 10 days uncovered differentially expressed genes (DEGs) associated with diverse biological processes including cell expansion and cell cycle, photosynthesis, phytohormone homeostasis and defense responses. To test the potential efficacy of TC09-mediated growth promotion on agricultural productivity, pepper plants (Capsicum annuum L.) of two different varieties, Cayenne and Minisweet, were pre-exposed to TC09 and planted in the greenhouse to monitor growth, flowering, and fruit production. Results showed that treated pepper plants flowered 20 days earlier and yielded up to 213% more fruit than untreated controls. Altogether the data suggest that exposure of young plants to C. sphaerospermum produced VOCs may provide a useful tool to improve crop productivity.

Antagonistic activities of volatiles produced by two Bacillus strains against Monilinia fructicola in peach fruit

BACKGROUND

Brown rot caused by Monilinia fructicola is one of most serious diseases of postharvest peach fruit. The objective of this study was to select effective antagonistic bacteria against Monilinia fructicola and evaluate the effects of these strains against brown rot.

RESULTS

Four bacterial strains producing inhibitory volatile gas against Monilinia fructicola were isolated from the peach rhizosphere soil. The volatiles produced by 12a (Bacillus vallismortis) and 14b (Bacillus altitudinis) showed considerable antagonistic activities. Monilinia fructicola showed 80.3% and 68.4% mycelial growth inhibition and cell damage in the presence of strains 12a and 14b, respectively. The inhibition rate of brown rot in peach fruit fumigated with the culture solution of 12a or 14b reached 77.1% and 50.0%, respectively. The volatile compounds produced by 12a and 14b were identified according to gas chromatographic-mass spectrometric analysis. Among them, 6-methyl-2-heptanone and 2-pentylfuran completely inhibited mycelial growth at 100 µL L^-1 concentration. Cedrol showed strong inhibitory activity against mycelial growth at 100 µg L^-1 and isodecyl methacrylate inhibited growth at high concentration. The inhibition rate of the 50 µL L^-1 artificial mixture of these four volatiles was 59.3% in vitro.

CONCLUSION

These results indicate that the two antagonistic bacteria and some volatiles produced by them have potential value in controlling brown rot in harvested peach fruit. © 2018 Society of Chemical Industry.

The Arabidopsis-Trichoderma interaction reveals that the fungal growth medium is an important factor in plant growth induction

Trichoderma spp colonizes the plant rhizosphere and provides pathogen resistance, abiotic stress tolerance, and enhance growth and development. We evaluated the Arabidopsis-Trichoderma interaction using a split system in which Trichoderma atroviride and Trichoderma virens were grown on PDA or MS medium. Arabidopsis growth was significantly increased at 3 and 5 days post-inoculation with both Trichoderma species, when the fungal strains were grown on PDA in split interaction. The analysis of DR5:uidA reporter line revealed a greater auxin accumulation in root tips when the fungi were grown on PDA in a split interaction. The root hair-defective phenotype of Arabidopsis rhd6 mutant was reverted with both Trichoderma species, even in split interactions. At 12 °C, Trichoderma species in split interactions were able to mitigate the effects of cold stress on the plant, and also Trichoderma induced the AtERD14 expression, a cold related gene. Volatile organic compounds analysis revealed that Trichoderma strains produce mainly sesquiterpenes, and that the type and abundance of these compounds was dependent on the fungal strain and the culture medium. Our results show that fungal nutrition is an important factor in plant growth in a split interaction.

From Acetoin to ( Z)-3-Hexen-1-ol: The Diversity of Volatile Organic Compounds that Induce Plant Responses

Evidence that plants can respond to volatile organic compounds (VOCs) was first presented 35 years ago. Since then, over 40 VOCs have been found to induce plant responses. These include VOCs that are produced not only by plants but also by microbes and insects. Here, we summarize what is known about how these VOCs are produced and how plants detect and respond to them. In doing so, we highlight notable observations we believe are worth greater consideration. For example, the VOCs that induce plant responses appear to have little in common. They are derived from many different biosynthetic pathways and have few distinguishing chemical or structural features. Likewise, plants appear to use several mechanisms to detect VOCs rather than a single dedicated "olfactory" system. Considering these observations, we advocate for more discovery-oriented experiments and propose that future research take a fresh look at the ways plants detect and respond to VOCs.

Fungi Indirectly Affect Plant Root Architecture by Modulating Soil Volatile Organic Compounds

The plant-growth modulating effect of microbial volatile organic compounds (VOCs) has been demonstrated repeatedly. This has most often been performed by exposing plants to VOC released by microbes grown on nutrient rich media. Here, we used soil instead to grow fungi of the Fusarium genus and investigate how VOCs emitted by this system influenced the development of Arabidopsis plants. The volatile profiles of Fusarium strains grown in soil and malt extract were also compared. Our results demonstrate that distinct volatile signatures can be attributed to different Fusarium genetic clades but also highlight a major influence of the growth medium on volatile emission. Furthermore, all soil-grown Fusarium isolates increased primary root length in Arabidopsis by decreasing VOC concentrations in soil. This result represents a major paradigm shift in plant-microbe interactions since growth modulating effects have been attributed so far to the emission and not the consumption of volatile signals.