In vitro antifungal activity of dimethyl trisulfide against Colletotrichum gloeosporioides from mango

Suppression of anthracnose disease by orsellinaldehyde isolated from the mushroom Coprinus comatus

AIMS

Anthracnose caused by Colletotrichum species is one of the most devastating diseases of fruits and crops. We isolated and identified an antifungal compound from the mushroom Coprinus comatus and investigated its inhibitory potential against anthracnose disease-causing fungi with the goal of discovering natural products that can suppress anthracnose-caused plant disease.

METHODS AND RESULTS

The culture filtrate of C. comatus was subjected to a bioassay-guided isolation of antifungal compounds. The active compound was identified as orsellinaldehyde (2,4-dihydroxy-6-methylbenzaldehyde) based on mass spectroscopy and nuclear magnetic resonance analyses. Orsellinaldehyde displayed broad-spectrum inhibitory activity against different plant pathogenic fungi. Among the tested Colletotrichum species, it exhibited the lowest IC50 values on conidial germination and germ tube elongation of Colletotrichum orbiculare. The compound also showed remarkable inhibitory activity against Colletotrichum gloeosporiodes. The staining of Colletotrichum conidia with fluorescein diacetate and propidium iodide demonstrated that the compound is fungicidal. The postharvest in-vivo detached fruit assay indicated that orsellinaldehyde suppressed anthracnose lesion symptoms on mango and cucumber fruits caused by C. gloeosporioides and C. orbiculare, respectively.

CONCLUSIONS

Orsellinaldehyde was identified as a potent antifungal compound from the culture filtrate of C. comatus. The inhibitory and fungicidal activities of orsellinaldehyde against different Colletotrichum species indicate its potential as a fungicide for protecting various fruits against anthracnose disease-causing fungi.

Dimethylpolysulfides production as the major mechanism behind wheat fungal pathogen biocontrol, by Arthrobacter and Microbacterium actinomycetes

IMPORTANCE

As the management of wheat fungal diseases becomes increasingly challenging, the use of bacterial agents with biocontrol potential against the two major wheat phytopathogens, Fusarium graminearum and Zymoseptoria tritici, may prove to be an interesting alternative to conventional pest management. Here, we have shown that dimethylpolysulfide volatiles are ubiquitously and predominantly produced by wheat-associated Microbacterium and Arthrobacter actinomycetes, displaying antifungal activity against both pathogens. By limiting pathogen growth and DON virulence factor production, the use of such DMPS-producing strains as soil biocontrol inoculants could limit the supply of pathogen inocula in soil and plant residues, providing an attractive alternative to dimethyldisulfide fumigant, which has many non-targeted toxicities. Notably, this study demonstrates the importance of bacterial volatile organic compound uptake by inhibited F. graminearum, providing new insights for the study of volatiles-mediated toxicity mechanisms within bacteria-fungus signaling crosstalk.

Transcriptomic and biochemical analyses revealed antifungal mechanism of trans-anethole on Aspergillus flavus growth

Plant volatile compounds have great potential for preventing and controlling fungal spoilage in post-harvest grains. Recently, we have reported the antifungal effects of trans-anethole, the main volatile constituent of the Illicium verum fruit, on Aspergillus flavus. In this study, the inhibitory mechanisms of trans-anethole against the growth of A. flavus mycelia were investigated using transcriptomic and biochemical analyses. Biochemical and transcriptomic changes in A. flavus mycelia were evaluated after exposure to 0.2 μL/mL trans-anethole. Scanning electron microscopy showed that trans-anethole treatment resulted in the surface wrinkling of A. flavus mycelia, and calcofluor white staining confirmed that trans-anethole treatment disrupted the mycelial cell wall structure. Annexin V-fluorescein isothiocyanate/propidium iodide double staining suggested that trans-anethole induced apoptosis in A. flavus mycelia. Reduced mitochondrial membrane potential and DNA damage were observed in trans-anethole-treated A. flavus mycelia using 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethyl-imidacarbocyanine and 4',6-diamidino-2-phenylindole staining, respectively. 2',7'- Dichloro-dihydro-fluorescein diacetate staining and biochemical assays demonstrated that trans-anethole treatment cause the accumulation of reactive oxygen species in the A. flavus mycelia. Transcriptome results showed that 1673 genes were differentially expressed in A. flavus mycelia exposed to trans-anethole, which were mainly associated with multidrug transport, oxidative phosphorylation, citric acid cycle, ribosomes, and cyclic adenosine monophosphate signaling. We propose that trans-anethole can inhibit the growth of A. flavus mycelia by disrupting the cell wall structure, blocking the multidrug transport process, disturbing the citric acid cycle, and inducing apoptosis. This study provides new insights into the inhibitory mechanism of trans-anethole on A. flavus mycelia and will be helpful for the development of natural fungicides. KEY POINTS: • Biochemical analyses of A. flavus mycelia exposed to trans-anethole were performed • Transcriptomic changes in trans-anethole-treated A. flavus mycelia were analyzed • An inhibitory mechanism of trans-anethole on the growth of A. flavus mycelia was proposed.

Inhibitory effects of hinokitiol on the development and pathogenicity of Colletotrichum gloeosporioides

Postharvest anthracnose of mango fruit caused by Colletotrichum gloeosporioides is a devastating fungal disease, which causes tremendous quality deterioration and economic losses. Hinokitiol, an environmentally friendly natural compound, is effective in controlling a variety of postharvest fungal diseases. However, there is still a lack of research on the inhibitory effect of hinokitiol on C. gloeosporioides and its possible modes of action. In the present study, the activity of hinokitiol against C. gloeosporioides and its potential mechanisms involved have been investigated. We found that hinokitiol treatment could effectively inhibit the virulence of C. gloeosporioides to harvested mango fruit. After treatment with 8 mg/L hinokitiol, the mycelial growth of C. gloeosporioides was completely inhibited. When the concentration of hinokitiol reached 9 mg/L, the spore germination rate of C. gloeosporioides decreased to 2.43% after 9 h of cultivation. The inhibitory effect is mainly due to the attenuation in cell viability, and impairment in plasma membrane followed by leakage of cytoplasmic contents such as nucleic acids, proteins, and soluble carbohydrates, which ultimately leads to the destruction of cell structure. Furthermore, hinokitiol suppressed the expression of pathogenicity-related genes, leading to reduced infection activity. Collectively, these results suggest that hinokitiol may be an excellent bio-fungicides for the management of mango anthracnose.

The Inhibitory Effect of Pseudomonas stutzeri YM6 on Aspergillus flavus Growth and Aflatoxins Production by the Production of Volatile Dimethyl Trisulfide

Aspergillus flavus and the produced aflatoxins cause great hazards to food security and human health across all countries. The control of A. flavus and aflatoxins in grains during storage is of great significance to humans. In the current study, bacteria strain YM6 isolated from sea sediment was demonstrated effective in controlling A. flavus by the production of anti-fungal volatiles. According to morphological characteristics and phylogenetic analysis, strain YM6 was identified as Pseudomonas stutzeri. YM6 can produce abundant volatile compounds which could inhibit mycelial growth and conidial germination of A. flavus. Moreover, it greatly prevented fungal infection and aflatoxin production on maize and peanuts during storage. The inhibition rate was 100%. Scanning electron microscopy further supported that the volatiles could destroy the cell structure of A. flavus and prevent conidia germination on the grain surface. Gas chromatography/mass spectrometry revealed that dimethyl trisulfide (DMTS) with a relative abundance of 13% is the most abundant fraction in the volatiles from strain YM6. The minimal inhibitory concentration of DMTS to A. flavus conidia is 200 µL/L (compound volume/airspace volume). Thus, we concluded that Pseudomonas stutzeri YM6 and the produced DMTS showed great inhibition to A. flavus, which could be considered as effective biocontrol agents in further application.

The Metabolic Potential of Endophytic Actinobacteria Associated with Medicinal Plant Thymus roseus as a Plant-Growth Stimulator

Bio-fertilizer practice considers not only economical but also environmentally friendly, sustainable agriculture. Endophytes can play important beneficiary roles in plant development, directly, indirectly, or synergistically. In this study, the majority of our endophytic actinobacteria were able to possess direct plant growth-promoting (PGP) traits, including auxin (88%), ammonia (96%), siderophore production (94%), and phosphate solubilization (24%), along with cell-wall degrading enzymes such as protease (75%), cellulase (81%), lipase (81%), and chitinase (18%). About 45% of tested strains have an inhibitory effect on the phytopathogen Fusarium oxysporum, followed by 26% for Verticillium dahlia. Overall, our results showed that strains XIEG63 and XIEG55 were the potent strains with various PGP traits that caused a higher significant increase (p ≤ 0.05) in length and biomass in the aerial part and roots of tomato and cotton, compared to the uninoculated plants. Our data showed that the greatest inhibition percentages of two phytopathogens were achieved due to treatment with strains XIEG05, XIEG07, XIEG45, and XIEG51. The GC-MS analysis showed that most of the compounds were mainly alkanes, fatty acid esters, phenols, alkenes, and aromatic chemicals and have been reported to have antifungal activity. Our investigation emphasizes that endophytic actinobacteria associated with medicinal plants might help reduce the use of chemical fertilization and potentially lead to increased agricultural productivity and sustainability.

Antifungal activity of non-conventional yeasts against Botrytis cinerea and non-Botrytis grape bunch rot fungi

Grapes harbour a plethora of non-conventional yeast species. Over the past two decades, several of the species have been extensively characterised and their contribution to wine quality is better understood. Beyond fermentation, some of the species have been investigated for their potential as alternative biological tools to reduce grape and wine spoilage. However, such studies remain limited to a few genera. This work aimed to evaluate the antagonistic activity of grape must-derived non-conventional yeasts against Botrytis cinerea and non-Botrytis bunch-rotting moulds and to further elucidate mechanisms conferring antifungal activity. A total of 31 yeast strains representing 21 species were screened on different agar media using a dual culture technique and liquid mixed cultures, respectively. Pichia kudriavzevii was the most potent with a minimum inhibitory concentration of 10² cells/mL against B. cinerea but it had a narrow activity spectrum. Twelve of the yeast strains displayed broad antagonistic activity, inhibiting three strains of B. cinerea (B05. 10, IWBT FF1 and IWBT FF2), a strain of Aspergillus niger and Alternaria alternata. Production of chitinases and glucanases in the presence of B. cinerea was a common feature in most of the antagonists. Volatile and non-volatile compounds produced by antagonistic yeast strains in the presence of B. cinerea were analysed and identified using gas and liquid chromatography mass spectrometry, respectively. The volatile compounds identified belonged mainly to higher alcohols, esters, organosulfur compounds and monoterpenes while the non-volatile compounds were cyclic peptides and diketopiperazine. To our knowledge, this is the first report to demonstrate inhibitory effect of the non-volatile compounds produced by various yeast species.

Synergistic Plant-Microbe Interactions between Endophytic Actinobacteria and Their Role in Plant Growth Promotion and Biological Control of Cotton under Salt Stress

Bacterial endophytes are well-acknowledged inoculants to promote plant growth and enhance their resistance toward various pathogens and environmental stresses. In the present study, 71 endophytic strains associated with the medicinal plant Thymus roseus were screened for their plant growth promotion (PGP), and the applicability of potent strains as bioinoculant has been evaluated. Regarding PGP traits, the percentage of strains were positive for the siderophore production (84%), auxin synthesis (69%), diazotrophs (76%), phosphate solubilization (79%), and production of lytic enzymes (i.e., cellulase (64%), lipase (62%), protease (61%), chitinase (34%), and displayed antagonistic activity against Verticillium dahliae (74%) in vitro. The inoculation of strain XIEG05 and XIEG12 enhanced plant tolerance to salt stress significantly (p < 0.05) through the promotion of shoot, root development, and reduced the activities of antioxidant enzymes (SOD, POD, and CAT), compared with uninoculated controls in vivo. Furthermore, inoculation of strain XIEG57 was capable of reducing cotton disease incidence (DI) symptoms caused by V. dahliae at all tested salt concentrations. The GC-MS analysis showed that many compounds are known to have antimicrobial and antifungal activity. Our findings provide valuable information for applying strains XIEG05 and XIEG12 as bioinoculant fertilizers and biological control agent of cotton under saline soil conditions.

Two strains of airborne Nocardiopsis alba producing different volatile organic compounds (VOCs) as biofungicide for Ganoderma boninense

Nocardiopsis are actinobacteria which produce active compounds, such as antifungals and volatile compounds. Ganoderma boninense is a pathogenic and aggressive fungus that decreases palm oil yield during production. In this study, we isolated two strains of Nocardia (GME01 and GME22) from airborne contaminants on the actinobacteria culture collection in the laboratory. The aim of this study is to identify two strains of Nocardiopsis and to obtain the antifungal potency of volatile organic compounds (VOCs) against G. boninese. We characterized the morphology using Scanning Electrone Microscope (SEM), molecular properties and whole-cell protein spectra using Matrix Assisted Laser Desorption Ionization Time of Flight Mass Spectrometry (MALDI-TOF MS), antifungal assay on G. boninense and VOCs analysis of Nocardia using solid phase micro extraction/gas chromatography (SPME/GC). The two Nocardiopsis strains had the similar characteristic such as white aerial mycelium and spores, aerobic, grow well on ISP-2, TSA and NA medium without diffusible pigment and had the highest similarity with Nocardiopsis alba DSM 43377 (99.63% and 99.55% similarity for GME01 and GME22, respectively), Different morphological feature was found in aerial mycelium and spores. GME22 has a clearly fragmented mycelium whereas GME01 has none. Other features also showed different on the whole-cell protein spectra, antifungal activity and VOCs profiles. Antifungal activity assay on G. boninense showed that N. alba GME22 has higher antifungal activity than GME01 related with the VOCs abundance in two strains. Almost 38.3% (18 VOCs) of N. alba GME22 and 25.5% (12 VOCs) of N. alba GME01 were found specifically in each strain, and 36.2% (the 17 same VOCs) produced by both. The known volatile antifungal compounds S-methyl ethanethioate, 1,2-dimethyldisulfane, acetic acid, 2-methyl propanoic acid, 3-methyl-butanoic acid, nonan-2-one, undecan-2-one and 2-isopropyl-5-methylcyclohexan-1-ol only produced by N. alba GME22 and 1,3-dimethyltrisulfane only produced by N. alba GME01. A total of two known antifungal compounds 1,2-dimethyldisulfane and 6-methylheptan-2-one were produced by both N. alba. The abundance of antifungal VOCs produced by these bacteria is potentially to be used as biocontrol agent for pathogenic fungi in plants.

Basidiomycetes Are Particularly Sensitive to Bacterial Volatile Compounds: Mechanistic Insight Into the Case Study of Pseudomonas protegens Volatilome Against Heterobasidion abietinum

Volatile organic compounds (VOCs) play an important role in the communication among organisms, including plants, beneficial or pathogenic microbes, and pests. In vitro, we observed that the growth of seven out of eight Basidiomycete species tested was inhibited by the VOCs of the biocontrol agent Pseudomonas protegens strain CHA0. In the Ascomycota phylum, only some species were sensitive (e.g., Sclerotinia sclerotiorum, Botrytis cinerea, etc.) but others were resistant (e.g., Fusarium oxysporum f. sp. cubense, Verticillium dahliae, etc.). We further discovered that CHA0 as well as other ten beneficial or phytopathogenic bacterial strains were all able to inhibit Heterobasidion abietinum, which was used in this research as a model species. Moreover, such an inhibition occurred only when bacteria grew on media containing digested proteins like peptone or tryptone (e.g., Luria-Bertani agar or LBA). Also, the inhibition co-occurred with a pH increase of the agar medium where the fungus grew. Therefore, biogenic ammonia originating from protein degradation by bacteria was hypothesized to play a major role in fungus inhibition. Indeed, when tested as a synthetic compound, it was highly toxic to H. abietinum (effective concentration 50% or EC₅₀ = 1.18 M; minimum inhibitory concentration or MIC = 2.14 M). Using gas chromatography coupled to mass spectrometry (GC/MS), eight VOCs were found specifically emitted by CHA0 grown on LBA compared to the bacterium grown on potato dextrose agar (PDA). Among them, two compounds were even more toxic than ammonia against H. abietinum: dimethyl trisulfide had EC₅₀ = 0.02 M and MIC = 0.2 M, and 2-ethylhexanol had EC₅₀ = 0.33 M and MIC = 0.77 M. The fungus growth inhibition was the result of severe cellular and sub-cellular alterations of hyphae occurring as early as 15 min of exposure to VOCs, as evidenced by transmission and scanning electron microscopy observations. Transcriptome reprogramming of H. abietinum induced by CHA0’s VOCs pointed out that detrimental effects occurred on ribosomes and protein synthesis while the cells tried to react by activating defense mechanisms, which required a lot of energy diverted from the growth and development (fitness cost).

Antifungal Effects of Volatile Organic Compounds Produced by Rahnella aquatilis JZ-GX1 Against Colletotrichum gloeosporioides in Liriodendron chinense × tulipifera

The use of volatile organic compounds (VOCs) produced by microorganisms for the biological control of plant diseases has attracted much attention in recent years. In this study, the antifungal activity and identity of VOCs produced by Rahnella aquatilis JZ-GX1 isolated from the rhizosphere soil of pine were determined and analyzed. The effect of the VOCs on the mycelial growth of Colletotrichum gloeosporioides, the pathogen of Liriodendron chinense × tulipifera black spot, was determined by a joined-petri dish fumigation method. An in vitro leaf inoculation method was used to determine the fumigation effect of the VOCs on Liriodendron black spot. VOCs with antifungal activity were collected by headspace solid-phase microextraction (SPME), and their components were analyzed by gas chromatography-mass spectrometry (GC-MS). The results showed that the VOCs secreted by JZ-GX1 inhibited the mycelial growth of the tested pathogen. The VOCs destroyed the morphology of the mycelium, significantly increased the permeability of the cell membrane and downregulated the expression of pathogenicity-related genes during mycelial infection, thus inhibiting the expansion of anthracnose disease spots in leaves. In the volatile compound profile, 3-methyl-1-butanol and 2-phenylethyl methyl ether significantly inhibited the mycelial growth and spore germination of C. gloeosporioides. This work provides a new strategy for the research and application of microorganisms and bioactive compounds to control plant anthracnose.