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Microbio/ogy
(1
994),
140,
1641-1 649
Printed in Great Britain
Metabolism of [14C]glutamate and
[14C]glutamine by the ectomycorrhizal fungus
Paxillus involutus
Michel Chalot, Annick Brun, Roger
D.
Finlay and Bengt Soderstrom
Author
for
correspondence:
M.
Chalot. Tel: +46 46
10
86 14. Fax: +46 46 10 41 58.
Department
of
Microbial
Ecology, University
of
Lund,
Ecology Building,
5-223 62
Lund, Sweden
To
examine pathways
of
glutamate and glutamine metabolism
in
the
ectomycorrhizal fungus
Paxillus
involutus,
tracer kinetic experiments were
performed
using
~-[U-l~C]glutamate and ~-[U-~~C]glutamine and the enzyme
inhibitors
methionine sulfoximine
(MSX),
azaserine (AZA) and
aminooxyacetate (AOA). When [14C]glutamate was supplied
to
fungal cultures,
25%
of
the radioactivity
of
the amino acid fraction was incorporated
into
glutamine after
5
min
feeding,
but
MSX
inhibited incorporation
of
14C
into
glutamine by
85
%,
suggesting the rapid operation
of
glutamine synthetase.
Conversely, when
P.
involutus
was fed
with
[14C]glutamine,
46%
of
the label
was
found
in
glutamate
within
30
min
of
feeding and AZA inhibited glutamate
formation by
90%.
Taken together, these data indicate that glutamate
synthase (GOGAT) is the major enzyme
of
glutamine degradation.
In
addition,
the
strong
inhibition
of
glutamine utilization by AOA indicates that glutamine
catabolism
in
P.
involutus
might
involve a transamination process as an
alternative pathway
to
GOGAT
for
glutamine degradation. The
high
l4CO,
evolution
shows
that glutamate and glutamine are further actively consumed
as respiratory substrates, being channelled
through
the tricarboxylic acid (TCA)
cycle and oxidized as CO,.
It
appears that synthesis of amino acid precursors
during
TCA cycle operation is an essential step
for
aspartate and alanine
synthesis
through
aminotransferase activities
in
P.
involutus.
Keywords
:
ectomycorrhizal fungi, glutamine metabolism, glutamate metabolism,
Paxillzts
involutu
INTRODUCTION
Symbiotic associations between roots and ecto-
mycorrhizal fungi play an integral role in the nitrogen
metabolism of most forest trees. Investigations based on
"N-labelling have indicated that glutamate and glutamine
are the main acceptors of inorganic nitrogen in ecto-
mycorrhizas and ectomycorrhizal fungi (Martin
et
al.,
1986; Finlay
etal.,
1989; Chalot
etal.,
1991b; Kershaw
&
Stewart, 1992). The potential enzymes for N transfer from
ammonium
to
amino acids are NADP-dependent glut-
amate dehydrogenase (NADP-GDH,
EC
1
.4.1.4), glut-
amine synthetase (GS,
EC
6.3.1.2)
and glutamate
synthase (GOGAT, EC 1 .4.7.1) (Miflin
&
Lea, 1980;
Stewart
et
al.,
1980).
Ammonium
is
assimilated by
Abbreviations:
AOA, aminooxyacetate; AZA, azaserine; GDH, glutamate
dehydrogenase; Gln-T, glutamine transaminase; GOGAT, glutamate syn-
thase;
GS,
glutamine synthetase; MSX, methionine sulfoximine; TCA,
tricarboxylic acid.
sequential GDH/GS activity in spruce ectomycorrhizas
(Dell
et
al.,
1989;
Chalot
et
al.,
1991b) and in rapidly
growing
Cenococcztm
geophilztm
(Genetet
et
al.,
1984)
whereas the GS/GOGAT cycle seems
to
predominate in
beech ectomycorrhizas (Martin
et
al.,
1986)
as well as in
the ectomycorrhizal fungus
Pisolithzts
tinctorizts
(Kershaw
&
Stewart, 1992). GOGAT
is
also the main enzyme of
glutamine degradation in yeasts (Holmes
et
al.,
1989),
Neztrospora
crassa
(Calderon
&
Mora,
1985,1989
;
Lomnitz
et
al.,
1987) and
Aspergzllzts
nidz4lan.r
(Kusnan
et
al.,
1987,
1989).
GDH, GS and GOGAT activities have been
detected in a range of ectomycorrhizal fungi (Vkzina
et
al.,
1989; Ahmad
et
al.,
1990).
NADP-GDH has been purified
to
electrophoretic homogeneity from
C.
geophilztm
(Martin
et
al.,
1983) and
Laccaria
laccata
(Brun
et
al.,
1992). GS has
also been purified and characterized from
L.
laccata
(Brun
et
al.
,
1992).
One of the major alternative pathways to the
GS/GOGAT cycle in
N.
crassa
is the cu-amidase pathway,
0001-8706
0
1994
SGM
1641
M.
CHALOT
and
OTHERS
in which glutamine is converted into 2-oxoglutarate and
ammonium by the sequential activities of glutamine
transaminase and co-amidase (Calderon
et
al.,
1985
;
Calderon
&
Mora, 1989). Glutamine transaminase activity
has been reported to be the major pathway for glutamine
catabolism in
Saccharomyes
cerevisiae
cu
1
tu re d unde r micro-
aerophilic conditions (Soberon
et
al.,
1989). In free-living
mycorrhizal mycelia and ectomycorrhizas, following in-
itial nitrogen assimilation into glutamate and glutamine,
the N is incorporated into a range of amino acids, mainly
alanine, aspartate and asparagine after short (Martin
et
a/.,
1986;
Chalot
et
al.,
1991b) or long (Finlay
et
al.,
1988,
1989)
incubation periods. These findings, supported by
the high aminotransferase activities measured in ecto-
mycorrhizas and ectomycorrhizal fungi (Dell
et
al.,
1989
;
Chalot
et
al.,
1990),
stress the central role of glutamate and
glutamine as
N
donors.
In addition, glutamate and glutamine can support biomass
production comparable to that on ammonium in different
ectomycorrhizal fungi (Abuzinadah
&
Read, 1988
;
Chalot
et
al.,
1991a; Finlay
et
al.,
1992). As pointed out by
Abuzinadah
&
Read
(1989),
assimilation of amino acids
derived from proteolytic activity can supply up to 10
YO
of
the total C gained by the host over a period of
50
d,
highlighting the importance of amino acids as a potential
C source. Data on the filamentous fungus
N.
cra~sa
(Calderon
&
Mora,
1989),
plants (Osaki
et
al.,
1992;
Muhitch, 1993), and root nodules (Ta
et
al.,
1988;
Kouchi
et
al.,
1991)
have clearly shown that [14C]gl~tamate arid
[14C]glutamine are used intensively as respiratory sub-
strates and a carbon source for organic acids, proteins and
sugars. However, little information has been obtained
concerning the utilization of their carbon skeletons by
ectomycorrhizal fungi or ectomycorrhizas. Indeed most
of the work on ectomycorrhizal fungi or ectomycorrhizas
has focused on the transfer of
N
from glutamate and
glutamine
to
other amino acids using the 15N isotope
(Martin
et
al.,
1986;
Finlay
et
a].,
1989;
Chalot
et
d.,
1991b;
Kershaw
&
Stewart, 1992) or on the transfer
of
C
from carbon dioxide or glucose to amino acids using I3C
(Martin
&
Canet,
1986)
or 14C (France
&
Reid, 1983)
isotopes.
The objectives of the present study were
(1)
to examine
14C-incorporation into amino acids from ~-[U-~~C]glu ta-
mate or ~-[U-~~C]glutamine by the ectomycorrhizal
fungus
Paxill,~
involutzi.~
and (2)
to
determine how the
transfer of C from newly-absorbed 14C-amino acids
to
newly-synthesized "C-labelled amino acids is affected by
the enzyme inhibitors methionine sulfoximine
(MS
X),
azaserine (AZA) and aminooxyacetate (AOA).
METHODS
growing edge of 10-d-old colonies and preincubated for 1 h in
a
nutrient solution containing either 2.5 mM MSX, 1 mM AZA
or 2 mM AOA prepared in modified MMN in which the
nitrogen source was omitted. These concentrations of inhibitors
were those giving complete inhibition of growth in test
experiments (Botton
&
Chalot, 1991). Their structure,
specificity and mode of action have been extensively reviewed
elsewhere (Miflin
&
Lea, 1980; Stewart
et
al.,
1980; Botton
&
Chalot, 1991).
A
control without inhibitor was also included.
The uptake of L-glutamate and L-glutamine was strongly
dependent on the external pH and was optimal
at
pH 4.1. The
initial pH of the MMN medium was 5-5 before addition of the
inhibitors and was adjusted to 4.1 after addition of the inhibitors
by using HC1 (in control, MSX and AZA treatments) or NaOH
(in AOA treatment). Fungal discs were then washed to remove
excess inhibitor and placed for between 5 and 120 min in
small dishes containing 1 ml nitrogen-free MMN supplemented
with either 3.7 kBq ~-[U-'~C]glutarnate (specific activity
10.4 MBq pmol-'; New England Nuclear) or 3.7 kBq
L-
[U-14C]glutamine (specific activity 7.77 MBq pmol-'
;
New
England Nuclear). At the end of the feeding period, the mycelial
discs were washed for 5 min with 0.1 mM CaSO, and freeze-
dried prior to analysis.
Separation
of
amino acids.
Amino acids were extracted from
lyophilized tissues in 70% (v/v) methanol. The extract was
centifuged for 20 min at 13
000
g
and filtered through
a
0.25
pm
membrane filter (Millipore). Samples were then evaporated to
dryness using
a
Speed Vac Concentrator (Savant, Speed Vac
Plus). The residues were taken up in
80
pl
50
mM sodium
acetate, pH 5.9, and a 60
pl
aliquot was used for chromato-
graphic separation.
Identification
of
amino acids.
Free amino acids were analysed
by reversed-phase high-performance liquid chromatography
(HPLC) in the methanol-soluble fraction after derivatization
with
o-phthaldialdehydelp-mercaptoethanol
reagent according
to Martin
et
a/.
(1986). Chromatographic separations were
performed using a Novapak
C18
column (39
x
150 mm). Amino
acid derivatives were separated with a gradient of solvent A
(water/methanol, 90
:
10,
v/v, containing 50 mM sodium
acetate, pH 5.9) and solvent B (methanol/acetonitrile, 95
:
5,
v/v). The gradient was varied as follows (flow rate:
1
ml min-')
:
0-35
YO
B, 26 min; 35-100
YO
B, 1 min; 100
YO
B, 3 min; 10&0
YO
B,
1 min;
0
YO
B, 4 min. The absorbance of the column eluate
was monitored at 340 nm. Amino acids were quantified using
the HPLC Manager Workstation (Pharmacia-LKB Bio-
technology).
Determination
of
radioactivity.
The radioactivity incorporated
into amino acids was measured by liquid scintillation spec-
troscopy of separate fractions corresponding to each amino acid
peak in the HPLC eluate collected at the outlet of the
spectrophotometric detector. Radioactivity was also determined
in an aliquot of the methanol-soluble fraction and in the
methanol-insoluble pellet after tissue solubilization with
Soluene 350 (Packard Instrument Co.). The 14C02 evolved
during [14C]glutamate or [14C]glutamine feeding was trapped in
methanol/ethanolamine (70/30, v/v) and the radioactivity
measured by scintillation spectroscopy.
Organism and
in
vivo
labelling.
Paxilltls involtlttls
(Batsch) Fr.
was grown on cellophane-covered agar medium containing
modified Melin-Norkrans (MMN) medium from which rnalt
extract was omitted. The MMN medium contained (mg
I-'):
I<H2P0,
(500), (NH,),HPO, (250), CaC1, (50), NaCl (25),
MgSO,
.
7H20 (150), thiamin hydrochloride (0*1), FeC1,.
6H20
(1).
This medium was used with 1
g
glucose
I-'.
Discs of fungal
inoculum were cut with a 25 mm diameter cork borer from ;he
RESULTS
Metabolism
of
[14C]glutamate and ['4C]glutamine
Following [14C]glutamate and [14C]glutamine feeding, 38
and
44%
respectively of the total radioactivity in the
control mycelium was found in the amino acid fraction
after 2 h incubation (Tables
1
and 2). Five to twelve
1642
Glutamate and glutamine metabolism by
P.
involutm
Table
I.
Total absorbed radioactivity, mycelium-
associated radioactivity and radioactivity in the amino
acid
pool
derived from metabolism of [14C]glutamate
.................
.
.
.........................
..............
..............
,
....................
......................
....
.
.............
..
.......
Mycelial discs were preincubated with either
2.5
mM
MSX,
1
mM
;\ZA
or
2
mM
AOA
and incubated with
3.7
kBq
L-
[U-14C]glutamate (specific activity
10.4
MBq pmol-l) for
30, 60
or
120
tnin. Each value is the mean
f
SE
of
at least three
replicates.
Treatment Time
x
Radioactivity [d.p.m.
(min) (mg dry wt)-']
Total Mycelium- Amino acid
associated
absorbed associated pool-
Control
30 27.8f2.4
60 78.0f 12.3
120 117.3f18.0
MSS
30 39.7+ 1.5
60 64*0+0*1
120 102.0
f
3.9
AZ:l
30 51.3f6-3
60 43.3f5-5
120 103.0
f
6.0
A0
h
30 34*5+3-3
60 70.6f2-8
120 77-7f2.2
27.1
f
2.8
33.3
f
4.8
61.8
f
9-0
32.7
f
1.2
40.9
f
1.2
50.4
f
5.4
24.4
f
4.9
33.0
f
0.9
59.2
f
1.8
46.2
f
4.6
54.6
f
5.1
80.7
f
8.4
12.6
f
1.3
16.8
f
5.2
23.4
&
8.7
16-5
f
1.3
20-4
f
3.4
21.6
f
2.6
12.4
f
4.0
16.8
f
0.6
23.5
f
5.4
28.8
f
3.6
31.8
f
3.0
44.4
f
6.0
Table
2.
Total absorbed radioactivity, mycelium-
associated radioactivity and radioactivity in the amino
acid
pool
derived from metabolism of [14C]glutamine
Mycelial discs were preincubated with either
2.5
mM MSX,
1
mhf AZA or
2
mM AOA and incubated with
3-7
kBq
L-
[U-"C]glutamine (specific activity
7.77
MBq pmol-l) for
30, 60
or
120
min. Each value is the mean
f
SE
of at least three
replicates.
Treatment Time
x
Radioactivity [d.p.m.
(min) (mg dry wt)-']
~~
Total Mycelium- Amino acid
absorbed associated
pool-
associated
(:ontrol
30
60
120
MSX
30
60
120
AZA
30
60
120
AOA
30
60
120
36.0
f
3.4
53.7
f
2.5
86.1
&
6.4
43.2
f
6.4
35.8
f
2-8
92.4
f
8.5
39.8
f
7.8
73.2
f
8.8
72.6
f
0.7
53.5
f
2.4
66.4 4-3
93-1
f
1.2
38.1 f6.6
44.5
f
4.2
51.6
f
2.5
46.6
f
5.7
39.1
f
3-3
61-9
f
4.0
38.1
f
10.3
59.5
f
5.4
67.5
f
5.4
56.0
f
2.5
58.3
f
7.2
102.7 9.1
27.5
f
3.9
26.7
f
4.6
22.9
f
2.1
29.1
f
5.2
18.1
f
1.9
23.8
f
2.4
22.8
f
4.5
37.6
f
4.2
40.6
+
1.2
34.0
&
0.7
32.1
f
4.0
56.7
f
4.6
percent of the radioactivity was associated with the
methanol-insoluble pellet fraction from fungal extracts,
indicating slow incorporation of '"C into proteins (data
not shown). The chemical form into which the remaining
activity in the methanol-soluble fraction was incorporated
was not investigated further but it is possible that the
activity was incorporated in carboxylic acids derived from
deamination of the amino acids. In preliminary experi-
ments the mycelium was fed with both the 14C source and
the inhibitor
(MSX
or AZA), thus blocking the uptake
system(s) for amino acids competitively. Under these
conditions, the level of radioactivity recovered in the
amino acid fraction of the mycelium was negligible,
indicating that the mycelium did not retain labelled amino
acids in the apoplastic space. Part of the radioactivity
removed from the feeding solution could not be found
inside the mycelium (Tables
1
and 2). This proportion
increased with time and may be due to formation of
volatile compounds in the control. We have not studied
'"CO, release in detail, but some observations are worth
noting. We found that, after 2 h incubation, 34% and
25% of '"C was lost as '"CO, during [14C]glutamate and
[14C]glutamine feeding respectively. This accounted for
approximately
80%
of the difference between the total
amount of absorbed radioactivity and the amount of
radioactivity associated with the mycelium.
Feeding [14C]glutamate to colonies of
P.
involzitzis
resulted
mainly in incorporation into glutamine. After 5 min,
[14C]glutamine accounted for 25
'YO
of the radioactivity in
the amino acid pool (Fig.
la)
while [14C]glutamate
represented 51
YO
of the radioactivity. These proportions
did not vary greatly during the 2 h feeding period. The
label was also detected in a range of amino acids including
aspartate, asparagine, alanine and y-aminobutyrate, which
represented 12.5,
2-9,
1.3
and 1.2% respectively of the
total radioactivity incorporated into amino acids (Fig.
1
b). Serine, glycine and citrulline were slightly labelled,
accounting for less than 1
YO
of the total radioactivity (not
shown). Arginine was not detected in the mycelium,
either in a labelled or unlabelled form, in our growth
conditions. The patterns of '"C-labelling found in free
amino acids in
P.
involzltus
were similar to those demon-
strated by Finlay
et
al.
(1989) using l5NH;, where
glutamate/glutamine, alanine, aspartate/asparagine and
y-aminobutyrate were the main acceptors of 15N whereas
no label could be detected in arginine. When [l"C]gluta-
mine was supplied to
P.
involutzi~
cultures,
46%
of the
radioactivity was found in glutamate within 30 min of
feeding, [14C]glutamine accounting for 41
%
(Fig. lc).
Twelve percent of the label in the amino acid fraction was
found in aspartate, 2.4% in alanine and
1.1
YO
in
y-
aminobutyrate after 2 h feeding (Fig. Id).
After 2 h feeding, there was no marked difference between
the distribution of '"C into amino acids of [14C]glutamate-
and [14C]glutamine-fed
P.
involtltus.
With both '"C-
sources, there was a rapid equilibrium between glutamate
and glutamine; glutamate accounted for 50-55
%
and
glutamine 25-30
'/o
of the total radioactivity in the amino
acid pool at the end of the experiment. However, the
[14C]glutamine-fed mycelium differed from the [14C]gluta-
1643
M.
CHALOT
and
OTHERS
Y
5
125-
.-
>
.-
3
100-
.-
-0
X
2
75-
N
z
50
25
0,
125
-
Q0:I
I I
I
I
-
-
-
i
--
IIIII
0
30 60 90
120
I
4
30
24
18
I
P
l2
2
6F
v
U
0
30
60
90
120
Time (min)
Fig.
1.
Accumulation of radioactivity from (a, b) [14C]glutamate,
and (c, d) [14C]glutamine into glutamate
(O),
glutamine
(01,
aspartate
(m),
asparagine
(a),
alanine
(A)
and y-aminobutyric
acid
(A)
by
P.
involutus.
Discs of fungal inoculum from 10-d-old
colonies were preincubated for
1
h in modified nitrogen-free
MMN and then placed in
a
solution containing nitrogen-free
MMN supplemented with 3.7 kBq ~-[U-’~C]gIutamate (specific
activity
10.4
MBq pmol-’) or with 3-7 kBq ~-[U-’~C]glutamine
(specific activity 7.77 MBq pmol-I). Data are expressed
as
means
of
triplicates. Vertical bars indicate
SE.
100
80
--
60
’
40
L,
20
I
U
v
€
no
d
Ill
I1
0
30
60
90
120
0
30
60
90
120
Time (min)
Fig.
2.
Effect of MSX on accumulation of radioactivity from
(a, b) [‘4C]glutamate, and
(c,
d)
[‘4C]glutamine into glutamate
(o),
glutamine
(o),
aspartate
(m),
asparagine
(n),
alanine
(A)
and y-aminobutyric acid
(A)
by
P.
involutus.
Discs of fungal
inoculum from 10-d-old colonies were preincubated for
1
h in
a
nutrient solution containing
2.5
mM
MSX prepared in modified
nitrogen-free MMN. Fungal discs were then washed to remove
excess inhibitor and placed in
a
solution containing nitrogen-
free MMN supplemented with
3.7
kBq ~-[U-’~C]glutamate
(specific activity 10.4 MBq pmol-’) or with
3.7
kBq L-[U-
14C]glutamine (specific activity
7.77
MBq pmol-’). Data are
expressed
as
means of triplicates. Vertical bars indicate
SE.
mate-fed mycelium in that, within the first 5-30 rnin of
incubation, significantly more radioactivity was incor-
porated into the amino acid fraction, reflecting a higher
absorption rate (Tables
1
and 2). The 14C in the amino
acids (except aspartate)
of
the [14C]glutamine-fed colonies
reached a maximum after 30 min (Table 2, Fig. lc, d)
whereas
it
continued
to
accumulate up to 2 h in the
[14C]glutamate-fed colonies (Table
1,
Fig. la, b).
Effect
of
MSX
Preincubation of cells with 2.5 mM L-MSX prior to the
addition of [14C]glutamate resulted in an immediate 85
‘/o
inhibition of the incorporation of radioactivity into the
glutamine fraction and a corresponding increase in the
[14C]glutamate pool (Fig. 2a) compared
to
the control
(Fig. la). However, the inhibitory effect decreased gradu-
ally throughout the 14C-feeding period (Table
3).
By the
end of the
2
h feeding the Glu
:
Gln ratio in MSX-treated
mycelia was similar
to
that of the control. This rapid
decrease in the inhibitory effect of MSX could possibly be
due
to
an
in
vivo
synthesis of
GS
that replaced the inhibited
enzyme, or to a detoxification process. Kusnan
et
al.
(1987)
reported that MSX had no effect
in
vivo
on
As-ergillm
nidzdans
whereas the extracted
GS
could be
fully inhibited by MSX, suggesting that the cells either
detoxified, or did not take up the inhibitor. This latter
hypothesis is not consistent with our data since high
GS
inhibition was found in the first 5-30 min of [14C]gluta-
mate feeding. Further studies are needed
to
clarify this
point. There was also a marked inhibition of 14C-
incorporation into aspartate and alanine under [14C]gluta-
mate feeding within the last 20-120 rnin (Fig. 2b). In
contrast, synthesis of y-aminobutyrate was not affected by
this inhibitor. Under MSX inhibition and [14C]glutamine
feeding, there was a 1.6-fold accumulation of [14C]gluta-
mine after 30 min feeding whereas the [14C]glutamate
remained unchanged (Fig. 2c) compared
to
the control
mycelium (Fig. lc). There was also a 1.6- and 2-fold
decrease
of
aspartate and alanine synthesis respectively,
after 2 h feeding (Fig. 2d).
Effect
of
AZA
When AZA-treated mycelia were given [14C]glutamate,
total mycelium-associated radioactivity as well as the total
amino acid pool radioactivity were similar
to
that found in
1644
Glutamate and glutamine metabolism by
P.
involzttzts
Table
3.
[
14C]
G
I
uta mat e
:
[
4C]
g
I
u ta
m
i
ne rat
i
0s
under
[
14C]
g
I
u
t
a m ate
or
[
4C]
g
I
u ta m
i
ne
feeding and inhibition treatments
Data were calculated from
Figs
1-4.
ND,
Not
determined.
Time
MSX-
AZA-
AOA-
(min) Control treated treated treated
l4C
source... Glu Gln Glu Gln Glu Gln Glu Gln
5
10
20
30
60
120
2.59
ND
15.64
ND
ND ND ND ND
2.61
ND
20-85
ND
ND ND ND ND
2.95
ND
11.51
ND ND ND ND ND
1.38
1.00
6-34 0.63 0.13 0.07 0.98 0.49
1.32 1.28 2.40 1.72 0.13 0.07 1.38 0.67
1.60 1-85 1-78 1-79 0.14 0.08 1.23 0.47
200
160
7
120
2
80
€
5
m
40
U
40
E
5
350
8
280
.-
>
.-
.-
U
2
210
N
k
140
70
0
1----
0
30
60
90
12(
I11
I1
u
0-
30 60
90
120
Time (min)
. . . . .
.
. .
,
. . . .
.
.
, ,
. . . .
,
,
. .
, , ,
. . .
, , , ,
.
,
,
, ,
.
,
,
, ,
.
,
,
,
.
.
. .
.
.
.
. . . .
.
.
,
. .
.
. . . .
.
.
, , ,
.
,
.
,
. .
, , ,
. . . . .
.
. .
.
.
.
. . . .
.
. .
.
.
.
.
,
.
.
. . .
,
. .
,
. .
.
. . .
,
.
.
,
.
.
.
, ,
. . . .
,
.
.
.
. .
. .
.
.
.
. .
Fig.
3.
Effect of
AZA
on accumulation of radioactivity from
(a, b) [’4C]glutamate, and (c,
d)
[14C]glutamine into glutamate
(e),
glutamine
(o),
aspartate
(m),
asparagine
(n),
alanine
(A)
and 11-aminobutyric acid
(A)
by
P.
involutus.
Incubations were
as in Fig.
2
except that
1
mM
AZA
was used as the inhibitor.
Data are expressed as means of triplicates. Vertical bars indicate
SE.
the control mycelium (Table 1). However, the proportion
of label incorporated into individual amino acids differed
greatly
to
that of the control. About 76
YO
of the label in
the amino acid fraction
of
AZA-treated mycelium was in
glutamine whereas
[
“C]glutamate accounted for only
11
O/O
of the radioactivity after 2
h
feeding (Fig. 3a), thus
giving a G1u:Gln ratio 10-fold lower under AZA
inhibition compared
to
the control (Table 3). Under these
conditions, the [14C]aspartate and [l‘clalanine pools
decreased by 3.5- and 4-6-fold respectively, representing
only 3.4 and 0.6% of the radioactivity after 2 h feeding
(Fig. 3b). In contrast, y-arnino[l4C] butyrate and
[l‘clasparagine increased by 2.3- and
1
-5-fold respectively,
representing 2.3 and 2.2% of the radioactivity in the
amino acid fraction, after 2 h incubation. When fed with
[14C]glutamine, the AZA-treated mycelium had about
double the radioactivity in the amino acid pool and the
total radioactivity increased by 16fold at
60
min (Table
2).
In
addition, assuming that most of the lost radioactivity
was evolved as 14C02, “CO, release was reduced eight-
fold in AZA-treated mycelium fed with
[
14C]glutamine
after
2
h feeding compared
to
the control (Table 2). AZA
also had strong and predicted effects on [14C]glutamine
metabolism since only
18
%
of the total [14C]glutamine in
the mycelium was metabolized after 2 h feeding (Fig. 3c),
in contrast
to
the control, where 72% was utilized (Fig.
lc). Seven percent of the amino acid label was in
glutamate, 2
%
in aspartate and 0.3
O/O
in alanine after 2 h
feeding (Fig. 3d). In contrast, y-amin~[’~C]butyrate and
[
“Clasparagine increased by 4.1
-
and 3.1 -fold, respect-
ively, representing 2-6 and 2.0% of radioactivity in the
amino acid fraction after 2 h feeding (Fig. 3d).
Effect
of
AOA
Preincubation with 2 mM- AOA prior
to
[14C]glutamate
feeding increased the total radioactivity associated with
the mycelium 1-7-fold after 30 min and doubled the
amount
of
“C-labelled amino acid after 30,60 and
120
min
(Table 1). Assuming that most
of
the lost radioactivity
was evolved as “C02, preincubation with AOA decreased
the amount of released “CO,
to
a negligible level (Table
1). Both glutamate and glutamine accounted for the
increase in radioactivity in the amino acid pool and
accumulation was double that in the control (Fig. 4a). The
G1u:Gln ratio did not differ greatly from the control
under [14C]gl~tamate feeding. In addition, a 3.2- and 9.2-
fold decrease in the label incorporated into aspartate and
alanine was observed (Fig. 4b), as expected if the reactions
1645
M.
CHALOT
and
OTHERS
250
200
-
150
I
9
100
eo
2
F
50
-0
v
4
T-l
z
.-
.Z’
350-
>
.-
+
280-
.-
-0
2
210-
N
2
140-
70
-
0-
0
30
60
90
120
0
30
60
90
120
Time (min)
Fig.
4.
Effect of AOA on accumulation of radioactivity from
(a, b) [14C]glutamate, and (c, d) [14C]glutamine into glutamate
(a),
glutamine
(O),
aspartate
(B),
asparagine
(a),
alanine
(A)
and y-aminobutyric acid
(A)
by
P.
involutus.
Incubations were
as
in Fig.
2
except that
2
mM AOA was used as the inhibitor.
Data are expressed
as
means of triplicates. Vertical bars indicate
SE.
catalysed by aminotransferases were blocked. By contrast,
synthesis of asparagine was not affected by this inhibitor
and y-amino[14C] butyrate increased 2.8-fold after 2 h
feeding. Exposure of AOA-treated mycelium
to
[
14C]glutamine gave similar results and revealed marked
accumulation of label associated with the mycelium or
with the amino acid pool correlated to a complete
reduction in the lost radioactivity, i.e.
of
14C0, (Table 2).
The [14C]glutamine pool increased approximately 6-fold
(Fig. 4c) and the amount of [14C]aspartate and [14C]alanine
decreased 2.2- and 2-3-fold after 2 h [14C]glutamine
feeding (Fig. 4d). However, in contrast
to
[14C]glutamate
feeding, the
[
14C]glutamate pool remained unchanged
(Fig. 4c) and the amount of y-aminobutyrate increased by
4.5-fold after
2
h.
As
Table
3
shows, the Glu: Gln ratio
under [14C]glutamine feeding was substantially lowered
compared
to
the control.
DISCUSSION
The results of the 14C tracer experiments suggest that the
carbon skeletons derived from newly-absorbed glutamate
were mainly used for the synthesis of glutamine. The
accumulation of [14C]glutamate and the marked decrease
of [14C]glutamine under MSX treatment are consistent
with rapid utilization of glutamate by GS in
Paxillus
GS
(MSX)
%
GS
(MSX)
glutamate
-
glutamine
4-
-
-
-
- - -
Gln-T
(AoA)
GOGAT
(AZA)
v
I
oxoglutaramate glutamate
-
-
-
-
-
-I
mamidase
(AZA)
GDH,
AAT,
AUT
?
oxoglutarate
cozy-
\m2
succinate
isocitrate
\
~~Acycie
1
citrate fumara te
. .
.
.
,
.
. .
.
.
.
,
.
.
. . .
. .
. . .
.
.
. .
.
.
,
.
.
.
,
.
.
.
. . .
.
. .
. . .
. .
.
.
, ,
.
, ,
. .
,
.
, ,
.
,
. .
.
.
. . .
.
.
. . .
. .
.
. . .
.
.
,
.
.
.
.
.
.
. . . . . .
.
.
.
.
.
.
. . .
.
, ,
,
.
,
,
,
.
, ,
. .
,
.
.
.
.
. .
.
. . . .
,
.
, , , , ,
.
, , ,
.
, , ,
. .
. . .
,
.
, ,
.
Fig.
5.
Possible pathways for metabolism
of
[14C]glutamate and
[14C]glutamine by
P.
involutus.
The newly-absorbed glutamate
is
actively metabolized by
GS
whereas GOGAT
is
the major
enzyme of glutamine degradation. In addition, the Gln-Th-
amidase sequence,
as
an alternative pathway to GOGAT, may
also be responsible for the production of oxoglutarate.
Glutamate and glutamine carbon skeletons are further actively
channelled through the TCA cycle, thus providing a carbon
source for mycelial respiration and for amino acid biosynthesis
through transamination reactions. AIAT, alanine
a m
i
not r
a
nsf e r ase
;
AAT,
as
pa rta
t
e
a
m
i
not r
a
nsf er ase
;
A0 A,
am
i
nooxyacetate
;
AZA, azase ri ne
;
G
D H,
g
I
utamate
dehydrogenase; Gln-T; glutamine transaminase; GOGAT,
glutamate synthase;
GS,
glutamine synthetase; MSX,
methionine sulfoximine.
involtltxr,
and support previous studies of ectomycorrhizal
fungi and ectomycorrhizas (Martin
et
al.,
1986,
1988;
Chalot
et
al.,
1991b; Kershaw
&
Stewart, 1992). The
newly-absorbed, as well as the newly-synthesized,
[
14C]glutamine were actively degraded into [14C]gluta-
mate, suggesting the rapid operation of the glutamine
transamidase GOGAT (Fig.
5).
This is also supported by
the striking accumulation of [‘4C]glutamine when col-
onies were preincubated with AZA. Recently, Kershaw
&
Stewart (1992) also suggested that GOGAT is involved
in the utilization
of
the 15N amido group of glutamine by
Pisolithw
tinctorius.
The unexpected accumulation
of
[14C]glutamine under MSX inhibition and [14C]glutamine
feeding suggests that GS might be involved in the
recycling of the newly-synthesized glutamate (Fig.
5),
the
non-utilization of glutamate then having a feedback
control effect on GOGAT activity.
The data presented also suggest direct involvement of
glutamate and glutamate carbon skeletons in the res-
piratory pathways. Rapid l4COZ evolution from
[14C]glutamate or [14C]glutamine indicates that glutamate
1646
Glutamate and glutamine metabolism by
P.
involutus
and glutamine carbon are rapidly metabolized
to
14C02,
presumably via the TCA cycle (Fig.
5).
Production of
the ke!. intermediate, oxoglutarate, from the newly-
synthesized glutamate is achieved by the putative anabolic
GDH reported
to
be present in ectomycorrhizal fungi
(Dell
et
a/.,
1989;
Vezina
et
al.,
1989).
Ultimately, carbon
entering the TCA cycle is metabolized to give oxaloacetate
and malate, which are used for aspartate and alanine (via
pyruvate) synthesis, respectively, by the amino-
transferases (Fig.
5).
Preincubation of
P.
involtitus
with
AOA, leading
to
marked reductions in the [14C]aspartate
and [14C]alanine pools, confirmed the rapid operation of
aminotransferases. In previous studies (Finlay
etal.,
1989),
aspartate and alanine (as free amino acids) were found
to
have high levels of 15N-enrichment when
P.
involztttis
was
fed with l5NH;, confirming the importance of amino-
transferases in this fungus. In contrast, in the present
study, under AOA treatment, y-amino[14C] butyrate in-
creased, suggesting that transamination is a possible route
for y-aminobutyrate degradation in
P.
involtitus,
as already
demonstrated in alfalfa nodules (Ta
et
a/.,
1988). More
surprising is the marked accumulation of
y-
amino[ ''C]butyrate under AZA treatment and [14C]gluta-
mine feeding, which suggests that y-aminobutyrate syn-
thesis
is
related
to
the glutamine pool. A similar re-
lationship is suggested by previous studies on spruce
ectomycorrhizas, in which a correlation was demonstrated
between the decrease in the glutamine pool due
to
MSX
and the decrease in y-aminobutyrate labelling during
l5NH; feeding (Chalot
et
al.,
1991b). This hypothesis is
supported by other findings that demonstrate a good
correlation between glutamine synthesis and y-amino-
butyrate synthesis in cultured rice cells (Kishinami
&
Ojima,
1980).
However, the mechanism involved remains
unclear. Similarly, the unexpectedly large accumulation of
[14C]asparagine under AZA treatment but not under
MSX
treatment, from either [14C]glutamate or [14C]gluta-
mine, might indicate that synthesis of asparagine is
glutamine-dependent but not sensitive
to
AZA in
P.
involtitus.
Snapp
&
Vance
(1986)
also reported that AZA
had little effect on asparagine synthesis in alfalfa root
nodules. Our data show that [14C]asparagine is syn-
thesized in higher quantities under conditions where the
necessary N donor, glutamine, is not rapidly used in
competing pathways, i.e. under AZA treatment. This is
also supported by the larger decrease in [14C]aspartate
pool, the carbon skeleton donor for asparagine synthesis,
under AZA inhibition.
In addition
to
having the predicted effects on [14C]aspar-
tate and [14C]alanine pools, AOA substantially decreased
14C0, release from [14C]glutamate or [14C]glutamine and
increased the [14C]glutamate or [14C]glutamine pools.
Similar effects of AOA on the utilization of [14C]glutamate
as a respiratory substrate have been observed in bacteroids
isolated from soybean root nodules (Kouchi
et
al.,
1991).
The addition of AOA
to
bacteroid suspensions resulted in
a
60
?&
decrease in 14C02 evolution from glutamate. It was
concluded that the degradation of glutamate might have
involved a transamination process as an essential step. In
P.
indtittis,
it seems rather that glutamine degradation
itself was inhibited by AOA since accumulation of
[14C]glutamine but not [14C]glutamate was observed in
AOA-treated and [14C]glutamine-fed mycelium. This led
us
to
the hypothesis that glutamine degradation can be
achieved by a glutamine transaminase (Gln-T) reported
to
be present in
Neuro.rpora
crassa
(Calderon
et
al.,
1985;
Calderon
&
Mora,
1989).
In this pathway glutamine is
transaminated
to
yield oxoglutaramate through the par-
ticipation of a Gln-T and oxoglutaramate is further
hydrolysed
to
oxoglutarate and ammonium by the action
of an o-amidase which has been reported
to
be inhibited
by amidotransferase inhibitors (Calderon
et
al.,
1985)
and
possibly also by AZA, a potent inhibitor of a wide range
of glutamine-utilizing enzymes that transfer amide groups
(Miflin
&
Lea, 1980). The accumulation
of
[14C]glutamine
observed in AOA-treated and
[
14C]glutamate- or
[
14C]glutamine-fed mycelium might be explained by
inhibition
of
Gln-T. Similar observations have been
reported in
N.
crassa,
where addition of AOA to
[14C]glutamine-fed cultures reduced the 14C02 release by
82
9'0
(Calderon
&
Mora, 1989). The Gln-T/w-amidase
sequence thus appears
to
be an alternative pathway
to
GOGAT for oxoglutarate production. However, if the
Gln-T/o-amidase pathway was solely responsible for the
degradation of glutamine, we would have expected
complete inhibition of glutamine degradation and gluta-
mate synthesis following AOA treatment, which did not
occur. The presence
of
enzymes involved in glutamate
and glutamine utilization (GS, GDH, GOGAT, Gln-T,
w-amidase) in
P.
involuttis
remains to be determined by
in
vitro
measurement of their activities. Using protein
immunoblots (with
GS
and NADP-GDH antibodies
raised against the enzyme from the ectomycorrhizal
fungus
Laccaria laccata),
we were able
to
demonstrate the
presence of GS in
P.
involtittis
whereas no NADP-GDH
could be detected (unpublished results). Moreover, some
GS activity has been detected in
P.
involtittis,
either free-
living or associated with
Pinus
qlvestris
(Sarjala, 1993).
However, the presence of abundant polyphenols is likely
to
have been an obstacle
to
the detection of the enzyme in
several ectomycorrhizal fungi (Botton
&
Chalot,
1991).
For instance, GOGAT activity was not detected in
Pisolithzts
tinctoritis
by Vkzina
et
al.
(1989)
whereas, using
the 15N isotope, the enzyme has been shown to be
essential for glutamate synthesis (Kershaw
&
Stewart,
1992).
Our results suggest also that the newly-absorbed
[14C]glutamate makes little contribution
to
the synthesis
of aspartate and alanine. Indeed when AZA-treated
mycelia were given [14C]glutamate, it failed
to
accumulate
and [14C]aspartate and [14C]alanine pools were substan-
tially lowered. In these conditions, where only the
[14C]glutamate pool from glutamine is reduced but not
the newly-absorbed [14C]glutamate, no reduction of the
[14C]aspartate and [14C]alanine pools would have occurred
if the newly-absorbed glutamate pool had been involved
in the synthesis of those two amino acids. Similar results
have been obtained from enzymic studies on
Cenococcum
geophiltim,
where aspartate aminotransferase was inhibited
in the presence of albizziine, an inhibitor of GOGAT
1647
M.
CHALOT
and
OTHERS
(B. Botton
&
A. Khalid, personal communication). It is
then possible that a very small but metabolically active
pool of glutamate, serving as a substrate in glutamine
synthesis, is tightly compartmentalized, away from the
other glutamate pool which channels the carbon
flow
from catabolism of glutamine and serves as a source of
carbon skeletons in the synthesis
of
organic acids and
amino acids. This is consistent with previous
work
showing clear compartmentation of GS in
L.
laccata
(Brun
et
d.,
1993).
However, conflicting results were obtained
from
MSX
experiments. When MSX-treated mycelia were
given [14C]glutamate there was no reduction of the
[14C]aspartate and [14C]alanine pools during high
G
S
inhibition, i.e. within the first
5-20
min, as would be
expected if glutamine synthesis was required for
[14C]aspartate and [14C]alanine synthesis. We suggest that
the slow but increasing glutamine synthesis rate might be
sufficient to provide carbon skeletons for the synthesis of
aspartate and alanine.
In conclusion, the present results provide direct evidence
for the utilization of glutamate and glutamine carbons v.a
GS and GOGAT activities by
P.
involutus.
The Gln-T/m-
amidase sequence, as an alternative pathway to GOGAT,
may also be responsible for the production of oxo-
glutarate, the key intermediate between amino acids and
oxoacids. Glutamate and glutamine carbon skeletons are
actively channelled through the TCA cycle, thus pro-
viding a carbon source for mycelial respiration and for
amino acid biosynthesis through transamination rc-
actions.
Financial support was obtained from the Swedish Natural
Science Research Council and the Swedish Council for Forestry
and Agricultural Research.
Abuzinadah,
R.
A.
&
Read, D.
1.
(1988).
Amino acids as nitrogeii
sources for ectomycorrhizal fungi.
Trans Br
Mycol
Soc
91, 473-479.
Abuzinadah,
R.
A.
&
Read, D.
1.
(1989).
Carbon transfer associated
with assimilation of inorganic nitrogen sources by silver birch
(Betda pendula
Roth.).
Trees
3, 17-23.
Ahmad,
I.,
Carleton, T.
J.,
Malloch, D.
W.
&
Hellebust,
1.
A. (1990).
Nitrogen metabolism in the ectomycorrhizal fungus
Laccaria
biculor
(R. Mre.) Orton.
New
Pbytol116, 431-441.
Botton,
B.
&
Chalot,
M.
(1991).
Techniques for the study
of
nitrogen metabolism in mycorrhizas.
Methods
Microbiol 23,203-252.
Brun, A., Chalot,
M.,
Martin,
F.
&
Botton,
B.
(1992).
Purification
and characterization of glutamine synthetase and NADP-glutamatc
dehydrogenase from the ectomycorrhizal fungus
Laccaria laccata.
Plant
Pbysiol99, 938-944.
Brun, A., Chalot,
M.
&
Botton,
B.
(1993).
Glutamate dehydrogenasc
and glutamine synthetase of the ectomycorrhizal fungus
Laccariu
laccata
:
occurrence and immunogold localization in the free-living
mycelium.
Plant
Pbysiol
(Lqe
Sci
Ah)
12, 53-60.
Calderon,
J.
&
Mora,
J.
(1985).
Glutamine cycling in
Neurosporu
crassa.
J
Gen
Microbioll31, 3237-3242.
Calderon,
1.81
Mora,
J.
(1989).
Glutamine assimilation pathways in
Neurospora crassa
growing
on
glutamine as sole nitrogen and carbon
source.
J
Gen
Microbioll35, 2699-2707.
Calderon,
J.,
Morett,
E.
&
Mora,
1.
(1985).
o-Amidase pathway in
the degradation of glutamine in
Neurospora crassa.
J
Bacteriol
161,
Chalot,
M.,
Brun, A., Khalid, A., Dell,
B.,
Rohr,
R.
&
Botton,
B.
(1990).
Occurrence and distribution
of
aspartate aminotransferases
in spruce and beech ectomycorrhizas.
Can
J
Bot
68, 1756-1762.
Chalot,
M.,
Brun, A., Debaud,
1.
C.
&
Botton,
B.
(1991a).
Ammonium-assimilating enzymes and their regulation in wild and
NADP-glutamate dehydrogenase-deficient strains of the ecto-
mycorrhizal fungus
Hebeloma
c_ylindrosporum.
Pbysiol
Plant
83,
Chalot,
M.,
Stewart, G.
R.,
Brun, A., Martin,
F.
&
Botton,
B.
(1991 b).
Ammonium assimilation by
spruce-Hebeloma
sp ecto-
mycorrhizas.
New
Pbytol119, 541-550.
Dell,
B.,
Botton,
B.,
Martin,
F.
&
Le Tacon,
F.
(1989).
Glutamate
dehydrogenase in ectomycorrhizas of spruce
(Picea excelsa
L.)
and
beech
(Fagus
sylvatica
L.).
New
Pbytol111, 683-692.
Finlay,
R.
D., Ek, H., Odham, G.
&
Saderstrom,
B.
(1988).
Mycelial
uptake, translocation and assimilation of nitrogen from 15N-labelled
ammonium by
Pinus
sylvestris
plants infected with four different
ectomycorrhizal fungi.
New
Pbytol110, 59-66.
Finlay,
R.
D., Ek, H., Odham, G.
&
Saderstrom,
B.
(1989).
Uptake,
translocation and assimilation of nitrogen from "N-labelled
ammonium and nitrate sources by intact ectomycorrhizal systems
of
Fagzts sylvatica
infected with
Paxillzts
involutus.
New
Pbytol
113,
47-55.
Finlay,
R.
D., FrostegHrd,
A.
&
Sonnerfeldt, A.
M.
(1992).
Utilization
of
organic and inorganic nitrogen sources by ecto-
mycorrhizal fungi in pure culture and in symbiosis with
Pinus
contorta
Dougl.
ex
Loud.
New
Pbytol120, 105-115.
France,
R.
C.
&
Reid, C.
P.
P.
(1983).
Interactions of nitrogen and
carbon in the physiology of ectomycorrhizae.
CanJBot
61,964-984.
Genetet,
I.,
Martin,
F.
&
Stewart, G.
R.
(1984).
Nitrogen as-
similation in mycorrhiza. Ammonium assimilation in the N-starved
ectomycorrhizal ascomycete
Cenococcum geophilum.
Plant
Pbysiol76,
395-399.
Holmes, A.
R.,
Collings, A., Farnden, K.
1.
F.
&
Shepherd,
M.
G.
(1989).
Ammonium assimilation by
Candida albicans
and other
yeasts
:
evidence for activity of glutamate synthase.
J
Gen
Microbiol
Kershaw,
J.
L.
&
Stewart, G.
R.
(1992).
Metabolism
of
15N-labelled
ammonium by the ectomycorrhizal fungus
Pisolitbus
tinctorius
(Pers.)
Coker and Couch.
Mycorrhixa
1, 71-77.
Kishinami,
1.
&
Ojima, K. (1980).
Accumulation of y-aminobutyric
acid due
to
adding ammonium or glutamine to cultured rice cells.
Plant
Cell
Pbysiol21, 581-589.
Kouchi, H., Fukai, K.
&
Kihara, A. (1991).
Metabolism of glutamate
and aspartate in bacteroids isolated from soybean root nodules.
J
Gen
Microbioll37, 2901-2910.
Kusnan,
M.
B.,
Berger,
M.
G.
&
Fock, H.
P.
(1987).
The involvement
of
glutamine synthetase/glutamate synthase in ammonium as-
similation by
Aspergillus nidulans.
J
Gen
Microbioll33, 1235-1242.
Kusnan,
M.
B.,
Klug, K.
&
Fock, H.
P.
(1989).
Ammonia assimilation
by
Aspergillus nidulans:
[15N] ammonia study.
J
Gen
Microbiol 135,
Lomnitz, A., Calderon,
1..
Hernandez, G.
&
Mora,
J.
(1987).
Functional analysis of ammonia assimilation enzymes in
Netrrospora
crassa.
J
Gen
Microbioll33, 2333-2340.
Martin,
F.
&
Canet, D. (1986).
Biospnthesis of amino acids during
807-809.
1
22-
1
28.
135, 1423-1430.
729-738.
1648
Glutamate and glutamine metabolism by
P.
involtrttls
['3C]glucose utilization by the ectomycorrhizal ascomycete
Cenococcm geopbilum
monitored by 13C nuclear magnetic resonance.
Martin,
F.,
Msatef,
Y.
&
Botton,
B.
(1983).
Nitrogen assimilation in
mycorrhizas.
I.
Purification and properties of the NADP-specific
glutamate dehydrogenase of the ectomycorrhizal fungus
Cenococcum
granifrme. New
Pbytol93,
41 5-422.
Martin,
F.,
Stewart, G.
R.,
Genetet,
1.
&
Le Tacon,
F.
(1986).
Assimilation of 15NH4f by beech
(Fagus yfvatica
L.)
ecto-
mycorrhizas.
New
Pbytol102,
85-94.
Martin,
F.,
Stewart, G.
R.,
Genetet,
1.
&
Mourot,
B.
(1988).
The
involvement of glutamate dehydrogenase and glutamine synthetase
in ammonium assimilation by the rapidly growing ectomycorrhizal
ascomycete,
Cenococcum geopbilum
Fr.
New
Pbytol110,
541-550.
Miflin,
B.
1.
81
Lea,
P.
1.
(1980).
Ammonia assimilation.
In
The
BiochemistTy
of
Plants
vol.
5,
pp.
169-202.
Edited
by
B.
J.
Miflin.
New
k'ork:
Academic Press.
Muhitch,
M.
1.
(1993).
In
vitra
metabolism
of
L-aspartate by maize
kernels.
P&ocbemistr_y
32,
1125-1 130.
Osaki,
M.,
Koyanagi,
A.
&
Tadano, T.
(1992).
Behavior of carbon
from asparagine, aspartic acid, glutamine, and glutamic acid after
introduction to flag leaf of rice plant during ripening.
Soil Sci Plant
Sarjala, T.
(1993).
Effect of ammonium on glutamine synthetase
Pbysiol
I
7&
24,
209-218.
Nutr
38,
537-543.
activity in ectomycorrhizal fungi, and in mycorrhizal and non-
mycorrhizal Scots pine seedlings.
Tree
Pbysioll2,
93-100.
Snapp,
5.5.
&
Vance,
C.
P.
(1986).
Asparagine biosynthesis in
Alfalfa
(Medicago sativa
L.)
root nodules.
Plant
Pbysiol82,
390-395.
Soberon,
M.,
Olamendi,
J.,
Rodriguez,
L.
&
Gonzales,
A.
(1989).
Role
of glutamine aminotransferase in glutamine catabolism by
Saccbaromyces cerevisiae
under microaerophilic conditions.
J
Gen
Microbioll35,
2693-2697.
Stewart, G.
R.,
Mann,
A.
F.
&
Fentem,
P.
A.
(1980).
Enzymes of
glutamate formation
:
glutamate dehydrogenase, glutamine syn-
thetase and glutamate synthase.
In
Tbe Biocbemistr_y
of
Plants
vol.
5,
pp.
271-327.
Edited by
B.
J.
Miflin. New York: Academic Press.
Ta, T.
C.,
McDowall,
F.
D. H., Faris,
M.
A.
&
Joy, K.
W.
(1988).
Metabolism of nitrogen fixed by root nodules of alfalfa
(Medicago
sativa
L.).
I.
Utilization of [14C,'5N]glutamate and ['4C,15N]gluta-
mine in the synthesis of y-aminobutyric acid.
Biocbem Cell
Biol66,
Vezina,
L.
P.,
Margolis,
H.
A.,
Mcafee,
B.
J.
&
Delaney,
5.
(1989).
Changes in the activity of enzymes involved with primary nitrogen
metabolism due to ectomycorrhizal symbiosis
on
jack pine seed-
lings.
Pbysiol Plant
75,
55-62.
1342-1 348.
Received
6
October
1993;
revised
26
November
1993;
accepted
31
January
1994.
1649
