Lectures in Plant Developmental Physiology, 2 cr.

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Lectures in Plant Developmental Physiology, 2 cr.

• Kurt Fagerstedt
• Department of Biological and Environmental
  Sciences
• Plant Biology
• Viikki Biocenter
• Spring 2006

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Embryo development Lecture 3
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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.
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Primary axis developmentradial 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.

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Thallus
Rhizoid
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Fucus
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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.

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Establishing polarity in the zygote

• Environmental cues affecting polarity are
  directional light, gravity, water currents, and
  temperature gradients.

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The Fucus zygote polarity
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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.

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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.

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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.

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Cell fate can be switched by cell wall contact in
Fucus
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HOW ABOUT DICOTYLEDONS?
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Major Hormones Regulating Angiosperm Embryogenesis
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Development of the sporophyte- embryogenesis
Embryogenesis in Arabidopsis thaliana
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Pattern formation in Arabidopsis embryo
Thick lines present division lines separating
apical (A), central (C) and basal (B) embryo
regions.
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Arabidopsis embryoasymmetry 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 gt abnormal
  division orientations in gnom embryos.

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Arabidopsis embryoThe polarity of the zygote gt
longitdinal axis of the embryo
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Arabidopsis gnom embryo
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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 gt there is a signal or a gradient
  defining the apical-basal axis that is
  independent of the cellular architecture of
  the-embryo.
• 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.

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Polar auxin transport is a prominent feature of
the shoot-to-root axis
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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-to-root 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.

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The polarization of PIN1 distribution in
Arabidopsis embryo
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PIN localisation and auxin transport in A.
thaliana embryo
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Role of polar auxin transport in the development
of embryogenic axes

• PIN1 / longitudinal axis in gnom embryos remains
  random
• gt GNOM is required for the polarization of PIN1
• gt gnom embryos have reduced auxin transport ?

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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.

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MP expression and the effects of the mp mutation
in Arabidopsis embryos.
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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. SHOOT-
  MERISTEMLESS (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.

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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.

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Roles of localised auxin transport
BDLAuxin repressor ARFAuxin Response
Element MPAuxin response factor
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Major regulators of maturation