High expression of a sucrose non-fermenting (SNF1)-related protein kinase from Colletotrichum gloeosporoides f. sp. malvae is associated with penetration of Malva pusilla

The serine-threonine protein kinase Snf1 orchestrates the expression of plant cell wall-degrading enzymes and is required for full virulence of the maize pathogen Colletotrichum graminicola

Colletotrichum graminicola, the causal agent of maize leaf anthracnose and stalk rot, differentiates a pressurized infection cell called an appressorium in order to invade the epidermal cell, and subsequently forms biotrophic and necrotrophic hyphae to colonize the host tissue. While the role of force in appressorial penetration is established (Bechinger et al., 1999), the involvement of cell wall-degrading enzymes (CWDEs) in this process and in tissue colonization is poorly understood, due to the enormous number and functional redundancy of these enzymes. The serine/threonine protein kinase gene SNF1 identified in Sucrose Non-Fermenting yeast mutants mediates de-repression of catabolite-repressed genes, including many genes encoding CWDEs. In this study, we identified and functionally characterized the SNF1 homolog of C. graminicola. Δsnf1 mutants showed reduced vegetative growth and asexual sporulation rates on media containing polymeric carbon sources. Microscopy revealed reduced efficacies in appressorial penetration of cuticle and epidermal cell wall, and formation of unusual medusa-like biotrophic hyphae by Δsnf1 mutants. Severe and moderate virulence reductions were observed on intact and wounded leaves, respectively. Employing RNA-sequencing we show for the first time that more than 2,500 genes are directly or indirectly controlled by Snf1 in necrotrophic hyphae of a plant pathogenic fungus, many of which encode xylan- and cellulose-degrading enzymes. The data presented show that Snf1 is a global regulator of gene expression and is required for full virulence.

Sucrose non-fermenting protein kinase gene UvSnf1 is required for virulence in Ustilaginoidea virens

Rice false smut caused by Ustilaginoidea virens is becoming one of the most devastating diseases in rice production areas in the world. Revealing U. virens potential pathogenic mechanisms provides ideas for formulating more effective prevention and control strategies. Sucrose non-fermenting 1 (Snf1) protein kinase plays a critical role in activating transcription and suppressing gene expression, as well as in cellular response to various stresses, such as nutrient limitation. In our study, we identified the Snf1 homolog UvSnf1 and analyzed its biological functions in U. virens. The expression level of UvSnf1 was dramatically up-regulated during invasion, indicating that UvSnf1 may participate in infection. Phenotypic analyses of UvSnf1 deletion mutants revealed that UvSnf1 is necessary for hyphae growth, spore production, and virulence in U. virens. Moreover, UvSnf1 promotes U. virens to use unfavorable carbon sources when the sucrose is insufficient. In addition, deletion of UvSnf1 down-regulates the expression of the cell wall-degrading enzymes (CWDEs) genes under sucrose limitation conditions in U. virens. Further analyses showed that CWDEs (UvCut1 and UvXyp1) are not only involved in growth, spore production, and virulence but are also required for the utilization of carbon sources. In conclusion, this study demonstrates that UvSnf1 plays vital roles in virulence and carbon source utilization in U. virens, and one of the possible mechanisms is playing a role in regulating the expression of CWDE genes.

Advances in understanding the mechanism of resistance to anthracnose and induced defence response in tea plants

The tea plant (Camellia sinensis) is susceptible to anthracnose disease that causes considerable crop loss and affects the yield and quality of tea. Multiple Colletotrichum spp. are the causative agents of this disease, which spreads quickly in warm and humid climates. During plant-pathogen interactions, resistant cultivars defend themselves against the hemibiotrophic pathogen by activating defence signalling pathways, whereas the pathogen suppresses plant defences in susceptible varieties. Various fungicides have been used to control this disease on susceptible plants, but these fungicide residues are dangerous to human health and cause fungicide resistance in pathogens. The problem-solving approaches to date are the development of resistant cultivars and ecofriendly biocontrol strategies to achieve sustainable tea cultivation and production. Understanding the infection stages of Colletotrichum, tea plant resistance mechanisms, and induced plant defence against Colletotrichum is essential to support sustainable disease management practices in the field. This review therefore summarizes the current knowledge of the identified causative agent of tea plant anthracnose, the infection strategies and pathogenicity of C. gloeosporioides, anthracnose disease resistance mechanisms, and the caffeine-induced defence response against Colletotrichum infection. The information reported in this review will advance our understanding of host-pathogen interactions and eventually help us to develop new disease control strategies.

Knock-down of glucose transporter and sucrose non-fermenting gene in the hemibiotrophic fungus Colletotrichum falcatum causing sugarcane red rot

Red rot caused by Colletotrichum falcatum, is one of the economically important disease of sugarcane and breeding for resistant varieties is considered to be the major solution to manage the disease. However, breakdown of red rot resistance become usual phenomenon due to development of newer races by culture adaptation on newly released varieties. Hence it is needed to characterize the genes responsible for pathogen virulence in order to take care of host resistance or to manage the disease by other methods. The transcript studies gave foundation to characterize the huge number of pathogenicity determinants and their role in pathogenesis. Here we studied role of two important genes viz., Glucose Transporter (GT) and Sucrose Non-Fermenting1 (SNF1) during pathogenesis of C. falcatum, which said to be involved in carbon source metabolism. Sugar metabolism has a vital role in disease progression of C. falcatum by regulating their cell growth, metabolism and development of the pathogen during various stages of infection. The present study was aimed to find out the role of GT and SNF1 genes in response to pathogenicity by RNA silencing (RNAi) approach. Knock-down of the target pathogenicity gene homologs in standard C. falcatum isolate Cf671 was carried out by amplifying sense and antisense fragments of targets individually using pSilent-1 vector. The expression cassette was cloned into the binary vector pCAMBIA1300 followed by fungal transformation through Agarobacterium mediated transformation. Resulted mutants of both the genes showed less virulence compared to wild type isolate. Simultaneously, both the mutants did not produce spores. Moreover, the molecular confirmation of the mutants displayed the expression of hygromycin gene with reduced expression of the target gene during host-pathogen interaction. Knockdown of the pathogenicity related genes (GT and SNF1) by RNAi approach corroborate the possible role of the genes in causing the disease.

Characterization of sucrose non-fermenting-1 (SNF1) homologue gene in Fusarium udum WSP-V2 and its regulation by the biocontrol agent Pseudomonas fluorescens OKC

Sucrose non-fermenting 1 (SNF1) is a protein kinase and plays an important role in the energy homeostasis of glucose repressible gene transcription. It derepresses glucose repressed genes and associated with pathogenesis and production of cell wall degrading enzymes in fungal species. In the present study, we identified and characterized SNF1 homologue FuSNF1 in the F. udum strain WSP-V2. Transcript analysis of FuSNF1 along with the MAP kinases and some cell wall degrading enzyme (CWDE) genes of F. udum during interaction with pigeonpea revealed that most MAP kinases and CWDE genes was positively correlated with the FuSNF1 gene. Interestingly, transcript accumulation of all these genes was lowered when pigeonpea seeds were bioprimed with a PGPR strain Pseudomonas fluorescens OKC. Transcript accumulation of FuSNF1 was observed from the day of inoculation and reached maximum level on day 7 in OKC non-bioprimed plants. However, transcript accumulation was low (1.5 fold) in F. udum inoculated with pigeonpea plants bioprimed with OKC. Transcript accumulation patterns of the F. udum MAP Kinases genes and CWDE genes also showed a similar trend and their transcript accumulation was lowered in the OKC bioprimed treatment. The results thus indicate a prime role of FuSNF1 in regulating pathogenicity and virulence of F. udum. The results further emphasize the importance of application of effective PGPR strains in regulating virulence of F. udum. In silico analysis of the SNF1 reference proteins from different fungal species showed that their homologue FuSNF1 is likely to be thermostable and acidic in nature.

Acetyl-coenzyme A synthetase gene ChAcs1 is essential for lipid metabolism, carbon utilization and virulence of the hemibiotrophic fungus Colletotrichum higginsianum

Acetyl-coenzyme A (acetyl-CoA) is a key molecule that participates in many biochemical reactions in amino acid, protein, carbohydrate and lipid metabolism. Here, we genetically dissected the distinct roles of two acetyl-CoA synthetase genes, ChAcs1 and ChAcs2, in the regulation of fermentation, lipid metabolism and virulence of the hemibiotrophic fungus Colletotrichum higginsianum. ChAcs1 and ChAcs2 are both highly expressed during appressorial development and the formation of primary hyphae, and are constitutively expressed in the cytoplasm throughout development. We found that C. higginsianum strains without ChAcs1 were non-viable in the presence of most non-fermentable carbon sources, including acetate, ethanol and acetaldehyde. Deletion of ChAcs1 also led to a decrease in lipid content of mycelia and delayed lipid mobilization in conidia to developing appressoria, which suggested that ChAcs1 contributes to lipid metabolism in C. higginsianum. Furthermore, a ChAcs1 deletion mutant was defective in the switch to invasive growth, which may have been directly responsible for its reduced virulence. Transcriptomic analysis and quantitative reverse transcription-polymerase chain reaction (qRT-PCR) revealed that ChAcs1 can affect the expression of genes involved in virulence and carbon metabolism, and that plant defence genes are up-regulated, all demonstrated during infection by a ChAcs1 deletion mutant. In contrast, deletion of ChAcs2 only conferred a slight delay in lipid mobilization, although it was highly expressed in infection stages. Our studies provide evidence for ChAcs1 as a key regulator governing lipid metabolism, carbon source utilization and virulence of this hemibiotrophic fungus.

FvSNF1, the sucrose non-fermenting protein kinase gene of Fusarium virguliforme, is required for cell-wall-degrading enzymes expression and sudden death syndrome development in soybean

Fusarium virguliforme is a soil-borne pathogenic fungus that causes sudden death syndrome (SDS) in soybean. Its pathogenicity is believed to require the activity of cell-wall-degrading enzymes (CWDEs). The sucrose non-fermenting protein kinase 1 gene (SNF1) is a key component of the glucose de-repression pathway in yeast, and a regulator of gene expression for CWDEs in some plant pathogenic fungi. To elucidate the functional role of the SNF1 homolog in F. virguliforme, FvSNF1 was disrupted using a split-marker strategy. Disruption of FvSNF1 in F. virguliforme abolishes galactose utilization and causes poor growth on xylose, arabinose and sucrose. However, the resulting Fvsnf1 mutant grew similar to wild-type and ectopic transformants on glucose, fructose, maltose, or pectin as the main source of carbon. The Fvsnf1 mutant displayed no expression of the gene-encoding galactose oxidase (GAO), a secretory enzyme that catalyzes oxidation of D-galactose. It also exhibited a significant reduction in the expression of several CWDE-coding genes in contrast to the wild-type strain. Greenhouse pathogenicity assays revealed that the Fvsnf1 mutant was severely impaired in its ability to cause SDS on challenged soybean plants. Microscopy and microtome studies on infected roots showed that the Fvsnf1 mutant was defective in colonizing vascular tissue of infected plants. Cross and longitudinal sections of infected roots stained with fluorescein-labeled wheat germ agglutinin and Congo red showed that the Fvsnf1 mutant failed to colonize the xylem vessels and phloem tissue at later stages of infection. Quantification of the fungal biomass in inoculated roots further confirmed a reduced colonization of roots by the Fvsnf1 mutant when compared to the wild type. These findings suggest that FvSNF1 regulates the expression of CWDEs in F. virguliforme, thus affecting the virulence of the fungus on soybean.

The LmSNF1 gene is required for pathogenicity in the canola blackleg pathogen Leptosphaeria maculans

Leptosphaeria maculans is a fungal pathogen causing blackleg in canola. Its virulence has been attributed, among other factors, to the activity of hydrolytic cell wall degrading enzymes (CWDEs). Studies on the pathogenicity function of CWDEs in plant pathogenic fungi have been difficult due to gene redundancy. In microorganisms many CWDE genes are repressed by glucose and derepressed by the function of the sucrose non-fermenting protein kinase 1 gene (SNF1). To address the molecular function of SNF1 in L. maculans, the ortholog of SNF1 (LmSNF1) was cloned and functionally characterized using a gene knockout strategy. Growth of the LmSNF1 knockout strains was severely disrupted, as was sporulation, spore germination and the ability to attach on the plant surface. When inoculated on canola cotyledons, the LmSNF1 knockout strains could not cause any symptoms, indicating the loss of pathogenicity. The expression of 11 selected CWDE genes and a pathogenicity gene (LopB) was significantly down-regulated in the LmSNF1 knockout strains. In conclusion, knockout of LmSNF1 prevents L. maculans from properly derepressing the production of CWDEs, compromises the utilization of certain carbon sources, and impairs fungal pathogenicity on canola.

Functional characterization of sucrose non-fermenting 1 protein kinase complex genes in the Ascomycete Fusarium graminearum

Sucrose non-fermenting 1 (SNF1) protein kinase complex is a heterotrimer that functions in energy homeostasis in eukaryotes by regulating transcription of glucose-repressible genes. Our previous study revealed that SNF1 of the homothallic ascomycete fungus Fusarium graminearum plays important roles in vegetative growth, sexual development, and virulence. In this study, we further identified the components of the SNF1 complex in F. graminearum and characterized their functions. We found that the SNF1 complex in F. graminearum consists of one alpha subunit (FgSNF1), one beta subunit (FgGAL83), and one gamma subunit (FgSNF4). Deletion of Fggal83 and Fgsnf4 resulted in alleviated phenotype changes in vegetative growth and sexual development as compared to those of the Fgsnf1 deletion mutant. However, all of the single, double, and triple deletion mutants among Fgsnf1, Fggal83, and Fgsnf4 had similar levels of decreased virulence. In addition, there was no synergistic effect of the mutant (single, double, or triple deletions of SNF1 complex component genes) phenotypes except for sucrose utilization. In this study, we revealed that FgSNF1 is mainly required for SNF1 complex functions, and the other two SNF1 complex components have adjunctive roles with FgSNF1 in sexual development and vegetative growth but have a major role in virulence in F. graminearum.

VdSNF1, the sucrose nonfermenting protein kinase gene of Verticillium dahliae, is required for virulence and expression of genes involved in cell-wall degradation

Verticillium dahliae is a soilborne fungus causing vascular wilt in a diverse array of plant species. Its virulence has been attributed, among other factors, to the activity of hydrolytic cell wall-degrading enzymes (CWDE). The sucrose nonfermenting 1 gene (VdSNF1), which regulates catabolic repression, was disrupted in V. dahliae tomato race 1. Expression of CWDE in the resulting mutants was not induced in inductive medium and in simulated xylem fluid medium. Growth of the mutants was significantly reduced when grown with pectin or galactose as a carbon source whereas, with glucose, sucrose, and xylose, they grew similarly to wild-type and ectopic transformants. The mutants were severely impaired in virulence on tomato and eggplant (final disease severity reduced by an average of 87%). Microscopic observation of the infection behavior of a green fluorescent protein (gfp)-labeled VdSNF1 mutant (70ΔSF-gfp1) showed that it was defective in initial colonization of roots. Cross sections of tomato stem at the cotyledonary level showed that 70ΔSF-gfp1 colonized xylem vessels considerably less than the wild-type strain. The wild-type strain heavily colonized xylem vessels and adjacent parenchyma cells. Quantification of fungal biomass in plant tissues further confirmed reduced colonization of roots, stems, and cotyledons by 70ΔSF-gfp1 relative to that by the wild-type strain.

GzSNF1 is required for normal sexual and asexual development in the ascomycete Gibberella zeae

The sucrose nonfermenting 1 (SNF1) protein kinase of yeast plays a central role in the transcription of glucose-repressible genes in response to glucose starvation. In this study, we deleted an ortholog of SNF1 from Gibberella zeae to characterize its functions by using a gene replacement strategy. The mycelial growth of deletion mutants (DeltaGzSNF1) was reduced by 21 to 74% on diverse carbon sources. The virulence of DeltaGzSNF1 mutants on barley decreased, and the expression of genes encoding cell-wall-degrading enzymes was reduced. The most distinct phenotypic changes were in sexual and asexual development. DeltaGzSNF1 mutants produced 30% fewer perithecia, which matured more slowly, and asci that contained one to eight abnormally shaped ascospores. Mutants in which only the GzSNF1 catalytic domain was deleted had the same phenotype changes as the DeltaGzSNF1 strains, but the phenotype was less extreme in the mutants with the regulatory domain deleted. In outcrosses between the DeltaGzSNF1 mutants, each perithecium contained approximately 70% of the abnormal ascospores, and approximately 50% of the asci showed unexpected segregation patterns in a single locus tested. The asexual spores of the DeltaGzSNF1 mutants were shorter and had fewer septa than those of the wild-type strain. The germination and nucleation of both ascospores and conidia were delayed in DeltaGzSNF1 mutants in comparison with those of the wild-type strain. GzSNF1 expression and localization depended on the developmental stage of the fungus. These results suggest that GzSNF1 is critical for normal sexual and asexual development in addition to virulence and the utilization of alternative carbon sources.

In vitro production of biotrophic-like cultures of Crinipellis perniciosa, the causal agent of witches' broom disease of Theobroma cacao

Witches' broom disease (WBD) of cacao, caused by the hemibiotrophic fungus, Crinipellis perniciosa, exhibits a succession of symptoms that are caused by the biotrophic phase of the fungus. However, the study of this biotrophic phase is limited by its exclusive growth inside the plant or in the presence of callus. Here we report for the first time a method for the growth and maintenance of the biotrophic-like phase of C. perniciosa on a defined medium with metabolites found in the diseased tissues. Our results suggest that glycerol is a key carbon source for this interaction. This is a crucial achievement toward understanding the biology of this fungus during the infectious phase of WBD.

Expression of a glycogen synthase protein kinase homolog from Colletotrichum gloeosporioides f.sp. malvae during infection of Malva pusilla

The potential role of a GSK3 protein kinase homolog, cggsk, was examined from Colletotrichum gloeosporioides f.sp. malvae, a fungal pathogen of Malva pusilla. A peak in cggsk expression relative to a constitutively expressed fungal actin gene occurred during host penetration and was followed by much lower expression levels during subsequent biotrophic and necrotrophic growth in host tissue. The peak level of cggsk expression observed during penetration was 21-fold greater than that during necrotrophic growth. Expression of cggsk showed small but reproducible changes during growth in culture; however, the levels were always similar to that during necrotrophic growth in the host. One possible role for cggsk could be to coordinate fungal development during host penetration.