The Coupling Between Cell Wall Integrity Mediated by MAPK Kinases and SsFkh1 Is Involved in Sclerotia Formation and Pathogenicity of Sclerotinia sclerotiorum

Recent advances in virulence of a broad host range plant pathogen Sclerotinia sclerotiorum: a mini-review

Sclerotinia sclerotiorum is a typical necrotrophic plant pathogenic fungus, which has a wide host range and can cause a variety of diseases, leading to serious loss of agricultural production around the world. It is difficult to control and completely eliminate the characteristics, chemical control methods is not ideal. Therefore, it is very important to know the pathogenic mechanism of S. sclerotiorum for improving host living environment, relieving agricultural pressure and promoting economic development. In this paper, the life cycle of S. sclerotiorum is introduced to understand the whole process of S. sclerotiorum infection. Through the analysis of the pathogenic mechanism, this paper summarized the reported content, mainly focused on the oxalic acid, cell wall degrading enzyme and effector protein in the process of infection and its mechanism. Besides, recent studies reported virulence-related genes in S. sclerotiorum have been summarized in the paper. According to analysis, those genes were related to the growth and development of the hypha and appressorium, the signaling and regulatory factors of S. sclerotiorum and so on, to further influence the ability to infect the host critically. The application of host-induced gene silencing (HIGS)is considered as a potential effective tool to control various fungi in crops, which provides an important reference for the study of pathogenesis and green control of S. sclerotiorum.

A MAP kinase cascade broadly regulates the lifestyle of Sclerotinia sclerotiorum and can be targeted by HIGS for disease control

Sclerotinia sclerotiorum causes white mold or stem rot in a wide range of economically important plants, bringing significant yield losses worldwide. Control of this pathogen is difficult as its resting structure sclerotia can survive in soil for years, and no Resistance genes have been identified in S. sclerotiorum hosts. Host-induced gene silencing (HIGS) has shown promising effects in controlling many fungal pathogens, including S. sclerotiorum. However, better molecular genetic understanding of signaling pathways involved in its development and pathogenicity is needed to provide effective HIGS gene targets. Here, by employing a forward genetic screen, we characterized an evolutionarily conserved mitogen-activated protein kinase (MAPK) cascade in S. sclerotiorum, consisting of SsSte50-SsSte11-SsSte7-Smk1, which controls mycelial growth, sclerotia development, compound appressoria formation, virulence, and hyphal fusion. Moreover, disruption of the putative downstream transcription factor SsSte12 led to normal sclerotia but deformed appressoria and attenuated host penetration, as well as impaired apothecia formation, suggestive of diverged regulation downstream of the MAPK cascade. Most importantly, targeting SsSte50 using host-expressed double-stranded RNA resulted in largely reduced virulence of S. sclerotiorum on both Nicotiana benthamiana leaves and transgenic Arabidopsis thaliana plants. Therefore, this MAPK signaling cascade is generally needed for its growth, development, and pathogenesis and can serve as ideal HIGS targets for mitigating economic damages caused by S. sclerotiorum infection.

Molecular Mechanism Underlying Pathogenicity Inhibition by Chitosan in Cochliobolus heterostrophus

Chitosan, as a natural nontoxic biomaterial, has been demonstrated to inhibit fungal growth and enhance plant defense against pathogen infection. However, the antifungal pattern and mechanism of how chitosan application evokes plant defense are poorly elucidated. Herein, we provide evidence that chitosan exposure is fungicidal to C. heterostrophus. Chitosan application impairs conidia germination and appressorium formation of C. heterostrophus and has a pronounced effect on reactive oxygen species production, thereby preventing infection in maize. In addition, the toxicity of chitosan to C. heterostrophus requires Mkk1 and Mps1, two key components in the cell wall integrity pathway. The Δmkk1 and Δmps1 mutants were more tolerant to chitosan than the wild-type. To dissect chitosan-mediated plant defense response to C. heterostrophus, we conducted a metabolomic analysis, and several antifungal compounds were upregulated in maize upon chitosan treatment. Taken together, our findings provide a comprehensive understanding of the mechanism of chitosan-alleviated infection of C. heterostrophus, which would promote the application of chitosan in plant protection in agriculture.

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 Snf5-Hsf1 transcription module synergistically regulates stress responses and pathogenicity by maintaining ROS homeostasis in Sclerotinia sclerotiorum

The SWI/SNF complex is guided to the promoters of designated genes by its co-operator to activate transcription in a timely and appropriate manner to govern development, pathogenesis, and stress responses in fungi. Nevertheless, knowledge of the complexes and their co-operator in phytopathogenic fungi is still fragmented. We demonstrate that the heat shock transcription factor SsHsf1 guides the SWI/SNF complex to promoters of heat shock protein (hsp) genes and antioxidant enzyme genes using biochemistry and pharmacology. This is accomplished through direct interaction with the complex subunit SsSnf5 under heat shock and oxidative stress. This results in the activation of their transcription and mediates histone displacement to maintain reactive oxygen species (ROS) homeostasis. Genetic results demonstrate that the transcription module formed by SsSnf5 and SsHsf1 is responsible for regulating morphogenesis, stress tolerance, and pathogenicity in Sclerotinia sclerotiorum, especially by directly activating the transcription of hsp genes and antioxidant enzyme genes counteracting plant-derived ROS. Furthermore, we show that stress-induced phosphorylation of SsSnf5 is necessary for the formation of the transcription module. This study establishes that the SWI/SNF complex and its co-operator cooperatively regulate the transcription of hsp genes and antioxidant enzyme genes to respond to host and environmental stress in the devastating phytopathogenic fungi.

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.

Two zinc finger proteins, VdZFP1 and VdZFP2, interact with VdCmr1 to promote melanized microsclerotia development and stress tolerance in Verticillium dahliae

BACKGROUND

Melanin plays important roles in morphological development, survival, host-pathogen interactions and in the virulence of phytopathogenic fungi. In Verticillum dahliae, increases in melanin are recognized as markers of maturation of microsclerotia which ensures the long-term survival and stress tolerance, while decreases in melanin are correlated with increased hyphal growth in the host. The conserved upstream components of the VdCmr1-regulated pathway controlling melanin production in V. dahliae have been extensively identified, but the direct activators of this pathway are still unclear.

RESULTS

We identified two genes encoding conserved C2H2-type zinc finger proteins VdZFP1 and VdZFP2 adjacent to VdPKS9, a gene encoding a negative regulator of both melanin biosynthesis and microsclerotia formation in V. dahliae. Both VdZFP1 and VdZFP2 were induced during microsclerotia development and were involved in melanin deposition. Their localization changed from cytoplasmic to nuclear in response to osmotic pressure. VdZFP1 and VdZFP2 act as modulators of microsclerotia melanization in V. dahliae, as confirmed by melanin biosynthesis inhibition and supplementation with the melanin pathway intermediate scytalone in albino strains. The results indicate that VdZFP1 and VdZFP2 participate in melanin biosynthesis by positively regulating VdCmr1. Based on the results obtained with yeast one- and two-hybrid (Y1H and Y2H) and bimolecular fluorescence complementation (BiFC) systems, we determined the melanin biosynthesis relies on the direct interactions among VdZFP1, VdZFP2 and VdCmr1, and these interactions occur on the cell walls of microsclerotia. Additionally, VdZFP1 and/or VdZFP2 mutants displayed increased sensitivity to stress factors rather than alterations in pathogenicity, reflecting the importance of melanin in stress tolerance of V. dahliae.

CONCLUSIONS

Our results revealed that VdZFP1 and VdZFP2 positively regulate VdCmr1 to promote melanin deposition during microsclerotia development, providing novel insight into the regulation of melanin biosynthesis in V. dahliae.

Effects of Different Nitrogen Levels on Lignocellulolytic Enzyme Production and Gene Expression under Straw-State Cultivation in Stropharia rugosoannulata

Stropharia rugosoannulata has been used in environmental engineering to degrade straw in China. The nitrogen and carbon metabolisms are the most important factors affecting mushroom growth, and the aim of this study was to understand the effects of different nitrogen levels on carbon metabolism in S. rugosoannulata using transcriptome analysis. The mycelia were highly branched and elongated rapidly in A3 (1.37% nitrogen). GO and KEGG enrichment analyses revealed that the differentially expressed genes (DEGs) were mainly involved in starch and sucrose metabolism; nitrogen metabolism; glycine, serine and threonine metabolism; the MAPK signaling pathway; hydrolase activity on glycosyl bonds; and hemicellulose metabolic processes. The activities of nitrogen metabolic enzymes were highest in A1 (0.39% nitrogen) during the three nitrogen levels (A1, A2 and A3). However, the activities of cellulose enzymes were highest in A3, while the hemicellulase xylanase activity was highest in A1. The DEGs associated with CAZymes, starch and sucrose metabolism and the MAPK signaling pathway were also most highly expressed in A3. These results suggested that increased nitrogen levels can upregulate carbon metabolism in S. rugosoannulata. This study could increase knowledge of the lignocellulose bioconversion pathways and improve biodegradation efficiency in Basidiomycetes.

Nitrate reductase is required for sclerotial development and virulence of Sclerotinia sclerotiorum

Sclerotinia sclerotiorum, the causal agent of Sclerotinia stem rot (SSR) on more than 450 plant species, is a notorious fungal pathogen. Nitrate reductase (NR) is required for nitrate assimilation that mediates the reduction of nitrate to nitrite and is the major enzymatic source for NO production in fungi. To explore the possible effects of nitrate reductase SsNR on the development, stress response, and virulence of S. sclerotiorum, RNA interference (RNAi) of SsNR was performed. The results showed that SsNR-silenced mutants showed abnormity in mycelia growth, sclerotia formation, infection cushion formation, reduced virulence on rapeseed and soybean with decreased oxalic acid production. Furthermore SsNR-silenced mutants are more sensitive to abiotic stresses such as Congo Red, SDS, H₂O₂, and NaCl. Importantly, the expression levels of pathogenicity-related genes SsGgt1, SsSac1, and SsSmk3 are down-regulated in SsNR-silenced mutants, while SsCyp is up-regulated. In summary, phenotypic changes in the gene silenced mutants indicate that SsNR plays important roles in the mycelia growth, sclerotia development, stress response and fungal virulence of S. sclerotiorum.

Sclerotinia sclerotiorum (Lib.) de Bary: Insights into the Pathogenomic Features of a Global Pathogen

Sclerotinia sclerotiorum (Lib.) de Bary is a broad host-range fungus that infects an inclusive array of plant species and afflicts significant yield losses globally. Despite being a notorious pathogen, it has an uncomplicated life cycle consisting of either basal infection from myceliogenically germinated sclerotia or aerial infection from ascospores of carpogenically germinated sclerotia. This fungus is unique among necrotrophic pathogens in that it inevitably colonizes aging tissues to initiate an infection, where a saprophytic stage follows the pathogenic phase. The release of cell wall-degrading enzymes, oxalic acid, and effector proteins are considered critical virulence factors necessary for the effective pathogenesis of S. sclerotiorum. Nevertheless, the molecular basis of S. sclerotiorum pathogenesis is still imprecise and remains a topic of continuing research. Previous comprehensive sequencing of the S. sclerotiorum genome has revealed new insights into its genome organization and provided a deeper comprehension of the sophisticated processes involved in its growth, development, and virulence. This review focuses on the genetic and genomic aspects of fungal biology and molecular pathogenicity to summarize current knowledge of the processes utilized by S. sclerotiorum to parasitize its hosts. Understanding the molecular mechanisms regulating the infection process of S. sclerotiorum will contribute to devising strategies for preventing infections caused by this destructive pathogen.

Cross-Talk and Multiple Control of Target of Rapamycin (TOR) in Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a necrotrophic phytopathogenic fungus that cross-talks with its hosts for control of cell-death pathways for colonization. Target of rapamycin (TOR) is a central regulator that controls cell growth, intracellular metabolism, and stress responses in a variety of eukaryotes, but little is known about TOR signaling in S. sclerotiorum. In this study, we identified a conserved TOR signaling pathway and characterized SsTOR as a critical component of this pathway. Hyphal growth of S. sclerotiorum was retarded by silencing SsTOR, moreover, sclerotia and compound appressoria formation were severely disrupted. Notably, pathogenicity assays of strains shows that the virulence of the SsTOR-silenced strains were dramatically decreased. SsTOR was determined to participate in cell wall integrity (CWI) by regulating the phosphorylation level of SsSmk3, a core MAP kinase in the CWI pathway. Importantly, the inactivation of SsTOR induced autophagy in S. sclerotiorum potentially through SsAtg1 and SsAtg13. Taken together, our results suggest that SsTOR is a global regulator controlling cell growth, stress responses, cell wall integrity, autophagy, and virulence of S. sclerotiorum. IMPORTANCE TOR is a conserved protein kinase that regulates cell growth and metabolism in response to growth factors and nutrient abundance. Here, we used gene silencing to characterize SsTOR, which is a critical component of TOR signaling pathway. SsTOR-silenced strains have limited mycelium growth, and the virulence of the SsTOR-silenced strains was decreased. Phosphorylation analysis indicated that SsTOR influenced CWI by regulating the phosphorylation level of SsSmk3. Autophagy is essential to preserve cellular homeostasis in response to cellular and environmental stresses. Inactivation of SsTOR induced autophagy in S. sclerotiorum potentially through SsAtg1 and SsAtg13. These findings further indicated that SsTOR is a global regulator of the growth, development, and pathogenicity of S. sclerotiorum in multiple ways.

Appressoria-Small but Incredibly Powerful Structures in Plant-Pathogen Interactions

Plant-pathogenic fungi are responsible for many of the most severe crop diseases in the world and remain very challenging to control. Improving current protection strategies or designating new measures based on an overall understanding of molecular host-pathogen interaction mechanisms could be helpful for disease management. The attachment and penetration of the plant surface are the most important events among diverse plant-fungi interactions. Fungi evolved as small but incredibly powerful infection structure appressoria to facilitate attachment and penetration. Appressoria are indispensable for many diseases, such as rusts, powdery mildews, and blast diseases, as well as devastating oomycete diseases. Investigation into the formation of plant-pathogen appressoria contributes to improving the understanding of the molecular mechanisms of plant-pathogen interactions. Fungal host attachment is a vital step of fungal pathogenesis. Here, we review recent advances in the molecular mechanisms regulating the formation of appressoria. Additionally, some biocontrol agents were revealed to act on appressorium. The regulation of fungal adhesion during the infective process by acting on appressoria formation is expected to prevent the occurrence of crop disease caused by some pathogenic fungi.

The SsAtg1 Activating Autophagy Is Required for Sclerotia Formation and Pathogenicity in Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a necrotrophic phytopathogenic fungus that produces sclerotia. Sclerotia are essential components of the survival and disease cycle of this devastating pathogen. In this study, we analyzed comparative transcriptomics of hyphae and sclerotia. A total of 1959 differentially expressed genes, 919 down-regulated and 1040 up-regulated, were identified. Transcriptomes data provide the possibility to precisely comprehend the sclerotia development. We further analyzed the differentially expressed genes (DEGs) in sclerotia to explore the molecular mechanism of sclerotia development, which include ribosome biogenesis and translation, melanin biosynthesis, autophagy and reactivate oxygen metabolism. Among these, the autophagy-related gene SsAtg1 was up-regulated in sclerotia. Atg1 homologs play critical roles in autophagy, a ubiquitous and evolutionarily highly conserved cellular mechanism for turnover of intracellular materials in eukaryotes. Therefore, we investigated the function of SsAtg1 to explore the function of the autophagy pathway in S. sclerotiorum. Deficiency of SsAtg1 inhibited autophagosome accumulation in the vacuoles of nitrogen-starved cells. Notably, ΔSsAtg1 was unable to form sclerotia and displayed defects in vegetative growth under conditions of nutrient restriction. Furthermore, the development and penetration of the compound appressoria in ΔSsAtg1 was abnormal. Pathogenicity analysis showed that SsAtg1 was required for full virulence of S. sclerotiorum. Taken together, these results indicate that SsAtg1 is a core autophagy-related gene that has vital functions in nutrient utilization, sclerotia development and pathogenicity in S. sclerotiorum.

SsAGM1-Mediated Uridine Diphosphate-N-Acetylglucosamine Synthesis Is Essential for Development, Stress Response, and Pathogenicity of Sclerotinia sclerotiorum

The necrotrophic fungus Sclerotinia sclerotiorum is a devastating pathogen. S. sclerotiorum can cause Sclerotinia stem rot in more than 600 species of plants, which results in serious economic losses every year. Chitin is one of the most important polysaccharides in fungal cell walls. Chitin and β-Glucan form a scaffold that wraps around the cell and determines the vegetative growth and pathogenicity of pathogens. UDP-GlcNAc is a direct precursor of chitin synthesis. During the synthesis of UDP-GlcNAc, the conversion of GlcNAc-6P to GlcNAc-1P that is catalyzed by AGM1 (N-acetylglucosamine-phosphate mutase) is a key step. However, the significance and role of AGM1 in phytopathogenic fungus are unclear. We identified a cytoplasm-localized SsAGM1 in S. sclerotiorum, which is homologous to AGM1 of Saccharomyces cerevisiae. We utilized RNA interference (RNAi) and overexpression to characterize the function of SsAGM1 in S. sclerotiorum. After reducing the expression of SsAGM1, the contents of chitin and UDP-GlcNAc decreased significantly. Concomitantly, the gene-silenced transformants of SsAGM1 slowed vegetative growth and, importantly, lost the ability to produce sclerotia and infection cushion; it also lost virulence, even on wounded leaves. In addition, SsAGM1 was also involved in the response to osmotic stress and inhibitors of cell wall synthesis. Our results revealed the function of SsAGM1 in the growth, development, stress response, and pathogenicity in S. sclerotiorum.