Plant Cells: Peroxisomes and
Glyoxysomes
Robert Paul Donaldson, The George Washington University, Washington DC, USA
Masoumeh Assadi, The George Washington University, Washington DC, USA
Konstantina Karyotou, The George Washington University, Washington DC, USA
Tulin Olcum, The George Washington University, Washington DC, USA
Tianqing Qiu, The George Washington University, Washington DC, USA
Secondary article
Article Contents
. Basic Structure, Basic Functions
. Photorespiration: Leaf Peroxisomes
. Fixed Nitrogen Conversion into Ureides: Root Nodule
Peroxisomes
. Breakdown of Fatty Acids during Germination:
Glyoxysomes
. Peroxisome Formation: Glyoxysome–Peroxisome
Conversions
. Conclusion
Peroxisomes and glyoxysomes are membrane enclosures, both referred to as microbodies,
which contain oxidative enzymes that participate in photorespiration in leaves, nitrogen
metabolism in root nodules, and fat conversions in seeds. The enzymes found within
microbodies are brought in from the cytosol by information described as peroxisomal
targeting sequences (PTS).
Basic Structure, Basic Functions
A peroxisome or a glyoxysome consists of a specific group
of enzymes or proteins enclosed by a single membrane.
These organelles, which are in the range of 1 mm in
diameter, are visible under the electron microscope and are
sometimes referred to as microbodies. In higher plants, at
least four classes of peroxisomes have been identified:
glyoxysomes, leaf peroxisomes, root nodule peroxisomes
and unspecialized peroxisomes. All classes of peroxisomes
have the following characteristics: (1) they have a single
membrane; (2) they have high equilibrium density of c.
1.25 g cm 2 2 in sucrose gradient centrifugation; and (3)
their matrix (internal content) is finely granular. Although
all classes possess the common characteristics, they have
distinct metabolic roles specified by the developmental
stage and type of cell. Catalase, which is by definition
always found in these organelles, can be stained black such
that the organelles are more obvious in electron micrographs. The proteins within this type of organelle are
visible as a granular matrix somewhat more dense than that
of the cytosol. This type of organelle does not have any
internal membranous structures but in some cases the
matrix includes a striking proteinaceous crystal or dense
aggregate, visible in electron microscopy. Peroxisomes or
glyoxysomes can also be visualized in fluorescence microscopy using antibodies specific to one of their proteins,
such as catalase. The relatively simple structure of the
internal matrix of microbodies distinguishes them from
chloroplasts or mitochondria, which have internal membranes that are folded or stacked. In the photosynthetic
cells of leaves the peroxisomes are often in contact with
chloroplasts and mitochondria; these three organelles
interact with each other in photorespiration. Glyoxysomes
are found in contact with lipid bodies in cotyledons or
endosperm where fatty acids are being converted to
carbohydrate (sugars) during germination. Images of
whole plant cells indicate that there may be a few hundred
microbodies in a cell. In some instances the microbodies
may be tubular or interconnected and appear to be
dividing.
All four known classes of microbodies found in plant
cells are organelles that, by definition, contain activities
that produce and destroy hydrogen peroxide (H2O2),
which is a toxic agent.
Glyoxysomes are specialized peroxisomes that are
present in postgerminative seedlings of oil seeds and
senescent organs. Glyoxysomes are involved in storage
lipid mobilization in growing seedlings via the glyoxylate
cycle. Succinate produced in glyoxysomes is ultimately
converted to sucrose in the cytosol. It is presumed that
presence of glyoxysomes in senescent organs is in response
to the mobilization of membrane lipids.
Leaf peroxisomes are present in green and photosynthetically active tissues, such as green cotyledons and leaves.
These peroxisomes contain enzymes that are required for
the light-dependent process of photorespiration.
Root nodule peroxisomes are present in the root nodules
of certain legumes and involved in nitrogen metabolism. In
many tropical legumes, nitrogen is transported in the form
of ureides, allantoin and allantoic acid. Reactions of ureide
biosynthesis take place in several subcellular compartments. One of the last steps of this pathway, the conversion
of urate to allantoin, is catalysed by urate oxidase in
peroxisomes.
Unspecialized peroxisomes are present in plant tissues
that are not photosynthetically active and that lack storage
lipids, such as the roots of most plants. Unspecialized
peroxisomes can be distinguished from other forms of
peroxisomes by their small size, low frequency and density
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net
1
Plant Cells: Peroxisomes and Glyoxysomes
compared to glyoxysomes and leaf-type peroxisomes.
Their specific role in the cellular metabolism is not known.
The metabolic processes that take place in peroxisomes
often bypass energy conservation steps. The prototypical
enzyme of these organelles is an oxidase that generates
H2O2, such as the glycolate oxidase (GO) of leaf
peroxisomes or the fatty acyl–CoA oxidase (AO) of
glyoxysomes. These oxidases contain flavin (as FAD),
which accepts two hydrogens from a substrate (e.g.
glycolate or acyl–CoA) and transfers them to oxygen,
resulting in H2O2. In the mitochondria this process would
be coupled to energy conservation, where the hydrogens
recovered from the substrate serve as a source of electrons
to power mitochondrial ATP generation. The advantage of
directly transferring the hydrogens to oxygen in peroxisomes is that the metabolic processes can take place even
when the cell is not consuming ATP and when additional
ATP does not need to be generated.
Photorespiration: Leaf Peroxisomes
Photorespiration and photosynthesis are opposing processes that occur in the cells of leaves or other green tissues.
Both processes are initiated in chloroplasts, but photorespiration involves a diversion into leaf peroxisomes and
mitochondria. As a cell of a young leaf expands, its vacuole
fills with water and spreads the cytoplasm around the
periphery of the cell. In the cytoplasm the chloroplasts,
peroxisomes and mitochondria become loaded with the
enzymes needed to absorb light and carbon dioxide to
create new organic molecules for the rest of the plant. The
carbon dioxide is taken in by the chloroplast enzyme,
ribulose-1,5-bisphosphate carboxylase (Rubisco), the
most abundant enzyme in the cell, and is normally
assimilated into the three-carbon molecule phosphoglyceric acid (PGA), which is used to synthesize sugars and other
organic molecules.
Photorespiration commences when oxygen replaces
carbon dioxide in Rubisco. This results in the two-carbon
molecule, phosphoglycolate, instead of PGA. The phosphoglycolate can be recycled back into PGA by a
circuitous process through leaf peroxisomes, mitochondria
and chloroplasts with the use of oxygen and the loss of one
carbon in four as carbon dioxide, hence the designation
‘photorespiration’. The glycolate, relieved of its phosphate, is passed from a chloroplast to a peroxisome. Here it
is subject to a typical peroxisomal enzyme, glycolate
oxidase, which transfers two hydrogens to oxygen,
resulting in hydrogen peroxide, which is broken down by
catalase. The result is glyoxylate, which accepts an amino
group (NH2) to become the amino acid glycine. This is
transported into the mitochondria. In a photosynthetic cell
the proteins of photorespiration are the most abundant in
the mitochondria. Here a complex of four proteins
2
combines two molecules of glycine to create a molecule
of serine with the release of a carbon dioxide molecule and
an amino group. The serine returns to a peroxisome where
additional enzymes complete its conversion to glycerate by
transferring its amino group to glyoxylate, followed by
reduction by hydroxypyruvate reductase using NADH2.
The glycerate re-enters a chloroplast where it is phosphorylated, finally yielding PGA.
These processes require the transport of metabolites
through the membranes of the various organelles including
the peroxisomes. There may be selective channel proteins
in the membranes that regulate the transport of metabolites. A porin protein discovered in the membranes of
peroxisomes may represent such a channel (Reumann et al.,
1998).
The process of photorespiration is very significant in
plants and becomes especially important when leaf
stomata close to reduce water loss. Then the supply of
carbon dioxide within the leaf diminishes as it is assimilated
and at the same time the concentration of oxygen increases,
favouring photorespiration. The carbon dioxide released
by photorespiration can be reassimilated by Rubisco, thus
allowing use of the light energy absorbed by the
chloroplast. Although photorespiration is counterproductive to photosynthesis, it may be necessary to protect the
leaf cells from damage due to light absorption.
Fixed Nitrogen Conversion into
Ureides: Root Nodule Peroxisomes
Root nodule peroxisomes of certain tropical legumes
synthesize allantoin, which serves as the major metabolite
for nitrogen transport in these plants. The ureides,
allantoin and allantoic acid, are the predominant form of
nitrogen transported in the xylem of soya bean and cowpea
plants growing symbiotically. The synthesis of allantoic
acid presumably derives from the degradation of purines.
Urate oxidase (UO), one enzyme in the purine degradation
pathway, is normally found in peroxisomes, along with
catalase, which consumes the hydrogen peroxide produced
by UO. Uric acid is oxidized by UO to allantoin within
peroxisomes. Small amounts of UO are present in
glyoxysomes of germinating oil seeds and of potato tubers,
while traces of UO are also present in peroxisomes from
other plant tissues. In all cases UO is easily solubilized and
is not part of the crystalline core of the peroxisome.
Allantoinase and allantoicase, enzymes participating in the
biogenesis of allantoin and allantoic acid, have been
reported to be present in peroxisomes from amphibian
and fish livers. Approximately one-half of the allantoinase
activity in castor bean endosperm is associated with
glyoxysomes; the remainder is in the proplastids (Hanks
et al., 1981).
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net
Plant Cells: Peroxisomes and Glyoxysomes
Breakdown of Fatty Acids during
Germination: Glyoxysomes
Metabolically, plant and animal cells differ in many
important respects. In particular, plant cells, along with
some microorganisms, can carry out the net synthesis of
carbohydrate from fat. This conversion is crucial in the
development of seeds, in which a great amount of energy is
stored in the form of triacylglycerols. As such seeds
germinate, triacylglycerol stored in lipid bodies is broken
down, transported to glyoxysomes and eventually converted to sugars, which provide energy and the raw
material needed for growth of the plant. In contrast,
animal cells cannot carry out the net synthesis of
carbohydrate from fat.
In plants, catabolism of triacylglycerols in the lipid
bodies yields fatty acids and glycerol. Fatty acids undergo
b-oxidation to yield acetyl–CoA, which can then be
incorporated into carbohydrate through the glyoxylate
cycle. The conversion of triacylglycerols into sugars
involves metabolism in glyoxysomes. While leaf peroxisomes have a key role in photorespiration, glyoxysomes are
the sites of b-oxidation of fatty acids and the glyoxylate
cycle (Cooper and Beevers, 1969). These pathways are
essential to the maintenance of gluconeogenesis initiated
by the degradation of reserve or structural lipids. bOxidation of fatty acids occurs in most plant microbodies,
but has a more important function in those organelles from
fat-storing tissues of oilseeds. In these organelles, fatty
acids are degraded via the b-oxidation pathway to acetyl–
CoA, which in turn is metabolized by the glyoxylate cycle
to succinate, bypassing the decarboxylating steps of the
Krebs cycle (Tolbert, 1981). Succinate is then used for
gluconeogenesis or synthesis of other metabolic intermediates.
The glyoxysomal b-oxidation of fatty acids is a recurring
sequence of four reactions shown in Figure 1: oxidation
(dehydrogenation) by acyl–CoA oxidase (AO), hydration
by enoyl–CoA hydratase combined with a second oxidation by 3-hydroxy acyl–CoA dehydrogenase, both catalysed within a bifunctional protein (BP), and finally
thiolysis by 3-ketoacyl–CoA thiolase (TH). The FADlinked acyl–CoA oxidase transfers electrons not to the
respiratory electron transport chain but directly to oxygen,
without recovery of chemical energy (ATP). The oxygen is
reduced to hydrogen peroxide, which in turn is scavenged
by catalase (CAT). The dehydrogenase produces NADH2
and the thiolase uses CoA to remove the last two carbons of
the 3-ketoacyl–CoA to yield acetyl–CoA.
As stored lipids are metabolized during seed germination, the acetyl–CoA produced by b-oxidation in glyoxysomes is transferred to the glyoxylate cycle, which can be
considered as an anabolic variant of the citric acid cycle.
The glyoxylate cycle converts two molecules of acetyl–
CoA into one molecule of succinate, as shown in Figure 1.
This involves two glyoxylate cycle-specific enzymes,
namely isocitrate lyase (IL) and malate synthase (MS),
and three enzyme activities similar to those from the citric
acid cycle, namely citrate synthase (CS), aconitase (AC)
and malate dehydrogenase (MD). Since the glyoxylate
pathway bypasses the two reactions of the Krebs cycle
where carbon is lost, each turn of the cycle involves
incorporation of two two-carbon molecules and results in
the net synthesis of the four-carbon molecule, succinate.
This is transported from the glyoxysome to the mitochondria where it is converted through the Krebs cycle to
oxalacetate, which is readily utilized by gluconeogenesis
for carbohydrate synthesis.
The reduced cofactors that are produced during both boxidation and glyoxylate cycle, namely NADH2 and
FADH2, do not have direct access to the mitochondrial
electron transport system. They must therefore be reoxidized in order for both pathways to remain functional.
The acyl–CoA oxidase of glyoxysomal b-oxidation avoids
that by transferring electrons from the FADH2 directly to
oxygen, resulting in hydrogen peroxide. Hydrogen peroxide is produced in abundance within glyoxysomes
during this process or from the disproportionation of
superoxide radicals by superoxide dismutase. Superoxide
radicals can be produced by the transfer of electrons from
NADH2 to oxygen via a protein in the membrane (Del Rio
et al., 1992). The hydrogen peroxide is decomposed either
by catalase (CAT) inside the glyoxysome or by an
ascorbate-specific peroxidase (AP) present at the glyoxysomal membrane. NADH2 produced by the 3-hydroxy
acyl–CoA dehydrogenase and by malate dehydrogenase in
the glyoxylate cycle also accumulates within glyoxysomes,
and is oxidized by the electron transport proteins in the
glyoxysomal membrane. These proteins include ascorbate
peroxidase (AP), ascorbate free radical reductase (AR)
and, possibly, cytochrome b5 and glutathione reductase.
Ascorbate peroxidase utilizes hydrogen peroxide to
catalyse a one-electron oxidation of ascorbate, resulting
in the formation of ascorbate free radicals. Regeneration of
ascorbate is achieved by ascorbate free radical reductase
(AR), using NADH2 as an electron donor. Overall,
glyoxysomal metabolism results in the production of a
variety of reactive species, such as O22 ., H2O2 and
ascorbate free radicals. At the same time the glyoxysomes
include the appropriate detoxifying enzymes, such as
catalase and the enzymes located in the membrane AP
and AR, which can protect against cell damage (Bunkelmann and Trelease, 1996; Ishikawa et al., 1998).
Peroxisome Formation: Glyoxysome–
Peroxisome Conversions
In the oil-storing cotyledons of seeds such as cotton,
cucumber or legumes, a population of glyoxysomes is
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net
3
Plant Cells: Peroxisomes and Glyoxysomes
Lipid body
Asc·
Asc
Triacyl glycerols
AP
Glycerol
Fatty acids
CAT
H 2O
AR
As
c
O2
Acyl–CoA
Fatty acid
H 2O 2
FAD
AO
H 2O
Enoyl–CoA
β-Oxidation
Glyoxysome
Succinate
BP
Asc
3-OH-acyl–CoA
NAD+
BP
AR
NADH2
3-ketoacyl–CoA
CoA-SH
TH
Asc·
Acetyl–CoA
Isocitrate
IL
Glyoxylate
MS
AC
Glyoxylate cycle
Malate
Citrate
Succinate
MD
CS
NAD+
Oxaloacetate
Acetyl–CoA
Mitochondrion
NADH2
AR
Cytosol
Figure 1 b-Oxidation and glyoxylate cycle enzymes in glyoxysomes. The b-oxidation enzymes are acyl–CoA oxidase (AO), enoyl–CoA hydratase
combined with 3-hydroxy acyl–CoA dehydrogenase in the bifunctional protein (BP) and 3-ketoacyl–CoA thiolase (TH). Glyoxylate-cycle enzymes are
isocitrate lyase (IL) and malate synthase (MS), citrate synthase (CS), aconitase (AC) and malate dehydrogenase (MD). Membrane enzymes include
ascorbate peroxidase (AP) and ascorbate free radical reductase (AR). Both catalase (CAT) and AP consume hydrogen peroxide. Asc, ascorbate; Asc.;
ascorbate free radical.
converted into a population of leaf peroxisomes following
exposure to light, resulting in greening of the tissue. The
functions of the microbodies are thus converted from lipid
metabolism to photorespiratory metabolism. Two ideas,
the one-population and the two-population hypotheses,
have been proposed for the interconversions and origins of
specialized peroxisomes. According to the first hypothesis,
leaf peroxisomes are formed from existing glyoxysomes by
insertion of newly synthesized leaf peroxisome-specific
enzymes and depletion of glyoxysomal-specific enzymes.
In contrast, the second hypothesis suggests de novo
formation of glyoxysomes and leaf peroxisomes. According to the one-population hypothesis, glyoxysomal-specific
enzymes and leaf peroxisome-specific enzymes are present
in single microbody species throughout seedling growth,
even after illumination. The second hypothesis suggests
that glyoxysomes and peroxisomes contain completely
different enzymes. However, several lines of evidence
support the first hypothesis. For example, microbodies
with both glyoxysomal-specific and leaf peroxisomalspecific enzymes have been identified during the transitional stage, using immunocytochemical analysis. This
indicates that glyoxysomes are directly transformed to leaf
4
peroxisomes during greening. Additional support for this
idea comes from the same kind of finding during senescence
of cotyledons or leaves, a stage in which leaf peroxisomes
are converted to glyoxysomes and the stores of carbon and
nitrogen are transferred to newly developing tissues.
Immunocytochemical analysis revealed that enzymes
specific to glyoxysomes and to leaf peroxisomes are both
present in microbodies of senescing cotyledons (Titus and
Becker, 1985).
Although the morphological appearance of microbodies
in cotyledons is the same during transitions from glyoxysomes to peroxisomes, their enzymatic contents are
changed drastically. As discussed above, each specialized
microbody contains different enzymes. Activities of
glyoxysomal-specific enzymes, such as malate synthase
and citrate synthase, increase with germination and
decrease gradually after lipid stores are depleted. At this
stage, activities of leaf peroxisome-specific enzymes are at
the lowest level. Rapid increase in activities of leaf
peroxisome-specific enzymes and decrease in activities of
glyoxysomal-specific enzymes occurs when seedlings are
transferred to the light. The transition of glyoxysomes to
leaf peroxisomes and the accumulations of new proteins in
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net
Plant Cells: Peroxisomes and Glyoxysomes
microbodies can be regulated in several ways: through gene
expression, protein transport, mRNA splicing and protein
degradation. These are discussed below.
Regulation of gene expression
The mRNA levels for glyoxysomal-specific enzymes –
malate synthase and citrate synthase – increase rapidly
during germination in the dark and decline markedly after
exposure of the cotyledons to the light. Expression of the
malate synthase gene is inhibited by sucrose, the end
product of the metabolism of stored lipid. On the other
hand mRNA levels of leaf peroxisome enzymes such as
glycolate oxidase are low during germination in the dark
and increase rapidly during greening. Regulation of leaf
peroxisome enzymes is light dependent and it has been
reported that phytochrome plays a role in signal transduction. Accordingly, it is suggested that light and levels of
metabolites are regulatory factors for microbody transition events.
Regulation of protein transport into
microbodies
All microbody enzymes are synthesized in the cytoplasm
and transported into microbodies posttranslationally. The
enzymes include certain sequences of amino acids that
serve as targeting information. Only two major types of
microbody targeting signals are known in plants as well as
in mammals, insects, fungi and protists. The peroxisomal
targeting signal-1, PTS-1, is a C-terminal tripeptide such as
Ser-Lys-Leu (SKL) that is present in mature proteins in
microbodies. Conservative variations in the amino acids of
the PTS-1 sequence can be tolerated without loss of
targeting activity. For example, the targeting sequences are
SRL for malate synthase from castor bean, PRL for
glycolate oxidase, and ARM or SRM for isocitrate lyases.
However, the removal of the ARM from castor bean
isocitrate lyase does not stop import of the protein,
suggesting that there is additional targeting information
in the protein (Gao et al., 1996). Experimental alterations
of the PTS-1 suggest that other amino acids may be
functional in the tripeptide and that additional amino acids
nearby may also contribute to recognition (Mullen et al.,
1997a; Wolins and Donaldson, 1997). The C-terminal
sequence of cotton seed glyoxysomal catalase is -NVKPSI
and the experimental evidence indicates that some of the
amino acids in addition to the PSI are necessary for import
(Mullen et al., 1997b). Many other proteins from a variety
of plant species (see Table 1) fit the rather relaxed PTS-1
consensus, but in the absence of experimental evidence it
cannot be assumed that each of these sequences functions
as a PTS-1. Comparisons of the amino acid sequences from
several enzymes indicate that each enzyme has a particular
version of the PTS-1 that is found in several species. A
cytosolic PTS-1 receptor that interacts with a peroxisomal
membrane docking protein has been described for human
and yeast peroxisomes. Some in vitro studies indicate that a
similar receptor exists in plants (Wolins and Donaldson,
1994; Brickner et al., 1997; Kragler et al., 1998).
Most of the microbody proteins have the C-terminal
PTS-1, but a few have a second type of targeting signal near
their N-terminal, PTS-2. This consists of a sequence such
as RLXXXXXHL, where X can be any amino acid (Gietl
et al., 1994). These proteins include glyoxysomal 3ketoacyl–CoA thiolase, malate dehydrogenase and citrate
synthase, which are synthesized with larger molecular mass
in the cytosol. Their N-terminal PTS-2 peptides are then
cleaved upon the targeting of the enzymes into microbodies. Experiments showed that a fusion protein composed of the N-terminal region of glyoxysomal citrate
synthase was transported to glyoxysomes, leaf peroxisomes and unspecialized microbodies and was subsequently processed. This suggests that microbodies use the
same transport mechanism and that differentiation of
microbodies is not regulated at the level of recognition of
the targeting information.
It has been observed that proteins that have had their
targeting information removed experimentally are imported into glyoxysomes if accompanied by proteins that
do have the targeting information (Lee et al., 1997). The
implication is that the protein lacking a PTS can ‘piggyback’ or associate with the protein having the PTS, and
that the two proteins can enter together. How such an
assemblage would traverse the membrane of the glyoxysome is not understood.
Regulation of mRNA splicing
Hydroxypyruvate reductase (HPR) is one of the leaf
peroxisome-specific enzymes that is induced and accumulates in microbodies during greening. cDNA analysis of
pumpkin cotyledons showed that two very similar cDNAs
encode for this enzyme. The only difference between the
two encoded proteins is that HPR-1 contains PTS-1 and
HPR-2 does not. Genomic DNA analysis suggested that
the HPR-1 and HPR-2 are encoded from the same gene by
alternative splicing. Accumulation during greening of
HPR-1 and HPR-2, in leaf peroxisomes and the cytosol,
respectively, suggests that alternative mRNA splicing may
play a regulatory role in microbody transition (Hayashi
et al., 1996).
Regulation at the level of protein degradation
During transition of glyoxysomes to leaf peroxisomes, de
novo synthesis of glyoxysomal-specific enzymes is prevented by depletion and degradation of mRNA for these
enzymes. Furthermore, preexisting glyoxysomal-specific
enzymes are degraded by proteolytic enzymes present in
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5
Plant Cells: Peroxisomes and Glyoxysomes
Table 1 Peroxisomal targeting sequences (PTS-1 and PTS-2)
Protein
Species
PTS-1
C-terminal amino acid sequence
Malate synthase
Cucumbera
Soya beana
Rapea
Castor beana
FLT
FLT
FLT
FLT
LDA
LDA
LDA
LAV
YNY
YNY
YNN
YDH
IVI
IVV
IVI
IVA
HHP
HHP
HYP
HYP
REL
RET
KGS
INA
SKL-C'
SKL
SRL
SRL
Glycolate oxidase
Spinach
Arabidopsisa
Cucumber
Rice
ISR
SEI
QEI
DIT
SHI
TRN
TRN
RAH
AAD
HIV
HIV
IYT
WDG
TEW
ADW
DAE
PSS
DIP
DTP
RLA
RAV
RHL
RVV
RPF
ARL
PRL
PRL
PRL
Isocitrate lyase
Tomato
Soya beana
Castor beana
Cucurbita
WTR
WTR
TRP
TRA
TGA
SGA
GAM
GAG
TNL
VNI
EMG
NLG
GDG
DRG
SAG
EEG
SVV
SIV
SEV
SVV
IAK
VAK
VAK
VAK
ARM
ARM
ARM
SRM
Hydroxypyruvate reductase
Arabidopsis
Pumpkin
PPN
PPA
ASP
ASP
SIV
SIV
NSK
NAK
ALG
ALE
LPV SKL
LPV SKL
Catalase
Tomato
Arabidopsis
Cottona
Cucurbit
SYL
SYW
SYW
SYW
SQA DKS CGQ KVA SRL TVK
LKA DRS LGQ KLA SRL NVR
SQA DKS LGQ KIA SRL NVR
SQA DRS LGQ KIA SRL NVR
PTM
PSI
PSI
PNI
PTS-2
N'- terminal amino acid sequence
Acyl–CoA oxidase
Pumpkin
Phalaenopsis
Arabidopsis
Malate dehydrogenase
Watermelona
Soya bean
Rape
Cucumber
Citrate synthase
Winter squash
Arabidopsis
N'-ASPGEPNRTAEDESQAAAR RIERLSLHL
MTKEAQMTSLASEHDTQQALR RIQKLSLHL
MESRREKNPMTEEESDGLIAAR RIQRLSLHL
TPI
LQP
SPS
MQPIPDVNQ
MEANSGASD
MPHK
MQPIPDVNQ
RIARISAHL
RISRIAGHL
RIAMISAHL
RIARISAHL
HPP
RPQ
QPS
HPP
MPTDMELSPSNVARH
MVFFRSVSAFTRLS
RLAVLAAHL
RVQGQQSSL
SAA
SNS
a
Indicates there is experimental evidence for the targeting function of this sequence. The bold types indicates the targeting sequence.
the matrix of glyoxysomes during the transitional stage. A
variety of proteases have been discovered in leaf peroxisomes but nothing is known about how these might
contribute to the selective degradation of enzymes in
microbodies (Distefano et al., 1997).
Conclusion
Since the 1980s considerable progress has been made
toward understanding the processes that take place in the
6
different types of microbodies in plant cells and how the
proteins that conduct these processes are directed from the
cytosol into peroxisomes and glyoxysomes. Yet little is
known about how cells maintain and propagate microbodies, how the proteins and lipid molecules of the
membrane are assembled, and how proteins pass through
the membrane. Nothing is known about how light and
levels of metabolites regulate the expression and delivery of
proteins to microbodies. Furthermore, there is little
knowledge of how metabolites such as fatty acids and
carboxylic acids or cofactors such as haem, CoA or/and
NAD enter or leave the organelle. Thus, there is much to be
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net
Plant Cells: Peroxisomes and Glyoxysomes
learned about how microbodies interact with other
subcellular molecules and processes.
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Further Reading
Nishimura M, Hayashi M, Kato A, Yamaguchi K and Mano S (1996)
Functional transformation of microbodies in higher plant cells. Cell
Structure and Function 21(5): 387–393.
Olsen LJ and Harada JJ (1995) Peroxisomes and their assembly in higher
plants. Annual Review of Plant Physiology and Plant Molecular Biology
46: 123–146.
Tolbert NE and Essner E (1981) Peroxisomes and glyoxysomes. Journal
of Cell Biology 91: 271s–283s.
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net
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