Suppression of Sclerotinia sclerotiorum by the Induction of Systemic Resistance and Regulation of Antioxidant Pathways in Tomato Using Fengycin Produced by Bacillus amyloliquefaciens FZB42

Genomic Analysis and Secondary Metabolites Production of the Endophytic Bacillus velezensis Bvel1: A Biocontrol Agent against Botrytis cinerea Causing Bunch Rot in Post-Harvest Table Grapes

Botrytis bunch rot caused by Botrytis cinerea is one of the most economically significant post-harvest diseases of grapes. In the present study, we showed that the bacterial strain Bvel1 is phylogenetically affiliated to Bacillus velezensis species. The strain Bvel1 and its secreted metabolites exerted an antifungal activity, under in vitro conditions, against B. cinerea. UHPLC-HRMS chemical analysis revealed that iturin A2, surfactin-C13 and -C15, oxydifficidin, bacillibactin, L-dihydroanticapsin, and azelaic acid were among the metabolites secreted by Bvel1. Treatment of wounded grape berries with Bacillus sp. Bvel1 cell culture was effective for controlling grey mold ingress and expansion in vivo. The effectiveness of this biological control agent was a function of the cell culture concentration of the antagonist applied, while preventive treatment proved to be more effective compared to curative. The strain Bvel1 exhibited an adequate colonization efficiency in wounded grapes. The whole-genome phylogeny, combined with ANI and dDDH analyses, provided compelling evidence that the strain Bvel1 should be taxonomically classified as Bacillus velezensis. Genome mining approaches showed that the strain Bvel1 harbors 13 antimicrobial biosynthetic gene clusters, including iturin A, fengycin, surfactin, bacilysin, difficidin, bacillaene, and bacillibactin. The results provide new insights into the understanding of the endophytic Bacillus velezensis Bvel1 biocontrol mechanism against post-harvest fungal pathogens, including bunch rot disease in grape berries.

Activity of Fengycin and Iturin A Isolated From Bacillus subtilis Z-14 on Gaeumannomyces graminis Var. tritici and Soil Microbial Diversity

Bacillus subtilis Z-14 can inhibit phytopathogenic fungi, and is used as a biocontrol agent for wheat take-all disease. The present study used the soil-borne fungus Gaeumannomyces graminis var. tritici (Ggt), which causes wheat take-all disease, and the soil microbial community as indicators, and investigated the antifungal effects of fengycin and iturin A purified from strain Z-14 using high performance liquid chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, respectively. The results showed that fengycin destroyed the internal structure of Ggt cells by digesting the cytoplasm and organelles, forming vacuoles, and inducing hyphal shrinkage and distortion. Iturin A induced cell wall disappearance, membrane degeneration, intracellular material shrinkage, and hyphal fragmentation. A biocontrol test demonstrated a 100% control effect on wheat take-all when wheat seedlings were treated with fengycin at 100 μg/ml or iturin A at 500 μg/ml. Iturin A and fengycin both reduced the relative abundance of Aspergillus and Gibberella. At the genus level, iturin A reduced the relative abundance of Mortierella and Myrothecium, while fengycin reduced that of Fusarium. Only fengycin treatment for 7 days had a significant effect on soil bacterial diversity.

Wuyiencin produced by Streptomyces albulus CK-15 displays biocontrol activities against cucumber powdery mildew

AIMS

Wuyiencin is a nucleoside antibiotic produced by Streptomyces albulus CK-15. The aim of this study was to determine whether wuyiencin can be used, as a suitable alternative to chemical pesticides, to protect cucumbers (Cucumis sativus L.) from powdery mildew caused by Sphaerotheca fuliginea. Further, the mechanisms underlying the control of cucumber powdery mildew by S. albulus CK-15 were preliminarily elucidated.

METHODS AND RESULTS

Wuyiencin solutions of different concentrations were used to treat infected cucumber plants under greenhouse conditions. The results indicated that wuyiencin could significantly reduce powdery mildew disease incidence, with a maximum prevention efficacy of 94·38%. Further, scanning electron micrographs and enzyme assays showed that wuyiencin inhibited S. fuliginea spore growth and elicited the activity of plant systemic resistance-related enzymes. Additionally, real-time quantitative reverse transcription PCR suggested that wuyiencin can activate a salicylic acid-dependent plant defence response.

CONCLUSIONS

Wuyiencin produced by S. albulus CK-15 possessed antifungal effects and was able to mitigate cucumber powdery mildew disease via antagonistic action. Wuyiencin also induced defence responses in the plants.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results reinforce the biotechnological potential of wuyiencin as both an antagonistic agent and an inducer of plant systemic resistance.

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.

Study on the Biocontrol Potential of Antifungal Peptides Produced by Bacillus velezensis against Fusarium solani That Infects the Passion Fruit Passiflora edulis

A bacterium identified as Bacillus velezensis with a growth inhibitory effect against Fusarium solani, a pathogen that caused basal stem rot in the passion fruit Passiflora edulis, was isolated in this study. From the fermentation broth of B. velezensis, a type of antifungal peptide (named BVAP) with a molecular weight of ca. 1.5 kDa was purified and found to be fengycin. BVAP suppressed mycelial growth in F. solani with an IC₅₀ of 5.58 μg/mL, which was superior to those of the chemical fungicides thiram (41.24 μg/mL) and hymexazol (343.31 μg/mL). The antifungal activity remained stable after exposure to 50-100 °C or following incubation with solutions at pH 1-3. Further research revealed that BVAP increased the permeability of the F. solani mycelial membrane, brought about swelling at the tips of hyphae, and elicited abnormal accumulation of nucleic acids and chitin at the sites of swelling. These findings indicate that BVAP possessed a remarkable biocontrol potential toward F. solani.

Isolation and yield optimization of lipopeptides from Bacillus subtilis Z-14 active against wheat take-all caused by Gaeumannomyces graminis var. tritici

Wheat take-all, caused by the soil-borne fungus Gaeumannomyces graminis var. tritici, is one of the major constraints on wheat production worldwide. Bacillus subtilis Z-14 exerts significant biocontrol activity against wheat take-all, and lipopeptide antibiotics are the main antifungal substances. Herein, lipopeptide antibiotics C14-C15 iturin A, C14-C16 fengycin A, and C15-C17 fengycin B from B. subtilis Z-14 culture filtrates were separated and identified by high-performance liquid chromatography, matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry, and mass spectrometry/mass spectrometry, respectively. The optimal medium components for Z-14 lipopeptide antibiotic production were 3.85 g/L corn flour, 1.57 g/L soybean meal, 0.03 g/L FeSO₄ ·7H₂ O, 0.2 g/L NaH₂ PO₄ ·2H₂ O, and 0.4 g/L Na₂ HPO₄ ·2H₂ O. Quantification analysis by high-performance liquid chromatography showed that fengycins played a main role in antifungal activity against Gaeumannomyces graminis var. tritici. Quantitative reverse transcription polymerase chain reaction showed that lipopeptide synthesis genes fenD and ituC reached maximum expression levels after 48 h of fermentation. The strongest control of wheat take-all by Z-14 was achieved by adding 30 mL of culture filtrate per 350 g of soil in pot experiments, during which disease reduction reached 88.15%. This study provides theoretical support and a material basis for the prevention and treatment of wheat take-all disease.

Isolation and Characterization of Fengycins Produced by Bacillus amyloliquefaciens JFL21 and Its Broad-Spectrum Antimicrobial Potential Against Multidrug-Resistant Foodborne Pathogens

The continuing emergence and development of pathogenic microorganisms that are resistant to antibiotics constitute an increasing global concern, and the effort in new antimicrobials discovery will remain relevant until a lasting solution is found. A new bacterial strain, designated JFL21, was isolated from seafood and identified as B. amyloliquefaciens. The antimicrobial substance produced by B. amyloliquefaciens JFL21 showed low toxicity to most probiotics but exhibited strong antimicrobial activities against multidrug-resistant foodborne pathogens. The partially purified antimicrobial substance, Anti-JFL21, was characterized to be a multiple lipopeptides mixture comprising the families of surfactin, fengycin, and iturin. Compared with commercially available polymyxin B and Nisin, Anti-JFL21 not only could exhibit a wider and stronger antibacterial activity toward Gram-positive pathogens but also inhibit the growth of a majority of fungal pathogens. After further separation through gel filtration chromatography (GFC), the family of surfactin, fengycin, and iturin were obtained, respectively. The results of the antimicrobial test pointed out that only fengycin family presented marked antimicrobial properties against the indicators of L. monocytogenes, A. hydrophila, and C. gloeosporioides, which demonstrated that fengycins might play a major role in the antibacterial and antifungal activity of Anti-JFL21. Additionally, the current study also showed that the fengycins produced by B. amyloliquefaciens JFL21 not only maintained stable anti-Listeria activity over a broad pH and temperature range, but also remained active after treatment with ultraviolet sterilization, chemical reagents, and proteolytic enzymes. Therefore, the results of this study suggest the new strain and its antimicrobials are potentially useful in food preservation for the biological control of the multidrug-resistant foodborne pathogens.

Antagonistic Potential of Novel Endophytic Bacillus Strains and Mediation of Plant Defense against Verticillium Wilt in Upland Cotton

Verticillium wilt caused by Verticillium dahliae is a threatening disease of cotton, causing economic loss worldwide. In this study, nine endophytic Bacillus strains isolated from cotton roots exhibited inhibitory activity against V. dahliae strain VD-080 in a dual culture assay. B. altitudinis HNH7 and B. velezensis HNH9 were chosen for further experiments based on their high antagonistic activity. The secondary metabolites of HNH7 and HNH9 also inhibited the growth of VD-080. Genetic marker-assisted detection revealed the presence of bacillibactin, surfactin, bacillomycin and fengycin encoding genes in the genome of HNH7 and HNH9 and their corresponding gene products were validated through LC-MS. Scanning electron microscopy revealed mycelial disintegration, curling and shrinkage of VD-080 hyphae after treatment with methanolic extracts of the isolated endophytes. Furthermore, a significant reduction in verticillium wilt severity was noticed in cotton plants treated with HNH7 and HNH9 as compared to control treatments. Moreover, the expression of defense-linked genes, viz., MPK3, GST, SOD, PAL, PPO and HMGR, was considerably higher in plants treated with endophytic Bacillus strains and inoculated with VD-080 as compared to control.

Elicitors of Plant Immunity Triggered by Beneficial Bacteria

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