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Departamento de Microbiología del Suelo y Sistemas Simbioticos, Estación Experimental del Zaidín, CSIC, Apd. 419, E-18008 Granada, Spain
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ABSTRACT |
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Different conditions of culture medium, incubation time, concentration and surfactant were tested to determine xyloglucanase activity. Trichoderma viride, Paecilomyces farinosus, Wardomyces inflatus and Pleurotus ostreatus showed increased xyloglucanase activities when the fungi grown on microcrystalline cellulose as the sole carbon source. Endoxyloglucanase activity increased with the growth of the fungi and reached a peak on day 14 of incubation, practically 95% of the activity was associated with the extracellular fraction. Precipitation with ammonium sulfate was the best concentration method for detection of endoxyloglucanase activity of the fungi. Endoxyloglucanase activity of the fungi was increased by 4 fold with the use of the non-ionic surfactant Tween 20. Six and three bands of xyloglucanase activities were observed in T. viride and P. ostreatus, respectively, whereas both P. farinosus and W. inflatus presented only one xyloglucanase activity band. These results indicate the presence of several xyloglucanases in the saprophytic fungi examined.
Key words: concentration, culture medium, incubation time, saprophytic, surfactant, xyloglucanase
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Although saprophytic fungi are important and common components of the soil rhizosphere, they have not been well studied. These fungi are important because of the large amount of microbial biomass they supply to the soil, and because of their role in plant residue decomposition (Domsch et al 1980
). Some saprophytic fungi are involved in complex interactions such as antibiosis (Cook and Baker 1983
), fungistasis (Pavlica et al 1978
) or mycoparasitism (Elad 1986
). Their metabolism may result in the production of substances that promote or inhibit the growth of other rhizosphere microorganisms (Dix and Webster 1995
).
Plant cell wall degradation may be important to fungi not only for penetration and ramification inside the plant tissue but also for releasing, from the wall polysaccharides, nutrients necessary for growth (Radford et al 1996
). Most fungi produce a wide array of enzymes capable of depolymerizing the polysaccharides of the plant cell wall. Many of these enzymes are extracellularly targeted glycoproteins, which are inducible upon exposure of the fungus to plant cell walls (De Lorenzo et al 1997
). Plant pathogenic fungi synthesize and secrete large quantities of cell-wall-degrading enzymes to invade the plant tissue and their regulation has been extensively studied (Deising et al 1995
). In contrast, saprophytic fungi produce strictly regulated amounts of enzymes in order to digest cellulose and to use it as the sole carbon source (Mendgen and Deising 1993
).
Xyloglucan is the major structural hemicellulose in primary cell walls of plants. In addition to its structural role, xyloglucan can be hydrolyzed by plant and fungal hydrolytic enzymes and the products used as a source of signalling molecules (Hayashi 1989
) and as a food reserve (Fry 1989
). Of the different hydrolytic enzymes, xyloglucanases are the least well known; however, they play an important role in plant cell wall degradation (Hoson et al 1995
). There is evidence that hemicellulases, including xyloglucanases, are involved in the colonization of root by the arbuscular mycorrhizal fungi (Rejón-Palomares et al 1996
). No studies on the production of xyloglucanases by saprophytic fungi have been described.
Because most fungi produce enzymes capable of hydrolyzing the plant cell wall and because xyloglucanase is one of the major structural hemicelluloses in primary cell walls, the purpose of this study was to determine the presence of xyloglucanase in several saprophytic fungi, their mode of action and the optimal conditions for their detection, localization and production.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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) modified as described by Fracchia et al 1998
. From the resulting colonies Wardomyces inflatus (Marchal) Hennebert (BAFC Cult. No. F8992; Hennebert 1968
) and Paecilomyces farinosus (Holm and Gray) A.H.S. Brown and G.Sm. (BAFC Cult. No. F8846; Samsom 1974
) were selected. Trichoderma viride and Pleurotus ostreatus present in the rhizosphere soil and black poplar wood, respectively, were isolated by the particle washing method using a multichamber washing apparatus (Widden and Bisset 1972
). The fungal isolates were maintained in tubes of potato dextrose agar (PDA). The fungi were grown in the following media: i) potato dextrose medium (PDB); ii) medium W composed of MgSO4·7H2O, 0.5 g; K2HPO4, 0.6 g; KH2PO4, 0.5 g; CuSO4·5H2O, 0.4 mg; MnCl2·4H2O, 0.09 mg; BO3H3, 0.07 mg; Na2MoO4, 0.02 mg; FeCl3, 1 mg; ZnCl2, 10 mg; biotin, 5 µg; thiamine-HCl, 0.1 mg; CaCl2, 0.11 g; distilled water to 1 L; iii) medium W plus glucose 1%; iv) medium W plus glucose 0.75% and microcrystalline cellulose (Avicel) 0.25%; v) medium W plus glucose 0.5% and Avicel 0.5%; vi) medium W plus 0.25% glucose and Avicel 0.75%; vii) medium W plus Avicel 1%. To analyze the effect of different surfactants on enzyme production, culture medium W plus Avicel 1% was supplied with polyoxyethylene sorbitan mono-laureate (Tween 20), polyoxyethylene sorbitan mono-oleate (Tween 80) or polyethylene glycol p-isooctylphenyl ether (Triton X-100).
The fungi were grown at 25 C with orbital shaking at 125 rpm in Erlenmeyer flasks (125 mL) containing 50 mL of culture medium. Each flask was inoculated by transferring a 3 mm plug cut out from the margin of a 5-d-old colony grown on 2% malt extract agar (MEA). The mycelium was harvested after 2, 4, 6, 8, 10, 12, and 14 d of growth. The culture liquid was separated from the mycelium by centrifugation (5 000 x g). The supernatant was used as crude or concentrate extracellular enzyme extract. The crude enzyme extract was concentrated by ammonium sulfate, acetone precipitation, or lyophilization. Ammonium sulfate was added up to 80% saturation; the solution was kept for 5 h at 4 C and centrifuged at 20 000 x g for 20 min. For the acetone precipitation 50 mL of acetone was added to 50 mL of supernatant; the solution was kept for 15 min at 4 C and centrifuged at 16 000 x g for 15 min. The supernatants obtained by the ammonium sulfate and acetone precipitation were discarded, the respective precipitate and the resulting powder obtained by lyophilization were dissolved in a small volume of distilled water and dialyzed against several hundred volumes of water for 16 h at 4 C.
For total enzyme assays (extracellular and hyphae-associated): 0.3% w/v Triton X-100 and 10 mM NaHCO3 were added to the Erlenmeyer flasks and the suspension of hyphae was then homogenized. After centrifugation for 20 min at 1000 x g the pellet was discarded and the supernatant was used as total enzyme source.
The culture solids (mycelium plus undegraded cellulose) were washed twice with distilled water, dried at 70 C overnight and weighed.
Total proteins (extracellular and mycelial) were measured by the method of Bradford (1976)
using a Bio-Rad kit with BSA as standard. Mycelial proteins were measured after hydrolysis of the culture solids in 1N NaOH for 30 min at 100 C with BSA in NaOH as standard.
Enzyme assays –
The extracts were assayed to determine the activities of endoxyloglucanase (endo-XG) and exoxyloglucanase (exo-XG). Endoxyloglucanase activity was assayed by the viscosity method, using xyloglucan as substrate from nasturtium seed (Tropaeolum majus L.) extracted as described by McDougall and Fry (1989)
. The reduction in viscosity was determined at 0–30 min intervals. Approximately 0.5 mL of the reaction mixture was sucked into a 1-mL syringe and the time taken for the meniscus to flow from the 0.70 mL to 0.20 mL mark was recorded. The reaction mixture contained 1 mL of 0.5% substrate in 50 mM citrate-phosphate buffer (pH 5) and 0.2 mL enzyme. Viscosity reduction was determined at 37 C. One unit of enzyme activity was expressed as specific activity (U/mg prot) (U reciprocal of time in h for 50% viscosity loss x 103) (Rejón-Palomares et al 1996
).
Exoxyloglucanase was quantified by measuring the reducing sugars with a 2,2'-bicinchoninate reagent (BCA) (Waffenschmidt and Jaenicke 1987
). Reaction mixtures at 40 C contained 400 µL of 0.5% substrate in 50 mM citrate-phosphate buffer (pH 5), 25 µL of the enzyme sample diluted to 400 µL with H2O, and 800 µL of 200 mM potassium phosphate-citric acid buffer (PCA, pH 5). Product formation was measured as described by Mateos et al (1992)
. A standard curve for reducing sugars was prepared with glucose in the range of 0–20 nmol. One unit of enzyme activity was defined as the amount of product released per h at 40 C and pH 5.
Polyacrylamide gel electrophoresis –
Xyloglucanase enzymes were separated by denaturing electrophoresis (SDS-PAGE) on 6% polyacrylamide slab minigels (MiniProtean II, Bio-Rad) amended with 0.05% xyloglucan in 50 mM Tris-HCl 1 M Glycine buffer (pH 8.8) (García-Garrido et al 1996
). The electrode tank contained the Tris-Glycine buffer (pH 8.8) as used in the gel. The wells were filled with 25 µL of fungus extract and 3 µL 0.05% bromophenol blue. Electrophoresis was done at 4 C and a constant current of 20 mA per gel for 5 h.
The gels were incubated with 50 mM citrate-phosphate buffer (pH 5) at 37 C for 8 h, after which they were stained with 0.1% Congo red for 15 min. This was followed by washing in 1 M NaCl until colorless bands became visible against a red background.
Statistical treatments – Each data point is the average of three replicate samples. The data were analyzed by the one-way ANOVA followed by Duncan's multiple range test (P = 0.05).
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RESULTS |
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The effect of carbon sources was studied after 14 days of incubations by adding different amounts of Avicel to the basal medium and xyloglucanase activity of T. viride, P. farinosus, W. inflatus and P. ostreatus was detected with all the culture media used (Table I ). The fungi grown on microcrystalline cellulose as the sole carbon source (medium W plus 1% Avicel) showed more xyloglucanase activity than those grown on other media. Quantitative differences referring to enzyme production were observed among the culture media. Endo- and exoxyloglucanase activities of all saprobes tested were increased in parallel with the microcrystalline cellulose concentration but most of the activity was found to be endoxyloglucanase. The endoxyloglucanase activities of P. farinosus and P. ostreatus grown in PDB culture medium were higher than the activity of the fungi grown in medium with low amount of Avicel. No significant differences in the dry weight of mycelia of fungi grown in the different media were found (Table I ). From the results obtained, medium W plus 1% Avicel was the most suitable growth medium for xyloglucanase detection, therefore this medium was used in all subsequent experiments.
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), various types of surfactants have been selected to study the effect on growth and endoxyloglucanase production of T. viride, P. farinosus, W. inflatus, and P. ostreatus. Tween 20 was the best compound for enhancing the endoxyloglucanase activity of the fungi. These activities were approximately three times higher than the control for T. viride and W. inflatus and four times for P. farinosus and P. ostreatus. The addition of Tween 80 and Triton X-100 to the culture medium also resulted in significant increases of endoxyloglucanase activity. Triton X-100 resulted in a rise in the extracellular proteins in the culture medium, however growth in the presence of Tween 20 and 80 resulted in the same levels of extracellular proteins as the controls. The growth of all four fungi, measured as mycelial protein, changed in parallel to their xyloglucanase activities (Table IV
).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Endoxyloglucanase activities of T. viride, P. farinosus, W. inflatus, and P. ostreatus were detected during their growth in medium with glucose as the only carbon source, but there were significant increases in activity when microcrystalline cellulose was included in the medium. Induction of ß-glucosidase of Aspergillus terreus by cellulose has also been reported (Pushalkar et al 1995
). The ability to digest cellulose and to use it as the sole carbon source is a property of a significant number of fungal species (Radford et al 1996
). The fact that P. farinosus and P. ostreatus show more activity in the PDB culture medium than in the medium with a low amount of Avicel may be due to the presence of inducing compounds originating from the potato extract in the medium. The lack of effect of the substrate on the mycelium dry weight in our experiments has also been observed in A. solani grown with different pectic substrates (Metha et al 1975
).
With regard to the localization of xyloglucanases, we can say that for all the fungi tested the endoxyloglucanase activity is primarily secreted into the culture medium. Contrary to the situation with bacterial hemicellulases, which are generally cell-bound or concentrated in microsomes, fungal hemicellulases seem to be readily secreted into the growth medium (Radford et al 1996
). The physical limitation of the xyloglucanolytic enzyme diffusion may be a potential adaptative mechanism. In this way the efficiency in polymeric insoluble substrate utilization, such as for microcrystalline cellulose, would be optimized. When competing with other microorganisms for a carbon source, it would require the direct contact of the cells to the substrate (Tate 1995
).
To detect and measure hydrolytic enzymes from microorganisms, the culture medium, incubation time and concentration used are important. The most suitable incubation time was found to be approximately 14 d and the highest specific activity was obtained when the enzyme solution was concentrated with ammonium sulfate precipitation. Acetone precipitation and lyophilization may have resulted in enzyme denaturation in the protein precipitate. A similar situation was reported for pectinases in plants colonized by arbuscular mycorrhizal fungus (García-Romera 1990
).
The stimulatory effects of surfactants on cellulolytic enzyme production and release have been described (Pardo 1996
). The stimulatory effect of Tween 20, Tween 80, and Triton X-100 on endoxyloglucanase activity may be a consequence of its action on cell membranes causing increased permeability (Reese et al 1969
) and/or by promoting the release of cell-bound enzymes (Reese and Manquire 1971
). It is increasingly believed that at least some fungal cellulolytic enzymes are either bound to the hyphal wall or held in close association with the hyphae (Messner et al 1990
). These two possibilities are in agreement with the fact that Triton X-100 cultures were higher in extracellular protein than the control. However, these possibilities are not true for the increased endoxyloglucanase activity with Tween 20 or 80, because no differences in extracellular protein content were observed relative to the control. Tween 20 and 80 may increase the enzymatic stability against the possible inactivation by shaking. Reese (1980)
found that some surfactant had a protective effect on the cellulases of T. reesei against shaking inactivation. The fact that the surfactants increase the T. viride, P. farinosus, W. inflatus, and P. ostreatus growth is interesting because the effect of surfactants in other systems was reported to be inhibitory due to a decrease in oxygen supply (Hulme and Stranks 1970
), or without effect on mycelial growth (Yazdi et al 1990
). Surfactants could also provoke an increase in cell membrane permeability leading to a more efficient nutrient uptake, without significant alteration in oxygen supply.
Trichoderma viride, P. farinosus, W. inflatus and P. ostreatus shared one band of xyloglucanase activity with the same electrophoretic mobility, indicating a possible relation between them. Different isozyme activities in T. viride and P. ostreatus may result from differences in the glucosylation of a common polypeptide chain, partial proteolysis of the enzymes, or different gene products (Pardo et al 1997
). Future studies should help to elucidate the various roles that xyloglucanase activity of these saprophytic fungi may play in the degradation of hemicellulolytic waste.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Accepted for publication November 27, 2001.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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