Pectin lyase enhances cotton resistance to Verticillium wilt by inducing cell apoptosis of Verticillium dahliae

Comparative single-cell transcriptomic map reveals divergence in leaves between two cotton species at cell type resolution

INTRODUCTION

Leaves are important functional organs in plants that determine yield and quality of crops. Upland cotton and sea-island cotton contribute more than 90% of the cotton fiber production annually. Deciphering and utilizing the diversity of leaf cells and functional genes underlying their divergences will be highly meaningful for cotton breeding.

OBJECTIVES

To investigate the conserved and divergent cell types of leaves between upland cotton and sea-island cotton, identify functional genes, and explore functional cell types in response to biotic and abiotic stresses in both species.

METHODS

Nuclei were isolated from leaves of upland cotton CRI12 and sea-island cotton XH21, respectively, and single-nucleus RNA-seq (snRNA-seq) was performed. Based on the orthologous genes, comparative single-cell transcriptomic map (CSCTM) of two cotton species was constructed to investigate conservation and divergence of cell types, and funtional genes were validated by virus induced gene silencing. Combining CSCTM, comparative genomic and transcriptomic analysis, functional cell types were identified in response to biotic and abiotic stresses.

RESULTS

A total of 22 and 20 distinct clusters were identified representing 6 main cell types in CRI12 and XH21, respectively. CSCTM analysis revealed a sea-island cotton-specific cell cluster, in which specifically expressed GbNF-YA7's role in pathogen resistance was validated. Meanwhile, the divergence of pigment gland development was revealed among cotton species and WRKY15 was identified to influence gossypol content without affecting pigment gland number. Moreover, integrated CSCTM and comparative genomic and transcriptomic analysis revealed genome variations could influence the gene expression in an elaborate cell type-specifc manner, highlighted the function of cotton leaf vascular tissue cells in Verticillium wilt resistance and putative functional differentiation of conserved abiotic stresses response genes. Additionally, different cell types might assume distinct roles in dealing with various stresses, forming a complex stress response system.

CONCLUSIONS

This study uncovered the conservation and divergence in cell types of leaves of upland cotton and sea-island cotton, which will provide a better understanding of phenotypic variation of the two species.

Detection of Verticillium infection in cotton leaves using ATR-FTIR spectroscopy coupled with machine learning algorithms

Verticillium wilt (VW) is a soil-borne vascular disease that affects upland cotton and is caused by Verticillium dahliae Kleb. A rapid and user-friendly early diagnostic technique is essential for the preventing and controlling VW disease. In this study, Fourier transform infrared (FTIR) spectroscopy with attenuated total reflectance (ATR) technology was used to detect VW infection in cotton leaves. About 1800 FTIR spectra were obtained from 348 cotton leaves. The cotton leaves were collected from three categories: VW group, infected group and control group (non-infected). The vibrational peak of chitins at 1558 cm-1 was identified through mean and differential analysis of FTIR spectra as a criterion to differentiate the VW or infected group from the control group. Classification models were constructed using various machine learning algorithms. The support vector machines (SVM) model exhibited the highest predictive accuracy (>96 %) in each group and a total accuracy (>97 %) for the three groups. These results provide a new approach for detecting Verticillium infection in cotton leaves and shows a promising potential for the future applications of the method in plant science.

MdMYB54 reduces disease severity caused by Fusarium solani in apple by modulating cell wall cellulose and pectate lyase-dependent defense

The plant cell wall is the first barrier against pathogen invasion. Fusarium solani is the primary pathogen responsible for apple replant disease. In this study, we identified an MYB protein, MdMYB54, which interacts with the positive regulator of F. solani resistance, MdERF114, and confers apple-increased tolerance against F. solani. The cellulose synthetase (CESA) gene MdCesA6 and pectin lyase-like (PLL) genes MdPLL8 and MdPLL12 were screened as three potential downstream target genes of MdMYB54 using DAP-seq. The results of electrophoretic mobility shift and yeast one-hybrid assays showed that MdMYB54 directly binds to the promoters of MdCesA6, MdPLL8, and MdPLL12 in vivo and in vitro. Dual-luciferase and β-glucuronidase assays showed that MdMYB54 activates the expression of these genes. The cellulose content and pectin lyase activity of MdMYB54-overexpressed roots were significantly higher than those of wild-type plants under F. solani treatment but were the opposite in MdMYB54-RNAi roots. The deposition of cellulose enhanced the physical barrier of the plant cell wall, whereas the activation of pectin lyase promoted the formation of oligogalacturonides and the production of reactive oxygen species. Overexpression of MdCesA6, MdPLL8, and MdPLL12 in the root system enhanced the tolerance of apple to F. solani. The direct interaction of MdERF114 with MdMYB54 enhanced MdMYB54-mediated cell wall defense response. These results suggest that modifying these candidate genes may provide a strategy for improving the resistance of apple to F. solani.

The Verticillium dahliae Effector VdPHB1 Promotes Pathogenicity in Cotton and Interacts with the Immune Protein GhMC4

Verticillium dahliae is a kind of pathogenic fungus that brings about wilt disease and great losses in cotton. The molecular mechanism of the effectors in V. dahliae regulating cotton immunity remains largely unknown. Here, we identified an effector of V. dahliae, VdPHB1, whose gene expression is highly induced by infection. The VdPHB1 protein is localized to the intercellular space of cotton plants. Knock-out of the VdPHB1 gene in V. dahliae had no effect on pathogen growth, but decreased the virulence in cotton. VdPHB1 ectopically expressed Arabidopsis plants were growth-inhibited and significantly susceptible to V. dahliae. Further, VdPHB1 interacted with the type II metacaspase GhMC4. GhMC4 gene-silenced cotton plants were more sensitive to V. dahliae with reduced expression of pathogen defense-related and programmed cell death genes. The accumulation of GhMC4 protein was concurrently repressed when VdPHB1 protein was expressed during infection. In summary, these results have revealed a novel molecular mechanism of virulence regulation that the secreted effector VdPHB1 represses the activity of cysteine protease for helping V. dahliae infection in cotton.

Mechanism of oxalate decarboxylase Oxd_S12 from Bacillus velezensis BvZ45-1 in defence against cotton verticillium wilt

Verticillium wilt, a soilborne vascular disease caused by Verticillium dahliae, strongly affects cotton yield and quality. In this study, an isolated rhizosphere bacterium, designated Bacillus velezensis BvZ45-1, exhibited >46% biocontrol efficacy against cotton verticillium wilt under greenhouse and field conditions. Moreover, through crude protein extraction and mass spectrometry analyses, we found many antifungal compounds present in the crude protein extract of BvZ45-1. The purified oxalate decarboxylase Odx_S12 from BvZ45-1 inhibited the growth of V. dahliae Vd080 by reducing the spore yield, causing mycelia to rupture, spore morphology changes, cell membrane rupture, and cell death. Subsequently, overexpression of Odx_S12 in Arabidopsis significantly improved plant resistance to V. dahliae. Through studies of the resistance mechanism of Odx_S12, V. dahliae was shown to produce oxalic acid (OA), which has a toxic effect on Arabidopsis leaves. Odx_S12 overexpression reduced Arabidopsis OA content, enhanced tolerance to OA, and improved resistance to verticillium wilt. Transcriptomics and quantitative real-time PCR analysis revealed that Odx_S12 promoted a reactive oxygen species burst and a salicylic acid- and abscisic acid-mediated defence response in Arabidopsis. In summary, this study not only identified B. velezensis BvZ45-1 as an efficient biological control agent, but also identified the resistance gene Odx_S12 as a candidate for cotton breeding against verticillium wilt.

The CfKOB1 gene related to cell apoptosis is required for pathogenicity and involved in mycovirus-induced hypovirulence in Colletotrichum fructicola

Colletotrichum fructicola is a globally significant phytopathogenic fungus. Mycovirus-induced hypovirulence has great potential for biological control and study of fungal pathogenic mechanisms. We previously reported that the mycovirus Colletotrichum alienum partitivirus 1 (CaPV1) is associated with the hypovirulence of C. fructicola, and the present study aimed to further investigate a host factor and its roles in mycovirus-induced hypovirulence. A gene named CfKOB1, which encodes putative protein homologous to the β-subunit of voltage-gated potassium channels and aldo-keto reductase, is downregulated upon CaPV1 infection and significantly upregulated during the early infection phase of Nicotiana benthamiana by C. fructicola. Deleting the CfKOB1 gene resulted in diminished vegetative growth, decreased production of asexual spores, hindered appressorium formation, reduced virulence, and altered tolerance to abiotic stresses. Transcriptome analysis revealed that CfKOB1 regulates many metabolic pathways as well as the cell cycle and apoptosis. Furthermore, enhanced apoptosis was observed in the ΔCfKOB1 mutants. Viral RNA accumulation was significantly increased in the CfKOB1 deletion mutant. Additionally, our findings demonstrated that CaPV1 infection in the WT strain also induced cell apoptosis. Collectively, these results highlight the diverse biological roles of the CfKOB1 gene in the fungus C. fructicola, while it also participates in mycovirus-induced hypovirulence by regulating apoptosis.

An effector of Erysiphe necator translocates to chloroplasts and plasma membrane to suppress host immunity in grapevine

The powdery mildew (Erysiphe necator) is a prevalent pathogen hampering grapevine growth in the vineyard. An arsenal of candidate secreted effector proteins (CSEPs) was encoded in the E. necator genome, but it is largely unclear what role CSEPs plays during the E. necator infection. In the present study, we identified a secreted effector CSEP080 of E. necator, which was located in plant chloroplasts and plasma membrane. Transient expressing CSEP080 promotes plant photosynthesis and inhibits INF1-induced cell death in tobacco leaves. We found that CSEP080 was a necessary effector for the E. necator pathogenicity, which interacted with grapevine chloroplast protein VviB6f (cytochrome b6-f complex iron-sulfur subunit), affecting plant photosynthesis. Transient silencing VviB6f increased the plant hydrogen peroxide production, and the plant resistance to powdery mildew. In addition, CSEP080 manipulated the VviPE (pectinesterase) to promote pectin degradation. Our results demonstrated the molecular mechanisms that an effector of E. necator translocates to host chloroplasts and plasma membrane, which suppresses with the grapevine immunity system by targeting the chloroplast protein VviB6f to suppress hydrogen peroxide accumulation and manipulating VviPE to promote pectin degradation.

The specificity of pectate lyase VdPelB from Verticilium dahliae is highlighted by structural, dynamical and biochemical characterizations

Pectins, complex polysaccharides and major components of the plant primary cell wall, can be degraded by pectate lyases (PLs). PLs cleave glycosidic bonds of homogalacturonans (HG), the main pectic domain, by β-elimination, releasing unsaturated oligogalacturonides (OGs). To understand the catalytic mechanism and structure/function of these enzymes, we characterized VdPelB from Verticillium dahliae. We first solved the crystal structure of VdPelB at 1.2 Å resolution showing that it is a right-handed parallel β-helix structure. Molecular dynamics (MD) simulations further highlighted the dynamics of the enzyme in complex with substrates that vary in their degree of methylesterification, identifying amino acids involved in substrate binding and cleavage of non-methylesterified pectins. We then biochemically characterized wild type and mutated forms of VdPelB. Pectate lyase VdPelB was most active on non-methylesterified pectins, at pH 8.0 in presence of Ca²⁺ ions. The VdPelB-G125R mutant was most active at pH 9.0 and showed higher relative activity compared to native enzyme. The OGs released by VdPelB differed to that of previously characterized PLs, showing its peculiar specificity in relation to its structure. OGs released from Verticillium-partially tolerant and sensitive flax cultivars differed which could facilitate the identification VdPelB-mediated elicitors of defence responses.

Inhibitory effect and mechanism of action of juniper essential oil on gray mold in cherry tomatoes

Juniper essential oil (JEO), which is mostly known as an immune system booster and effective detoxifier, has substantial antimicrobial activity. A comparison of the inhibitory effects of three plant essential oils from juniper (Juniperus rigida), cedarwood (Juniperus virginiana), and cypress (Crupressus sempervirens) on four plant pathogenic fungi indicated that JEO was the most effective at inhibiting the growth of gray mold (Botrytis cinerea). Additional studies were subsequently conducted to explore the in vivo and in vitro antifungal activity and possible mechanism of JEO against B. cinerea. The results show that JEO inhibited the germination of spores and mycelial growth of B. cinerea in a concentration-dependent manner and exhibited strong inhibition when its concentration exceeded 10 μL/mL. JEO also significantly inhibited the incidence of disease and diameters of gray mold lesions on cherry tomato fruit (Solanum lycopersicum). After 12 h of treatment with JEO, the extracellular conductivity, and the contents of soluble protein, malondialdehyde, and hydrogen peroxide were 3.1, 1.2, 7.2, and 4.7 folds higher than those of the control group, respectively (P < 0.05), which indicated that JEO can damage membranes. Scanning electron microscopy observations revealed that JEO affected the morphology of mycelia, causing them to shrivel, twist and distort. Furthermore, JEO significantly improved the activities of the antioxidant-related enzymes superoxide dismutase and catalase but reduced the pathogenicity-related enzymes polygalacturonase (PG), pectin lyase and endoglucanase of B. cinerea (P < 0.05). In particular, PG was reduced by 93% after treatment with JEO for 12 h. Moreover, the 18 constituents of JEO were identified by gas chromatography/mass spectrometry (GC-MS) analysis, mainly limonene (15.17%), γ-terpinene (8.3%), β-myrcene (4.56%), terpinen-4-ol (24.26%), linalool (8.73%), α-terpineol (1.03%), o-cymene (8.35%) and other substances with antimicrobial activity. Therefore, JEO can be an effective alternative to prevent and control gray mold on cherry tomato fruit.

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.

Advanced genes expression pattern greatly contributes to divergence in Verticillium wilt resistance between Gossypium barbadense and Gossupium hirsutum

Verticillium, representing one of the world's major pathogens, causes Verticillium wilt in important woody species, ornamentals, agricultural, etc., consequently resulting in a serious decline in production and quality, especially in cotton. Gossupium hirutum and Gossypium barbadense are two kinds of widely cultivated cotton species that suffer from Verticillium wilt, while G. barbadense has much higher resistance toward it than G. hirsutum. However, the molecular mechanism regarding their divergence in Verticillium wilt resistance remains largely unknown. In the current study, G. barbadense cv. Hai7124 and G. hirsutum acc. TM-1 were compared at 0, 12, 24, 48, 72, 96, 120, and 144 h post-inoculation (hpi) utilizing high throughput RNA-Sequencing. As a result, a total of 3,549 and 4,725 differentially expressed genes (DEGs) were identified, respectively. In particular, the resistant type Hai7124 displayed an earlier and faster detection and signaling response to the Verticillium dahliae infection and demonstrated higher expression levels of defense-related genes over TM-1 with respect to transcription factors, plant hormone signal transduction, plant-pathogen interaction, and nucleotide-binding leucine-rich repeat (NLR) genes. This study provides new insights into the molecular mechanisms of divergence in Verticillium wilt resistance between G. barbadense and G. hirsutum and important candidate genes for breeding V. dahliae resistant cotton cultivars.

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.

GhENODL6 Isoforms from the Phytocyanin Gene Family Regulated Verticillium Wilt Resistance in Cotton

Verticillium wilt (VW), a fungal disease caused by Verticillium dahliae, currently devastates cotton fiber yield and quality seriously, yet few resistance germplasm resources have been discovered in Gossypium hirsutum. The cotton variety Nongda601 with suitable VW resistance and high yield was developed in our lab, which supplied elite resources for discovering resistant genes. Early nodulin-like protein (ENODL) is mainly related to nodule formation, and its role in regulating defense response has been seldom studied. Here, 41 conserved ENODLs in G. hirsutum were identified and characterized, which could divide into four subgroups. We found that GhENODL6 was upregulated under V. dahliae stress and hormonal signal and displayed higher transcript levels in resistant cottons than the susceptible. The GhENODL6 was proved to positively regulate VW resistance via overexpression and gene silencing experiments. Overexpression of GhENODL6 significantly enhanced the expressions of salicylic acid (SA) hormone-related transcription factors and pathogenicity-related (PR) protein genes, as well as hydrogen peroxide (H₂O₂) and SA contents, resulting in improved VW resistance in transgenic Arabidopsis. Correspondingly, in the GhENODL6 silenced cotton, the expression levels of both phenylalanine ammonia lyase (PAL) and 4-coumarate-CoA ligase (4CL) genes significantly decreased, leading to the reduced SA content mediating by the phenylalanine ammonia lyase pathway. Taken together, GhENODL6 played a crucial role in VW resistance by inducing SA signaling pathway and regulating the production of reactive oxygen species (ROS). These findings broaden our understanding of the biological roles of GhENODL and the molecular mechanisms underlying cotton disease resistance.

Antifungal Activity of Chaetoviridin A from Chaetomium globosum CEF-082 Metabolites Against Verticillium dahliae in Cotton

Cotton Verticillium wilt (CVW) is a severe soilborne disease caused by the pathogen Verticillium dahliae, and it has a great impact on cotton production. Previous studies found that the biocontrol agent Chaetomium globosum CEF-082 and its metabolic filtrate could reduce the incidence of CVW; however, the underlying mechanism remains unclear. The metabolic crude extract of CEF-082 increased the sensitivity of V. dahliae to stress, degraded the cell wall of V. dahliae, and increased the emergence and plant height of cotton. Through separation and purification of the metabolic crude extract of CEF-082, chaetoviridin A was identified and found to be highly active against V. dahliae. The compound caused cell necrosis and mycelial deformation, increased the production of reactive oxygen species and nitrous oxide, and inhibited the germination of microsclerotia of V. dahliae, enhancing the cotton plant defense response. In addition, CEF-082 also colonized cotton plants.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Genome-Wide Analysis of Ribosomal Protein GhRPS6 and Its Role in Cotton Verticillium Wilt Resistance

Verticillium wilt is threatening the world’s cotton production. The pathogenic fungus Verticillium dahliae can survive in the soil in the form of microsclerotia for a long time, colonize through the root of cotton, and invade into vascular bundles, causing yellowing and wilting of cotton leaves, and in serious cases, leading to plant death. Breeding resistant varieties is the most economical and effective method to control Verticillium wilt. In previous studies, proteomic analysis was carried out on different cotton varieties inoculated with V. dahliae strain Vd080. It was found that GhRPS6 was phosphorylated after inoculation, and the phosphorylation level in resistant cultivars was 1.5 times than that in susceptible cultivars. In this study, knockdown of GhRPS6 expression results in the reduction of SA and JA content, and suppresses a series of defensive response, enhancing cotton plants susceptibility to V. dahliae. Overexpression in Arabidopsis thaliana transgenic plants was found to be more resistant to V. dahliae. Further, serines at 237 and 240 were mutated to phenylalanine, respectively and jointly. The transgenic Arabidopsis plants demonstrated that seri-237 compromised the plant resistance to V. dahliae. Subcellular localization in Nicotiana benthamiana showed that GhRPS6 was localized in the nucleus. Additionally, the pathogen inoculation and phosphorylation site mutation did not change its localization. These results indicate that GhRPS6 is a potential molecular target for improving resistance to Verticillium wilt in cotton. This lays a foundation for breeding disease-resistant varieties.

The Role of a New Compound Micronutrient Multifunctional Fertilizer against Verticillium dahliae on Cotton

Verticillium dahliae Kleb., the causal pathogen of vascular wilt, can seriously reduce the yield and quality of many crops, including cotton (Gossypium hirsutum). To control the harm caused by V. dahliae, considering the environmental pollution of chemical fungicides and their residues, the strategy of plant nutrition regulation is becoming increasingly important as an eco-friendly method for disease control. A new compound micronutrient fertilizer (CMF) found in our previous study could reduce the damage of cotton Verticillium wilt and increase yield. However, there is little information about the mode of action of CMF to control this disease. In the present study, we evaluated the role of CMF against V. dahliae and its mechanism of action in vitro and in vivo. In the laboratory tests, we observed that CMF could inhibit hyphal growth, microsclerotia germination, and reduce sporulation of V. dahliae. Further studies revealed that the biomass of V. dahliae in the root and hypocotyl of cotton seedlings treated with CMF were significantly reduced compared with the control, and these results could explain the decline in the disease index of cotton Verticillium wilt. Furthermore, those key genes involved in the phenylpropanoid metabolism pathway, resistance-related genes defense, and nitric oxide signaling pathway were induced in cotton root and hypocotyl tissue when treated with CMF. These results suggest that CMF is a multifaceted micronutrient fertilizer with roles in inhibiting the growth, development, and pathogenicity of V. dahliae and promoting cotton growth.

Bacillus circulans GN03 Alters the Microbiota, Promotes Cotton Seedling Growth and Disease Resistance, and Increases the Expression of Phytohormone Synthesis and Disease Resistance-Related Genes

Plant growth-promoting bacteria (PGPB) are components of the plant rhizosphere that promote plant growth and/or inhibit pathogen activity. To explore the cotton seedlings response to Bacillus circulans GN03 with high efficiency of plant growth promotion and disease resistance, a pot experiment was carried out, in which inoculations levels of GN03 were set at 10⁴ and 10⁸ cfu^⋅mL^-1. The results showed that GN03 inoculation remarkably enhanced growth promotion as well as disease resistance of cotton seedlings. GN03 inoculation altered the microbiota in and around the plant roots, led to a significant accumulation of growth-related hormones (indole acetic acid, gibberellic acid, and brassinosteroid) and disease resistance-related hormones (salicylic acid and jasmonic acid) in cotton seedlings, as determined with ELISA, up-regulated the expression of phytohormone synthesis-related genes (EDS1, AOC1, BES1, and GA20ox), auxin transporter gene (Aux1), and disease-resistance genes (NPR1 and PR1). Comparative genomic analyses was performed between GN03 and four similar species, with regards to phenotype, biochemical characteristics, and gene function. This study provides valuable information for applying the PGPB alternative, GN03, as a plant growth and disease-resistance promoting fertilizer.

The Lethal Effect of Bacillus subtilis Z15 Secondary Metabolites on Verticillium dahliae

Bacillus subtilis Z15 (BS-Z15), isolated from cotton rhizosphere soil, inhibits Verticillium dahliae and suppresses cotton Verticillium wilt in pot experiments. We investigated the influence of environmental factors, pH, temperature, ultraviolet light, protease, and incubation time on the stability of BS-Z15 secondary metabolites (SMs), and the mechanism underlying the cytotoxicity of BS-Z15 SMs on V. dahliae. BS-Z15 and its fermentation broth inhibited V. dahliae, and this effect was mediated by its SMs. These were shown to be stable to the influence of the above environmental factors. BS-Z15 SMs decreased the viability of V. dahliae conidia in a time-dependent manner. Scanning electron microscopy showed that BS-Z15 and its SMs resulted in flattened and depressed conidia. BS-Z15 SMs induced morphological abnormalities in the hyphae, which showed rough aberrant structures, reduced conidiophore production, and accelerated aging. Flow cytometry using Hoechst/propidium iodide double staining revealed that BS-Z15 SMs induced necrosis in V. dahliae in a time-dependent manner. Fluorescence microscopy showed that BS-Z15 SMs did not induce apoptotic bodies in the conidia of V. dahliae but caused significant changes in karyotypes, accompanied by nuclear lysis and nucleic-acid diffusion, which may play important roles in necrosis. In addition, 0.3 mg/mL BS-Z15 SMs had no effect on either the mitochondrial membrane potential or the synthesis of proapoptotic proteins, indicating that the SMs did not induce apoptosis in V. dahliae. Their lethal effect on V. dahliae was by inducing necrosis in its conidia and hyphae. BS-Z15 SMs thus have potential as biological pesticides to control Verticillium wilt in cotton.