Received for publication December 26, 1990
Accepted March 21, 1991
Plant Physiol. (1991) 96, 952-956
0032-0889/91/96/0952/05/$01 .00/0
Changes to the Stoichiometry of Glycine Decarboxylase
Subunits during Wheat (Triticum aestivum L.) and Pea
(Pisum sativum L.) Leaf Development'
W. John Rogers, Brian R. Jordan, Stephen Rawsthorne, and Alyson K. Tobin*
Biology Department, School of Biological Sciences, University of Sussex, Falmer, Brighton, Sussex, BN1 9QG,
United Kingdom (W.J.R.); Department of Molecular Biology, Horticultural Research International, Worthing
Road, Littlehampton, West Sussex, BN1 7 6LP, United Kingdom (B.R.J.); Cambridge Laboratory, John Innes
Centre for Plant Science Research, Colney Lane, Norwich, NR4 7UH, United Kingdom (S.R.); Plant
Metabolism Research Unit, Department of Cell and Structural Biology, Williamson Building, University
of Manchester, Manchester, M13 9PL, United Kingdom (A.K. T.)
ABSTRACT
Changes In the levels of the four subunits of the mitochondrial
glycine decarboxylase (EC 2.1.2.10) have been investigated during development in the 8 day old primary leaf of wheat
(Triticum aestivum L.). Proteins were extracted from wheat leaf
sections between the basal meristem and 8.5 centimeters. The
individual glycine decarboxylase subunits were detected by
Western blotting, using subunit-specific polyclonal antibodies,
and quantified by laser densitometry. P, T, and H subunits showed
similar developmental patterns along the leaf. All were below the
level of detection up to 1.5 centimeters from the meristem, but
then increased over the leaf length examined. In contrast, the
increase in the L protein (lipoamide dehydrogenase) was more
gradual, and levels in the youngest regions of the leaf were
maintained at approximately 14% of those at 8.5 centimeters. In
a complementary study, levels of the four subunits in light-grown
leaf tissues were compared to those in etiolated leaves from
wheat and pea (Pisum sativum L.), using the activity of the
mitochondrial marker enzyme fumarase as the basis for comparison. For both wheat and pea, levels of P, T, and H proteins in
etiolated tissues were between 25 and 30% of those in lightgrown tissue. However, in etiolated tissues L protein was present
at levels of 60 to 70% of that in light-grown tissues. The results
indicate that discrete mechanisms may control the synthesis of
L, as compared to P, T, and H proteins.
enzyme
Within the photosynthetic cell, the photorespiratory pathis a major series of reactions linking photosynthetic
activity with metabolic processes in the mitochondria and
peroxisomes (12). As such, the control of the biosynthesis of
enzymes involved in the pathway, during the maturation of
leaf cells, may be expected to represent a response both to the
development of photosynthetic capacity, and to that of the
other processes in which the intermediate enzymes participate. The initial substrate for the pathway is phosphoglycolate,
originating from the oxygenase reaction of the chloroplast
way
' Supported by the Agricultural and Food Research Council (grant
number PG85/500 to A. K. T.) and by The Royal Society (University
Research Fellowship to A. K. T.).
enzyme Rubisco, and the pathway is restricted to photosynthetic tissues (13). Within the pathway, the enzyme GDC2
(EC 2.1.2.10) is responsible for the oxidation of the intermediate compound glycine, with the subsequent release of CO2
and NH3.
The GDC complex comprises four subunit proteins, P (98
kD), L (59 kD), T (45 kD), and H (15.5 kD), and is localized
in the mitochondria (10, 23). Studies which have investigated
the distributions of glycine oxidation capacity (5), of the
activity of GDC, and of the P subunit (9, 15, 18, 22), have
shown that these are concentrated in photosynthetic cells.
However, little is known about the mechanisms which control
either the development of GDC activity in the maturing
photosynthetic cell, or the biosynthesis of the enzyme.
The monocotyledenous leaf provides an excellent system
for the study of the development of photosynthetic tissue. All
cell division occurs in the leaf basal meristem, and sequential
sections of the leaf then provide a linear model of cell maturation from the basal meristem to the tip. Using this system,
it has been shown that in the wheat leaf GDC activity increases
in parallel with other photorespiratory enzymes in the mesophyll cells (21).
The present study has utilised the wheat leaf system to
compare the change in levels of the four subunits of GDC as
the tissue matures. This has been done using polyclonal
antibodies specific to the separate subunits. In addition, as
synthesis of the subunits has been shown to be light-induced
in pea leaves (24), the study also includes a comparison of
subunit levels in light-grown and etiolated leaves of wheat
and pea. The results are discussed with respect to their implications regarding the biosynthesis of the GDC complex, and
also in relation to the developing photorespiratory pathway
and mitochondrial metabolism.
MATERIALS AND METHODS
Growth of Plants and Leaf Sectioning
Wheat (Triticum aestivum L., cv Maris Huntsman) was
grown in ambient CO2 and 02 conditions in a Saxcil Environ2Abbreviations: GDC, glycine decarboxylase; PDH, pyruvate
dehydrogenase.
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952
Copyright © 1991 American Society of Plant Biologists. All rights reserved.
DEVELOPMENTAL CHANGES IN GLYCINE DECARBOXYLASE STOICHIOMETRY
mental Growth Cabinet (RK Saxton Sax-Air Ltd, London,
UK) under a regime of 16 h light, 8 h darkness, and temperatures of 20 and 10C, respectively. Light was supplied at 350
,umol m-2 s- PAR. Primary leaves were harvested at 8 d,
when the growth rate is at a maximum (21); only leaves of
between 9 and 11 cm from the basal meristem to the tip were
used. Leaves were cut into 0.5 cm sections, immediately
frozen in liquid nitrogen, and stored at -60TC. For the comparative study of etiolated and green leaves, the green wheat
leaf tissue was taken from the mature region (8-8.5 cm).
Light-grown pea leaves (Pisum sativum L. cv Birte) were
grown under identical conditions to the wheat leaves; these
were used whole after being harvested at 10 d. Etiolated leaf
tissue was sampled from pea and wheat plants that were
germinated and grown in complete darkness for 8 d at room
temperature. Whole leaves were harvested for both.
Protein Determination
The protein content of samples was determined by the
method of Peterson (17).
Fumarase Assay
Fumarase activity was monitored according to Hill and
Bradshaw (6). Samples were prepared as follows: tissue (0.3 g
fresh weight) was harvested and immediately ground, over
ice, in 3 mL ice-cold buffer (50 mm Na-phosphate [pH 7.5],
3 mM 2-mercaptoethanol, and 2 mm NaEDTA). The homogenate was centrifuged at l0,OOOg for 10 min at 5C, and 0.1
mL aliquots of supernatant were assayed at room temperature
in 2 mL assay buffer.
Quantitation of Mitochondrial Volume per Mesophyll
Cell and Mesophyll Cell Numbers per Leaf Section
Mesophyll cell numbers per 0.5 cm leaf section were taken
from previously published data (20). Mean mitochondrial
volumes per mesophyll cell at 1, 15, and 55 mm from the
basal meristem were taken from a morphometric analysis of
the ultrastructure of developing wheat leaf mesophyll cells
(our manuscript in preparation).
Protein Separation and Western Blotting
Leaf tissue was ground at room temperature with mortar
and pestle in extraction medium (8 M urea, 1% [w/v] SDS
and 5% [v/v] 2-mercaptoethanol), maintaining a constant
ratio of the number of 0.5 cm leaf pieces to volume for each
developmental section. This was normally 40 pieces per 2 mL
extraction medium. The extract was then centrifuged at lOOOg
for 5 min at 5°C; 2.5 volumes of ice-cold acetone were added
to the supernatant, and after standing on ice for 1 h, protein
was precipitated at 1750g for 10 min at 5°C. The final pellet
was resuspended in extraction medium, 0.5 mL for every 40
pieces of tissue used, giving a final protein concentration of
10 to 30 mg protein per mL, depending on the developmental
section. The extract was mixed with an equal volume of
loading buffer (20% [v/v] glycerol, 10% [w/v] SDS, 22% [v/
v], saturated bromophenol blue, 1 mm EDTA, 125 mM TrisHCl [pH 6.8]), and 0.1 volume of 2-mercaptoethanol, and
heated to 80°C for 5 min. before being stored at -1 5°C.
953
Protein separation and transfer were carried out using BioRad Mini-Protean and Mini Trans-Blot Cells and Trans-Blot
Transfer Medium (Bio-Rad Laboratories Ltd., Hemel Hempstead, Herts., UK). Proteins were separated by electrophoresis
on 13%, 1 mm thick SDS-PAGE gels, as described previously
(1 1), normally at 200 V. Proteins were transferred onto nitrocellulose at 100 V for 1 h. After transfer, immunodevelopment
was carried out essentially as described previously (1), with a
blocking solution supplemented with 3% (w/v) BSA and 2%
(w/v) dried milk powder, and intermediate washing stages
supplemented with an additional high-salt wash (PBS-Tween
solution, as described in Blake et al. [ 1], containing 1 M NaCl);
all reactions were carried out at room temperature.
Antisera
Polyclonal antibodies were raised, in rabbits, against individual GDC subunits. The subunits were isolated and purified
from pea leaf mitochondria, and antibodies were used during
Western blotting as crude sera at dilutions of 1/1000 (v/v).
Secondary antibody (goat anti-rabbit IgG-alkaline phosphatase conjugate, Sigma, UK) was also used at a dilution of 1/
1000 (v/v). The bromo-chloro-indolyl phosphate, nitroblue
tetrazolium system was used as alkaline phosphatase
substrate.
Comparative Quantification of Western Blotting
For the comparison of wheat leaf sections, each SDS-PAGE
well was loaded to represent equal numbers of mesophyll
cells, while in order to compare etiolated and green tissues,
each well was loaded to represent an equal amount of the
mitochondrial marker fumarase. Western blots of serial dilutions of extracts from mature wheat, mature pea, and etiolated
tissues were scanned (see below) to determine the range of
linear response between the final absorbance and the original
amount of protein loaded per well. The linear range was from
20 to 250 ,g protein per well for each subunit, and this was
used in all subsequent experiments.
Scanning of Western Blots
Band densities were quantified by scanning with an LKB
Ultroscan XL Enhanced Laser Densitometer, using Pharmacia-LKB 2400 Gel Scan XL software. Areas under the absorbance peaks were measured and expressed on a relative basis,
absorbance for the leaf section 8 to 8.5 cm being defined as
100%. Scan results are expressed as the mean and standard
error of at least four Western blots for each subunit, from
three separate protein extractions.
RESULTS
During Western blotting of total-protein extracts only one
protein band was stained by each of the four antibodies over
normal developing periods (5-10 min). The bands corresponded to the mol wt previously determined for the GDC
subunits of pea (23), suggesting that the antibodies were
specific to the respective subunits (Fig. 1).
Examples of Western blots of sequential leaf sections for
the four subunits are shown juxtaposed in Figure 2. From
densitometric data, it was apparent that no significant differ-
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Copyright © 1991 American Society of Plant Biologists. All rights reserved.
954
. -_
detectable in the manner in which the P, T, and H
subunits increased up the leaf length. The pattern of development also suggests that the increase continues beyond the
most mature region of the leaf studied (Fig. 3). These three
subunits were below the level of detection between the basal
meristem and 1.5 cm. The amount of L protein also increased
throughout development, but in contrast to the other three
subunits it was readily detected in the youngest sections of
the leaf, where it represented 14% of that present in the most
mature region studied (Figs. 2 and 3). By comparison, increases in GDC activity (data from ref. 22, and incorporated
into Fig. 3) were only detectable in leaf regions above 4 cm
from the basal meristem.
A comparison of levels of the four subunits in green and
etiolated leaf tissue of wheat and pea revealed that the P, T,
and H subunits'were maintained at between 25 and 30% of
those in green tissue, for both species. However, the amount
of L protein in etiolated tissue was between 60 and 70% of
that in green tissue (Fig. 4).
When calculations for the total mitochondrial volume per
ence was
Plant Physiol. Vol. 96, 1991
ROGERS ET AL.
mesophyll cell at three stages of development along the wheat
leaf length are combined with the relative amount of each
subunit per cell at the respective stages (Table I), the results
suggest that the increases in all four subunits represent increases in concentration within the mitochondria, and are not
the result of an increase in the total mitochondrial volume as
the leaf matures.
DISCUSSION
The present work has investigated the temporal pattern of
development of the four GDC subunits in wheat. The P, T,
and H subunits, which were undetectable in the youngest
sections of the leaf, show no difference between the patterns
of increase with advancing leaf cell age. L protein shows a
different temporal pattern, being maintained at higher levels
in the youngest sections (Fig. 3).
In contrast to the gradual increases in all four GDC subunits
along the leaf, the rise in GDC activity above 4 cm (Fig. 3) is
relatively rapid. This may suggest that the parabolic relationship which has been demonstrated between activity and subunit concentrations in vitro (2), also applies in vivo.
Although major increases in protein levels occur for the P
subunit before 4 cm (Fig. 3), the density of immunogold
labeling for P protein per mitochondrion (22) suggests that
the concentration of this subunit is invariant between 2 and
4 cm. This suggests that early changes in protein levels for the
four subunits may be primarily the result of increases in
mitochondrial number during this stage of cell development.
Morphometric analysis of wheat leaves (our manuscript in
preparation) has shown that an increase in mitochondrial
number per mesophyll cell does occur between the basal
0:
100 A, v,
T, H & P proteins
0: L protein
. :GDC activity
075
-
C
60 0
C>
2
40
C)14
50
-
C
/~~~~~~~~~~~2
cm From Leaf Basal Meristem
Figure 3. Comparative development of the four GDC subunit proteins, P. L, T. and H. along the primary wheat leaf, from the basal
meristem to 8.5 cm. The graph represents the combined results of
laser densitometer scanning of Westerrl blots for the four GDC
subunits, with bands of the 8 to 8.5 cm section set at 1 00% for each
subunit. The development of GDC activity (measured as 14Cp2 released from [1 -14C]glycine) along the wheat leaf is included (Tobin et
a/. 22). Bands for the sections 0, 0.5, and 1.0 cm were below the
minimum level of detection in the case of proteins P, T, and H. Results
represent the mean ± SE for protein extractions from three separate
wheat harvests.
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Copyright © 1991 American Society of Plant Biologists. All rights reserved.
DEVELOPMENTAL CHANGES IN GLYCINE DECARBOXYLASE STOICHIOMETRY
100
Z.
._4
Respective green tissue
.
contrast, levels of P, T, and H proteins are present at levels
of 25 to 30% in etiolated tissues (Fig. 4). That all four GDC
subunits are maintained at relatively high levels in etiolated
set at 100 %
75.
0
50-
l
c
\",
bq 25-
0
\1
K
H
\i
\\1
'<XI
*\
P
T
'.1
\
\
\I
H
L
X
'.
T
P
\.
L
Wheat
Pea
GDC Subunits
\
-
Etiolated Tissue
Figure 4. Comparative levels of the four GDC subunits in etiolated,
compared with light-grown, leaf tissues from wheat and pea. The
results, obtained by laser densitometer scanning of Westem blots as
described in "Materials and Methods," are based on the loading of
SDS-PAGE wells to represent equal amounts of activity of the mitochondrial marker enzyme fumarase. Results represent the mean ±
SE for protein extractions from three separate wheat harvests. Levels
in light-grown tissues are set at 100%.
meristem and 1.5
cm,
although above this developmental
stage mitochondrial numbers either remain constant or decrease. Further investigation is therefore necessary to ascertain
the precise pattern of increase in subunit concentrations at
the individual organelle level. The morphometric analysis has
also shown that mitochondrial volume fractions per mesophyll cell, between the basal meristem and 6 cm, remain
constant, and it can therefore be inferred that substantial
increases in subunit concentrations per total mitochondrial
volume occur during mesophyll cell development.
The discrete developmental pattern of the L protein, compared to the other three subunits, along the light-grown wheat
leaves indicates a distinct regulatory mechanism for this protein. This is supported by the finding that, for both pea and
wheat, compared to P, H, and T proteins, much higher levels
of L protein (up to 70% of those present in mature lightgrown leaf tissues) are maintained in etiolated leaf tissues. In
tissues is indicative of developmental control of GDC synthesis, in addition to any light-regulated control which may exist.
However, the maintenance of relatively high subunit levels
does not necessarily represent physiologically significant GDC
activity in vivo. As mentioned above, the parabolic relationship between GDC activity and subunit levels suggested by in
vitro studies (2) may indicate that, below certain threshold
subunit concentrations, GDC activity is negligible. It has been
shown, for example, that, in vitro, a decrease in GDC activity
from 60 to 70% of the maximum, to between 5 and 10%,
occurs during a reduction in H subunit concentration of threeto fourfold (23).
The four subunits of GDC have been isolated and characterized in detail from both plant and animal sources (2, 7, 8,
10, 19, 23). Of the four proteins, the L protein, lipoamide
dehydrogenase, is the only one known to be involved in other
enzyme reactions, notably the a-keto acid dehydrogenase
complexes, PDH, and 2-oxoglutarate dehydrogenase. It is not
known whether these complexes, together with GDC, share a
common lipoamide dehydrogenase, or whether several isoenzymes are involved. In support of the former theory, it has
been reported that a monoclonal antibody which inhibits
GDC activity via the L protein equally inhibits PDH from
pea leaves (23). If this is the case, the pattern of development
for lipoamide dehydrogenase would be expected to reflect the
different development of the activities of the various enzyme
systems. For example, as a key enzyme linked to carbon entry
into the TCA cycle (3), PDH activity would be expected to
be at a maximum in tissues having a relatively high metabolic
rate, such as the areas of cell division and expansion in the
lower regions of the monocotyledenous leaf (16), and in
etiolated tissues. Further study is required to determine
whether this is a factor contributing to the developmental
pattern of the L protein detected in the present study. The
issue is complicated by varying reports regarding lipoamide
dehydrogenase from the two species of a-keto acid dehydrogenase complex. This subunit has been shown to be functionally interchangeable and of similar mol wt irrespective of
Table I. Estimated Changes in the Relative Concentrations of GDC Subunits per Mitochondrial Volume
during Mesophyll Cell Development in the Primary Leaf of Wheat
Mean mitochondrial volumes per mesophyll cell have been estimated at three leaf levels from the
basal meristem (1, 15, and 55 mm) by morphometnc methods (our manuscript in preparation). These
volumes are combined with the relative percentage levels of the GDC subunits (Fig. 3), enabling them
to be expressed in terms of mean mitochondrial volume at each level.
Wheat Leaf
Section
cm from
meristem
0-0.5
1.5-2.0
5.5-6.0
955
Mitochondrial
Volume per
Mesophyll Cell
Subunit Percent per Total
Mitochondnal Volume per
Mesophyll Cell
L
P,H,T
3
162.4
230.3
196.8
0.09
0.10
0.30
0
0.08
0.25
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Copyright © 1991 American Society of Plant Biologists. All rights reserved.
ROGERS ET AL.
956
source (4), or consisting ofseveral isoenzymes, distinguishable
by both mol wt and isoelectric focusing (14).
The relative amounts of the four subunits reported here for
etiolated tissue are not in complete agreement with previous
results concerning the individual activities of the subunits
(24). Activity ratios for individual subunits (light-grown compared to dark-grown pea leaf tissue) were reported as 12 for
P, 3.6 for L, 10 for T, and 4.2 for H. Thus, the activities of P
and T subunits were considerably more reduced in etiolated
tissue compared to the approximately fourfold decreases reported here, while both the L and the H subunit activities
were maintained at elevated levels compared to P and T
(rather than the L subunit only). As discussed above with
respect to total GDC activity, such discrepancies may underline the necessary distinction between protein levels and activities. The present data provide evidence that, during development, the GDC L protein is regulated separately from the
P. T, and H subunits, as has been previously reported in
relation to the effects of light (24).
ACKNOWLEDGMENT
We would like to thank Prof. T. J. Flowers for his support of the
research carried out at Sussex University, including the use of space
and equipment within the Plant Physiology Group.
Plant Physiol. Vol. 96, 1991
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Copyright © 1991 American Society of Plant Biologists. All rights reserved.