Transcript Embryo development Lecture 3

Lectures in Plant Developmental
Physiology, 2 cr.
Kurt Fagerstedt
Department of Biological and Environmental Sciences
Plant Biology
Viikki Biocenter
Spring 2006
Embryo development
Lecture 3
Time-table and organisation
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.
Primary axis development
radial axis & longitudinal axis
• axes are polar (mature as well as developing
axes).
• the acquisition of polarity has been studied
extensively in the seaweed Fucus.
• Fucus produces free-floating eggs which are
fertilized by motile sperm. After fertilization the
zygote attaches itself to a rock and
commences embryogenesis.
• The Fucus egg is spherical and apolar.
• The zygote acquires longitudinal polarity
largely in response to environmental cues.
Thallus
Rhizoid
Fucus
Establishing polarity in the
zygote
• The Fucus egg has no cell wall and is
apolar.
• The first sign of polarity in the zygote
occurs within minutes of fertilization as a
patch of F-actin accumulates at the site of
sperm entry. In the absence of polarized
environmental cues, this site will become
the rhizoid pole of the zygote. Usually the
longitudinal axis is oriented relative to
external information.
Establishing polarity in the zygote
• Environmental cues affecting polarity are
directional light, gravity, water currents, and
temperature gradients.
The Fucus zygote polarity
Establishing polarity in the zygote the mechanism ?
• If environmentally determined axis is oriented
differently from the axis defined by site of sperm
entry, the F-acting patch marking the sperm
entry is disassembeld and a new F-actin patch
accumulates at the new rhizoid pole.
• Electric current that flows out of the zygote at
the thallus pole and into the zygote at the rhizoid
pole can be detected. Current involves
movement of calcium and / or hydrogen ions
through asymmetrically distributed pumps
and channels in the zygote plasma membrane.
• Asymmetric distribution is directed by the
polarized distribution of F-actin i.e. cytoskeleton
is in a central role.
The role of cell wall in
maintaining longitudinal axis
• Axis fixation: Several hours after
fertilization, the longitudinal axis becomes
fixed and the positions of the future thallus
and rhizoid cannot be altered by external
cues.
• Axis fixation involves interactions between
the cytoplasm and cell wall.
• Axis stabilizing complex, actin filaments
and substances in the cell wall.
The role of cell wall in
maintaining longitudinal axis
• A simple axis-stabilizing complex might explain
how the polarity of the zygote is fixed but more
complex positional information is also secreted
into the cell wall.
Cell fate can be
switched by cell
wall contact in
Fucus
HOW ABOUT
DICOTYLEDONS
?
Major Hormones Regulating
Angiosperm Embryogenesis
Development of the sporophyte
- embryogenesis
Embryogenesis in Arabidopsis thaliana
Pattern formation in
Arabidopsis embryo
Thick lines present
division lines
separating apical
(A), central (C) and
basal (B) embryo
regions.
Arabidopsis embryo
asymmetry of the zygotic division is not required
to establish the longitudinal axis of the embryo
proper
• AtLTP1 in protoderm, encodes a lipid transfer
protein involved in formation of cuticle. Marker
of embryo polarity.
• GNOM gene encodes a protein with similarity of
yeast proteins involved in secretion.
• gnom-phenotype, GNOM protein is required to
direct wall materials to the sites of cell wall
deposition. If not directed accurately > abnormal
division orientations in gnom embryos.
Arabidopsis embryo
The polarity of the zygote >
longitdinal axis of the embryo
Arabidopsis gnom embryo
Asymmetry of the zygotic division is not required to
establish the longitudinal axis of the embryo proper
• Embryo polarity can be expressed despite
abnormal division plane > there is a signal or a
gradient defining the apical-basal axis that is
independent of the cellular architecture of theembryo.
• initial orientation of the signal or gradient
depends on polarity inherited from the egg cells.
• reverse longitudinal axes suggest that after
original embryo polarity has been lost, a new
longitudinal axis can arise de novo.
Polar auxin transport is a prominent feature of the shoot-to-root axis
Role of polar auxin transport in the embryo
•
Auxin transport along longitudinal axis is
a universal feature of higher plant embryos.
•
From the globular stage onwards, auxin
transport can be detected in the shoot-toroot direction and this is in correlation in the
distribution of PIN1 (PIN-FORMED1)
protein, which is a component of an auxin
efflux carrier.
•
In early embryos PIN1 has an apolar
distribution.
The polarization of PIN1 distribution
in Arabidopsis embryo
PIN localisation and auxin
transport in A. thaliana embryo
Role of polar auxin transport in the
development of embryogenic axes
• PIN1 / longitudinal axis in gnom
embryos remains random
• > GNOM is required for the polarization
of PIN1
• > gnom embryos have reduced auxin
transport ?
PIN1 / longitudinal axis
in mp embryos
• The development of mp embryos is abnormal
from the two-cell stage onwards.
MONOPTEROS (MP) gene encodes an Auxin
Response Factor (ARF). ARFs bind to promoters
of auxin-inducible genes and regulate their
transcription.
• Rate of auxin transport is significantly less than
in wild type i.e. it might be that a reduction in
auxin transport causes the partial failure of the
longitudinal axis in mp embryos.
MP expression
and the effects
of the mp
mutation in
Arabidopsis
embryos.
RAM & SAM apical meristem formation
• Auxin concentration gradient either induces or
modulates the development of RAM= root apical
meristem.
• ”Auxin maximum” is required for RAM development.
• Several genes necessary for shoot apical meristem
(SAM) function have been identified. SHOOTMERISTEMLESS (STM), WUSCHEL (WUS), CLAVATA1
(CLV1) & CLAVATA3 (CLV3).
• The first indication of of SAM development is the
expression of WUS in cells at the apex at the early
globular stage. STM is expressed in the apex of late
globular embryo. CLV1 and CLV3 are expressed at the
site of presumptive SAM in the early heart-shaped
embryos.
• SAM becomes histologically distinquishable at the
torpedo stage.
Radial axis of A. embryos
• Radial axis becomes apparent later
than longitudinal axis
• Radial axis in all parts of the plant is
under similar control during both
embryonic and post-embryonic
development.
Roles of localised auxin
transport
BDL=Auxin repressor
ARF=Auxin Response
Element
MP=Auxin response
factor
Major regulators of maturation