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Previous Article | Next Article 
Applied and Environmental Microbiology, December 2001, p. 5833-5839, Vol. 67, No. 12
0099-2240/01/$04.00+0 DOI: 10.1128/AEM.67.12.5833-5839.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
An Antifungal Exo-
-1,3-Glucanase (AGN13.1) from
the Biocontrol Fungus Trichoderma harzianum
Hassane
Ait-Lahsen,1
Andrés
Soler,1
Manuel
Rey,1
Jesús
de
la Cruz,1,
Enrique
Monte,2 and
Antonio
Llobell1,*
Instituto de Bioquímica Vegetal y
Fotosíntesis, Universidad de Sevilla-CSIC,
Seville,1 and Centro Hispano-Luso de
Investigaciones Agrarias, Universidad de Salamanca,
Salamanca,2 Spain
Received 15 June 2001/Accepted 30 September 2001
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ABSTRACT |
Trichoderma harzianum secretes 
-1,3-glucanases
when it is grown on polysaccharides, fungal cell walls, or autoclaved
mycelium as a carbon source (simulated antagonistic conditions). We
have purified and characterized one of these enzymes, named AGN13.1. The enzyme was monomeric and slightly basic. AGN13.1 was an exo-type -1,3-glucanase and showed lytic and antifungal activity against fungal plant pathogens. Northern and Western analyses indicated that
AGN13.1 is induced by conditions that simulated antagonism. We propose
that AGN13.1 contributes to the antagonistic response of T.
harzianum.
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TEXT |
Biological control by antagonistic
organisms is a potential nonchemical tool for crop protection against
phytopathogenic fungi (33). Several strains from the genus
Trichoderma have been described as antagonistic fungi able
to control a wide range of phytopathogenic fungi. The antifungal
activity of Trichoderma involves production of antibiotics,
including compounds affecting the integrity of fungal membranes,
competition for key nutrients, and production of fungal cell
wall-degrading enzymes (CWDEs) (14, 23). Although none of
these mechanisms have been convincingly proven, the degradation and
further assimilation of fungal structures and contents have been
proposed as the major mechanism accounting for the antagonistic process
against fungal plant pathogens (7).
A number of Trichoderma isolates produce a wide variety of
CWDEs, among them, chitinases, 
-1,3- and -1,6-glucanases, and proteases, when grown on polysaccharides, fungal cell walls, or autoclaved mycelium as a carbon source (4, 10, 29). These conditions have been described as "simulated antagonism" (13, 14, 30, 45). These observations, together with the fact that
chitin and -1,3-glucan are the main skeletal polysaccharides of
fungal cell walls (except those from Oomycetes, which
contain -glucans and cellulose) (3, 34), suggest that
chitinases and -1,3-glucanases act as the key enzymes in the lysis
of phytopathogenic fungal cell walls during the antagonistic action of
Trichoderma. However, other CWDEs, including those
hydrolyzing minor polymers (i.e., -1,6-glucans or proteins), may
also be involved in the antagonistic response of Trichoderma
(10, 11, 18).
Polysaccharides, which consist of -glycosidic linkages, also appear
to be important in the cell wall composition and architecture. The
so-called alkali-soluble glucans (S-glucans) range from polymers containing nearly pure -1,3-linkages to polymers regularly
alternating -1,3- and -1,4-linkages (2, 3).
S-glucans represent the major matrix polysaccharides for most fungi
(3, 34); in some instances, like in Aspergillus
nidulans, S-glucans account for approximately 25% of the dry
weight of the cell wall (47). Despite the importance of
S-glucans, there have been few reports describing the presence of crude
enzyme activities capable of hydrolyzing this polysaccharide, and there
is almost no biochemical and molecular information on
-1,3-glucanases (17, 19, 22, 41, 43, 49).
Different applications have been envisaged for antifungal CWDEs and
their genes from Trichoderma strains, ranging from the improvement of biocontrol agents (4, 16, 28) to their use as heterologous genes for plant resistance against phytopathogenic fungi (31). In the search for new CWDEs, no attention has
been devoted to -1,3-glucanases to date.
In this article, we report on the purification and molecular
characterization of an exo--1,3-glucanase (glucan
1,3--glucosidase [EC 3.2.1.84]), namely AGN13.1, from the
antagonistic fungus T. harzianum. During the progress of
this work, AGN13.1 from T. harzianum was identified
independently as an enzyme able to degrade the mutan, an extracellular
-1,3-glucan produced by tooth-colonizing streptococci
(17). Herein, we show that the expression of the gene and
the enzyme secretion occur when T. harzianum grows under conditions that simulate antagonism. The enzyme is able to degrade cell
walls of some phytopathogenic fungi. Antifungal assays reinforce the
possible role of AGN13.1 during the antagonistic action of T. harzianum.
Microorganisms and culture conditions.
T.
harzianum CECT 2413 was obtained from the Spanish Type
Culture Collection (Burjasot, Valencia, Spain) and maintained on potato-dextrose-agar medium (Difco, Detroit Mich.). For enzyme production, this strain was grown in two-step liquid cultures as
exactly described in reference 13.
To obtain cell walls, other fungi were also purchased from CECT:
Aspergillus niger CECT 2574, Botrytys cinerea
CECT 2100, Colletotrichum acutatum 74, Fusarium
oxysporum, Penicillium aurantiogriseum IMI 374515, Phytophthora syringae CECT 2351, Rhizoctonia
solani CECT 2815, Schizosaccharomyces pombe and
Saccharomyces cerevisiae (La Cinta Roja, Spain). S-glucan
from A. niger was prepared as described previously
(22, 47). The composition of this S-glucan was determined
by infrared (2) and 13C nuclear
magnetic resonance (13C-NMR) spectra (5,
44), resulting in a polymer with nearly pure -1,3 linkages.
Mutan from Streptococcus mutans (an -1,3-glucan with some
-1,6-glucan side chains) was prepared exactly as described previously (17). Fungal cell walls and -glucan from
S. cerevisiae were prepared as previously described
(15, 37).
-1,3-Glucanase assay and protein determination.
-1,3-Glucanase activity was routinely assayed by incubating 0.2 ml
of 5 mg of S-glucan per ml from A. niger (-1,3-glucan) in
50 mM potassium acetate buffer (pH 5.5) (buffer A) with 50 µl of
enzyme solution appropriately diluted in the same buffer. Reaction
mixtures were incubated at 37°C for 30 min and were stopped by
boiling for 5 min. Samples were centrifuged (5,000 × g, 5 min), 0.15 ml of supernatant per reaction was taken,
and reducing sugars were determined (32, 40), with glucose
as a standard. Enzyme and substrate blanks were included. One unit of
-1,3-glucanase activity was defined as the amount of enzyme that
releases 1 µmol of reducing sugar equivalents (expressed as glucose)
per min under the standard assay conditions. Protein concentration was
determined by the method of Bradford (6), with ovalbumin
as a standard.
Enzyme production and purification.
T. harzianum
was grown on rich medium and transferred to fresh minimal medium
supplemented with different carbon sources (induced conditions). Enzyme
production was determined in filtrates from T. harzianum
transferred to the induced conditions. As reported for other CWDEs
(13, 46), we found that the -1,3-glucanase activity is
repressed at a high glucose concentration (2 to 10% glucose, 50 mU/mg
of protein) and produced under carbon starvation conditions (0.1%
glucose, 100 mU/mg of protein) and under induced conditions with 0.5%
(wt/vol) fungal cell walls or their polymers (100 to 400 mU/mg of
protein). We conclude that production of -1,3-glucanase by T. harzianum is dependent on the carbon source available.
Three consecutive steps were used for the purification of AGN13.1. (i)
T. harzianum cultures were grown as described above and
transferred for 48 h to minimal medium with 0.5% A. niger cell walls, filtrated through Whatman no. 1 paper, and
centrifuged (8,000 × g, 10 min). The supernatant
(about 600 ml) was precipitated with solid ammonium sulfate (80%
saturation), and the pellet was recovered by centrifugation
(12,000 × g, 20 min), resuspended in distilled water,
and dialyzed against 50 mM buffer A. The dialyzed fraction was
centrifuged (12,000 × g, 20 min), and the supernatant (about 18 ml) was recovered. (ii) Aliquots of 2 ml of this sample were
adsorbed to S-glucan (5 mg/ml) for 2 h with magnetic stirring, and
the pellet was collected by centrifugation at 12,000 × g for 10 min. Adsorbed fractions were washed three times
with 50 mM buffer A containing 1 M NaCl, resuspended in 50 mM buffer A
with 1 mM phenylmethylsulfonyl fluoride (PMSF) and 1 mM sodium azide, and incubated overnight at 37°C for S-glucan digestion. (iii) Aliquots of 0.5 ml of S-glucan digestion were repeatedly applied to a
PAK125 gel filtration column (7.8 mm by 30 cm; Millipore) equilibrated
with 100 mM buffer A containing 0.5 M NaCl, and eluted with the same
buffer at a flow rate of 0.2 ml/min. Fractions of 0.2 ml were collected
and monitored for protein (A280) and
for -1,3-glucanase activity. Most active fractions were pooled,
washed in 100 mM buffer A, and concentrated to approximately 1 ml on Centricon 10 concentrators (Amicon, Beverley, Mass.). The purified protein was stored in 100 mM buffer A at 
20°C.
Following this procedure, AGN13.1 was purified sevenfold with a minimal
estimated recovery of approximately 10%. As shown in Fig.
1, the final purified preparation
migrated as a single band on sodium dodecyl sulfate-polyacrylamide gel
electrophoresis (SDS-PAGE [12% polyacrylamide]) with an apparent
molecular mass of 72 kDa after Coomassie staining (27).
When the molecular mass was calculated by gel filtration, the value
obtained was 67 kDa (data not shown). We conclude that this procedure
achieved purification of AGN13.1 to apparent electrophoretic
homogeneity and that the enzyme is a monomeric protein.
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FIG. 1.
Purification of AGN13.1 from T.
harzianum. Samples containing -1,3-glucanase from each
purification step were analyzed by SDS-PAGE. Lanes: std., molecular
mass standards; 1, ammonium sulfate precipitation from a culture
filtrate from T. harzianum grown on 0.5% A.
niger cell walls (10 µg of protein); 2, adsorption and
digestion of S-glucan (5 µg of protein); 3, gel filtration pool (5 µg of protein).
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Biochemical properties.
The pIs of the pure enzyme, determined
by isoelectrofocusing (36) and basic chromatofocusing
(11), were estimated to be 6.7 and 7.5, respectively (data
not shown). The Michaelis-Menten constant resulted in
Kms of 1.76 mg/ml (S-glucan) and 1.69 mg/ml (mutan). The optimal temperature for the enzyme was 55°C at pH 5.5. After 30 min of preincubation time at different temperatures, the
inactivation temperature was calculated as 50°C. Therefore, S-glucan
seems to stabilize the enzyme.
Substrate specificity and action pattern.
The activity of
AGN13.1 was tested on different polymers with - or -glycosidic
bonds with the standard assay described above. Chitinase and protease
activities were determined as previously described (9,
39). As shown in Table 1, the
maximal activity was detected against mutan and S-glucan, which are
linear -1,3-glucans (17) (and data not shown). No
activity was detected with nigeran as substrate, which is a linear
polyglucan containing alternating -1,3 and -1,4 bonds
(34) or against other - or -glucans or chitin.
Furthermore, the enzyme preparation lacks protease activity (data not
shown). We conclude that the AGN13.1 enzyme is a specific
-1,3-glucanase.
The action pattern of AGN13.1 toward S-glucan was examined by comparing
the rate of glucose production to the rate of reducing sugar production
at different times, comparing the activities of the purified enzyme
against S-glucan and periodate-oxidized S-glucan (unable to be
hydrolyzed by exo--1,3-glucanases), and finally analyzing the
corresponding hydrolysis products by high-performance liquid
chromatography (HPLC). The reaction products were separated on an
Aminex HPX 42-A column (Bio-Rad) maintained at 40°C. Water was used
as an eluant at a flow rate of 0.4 ml/min. Products were detected on
the basis of their refraction index and identified by comparison with
glucose and cellulose oligosaccharide. Incubations of S-glucan with the
enzyme resulted in a 1:1 correlation between the production of reducing
sugars and glucose (data not shown). Therefore, glucose accounts for
almost all of the reducing sugar and is the major hydrolysis product of
the enzyme. AGN13.1 was unable to hydrolyze periodate-oxidized S-glucan
(Table 1). By HPLC analysis, glucose was detected at the initial stage
of the enzymatic reaction (data not shown). At longer time points, the peak of glucose increased, since glucose was the final product of
hydrolysis (Fig. 2). Identical results
were obtained when mutan was used as the substrate (data not shown).
Altogether, these results indicate an exo-type mode of action for the
purified AGN13.1 enzyme.
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FIG. 2.
HPLC profiles of the reaction products of AGN13.1 acting
on S-glucan. S-glucan (4 mg/ml) was incubated for 14 h at 37°C
in the absence of enzyme (A) or with 2 µg of the purified enzyme (B).
In the abbreviation "Gn," n represents the degree of
polymerization.
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Antifungal properties.
AGN13.1 was assayed for fungal cell
wall binding and degrading activity as previously described
(13). The enzyme was able to bind to cell walls of various
phytopathogenic fungi, such as A. niger, B. cinerea, C. acutatum, F. oxysporum, P. aurantiogriseum, or R. solani, but not those of
P. syringae, S. cerevisiae, or S. pombe (data not shown). In addition, the enzyme alone was quite active against cell walls from A. niger, P. aurantiogriseum, B. cinerea, C. acutatum,
and F. oxysporum (in descending order of efficacy), but
showed no activity against cell walls from R. solani, P. syringae, S. cerevisiae or S. pombe
(Table 1). Moreover, when the mode of action of AGN13.1 toward cell
walls from A. niger or P. aurantiogriseum was
studied, an exo-type mechanism was detected, with glucose as the major
final product of hydrolysis (data not shown). We conclude that AGN13.1
acts as a lytic enzyme against phytopathogenic fungal cell walls, which
may therefore contain -1,3-glucans.
Antifungal activity for conidia or hyphae from A. niger,
B. cinerea, and P. aurantiogriseum was also
detected. Antifungal assays were performed in microtiter plates
(Nunclon Microwell Minitray; 60 by 80 mm) at 24°C. Each
microwell contained 1 µl of 5× potato dextrose broth, 2 µl
of a spore suspension of different phytopathogenic fungi (60 total
spores), and 13 µl of distilled water (control) or purified enzyme in
distilled water. To determine the effects of the enzyme on fungal
germination, the enzyme solution was added simultaneously to the spore
suspension. At different times, the number of germinated spores was
counted and compared to that of the control. To determine the effects
of the enzyme on hyphal growth, the enzyme solution was added after
most spores were germinated. At different times, microscopic
observations of the microwells were done and compared to those of the
control. In this case, hyphal growth was calculated automatically as a percentage of the covered area of the microwells (Microimage program 3.0; Windows). The results represent the mean of three replicates. Our
results indicate that although germination was slightly affected when
A. niger and B. cinerea were used, around 70%
inhibition of P. aurantiogriseum spore germination was
determined in the presence of the enzyme at concentrations around 270 µg/ml (Fig. 3A).
However, growth was efficiently inhibited after the addition of AGN13.1
to germinated spores from the three fungi, at concentrations around and
greater than 90 µg/ml (data not shown) (Fig. 3B and C). Moreover,
aberrant morphology of hyphal tips was observed under the light
microscope, as shown for A. niger in Fig. 3D.
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FIG. 3.
Antifungal properties of AGN13.1. (A) Inhibition of
spore germination of P. aurantiogriseum by AGN13.1 at
different concentrations. (B) Inhibition of hyphal growth of A.
niger by AGN13.1 at different concentrations: solid circles, 0 µg/ml; open circles, 90 µg/ml; open squares, 180 µg/ml; open
triangles, 270 µg/ml. In panels A and B, error bars indicate standard
deviations. The experiment was performed three times with similar
results. (C) Photographs of A. niger under the light
microscope growing in the absence (left panel) or presence (right
panel) of 270 µg of AGN13.1 per ml. Magnification, 100-fold. (D)
Detail of the photographs shown in panel C at 400-fold magnification.
As indicated above, the left panel corresponds to the control without
AGN13.1, and the right panel corresponds to 270 µg of AGN13.1 per
ml.
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Expression pattern of agn13.1 mRNA and AGN13.1
protein.
We have determined that the N-terminal amino acid
sequence of the purified AGN13.1 is ASSADRLVFSHFMIGIVGD.
Based on this sequence, we have cloned a complete agn13.1
cDNA (M. Rey, unpublished results). The nucleotide sequence of the full
agn13.1 cDNA was identical to that of a previously described
cDNA coding for a mutanase (GenBank/EBI accession no. AF214480)
(17). Northern and Western blot analyses were not covered
by Fuglsang et al. (17), but they are pertinent to
establish whether the induction of agn13.1 preferentially
occurs under conditions similar to those involving the antagonistic
action by T. harzianum.
T. harzianum was grown on rich medium and transferred to
fresh minimal medium supplemented with different carbon sources. Samples of mycelia were then collected at different times, and RNA was
extracted and subjected to Northern blot analyses according to standard
procedures (8, 38). The radioactivity of the bands was
quantified by using a Cyclone device with OptiQuant software (Packard
Instrument Co.). Expression levels were normalized against the signal
obtained by hybridizing the blots with 18S radish ribosomal DNA (rDNA).
As shown in Fig. 4A, the
agn13.1 mRNA is detectable as early as 9 h after
transfer to chitin or fungal cell walls (Fig. 4A, lanes 3 to 5). Other
lower and less intense bands were also detected, but they might
correspond to partial degradation of the full-length agn13.1
mRNA. Densitometric analyses of the Northern blots indicated that the
agn13.1 mRNA accumulated at its highest levels when A. niger cell walls were used as an inducer (relative intensity,
0.5), instead of chitin (relative intensity, 0.25) or P. syringae cell walls (relative intensity, 0.15). Other
fungal cell walls induced agn13.1 expression to
intermediate levels between those from A. niger and P. syringae cell walls (data not shown). No expression was found at
this time on glucose-supplemented media and with carbon or nitrogen
starvation (Fig. 4A, lanes 1, 2, and 6). However, when the expression
was investigated at longer time points, agn13.1 mRNA began
to accumulate in mycelia transferred to carbon starvation conditions
(relative intensity, 0.10) (data not shown).
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FIG. 4.
Expression pattern of agn13.1. (A)
Northern blot analysis. Total RNA was extracted from cultures grown for
9 h on 2% glucose (lane 1), 0.1% glucose (lane 2), 1.5% chitin
(lane 3), 0.5% P. syringae cell walls (lane 4), 0.5%
A. niger cell walls (lane 5), and with nitrogen
starvation (lane 6). Total RNA (50 µg) was then subjected to
electrophoresis on an agarose gel under denaturing conditions,
transferred to a nylon membrane, and hybridized against a 1.9-kb probe
of the coding sequence of agn13.1 (upper panel) or 18S
rDNA (bottom panel). (B) Western blot analysis. Filtrates were obtained
from cultures grown for 48 h on 2% glucose (lane 1), 0.1%
glucose (lane 2), 1.5% chitin (lane 3), 0.5% P.
syringae cell walls (lane 4), 0.5% P.
aurantiogriseum cell walls (lane 5), 0.5% A.
niger cell walls (lane 6), and with nitrogen starvation (lane
7). Samples (25 µg) were subjected to SDS-PAGE on a 12%
polyacrylamide gel, and protein was transferred to a nitrocellulose
membrane and probed with specific rabbit polyclonal AGN13.1 antibodies
(dilution 1:1,000). AGN13.1 was visualized with peroxidase-conjugated
goat anti-rabbit secondary antibodies (dilution of 1:1,000).
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The steady-state levels of extracellular AGN13.1 protein were measured
by Western blot analyses in culture filtrates from cultures growing in
the presence of different carbon sources. Western blots were done
according to standard procedures (1) with rabbit
polyclonal antibodies raised against the purified AGN13.1 protein.
Figure 4B shows that AGN13.1 is present at almost similar levels in
filtrates from T. harzianum grown for 48 h on chitin
and several fungal cell walls. Some protein was detected in filtrates
from T. harzianum transferred to carbon starvation conditions. In all of these cases, increased -1,3-glucanase activity in filtrates correlates with the presence of the AGN13.1 protein. However, no signal at an AGN13.1 position, determined with purified preparations, was detected in samples of T. harzianum grown
on 2% glucose or under nitrogen starvation conditions. Therefore, AGN13.1 did not account for the basal extracellular activity found under these conditions (data not shown) (Fig. 4B, lanes 1 and 7). Other
minor bands were also detected, but they might correspond to partial
degradation of the full-length AGN13.1 protein and/or unspecific
binding of the primary antibody. We conclude that agn13.1 expression is mainly triggered at the transcriptional level by products
derived from fungal cell walls or isolated polymers such as chitin.
Conclusions.
T. harzianum antagonizes a large
variety of plant pathogenic fungi responsible for major crop diseases
(7, 33). Among different mechanisms for antagonism,
T. harzianum is able to produce lytic enzymes for the
degradation of the fungal cell wall to further assimilate the
intracellular contents of its hosts (4, 7). Physically,
the fungal cell wall is a fabric of interwoven microfibrils embedded in
or cemented by amorphous matrix substances (3). In most
fungi, chitin and non-cellulosic -glucans are the most abundant
skeletal or microfibrilar components, while proteins and -glucans
are the main cementing components (34). A considerable amount of recent research has been devoted to study of the lytic systems produced by T. harzianum, including chitinases
(9, 21, 30), -glucanases (4, 11, 12), and
proteases (18), and the relative importance of any of
these systems in the antagonistic process. However, the
-glucanolytic system has neither been biochemically well
characterized nor tested for its antifungal potential.
In this paper, we have described the induction of -1,3-glucanase
activity in filtrates of T. harzianum growing on
simulated antagonistic conditions. Moreover, we have purified
and biochemically characterized an -1,3-glucanase protein,
AGN13.1. The purification method was based on the strong affinity of
AGN13.1 to insoluble S-glucan from A. niger cell walls.
After gel filtration, the enzyme was recovered to apparent
electrophoretic homogeneity. The yield of the purification was only
10%; this apparent low yield can be explained by the overestimation of
the -1,3-glucanase activity in crude preparations, perhaps due to
synergistic effects of AGN13.1 with other -1,3-glucanolytic enzymes
present in the filtrates. The enzyme was purified only seven times with
respect to the crude filtrates. This result indicates that AGN13.1 is
an abundant protein in the filtrates of T. harzianum growing
on A. niger cell walls, as demonstrated by visualizing the
profiles of protein secreted by T. harzianum under these
conditions (Fig. 1, lane 1). The purified protein has a molecular mass
of 72 kDa, as calculated by SDS-PAGE. This value was 67 kDa according
to gel filtration, strongly suggesting a native monomeric form, as
previously reported for a large variety of extracellular CWDEs from
T. harzianum (4, 9, 25, 42). These data are not
too disparate from the predicted molecular mass for the mature protein
(ca. 63.8 kDa) and data published by Fuglsang and coworkers
(17). The optimal and inactivation temperatures (both
around 50°C) were similar to those found for other CWDEs from
T. harzianum and other fungi. The
Km values for S-glucan and mutan (1.76 and
1.69 mg/ml, respectively) were high, but these values are common among
enzymes attacking insoluble substrates. The
Km values are in the range calculated for
other bacterial and fungal -1,3-glucanases (17, 19, 22, 41, 43, 49).
The purified AGN13.1 enzyme was found to be highly specific for
-1,3-linkages in polysaccharides, hydrolyzing S-glucan and mutan
very efficiently. Our experiments show unequivocally that AGN13.1
exhibits an exo type of action, with glucose as the main early and
final hydrolytic product. Oligosaccharides containing two or three
residues of glucose were also detected as final reaction products by
HPLC. This result strongly suggests that AGN13.1 cannot recognize
and/or split -1,3-glucans with a degree of polymerization less than
3. Both, the high specificity for -1,3-linkages and the exo-type
mechanism explain the lack of activity against -1,3--1,4-glucans as nigeran.
Finally, our results strongly support that AGN13.1 plays an important
role in the antagonism of T. harzianum. (i) First,
expression of agn13.1 mRNA and protein is repressed by
glucose and induced in cultures with chitin or fungal cell walls as a
nutrient source, which may represent a good simulation of antagonism.
This expression pattern is almost identical to that found for other
CWDEs from T. harzianum (4, 13, 18, 21), and it
may reflect a coordinate induction of CWDEs for optimal establishment
of antagonism, as discussed in references 12 and
13. In addition, AGN13.1 is derepressed late by low
glucose levels, suggesting some mobilization of -glucans from the
cell wall of T. harzianum under carbon starvation conditions, as previously indicated for A. nidulans
(48). (ii) AGN13.1 is able to bind and release reducing
sugar from the cell walls of a variety of fungal phytopathogens. This
activity depends on the cell wall source and may reflect different
proportions of accessible -1,3-glucan in those cell walls.
Intriguingly, AGN13.1 bound efficiently, but failed to attack the cell
walls from R. solani. Rather than the lack of an
-1,3-glucan substrate in these cell walls, this result is likely due
to the presence of substances, such as melanin-like compounds, able to
inhibit CWDEs (35). (iii) Most importantly, we observed
different rates of spore germination and growth inhibition by AGN13.1
of phytopathogenic fungi A. niger, B. cinerea,
and P. aurantiogriseum. Differences in sensitivity to the
enzyme among the fungi might be due to the different molecular
architectures of the spore and hyphal cell walls and/or the presence of
specific inhibitors of CWDEs.
Altogether, the expression pattern of agn13.1 gene and
protein, the lytic properties of AGN13.1 against fungal cell walls, and
AGN13.1's antifungal activity strongly suggest that AGN13.1 contributes to the antagonism of T. harzianum. In addition
to its possible medical applications against dental caries (17, 20, 24), important agricultural applications can be envisaged for this enzyme. Overexpression in recombinant T. harzianum
could result in the generation of more effective biocontrol strains. Plant cell walls lack -1,3-glucans (26), and to our
knowledge, no -1,3-glucanase activity has been described among the
defense response systems in plants. Expression of the
agn13.1 gene in transgenic plants might therefore improve
resistance to fungal pathogens.
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ACKNOWLEDGMENTS |
We gratefully acknowledge F. Domínguez for helpful
assistance during the first period of the experimental work. We thank R. Sánchez for help with HPLC experiments and J. M. García and M. López-Reyes for 13C-NMR.
J. de la Cruz thanks A. Vioque for encouragement.
A. Soler is a recipient of a fellowship from the MEC. J. de la
Cruz thanks the MEC (Spain) for financial support. This work was
supported by grants FAIR6-CT98-4140 from EU and FEDER
IFD97-0843-C05-01 from EU and Plan Nacional I+D.
H. Ait-Lahsen and A. Soler contributed equally to this work.
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FOOTNOTES |
*
Corresponding author. Mailing address: Instituto de
Bioquímica Vegetal y Fotosíntesis, Centro de
Investigaciones Científicas Isla de la Cartuja, Avda.
Américo Vespucio s/n, Isla de la Cartuja, E-41092 Seville, Spain.
Phone: 34 95 4489521. Fax: 34 95 4460065. E-mail:
Llobell{at}cica.es.
Present address: Departamento de Genética, Facultad de
Biología, Universidad de Sevilla. Seville, Spain.
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Applied and Environmental Microbiology, December 2001, p. 5833-5839, Vol. 67, No. 12
0099-2240/01/$04.00+0 DOI: 10.1128/AEM.67.12.5833-5839.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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