Volatile organic compounds (VOCs) from Bacillus subtilis CF-3 reduce anthracnose and elicit active defense responses in harvested litchi fruits

Study of Pathogenicity Test, Antifungal Activity, and Secondary Metabolites of Bacillus spp. from Lake Bogoria as Biocontrol of Rhizoctonia solani Kühn in Phaseolus vulgaris L

The common bean (Phaseolus vulgaris L.) is a yearly herbaceous plant grown for its edible dry seeds. Despite that, pests and diseases have contributed to the decline of common bean production in Kenya. Therefore, the study aimed to identify bacteria from Lake Bogoria, assess the pathogenicity of Rhizoctonia solani Kühn, screen for effective antifungal agents, and determine secondary metabolites for the biocontrol of R. solani. A total of 49 bacteria were isolated, of which 10 isolates had varied mycelial inhibition rates of R. solani in the co-culture technique. The efficacy of volatile compounds of the three selected bacterial strains had varied mycelial growth and percent reduction against R. solani. The pathogenicity assay showed varied plant parameters and biomass of R. solani on common bean plantlets. The molecular characterization based on 16 S ribosomal RNA confirmed the selected bacterial strains' identity with a diversity similar to the Bacillus genus. Gas chromatography-mass spectrometry analysis of secondary metabolites showed different antimicrobial compounds produced by Bacillus subtilis strain TW21. In conclusion, Lake Bogoria harbors useful microbes as biocontrol agents against plant pathogens. The current study discovers the potential biocontrol bacteria isolates from Lake Bogoria as alternative bioagents against R. solani. Therefore, the isolate Bacillus subtilis strain TW21 can be assessed further for toxicological and ecotoxicological studies and registered by the Pest Control Products Board (PCPB), Kenya, as a biocontrol product against common diseases affecting common beans' production.

Characterization and evaluation of Bacillus subtilis GYUN-2311 as a biocontrol agent against Colletotrichum spp. on apple and hot pepper in Korea

Crop plants are vulnerable to a variety of diseases, including anthracnose, caused by various species of Colletotrichum fungi that damages major crops, including apples and hot peppers. The use of chemical fungicides for pathogen control may lead to environmental pollution and disease resistance. Therefore, we conducted this research to develop a Bacillus subtilis-based biological control agent (BCA). B. subtilis GYUN-2311 (GYUN-2311), isolated from the rhizosphere soil of an apple orchard, exhibited antagonistic activity against a total of 12 fungal pathogens, including eight Colletotrichum species. The volatile organic compounds (VOCs) and culture filtrate (CF) from GYUN-2311 displayed antifungal activity against all 12 pathogens, with 81% control efficiency against Fusarium oxysporum for VOCs and 81.4% control efficacy against Botryosphaeria dothidea for CF. CF also inhibited germination and appressorium formation in Colletotrichum siamense and C. acutatum. The CF from GYUN-2311 showed antifungal activity against all 12 pathogens in different media, particularly in LB medium. It also exhibited plant growth-promoting (PGP) activity, lytic enzyme activity, siderophore production, and the ability to solubilize insoluble phosphate. In trials on apples and hot peppers, GYUN-2311 effectively controlled disease, with 75 and 70% control efficacies against C. siamense in wounded and unwounded apples, respectively. Similarly, the control efficacy of hot pepper against C. acutatum in wounded inoculation was 72%. Combined application of GYUN-2311 and chemical suppressed hot pepper anthracnose to a larger extent than other treatments, such as chemical control, pyraclostrobin, TK®, GYUN-2311 and cross-spraying of chemical and GYUN-2311 under field conditions. The genome analysis of GYUN-2311 identified a circular chromosome comprising 4,043 predicted protein-coding sequences (CDSs) and 4,096,969 bp. B. subtilis SRCM104005 was the strain with the highest average nucleotide identity (ANI) to GYUN-2311. AntiSMASH analysis identified secondary metabolite biosynthetic genes, such as subtilomycin, bacillaene, fengycin, bacillibactin, pulcherriminic acid, subtilosin A, and bacilysin, whereas BAGEL analysis confirmed the presence of competence (ComX). Six secondary metabolite biosynthetic genes were induced during dual culture in the presence of C. siamense. These findings demonstrate the biological control potential of GYUN-2311 against apple and hot pepper anthracnose.

Volatile organic compounds produced by Bacillus aryabhattai AYG1023 against Penicillium expansum causing blue mold on the Huangguan pear

Previous research on Bacillus aryabhattai has mainly focused on bioremediation, biosynthesis, and promotion of plant growth, whereas the function of B. aryabhattai on antifungal activity remains to be explored. In this study, we isolated a biocontrol bacterium with antagonistic activity against post-harvest pathogenic fungi by releasing volatile organic compounds (VOCs). We aimed to assess the effectiveness of VOCs produced by B. aryabhattai in prevention of the development of blue mold caused by Penicillium expansum in the Huangguan pear, and reveal the inhibitory mechanism against the pathogenic fungi. Using molecular methods, the biocontrol bacterium was identified as Bacillus aryabhattai AYG1023. 2-Nonanol was identified as the main VOC by solid-phase microextraction gas chromatography-mass spectrometry (SPME-GC/MS). It showed strong inhibition of mycelial growth and conidial germination when treated at the minimum inhibitory concentration (MIC). Scanning and transmission electron microscopy showed that 2-nonanol caused abnormal changes in mycelial and conidial ultrastructure. 2-Nonanol also damaged the integrity of fungal cell membranes and reduced the ergosterol content to 44.77% of P. expansum. In addition, the production of secondary metabolites in P. expansum including patulin and citrinin was significantly reduced by 2-nonanol. Transcriptome analysis revealed that 2-nonanol modulated the expression of genes involved in development, and conidiation pathways, as well as secondary metabolite biosynthesis including steroid biosynthesis, citrinin production and patulin production. Furthermore, blue mold was completely inhibited by treatment with 0.04 μL mL-1 2-nonanol for 48 h on the Huangguan pear. In conclusion, Bacillus aryabhattai AYG1023 was identified as a promising and efficient agent for controlling post-harvest diseases via the release of VOCs, and the outcome of this study lays a theoretical foundation for future applications.

The Prospect of Hydrolytic Enzymes from Bacillus Species in the Biological Control of Pests and Diseases in Forest and Fruit Tree Production

Plant diseases and insect pest damage cause tremendous losses in forestry and fruit tree production. Even though chemical pesticides have been effective in the control of plant diseases and insect pests for several decades, they are increasingly becoming undesirable due to their toxic residues that affect human life, animals, and the environment, as well as the growing challenge of pesticide resistance. In this study, we review the potential of hydrolytic enzymes from Bacillus species such as chitinases, β-1,3-glucanases, proteases, lipases, amylases, and cellulases in the biological control of phytopathogens and insect pests, which could be a more sustainable alternative to chemical pesticides. This study highlights the application potential of the hydrolytic enzymes from different Bacillus sp. as effective biocontrol alternatives against phytopathogens/insect pests through the degradation of cell wall/insect cuticles, which are mainly composed of structural polysaccharides like chitins, β-glucans, glycoproteins, and lipids. This study demonstrates the prospects for applying hydrolytic enzymes from Bacillus sp. as effective biopesticides in forest and fruit tree production, their mode of biocidal activity and dual antimicrobial/insecticidal potential, which indicates a great prospect for the simultaneous biocontrol of pests/diseases. Further research should focus on optimizing the production of hydrolytic enzymes, and the antimicrobial/insecticidal synergism of different Bacillus sp. which could facilitate the simultaneous biocontrol of pests and diseases in forest and fruit tree production.

Unveiling the Potential of Bacillus safensis Y246 for Enhanced Suppression of Rhizoctonia solani

Rhizoctonia solani is a significant pathogen affecting various crops, including tobacco. In this study, a bacterial strain, namely Y246, was isolated from the soil of healthy plants and exhibited high antifungal activity. Based on morphological identification and DNA sequencing, this bacterial strain was identified as Bacillus safensis. The aim of this investigation was to explore the antifungal potential of strain Y246, to test the antifungal stability of Y246 by adjusting different cultivation conditions, and to utilize gas chromatography-mass spectrometry (GC-MS) to predict the volatile compounds related to antifungal activity in Y246. In vitro assays demonstrated that strain Y246 exhibited a high fungal inhibition rate of 76.3%. The fermentation broth and suspension of strain Y246 inhibited the mycelial growth of R. solani by 66.59% and 63.75%, respectively. Interestingly, treatment with volatile compounds derived from the fermentation broth of strain Y246 resulted in abnormal mycelial growth of R. solani. Scanning electron microscopy analysis revealed bent and deformed mycelium structures with a rough surface. Furthermore, the stability of antifungal activity of the fermentation broth of strain Y246 was assessed. Changes in temperature, pH value, and UV irradiation time had minimal impact on the antifungal activity, indicating the stability of the antifungal activity of strain Y246. A GC-MS analysis of the volatile organic compounds (VOCs) produced by strain Y246 identified a total of 34 compounds with inhibitory effects against different fungi. Notably, the strain demonstrated broad-spectrum activity, exhibiting varying degrees of inhibition against seven pathogens (Alternaria alternata, Phomopsis. sp., Gloeosporium musarum, Dwiroopa punicae, Colletotrichum karstii, Botryosphaeria auasmontanum, and Botrytis cinerea). In our extensive experiments, strain Y246 not only exhibited strong inhibition against R. solani but also demonstrated remarkable inhibitory effects on A. alternata-induced tobacco brown spot and kiwifruit black spot, with impressive inhibition rates of 62.96% and 46.23%, respectively. Overall, these findings highlight the significant antifungal activity of B. safensis Y246 against R. solani. In addition, Y246 has an excellent antifungal stability, with an inhibition rate > 30% under different treatments (temperature, pH, UV). The results showed that the VOCs of strain Y246 had a strong inhibitory effect on the colony growth of R. solani, and the volatile substances produced by strain Y246 had an inhibitory effect on R. solani at rate of 70.19%. Based on these results, we can conclude that Y246 inhibits the normal growth of R. solani. These findings can provide valuable insights for developing sustainable agricultural strategies.

Plant-Microbes Interaction: Exploring the Impact of Cold-Tolerant Bacillus Strains RJGP41 and GBAC46 Volatiles on Tomato Growth Promotion through Different Mechanisms

The interaction between plant and bacterial VOCs has been extensively studied, but the role of VOCs in growth promotion still needs to be explored. In the current study, we aim to explore the growth promotion mechanisms of cold-tolerant Bacillus strains GBAC46 and RJGP41 and the well-known PGPR strain FZB42 and their VOCs on tomato plants. The result showed that the activity of phytohormone (IAA) production was greatly improved in GBAC46 and RJGP41 as compared to FZB42 strains. The in vitro and in-pot experiment results showed that the Bacillus VOCs improved plant growth traits in terms of physiological parameters as compared to the CK. The VOCs identified through gas chromatography-mass spectrometry (GC-MS) analysis, namely 2 pentanone, 3-ethyl (2P3E) from GBAC46, 1,3-cyclobutanediol,2,2,4,4-tetramethyl (CBDO) from RJGP41, and benzaldehyde (BDH) from FZB42, were used for plant growth promotion. The results of the partition plate (I-plate) and in-pot experiments showed that all the selected VOCs (2P3E, CBDO, and BDH) promoted plant growth parameters as compared to CK. Furthermore, the root morphological factors also revealed that the selected VOCs improved the root physiological traits in tomato plants. The plant defense enzymes (POD, APX, SOD, and CAT) and total protein contents were studied, and the results showed that the antioxidant enzymes and protein contents significantly increased as compared to CK. Similarly, plant growth promotion expression genes (IAA4, ARF10A, GA2OX2, CKX2, and EXP1) were significantly upregulated and the ERF gene was downregulated as compared to CK. The overall findings suggest that both Bacillus isolates and their pure VOCs positively improved plant growth promotion activities by triggering the antioxidant enzyme activity, protein contents, and relative gene expressions in tomato plants.

Identification and characterization of Brevibacillus halotolerans B-4359: a potential antagonistic bacterium against red pepper anthracnose in Korea

Our study aimed to identify potential biocontrol agents (BCAs) against major phytopathogens under in vitro conditions by screening the Freshwater Bioresources Culture Collection (FBCC), Korea. Of the identified 856 strains, only 65 exhibited antagonistic activity, among which only one representative isolation, Brevibacillus halotolerans B-4359 was selected based on its in vitro antagonistic activity and enzyme production. Cell-free culture filtrate (CF) and volatile organic compounds (VOCs) of B-4359 were shown to be effective against the mycelial growth of Colletotrichum acutatum. Notably, B-4359 was found to promote spore germination in C. acutatum instead of exhibiting a suppressive effect when the bacterial suspension was mixed with the spore suspension of C. acutatum. However, B-4359 showed an excellent biological control effect on the anthracnose of red pepper fruits. Compared to other treatments and untreated control, B-4359 played a more effective role in controlling anthracnose disease under field conditions. The strain was identified as B. halotolerans using BIOLOG and 16S rDNA sequencing analyses. The genetic mechanism underlying the biocontrol traits of B-4359 was characterized using the whole-genome sequence of B-4359, which was closely compared with related strains. The whole-genome sequence of B-4359 consisted of 5,761,776 bp with a GC content of 41.0%, including 5,118 coding sequences, 117 tRNA, and 36 rRNA genes. The genomic analysis identified 23 putative secondary metabolite biosynthetic gene clusters. Our results provide a deep understanding of B-4359 as an effective biocontrol agent against red pepper anthracnose for sustainable agriculture.

Bacillus VOCs in the Context of Biological Control

A contemporary agricultural production system relying on heavy usage of agrochemicals represents a questionable outlook for sustainable food supply in the future. The visible negative environmental impacts and unforeseen consequences to human and animal health have been requiring a shift towards the novel eco-friendly alternatives for chemical pesticides for a while now. Microbial-based biocontrol agents have shown a promising potential for plant disease management. The bacteria of the genus Bacillus have been among the most exploited microbial active components due to several highly efficient mechanisms of action against plant pathogens, as well as a palette of additional plant-beneficial mechanisms, together with their suitable properties for microbial biopesticide formulations. Among other bioactive metabolites, volatile organic compounds (VOCs) have been investigated for their biocontrol applications, exhibiting the main advantage of long-distance effect without the necessity for direct contact with plants or pathogens. The aim of this study is to give an overview of the state-of-the-art in the field of Bacillus-based VOCs, especially in terms of their antibacterial, antifungal, and nematicidal action as the main segments determining their potential for biocontrol applications in sustainable agriculture.

Fungal pathogens causing postharvest fruit rot of wolfberry and inhibitory effect of 2,3-butanedione

Fungal pathogen contamination is one of the most important factors affecting the postharvest quality and shelf life of wolfberry fruits. Therefore, the prevention and control of fungal pathogens that cause fruit rot has become particularly important. Volatile antifungal agents of biological origin have broad application prospects. They may be safer and more efficient than traditional physical and chemical methods. Four pathogenic fungi were isolated and purified from rotting wolfberry. These pathogenic fungi were determined to be Mucor circinelloides LB1, Fusarium arcuatisporum LB5, Alternaria iridiaustralis LB7, and Colletotrichum fioriniae LB8. In vitro fumigation experiments showed that 2,3-butanedione can effectively inhibit the mycelial growth, spore germination, and sporulation ability of pathogenic fungi. The scanning electron microscope (SEM) showed morphological changes in hyphae. Propidium iodide (PI) Staining and leakage of 260 and 280 nm-absorbing increased, suggesting damage to cell membranes. Furthermore, 2,3-butanedione was found to significantly improve fruit firmness, soluble solid, total phenol, flavonoid, and soluble sugar content, as well as higher SOD enzyme activity and lower PPO and POD enzyme activity in the treated fruit, indicating that 2,3-butanedione can effectively reduce the adverse effects of pathogenic fungi in wolfberry. Based on these results, we conclude that 2,3-butanedione is effective against infection by pathogenic fungi in post-harvest wolfberry. 2,3-butanedione should be considered a viable substitute for conventional fungicides that are currently used to control rot in wolfberry.

Effect of volatile compounds produced by the cotton endophytic bacterial strain Bacillus sp. T6 against Verticillium wilt

BACKGROUND

Verticillium wilt, caused by the fungus Verticillium dahliae, leads to significant losses in cotton yield worldwide. Biocontrol management is a promising means of suppressing verticillium wilt. The purpose of the study was to obtain and analyze endophytic bacteria with Verticillium wilt-resistant activities from the roots of Gossypium barbadense 'Xinhai15' and to explore the interactions between the soil and plants.

RESULTS

An endophytic bacterium Bacillus sp. T6 was obtained from the Verticillium wilt-resistant cotton G. barbadense 'Xinhai15', which showed significant antagonistic abilities against cotton Verticillium wilt. The bioassay results indicated that the strain possessed strong antagonistic abilities that inhibited V. dahliae spore germination and mycelial growth without contact, and thus it was speculated that the active factor of the bacteria might be volatile compounds. A total of 46 volatile substances were detected via headspace solid-phase microextraction and gas chromatography-mass spectrometry analysis. The pure product verification experiment confirmed that the styrene produced by the T6 strain was the main virulence factor. Transcriptome analysis showed that following styrene induction, 247 genes in V. dahliae, including four hydrolase genes, eight dehydrogenase genes, 11 reductase genes, 17 genes related to transport and transfer were upregulated. Additionally, 72 genes, including two chitinase genes, two protease genes, five transport-related genes, and 33 hypothetical protein genes, were downregulated. The quantitative real-time PCR results confirmed that the expression of the four genes VDAG_02838, VDAG_09554, VDAG_045572, and VDAG_08251 was increased by 3.18, 78.83, 2.71, and 2.92 times, respectively, compared with the uninduced control group.

CONCLUSIONS

The research provides a new reference for the development and application of the volatile compounds of endophytic bacteria as new biocontrol agents for the control of Verticillium wilt and as biological preservatives for agricultural products.

Antagonism of volatile organic compounds of the Bacillus sp. against Fusarium kalimantanense

Fusarium kalimantanense is a genetic lineage of Fusarium oxysporum f. sp. cubense (Foc) and belongs to the Fusarium oxysporum species complex (FOSC). This pathogen is a causative agent of Panama disease, an infection that has caused damage to the banana crop worldwide. Bacillus sp. (LPPC170) showed preliminary antagonist activity against F. kalimantanense (LPPC130) in vitro tests from the cultivation of axenic culture and co-culture with inhibition of mycelial growth of phytopathogen of 41.23%. According to these findings, volatile organic compounds (VOCs) emitted from Bacillus sp. were obtained by solid-phase microextraction and identified by gas chromatography coupled with a mass spectrometer (GC-MS). The multivariate data analysis tool (PLS-DA and Heatmap) identified short-chain organic acids as the main antagonistic VOCs responsible for inhibiting the mycelial growth of LPPC130. Acetic acid, propanoic acid, butanoic acid, valeric acid, and isovaleric acid exhibited a strong inhibitory effect on the mycelial growth of LPPC130, with inhibition of 20.68%, 33.30%, 26.87%, 43.71%, and 53.10%, respectively. Scanning electron microscopy revealed that VOCs caused damage to the vegetative and reproductive structures of the fungus. These results suggest Bacillus LPPC170 as an excellent biocontrol tool against the phytopathogen causative agents of Panama disease.

Biocontrol yeast T-2 improves the postharvest disease resistance of grape by stimulation of the antioxidant system

Table grapes are susceptible to external pathogens during postharvest storage. The resulting continuous oxidative stress causes damage and aging, thereby reducing the defense against disease. In this study, the effect of biocontrol yeast T-2 on the storage performance of grapes was evaluated. After T-2 treatment, the grapefruits rot rate and lesion diameter caused by Botrytis cinerea (B. cinerea) were significantly decreased at 2-5 days after inoculation (DAI). Additionally, the browning rate and shedding rate of grapefruit during storage were significantly reduced at 2-5 DAI, and the weight loss rate was significantly reduced at 3-5 DAI. The decreased malondialdehyde (MDA) content in grapefruits at 1-5 DAI with T-2 indicated a reduction in oxidative damage. Furthermore, the activities of antioxidant enzymes such as peroxidase (POD), catalase (CAT), phenylalanin ammonia-lyase (PAL) were significantly increased during most storage time after being treated with T-2. Moreover, the contents of total phenolics and flavonoids and the expression levels of key enzyme genes in metabolic pathways were increased after T-2 treatment during most postharvest storage time, providing evidence that T-2 changed the biological process of phenolic flavonoid metabolism. The increase in enzymatic and nonenzymatic antioxidants after treatment with T-2 reflected the strengthening of the antioxidant system, hence postponing fruit senescence and promoting storage performance under the stress of B. cinerea.

Volatile organic compounds produced by Bacillus velezensis L1 as a potential biocontrol agent against postharvest diseases of wolfberry

Volatile organic compounds (VOCs) produced by antagonistic microorganisms have good biocontrol prospects against postharvest diseases. Infection caused by Alternaria iridiaustralis and 10 other significant fungal diseases can be successfully inhibited by VOCs produced by an identified and screened endophytic strain L1 (Bacillus velezensis). This study revealed the in vivo and in vitro biocontrol effects of VOCs released by B. velezensis L1 on A. iridiaustralis, a pathogenic fungus responsible for rot of wolfberry fruit. The inhibition rates of VOCs of B. velezensis L1 on the mycelial growth of A. iridiaustralis in vitro were 92.86 and 90.30%, respectively, when the initial inoculum concentration on the plate was 1 × 10⁹ colony forming unit (CFU)/ml. Spore germination and sporulation were 66.89 and 87.96%, respectively. VOCs considerably decreased the wolfberry’s disease index and decay incidence in vivo. Scanning electron microscopy revealed that the morphological and structural characteristics of A. iridiaustralis could be altered by VOCs. Ten VOCs were identified through headspace-gas chromatography-ion mobility spectrometry. Pure chemical tests revealed that 2.3-butanedione had the strongest antifungal effects, totally inhibiting A. iridiaustralis in wolfberry fruit at a 60 μl/L concentration. The theory underpinning the potential application of VOCs from B. velezensis is provided herein. This is also the first study to document the antifungal capabilities of the B. velezensis strain on postharvest wolfberry fruit.

GRAPHICAL ABSTRACT

Discovery of Tropolone Stipitaldehyde as a Potential Agent for Controlling Phytophthora Blight and Its Action Mechanism Research

The fermentation of endophytic Nigrospora chinensis GGY-3 resulted in the isolation of tropolone stipitaldehyde (1), which exhibited broad-spectrum inhibition activity against fungi and bacteria, especially against Phytophthora capsici, with an EC₅₀ value of 0.83 μg/mL and Xanthomonas oryzae pv. oryzicola, with a minimum inhibitory concentration value of 4.0 μg/mL. The in vitro and in vivo assays demonstrated that 1 had a significant protective effect on P. capsici. Furthermore, 1 inhibited the spore germination of P. capsici and damaged the plasma membrane structure. As observed by SEM and TEM, after exposure to 1, mycelia exhibited swelling, shrunken, branch-increasing phenomena, cell wall and membrane damage, and disordered content. Transcriptome analysis revealed that 1 might affect starch and sucrose metabolism and fatty acid biosynthesis by suppressing the expression of genes relevant to cell wall synthetases and cell membrane-associated genes. These findings indicate that 1 may be a potential agrochemical fungicide for controlling phytophthora blight.

Antifungal volatile organic compounds from Streptomyces setonii WY228 control black spot disease of sweet potato

Volatile organic compounds (VOCs) produced by microorganisms are considered as promising environmental-safety fumigants for controlling postharvest diseases. Ceratocystis fimbriata, the pathogen of black spot disease, seriously affects the quality and yield of sweet potato in the field and postharvest. This study tested the effects of VOCs produced by Streptomyces setonii WY228 on the control of C. fimbriata in vitro and in vivo. The VOCs exhibited strong antifungal activity and significantly inhibited the growth of C. fimbriata. During the 20-days storage, VOCs fumigation significantly controlled the occurrence of pathogen, increased the content of antioxidant and defense-related enzymes and flavonoids, and boosted the starch content so as to maintain the quality of sweet potato. Headspace analysis showed that volatiles 2-ethyl-5-methylpyrazine and dimethyl disulfide significantly inhibited the mycelial growth and spore germination of C. fimbriata in a dose dependent manner. Fumigation with 100 μL/L 2-ethyl-5-methylpyrazine completely controlled the pathogen in vivo after 10-days storage. Transcriptome analysis showed that volatiles mainly downregulated the ribosomal synthesis genes and activated the proteasome system of pathogen in response to VOCs stress, while the genes related to spore development, cell membrane synthesis, mitochondrial function, as well as hydrolase and toxin synthesis were also downregulated, indicating that WY228-produced VOCs act diverse modes of action for pathogen control. Our study demonstrates that fumigation of sweet potato tuberous roots with S. setonii WY228 or use of formulations based on the VOCs is a promising new strategy to control sweet potato and other food and fruit pathogens during storage and shipment. Importance Black spot disease caused by Ceratocystis fimbriata has caused huge economic losses to worldwide sweet potato production. At present, the control of C. fimbriata mainly depends on toxic fungicides, and there is a lack of effective alternative strategies. The research on biological control of sweet potato black spot disease is also very limited. The development of efficient biocontrol technique against pathogens using microbial volatile organic compounds could be an alternative method to control this disease. Our study revealed the significant biological control effect of volatile organic compounds of Streptomyces setonii WY228 on black spot disease of postharvest sweet potato and the complex antifungal mechanism against C. fimbriata. Our data demonstrated that Streptomyces setonii WY228 and its volatile 2-ethyl-5-methylpyrazine could be candidate strain and compound for the creation of fumigants, and showed the important potential of biotechnology application in the field of food and agriculture.

Phytostimulation and biocontrol potential of Gram-positive endospore-forming Bacilli

The spore-forming Bacillus and Paenibacillus species represent the phyla of beneficial bacteria for application as agricultural inputs in form of effective phytostimulators, biofertilizers, and biocontrol agents. The members of the genera Bacillus and Paenibacillus isolated from several ecological habitats are been thoroughly dissected for their effective application in the development of sustainable and eco-friendly agriculture. Numerous Bacillus and Paenibacillus species are reported as plant growth-promoting bacteria influencing the health and productivity of the food crops. This review narrates the mechanisms utilized by these species to enhance bioavailability and/or facilitate the acquisition of nutrients by the host plant, modulate plant hormones, stimulate host defense and stress resistance mechanisms, exert antagonistic action against soil and airborne pathogens, and alleviate the plant health. The mechanisms employed by Bacillus and Paenibacillus are seldom mutually exclusive. The comprehensive and systematic exploration of the aforementioned mechanisms in conjunction with the field investigations may assist in the exploration and selection of an effective biofertilizer and a biocontrol agent. This review aims to gather and discuss the literature citing the applications of Bacillus and Paenibacillus in the management of sustainable agriculture.

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.

Antifungal Mechanism of Volatile Organic Compounds Produced by Bacillus subtilis CF-3 on Colletotrichum gloeosporioides Assessed Using Omics Technology

Bacillus subtilis is commonly used as a biocontrol bacterium owing to its strong antifungal activity, broad-spectrum inhibition, and general safety. In this study, the inhibitory effects of volatile organic compounds (VOCs) produced by B. subtilis CF-3 on Colletotrichum gloeosporioides, a major destructive phytopathogen of litchi anthracnose, were analyzed using proteomics and transcriptomics. Differentially expressed genes (DEGs) and proteins (DEPs) indicated that the inhibition of C. gloeosporioides by B. subtilis CF-3 VOCs downregulated the expression of genes related to cell membrane fluidity, cell wall integrity, energy metabolism, and production of cell wall-degrading enzymes. Particularly, those important DEGs and DEPs related to the ergosterol biosynthetic and biosynthesis of unsaturated fatty acids are most significantly influenced. 2,4-di-tert-butylphenol, a characteristic component of B. subtilis CF-3 VOCs, also showed a similar effect on C. gloeosporioides. Our results provide a theoretical basis for the potential application of B. subtilis CF-3 in the postharvest protection of fruits and vegetables.

Belowground fungal volatiles perception in okra (Abelmoschus esculentus) facilitates plant growth under biotic stress

Microbial volatile organic compounds (mVOCs) have great potential in plant ecophysiology, yet the role of belowground VOCs in plant stress management remains largely obscure. Analysis of biocontrol producing VOCs into the soil allow detailed insight into their interaction with soil borne pathogens for plant disease management. A root interaction trial was set up to evaluate the effects of VOCs released from Trichoderma viride BHU-V2 on soil-inhabiting fungal pathogen and okra plant growth. VOCs released into soil by T. viride BHU-V2 inhibited the growth of collar rot pathogen, Sclerotium rolfsii. Okra plants responded to VOCs by increasing the root growth (lateral roots) and total biomass content. VOCs exposure increased defense mechanism in okra plants by inducing different enzyme activities i.e. chitinase (0.89 fold), β-1,3-glucanase (0.42 fold), peroxidase (0.29 fold), polyphenol oxidase (0.33 fold) and phenylalanine lyase (0.7 fold) when inoculated with S. rolfsii. In addition, T. viride BHU-V2 secreted VOCs reduced lipid peroxidation and cell death in okra plants under pathogen inoculated condition. GC/MS analysis of VOCs blend revealed that T. viride BHU-V2 produced more number of antifungal compounds in soil medium as compared to standard medium. Based on the above observations it is concluded that okra plant roots perceive VOCs secreted by T. viride BHU-V2 into soil that involved in induction of plant defense system against S. rolfsii. In an ecological context, the findings reveal that belowground microbial VOCs may play an important role in stress signaling mechanism to interact with plants.

The inhibitory effect of volatile organic compounds produced by Bacillus subtilis CL2 on pathogenic fungi of wolfberry

Bacillus subtilis strain CL2 is antagonistic to wolfberry postharvest pathogenic fungi. In this study, we isolated and screened this strain for in vitro experiments. The result of the two-sealed-base-plates method revealed that volatile organic compounds (VOCs) emitted from the strain CL2 inhibited the hyphal growth of four pathogenic fungi Mucor circinelloides LB1, Fusarium arcuatisporum LB5, Alternaria iridiaustralis LB7, and Colletotrichum fioriniae LB8. After exposure to VOCs for 5 days, the hyphal growth of the pathogen C. fioriniae LB8 was inhibited by 73%. Scanning electron microscopy revealed that the VOCs produced by B. subtilis CL2 caused the mycelium morphology of the pathogenic fungi to deform, twist, fold, and shrink. In the in vivo experiments, we noticed that VOCs could significantly reduce the weight loss rate of wolfberry fruits caused by the pathogenic fungus M. circinelloides LB1 and that the decay incidence rate were caused by the pathogenic fungi F. arcuatisporum LB5, A. iridiaustralis LB7, and C. fioriniae LB8. On the basis of the headspace-gas chromatography-ion mobility spectrometry analysis, seven VOCs produced by strain CL2 were identified. Among them, 2,3-butanedione and 3-methylbutyric acid are the main antifungal active substances. This study investigated the antifungal properties of VOCs produced by the strain CL2 on postharvest pathogenic fungi isolated from wolfberry fruits both in vivo and in vitro, thereby providing the theoretical basis for its future applications.

Bacillus mycoides A1 and Bacillus tequilensis A3 inhibit the growth of a member of the phytopathogen Colletotrichum gloeosporioides species complex in avocado

BACKGROUND

Avocado is affected by Colletotrichum gloeosporioides causing anthracnose. Antagonistic microorganisms against C. gloeosporioides represent an alternative for biological control. Accordingly, in the present study, we focused on the isolation and characterization of potential antagonist bacteria against a member of the C. gloeosporioides species complex with respect to their possible future application.

RESULTS

Samples of avocado rhizospheric soil were aquired from an orchard located in Ocuituco, Morelos, Mexico, aiming to obtain bacterial isolates with potential antifungal activity. From the soil samples, 136 bacteria were isolated and they were then challenged against a member of the C. gloeosporioides species complex; only three bacterial isolates A1, A2 and A3 significantly diminished mycelial fungal growth by 75%, 70% and 60%, respectively. Two of these isolates were identified by 16S rRNA as Bacillus mycoides (A1 and A2) and the third was identified as Bacillus tequilensis (A3). Bacillus mycoides bacterial cell-free supernatant reduced the mycelial growth of a member of the C. gloeosporioides species complex isolated from avocado by 65%, whereas Bacillus tequilensis A3 supernatant did so by 25% after 3 days post inoculation. Bacillus tequilensis mycoides A1 was a producer of proteases, indolacetic acid and siderophores. Preventive treatment using a cell-free supernatant of B. mycoides A1 diminished the severity of anthracnose disease (41.9%) on avocado fruit.

CONCLUSION

These results reveal the possibility of using B. mycoides A1 as a potential biological control agent. © 2020 Society of Chemical Industry.

Effect of Marine Bacteria and Ulvan on the Activity of Antioxidant Defense Enzymes and the Bio-Protection of Papaya Fruit against Colletotrichum gloeosporioides

Anthracnose, caused by Colletotrichum gloeosporioides, is one of the most important diseases in papaya fruit. Its control has been achieved with synthetic fungicides, but the application of marine bacteria and the sulphated polysaccharide ulvan (structural description: β[1,4]-D-GlcA-α[1,4]-L-Rha 3 sulfate, β[1,4]-L-IdoA-α[1,4]-L-Rha 3 sulfate, β[1,4]-D-Xyl-α[1,4]-L-Rha 3 sulfate, and β[1,4]-D-Xyl 2-sulfate-α[1,4]-L-Rha 3 sulfate) from Ulva sp. can be an alternative in the use of agrochemicals. Thus, the objective of this study was to assess the effect in vitro and in vivo of two marine bacteria, Stenotrophomonas rhizophila and Bacillus amyloliquefaciens, and ulvan in papaya fruit’s bio-protection against C. gloeosporioides. The capacity of marine bacteria to inhibit mycelial growth and phytopathogen spore germination in vitro through volatile organic compounds (VOCs) and carbohydrate competition was evaluated. Fruit was inoculated with bacteria, ulvan, and C. gloeosporioides and incubated at 25 °C and 90% of relative humidity (RH) for seven days. Disease incidence (%), lesion diameter (mm), and antioxidant defense enzyme activity (such as superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) were quantified. In vitro, C. gloeosporioides was inhibited by S. rhizophila and B. amyloliquefaciens. In vivo, disease incidence and the lesion diameter of anthracnose on papaya fruit were significantly reduced by marine bacteria and ulvan. Antioxidant defense enzyme activity played an important role in fruit bio-protection against C. gloeosporioides. The application of marine bacteria and ulvan can be an alternative in the sustainable postharvest management of papaya.