VdPKS1 is required for melanin formation and virulence in a cotton wilt pathogen Verticillium dahliae

The potential of Burkholderia gladioli KRS027 in plant growth promotion and biocontrol against Verticillium dahliae revealed by dual transcriptome of pathogen and host

Verticillium dahliae is a destructive, soil-borne pathogen that causes significant losses on numerous important dicots. Recently, beneficial microbes inhabiting the rhizosphere have been exploited and used to control plant diseases. In the present study, Burkholderia gladioli KRS027 demonstrated excellent inhibitory effects against Verticillium wilt in cotton seedlings. Plant growth and development was promoted by affecting the biosynthesis and signaling pathways of brassinosteroids (BRs), gibberellins (GAs), and auxins, consequently promoting stem elongation, shoot apical meristem, and root apical tissue division in cotton. Furthermore, based on the host transcriptional response to V. dahliae infection, it was found that KRS027 modulates the plants to maintain cell homeostasis and respond to other pathogen stress. Moreover, KRS027 induced disruption of V. dahliae cellular structures, as evidenced by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses. Based on the comparative transcriptomic analysis between KRS027 treated and control group of V. dahliae, KRS027 induced substantial alterations in the transcriptome, particularly affecting genes encoding secreted proteins, small cysteine-rich proteins (SCRPs), and protein kinases. In addition, KRS027 suppressed the growth of different clonal lineages of V. dahliae strains through metabolites, and volatile organic compounds (VOCs) released by KRS027 inhibited melanin biosynthesis and microsclerotia development. These findings provide valuable insights into an alternative biocontrol strategy for Verticillium wilt, demonstrating that the antagonistic bacterium KRS027 holds promise as a biocontrol agent for promoting plant growth and managing disease occurrence.

The HD-Zip I transcription factor MdHB-7 negatively regulates resistance to Glomerella leaf spot in apple

Glomerella leaf spot (GLS), caused by Colletotrichum fructicola (Cf), has been one of the main fungal diseases afflicting apple-producing areas across the world for many years, and it has led to substantial reductions in apple output and quality. HD-Zip transcription factors have been identified in several species, and they are involved in the immune response of plants to various types of biotic stress. In this study, inoculation of MdHB-7 overexpressing (MdHB-7-OE) and interference (MdHB-7-RNAi) transgenic plants with Cf revealed that MdHB-7, which encodes an HD-Zip transcription factor, adversely affects GLS resistance. The SA content and the expression of SA pathway-related genes were lower in MdHB-7-OE plants than in 'GL-3' plants; the content of ABA and the expression of ABA biosynthesis genes were higher in MdHB-7-OE plants than in 'GL-3' plants. Further analysis indicated that the content of phenolics and chitinase and β-1, 3 glucanase activities were lower and H2O2 accumulation was higher in MdHB-7-OE plants than in 'GL-3' plants. The opposite patterns were observed in MdHB-7-RNAi apple plants. Overall, our results indicate that MdHB-7 plays a negative role in regulating defense against GLS in apple, which is likely achieved by altering the content of SA, ABA, polyphenols, the activities of defense-related enzymes, and the content of H2O2.

Genome sequencing and comparative genomics reveal insights into pathogenicity and evolution of Fusarium zanthoxyli, the causal agent of stem canker in prickly ash

BACKGROUND

Fusarium zanthoxyli is a destructive pathogen causing stem canker in prickly ash, an ecologically and economically important forest tree. However, the genome lack of F. zanthoxyli has hindered research on its interaction with prickly ash and the development of precise control strategies for stem canker.

RESULTS

In this study, we sequenced and annotated a relatively high-quality genome of F. zanthoxyli with a size of 43.39 Mb, encoding 11,316 putative genes. Pathogenicity-related factors are predicted, comprising 495 CAZymes, 217 effectors, 156 CYP450s, and 202 enzymes associated with secondary metabolism. Besides, a comparative genomics analysis revealed Fusarium and Colletotrichum diverged from a shared ancestor approximately 141.1 ~ 88.4 million years ago (MYA). Additionally, a phylogenomic investigation of 12 different phytopathogens within Fusarium indicated that F. zanthoxyli originated approximately 34.6 ~ 26.9 MYA, and events of gene expansion and contraction within them were also unveiled. Finally, utilizing conserved domain prediction, the results revealed that among the 59 unique genes, the most enriched domains were PnbA and ULP1. Among the 783 expanded genes, the most enriched domains were PKc_like kinases and those belonging to the APH_ChoK_Like family.

CONCLUSION

This study sheds light on the genetic basis of F. zanthoxyli's pathogenicity and evolution which provides valuable information for future research on its molecular interactions with prickly ash and the development of effective strategies to combat stem canker.

Kss1 of Verticillium dahliae regulates virulence, microsclerotia formation, and nitrogen metabolism

Verticillium dahliae causes destructive vascular wilt diseases on more than 200 plant species, including economically important crops and ornamental trees worldwide. The melanized microsclerotia (MS) enable V. dahliae to survive for years in soil, thus the fungus is especially difficult to control once it has become established. Previously, we found that the mitogen activated protein kinase VdSte11 (MAPKKK) plays key roles in MS formation, penetration, and virulence in V. dahliae. In this study, two MAPK homologs of the yeast Ste7p and Kss1p were identified and characterized in V. dahliae. Deletion of VdSte7 or VdKss1 reuslted in severe defects in melaninized MS formation and virulence. Furthermore, phosphorylation assays demonstrated that VdSte11 and VdSte7 can phosphorylate VdKss1 in V. dahliae. Proteomic analysis revealed a significant change in sterol biosynthesis with a fold change of ≥ 1.2 after the deletion of VdKss1. In addition, phosphoproteomic analysis showed that VdKss1 was involved in the regulation of nitrogen metabolism. Finally, we identified VdRlm1 as a potentially downstream target of VdKss1, which is involved in regulating ammonium nitrogen utilization. This study sheds light on the network of regulatory proteins in V. dahliae that affect MS formation and nitrogen metabolism.

Protein Kinase C Is Involved in Vegetative Development, Stress Response and Pathogenicity in Verticillium dahliae

Potato Verticillium wilt, caused by Verticillium dahliae, is a serious soil-borne vascular disease, which restricts the sustainable development of the potato industry, and the pathogenic mechanism of the fungus is complex. Therefore, it is of great significance to explore the important pathogenic factors of V. dahliae to expand the understanding of its pathology. Protein kinase C (PKC) gene is located in the Ca2+ signaling pathway, which is highly conserved in filamentous fungi and involved in the regulation of a variety of biological processes. In the current study, the PKC gene in V. dahliae (VdPKC) was characterized, and its effects on the fungal pathogenicity and tolerance to fungicide stress were further studied. The results showed that the VdPKC positively regulated the growth and development, conidial germination, and production of V. dahliae, which was necessary for the fungus to achieve pathogenicity. It also affected the formation of melanin and microsclerotia and changed the adaptability of V. dahliae to different environmental stresses. In addition, VdPKC altered the tolerance of V. dahliae to different fungicides, which may be a potential target for polyoxin. Therefore, our results strongly suggest that VdPKC gene is necessary for the vegetative growth, stress response, and pathogenicity of V. dahliae.

Comparative Transcriptome Analysis Reveals the Effect of the DHN Melanin Biosynthesis Pathway on the Appressorium Turgor Pressure of the Poplar Anthracnose-Causing Fungus Colletotrichum gloeosporioides

Anthracnose of poplar caused by Colletotrichum gloeosporioides is a leaf disease that seriously affects poplar growth. The pathogen invades the host in the form of adherent cells, which generate turgor pressure through the metabolism of intracellular substances prior to penetrating the epidermis of poplar leaves. In this study, the expansion-related pressure of the mature appressorium of the wild-type C. gloeosporioides was approximately 13.02 ± 1.54 MPa at 12 h, whereas it was 7.34 ± 1.23 MPa and 9.34 ± 2.22 MPa in the melanin synthesis-related gene knockout mutants ΔCgCmr1 and ΔCgPks1, respectively. The CgCmr1 and CgPks1 genes were highly expressed at 12 h in the wild-type control, implying that the DHN melanin biosynthesis pathway may play an important role in the mature appressorium stage. The transcriptome sequencing analysis indicated that the upregulated melanin biosynthesis genes in C. gloeosporioides, such as CgScd1, CgAyg1, CgThr1, CgThr2, and CgLac1, are involved in specific KEGG pathways (i.e., fatty acid biosynthesis, fatty acid metabolism, and biotin metabolism). Therefore, we speculate that the melanin synthesis-related genes and fatty acid metabolism pathway genes contribute to the regulation of the turgor pressure in the mature C. gloeosporioides appressorium, ultimately leading to the formation of infection pegs that enter plant tissues. These observations may reflect the co-evolution of C. gloeosporioides and its host.

Tomato Xylem Sap Hydrophobins Vdh4 and Vdh5 Are Important for Late Stages of Verticillium dahliae Plant Infection

Verticillium dahliae causes economic losses to a wide range of crops as a vascular fungal pathogen. This filamentous ascomycete spends long periods of its life cycle in the plant xylem, a unique environment that requires adaptive processes. Specifically, fungal proteins produced in the xylem sap of the plant host may play important roles in colonizing the plant vasculature and in inducing disease symptoms. RNA sequencing revealed over 1500 fungal transcripts that are significantly more abundant in cells grown in tomato xylem sap compared with pectin-rich medium. Of the 85 genes that are strongly induced in the xylem sap, four genes encode the hydrophobins Vdh1, Vdh2, Vdh4 and Vdh5. Vdh4 and Vhd5 are structurally distinct from each other and from the three other hydrophobins (Vdh1-3) annotated in V. dahliae JR2. Their functions in the life cycle and virulence of V. dahliae were explored using genetics, cell biology and plant infection experiments. Our data revealed that Vdh4 and Vdh5 are dispensable for V. dahliae development and stress response, while both contribute to full disease development in tomato plants by acting at later colonization stages. We conclude that Vdh4 and Vdh5 are functionally specialized fungal hydrophobins that support pathogenicity against plants.

Loss of function of VdDrs2, a P4-ATPase, impairs the toxin secretion and microsclerotia formation, and decreases the pathogenicity of Verticillium dahliae

Four P4-ATPase flippase genes, VdDrs2, VdNeo1, VdP4-4, and VdDnf1 were identified in Verticillium dahliae, one of the most devastating phytopathogenic fungi in the world. Knock out of VdDrs2, VdNeo1, and VdP4-4, or knock down of VdDnf1 significantly decreased the pathogenicity of the mutants in cotton. Among the mutants, the greatest decrease in pathogenicity was observed in ΔVdDrs2. VdDrs2 was localized to plasma membrane, vacuoles, and trans-Golgi network (TGN). In vivo observation showed that the infection of the cotton by ΔVdDrs2 was significantly delayed. The amount of two known Verticillium toxins, sulfacetamide, and fumonisin B1 in the fermentation broth produced by the ΔVdDrs2 strain was significantly reduced, and the toxicity of the crude Verticillium wilt toxins to cotton cells was attenuated. In addition, the defect of VdDrs2 impaired the synthesis of melanin and the formation of microsclerotia, and decreased the sporulation of V. dahliae. Our data indicate a key role of P4 ATPases-associated vesicle transport in toxin secretion of disease fungi and support the importance of mycotoxins in the pathogenicity of V. dahliae.

Deletion C-terminal thioesterase abolishes melanin biosynthesis, affects metabolism and reduces the pathogenesis of Fonsecaea monophora

Dematiaceous Fonsecaea monophora is one of the major pathogens of chromoblastomycosis. It has been well established that melanization is catalyzed by the type I polyketide synthase (PKS) in F. monophora. Multidomain protein Type I PKS is encoded by six genes, in which the last enzyme thioesterase (TE) catalyzes the cyclization and releases polyketide. Two PKS genes AYO21_03016 (pks1) and AYO21_10638 have been found in F. monophora and both PKS loci have the same gene arrangement but the TE domain in AYO21_10638 is truncated at 3’- end. TE may be the key enzyme to maintain the function of pks1. To test this hypothesis, we constructed a 3’-end 500 bp deletion mutant of AYO21_03016 (Δpks1-TE-C500) and its complemented strain. We profiled metabolome of this mutant and analyzed the consequences of impaired metabolism in this mutant by fungal growth in vitro and by pathogenesis in vivo. Compared with wild-type strain, we found that the mutant repressed pks1 expression and other 5 genes expression levels were reduced by more than 50%, perhaps leading to a corresponding melanin loss. The mutant also reduced sporulation and delayed germination, became vulnerable to various environmental stresses and was less resistance to macrophage or neutrophil killings in vitro, and less virulence in mice footpad model. Metabolomic analysis indicated that many metabolites were remarkably affected in Δpks1-TE-C500, in particular, an increased nicotinamide and antioxidant glutathione. In conclusion, we confirmed the crucial role of C-terminal TE in maintaining fully function of pks1 in F. monophora. Deletion of TE negatively impacts on the synthesis of melanin and metabolites that eventually affect growth and virulence of F. monophora. Any potential inhibitor of TE then could be a novel antifungal target for drug development.

F. monophora is a fungal pathogen that causes chromoblastomycosis. Melanin of F. monophora was synthesized through PKS in which TE is the last enzyme to catalyze the cyclization and release polyketide. Few studies have investigated the effect of TE on the metabolism and pathogenesis of F. monophora. In this study, TE deletion leads to albino phenotype, decreases the expression of other domains of the pks1, and reduces biosynthesis of metabolites. The Δpks1-TE-C500 strain exhibits a changed morphogenesis and becomes less resistant to various environmental stresses. In vitro study, the Δpks1-TE-C500 strain is avirulent and less resistant to macrophages and neutrophils. In conclusion, we demonstrate that the 500 bp C-terminal of TE is essential for the function of pks1, perhaps through its effects on melanin and metabolites to regulate the growth and virulence of F. monophora. Data from this study could inspire an exploration in development of clinical therapy for CBM.

The essential role of fungal peroxisomes in plant infection

Several filamentous fungi are ecologically and economically important plant pathogens that infect a broad variety of crops. They cause high annual yield losses and contaminate seeds and fruits with mycotoxins. Not only powerful infection structures and detrimental toxins, but also cell organelles, such as peroxisomes, play important roles in plant infection. In this review, we summarize recent research results that revealed novel peroxisomal functions of filamentous fungi and highlight the importance of peroxisomes for infection of host plants. Central for fungal virulence are two primary metabolic pathways, fatty acid β-oxidation and the glyoxylate cycle, both of which are required to produce energy, acetyl-CoA, and carbohydrates. These are ultimately needed for the synthesis of cell wall polymers and for turgor generation in infection structures. Most novel results stem from different routes of secondary metabolism and demonstrate that peroxisomes produce important precursors and house various enzymes needed for toxin production and melanization of appressoria. All these peroxisomal functions in fungal virulence might represent elegant targets for improved crop protection.

The bZip Transcription Factor VdMRTF1 is a Negative Regulator of Melanin Biosynthesis and Virulence in Verticillium dahliae

The ascomycete fungus Verticillium dahliae infects over 400 plant species and causes serious losses of economically important crops, such as cotton and tomato, and also of woody plants, such as smoke tree, maple, and olive. Melanized long-term survival structures known as microsclerotia play crucial roles in the disease cycle of V. dahliae, enabling this soilborne fungus to survive for years in the soil in the absence of a host. Previously, we identified VdMRTF1 (microsclerotia-related transcription factor) encoding a bZip transcription factor which is downregulated during microsclerotial development in V. dahliae. In the present study, we showed that VdMRTF1 negatively controls melanin production and virulence by genetic, biological, and transcriptomic analyses. The mutant strain lacking VdMRTF1 (ΔVdMRTF1) exhibited increased melanin biosynthesis and the defect also promoted microsclerotial development and sensitivity to Ca²⁺. In comparison with the wild-type strain, the ΔVdMRTF1 strain showed a significant enhancement in virulence and displayed an increased capacity to eliminate reactive oxygen species in planta. Furthermore, analyses of transcriptomic profiles between the ΔVdMRTF1 and wild-type strains indicated that VdMRTF1 regulates the differential expression of genes associated with melanin biosynthesis, tyrosine metabolism, hydrogen peroxide catabolic processes, and oxidoreductase activity in V. dahliae. Taken together, these data show that VdMRTF1 is a negative transcriptional regulator of melanin biosynthesis, microsclerotia formation, and virulence in V. dahliae.

IMPORTANCE Verticillium wilt is difficult to manage because the pathogen colonizes the plant xylem tissue and produces melanized microsclerotia which survive for more than 10 years in soil without a host. The molecular mechanisms underlying microsclerotia formation are of great importance to control the disease. Here, we provide evidence that a bZip transcription factor, VdMRTF1, plays important roles in melanin biosynthesis, microsclerotial development, resistance to elevated Ca²⁺ levels, and fungal virulence of V. dahliae. The findings extend and deepen our understanding of the complexities of melanin biosynthesis, microsclerotia formation, and virulence that are regulated by bZip transcription factors in V. dahliae.

The Verticillium dahliae Spt-Ada-Gcn5 Acetyltransferase Complex Subunit Ada1 Is Essential for Conidia and Microsclerotia Production and Contributes to Virulence

Verticillium dahliae is a destructive soil-borne pathogen of many economically important dicots. The genetics of pathogenesis in V. dahliae has been extensively studied. Spt-Ada-Gcn5 acetyltransferase complex (SAGA) is an ATP-independent multifunctional chromatin remodeling complex that contributes to diverse transcriptional regulatory functions. As members of the core module in the SAGA complex in Saccharomyces cerevisiae, Ada1, together with Spt7 and Spt20, play an important role in maintaining the integrity of the complex. In this study, we identified homologs of the SAGA complex in V. dahliae and found that deletion of the Ada1 subunit (VdAda1) causes severe defects in the formation of conidia and microsclerotia, and in melanin biosynthesis and virulence. The effect of VdAda1 on histone acetylation in V. dahliae was confirmed by western blot analysis. The deletion of VdAda1 resulted in genome-wide alteration of the V. dahliae transcriptome, including genes encoding transcription factors and secreted proteins, suggesting its prominent role in the regulation of transcription and virulence. Overall, we demonstrated that VdAda1, a member of the SAGA complex, modulates multiple physiological processes by regulating global gene expression that impinge on virulence and survival in V. dahliae.

Acetolactate synthases regulatory subunit and catalytic subunit genes VdILVs are involved in BCAA biosynthesis, microscletotial and conidial formation and virulence in Verticillium dahliae

Acetolactate synthase (AHAS) catalyses the first common step in the biosynthesis pathways of three branched-chain amino acids (BCAAs) of valine, isoleucine and leucine. Here, we characterized one regulatory subunit (VdILV6) and three catalytic subunits (VdILV2A, VdILV2B and VdILV2C) of AHAS from the important cotton Verticillium wilt fungus Verticillium dahliae. Phenotypic analysis showed that VdILV6 knockout mutants were auxotrophic for valine and isoleucine and were defective in conidial morphogenesis, hypha penetration and virulence to cotton, and lost ability of microscletotial formation. The growth of single catalytic subunit gene knockout mutants were significantly inhibited by leucine at higher concentration and single catalytic subunit gene knockout mutants showed significantly reduced virulence to cotton. VdILV2B knockout also led to obviously reduced microscletotial formation and conidial production, VdILV2C knockout led to reduced conidial production. Further studies suggested that both feedback inhibition by leucine and the inhibition by AHAS inhibiting herbicides of tribenuron and bispyribac resulted in significantly down-regulated expression of the four subunit VdILVs genes (VdILV2A, VdILV2B, VdILV2C and VdILV6). Any single catalytic subunit gene knockout led to reduced expression of the other three subunit genes, whereas VdILV6 knckout induced increased expression of the three catalytic subunit genes. VdILV2B, VdILV2C and VdILV6 knockout resulted in increased expression of VdCPC1 regulator gene of the cross-pathway control of amino acid biosynthesis. Taken together, these results indicate multiple roles of four VdILVs genes in the biosynthesis of BCAAs, virulence, fungal growth and development in the filamentous fungi V. dahliae.

Analysis of melanin biosynthesis in the plant pathogenic fungus Colletotrichum gloeosporioides

Melanin is recognized as a dark pigment that can protect fungi from the harm of environmental stresses. To investigate what roles of melanin played in the pathogenicity and development of Colletotrichum gloeosporioides, a causal agent of poplar anthracnose, genes encoding a transcription factor CgCmr1 and a polyketide synthase CgPks1 were isolated as the ortholog of Magnaporthe oryzae Pig1 and Pks1 respectively. Deletion of CgCmr1 or CgPks1 resulted in melanin-deficient fungal colony. The ΔCgPks1 mutant showed no melanin accumulation in appressoria, and lack of CgCmr1 also resulted in the delayed and decreased melanization of appressoria. In addition, the turgor pressure of the appressorium was lower in ΔCgPks1 and ΔCgCmr1 than in the wild-type (WT). However, DHN melanin was not a vital factor for virulence in C. gloeosporioides. Moreover, deletion of CgCmr1 and CgPks1 resulted in the hypersensitivity to hydrogen peroxide (H₂O₂) oxidative stress but not to other abiotic stresses. Collectively, these results suggest that CgCmr1 and CgPks1 play an important role in DHN melanin biosynthesis, and melanin was not an essential factor in penetration and pathogenicity in C. gloeosporioides. The data presented in this study will facilitate future evaluations of the melanin biosynthetic pathway and development in filamentous fungi.

The GhSWEET42 Glucose Transporter Participates in Verticillium dahliae Infection in Cotton

The SWEET (sugars will eventually be exported transporter) proteins, a family of sugar transporters, mediate sugar diffusion across cell membranes. Pathogenic fungi can acquire sugars from plant cells to satisfy their nutritional demands for growth and infection by exploiting plant SWEET sugar transporters. However, the mechanism underlying the sugar allocation in cotton plants infected by Verticillium dahliae, the causative agent of Verticillium wilt, remains unclear. In this study, observations of the colonization of cotton roots by V. dahliae revealed that a large number of conidia had germinated at 48-hour post-inoculation (hpi) and massive hyphae had appeared at 96 hpi. The glucose content in the infected roots was significantly increased at 48 hpi. On the basis of an evolutionary analysis, an association analysis, and qRT-PCR assays, GhSWEET42 was found to be closely associated with V. dahliae infection in cotton. Furthermore, GhSWEET42 was shown to encode a glucose transporter localized to the plasma membrane. The overexpression of GhSWEET42 in Arabidopsis thaliana plants led to increased glucose content, and compromised their resistance to V. dahliae. In contrast, knockdown of GhSWEET42 expression in cotton plants by virus-induced gene silencing (VIGS) led to a decrease in glucose content, and enhanced their resistance to V. dahliae. Together, these results suggest that GhSWEET42 plays a key role in V. dahliae infection in cotton through glucose translocation, and that manipulation of GhSWEET42 expression to control the glucose level at the infected site is a useful method for inhibiting V. dahliae infection.

Deletion of pksA attenuates the melanogenesis, growth and sporulation ability and causes increased sensitivity to stress response and antifungal drugs in the human pathogenic fungus Fonsecaea monophora

Fonsecaea monophora, which is very similar to Fonsecaea pedrosoi in morphological features, has been commonly misdiagnosed as F. pedrosoi. Like F. pedrosoi, F. monophora has been also identified as a predominant pathogen of Chromoblastomycosis (CBM). Melanin has been recognized as a virulence factor in several fungi, however, it is still largely unknown about the biological role of melanin and how melanin is synthesized in F. monophora. In this study, we identified two putative polyketide synthase genes (pks), AYO21_03016 (pksA) and AYO21_10638, by searching against the genome of F. monophora. AYO21_03016 and AYO21_10638 were further targeted disrupted by Agrobacterium tumefaciens-mediated transformation (ATMT). We discovered that pksA gene was the major polyketide synthase required for melanin synthesis in F. monophora, rather than AYO21_10638. Phenotypic analysis showed that, knocking out of the pksA gene attenuated melanogenesis, growth rate, sporulation ability and virulence of F. monophora, as compared with wild-type and complementation strain (pksA-C). Furthermore, the ΔpksA mutant was confirmed to be more sensitive to the oxidative stress, extreme pH environment, and antifungal drugs including itraconazole (ITC), terbinafine (TER), and amphotericin B (AMB). Taken together, these findings enabled us to comprehend the role of pksA in regulating DHN-melanin pathway and its effect on the biological function of F. monophora.

Opportunities and Challenges in Studies of Host-Pathogen Interactions and Management of Verticillium dahliae in Tomatoes

Tomatoes (Solanum lycopersicum L.) are a valuable horticultural crop that are grown and consumed worldwide. Optimal production is hindered by several factors, among which Verticillium dahliae, the cause of Verticillium wilt, is considered a major biological constraint in temperate production regions. V. dahliae is difficult to mitigate because it is a vascular pathogen, has a broad host range and worldwide distribution, and can persist in soil for years. Understanding pathogen virulence and genetic diversity, host resistance, and plant-pathogen interactions could ultimately inform the development of integrated strategies to manage the disease. In recent years, considerable research has focused on providing new insights into these processes, as well as the development and integration of environment-friendly management approaches. Here, we discuss the current knowledge on the race and population structure of V. dahliae, including pathogenicity factors, host genes, proteins, enzymes involved in defense, and the emergent management strategies and future research directions for managing Verticillium wilt in tomatoes.

Contributions of Spore Secondary Metabolites to UV-C Protection and Virulence Vary in Different Aspergillus fumigatus Strains

Fungal spores contain secondary metabolites that can protect them from a multitude of abiotic and biotic stresses. Conidia (asexual spores) of the human pathogen Aspergillus fumigatus synthesize several metabolites, including melanin, which has been reported to be important for virulence in this species and to be protective against UV radiation in other fungi. Here, we investigate the role of melanin in diverse isolates of A. fumigatus and find variability in its ability to protect spores from UV-C radiation or impact virulence in a zebrafish model of invasive aspergillosis in two clinical strains and one ISS strain. Further, we assess the role of other spore metabolites in a clinical strain of A. fumigatus and identify fumiquinazoline as an additional UV-C-protective molecule but not a virulence determinant. The results show differential roles of secondary metabolites in spore protection dependent on the environmental stress and strain of A. fumigatus. As protection from elevated levels of radiation is of paramount importance for future human outer space explorations, the discovery of small molecules with radiation-protective potential may result in developing novel safety measures for astronauts.

Fungi are versatile organisms which thrive in hostile environments, including the International Space Station (ISS). Several isolates of the human pathogen Aspergillus fumigatus have been found contaminating the ISS, an environment with increased exposure to UV radiation. Secondary metabolites (SMs) in spores, such as melanins, have been shown to protect spores from UV radiation in other fungi. To test the hypothesis that melanin and other known spore SMs provide UV protection to A. fumigatus isolates, we subjected SM spore mutants to UV-C radiation. We found that 1,8-dihydroxynaphthalene (DHN)-melanin mutants of two clinical A. fumigatus strains (Af293 and CEA17) but not an ISS-isolated strain (IF1SW-F4) were more sensitive to UV-C than their respective wild-type (WT) strains. Because DHN-melanin has been shown to shield A. fumigatus from the host immune system, we examined all DHN mutants for virulence in the zebrafish model of invasive aspergillosis. Following recent studies highlighting the pathogenic variability of different A. fumigatus isolates, we found DHN-melanin to be a virulence factor in CEA17 and IF1SW-F4 but not Af293. Three additional spore metabolites were examined in Af293, where fumiquinazoline also showed UV-C-protective properties, but two other spore metabolites, monomethylsulochrin and fumigaclavine, provided no UV-C-protective properties. Virulence tests of these three SM spore mutants indicated a slight increase in virulence of the monomethylsulochrin deletion strain. Taken together, this work suggests differential roles of specific spore metabolites across Aspergillus isolates and by types of environmental stress.

The Vta1 transcriptional regulator is required for microsclerotia melanization in Verticillium dahliae

Many fungi are able to produce resting structures, which ensure survival and protect them against various stresses in their habitat such as exposure to UV light, temperature variations, drought as well as changing pH and nutrient conditions. Verticillium dahliae is a plant pathogenic fungus that forms melanized resting structures, called microsclerotia, for survival of time periods without a host. These highly stress resistant microsclerotia persist in the soil for many years and are therefore problematic for an effective treatment of the fungus. The Verticillium transcription activator of adhesion 1 (Vta1) was initially identified as one of several transcriptional regulators that rescue adhesion in non-adhesive Saccharomyces cerevisiae cells. Vta2 and Vta3 are required for early steps in plant infection and colonization and additionally control microsclerotia formation. Here, we show that Vta1 function is different, because it is dispensable for root colonization and infection. Vta1 is produced in the fungal cell during microsclerotia development. Analysis of the deletion mutant revealed that the absence of Vta1 allows microsclerotia production, but they are colorless and no more melanized. Vta1 is required for melanin production and activates transcription of melanin biosynthesis genes including the polyketide synthase encoding PKS1 and the laccase LAC1. The primary function of Vta1 in melanin production is important for survival of microsclerotia as resting structures of V. dahliae.

The Verticillium dahliae Sho1-MAPK pathway regulates melanin biosynthesis and is required for cotton infection

Verticillium dahliae is a soil‐borne fungus that causes vascular wilt on numerous plants worldwide. The fungus survives in the soil for up to 14 years by producing melanized microsclerotia. The protective function of melanin in abiotic stresses is well documented. Here, we found that the V. dahliae tetraspan transmembrane protein VdSho1, a homolog of the Saccharomyces cerevisiae Sho1, acts as an osmosensor, and is required for plant penetration and melanin biosynthesis. The deletion mutant ΔSho1 was incubated on a cellophane membrane substrate that mimics the plant epidermis, revealing that the penetration of ΔSho1 strain was reduced compared to the wild‐type strain. Furthermore, VdSho1 regulates melanin biosynthesis by a signalling mechanism requiring a kinase‐kinase signalling module of Vst50‐Vst11‐Vst7. Strains, ΔVst50, ΔVst7 and ΔVst11 also displayed defective penetration and melanin production like the ΔSho1 strain. Defects in penetration and melanin production in ΔSho1 were restored by overexpression of Vst50, suggesting that Vst50 lies downstream of VdSho1 in the regulatory pathway governing penetration and melanin biosynthesis. Data analyses revealed that the transmembrane portion of VdSho1 was essential for both membrane penetration and melanin production. This study demonstrates that Vst50‐Vst11‐Vst7 module regulates VdSho1‐mediated plant penetration and melanin production in V. dahliae, contributing to virulence.

Production of melanin pigments in saprophytic fungi in vitro and during infection

Melanins are one of the great natural pigments produced by a wide variety of fungal species that promote fitness and cell survival in diverse hostile environments, including during mammalian infection. In this study, we sought to demonstrate the production of melanin in the conidia and hyphae of saprophytic fungi, including dematiaceous and hyaline fungi. We showed that a melanin-specific monoclonal antibody (MAb) avidly labeled the cell walls of hyphae and conidia, consistent with the presence of melanin in these structures, in 14 diverse fungal species. The conidia of saprophytic fungi were treated with proteolytic enzymes, denaturant, and concentrated hot acid to yield dark particles, which were shown to be stable free radicals, consistent with their identification as melanins. Samples obtained from patients with fungal keratitis due to Fusarium falciforme, Aspergillus fumigatus, Aspergillus flavus, Curvularia lunata, Exserohilum rostratum, or Fonsecaea pedrosoi were found to be intensely labeled by the melanin-specific MAb at the fungal hyphal cell walls. These results support the hypothesis that melanin is a common component that promotes survival under harsh conditions and facilitates fungal virulence. Increased understanding of the processes of melanization and the development of methods to interfere with pigment formation may lead to novel approaches to combat these complex pathogens that are associated with high rates of morbidity and mortality.

Insights into VdCmr1-mediated protection against high temperature stress and UV irradiation in Verticillium dahliae

The fungus Verticillium dahliae causes vascular wilt disease on more than 200 plant species worldwide. This fungus can survive for years in soil as melanized microsclerotia. We found that VdCmr1, a transcription factor, is required for the melanin production and increased survival following UV irradiation in V. dahliae but not for microsclerotia production or virulence. Here, we provided evidence how VdCmr1 protects against high temperature (HT) and UV irradiation in V. dahliae. The results indicate that VdCmr1 mediates entry to the diapause period in V. dahliae in response to HT and contributes to the expression of proteins to minimize protein misfolding and denaturation. VdCmr1 deletion results in the misregulation of DNA repair machinery, suggestive of reduced DNA repair capacity following UV irradiation and in correlation with the low survival rate of UV-treated VdCmr1 mutants. We discovered a putative VdCmr1-dependent gene cluster associated with secondary metabolism and stress responses. We also functionally characterized two VdCmr1-responsive genes participating in HT and UV response. These results shed further light on the roles of VdCmr1 in protection from HT or UV irradiation, and the additional insights into the mechanisms of this protection may be useful to exploit for more effective disease control.

The α-1,6-mannosyltransferase VdOCH1 plays a major role in microsclerotium formation and virulence in the soil-borne pathogen Verticillium dahliae

Sunflower yellow wilt is a widespread and destructive disease caused by the soil-borne pathogen Verticillium dahliae (V. dahliae). To better understand the pathogenesis mechanism of V. dahliae in sunflower, T-DNA insertion library was generated via Agrobacterium tumefaciens mediated transformation system (ATMT). Eight hundred positive transformants were obtained. Transformants varied in colony morphology, growth rate, conidia production and pathogenicity in sunflower compared to the wild type strain. A mutant, named VdGn3-L2, was chosen for further analysis based on its deprivation on microsclerotia formation. The flanking sequence of T-DNA insertion site of VdGn3-L2 was identified via hiTAIL-PCR, and the interrupted gene encoded an initiation-specific α-1, 6-mannosyltransferase, named as VdOCH1. The deletion mutant ΔVdOCH1 was impaired in certain characteristics such as fungal growth, conidia production, and microsclerotia formation. Also, ΔVdOCH1 mutants were more sensitive to the cell wall perturbing reagents, such as SDS and Congo red, lost their penetration ability through cellophane membrane, and exhibited dramatically decreased pathogenicity to sunflower. The impaired phenotypes could be restored to the wild type level by complementation of the deletion mutant with full-length VdOCH1 gene. In conclusion, VdOCH1, encoded α-1,6-mannosyltransferase, manipulating the biological characteristics, microsclerotia formation and pathogenic ability of V. dahliae in sunflower.

Fungal microsclerotia development: essential prerequisites, influencing factors, and molecular mechanism

Microsclerotia (MS) consist of an outer layer of pigment parenchyma cells and an inner layer of colorless medulla cells. In nature, MS are formed as overwintering and spreading structures in phytopathogenic fungi. For biological applications, MS can be induced in artificial liquid medium. To understand the complicated structure of MS and molecular mechanism of MS development in entomopathogenic and phytopathogenic fungi, data from different studies can be integrated. In this review, the essential prerequisites, environmental cues, and internal stimulating factors for MS development are explored. Emerging knowledges about the association between transcriptional regulatory circuits and signaling pathways involved in MS development in entomopathogenic and phytopathogenic fungi is also highlighted.

Physiological and molecular mechanism of defense in cotton against Verticillium dahliae

Cotton, a natural fiber producing crop of huge importance for textile industry, has been reckoned as the backbone in the economy of many developing countries. Verticillium wilt caused by Verticillium dahliae reflected as the most devastating disease of cotton crop in several parts of the world. Average losses due to attack of this disease are tremendous every year. There is urgent need to develop strategies for effective control of this disease. In the last decade, progress has been made to understand the interaction between cotton-V. dahliae and several growth and pathogenicity related genes were identified. Still, most of the molecular components and mechanisms of cotton defense against Verticillium wilt are poorly understood. However, from existing knowledge, it is perceived that cotton defense mechanism primarily depends on the pre-formed defense structures including thick cuticle, synthesis of phenolic compounds and delaying or hindering the expansion of the invader through advanced measures such as reinforcement of cell wall structure, accumulation of reactive oxygen species (ROS), release of phytoalexins, the hypersensitive response and the development of broad spectrum resistance named as, systemic acquired resistance (SAR). Investigation of these defense tactics provide valuable information about the improvement of cotton breeding strategies for the development of durable, cost effective, and broad spectrum resistant varieties. Consequently, this management approach will help to reduce the use of fungicides and also minimize other environmental hazards. In the present paper, we summarized the V. dahliae virulence mechanism and comprehensively discussed the cotton molecular mechanisms of defense such as physiological, biochemical responses with the addition of signaling pathways that are implicated towards attaining resistance against Verticillium wilt.

Transcription factor VdCmr1 is required for pigment production, protection from UV irradiation, and regulates expression of melanin biosynthetic genes in Verticillium dahliae

Verticillium dahliae is a soilborne fungus that causes vascular wilt diseases on numerous plant species worldwide. The production of darkly melanized microsclerotia is crucial in the disease cycle of V. dahliae, as these structures allow for long-term survival in soil. Previously, transcriptomic and genomic analysis identified a cluster of genes in V. dahliae that encodes some dihydroxynaphthalene (DHN) melanin biosynthetic pathway homologues found in related fungi. In this study, we explored the roles of cluster-specific transcription factor VdCmr1, as well as two other genes within the cluster encoding a polyketide synthase (VdPKS1) and a laccase (VdLac1), enzymes at initial and endpoint steps in DHN melanin production. The results revealed that VdCmr1 and VdPKS1 are required for melanin production, but neither is required for microsclerotia production. None of the three genes were required for pathogenesis on tobacco and lettuce. Exposure of ΔVdCmr1 and wild-type strains to UV irradiation, or to high temperature (40 °C), revealed an approx. 50 % reduction of survival in the ΔVdCmr1 strain, relative to the wild-type strain, in response to either condition. Expression profiles revealed that expression of some melanin biosynthetic genes are in part dependent on VdCmr1. Combined data indicate VdCmr1 is a key regulator of melanin biosynthesis, and that via regulation of melanogenesis, VdCmr1 affects survival of V. dahliae in response to abiotic threats. We conclude with a model showing regulation of VdCmr1 by a high osmolarity glycerol response (Hog)-type MAP kinase pathway.

VdPLP, A Patatin-Like Phospholipase in Verticillium dahliae, Is Involved in Cell Wall Integrity and Required for Pathogenicity

The soil-borne ascomycete fungus Verticillium dahliae causes vascular wilt disease and can seriously diminish the yield and quality of important crops. Functional analysis of growth- and pathogenicity-related genes is essential for revealing the pathogenic molecular mechanism of V. dahliae. Phospholipase is an important virulence factor in fungi that hydrolyzes phospholipids into fatty acid and other lipophilic substances and is involved in hyphal development. Thus far, only a few V. dahliae phospholipases have been identified, and their involvement in V. dahliae development and pathogenicity remains unknown. In this study, the function of the patatin-like phospholipase gene in V. dahliae (VdPLP, VDAG_00942) is characterized by generating gene knockout and complementary mutants. Vegetative growth and conidiation of VdPLP deletion mutants (ΔVdPLP) were significantly reduced compared with wild type and complementary strains, but more microsclerotia formed. The ΔVdPLP mutants were very sensitive to the cell-wall-perturbing agents: calcofluor white (CFW) and Congo red (CR). The transcriptional level of genes related to the cell wall integrity (CWI) pathway and chitin synthesis were downregulated, suggesting that VdPLP has a pivotal role in the CWI pathway and chitin synthesis in V. dahliae. ΔVdPLP strains were distinctly impaired in in their virulence and ability to colonize Nicotiana benthamiana roots. Our results demonstrate that VdPLP regulates hyphal growth and conidial production and is involved in stabilizing the cell wall, thus mediating the pathogenicity of V. dahliae.

Verticillium dahliae-Arabidopsis Interaction Causes Changes in Gene Expression Profiles and Jasmonate Levels on Different Time Scales

Verticillium dahliae is a soil-borne vascular pathogen that causes severe wilt symptoms in a wide range of plants. Co-culture of the fungus with Arabidopsis roots for 24 h induces many changes in the gene expression profiles of both partners, even before defense-related phytohormone levels are induced in the plant. Both partners reprogram sugar and amino acid metabolism, activate genes for signal perception and transduction, and induce defense- and stress-responsive genes. Furthermore, analysis of Arabidopsis expression profiles suggests a redirection from growth to defense. After 3 weeks, severe disease symptoms can be detected for wild-type plants while mutants impaired in jasmonate synthesis and perception perform much better. Thus, plant jasmonates have an important influence on the interaction, which is already visible at the mRNA level before hormone changes occur. The plant and fungal genes that rapidly respond to the presence of the partner might be crucial for early recognition steps and the future development of the interaction. Thus they are potential targets for the control of V. dahliae-induced wilt diseases.