Endophytic Bacillus subtilis antagonize soil-borne fungal pathogens and suppress wilt complex disease in chickpea plants (Cicer arietinum L.)

Burkholderia ambifaria H8 as an effective biocontrol strain against maize stalk rot via producing volatile dimethyl disulfide

BACKGROUND

Maize stalk rot (MSR) caused by Fusarium graminearum is the primary factor contributing to the reduction in maize yield and quality. However, this soil-borne disease presents a significant challenge for sustainable control through field management and chemical agents. The screening of novel biocontrol agents can aid in developing innovative and successful strategies for MSR control.

RESULTS

A total of 407 strains of bacteria were isolated from the rhizosphere soil of a resistant maize inbred line. One strain exhibited significant antagonistic activity in plate and pot experiments, and was identified as Burkholderia ambifaria H8. The strain could significantly inhibit the mycelial growth and spore germination of F. graminearum, induce resistance to stalk rot, and promote plant growth. The volatile compounds produced by strain H8 and its secondary metabolites in the sterile fermentation broth exhibited antagonistic activity. The primary volatile compound produced by strain H8 was identified as dimethyl disulfide (DMDS) using gas chromatography tandem mass spectrometry. Through in vitro antagonistic activity assays and microscopic observation, it was confirmed that DMDS was capable of inhibiting mycelial growth and disrupting the mycelial structure of F. graminearum, suggesting it may be the major active compound for strain H8. The transcriptome data of F. graminearum further indicated that strain H8 and its volatile compounds could alter pathogenic fungi metabolism, influence the related metabolic pathways, and potentially induce cell apoptosis within F. graminearum.

CONCLUSION

Our results showed that B. ambifaria H8 was capable of producing the volatile substance dimethyl disulfide, which influenced the synthesis and permeability of cell membranes in pathogens. Thus, B. ambifaria H8 was found to be a promising biological control agent against MSR. © 2024 Society of Chemical Industry.

Evaluation of Tunisian wheat endophytes as plant growth promoting bacteria and biological control agents against Fusarium culmorum

Plant growth-promoting rhizobacteria (PGPR) applications have emerged as an ideal substitute for synthetic chemicals by their ability to improve plant nutrition and resistance against pathogens. In this study, we isolated fourteen root endophytes from healthy wheat roots cultivated in Tunisia. The isolates were identified based from their 16S rRNA gene sequences. They belonged to Bacillota and Pseudomonadota taxa. Fourteen strains were tested for their growth-promoting and defense-eliciting potentials on durum wheat under greenhouse conditions, and for their in vitro biocontrol power against Fusarium culmorum, an ascomycete responsible for seedling blight, foot and root rot, and head blight diseases of wheat. We found that all the strains improved shoot and/or root biomass accumulation, with Bacillus mojavensis, Paenibacillus peoriae and Variovorax paradoxus showing the strongest promoting effects. These physiological effects were correlated with the plant growth-promoting traits of the bacterial endophytes, which produced indole-related compounds, ammonia, and hydrogen cyanide (HCN), and solubilized phosphate and zinc. Likewise, plant defense accumulations were modulated lastingly and systematically in roots and leaves by all the strains. Testing in vitro antagonism against F. culmorum revealed an inhibition activity exceeding 40% for five strains: Bacillus cereus, Paenibacillus peoriae, Paenibacillus polymyxa, Pantoae agglomerans, and Pseudomonas aeruginosa. These strains exhibited significant inhibitory effects on F. culmorum mycelia growth, sporulation, and/or macroconidia germination. P. peoriae performed best, with total inhibition of sporulation and macroconidia germination. These finding highlight the effectiveness of root bacterial endophytes in promoting plant growth and resistance, and in controlling phytopathogens such as F. culmorum. This is the first report identifying 14 bacterial candidates as potential agents for the control of F. culmorum, of which Paenibacillus peoriae and/or its intracellular metabolites have potential for development as biopesticides.

Enhancing phytoremediation of cadmium and arsenic in alkaline soil by Miscanthus sinensis: A study on the synergistic effect of endophytic fungi and biochar

Endophytic fungi (Trichoderma harzianum (TH) and Paecilomyces lilacinus (PL)) showed potential in phytoremediation for soils contaminated with potentially toxic elements (PTEs (Cd and As)). However, their efficiency is limited, which can be enhanced with the assistance of biochar. This study sought to investigate the effects of TH at two application rates (T1: 4.5 g m-2; T2: 9 g m-2), PL at two application rates (P1: 4.5 g m-2; P2: 9 g m-2), in conjunction with biochar (BC) at 750 g m-2 on the phytoremediation of PTEs by Miscanthus sinensis (M. sinensis). The results showed that the integration of endophytic fungi with biochar notably enhanced the accumulation of Cd and As in M. sinensis by 59.60 %-114.38 % and 49.91 %-134.60 %, respectively. The treatments T2BC and P2BC emerged as the most effective. Specifically, the P2BC treatment significantly enhanced the soil quality index (SQI > 0.55) across all examined soil layers, markedly improving the overall soil condition. It was observed that T2BC treatment could elevate the SQI to 0.56 at the 0-15 cm depth. The combined amendment shifted the primary influences on plant PTEs accumulation from fungal diversity and soil nutrients to bacterial diversity and the availability of soil PTEs. Characteristic microorganisms identified under the combined treatments were RB41 and Pezizaceae, indicating an increase in both bacterial and fungal diversity. This combination altered the soil microbial community, influencing key metabolic pathways. The combined application of PL and biochar was superior to the TH and biochar combination for the phytoremediation of M. sinensis. This approach not only enhanced the phytoremediation potential but also positively impacted soil health and microbial community, suggesting that the synergistic use of endophytic fungi and biochar is an effective strategy for improving the condition of alkaline soils contaminated with PTEs.

Characterization of Bacillus velezensis 32a metabolites and their synergistic bioactivity against crown gall disease

Crown gall disease caused by Agrobacterium tumefaciens is considered to be the main bacterial threat of stone fruit plants in Mediterranean countries. In a previous study, Bacillus velezensis strain 32a was isolated from Tunisian rhizosphere soil and revealed high antagonistic potential against A. tumefaciens strains. In order to better characterize the antagonistic activity of this strain against this important plant pathogen, the production of secondary metabolites was analyzed using liquid chromatography coupled with mass spectrometry. The results revealed the production of different compounds identified as surfactins, fengycins, iturins and bacillibactin belonging to the lipopeptide group, three polyketides (macrolactins, oxydifficidin and bacillaenes), bacilysin and its chlorinated derivative; chlorotetaine. The involvement of lipopeptides in this antagonistic activity was ruled out by performing agar and broth dilution tests with pure molecules. Thus, the construction of B. velezensis 32a mutants defective in polyketides and bacilysin biosynthesis and their antagonistic activity was performed and compared to a set of derivative mutants of a comparable strain, B. velezensis GA1. The defective difficidin mutants (△dfnA and △dfnD) were unable to inhibit the growth of A. tumefaciens, indicating the high-level contribution of difficidin in the antagonism process. While the macrolactin deficient mutant (∆mlnA) slightly decreased the activity, suggesting a synergetic effect with difficidin. Remarkably, the mutant △dhbC only deficient in bacillibactin production showed significant reduction in its capacity to inhibit the growth of Agrobacterium.Taken collectively, our results showed the strong synergetic effect of difficidin and macrolactins and the significant implication of siderophore to manage crown gall disease.

Isolation and Identification of Endophytic Bacteria Bacillus sp. ME9 That Exhibits Biocontrol Activity against Xanthomonas phaseoli pv. manihotis

In recent years, the bacterial blight of cassava has caused substantial economic losses to the Chinese cassava industry. Chemical control methods have become the primary approach to control this disease; however, their widespread usage and harmful residues have raised concerns about environmental pollution. In order to avoid this, it is urgent to seek a green ecological method to prevent and control it. Biological control through the utilization of microorganisms not only effectively inhibits the disease, but also gives consideration to environmental friendliness. Therefore, investigating an endophytic biological control method for cassava bacterial blight is of great importance. In this study, cassava leaf tissues were used as test specimens in order to isolate endophytic bacteria by using dilution and separation methods. Bacillus ME9, derived from cassava endophytic bacteria, exhibits good antagonism against a diverse range of pathogens, including Xpm11. Its genome consists of a series of genes encoding antibacterial lipopeptides, which may be directly related to its antibacterial capabilities. Furthermore, inoculation resulted in a substantial change in the diversity of the endophytic bacterial community, characterized by improved diversity, and displayed an obvious inhibition of pathogenic bacterial growth, demonstrating successful colonization within plants. The results laid a foundation and provided theoretical support for the development and utilization of cassava endophytic bacterial diversity and endogenous disease control strategies.

Screening of Reference Genes under Biotic Stress and Hormone Treatment of Mung Bean (Vigna radiata) by Quantitative Real-Time PCR

Mung bean (Vigna radiata) production has been greatly threatened by numerous diseases. Infection with these pathogens causes extensive changes in gene expression and the activation of hormone signal transduction. Quantitative real-time PCR (qRT-PCR) is the most common technique used for gene expression validation. Screening proper reference genes for mung bean under pathogen infection and hormone treatment is a prerequisite for ensuring the accuracy of qRT-PCR data in mung bean disease-resistance research. In this study, six candidate reference genes (Cons4, ACT, TUA, TUB, GAPDH, and EF1α) were selected to evaluate the expression stability under four soil-borne disease pathogens (Pythium myriotylum, Pythium aphanidermatum, Fusarium oxysporum, and Rhizoctonia solani) and five hormone treatments (SA, MeJA, ETH, ABA, and GA3). In the samples from different treatments, the Ct value distribution of the six candidate reference genes was different. Under the condition of hormone treatment, the Ct value ranged from a minimum of 17.87 for EF1α to a maximum of 29.63 for GAPDH. Under the condition of pathogen infection, the Ct value ranged from a minimum of 19.43 for EF1α to a maximum of 31.82 for GAPDH. After primer specificity analysis, it was found that GAPDH was not specific, so the five reference genes Cons4, ACT, TUA, TUB, and EF1α were used in subsequent experiments. The software products GeNorm, NormFinder, BestKeeper and RefFinder were used for qRT-PCR data analysis. In general, the best candidates reference genes were: TUA for SA, ABA, GA3, and Pythium myriotylum treatment; TUB for ETH treatment; ACT for MeJA and Fusarium oxysporum treatment; and EF1α for Pythium aphanidermatum and Rhizoctonia solani treatment. The most stably expressed genes in all samples were TUA, while Cons4 was the least stable reference gene. Finally, the reliability of the reference gene was further validated by analysis of the expression profiles of four mung bean genes (Vradi0146s00260, Vradi0158s00480, Vradi07g23860, and Vradi11g03350) selected from transcriptome data. Our results provide more accurate information for the normalization of qRT-PCR data in mung bean response to pathogen interaction.

Seasonal changes in the abundance Fusarium proliferatium, microbial endophytes and nutrient levels in the roots of hybrid bamboo Bambusa pervariabilis × Dendrocalamopsis grandis

Plant root pathogens invade the soil around plant roots, disturbing the systemic balance, reducing plant defenses, and causing severe disease. At present, there are few studies on the severity of plant diseases caused by pathogen invasion in different seasons and how pathogens affect root microecology. In this study, we compared the levels of nutrients in the root tissues of the two groups of plants. We used 16S and ITS amplicon sequencing with Illumina NovaSeq 6000 to compare seasonal changes in the composition and structure of microbial communities from healthy roots of bamboo Bambusa pervariabilis × Dendrocalamopsis grandis and roots infected by the soilborne pathogen Fusarium proliferatum. We have found that the invasion of the pathogen led to a substantial decrease in nutrient elements in bamboo roots, except for nitrogen. The pathogen presence correlated with seasonal changes in the bamboo root microbiome and decreased bacterial richness in diseased plants. The root microbial community structure of healthy plants was more stable than that of their diseased counterparts. Furthermore, we identified the lesion area and relative abundance of F. proliferatum were significant predictors of disease progression. The potassium tissue content and the disease lesion area were identified as factors linked with the observed changes in the bamboo root microbiome. This study provides a theoretical foundation for understanding the seasonal dynamics F. proliferatum, an economically important soilborne pathogen of hybrid bamboo grown in Sichuan Province, China.

Antagonistic Strain Bacillus halotolerans Jk-25 Mediates the Biocontrol of Wheat Common Root Rot Caused by Bipolaris sorokiniana

Common root rot caused by Bipolaris sorokiniana infestation in wheat is one of the main reasons for yield reduction in wheat crops worldwide. The bacterium strain JK-25 used in the current investigation was isolated from wheat rhizosphere soil and was later identified as Bacillus halotolerans based on its morphological, physiological, biochemical, and molecular properties. The strain showed significant antagonism to B. sorokiniana, Fusarium oxysporum, Fusarium graminearum, and Rhizoctonia zeae. Inhibition of B. sorokiniana mycelial dry weight and spore germination rate by JK-25 fermentation supernatant reached 60% and 88%, respectively. The crude extract of JK-25 was found, by Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), to contain the surfactin that exerted an inhibitory effect on B. sorokiniana. The disruption of mycelial cell membranes was observed under laser scanning confocal microscope (LSCM) after treatment of B. sorokiniana mycelium with the crude extract. The antioxidant enzyme activity of B. sorokiniana was significantly reduced and the oxidation product malondialdehyde (MDA) content increased after treatment with the crude extract. The incidence of root rot was significantly reduced in pot experiments with the addition of JK-25 culture fermentation supernatant, which had a significant biological control effect of 72.06%. Its ability to produce siderophores may help to promote wheat growth and the production of proteases and pectinases may also be part of the strain's role in suppressing pathogens. These results demonstrate the excellent antagonistic effect of JK-25 against B. sorokiniana and suggest that this strain has great potential as a resource for biological control of wheat root rot strains.

Antifungal properties of rhizobacterial strains in relation to fungi of agricultural crops

On earth, pathogenic fungal infections are considered one of the most common crop problems, accounting for more than 80% of all plant diseases. The production of biologics and the fight against pathogens are highly relevant priorities. In this article, rhizosphere cultures isolated from wheat root tubers have been studied in detail for the development of pathogenic fungi. In the course of research, the inhibitory properties of rhizobacteria against common pathogenic fungi were studied. The main mechanism of antagonism is that bacteria directly affect phytopathogens or increase plant resistance to diseases by enhancing immunity. The antagonistic activity of rhizospheric microorganisms was studied and the level of their action was determined. It has been established that wheat rhizospheric bacteria Escherichia hermannii, Enterobacter cloacae and Rahnella aquatilis affect the development of pathogens that cause Alternaria and Fusarium wilt. In the experiments, 46 strains of rhizobacteria of the genus Enterobacter (strain E. cloacae CCIM1022), Rahnella (R. aquatilis CCIM1023), Pantoea, Pseudomonas, Bacillus, Escherichia were isolated and the effectiveness of their antifungal properties on the reproduction of pathogenic fungi was tested. Pathogenic fungi Alternaria, Fusarium, causing diseases of vegetable and melon crops, were isolated from host plants. It is noted that rhizobacteria have a negative impact and block the development of pure cultures of fungi isolated from tomatoes, zucchini, figs, melons, causing fungal diseases on cups. The 46 isolated and tested strains of rhizobacteria effectively inhibited the development of pathogenic fungi. Of these, Rahnella is the first bacterium studied for this purpose, and has been given great importance in experiments to block the development of Alternaria (A. alternata CCIM 1021). The causative agents of alternariosis are widely open. However, large-scale analysis of their presence and control of pathogens has always been difficult. In experiments to combat the pathogen in the laboratory, a new agent was used to spread nitrogen-fixing bacteria.