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The cover illustrates somatic embryogenesis in pea. Photograph taken by M. Griga, pp. 238
Library of Congress Cataloging in Publication Data
Advances in Regulation of Plant Growth and Development
Pavel Pec, Erwin Beck. -Prague:
Peres, 1999,258 s.
ISBN 80-86360-06- 7
581.1 * 581.143 * 581.14
.plant
.plant
.plant
2
physiology
growth
development
/ Edited by Miroslav
Strnad,
Advances in Regulation of Plant Growth and D~~m~-t{1999119~212
Programmed
L.ADISLA V HA VEL 1 &
cell death
in plant
development
DON JOHN DURZAN: 2
IDepartment of Botany and Plant Physiology, Mendel University of Agriculture and Forestry, Zemedelskli I, 61300 Brno, Czech Republic
2Department of Environmental Horticulture,
University of California, One Shields Ave., Davis, CA 95616-8587, USA
Abstract
Plant development involves the elimination of cell organelles, protoplasts, tissues and organs. The concept of programmed cell death elaborated in medical and animal sciences has become suitable for explanation of these eliminations which must be highly co-ordinated to maintain plant integrity .Characteristic features of apoptosis, a form of
programmed cell death, were found e.g. in leaf senescence,abscission of flower parts, reproduction processes, tracheary element formation, and responses to various biotic and abiotic stresses.The role of phytohormones in programmed cell death is becoming evident. Apoptosis in meristems influences longevity and overall development of
plants.
Introduction
Plants eliminate cells, organs, and parts during responses to stress and expressions of various developmental programs. Leaves that are not lighted enough are
shed. Most of broad leaf trees in zones with adverse
winter conditions shed leaves in autumn. The unpollinated flowers are fully thrown away. Ovaries with fertilised egg cells in ovules on the same plant are retained
forming fruits while the other parts of flowers, e.g. petals and sepals or tepals falloff. Stigmas and pistils may
also be eliminated. In apomictic species, the fruits develop without fertilisation which means that the ovaries
with ovules are retained forming fruit, while the other
flower parts are eliminated. In parthenocarpic species
that bear fruits without seeds, only walls of ovaries continue development. All other flower parts are removed.
This elimination must be highly controlled by internal
factors or, in some cases, in combination with external
stimuli that involve an array of cellular and subcellular
activities.
A typical plant cell consists of the cell wall and the
protoplast. MAUSETH (1995) emphasises that metabolism also occurs in cell walls and they should be considered dynamic, active parts of plant cells. Generally, the
protoplast is more metabolically active in comparison
with the cell wall. The plant is able to abolish not only
whole organs or their parts, the elimination can occur
also on the cellular and even subcellular level. Whole
cells, protoplasts and cell walls are eliminated too.
These events are known in spores that do not participate
in the megagametophyte development of gymnosperms
and angiosperms. Another example is the elimination of
suspensor cells during zygotic embryogenesis (NAGL
1976). Dead cells can also be filled with storage mate-
rial as in most endosperm cells in the caryopsis of
grasses(BEWLEY& BLACK 1984).
Plants live very economically. When the cell wall itself is able to accomplish a specific function, the protoplast is eliminated. Sclerenchyma cells are dead because
thick cell walls perform the mechanical function.
Phellem, commonly known as cork, is constituted of
characteristic cells with a thick suberinised layer of the
cell wall. Suberin, combined with lack of intercellular
spaces,protects internal tissues against desiccation. The
protoplast is no longer needed. In xylem, tracheary elements also lack protoplasts. Water and nutrients flow
through spaces, where protoplasts were originally
placed, surrounded by modified, lignin impregnated cell
walls. Cells performing these functions are dead.
Not only whole protoplasts but also their parts can
be eliminated -even such an important organelle as the
nucleus. This process, which has been described in sieve
tube elements of phloem, is completed through dechromatisation in secondary phloem or through pycnosis in
primary phloem (c.f. BUVAT 1989). The nuclear genome
is sometimes fragmented into unequal parts by amitosis.
The resultant micronuclei mayor may not persist (SINGH
1993; KIPLING 1995). The plastids can be eliminated as
well. The formation of neocytoplasm after fertilisation
in conifers, where maternal chloroplasts are deleted, and
exclusion of one type chloroplast after somatic hybridisation can serve as examples (c.f. CAMEFORT 1969;
MANTELet al. 1985).
Many variations in plant cell elimination are evident. It is certain that these processesmust be controlled
ontogenetically and spatially. In other words, the elimination or death of cells must be programmed. Variations
203
L. HAVEL & D. J. DURZAN
in programmed cell death, well known from animal and
medical sciences,represent comparable events.
Programmed
ences
cell death
in animal
and medical
sci-
The death of single cells as integral parts of coordinated living processes has been neglected for very
long time. In early seventies KERR et at. (1972) described apoptosis as a distinct fonn of cell deletion or
programmed cell death that plays a major role during
development, homeostasis, and even in the expressions
of many diseases.Concepts of cell death became one of
the fast developing areas of animal and medical sciences
especially in pathology (KORSMEYER1995). No wonder
that most of the infonnation has been acquired here.
Different genes and their products that control cell
death by signalling and executing pathways were characterised or presumed. Multisignalling events have been
implicated in the regulation of apoptosis (e.g. HANNUN
1996; BAYLY et at. 1997). The proposed molecular
network of mammalian apoptosis pathways and their use
in metabolic engineering is very complex at present ( e.g.
FUSSENEGGER
& BAILEY 1998).
Several cell death suppressor genes have been identified, e.g. Bcl-2 gene family or DADJ, products of
which are capable of protecting the cells from programmed cell death ( c.f. V AUX & STRASSER1996;
NAKSHlMAet al. 1993).
Parts of cell death programs have been conserved
among wonns, insects, and vertebrates and in all cell
types (STELLER1995).
The link between divisional cycles and programmed
cell death was also suggested ( c.f. HA VEL & DURZAN
1996a; FUSSENEGGER
& BAILEY 1998). New studies and
reinterpretation of the old data from the experimental
work with the oncogenes provided substantial evidence
that cell renewal and cell death are linked even if they
appear to be opposing and mutually contradictory
(EVAN& LITTLEWOOD1998).
cells. In plants, apoptosis is also characterised as a phenotypically distinct form of controlled cell deletion
(RAVEL & DURZAN 1996a). GILCHRIST(1998) characterised apoptosis as genetically regulated, signal transduction-dependent programmed cell death. Logically,
other forms of programmed cell death exist in plants.
Nevertheless, many authors do not distinguish between
programmed cell death and apoptosis using both terms
as synonym. Keeping in mind that apoptosis represents
the predominant form of cell death (SCHULZE-OSTHOFF
et al. 1994), false understanding will be rare. The term
"cell or cellular suicide" is also often used instead, or
together, with programmed cell death ( e.g. STELLER
1995). Also terms "physiological cell death" or "developmentally regulated cell death" in the meaning of programmed cell death are used (e.g. KROEMERet al. 1998;
RAMMOND-KOSACKet al. 1994). Yet another used term
is "apoptotic cell death" (e.g. BRUNE et al. 1998). Taking in mind that apoptosis is a kind of programmed cell
death this term seemsto be redundant.
Another term -"necrosis" -has been also used for
the description of cell or tissue death. At present necrosis is often considered as an opposite to apoptosis. It is
unprogrammed and involves the decay of injured groups
of dying cells. The "term accidental death" is used in
this sense. Several features are used to distinguish
apoptosis from necrosis (Tab. I).
MAJNO & JORIS(1995) recommended preservation
of the original meaning for "necrosis" which has been
used in life sciences for very long time. Originally, necrosis meant drastic tissue changes visible by naked eye
and therefore occurring well after the cell death. They
offer the term "oncosis" for unprogrammed cell death as
an opposite to apoptosis (c.f. MAJNo& JORIS1995).
"Terminal differentiation" is used for cell elimination where differentiation leads to cell death (c.f. RAVEL
& DURZAN 1996a). In plants almost every protoplast is
eliminated earlier or later after cell differentiation while
the cell wall can persist. The cells that dedifferentiate
and reestablish divisional cycles are an exception.
Morphological
Terminology
Historically, the fact that cells can perish was discussed
in the Lecture XV. of Virchow's Cellular Pathology
among "passive processes and degenerations" in the
middle of the last century (MAJNo & JORIS1995). It is
clear that cell death leads to the point of no return but
this point can be achieved in many different ways. This
is one of the reasons why, till now, a lot of different
terms were used in concepts for a single pathway to cell
elimination. Moreover, the same terms have been used
for different processesor features or vice versa.
The pioneers, KERR et al. (1972), defined a distinct
form of programmed cell death as apoptosis and described characteristic markers of this process in animal
204
markers of apoptosis
The morphological markers are mainly based on studies
of animal and human cells observed in viva and in vitra.
At present, however, more of the new information is
being found in plant cells.
In animals, dying cells shrink and separate from
their neighbours. The cytoplasm contracts and dilatations with some vesiculation of the endoplasmatic reticulum can occur, The nuclear changes are the most
studied morphological marker. The chromatin condenses
into dense compact masses that may coalescence into a
crescent inner cap lining the nuclear membrane. The
nucleolus fragments. Invaginations of the nuclear membrane may further divide the nucleus (HA YLYet al.
1997). The degradation of the nuclear lamina has been
described (UCKER et al. 1992; LAZEBNIK et al. 1993)
Programmed
cell death in plant development
Tab. I.: Comparison of morphological, biochemical and molecular features and physiological significance of apoptosis and necrosis.
Necrosis
Apoptosis
-Membrane
blebbing,
but no loss of integrity
-Aggregation
of chromatin
at the nuclear membrane
-Cellular
condensation (cell shrinkage)
-Formation
of membrane bound vesicles
-Loss
of membrane
-Flocculation
-Swelling
-No
integrity
of chromatin
of the cell and lysis
vesicle
formation,
complete
-Disintegration(swelling)
lysis
of organelles
(apoptotic bodies)
-No disintegration of organelles,
organelles remain intact
Biochemical and molecular features
-Tightly
regulated process involving
activation and enzymatic steps
-Loss
-Energy
(A TP)- dependent
-Non-random
mono- and oligonucleosomal
length fragmentation of DNA (ladder
pattern after agarose gel electrophoresis)
-Prelytic
DNA fragmentation (early event
of cell death)
-Random
digestion
agarose
-Postlytic
gel electrophoresis)
DNA fragmentation
-No
of regulation
energy
of ion homeostasis
requirement
of DNA
(smear
(late
after
event
of death)
Physiological significance
-Death of single, individual cells
-Induced by physiological stimuli
-Phagocytosis by adjacent cells or
-Death of cell groups
-Evoked by non-physiological disturbances
-Phagocytosis by macrophages
macrophages
-No inflammatory response
-Significant
inflammatory response
while other organelles remain intact. Later a characteristic bubbling and blebbing of the cytoplasmic membrane and the formation the membrane-bound fragments, apoptotic bodies, occurs. The apoptotic bodiesare then phagocytosed by neighbouring cells (KERR et
a/. 1972, Fig. 1). The loss of structural organisation is
energy dependent, often causing an increase in respiratory rate (NEWMEYERet a/. 1994)
(KORSMEYER1995). Over the past five or six years
about 30 new molecules have been discovered that initiate or regulate apoptosis. At least 20 other molecules
associated with signalling or DNA replication, transcription or repair, have been recognised as affecting the
regulation of apoptosis (WILLIE 1998).
One of the first signal for apoptosis, known at present, is a decrease in mitochondrial transmembrane potential, irrespectiveof any apoptosis-inducingstimulus ( c.f.
KROEMERet al. 1998). The aberrant exposure of phosphatidylserine in the plasma membrane is another early
Biochemical and molecular markers of apoptosis
marker of the apoptotic process (KROEMERet al. 1998).
These events are followed by the activation of nucleThe process of programmed cell death can be schemati- ases, proteases, phospholipases and phosphatases. The
cally subdivided into three steps: a signalling phase, an participation of calcium was also well documented
execution phase and a dismantling phase (DEPREATERE (SCHWARZTMAN
& CIDLOWSKI1993).
& GOLSTEIN 1998). The regulation of apoptosis is
The activation of nucleases leads to a non-random
known mainly from the work with neoplastic tissues cleavage of nuclear DNA (EA YLYet al. 1997). Cleavage
...
./
.
"
@\
~
.
.=.-/
/
(A)
(8)
(C)
(D)
(E)
(F)
Fig. 1.: Apoptosis in animal cells. After receipt of a signal to undergo apoptosis, an adherent cell (A) rounds up (8). Nuclear DNA rapidly condenses (C).
Nucleus is separated into discrete masses of condensed chromatin (D) and, finally, fragmentation of the cell into several membrane-bound vesicles apoptotic bodies -follows (E). These bodies are rapidly recognised and phagocytosed by macrophages or neighbouring cells (F).
205
L. HAVEL & D. J. DURZAN
usually results in formation of small fragments of double
stranded DNA (size 180-200 bp) that can form a typical
ladder on agarose gels (e.g. WALKER et al. 1993). The
individual apoptotic nuclei are detected with terminal
deoxynucleotidyl transferase-mediated dUTP nick end
labelling (TUNEL) in situ (GAVRIELI et al. 1992, (Fig.
2). Larger fragments and single-strand DNA cuts were
also characteristic for apoptotic degradation of the nucleus (e.g. PEITSCHet al. 1993; BORTNERet al. 1995).
Larger fragments (50 kbp) are generated by the release
of DNA from the nuclear matrix. SubsequentinternucleosomalDNA cleavageresults in the formation of small
fragments (OBERHAMMERet al. 1993). These findings
suggestthat the apoptotic degradation of the nucleus is a
gradual process. The patterns of endonucleases activity
cannot be a sole criterion for apoptosis, as nucleases can
be activated by several processes. Other observations
indicate that endonucleolytic DNA degradation is neither required nor sufficient evidence of apoptosis in
certain cells (SCHULZet al. 1998).
Programmed cell death in plants
The term "apoptosis" comes from plant kingdom from
old Greek "apoptosis" which originally means the loss
of petals or leaves. Surprisingly, despite the obvious role
of cell death in plants the concept of programmed cell
death was developed and pioneered in animal and medical sciences. Now it is becoming obvious that this concept will better explain many events of plant biology .A
model for plant apoptosis in the life cycle has been already proposed. This model embodied predisposing
physiological states, divisional cycling, salvage of metabolic degradation products, terminal differentiation,
disease resistance, and renewed growth (HA VEL &
DURZAN1996a;b).
Markers
The basic model for apoptosis as it was accepted in
animal and human sciences was applied in plant sciences, and the same markers were employed. The localised collapse of nuclear domains (pycnosis), loss of
nuclear membrane, nucleolar release, and fragmentation
of nuclei and cytoplasm into the distinct bodies was also
described in plants (RAVEL & DURZAN 1996c; 1999;
W ANG et al. 1996a;b ). The TUNEL assay showed DNA
fragmentation in situ in nuclei of cells where the programmed cell death has been expected (e.g. MITTLER &
LAM 1995; MITTLER et al. 1995; RAVEL & DURZAN
1996c; W ANG et al. 1996a). When TUNEL is combined
with other fluorescent dyes that are specific for DNA e.g.
4,6-diamidino-2-phenylindole
dihydrochloride
(DAP1),
the nuclei that are or are not destined for elimination can
be distinguished (RAVEL& DURZAN 1996a;b).
206
Another feature of apoptosis -DNA ladder formation on agarose gels -was more difficult to prove. The
number of cells with apoptotic nuclei in tissues of
growing plants is relatively low. The expected laddering
was detected in cultures and organs where the external
stimuli provide the means for an increase of apoptotic
cells in certain time (synchronisation of apoptosis). In
vitro cultured cell and toxic and/or abiotic stimuli were
used first (W ANG et al. 1996a;b; KouKALOVA et al.
1997). The naturally synchronised elimination of cell
populations, that form carpels, petals and foliage leaves,
followed (ORZAEz & GRANELL 1997a;b; YEN & Y ANG
1998).
O'BRIEN et al. (1998) showed that annexin V-binding, an indicator of exposure of phosphatidyl serine on
the outer cell plasma membrane of mammalian cells, can
also be used to detect apoptosis in plants. They used the
isolated protoplasts from a cell suspension culture of Nicotiana plumbaginifolia. It has not been proven yet
whether the cells with cell walls are sensitive to this assay.
Reproduction
processes
Many different processes participate in the formation of
new individual plants. These processes comprise the
formation of spores, gametophytes, sporophytes, gametes and zygotes. There are many specialised developmental pathways in different plant groups. It has already
been recognised that programmed cell death can participate in reproduction processes.
Programmed cell death is a normal part of ovule and
seed development which is regularly accompanied by
the degeneration of supernumerary archesporial cells
and megaspores, nucellus and, in angiosperms, certain
cells in the embryonal sac and endosperm (ERDELSKA
1998). Also individual layers of seed coats die.
SCHWARTZet al. (1997) stressed that the only known
example of programmed cell death during plant embryogenesis is during the degeneration of the suspensor.
Based on older results BELL (1996a) suggested that
abortion of certain megaspores in angiosperms is a form
of programmed cell death, probably apoptosis. The
older results obtained by CORRENS(1900) conformed
remarkably well with the hypothesis that regular elimination of three of the megaspores is genetically determined. The same genetic background is expressed in
macrosporangia (carpels) and not in micro sporangia
(anther) where all four meiotic products survive (BELL
1996a).
Apoptosis was described in suspensor cells of the
early somatic embryos of Norway spruce (HA VEL &
DURZAN1996c). The early somatic embryo, which has a
longitudinal structure, comprises three parts: embryonal
group, embryonal tubes and embryonal suspensor. The
cells of the embryonal group are actively dividing with
no signs of apoptosis. The first TUNEL positive nuclei
were observed in the embryonal tubes quite close to the
~
Programmed
,
cell death
in plant development
(A)
"""""'-
~
~
~
(C)
(D)
"""""-.
0
~
labeled
dU
TP
labeled streptavidin
color product
Fig. 2.: TUNEl (terminaldeoxynucleotidyl
transferase-mediated
dUTP-biotinnick end labelling).The ONAis cleavedInto nucleosomes(A). 3 OH endsof ONAfragmenls
are labelledby terminaldeoxynucleotidyl
transferasewith dum which Is conjugatedwith a fluorescentdye (for Immediateobservation)or with biotin(B). Biotinis specifIcally boundto streptavldln(C). Streptavldlncan be labelledwith a fluorescentdye for observationor conjugatedwith an enzyme which reactswith its substrategiving a
color product(D).
207
L. HAVEL & D. J. DURZAN
embryonal group. More distal nuclei became pycnotic
and disintegrated with the release of nucleoli and nuclear fragments into the cytoplasm. The morphology and
development of the early zygotic embryos (c.f. SINGH
1978) is similar to, if not the same as, the early somatic
embryos and thus apoptosis may be also expected in the
zygotic embryogenesis of Norway spruce. The same
features can be expected in embryogenesis of more
species in conifers because HA VEL & DURZAN ( 1999)
observed the presence of apoptosis in tubes and suspensor of early somatic embryos of blue spruce.
The cultured early somatic embryos of conifers
multiply via cleavage of the embryonal group and so
called diploid parthenogenesis which was described in
Norway and blue spruces (c.f. RAVEL & DURZAN 1992;
1999; DURZANet al. 1994). The participation of apoptosis in this process was shown as well ~RAVEL &
DURZAN1996c; 1999).
During development and germination of an embryo
its nutrition is important. The role of apoptosis in this
process was detected in the aleurone layer -the outer
part of the endosperm -of barley grains (W ANG et al.
1996c). Here the cells do not store many reserves but
may be responsible for the release of a certain amount of
enzymes that mobilise the nutrients from the rest of
endosperm that has non-living cells fully filled with
nutrients (BEWLEY& BLACK 1994). Apoptosis appeared
to be important for the spatial and temporal control of
the aleurone layer activities during grain germination
(W ANGet al. 1996c).
Tissue development
As mentioned above, programmed cell death can occur
where cells eliminate their nucleated protoplasts to perform structural and translocatory functions. The tracheary elements may be the best known examples of
such cells. The cells with TUNEL positive nuclei were
detected in developing xylem of intact roots ( e.g.
MITTLER& LAM 1995; MITTLERet al. 1995). The gradual DNA fragmentation in nuclei in developing tracheary elements in in vitro cultured callus was also
observed (HA VEL et al. 1997).
The lateral root primordia develop in the pericycle
of the main root. Lateral roots grow through the cortex,
reach the main root surface, and continue their elongation. Apoptotic cells were observed in the inner cortical
cells of soybean, that overlay the root primordia. This
elimination frees the space for the undisturbed growth
and development of lateral roots (KOSSLAKet al. 1997).
In a mutant of the same species the apoptotic cells were
spread through the whole root although a spatial control
existed here. Typical nuclear DNA fragmentation was
detected opposite the xylem poles of the root vascular
bundle (KOSSLAKet al. 1997).
208
The root cap protects the root apical meristem and
facilitates its growth through the soil or substrate. The
destruction of the most distal cells, which results in
mucilage production, facilitates the smooth growth of
the root tip in harsh conditions. A typical DNA cleavage
proved by TUNEL was shown in nuclei of dying cells of
the root cap (W ANGet al. 1996a).
Senescence
Senescence occurs in individual cells, tissues, organs,
and the whole organism. Different opinions exist as to
the meaning of the term "senescence". It is generally
accepted that senescence is a genetically
controlled
developmental process which is internally programmed
(NOOOEN & GUIAMET 1996). Senescence results in the
loss of homeostasis at the cell or organism level. Ultrastructural studies provide a valuable overview of cell
senescence (c.f. NoooEN 1988). Several features resemble typical markers of programmed cell death.
ORZAEz & GRANELL (1997a) detected typical DNA
fragmentation during the senescence of unpollinated
pistils of Pisum sativum. The apoptotic event was also
observed during petal senescence in the same species
(ORZAEZ & GRANELL 1997b). Moreover,
the DNA
fragmentation
which
accompanies
senescence was
regulated by ethylene (ORZAEz & GRANELL 1997a).
Recently, YEN & Y ANG (1998) reported the detection of
programmed cell death in the senescent leaf tissue from
five plant species. The use of gel electrophoresis and
Southern hybridisation
detected DNA ladders only in
senescent leaves but not in green leaves. DNA fragmentation and nuclear DNA condensation were further
confirmed in situ by the use of TUNEL assay. These
results provide direct evidence to support the notion that
the natural senescence of the leaves is indeed an apoptotic process (YEN & Y ANG 1998).
GALLOIS et al. (1997) isolated a clone from an
Arabidopsis
thaliana cDNA library whose predicted
translation product showed highly significant similarity
to a mammalian defender against apoptotic death 1 protein (DAD 1) -product
of a gene which was mentioned
above. Their experimental data indicated that two such
genes (AtDAD) exist in this species in contrast to mammals having only one such gene. The transcripts of AtDAD genes are found in root, stem, leaves, flowers and
siliques at different stages of the plant development. The
abundance of transcripts is reduced in siliques during
their maturation and desiccation (GALLOIS et al. 1997).
This is natural considering that the role of the DAD gene
suppresses programmed cell death (NAKSHIMA et al.
1993). The protoplasts of siliques cells die while their
persisting cell walls desiccate. The whole process must
be highly co-ordinated to ensure the opening of a ripe
silique.
Programmed cell death in plant development
Phytohormones
The role of phytohormones in plant growth and development is documented in detail in other chapters of this
book. First observations on the role of these compounds
in programmed cell death have been already published.
Apoptosis in the aleurone which takes place during
germination was inhibited by abscisic acid (W ANGet al.
1996c). Ethylene is known by its role in senescenceand
it was also shown to regulate DNA fragmentation, a
hallmark of apoptosis, in pistil senescence in pea
(ORZAEz& GRANELL1997a).
Longevity
The development of plants lasts for different time periods depending on species. Some coniferous species live
several thousands of years e.g. individuals of Pinus
aristata in California are more than 4 300 years old
(FoSTER & GIFFORD 1974). Herbaceous species may
develop and die in several weeks. In general, plant longevities fall into several categories relative to the annual
seasonal cycle (annuals, biennials, and perennials). The
longevity of plants is not always limited by endogenous
programs but can be terminated by environmental or
pathological factors. If no such factors are present the
longevity depends on the continuation of meristematic
activity. The persistence of cell divisions for thousands
of years depends on the ability of meristems to repair
and recover from the damage of the genome (HA VEL &
DURZAN 1996a). The meristematic cells must also suppress the genes or their products that trigger programmed cell death to maintain their division.
Recently, SCHUBERTet al. (1998) showed that terminal deletions of artificially elongated chromosome
arms triggered apoptosis (TUNEL positive nuclei) in
foot tip meristems of faba bean. If the number of damaged meristematic cells surpasseda threshold, the plants
showed developmental disturbances. Extensive cell
death in meristems was eventually responsible for reduced growth, disturbed development and reduced seed
set. This observation suggests that apoptosis in meristems influences longevity and the overall development
of plants.
Pathological events
Phytopatological
events are not parts of nonnal plant
development but we mention it here, because the concept of programmed cell death in plants emerged several
years ago in pathology studies (DIETRICH et al. 1994;
GREENBERG et al. 1994; HAMMOND-KOSACK et a/.
1994; LEVINE et a/. 1994; 1996; PONTIER et a/. 1994;
MITTLER & LAM 1995; MITTLER et a/. 1995; JONES &
DANGL 1996; RyERSON & HEATH 1996; WANG et a/.
1996a).
Sacrificing an infected cell in order to prevent systemic spread of a pathogen appears to be a conserved
strategy in both plants and animals (MITTLER et al.
1997). Recognition of invading pathogens and activation
of the cell death result in the formation of dead cells
around the site of the attack. This process, termed a
hypersensitive response, prevents the systemic spread of
some pathogens. Several lines of evidence suggested
that cell death during the hypersensitive response results
from activation of a programmed cell death pathway
(MITTLERet al. 1997).
Current data suggest that activation or suppression
of programmed cell death may underlie diseases in
plants as it does in animals (GREENBERG 1997;
GILCHRIST1998). Programmed cell death can act as an
endogenous "secondary signal" for the induction of
localised defences and signals triggering systemic defence (HEATH 1998).
The disease resistance gene Cf-9 together with a
fungal avirulence gene regulates cell death in tomato
seedlings (HAMMONO-KOSACKet a/. 1994). Another
gene, hstr203J, with rapid activation, was highly localised and specific for incompatible plant/pathogen interaction was found in tobacco (PONTIERet a/. 1994). The
accelerated cell death gene (ACD2) acts as a negative
regulator of programmed cell death triggered by pathogens (GREENBERG
et a/. 1994). Calcium in apoptosis in
plants was also studied in this context (LEVINE et a/.
1996).
Conclusions
The differences in details of programmed cell death
between plants and animals are now evident, however,
many common features exist too (HA VEL & DURZAN
1996a; WANG et al. 1996a; GILCHRIST1997; PENNELL
& LAMB 1997). E.g. the same external agents can trigger
apoptosis in both, animal and plant cells (W ANG et al.
1996b).
The more precise evidence for the similarity between the animal and the plant mechanism of programmed cell death is based on findings that homologues of an animal gene involved in apoptosis also
exists in plants (genes AtDAD -see above). Transformation of mutant hamster cells, which undergo apoptosis at a restrictive temperature, demonstrated the efficiency of the plant gene products in rescuing these
mammalian cells from apoptosis (GALLOISet al. 1997).
JANICKE et al. (1998) recently showed that the
regulation of cell death crosses evolutionary boundaries.
They cloned and characterised an oxidative stress induced gene (oxy5) from the same species (Arabidopsis
thaliana). The product of this gene protects transformed
human tumor cells (HeLa) from tumor necrosis factor-induced apoptosis. The expression of this gene also
protected bacterial cells from death caused by oxidative
stress. HEATH (1998) noted the strong similarity of the
responses of isolated protoplasts to mammalian apopto-
209
L. HAVEL & D. J. DURZAN
sis. This suggeststhat unwalled plant cells exhibit more
animal-like programmed cell death that do walled cells
in intact plants.
The differences between animal and plant programmed cell death can be also detected. It seems that
the extent of chromatin condensation is substantially
greater in plant cells and is reversible in the early stages.
Reversibility has been confmned by using the chemicals
that cause chromatin condensation at sublethal levels,
and by removing the agent from treated cells by washing
(O'BRIEN et al. 1998).
The study of apoptosis depends on suitable experimental systems. Models that predict "synchronised"
apoptosis for senescing leaves or parts of flowers or cell
cultures under the biotic or abiotic stresses revealed
similarities between plant and animal apoptosis (see
above). In animal cells a lot of knowledge has been
acquired in studies with malignant cells in situ or in
vitro. It is promising that the concept of plant cancers
has emerged (GASPAR 1995). The results with habituated (hormone independent) cells and with vitrification
(hyperhydric malformations) of sugar beet were viewed
as a form of plant cancer by GASPAR (1998) and
GASPARet al. (1998). They defined a cancerous state as
an irreversible loss of organogenic totipotency at the end
of a neoplastic progression. The authors presumed cancerous plant cells could be very useful in studies of
programmed cell death in plants.
The old ideasmay also be reintroduced. BELL (1996b)
noted that the recognition of apoptosis(or programmedcell
death) as an accompaniment of normal development
stimulates renewed interest in Haberlandt's concept of
"wound hormones" or "necrohormones" in apomictic reproduction (e.g. HABERLANDT 1923). In these experiments, the injury of ovules caused the apomictic
development of embryos. As we assume the injury of
some cells can be accompanied by apoptosis of adjacent
cells. Apoptotic degradation products are not the result
Qf a genetically unprogrammed disaster (i.e. accident)
but of terminal events offering continuity, survival, and
protection of plant life cycle (HA VEL & DURZAN1996a).
Products released by the protoplasts undergoing self-destruction are utili sed by living cells and stimulate
their division and induce development of new structures
(BELL 1996).
Many features of plant biology will need reinterpretation (HA VEL & DURZAN 1996b) and the new conclusions will result in better understanding of plant developmental processesand their regulation.
Acknowledgements
Dr. Martin Truksa's reading of the manuscript is highly
appreciated. The preparation of the manuscript was
supported by the Ministry of Education of the Czech
Republic (project number vs 96082).
210
References
Programmed
cell death in plant development
GAVRIELI, Z., SHERMAN, Y. & BEN-SASSON,B.: Identification of
programmed cell death in situ via specific labelling of nuclear
DNA fragmentations. -J. Cell BioI. 119 493-501,1992.
KROEMER, G., DALLAPORTA, B. & RESCHE-RIGON,M.. The mitochondrial death/life regulator in apoptosis and necrosis. -Ann.
Rev. Physiol. 60: 619-642,1998.
GILCHRIST,D.G.. Mycotoxins reveal connections between plants and
animals in apoptosis and ceramide signalling. -Cell Death Differ. 4: 689-698,1997.
LAZEBNIK, Y.A., COLE, S., COOKE, C.A., NELSON, W.G &
EARNSHAW,W.C.: Nuclear events of apoptosis in vilro in cell-free mitotic extract. A model system for analysis of the active
phase ofapoptosis. -l. Cell BioI. 123.7-22,1993.
GILCHRIST,D.G.: Programmed cell death in plant disease. The purpose and promise of cellular suicide. -Ann. Rev. Phytopathol.
36: 393-414,1998.
GREENBERG,J.T.: Programmed cell death in plant-pathogen interactions. -Ann. Rev. Plant Physiol. Plant Mol. BioI. 48. 525-545,
1997.
GREENBERG,J.T., Guo, A., KLESSING, D.F. & AUSUBEL, F.M..
Programmed cell death in plants, a pathogen-triggerd response
activated coordinately with multiple defence functions. -Cell
77.551-563,1994.
HABERLANOT, G.. Wundhormone als Erreger von Zellteilungen.
-Beitr.Allg.Bot.
2: 1-53,1923.
HAMMONO-KOSACK,K.E., HARRISON,K. & JONES,J.D.G.: Developmentally regulated cell death on expression of the fungal avirulence gene Avr9 in tomato seedlings carrying the disease resistance gene Cf9. -Proc. Natl. Acad. Sci. USA 91. 10445-10449,
1994.
LEVINE, A., TENHAKEN, R., DIXON, R. & LAMB, C.. H2O2 from the
oxidative burst orchestrates the plant hypersensitive disease resistance response. -Cell 79. 583-593, 1994.
LEVINE, A., PENNEL, R.I., ALVARE, M.E., PALMER, R. & LAMB, C.:
Calcium-CUff.
mediated
apoptosis
in a plant hypersensitive disease
response.
BioI.
6: 427-437,1996.
,
MAJNO, G. & lORIS, I.. Apoptosis, oncosis, and necrosis. -An
view of cell death. -Amer. l. Pathol. 146: 3-15, 1995.
over-
MANTEL, S.H., MATTHEWS,1.A. & McKEE, R.A.. Principles of Plant
Biotechnology. Blackwell Scientific Publ., Oxford, 1985.
MAUSETH, 1.D.. Botany. An Introduction to Plant Biology. -Saunders College Publishing, Philadelphia, 1995.
MITTLER, R. & LAM, E.: Identification, characterization, and purification of a tobacco endonuclease activity induced upon hypersensitive response cell death. -Plant Cell 7: 1951-1962,1995.
HANNUN, Y.A.. Functions of ceramide in coordinating cellular responses to stress. -Science 274: 1855-1859,1996.
MITTLER, R., SHULAEV, V. & LAM, E.. Coordinated activation of
programmed cell death and defense in transgenic tobacco plants
expressing a bacterial proton pump. -Plant Cell 7. 29-42, 1995.
HAVEL, L. & DURZAN, D.J.: Origin of somatic embryos from single
cells in vitro in Norway spruce (Picea abies (L.) Karst.). -BioI.
Plant. 34 (Suppl.)" 537,1992.
MITTLER, R., SIMON, L. & LAM, E.: Pathogen induced programmed
cell death in tobacco. -1. Cell Bioi. 110.1333-1334,1997.
HAVEL, L. & DURZAN, D.J.. Apoptosis in plants. -Bot.
Acta 109.
268-277,1996a.
HAVEL, L. & DURZAN, QJ. Apoptosis -A new reinterpretation for
plant physiology. -Plant Physiol. Biochem Spec. 1ssue.pp. 23-24,
1996b.
HAVEL, L. & DURZAN, QJ.: Apoptosis during diploid parthenogenesis and early somatic embryogenesis of Norway spruce. -Int J.
Plant Sci. 157: 8-16, 1996c.
HAVEL, L. & DURZAN, QJ.. Apoptosis during early somatic embryogenesis in Picea spp. -In: JAIN, S.M., GUPTA, P.K. & NEWTON,
R.J. (eds.)" Somatic Embryogenesis in Woody Plants. Kluwer
Acad. Publ., in press.
HAVEL, L., SCARANO,M.T. & DURZAN, QJ.: Xylogenesis in Cupressus cells involves apoptosis. -Adv. Hort. Sci. 11: 37-40,1997.
HEATH, M. C.: Apoptosis, programmed cell death and the hypersensitiveresponse.-Eur.J.Plant
Pathol.104. 117-124,1998.
JANICKE,R.U., POTER,A.G. & KUSH, A.. A novel Arabidopsis thaliana protein protects tumor cells from tumor necrosis factor-induced apoptosis. -Biochim.
Biophys. Acta 1402. 70-78,
1998.
JONES,A.M. & DANGL, J.L.. Logjam at the Styx. programmed cell
death in plants.-TrendsPlant
Sci.1: 114-119,1996.
KERR, J.F.R., WVLLIE, A.H. & CURRIE, A.R.: Apoptosis: a basic
biological phenomenon with wide-ranging implications in tissue
kinetics. -Br. J. Cancer 26: 239-257,1972.
KIPLING, Q. The Telomere. Oxford Univ. Press, Oxford, 1995.
KORSMEVER,S.J.. Regulation of cell death. -Trends
101-105,1995.
Genet 11.
KOSSLAK,R.M., CHAMBERLIN,M.A., PALMER,R.G. & BOWEN, B.A.:
Programmed cell death in the root cortex of soybean root necrosis mutant -Plant J. 11: 729-745,1997.
KOUKALOVA, B., KOVARtK, A., FALKUS, J. & SIROKY, J.: Chromatin
fragmentation associated with apoptotic changes in tobacco cells
exposed to cold stress. -FEBS Let 414: 289-292,1997.
NAKSHlMA, T., SEKIGUCHI, T., KURAOKA, A., FUKUSHlMA, K.,
SHIBATA, Y., KOMIYAMA, S. & NISHIMOTO, T.. Molecular
cloning of a human cDNA encoding a novel protein, DADI,
whose defect causes apoptotic cell death in hamster BHK21
cells. -Mol. Cell. BioI. 13.6367-6374, 1993.
NAGL, W.. Ultrastructural and developmental aspects of autolysis in
embryo suspensors. -Ber.
Deutsch. Bot. Ges. 89: 301-31 I,
1976.
NEWMEYER,D.D., FARSCHON,D.M. & REED, 1.C.: Cell-free apoptosis in Xenopus /evis egg extracts. Initiation by Bcl2 and requirement for an organelle fraction enrichment in mitochondria.
-Cell 79. 353-364, 1994.
NOODEN, L. D.. The phenomena of senescence and aging. -In.
NOODEN, L. D. & LEOPOLD,A.C., (eds.)" Senescence and Aging
in Plants. Pp. 2-50. Academic Press, San Diego, 1988.
NOODEN, L. D. & GUIAMET, 1.1.. Genetic control of senescence and
aging in plants. -In: Handbook of the Biology of Aging. Pp
94- 118. Academic Press, 1996.
OBERHAMMER,F., HOCHEGGER,K., FROSCHL,G., TIEFENBACHER,R.
& PAVELKA, M.. Chromatin condensation during apoptosis is
accompanied by degradation of lamin A+B, without enhanced
activation ofcdc2 kinase. -l. Cell Bioi. 126.827-837,1993.
O'BRIEN, I. E.W., BAGULEY, B.C., MURRAY, B.G., MORRIS, B.A., M.
& FERGUSON,I.B.. Early stages of the apoptotic pathway in
plant cells are reversible. -Plant l. 13. 803-814, 1998.
ORZAEZ, D. & GRANELL, A.: DNA fragmentation is regulated by
ethylene during carpel senescence in Pisum salivum. -Plant l.
II. 137-144, 1997a.
ORZAEZ, D. & GRANELL, A.: The plant homologue of the defender
against apoptotic death gene is down-regulated during senescence of flower petals. -FEBS Lett. 404.275-278, 1997b.
PEITSCH, MC., MULLER, C. & TSCHOPP,l.: DNA fragmentation
during apoptosis is caused by frequent single-strand cuts. -Nucleic Acid Res. 21: 4206-4209,1993.
21 1
Lo
HAVEL
&
Do
Jo
DURZAN
PENNELL,R.I. & LAMB, C.: Programmed cell death in plants. -Plant
Cell 9: 1157-1168, 1997.
STELLER, H.: Mechanism and genes of cellular suicide. -Science
267. 1445-1449, 1995.
PONTIER, D., GoDIARD, L., MARCO, Z. & ROBY, D.: hstr203J, a
tobacco gene whose activation is rapid, highly localized and
specific for incompatible plant/pathogen interaction. -Plant 1.
5: 507-521, 1994.
UCKER, D.S., OBERMILLER, P.S., ECKHART, W., APGAR, J.R.,
BERGER,N.A. & MEYERS,J.. Genome digestion is a dispensable
consequence of physiological cell death mediated by cytotoxic T
lymphocytes functional paradigm for programmed cell death induced by a host-selective phytotoxin and invoked. -Mol.
Cell
Bioi. 12.3060-3069,1992.
RYERSON, D.E. & HEATH, M.C.: Cleavage of nuclear DNA into
oligonucleosomal fragments during cell death induced by fungal
infection or by abiotic treatments. -Plant
Cell 8: 393-402,
1996.
VAOX, D.I. & STRASSER,A.: The molecular biology of apoptosis.
-Proc. Natl. Acad. Sci. USA258: 1955-1957,1996.
SCHUBERT,I., OUD, 0.1. & PICH, U.: Unscheduled apoptosis in
meristematic cells is triggered via terminal deletions in artificially elongated chromosome arms. -Theor. Appl. Genet. 96.
1022-1026, 1998.
WANG, H., LI, J., BOSTOCK,R.M. & GILCHRIST,D.G.: Apoptosis. A
functional paradigm for programmed cell death induced by a
host-selective phytotoxin and invoked during development.
-Plant Cell 8: 375-391, 1996a.
SCHULZ, 1., B., BEINROTH, S., WELLER, M., WULLNER, U. &
KLOCKGETHER,T.. Endonucleolytic DNA fragmentation is not
required for apoptosis of cultured rat cerebellar granule neurons.
-Neuroscience Lett. 245.9-12, 1998.
WANG, H., JONES,C., CIACCIZANELLA,J., HOLT, T., GILCHRIST,D.G.
& DICKMAN, M.B.. Fumonisins and Alternaria alternata Iycopersici toxins -sphinganine analog mycotoxins induce apoptosis in monkey kidney cells. -Proc. Natl. Acad. Sci. USA 258.
3461-3465,1996b.
SCHULZE-OSTHOFF,K., WALCZAK, H., DROGE, W. & KRAMMER,
P.H.: Cell nucleus and DNA fragmentation are not required for
apoptosis. -1. Cell BioI. 127. 15-20, 1994.
SCHWARTZ,B.W., VERNON,D.M. & MEINKE, D.W.. Development of
the suspensor: Differentiation, communication, and programmed
cell death during plant embryogenesis. -In:
LARKINS, B. &
V ASIL, I.K. (eds.): Cellular and Molecular Biology of Plant Seed
Development. Pp. 53-72. Kluwer Acad. Publ., The Netherlands,
1997.
SCHWARZTMAN,R.A. & CmLowsKI, 1.A.: Apoptosis: the biochemistry and molecular biology of programmed cell death. -EndocrineRevs.14: 133-151, 1993.
SINGH, H.: Embryology of Gymnosperms. -Begruder
Berlin, Stuttgart, 1978.
Bomantrager,
SINGH,R.1.: Plant Cytogenetics. CRC Press, Boca Raton, 1993.
212
WANG, M., OPPEDIJK,B.J., Lo X., VAN DOIJN, B. & SCHILPEROORT,
R.A. .Apoptosis in barley aleurone during germination and its
inhibition by abscisic acid. -Plant Mol. Bioi. 32. 1125-1134,
1996c.
WALKER, R.R., KOKILEVA, L., LEBLANC, J. & SIKORSKA,M.. Detection of the initial stages of DNA fragmentation in apoptosis.
-Biotechniques 15: 1032-1035, 1993.
WILLIE, A.H.. Cell death. -In: Apoptosis and Cell Proliferation, 2nd
edition. Pp. VI-VII. Boehringer, Mannheim, 1998.
YEN, CH. & YANG, CH.: Evidence for programmed cell death during
leaf senescence in plants. -Plant Cell Physiol. 39. 922-927,
1998.