Purification and characterization of a glucoamylase secreted by the plant pathogen Scierotinia sclerotiorum

Identification, molecular and biochemical characterization of a novel thermoactive and thermostable glucoamylase from Thermoanaerobacter ethanolicus

PURPOSE

We identified a new glucoamylase (TeGA) from Thermoanaerobacter ethanolicus, a thermophilic anaerobic bacterium. Structural studies suggest that TeGA belongs to the family 15 of glycosylhydrolases (GH15).

METHODS

The expression of this enzyme was optimized in E. coli (BL21) cells in order to have the highest amount of soluble protein (around 3 mg/l of culture medium).

RESULTS

TeGA showed a high optimum temperature of 75 °C. It also showed one of the highest specific activities reported for a bacterial glucoamylase (75.3 U/mg) and was also stable in a wide pH range (3.0-10.0). Although the enzyme was preferentially active with maltose, it was also able to hydrolyze different soluble starches such as those from potato, corn or rice. TeGA showed a high thermostability up to around 70 °C, which was increased in the presence of PEG8000, and also showed to be stable in the presence of moderate concentrations of ethanol.

CONCLUSION

We propose that TeGA could be suitable for use in different industrial processes such as biofuel production and food processing.

Purification and biochemical characterization of extracellular glucoamylase from Paenibacillus amylolyticus strain

In the present study, glucoamylase produced from a soil bacterium Paenibacillus amylolyticus NEO03 was cultured under submerged fermentation conditions. The extracellular enzyme was purified by starch adsorption chromatography and further by gel filtration, with 2.73-fold and recovery of 40.02%. The protein exhibited molecular mass of ∼66,000 Da as estimated by SDS-PAGE and depicted to be a monomer. The enzyme demonstrated optimum activity at pH range 6.0-7.0 and temperature range 30-40 °C. Glucoamylase was mostly activated by Mn²⁺ metal ions and depicted no dependency on Ca²⁺ ions. The enzyme preferentially hydrolyzed all the starch substrates. High substrate specificity was demonstrated towards soluble starch and kinetic values K_m and V_max were 2.84 mg/ml and 239.2 U/ml, respectively. The products of hydrolysis of soluble starch were detected by thin layer chromatography which showed only _D -glucose, indicating a true glucoamylase. The secreted glucoamylase from P. amylolyticus strain possesses properties suitable for saccharification processes such as biofuel production.

Disruption of the Gene Encoding Endo-β-1, 4-Xylanase Affects the Growth and Virulence of Sclerotinia sclerotiorum

Sclerotinia sclerotiorum (Lib.) de Bary is a devastating fungal pathogen with worldwide distribution. S. sclerotiorum is a necrotrophic fungus that secretes many cell wall-degrading enzymes (CWDEs) that destroy plant's cell-wall components. Functional analyses of the genes that encode CWDEs will help explain the mechanisms of growth and pathogenicity of S. sclerotiorum. Here, we isolated and characterized a gene SsXyl1 that encoded an endo-β-1, 4-xylanase in S. sclerotiorum. The SsXyl1 expression showed a slight increase during the development and germination stages of sclerotia and a dramatic increase during infection. The expression of SsXyl1 was induced by xylan. The SsXyl1 deletion strains produce aberrant sclerotia that could not germinate to form apothecia. The SsXyl1 deletion strains also lost virulence to the hosts. This study demonstrates the important roles of endo-β-1, 4-xylanase in the growth and virulence of S. sclerotiorum.

Identification of genes differentially expressed during early interactions between the stem rot fungus (Sclerotium rolfsii) and peanut (Arachis hypogaea) cultivars with increasing disease resistance levels

Sclerotium rolfsii, a destructive soil-borne fungal pathogen causes stem rot of the cultivated peanut, Arachis hypogaea. This study aimed to identify differentially expressed genes associated with peanut resistance and fungal virulence. Four peanut cultivars (A100-32, Georgia Green, GA-07W and York) with increasing resistance levels were inoculated with a virulent S. rolfsii strain to study the early plant-pathogen interaction. 454 sequencing was performed on RNAs from infected tissue collected at 4 days post inoculation, generating 225,793 high-quality reads. Normalized read counts and fold changes were calculated and statistical analysis used to identify differentially expressed genes. Several genes identified as differential in the RNA-seq experiment were selected based on functions of interest and real-time PCR employed to corroborate their differential expression. Expanding the analysis to include all four cultivars revealed a small but interesting set of genes showing colinearity between cultivar resistance and expression levels. This study identified a set of genes possibly related to pathogen response that may be useful marker assisted selection or transgenic disease control strategies. Additionally, a set of differentially expressed genes that have not been functionally characterized in peanut or other plants and warrant additional investigation were identified.

Gene disruption of an arabinofuranosidase/beta-xylosidase precursor decreases Sclerotinia sclerotiorum virulence on canola tissue

Although Sclerotinia sclerotiorum (Lib.) de Bary has been studied extensively, there are still aspects of this important phytopathogen's ability to cause disease in susceptible plants that remain unclear. A recent comprehensive proteome-level investigation of this fungus identified a number of proteins whose functions in disease initiation and progression have not been clearly established. Included among these proteins was an arabinofuranosidase/beta-xylosidase precursor whose role as a potential virulence factor had not been investigated previously. This article describes the generation of gene-disrupted mutant S. sclerotiorum unable to produce this arabinofuranosidase/beta-xylosidase precursor as well as the comparison of the virulence of this mutant with that of wild-type mycelia on susceptible canola leaves and stems. At all time points tested, the degree of necrosis was observed to be significantly greater on the plant tissue inoculated with wild-type mycelia. To our knowledge, this is the first report that clearly demonstrates that this arabinofuranosidase/beta-xylosidase precursor is a virulence factor for S. sclerotiorum.

Role of swollenin, an expansin-like protein from Trichoderma, in plant root colonization

Swollenin, a protein first characterized in the saprophytic fungus Trichoderma reesei, contains an N-terminal carbohydrate-binding module family 1 domain (CBD) with cellulose-binding function and a C-terminal expansin-like domain. This protein was identified by liquid chromatography-mass spectrometry among many other cellulolytic proteins secreted in the coculture hydroponics medium of cucumber (Cucumis sativus) seedlings and Trichoderma asperellum, a well-known biocontrol agent and inducer of plant defense responses. The swollenin gene was isolated and its coding region was overexpressed in the same strain under the control of the constitutive pki1 promoter. Trichoderma transformants showed a remarkably increased ability to colonize cucumber roots within 6 h after inoculation. On the other hand, overexpressors of a truncated swollenin sequence bearing a 36-amino acid deletion of the CBD did not differ from the wild type, showing in vivo that this domain is necessary for full protein activity. Root colonization rates were reduced in transformants silenced in swollenin gene expression. A synthetic 36-mer swollenin CBD peptide was shown to be capable of stimulating local defense responses in cucumber roots and leaves and to afford local protection toward Botrytis cinerea and Pseudomonas syringae pv lachrymans infection. This indicates that the CBD domain might be recognized by the plant as a microbe-associated molecular pattern in the Trichoderma-plant interaction.

Sclerotinia sclerotiorum (Lib.) de Bary: biology and molecular traits of a cosmopolitan pathogen

SUMMARY Sclerotinia sclerotiorum (Lib.) de Bary is a necrotrophic fungal pathogen causing disease in a wide range of plants. This review summarizes current knowledge of mechanisms employed by the fungus to parasitize its host with emphasis on biology, physiology and molecular aspects of pathogenicity. In addition, current tools for research and strategies to combat S. sclerotiorum are discussed.

TAXONOMY

Sclerotinia sclerotiorum (Lib.) de Bary: kingdom Fungi, phylum Ascomycota, class Discomycetes, order Helotiales, family Sclerotiniaceae, genus Sclerotinia.

IDENTIFICATION

Hyphae are hyaline, septate, branched and multinucleate. Mycelium may appear white to tan in culture and in planta. No asexual conidia are produced. Long-term survival is mediated through the sclerotium; a pigmented, multi-hyphal structure that can remain viable over long periods of time under unfavourable conditions for growth. Sclerotia can germinate to produce mycelia or apothecia depending on environmental conditions. Apothecia produce ascospores, which are the primary means of infection in most host plants.

HOST RANGE

S. sclerotiorum is capable of colonizing over 400 plant species found worldwide. The majority of these species are dicotyledonous, although a number of agriculturally significant monocotyledonous plants are also hosts. Disease symptoms: Leaves usually have water-soaked lesions that expand rapidly and move down the petiole into the stem. Infected stems of some species will first develop dark lesions whereas the initial indication in other hosts is the appearance of water-soaked stem lesions. Lesions usually develop into necrotic tissues that subsequently develop patches of fluffy white mycelium, often with sclerotia, which is the most obvious sign of plants infected with S. sclerotiorum.

USEFUL WEBSITES

http://www.whitemoldresearch.com; http://www.broad.mit.edu/annotation/fungi/sclerotinia_sclerotiorum.

Diversity of the exoproteome of Fusarium graminearum grown on plant cell wall

The exoproteome of the fungus Fusarium graminearum grown on glucose and on hop (Humulus lupulus, L.) cell wall has been investigated. The culture medium was found to contain a higher quantity of proteins and the proteins are more diverse when the fungus is grown on cell wall. Using both 1D and 2D electrophoresis followed by mass spectrometry analysis and protein identification based on similarity searches, 84 unique proteins were identified in the cell wall-grown fungal exoproteome. Many are putatively implicated in carbohydrate metabolism, mainly in cell wall polysaccharide degradation. The predicted carbohydrate-active enzymes fell into 24 different enzymes classes, and up to eight different proteins within a same class are secreted. This indicates that fungal metabolism becomes oriented towards synthesis and secretion of a whole arsenal of enzymes able to digest almost the complete plant cell wall. Cellobiohydrolase is one of the only four proteins found both after growth on glucose and on plant cell wall and we propose that this enzyme could act as a sensor of the extracellular environment. Extensive knowledge of this very diverse F. graminearum exoproteome is an important step towards the full understanding of Fusarium/plants interactions.

Purification and characterization of a thermostable glucoamylase from Chaetomium thermophilum

Thermostable amylolytic enzymes are currently being investigated to improve industrial processes of starch degradation. A thermostable extracellular glucoamylase (exo-1, 4-alpha-D-glucanohydrolase, E.C.3.2.1.3) from the culture supernatant of a thermophilic fungus Chaetomium thermophilum was purified to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) homogeneity by using ammonium sulfate fraction, DEAE-Sepharose Fast Flow chromatography, and Phenyl-Sepharose Fast Flow chromatography. SDS-PAGE of the purified enzyme showed a single protein band of molecular weight 64 kDa. The glucoamylase exhibited optimum catalytic activity at pH 4.0 and 65 degrees C. It was thermostable at 50 degrees C and 60 degrees C, and retained 50% activity after 60 min at 65 degrees C. The half-life of the enzyme at 70 degrees C was 20 min. N-terminal amino acid sequencing (15 residues) was AVDSYIERETPIAWN. Different metal ions showed different effects on the glucoamylase activity. Ca2+, Mg2+, Na+, and K+ enhanced the enzyme activity, whereas Fe2+, Ag+, and Hg2+ cause obvious inhibition. These properties make it applicable to other biotechnological purposes.