Proteomics Analysis of SsNsd1-Mediated Compound Appressoria Formation in Sclerotinia sclerotiorum

The Transcription Factor SsZNC1 Mediates Virulence, Sclerotial Development, and Osmotic Stress Response in Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a fungal pathogen with a broad range of hosts, which can cause diseases and pose a great threat to many crops. Fungal-specific Zn2Cys6 transcription factors (TFs) constitute a large family prevalent among plant pathogens. However, the function of Zn2Cys6 TFs remains largely unknown. In this study, we identified and characterized SsZNC1, a Zn2Cys6 TF in S. sclerotiorum, which is involved in virulence, sclerotial development, and osmotic stress response. The expression of SsZNC1 was significantly up-regulated in the early stages of S. sclerotiorum infection on Arabidopsis leaves. The target deletion of SsZNC1 resulted in reduced virulence on Arabidopsis and oilseed rape. In addition, sclerotial development ability and growth ability under hyperosmotic conditions of SsZNC1 knockout transformants were reduced. A transcriptomic analysis unveiled its regulatory role in key cellular functions, including cellulose catabolic process, methyltransferase activity, and virulence, etc. Together, our results indicated that SsZNC1, a core regulatory gene involved in virulence, sclerotial development and stress response, provides new insight into the transcription regulation and pathogenesis of S. sclerotiorum.

The Sclerotinia sclerotiorum ADP-Ribosylation Factor 6 Plays an Essential Role in Abiotic Stress Response and Fungal Virulence to Host Plants

The ADP-ribosylation factor 6 (Arf6), as the only member of the Arf family III protein, has been extensively studied for its diverse biological functions in animals. Previously, the Arf6 protein in Magnaporthe oryzae was found to be crucial for endocytosis and polarity establishment during asexual development. However, its role remains unclear in S. sclerotiorum. Here, we identified and characterized SsArf6 in S. sclerotiorum using a reverse genetic approach. Deletion of SsArf6 impaired hyphal growth and development and produced more branches. Interestingly, knockout of SsArf6 resulted in an augmented tolerance of S. sclerotiorum towards oxidative stress, and increased its sensitivity towards osmotic stress, indicative of the different roles of SsArf6 in various stress responses. Simultaneously, SsArf6 deletion led to an elevation in melanin accumulation. Moreover, the appressorium formation was severely impaired, and fungal virulence to host plants was significantly reduced. Overall, our findings demonstrate the essential role of SsArf6 in hyphal development, stress responses, appressorium formation, and fungal virulence to host plants.

Draft genome sequencing and secretome profiling of Sclerotinia sclerotiorum revealed effector repertoire diversity and allied broad-host range necrotrophy

White mold commonly known as Sclerotinia sclerotiorum causes stem rot disease and has emerged as one of the major fungal pathogens of oilseed Brassica across the world. In the present study, consistently virulent S. sclerotiorum isolate "ESR-01" was sequenced and an assembly size of ~ 41 Mb with 328 scaffolds having N50 of 447,128 was obtained. Additionally, 27,450 single nucleotide polymorphisms (SNPs) were identified from 155 scaffolds against S. sclerotiorum 1980 isolate, with an average SNP density of ~ 1.5 per kb genome. 667 repetitive elements were identified and approximately comprised 7% of the total annotated genes. The DDE_1 with 454 in numbers was found to be the most abundant and accounts for 68% of the total predicted repetitive elements. In total, 3844 simple sequence repeats are identified in the 328 scaffolds. A total of 9469 protein-coding genes were predicted from the whole genome assembly with an average gene length of 1587 bp and their distribution as 230.95 genes per Mb in the genome. Out of 9469 predicted protein-coding genes, 529 genes were observed encoding the CAZymes (Carbohydrate-Active enzymes) capable of degradation of the complex polysaccharides. Glycosyltransferase (GT) families were most abundant (49.71%) among the predicted CAZymes and GT2 (23%), GT4 (20%), and glycoside hydrolase (GH) 23% with GH18 (11%) were the prominent cell wall degrading enzyme families in the ESR-01 secretome. Besides this, 156 genes essential for the pathogen-host interactions were also identified. The effector analysis in the whole genome proteomics dataset revealed a total of 57 effector candidates (ECs) and 27 of them were having their analogs whereas the remaining 30 were novel ones. Eleven selected ECs were validated experimentally by analyzing the expression profile of the ESR-01 isolate of S. sclerotiorum. Together, the present investigation offers a better understanding of the S. sclerotiorum genome, secretome, and its effector repertoire which will help in refining the present knowledge on S. sclerotiorum-Brassica interactions and necrotrophic lifestyle of the phytopathogen in general.

Diversity and Function of Appressoria

Endophytic, saprobic, and pathogenic fungi have evolved elaborate strategies to obtain nutrients from plants. Among the diverse plant-fungi interactions, the most crucial event is the attachment and penetration of the plant surface. Appressoria, specialized infection structures, have been evolved to facilitate this purpose. In this review, we describe the diversity of these appressoria and classify them into two main groups: single-celled appressoria (proto-appressoria, hyaline appressoria, melanized (dark) appressoria) and compound appressoria. The ultrastructure of appressoria, their initiation, their formation, and their function in fungi are discussed. We reviewed the molecular mechanisms regulating the formation and function of appressoria, their strategies to evade host defenses, and the related genomics and transcriptomics. The current review provides a foundation for comprehensive studies regarding their evolution and diversity in different fungal groups.

Secretome-Wide Analysis of Lysine Acetylation in Fusarium oxysporum f. sp. lycopersici Provides Novel Insights Into Infection-Related Proteins

Fusarium oxysporum f. sp. lycopersici (Fol) is the causal agent of Fusarium wilt disease in tomato. Proteins secreted by this pathogen during initial host colonization largely determine the outcome of pathogen-host interactions. Lysine acetylation (Kac) plays a vital role in the functions of many proteins, but little is known about Kac in Fol secreted proteins. In this study, we analyzed lysine acetylation of the entire Fol secretome. Using high affinity enrichment of Kac peptides and LC-MS/MS analysis, 50 potentially secreted Fol proteins were identified and acetylation sites determined. Bioinformatics analysis revealed 32 proteins with canonical N-terminal signal peptide leaders, and most of them were predicted to be enzymes involved in a variety of biological processes and metabolic pathways. Remarkably, all 32 predicted secreted proteins were novel and encoded on the core chromosomes rather than on the previously identified LS pathogenicity chromosomes. Homolog scanning of the secreted proteins among 40 different species revealed 4 proteins that were species specific, 3 proteins that were class-specific in the Ascomycota phylum, and 25 proteins that were more widely conserved genes. These secreted proteins provide a starting resource for investigating putative novel pathogenic genes, with 26 up-regulated genes encoding Kac proteins that may play an important role during initial symptomless infection stages.

Microbial Virulence Factors

Microbial virulence factors encompass a wide range of molecules produced by pathogenic microorganisms, enhancing their ability to evade their host defenses and cause disease [...].

The Formaldehyde Dehydrogenase SsFdh1 Is Regulated by and Functionally Cooperates with the GATA Transcription Factor SsNsd1 in Sclerotinia sclerotiorum

S. sclerotiorum is a pathogenic fungus with sclerotium and infection cushion development, making S. sclerotiorum one of the most challenging agricultural pathogens with no effective control method. We identified important sclerotium and compound appressorium formation determinants, SsNsd1 and SsFdh1, and investigated their regulatory mechanism at the molecular level. SsNsd1 and SsFdh1 are zinc finger motif-containing proteins and associate with each other in the nucleus. On other hand, SsNsd1, as a GATA transcription factor, directly binds to GATA-box DNA in the promoter region of Ssfdh1. The SsNsd1-SsFdh1 interaction and nuclear translocation were found to prevent efficient binding of SsNsd1 to GATA-box DNA. Our results provide insights into the role of the GATA transcription factor and its regulation of formaldehyde dehydrogenase in stress resistance, fungal sclerotium and compound appressorium development, and pathogenicity.

GATA transcription factors (TFs) are common eukaryotic regulators, and glutathione-dependent formaldehyde dehydrogenases (GD-FDH) are ubiquitous enzymes with formaldehyde detoxification activity. In this study, the formaldehyde dehydrogenase Sclerotinia sclerotiorum Fdh1 (SsFdh1) was first characterized as an interacting partner of a GATA TF, SsNsd1, in S. sclerotiorum. Genetic analysis reveals that SsFdh1 functions in formaldehyde detoxification, nitrogen metabolism, sclerotium development, and pathogenicity. Both SsNsd1 and SsFdh1 harbor typical zinc finger motifs with conserved cysteine residues. SsNsd1 regulates SsFdh1 in two distinct manners. SsNsd1 directly binds to GATA-box DNA in the promoter region of Ssfdh1; SsNsd1 associates with SsFdh1 through disulfide bonds formed by conserved Cys residues. The SsNsd1-SsFdh1 interaction and nuclear translocation were found to prevent efficient binding of SsNsd1 to GATA-box DNA. Site-directed point mutation of these Cys residues influences the SsNsd1-SsFdh1 interaction and SsNsd1 DNA binding capacity. SsFdh1 is regulated by and functions jointly with the SsNsd1 factor, providing new insights into the complex transcriptional regulatory mechanisms of GATA factors.

IMPORTANCES. sclerotiorum is a pathogenic fungus with sclerotium and infection cushion development, making S. sclerotiorum one of the most challenging agricultural pathogens with no effective control method. We identified important sclerotium and compound appressorium formation determinants, SsNsd1 and SsFdh1, and investigated their regulatory mechanism at the molecular level. SsNsd1 and SsFdh1 are zinc finger motif-containing proteins and associate with each other in the nucleus. On other hand, SsNsd1, as a GATA transcription factor, directly binds to GATA-box DNA in the promoter region of Ssfdh1. The SsNsd1-SsFdh1 interaction and nuclear translocation were found to prevent efficient binding of SsNsd1 to GATA-box DNA. Our results provide insights into the role of the GATA transcription factor and its regulation of formaldehyde dehydrogenase in stress resistance, fungal sclerotium and compound appressorium development, and pathogenicity.