Novel screening strategy reveals a potent Bacillus antagonist capable of mitigating wheat take-all disease caused by Gaeumannomyces graminis var. tritici

Antifungal mechanisms of the antagonistic bacterium Bacillus mojavensis UTF-33 and its potential as a new biopesticide

Biological control has gradually become the dominant means of controlling fungal disease over recent years. In this study, an endophytic strain of UTF-33 was isolated from acid mold (Rumex acetosa L.) leaves. Based on 16S rDNA gene sequence comparison, and biochemical and physiological characteristics, this strain was formally identified as Bacillus mojavensis. Bacillus mojavensis UTF-33 was sensitive to most of the antibiotics tested except neomycin. Moreover, the filtrate fermentation solution of Bacillus mojavensis UTF-33 had a significant inhibitory effect on the growth of rice blast and was used in field evaluation tests, which reduced the infestation of rice blast effectively. Rice treated with filtrate fermentation broth exhibited multiple defense mechanisms in response, including the enhanced expression of disease process-related genes and transcription factor genes, and significantly upregulated the gene expression of titin, salicylic acid pathway-related genes, and H₂O₂ accumulation, in plants; this may directly or indirectly act as an antagonist to pathogenic infestation. Further analysis revealed that the n-butanol crude extract of Bacillus mojavensis UTF-33 could retard or even inhibit conidial germination and prevent the formation of adherent cells both in vitro and in vivo. In addition, the amplification of functional genes for biocontrol using specific primers showed that Bacillus mojavensis UTF-33 expresses genes that can direct the synthesis of bioA, bmyB, fenB, ituD, srfAA and other substances; this information can help us to determine the extraction direction and purification method for inhibitory substances at a later stage. In conclusion, this is the first study to identify Bacillus mojavensis as a potential agent for the control of rice diseases; this strain, and its bioactive substances, have the potential to be developed as biopesticides.

Metabolomic and Transcriptome Analysis of the Inhibitory Effects of Bacillus subtilis Strain Z-14 against Fusarium oxysporum Causing Vascular Wilt Diseases in Cucumber

Controlling cucumber Fusarium wilt caused by Fusarium oxysporum f. sp. cucumerinum (FOC) with Bacillus strains is a hot research topic. However, the molecular mechanism of Bacillus underlying the biocontrol of cucumber wilt is rarely reported. In this study, B. subtilis strain Z-14 showed significant antagonistic activity against FOC, and the control effect reached 88.46% via pot experiment. Microscopic observations showed that strain Z-14 induced the expansion and breakage of FOC hyphae. The cell wall thickness was uneven, and the organelle structure was degraded. The combined analysis of metabolome and transcriptome showed that strain Z-14 inhibited the FOC infection by inhibiting the synthesis of cell wall and cell membrane, energy metabolism, and amino acid synthesis of FOC mycelium, inhibiting the clearance of reactive oxygen species (ROS) and the secretion of cell wall-degrading enzymes (CWDEs), thereby affecting mitogen-activated protein kinase (MAPK) signal transduction and inhibiting the transport function.

Influence of Bacillus subtilis strain Z-14 on microbial communities of wheat rhizospheric soil infested with Gaeumannomyces graminis var. tritici

Wheat take-all disease caused by Gaeumannomyces graminis var. tritici (Ggt) spreads rapidly and is highly destructive, causing severe reductions in wheat yield. Bacillus subtilis strain Z-14 that significantly controlled wheat take-all disease effectively colonized the roots of wheat seedlings. Z-14 increased the metabolic activity and carbon source utilization of rhizospheric microorganisms, thus elevating average well-color development (AWCD) values and functional diversity indexes of soil microbial communities. Z-14 increased the abundance of Bacillus in the rhizosphere, which was positively correlated with AWCD and functional diversity indexes. The Z-14-treated samples acquired more linkages and relative connections between bacterial communities according to co-occurrence network analyses. After the application of Ggt, the number of linkages between fungal communities increased but later decreased, whereas Z-14 increased such interactions. Whole-genome sequencing uncovered 113 functional genes related to Z-14’s colonization ability and 10 secondary metabolite gene clusters in the strain, of which nine substances have antimicrobial activity. This study clarifies how bacterial agents like Z-14 act against phytopathogenic fungi and lays a foundation for the effective application of biocontrol agents.

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.

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, heterologous expression, and purification of a novel antifungal protein from Bacillus subtilis strain Z-14

BACKGROUND

Wheat sheath blight, a soil borne fungal disease caused by Rhizoctonia cerealis, is considered as one of the most serious threats to wheat worldwide. Bacillus subtilis Z-14 was isolated from soil sampled from a wheat rhizosphere and was confirmed to have strong antifungal activity against R. cerealis.

RESULTS

An antifungal protein, termed F2, was isolated from the culture supernatant of Z-14 strain using precipitation with ammonium sulfate, anion exchange chromatography, and reverse phase chromatography. Purified F2 had a molecular mass of approximately 8 kDa, as assessed using sodium dodecyl sulfate polyacrylamide gel electrophoresis. Edman degradation was used to determine the amino acid sequence of the N-terminus, which was NH₂ASGGTVGIYGANMRS. This sequence is identical to a hypothetical protein RBAM_004680 (YP_001420098.1) synthesized by B. amyloliquefaciens FZB42. The recombinant F2 protein (rF2) was heterologously expressed in the yeast host Pichia pastoris, purified using a Niaffinity column, and demonstrated significant antifungal activity against R. cerealis. The purified rF2 demonstrated broad spectrum antifungal activity against different varieties of fungi such as Fusarium oxysporum, Verticillium dahliae, Bipolaris papendorfii, and Fusarium proliferatum. rF2 was thermostable, retaining 91.5% of its activity when incubated for 30 min at 100 °C. Meanwhile, rF2 maintained its activity under treatment by proteinase K and trypsin and over a wide pH range from 5 to 10.

CONCLUSIONS

A novel antifungal protein, F2, was purified from biocontrol Bacillus subtilis Z-14 strain fermentation supernatant and heterologously expressed in Pichia pastoris to verify its antifungal activity against R. cerealis and the validity of the gene encoding F2. Considering its significant antifungal activity and stable characteristics, protein F2 presents an alternative compound to resist fungal infections caused by R. cerealis.

Synthesis of 1, 2, 4-triazole benzoyl arylamine derivatives and their high antifungal activities

Two series of novel 1, 2, 4-triazole benzoyl arylamine derivatives were prepared and screened for their activities against three pathogens of Gaeumannomyces graminis var.tritici, Sclerotinia sclerotiorum and Fusarium graminearum using the mycelium growth inhibition method in vitro. The results indicated that most of the synthesized derivatives displayed antifungal activities. Compounds 6c-d and 6f-g exhibited lower EC₅₀s against all the three pathogens. Among of them, the compound 6g displayed the most potent antifungal activities with EC₅₀ values of 0.01, 0.19 and 0.12 μg mL^-1 respectively. The structure and activity relationship showed that election-withdrawing group at pata-position of aniline was favorable for high activities, and the preferred groups were alkoxy carbonyls. These results proposed that the compound 6g can be a lead compound for development of novel fungicide.

Bacillus lipopeptides as powerful pest control agents for a more sustainable and healthy agriculture: recent studies and innovations

Lipopeptides could help to overcome a large concern in agriculture: resistance against chemical pesticides. These molecules have activity against various phytopathogens and a potential to be transformed by genetic engineering. The exponential rise of pest resistances to different chemical pesticides and the global appeal of consumers for a sustainable agriculture and healthy nutrition have led to the search of new solutions for pest control. Furthermore, new laws require a different stance of producers. Based on that, bacteria of the genus Bacillus present a great agricultural potential, producing lipopeptides (LPs) that have high activity against insects, mites, nematodes, and/or phytopathogens that are harmful to plant cultures. Biopesticide activity can be found mainly in three families of Bacillus lipopeptides: surfactin, iturin, and fengycin. These molecules have an amphiphilic nature, interfering with biological membrane structures. Their antimicrobial properties include activity against bacteria, fungi, oomycetes, and viruses. Recent studies also highlight the ability of these compounds to stimulate defense mechanisms of plants and biofilm formation, which is a key factor for the successful colonization of biocontrol organisms. The use of molecular biology has also recently been researched for continuous advances and discoveries of new LPs, avoiding possible future problems of resistance against these molecules. As a consequence of the properties and possibilities of LPs, numerous studies and developments as well as the attention of large companies in the field is expected in the near future.

Inhibitory Abilities of Bacillus Isolates and Their Culture Filtrates against the Gray Mold Caused by Botrytis cinerea on Postharvest Fruit

Botrytis cinerea, a major phytopathogenic fungus, has been reported to infect more than 200 crop species worldwide, and it causes massive losses in yield. The aim of this study was to evaluate the inhibitory abilities and effects of Bacillus amyloliquefaciens RS-25, Bacillus licheniformis MG-4, Bacillus subtilis Z-14, and Bacillus subtilis Pnf-4 and their culture filtrates and extracts against the gray mold caused by B. cinerea on postharvest tomato, strawberry, and grapefruit. The results revealed that the cells of Z-14, culture filtrate of RS-25, and cells of Z-14 showed the strongest biocontrol activity against the gray mold on the strawberry, grape, and tomato fruit, respectively. All the strains produced volatile organic compounds (VOCs), and the VOCs of Pnf-4 displayed the highest inhibition values. Based on headspace solid-phase microextraction in combination with gas chromatography-mass spectrometry, esters accounted for the largest percentage of the VOCs produced by RS-25, MG-4, Z-14, and Pnf-4 (36.80%, 29.58%, 30.78%, and 36.26%, respectively). All the strains showed potent cellulase and protease activities, but no chitinase activity. RS-25, Z-14, and MG-4, but not Pnf-4, grew on chrome azurol S agar, and an orange halo was formed around the colonies. All the strains showed biofilm formation, fruit colonization, and lipopeptide production, which may be the main modes of action of the antagonists against B. cinerea on the fruit. This study provides the basis for developing natural biocontrol agents against the gray mold caused by B. cinerea on postharvest fruit.

The Highly Conserved Barley Powdery Mildew Effector BEC1019 Confers Susceptibility to Biotrophic and Necrotrophic Pathogens in Wheat

Effector proteins secreted by plant pathogens play important roles in promoting colonization. Blumeria effector candidate (BEC) 1019, a highly conserved metalloprotease of Blumeria graminis f. sp. hordei (Bgh), is essential for fungal haustorium formation, and silencing BEC1019 significantly reduces Bgh virulence. In this study, we found that BEC1019 homologs in B. graminis f. sp. tritici (Bgt) and Gaeumannomyces graminis var. tritici (Ggt) have complete sequence identity with those in Bgh, prompting us to investigate their functions. Transcript levels of BEC1019 were abundantly induced concomitant with haustorium formation in Bgt and necrosis development in Ggt-infected plants. BEC1019 overexpression considerably increased wheat susceptibility to Bgt and Ggt, whereas silencing this gene using host-induced gene silencing significantly enhanced wheat resistance to Bgt and Ggt, which was associated with hydrogen peroxide accumulation, cell death, and pathogenesis-related gene expression. Additionally, we found that the full and partial sequences of BEC1019 can trigger cell death in Nicotiana benthamiana leaves. These results indicate that Bgt and Ggt can utilize BEC1019 as a virulence effector to promote plant colonization, and thus these genes represent promising new targets in breeding wheat cultivars with broad-spectrum resistance.