Transcript Cell intrinsic information Lecture 2

Lectures in Plant Developmental
Physiology, 3 cr.
Kurt Fagerstedt
Department of Biological and Environmental Sciences
Plant Biology
Viikki Biocenter
Spring 2006
Cell intrinsic information:
The cell and the cell cycle
Lecture 2
Contents
Mon 13.3.
Orienteering and Introduction to plant developmental biology.
Cell-intrinsic information. Prof. mvs. Kurt Fagerstedt
Wed 15.3.
Embryo development (primary axis development).
Prof. mvs. Kurt Fagerstedt
Mon 20.3.
Shoot apical meristems. Prof. mvs. Kurt Fagerstedt
Wed 22.3.
Leaf development, stomata. Prof. Jaakko Kangasjärvi
Mon 27.3.
Root apical meristems, root development.
Prof. Ykä Helariutta
Wed 29.3.
Flower development. Prof. Teemu Teeri
Mon 3.4.
Hormonal control of development, Prof. Ykä Helariutta
Wed 5.4.
Developmental responses to light. Prof. Jaakko Kangasjärvi
Mon 10.4.
Environmental information other than light.
Prof. mvs. Kurt Fagerstedt
Wed 12.4.
Coordination of development, Prof. mvs. Kurt Fagerstedt
Mon 17.4.
No lecture (Easter)
Wed 19.4.
Open examination on the lectures and additional reading.
References / Further
Reading
• Dewitte, W. & Murray, J.A.H. 2003. The
Plant Cell Cycle. Annu. Rev. Plant Biol.
54: 235-264.
• Buchanan, Gruissem & Jones (eds.)
2000. Biochemistry & Molecular Biology
of Plants. Chapter 11. Cell Division
Regulation.
’Omnis cellula e cellula’
• similar fundamental mechanisms are
operational at the core of the cell-division
cycle of all eukaryotes.
• the mechanisms of pattern formation
have evolved independently in plants and
animals.
How crucial is the plant cell cycle
as a point of control in plant
development?
• Cell proliferation – meristems in some trees.
How do they avoid the cell cycle running out of
control?
• Cell cycle genes –the complement of cell cycle
genes in plants is more complex than in
animals
– plants’ sessile lifestyle i.e. well adapted to adjust
their development in response to changing
environmental conditions? Developmental fine
tuning?
• Cell cycle and development
– cellular theory, organismal theory > new and
more complex concepts will emerge
• The cell wall
Cell Lineage and Cell Position
• Since plant cells do not migrate within the tissues, their
position is of vital importance in the development of the
tissues. However, lineage does not restrict development to
a specific fate.
• Examples can be found in mutation studies in developing
leaves or in ablation experiments in the root tip.
Examples: tangled1 mutant of maize
and fass mutant of Arabidopsis
Mitotic cell cycle
• Within the shoot apices of intact plants cell
division is essentially asynchronous with little or
no coordination of division timing between cells.
• Most suitable for detailed analysis are in vitro
suspension cultures of plant cells i.e. cell
division is removed from any developmental
context.
– cultures capable for various degrees of
synchronization have been developed for
Acer, Catharanthus, alfalfa, Nicotiana.
– the best levels of synchronization – tobacco
Bright Yellow-2 (BY-2) cell line.
Mitotic cell cycle
• The gap phases allow
the operation of
controls that ensure
that the previous
phase has been
accurately and fully
completed. The major
regulatory points are
G1/S, G2/M &
metaphase/anaphase
boundaries.
Control point
Control point
Control points of the Cell
Cycle
DNA replication is strictly
controlled during the cell cycle
• Initiation of DNA synthesis is inhibited in wildtype cells during G2, M and G1
• DNA synthesis in S-phase is initiated at
discrete origins of replication distributed at
regular intervals throughout the genome.
They occur on average every 36 kb in yeast,
66 kb in dicots and 47 kb in monocots.
DNA replication is strictly
controlled during the cell cycle
A large number of proteins interact directly and indirectly
with the origins to control progression through the
chromosomal cycle. ORC = origin recognition complex
DNA replication is strictly
controlled during the cell cycle
• During late M & G1, the assembly of the
protein complexes that mediate
initiation of DNA synthesis is promoted
= the cells become competent to initiate
DNA synthesis.
DNA replication is strictly controlled
during the cell cycle
• To prevent premature DNA replication, the replication
proteins that associate with ORC are assembled in steps.
– Cdc6 protein binds first
– then MCM and Cdc45
– prereplication complex is activated by protein
phosphorylation at the restriction point (START of S
phase)
– phosphorylation is achieved by Cdc7/Dbf4p protein
kinase complex > MCM complex releases from ORC &
facilitates access of DNA polymerases to the template.
– if mitotic kinase activity is suppressed in S or G2 phase,
rereplication will occur without intervening mitosis
Mitosis
- Mitosis is suppressed
during G1, S and G2
- Onset of M-phase /
initiation of chromosome
condensation and the
disassembly of the
nuclear matrix from the
cytoplasm
- the cells are not yet
competent for
chromosome
segregation
Regulation of mitosis
• condensins and cohesins > assemble long chromatin
fibres into chromosomes > replicated DNA is able to
be segregated without damage
• cohesion proteins Scc1p & Smc1p are synthesized
during S phase (sister chromatids need to be joined
in S phase)
• Inhibitory proteins such as Pds1p accumulate and
bind and antagonize to Cut1p protein
– Cut1p breaks the linkage between sister
chromatids
– anaphase-promoting complex activated by protein
phosphorylation tags the mitosis inhibitor Pds1p
for proteolysis > destruction of cohesion proteins
Chromosome
condensation
and
kinetochore
complex
Chromosome segregation
• segregation in suppressed during metaphase
by Pds1p (inhibitor of Cut1p)
• metaphase –anaphase transition is activated
by phosphorylation of APC which is catalyzed
by CDKs & CDC5
• Activation of APC results in ubiquination of
Pds1p, which is recognized by the 26S
proteasome and will be degraded ………
Cell cycle is controlled in multiple points
by CDKs
• cyclin dependent kinases
• yeast have a single CDK which has
PSTAIRE sequence within its cyclin-binding
domain
• In higher eukaryotes there are multiple
additional CDKs that have roles at different
points in the cell cycle
• In plants CDKA to CDKE
Cyclins
• diverse group of proteins with low
homology that share a large, poorly
conserved region (cyclin core), which is
responsible for their interaction with
CDKs.
• In plants there are cyclins A, B, C, D & H.
Cyclins
• A-type cyclins (also known as S cyclins) appear at the
beginning of S-phase and will be destroyed at G2/M
transition. Cyclic expression / abundance.
• B-type cyclins (also known as M cyclins, mitotic cyclins)
appear at G2, control G2/M and are destroyed at
anaphase. Cyclic expression / abundance.
• D-type cyclins control progression through G1 and Sphase. Presence depends on extracellular signals that
stimulate or maintain division.
CDK activity is regulated
at multiple levels
Activity depends on several factors:
• levels of cyclins and CDK (transcription,
translation, protein turnover, sequestration,
intracellular localization)
• CAK activates phosphorylation
• inhibitory WEE1-mediated phosphorylation
CDK inhibitors have a key role in
controlling cell cycle progress
• ICK = CKI =inhibitors of CDK or KRP (Kip-related
proteins) which bind both CDK and cyclin
subunits.
• ICKs are used by the cell to control CDK-cyclin
complex activity before undergoing cell cycle
transitions and to temporarily arrest the cell cycle
in response to DNA damage or to other signaling
pathways.
• In Arabidopsis ICK1 gene is induced by treatment
with ABA and probably mediates the cell cycle
arrest.
Cell Cycle
Inhibitors and
Root Tip
Development
CDK activity is regulated
at multiple levels
CDK subunit proteins
(CKS) scaffold
interactions
with target
substrates
by WEE1 kinase
CAK = CDK activating
kinase
by binding with
inhibitory proteins e.g. KRP
Cell cycle control
• domino model -”clock” model > combination
model
• In living systems, biochemical reactions do
not always proceed to completion and a cell
undergoing division can experience adverse
conditions that could damage DNA or spindle
apparatus > checkpoints at which cell can
monitor completion of specific reactions are
important.
The Cell Cycle
Once cell division is
completed, how does the cell
regulate its entry into a new
cycle?
• In somatic cells G1 cyclins (D-type)
play an important role and their
synthesis is coupled with growth.
• cyclin D dependent CDK activity
increases.
E2Ftranscription factor family
& Rb = retinoblastoma gene family are
accessory proteins required to enforce CDK
control of cell cycle progression
• E2F essential during DNA replication,
critical effectors of the decision to pass
the G1-to-S and allow the cell to
procede into S phase.
• autocatalytic transcription loop creates
a need for an inhibitor =Rb to inactivate
E2F.
E2F
transcription
factor
activation
E2F
transcription
factor
activation
Intercellular communication controls the
cell cycle during growth and
development
• Nutrition limitation is the most important
factor restricting cell growth in single-cell
organisms and the cell size is the major cue
for division.
• In plants only a few stem cells organized into
meristems divide to produce the plant body.
• In most multicellular organisms, social control
prevails over nutritional control of the cell
cycle.
Intercellular communication controls the
cell cycle during growth and
development
• Cell cycle control is integrated with the regulation of
cell expansion, differentiation and cell death.
• Cells in multicellular organisms are normally
quiescent and they do not proliferate without
stimulation.
• Stimuli provide the cell with information about the
status of the whole organism (rather than just its
individual cells).
• If only a few cells permanently retain the capacity to
divide, the majority of cells that have lost this
capacity must be instructed how to do so.
Cells need stimulation to
proceed with proliferation
Plant growth regulators and cell cycle
• Plant hormones affect cell proliferation.
• Because most hormones also provoke
morphogenetic effects, the cell-cycle
consequences may be direct or part of
the morphogenetic response.
Plant growth regulators and cell cycle
• Auxins and cytokinins have direct roles in cell
proliferation:
– withdrawal of auxin from tobacco cell cultures
arrests cell cycle in G1.
– withdrawal of cytokinin from tobacco cell cultures
arrests cell cycle in G2.
– In Arabidopsis cytokinin is required to stimulate
CYCD. Auxins and cytokinins have direct roles in
cell proliferation.
• Cytokinins have effects on G1/S and G2/M as well as
progression through S phase.
• ABA – expression of the CDK-cyclin complex inhibitor
(ICK1) is induced in the root during water stress and
probably mediates the cell cycle arrest in the apical
meristem.
Plant hormones and cell cycle control
Sucrose, auxin
and cytokinin
promote
accumulation of
CYCD, and
therefore
support entry
into new cell
cycle. Cytokinin
promotes entry
into M-phase.
ICK = CKI = inhibitors of KRP (Kiprelated proteins) which bind both CDK and
cyclin subunits.
Auxin and cell cycle control
Endoreduplication
• Dewitte, W. & Murray,
J.A.H. 2003. The Plant
Cell Cycle. Annu. Rev.
Plant Biol. 54: 235-264.
• Sugimoto-Shirasu, K. &
Roberts, K. 2003.
Current Opinion in
Plant Biology 6: 544553.
An alternative cycle
endoreduplication
• in differentiating plant cells.
• characterized by an increase in the
nuclear ploidy level that results
from repeated S phases with no
intervening mitosis.
• This occurs only after cells have
ceased normal mitotic cycles.
• e.g. in Arabidopsis this produces
ploidy levels up to 32 C in the final
stages of leaf development.
• takes place also in trichomes and in
leguminous nodules.
An alternative cycle
endoreduplication
• The entire complement of
chromosomes is usually rereplicated.
– chromosomes go through
condensation and decondensation
stages after replication and sister
chromatids separate > polyploidy
– chromosomes replicate without
undergoing such condensation stages
and sister chromatids remain closely
associated > polyteny
• The term polyploidy is often used
more generally to describe all types
of endoreduplicated chromosomes.
Endoreduplication in
Arabidopsis (from
Sugimoto-Shirasu, K. &
Roberts, K. 2003.)
a. a pair of stomatal
quard cells 2C
b. a small epidermal cell
4C
c. a large epidermal
cell 8C
d. trichome 32C
heterochromatic
centromeric areas
are few in number >
polytene chromosomes
Ploidy and cell size
Datura stramonium
Different cell layers have different ploidies
green nuclei 8C
yellow nuclei 2C
Switching from cell cycle to
endocycle
• During the normal mitotic cell cycle, cells have a
mechanism that licenses chromosomes to replicate
only once each cycle, after an intervening mitosis.
• The key step in the switch to endoreduplication is to
allow cells to start another round of DNA replication
while at the same time inhibiting mitosis.
• G1-S transition; endocycle appears to use much of
the same machinery to re-enter S-phase.
ORC = origin of
recognition complex
Cdc6 = cell division
cycle 6 protein
MCM =
minichromosome
maintenance
complex
when replication
starts all proteins
dissociate except ORC
In yeast, mitotic cyclin B (CycB/Cdc2) then binds
to replication origins via ORCs during G2 and early
mitosis thus preventing rereplication. In mitotic cell
cycle CycB is degradaded in late metaphase >
reassembly of prereplication complex & licensing of next
round of DNA replication.
An alternative cycle
endoreduplication
• a general view is the
downregulation of CYCA1,
CYCA2,CYCBs and CDKB in
endoreduplicating cells.
• CYCB1 overexpression induced
mitosis i.e. this may be a limiting
factor for entry into mitosis.
• The loss of mitotic cyclins is
involved in the switch from mitotic
cycles to endocycles but the
upstream mechanisms are still
unknown.
Endoreduplication
Transcriptional control of endocycles:
• Presumably endocycling cells need
to replicate far more DNA than
mitotic cells. The same machinery?
• Topoisomerase VI – role in plant
endoreduplication?
Polyploidy and speciation
Contents
Mon 13.3.
Orienteering and Introduction to plant developmental biology.
Cell-intrinsic information. Prof. mvs. Kurt Fagerstedt
Wed 15.3.
Embryo development (primary axis development).
Prof. mvs. Kurt Fagerstedt
Mon 20.3.
Shoot apical meristems. Prof. mvs. Kurt Fagerstedt
Wed 22.3.
Leaf development, stomata. Prof. Jaakko Kangasjärvi
Mon 27.3.
Root apical meristems, root development.
Prof. Ykä Helariutta
Wed 29.3.
Flower development. Prof. Teemu Teeri
Mon 3.4.
Hormonal control of development, Prof. Ykä Helariutta
Wed 5.4.
Developmental responses to light. Prof. Jaakko Kangasjärvi
Mon 10.4.
Environmental information other than light.
Prof. mvs. Kurt Fagerstedt
Wed 12.4.
Coordination of development, Prof. mvs. Kurt Fagerstedt
Mon 17.4.
No lecture (Easter)
Wed 19.4.
Open examination on the lectures and additional reading.
Article to be read by
Wednesday
Tzafrir et al. 2004: Identification of
genes required for embryo
development in Arabidopsis. – Plant
Physiology 135: 1206-1220.