Rapid and Sensitive Detection of Verticillium dahliae from Soil Using LAMP-CRISPR/Cas12a Technology

Cotton Verticillium wilt is mainly caused by the fungus Verticillium dahliae, which threatens the production of cotton. Its pathogen can survive in the soil for several years in the form of microsclerotia, making it a destructive soil-borne disease. The accurate, sensitive, and rapid detection of V. dahliae from complex soil samples is of great significance for the early warning and management of cotton Verticillium wilt. In this study, we combined the loop-mediated isothermal amplification (LAMP) with CRISPR/Cas12a technology to develop an accurate, sensitive, and rapid detection method for V. dahliae. Initially, LAMP primers and CRISPR RNA (crRNA) were designed based on a specific DNA sequence of V. dahliae, which was validated using several closely related Verticillium spp. The lower detection limit of the LAMP-CRISPR/Cas12a combined with the fluorescent visualization detection system is approximately ~10 fg/μL genomic DNA per reaction. When combined with crude DNA-extraction methods, it is possible to detect as few as two microsclerotia per gram of soil, with the total detection process taking less than 90 min. Furthermore, to improve the method's user and field friendliness, the field detection results were visualized using lateral flow strips (LFS). The LAMP-CRISPR/Cas12a-LFS system has a lower detection limit of ~1 fg/μL genomic DNA of the V. dahliae, and when combined with the field crude DNA-extraction method, it can detect as few as six microsclerotia per gram of soil, with the total detection process taking less than 2 h. In summary, this study expands the application of LAMP-CRISPR/Cas12a nucleic acid detection in V. dahliae and will contribute to the development of field-deployable diagnostic productions.

The novel bacteriocin BacYB1 produced by Leuconostoc mesenteroides YB1: from recent analytical characterization to biocontrol Verticillium dahliae and Agrobacterium tumefaciens

Biocontrol of phytopathogens involving the use of bioactive compounds produced by lactic acid bacteria (LAB), is a promising approach to manage many diseases in agriculture. In this study, a lactic acid bacterium designated YB1 was isolated from fermented olives and selected for its antagonistic activity against Verticillium dahliae (V. dahliae) and Agrobacterium tumefaciens (A. tumefaciens). Based on the 16S rRNA gene nucleotide sequence analysis (1565 pb, accession number: OR714267), the new isolate YB1 bacterium was assigned as Leuconostoc mesenteroides YB1 (OR714267) strain. This bacterium produces an active peptide "bacteriocin" called BacYB1, which was purified in four steps. Matrix-assisted lasers desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) based approach was performed to identify and characterize BacYB1. The exact mass was 5470.75 Da, and the analysis of the N-terminal sequence (VTRASGASTPPGTASPFKTL) of BacYB1 revealed no significant similarity to currently available antimicrobial peptides. The BacYB1 displayed a bactericidal mode of action against A. tumefaciens. The potentiel role of BacYB1 to supress the growth of A. tumefaciens was confirmed by live-dead cells viability assay. In pot experiments, the biocontrol efficacy of BacYB1 against V. dahliae wilt on young olive trees was studied. The percentage of dead plants (PDP) and the final mean symptomes severity (FMS) of plants articifialy infected by V. dahliae and treated with the pre-purified peptide BacYB1 (preventive and curative treatments) were significantly inferior to untreated plants. Biochemical analysis of leaves of the plants has shown that polyophenols contents were highly detected in plants infected by V. dahliae and the highest contents of chlorophyl a, b and total chlorophyll were recorded in plants treated with the combination of BacYB1 with the biofertilisant Humivital. BacYB1 presents a promising alternative for the control of Verticillium wilt and crown gall diseases.

Genome-Wide Identification and Expression Analysis of Respiratory Burst Oxidase Homolog (RBOH) Gene Family in Eggplant (Solanum melongena L.) under Abiotic and Biotic Stress

Respiratory burst oxidase homologs (RBOHs) are important proteins that catalyze the production of reactive oxygen species (ROS), which play important roles in growth and stress response. For a comprehensive analysis of SmRBOH genes, we conducted genome-wide identification of the SmRBOH gene family in eggplant (Solanum melongena L.) and analyzed the expression of SmRBOHs under abiotic (salt, high-temperature, and low-temperature) and biotic stress (Verticillium dahliae inoculation) by quantitative real-time PCR (qRT-PCR). The result showed that a total of eight SmRBOH members were identified from the genome database of eggplant, and they were relatively evenly distributed across seven chromosomes. The analysis of Motif and the conserved domain showed that SmRBOHs have high similarity in protein sequences and functions. Based on phylogenetics, SmRBOHs were classified into three distinct clades. Furthermore, the promoter regions of SmRBOHs were found to contain different cis-elements. Additionally, the results of the qRT-PCR demonstrated differential expression patterns of SmRBOHs in different tissues (the roots, stems, and leaves) and stress conditions. SmRBOHB, SmRBOHD, SmRBOHH1, and SmRBOHH2 showed significant upregulation (>20-fold) under at least one stress condition. Subcellular localization analysis of the above four members further confirmed that they localized on the plasma membrane. This study provides a theoretical foundation for understanding the functions of SmRBOHs in eggplant.

Verticillium dahliae Secretes Small RNA to Target Host MIR157d and Retard Plant Floral Transition During Infection

Bidirectional trans-kingdom RNA silencing [or RNA interference (RNAi)] plays a key role in plant-pathogen interactions. It has been shown that plant hosts export specific endogenous miRNAs into pathogens to inhibit their virulence, whereas pathogens deliver small RNAs (sRNAs) into plant cells to disturb host immunity. Here, we report a trans-kingdom fungal sRNA retarding host plant floral transition by targeting a miRNA precursor. From Arabidopsis plants infected with Verticillium dahliae, a soil-borne hemibiotrophic pathogenic fungus that causes wilt diseases in a wide range of plant hosts, we obtained a number of possible trans-kingdom V. dahliae sRNAs (VdsRNAs) by sequencing AGO1-immunoprecipitated sRNAs. Among these, a 24-nt VdsRNA derived from V. dahliae rRNA, VdrsR-1, was shown to be an actual trans-kingdom VdsRNA that targets the miR157d precursor MIR157d, resulting in increased rather than reduced miR157d accumulation in V. dahliae-infected plants. Consistent with the miR157 family in the regulation of vegetative and floral transitions by targeting SPL genes in several plant species, we detected two SPL genes, SPL13A/B, that were notably reduced in V. dahliae-infected and VdrsR-1-expressing plants compared with control plants. Furthermore, V. dahliae-infected and VdrsR-1-expressing plants also displayed delayed vegetative phase change and floral transition compared to control plants. Taken together, we disclosed a novel mode of action for a trans-kingdom fungal sRNA, VdrsR-1, which was secreted into host cells to modulate plant floral transition by employing the miR157d/SPL13A/B regulatory module, leading to prolonged host vegetative growth that would undoubtedly benefit fungal propagation.

A fungal effector suppresses the nuclear export of AGO1-miRNA complex to promote infection in plants

SignificanceIncreasing evidence demonstrates that small RNAs can serve as trafficking effectors to mediate bidirectional transkingdom RNA interference (RNAi) in interacting organisms, including plant-pathogenic fungi systems. Previous findings demonstrated that plants can send microRNAs (miRNAs) to fungal pathogen Verticillium dahliae to trigger antifungal RNAi. Here we report that V. dahliae is able to secret an effector to the plant nucleus to interfere with the nuclear export of AGO1-miRNA complexes, leading to an inhibition in antifungal RNAi and increased virulence in plants. Thus, we reveal an antagonistic mechanism that can be exploited by fungal pathogens to counteract antifungal RNAi immunity via manipulation of plant small RNA function.

The Respiratory Burst Oxidase Homolog Protein D (GhRbohD) Positively Regulates the Cotton Resistance to Verticillium dahliae

Verticillium wilt, mainly caused by a soil-inhabiting fungus Verticillium dahliae, can seriously reduce the yield and quality of cotton. The complex mechanism underlying cotton resistance to Verticillium wilt remains largely unknown. In plants, reactive oxygen species (ROS) mediated by Rbohs is one of the earliest responses of plants to biotic and abiotic stresses. In our previous study, we performed a time-course phospho-proteomic analysis of roots of resistant and susceptible cotton varieties in response to V. dahliae, and found early differentially expressed protein burst oxidase homolog protein D (GhRbohD). However, the role of GhRbohD-mediated ROS in cotton defense against V. dahliae needs further investigation. In this study, we analyzed the function of GhRbohD-mediated resistance of cotton against V. dahliae in vitro and in vivo. Bioinformatics analysis showed that GhRbohD possessed the conservative structural attributes of Rbohs family, 12 members of RbohD out of 57 Rbohs in cotton. The expression of GhRbohD was significantly upregulated after V. dahliae inoculation, peaking at 6 hpi, and the phosphorylation level was also increased. A VIGS test demonstrated that ROS production, NO, H₂O₂ and Ca²⁺ contents of GhRbohD-silenced cotton plants were significantly reduced, and lignin synthesis and callose accumulation were damaged, important reasons for the impairment of GhRbohD-silenced cotton’s defense against V. dahliae. The expression levels of resistance-related genes were downregulated in GhRbohD-silenced cotton by qRT-PCR, mainly involving the lignin metabolism pathway and the jasmonic acid signaling pathway. However, overexpression of GhRbohD enhanced resistance of transgenic Arabidopsis to V. dahliae challenge. Furthermore, Y2H assays were applied to find that GhPBL9 and GhRPL12C may interact with GhRbohD. These results strongly support that GhRbohD activates ROS production to positively regulate the resistance of plants against V. dahliae.

Application of garlic allelochemicals improves growth and induces defense responses in eggplant (Solanum melongena) against Verticillium dahliae

Aqueous garlic extracts (AGE) and garlic allelochemical diallyl disulfide (DADS) have been recently reported to bear bioactive properties to stimulate plant growth and development and alter defense-related physiology. We, therefore, performed a bioassay to study these chemicals as possible biostimulants for defense against Verticillium dahliae in eggplant seedlings. AGE and DADS were applied as a foliar application to the eggplants and samples were collected before and after pathogen inoculation at various intervals to analyze the defense mechanism. The obtained data revealed that with the application of AGE and DADS, the seedlings showed responses including activation of antioxidant enzymes, an abundance of chlorophyll contents, alteration of photosynthesis system, and accumulation of plant hormones compared to the control plants. Furthermore, the microscopic analysis of the AGE or DADS treated plants showed high variability in pathogen density within the root crown at 28 days post-inoculation. The low abundance of reactive oxygen species was noticed in AGE or DADS treated plants, which indicates that the plants were able to successfully encounter pathogen attacks. The AGE and DADS treated plants exhibited a lower disease severity index (32.4% and 24.8% vs 87.1% in controls), indicating successful defense against Verticillium infection. Our results were therefore among the first to address the biostimulatory effects of AGE or DADS to induce resistance in eggplant seedlings against V. dahliae and may be used to establish preparation for garlic-derived bioactive compounds to improve growth and defense responses of eggplants under-protected horticultural situations such as glasshouse or plastic tunnels system.

Calcium-Dependent Protein Kinase GhCDPK28 Was Dentified and Involved in Verticillium Wilt Resistance in Cotton

Verticillium dahliae is a soil-borne fungus that causes vascular wilt through the roots of plants. Verticillium wilt caused by V. dahliae is one of the main diseases in cotton producing areas of the world, resulting in huge economic losses. Breeding resistant varieties is the most economical and effective method to control Verticillium wilt. Calcium-dependent protein kinases (CDPKs) play a pivotal role in plant innate immunity, including regulation of oxidative burst, gene expression as well as hormone signal transduction. However, the function of cotton CDPKs in response to V. dahliae stress remains unexplored. In this study, 96, 44 and 57 CDPKs were identified from Gossypium hirsutum, Gossypium raimondii and Gossypium arboretum, respectively. Phylogenetic analysis showed that these CDPKs could be divided into four branches. All GhCDPKs of the same clade are generally similar in gene structure and conserved domain arrangement. Cis-acting elements related to hormones, stress response, cell cycle and development were predicted in the promoter region. The expression of GhCDPKs could be regulated by various stresses. Gh_D11G188500.1 and Gh_A11G186100.1 was up-regulated under Vd0738 and Vd991 stress. Further phosphoproteomics analysis showed that Gh_A11G186100.1 (named as GhCDPK28-6) was phosphorylated under the stress of V. dahliae. Knockdown of GhCDPK28-6 expression, the content of reactive oxygen species was increased, a series of defense responses were enhanced, and the sensitivity of cotton to V. dahliae was reduced. Moreover, overexpression of GhCDPK28-6 in Arabidopsis thaliana weakened the resistance of plants to this pathogen. Subcellular localization revealed that GhCDPK28-6 was localized in the cell membrane. We also found that GhPBL9 and GhRPL12C may interact with GhCDPK28-6. These results indicate that GhCDPK28-6 is a potential molecular target for improving resistance to Verticillium wilt in cotton. This lays a foundation for breeding disease-resistant varieties.

Comprehensive Analysis of Respiratory Burst Oxidase Homologs (Rboh) Gene Family and Function of GbRboh5/18 on Verticillium Wilt Resistance in Gossypium barbadense

Respiratory burst oxidase homologs (Rbohs) play a predominant role in reactive oxygen species (ROS) production, which is crucial in plant growth, differentiation, as well as their responses to biotic and abiotic stresses. To date, however, there is little knowledge about the function of cotton Rboh genes. Here, we identified a total of 87 Rbohs from five sequenced Gossypium species (the diploids Gossypium arboreum, Gossypium raimondii, and Gossypium australe, and the allotetraploids Gossypium hirsutum and Gossypium barbadense) via BLAST searching their genomes. Phylogenetic analysis of the putative 87 cotton Rbohs revealed that they were divided into seven clades. All members within the same clade are generally similar to each other in terms of gene structure and conserved domain arrangement. In G. barbadense, the expression levels of GbRbohs in the CladeD were induced in response to a fungal pathogen and to hormones (i.e., jasmonic acid and abscisic acid), based upon which the main functional member in CladeD was discerned to be GbRboh5/18. Further functional and physiological analyses showed that the knock-down of GbRboh5/18 expression attenuates plant resistance to Verticillium dahliae infection. Combined with the molecular and biochemical analyses, we found less ROS accumulation in GbRboh5/18-VIGS plants than in control plants after V. dahliae infection. Overexpression of GbRboh5/18 in G. barbadense resulted in more ROS accumulation than in control plants. These results suggest that GbRboh5/18 enhances the cotton plants' resistance against V. dahliae by elevating the levels of ROS accumulation. By integrating phylogenetic, molecular, and biochemical approaches, this comprehensive study provides a detailed overview of the number, phylogeny, and evolution of the Rboh gene family from five sequenced Gossypium species, as well as elucidating the function of GbRboh5/18 for plant resistance against V. dahliae. This study sheds fresh light on the molecular evolutionary properties and function of Rboh genes in cotton, and provides a reference for improving cotton's responses to the pathogen V. dahliae.

Molecular characterization and functional analysis of a specific secreted protein from highly virulent defoliating Verticillium dahliae

Verticillium dahliae Kleb. is a phytopathogenic fungus that causes wilt diseases in hundreds of dicotyledonous plant species. Previous research has demonstrated that the secretome plays an important role in the pathogenicity of V. dahliae. In this study, the specific secreted protein gene (VdSSP1) in highly virulent defoliating V. dahliae strain VDG1 was cloned, and considered to be a secreted protein by signal peptide activity assay. VdSSP1 deletion mutants in VDG1 significantly compromised virulence, and the fungal growth decreased in media with pectin and starch as carbon sources. Pathogenicity and carbon utilization were restored upon complementation of the VdSSP1 deletion strains or low virulence non-defoliating strain VDG2, which lacks VdSSP1. It is indicated that the virulence role of VdSSP1 is associated with plant cell wall degradation. In conclusion, our data suggested that VdSSP1 is a secreted protein that is engaged in the pathogenicity of the highly virulent defoliating V. dahliae.