Lectures in Plant Developmental Physiology, 3 cr.

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Lectures in Plant Developmental Physiology, 3 cr.

• Kurt Fagerstedt
• Department of Biological and Environmental
  Sciences
• Plant Biology
• Viikki Biocenter 3

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Leaf Development Lecture 5
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Emergence of the leaf primordia
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Hairy leaf of Coleus

• adaxial epidermis
• abaxial epidermis
• epidermal cells
• trichomes
• spongy parenchyma
• intercellular spaces
• palisade parenchyma

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Axis development in the leaf

• leaves are lateral organs.
• leaves display consistent orientation and
  polarity relative to the shoot i.e. axial
  information in the leaf does not arise de novo
  but depends on existing axial information.
• Angiosperm leaf is almost always a determinate
  organ.

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Structural symmetry in the leaf

• simple leaves have three axes of symmetry.
• proximodistal axis from base of the leaf to the
  tip.
• adaxial-abaxial axis from the upper to the lower
  epidermis.
• centrolateral axis from the midrib to the margin.

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Structural symmetry in the leaf
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Adaxial-abaxial axis(dorsoventral axis)

• adaxial-abaxial asymmetry.
• Dicot leaf primordium is initiated as a radially
  symmetric outgrowth that rapidly acquires
  adaxial-abaxial asymmetry
• In tobacco P1 (the youngest visible leaf
  primordium) is cylindrical whereas P2 has a
  flattened adaxial surface
• adaxial-abaxial polarity in the leaf depends on
  the radial axis of the shoot apical meristem.

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Symmetry development in the leaf
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Adaxial-abaxial polarity

• adaxial-abaxial polarity in the leaf depends on
  the radial axis of the shoot apical meristem.
• PHANTASTICA (from Antirrhinum)
• PINHEAD
• ARGONAUTE1
• PHABULOSA
• YABBY

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PHANTASTICA (from Antirrhinum), encodes a
MYB-type transcription factor

• loss-of-function phan mutants develop leaves with
  variable loss of adaxial-abaxial asymmetry.
• it is expressed in apical meristems at the future
  sites of leaf initiation and in leaf primordia up
  until the P3 stage.
• PHAN expression is uniform along the
  adaxial-abaxial axis.
• PHAN does not itself provide adaxial-abaxial
  information but it might be that expression is
  needed by the primordia to be able to respond to
  the polarizing signal produced by the apical
  meristem.

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Wild type Antirrhinum Radially symmetric phan
leaf
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PINHEAD ARGONAUTE1

• PNH AGO1 are needed for the development of
  adaxial leaf tissue.
• encode proteins with similarity to eukaryotic
  translation initiation factors but the
  biochemical functions are unknown.
• AGO1 is expressed ubiquitously in plants but PNH
  relates to the adaxial-abaxial axis.

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PHABULOSA(transcription factor with homeodomain,
leucin zipper, sterol/lipid-binding domains)

• PHAB gene is also believed to act in promotion of
  adaxial leaf fates.
• In wild type plants PHAB is expressed uniformely
  across I1 (future leaf primordia, incipients) but
  becomes restricted to the adaxial region of the
  leaf by P2.

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YABBY (transcription factors)

• YABBY gene family is required for the development
  of abaxial leaf tissue in Arabidopsis
• FILAMENTOUS FLOWER (FIL)
• YABBY2 (YAB2)
• YABBY3 (YAB3)
• Uniform expression begins at I2 in subepidermal
  cells but at P1 expression becomes restricted to
  the abaxial side. Expression disappears in the
  mature leaf.
• Signal from apical meristem promoting adaxial
  leaf fate inhibits directly or indirectly YABBY
  gene family expression in adaxial tissues.

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Maintenance of adaxial-abaxial axis

• The mechanims maintaining the axis are probably
  intrinsic to the leaf but little is known about
  this.
• LAM1 (DNA sequence?) is probably needed.
• In lam1 leaf primordia are indistinguishable from
  wild types. However, adaxial cell types are
  replaced by abaxial ones (and lamina fails to
  grow along the centrolateral axis).

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Centrolateral axis

• In dicots, the transition from a radially
  symmetric P1 leaf primordium to a flattened P2
  primordium results in bilateral symmetry.
• At this stage centrolateral axis becomes
  apparent.
• The extension of lamina along the cenrolateral
  axis requires the juxtaposition of adaxial and
  abaxial cell types.
• phan example

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Adaxial leaf tissue and SAM

• Adaxial leaf tissue promotes the formation of
  axillary meristems and maintains the development
  of the primary shoot apical meristem.
• In wild type Arabidopsis leaf, an axillary
  meristem develops from adaxial cells at leaf
  base.

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Proximodistal axis of the leaf

• Proximodistal differences between leaf cells are
  visible at the P3 stage.
• Leaf matures in a tip-to-base (basipetal) wave.
• knotted 1, consequence of gain of function.

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Leaf development determinate

• Loss-of function mutations in STM
  (SHOOTMERISTEMLESS, KNOX gene) lead to failure of
  meristem initiation during embryogenesis or
  premature meristem termination. KNOX genes are
  required to maintain indeterminate state of the
  apical meristems.
• High KNOX activity may induce SAMs on the leaf.
• Absence of KNOX activity contributes to the
  determinate nature of leaf development.
• compound leaves follow a less determinate pattern
  of development than simple leaves.

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Stomatal development

• Epidermis the interface between plant and the
  world.
• To maximize photosynthetic efficiency while
  minimizing water loss, stomatal pore size is
  modulated by the ion-driven swelling of the quard
  cells. Optimal gas exchange requires regulation
  of
• numbers and positions of stomata
• the ability to open and close stomata

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Cell signaling is critical to establisment of
stomatal pattern

• Stomata are formed through a stereotyped lineage
  of asymmetric cell divisions.
• Patterned locally so that two stomatal complexes
  never adjacent to each other the
  one-cell-spacing rule.
• Overall numbers of stomatal complexes are
  controlled in response to environmental cues.
  (e.g. CO2)

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Lineage pattern for quard cell formation in
Arabidopsis
Current Opinion in Plant Biology 2004, 726-32.
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Cell fate and cell signaling

• Protodermal cells distributed throughout the
  young leaf epidermis enter into the lineage
  pathway that leads to the formation of stomata.
• Lineage alone is not sufficient to ensure
  adherence to the one-cell-spacing rule.

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Cell fate and cell signaling

• The major factors in determining the pattern of
  stomata are the signals from mature quard cells
  (or their precursors GMCs or meristemoids) to
  their neighboring cells.
• cells that are in contact with a single stoma are
  instructed to orient their future division planes
  such that divisions place the smaller cell distal
  to the pre-existing stoma.
• cells that are in contact with two or more
  stomata are instructed not to divide.

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Development of stomata in Arabidopsis epidermis
Meristemoids yellow GMCs pink quard cells blue
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Cell fate and cell signaling

• The gene products required for stomatal
  patterning are
• the leucine-rich-repeat receptor-like protein
  encoded by TMM (two many mouths).
• serine protease encoded by SDD1 (stomatal density
  and distribution).
• Mutations in TMM or SDD1 lead to an increase in
  stomatal index and a breakdown of the
  one-cell-spacing rule.
• TMM serves as a receptor for a signal generated
  by SDD1.

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Hormonal control of stomata

• Application of GA in combination with auxin or
  ethylene gt overproduction of stomata
• GA inhibitor gt stomata were eliminated in
  hypocotyl but not in leaves
• hypocotyl and leaves regulate differently cell
  identity

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Signals that direct stomatal pattern
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Growth and cell size control in plants

• Cell-size increase in plants is driven by two
  very distinct processes
• cell growth involving an increase in total
  cytoplasmic macromolecular mass.
• cell expansion involving increased cell volume
  through vacuolation.

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Cell growth and cell expansion
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Cell growth and cell expansion
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Model of some of the key processes that
regulate cell size.
cell size is independent of the cell number.
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Leaf size

• The final size and shape of a leaf depends on the
  position of the leaf on the shoot and on
  environmental conditions.
• How a developing leaf can regulate its absolute
  size?
• axis specific mechanisms?

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Leaf size / competitionchimaeric Pelargonium

• Leaf cells compete to contribute to the leaf

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Cell intrinsic information

• plasmodesmata, symplastic domains

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