Transcript Leaf Development Lecture 7

Lectures in Plant Developmental Physiology, 3 cr.
Kurt Fagerstedt
Department of Biological and Environmental Sciences
Plant Biology
Viikki Biocenter 3
Leaf Development
Lecture 5
Emergence of the leaf
primordia
Hairy leaf of Coleus
1.
2.
3.
4.
5.
6.
7.
adaxial epidermis
abaxial epidermis
epidermal cells
trichomes
spongy parenchyma
intercellular spaces
palisade
parenchyma
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.
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.
Structural symmetry in the leaf
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.
Symmetry development in the leaf
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
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 adaxialabaxial 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.
Wild type Antirrhinum
Radially symmetric phan leaf
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.
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.
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.
Maintenance of adaxialabaxial 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).
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
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.
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.
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.
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
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)
Lineage pattern for quard cell
formation in Arabidopsis
Current Opinion in Plant Biology 2004, 7:26-32.
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.
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 preexisting stoma.
– cells that are in contact with two or more stomata are
instructed not to divide.
Development of stomata in Arabidopsis
epidermis
Meristemoids
yellow
GMCs pink
quard cells blue
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 onecell-spacing rule.
• TMM serves as a receptor for a signal
generated by SDD1.
Hormonal control of stomata
• Application of GA in combination with
auxin or ethylene > overproduction of
stomata
• GA inhibitor > stomata were eliminated in
hypocotyl but not in leaves
– hypocotyl and leaves regulate differently cell
identity
Signals that direct stomatal pattern
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.
Cell growth
and cell
expansion
Cell growth
and cell
expansion
Model of
some of the
key
processes
that regulate
cell size.
cell size is
independent
of the cell
number.
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?
Leaf size / competition
chimaeric Pelargonium
• Leaf cells compete to contribute to the leaf
Cell intrinsic information
• plasmodesmata,
symplastic domains