Download Leaf has 3 axes:1) proximodistal, 2) centrolateral, 3) ab

Leaf has 3 axes:1) proximodistal, 2) centrolateral, 3) ab-adaxial
Primordium starts out as a peglike outgrowth that is radially
symmetrical
Figure 5-1
Figure 1-7
1) Proximodistal axis:
•cell divisions end first at the tip, then stop proximally
• cell differentiation is complete first at the tip
•petiole is distinct from the blade
2) Centrolateral
•Midrib is a thickened region surrounding the midvein
•Leaf margin is at the leaf edge
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3) ab-adaxial
•Growth rate is different - cells on adaxial side divide more, resulting in
the leaf flattening
•Epidermal cells are different on ab and adaxial sides (trichomes,
stomata, cell size and shape)
•Internal cell arrangement is polarized (palisade mesophyll on adaxial,
spongy on abaxial, xylem adaxial, phloem abaxial)_
•Incision is made isolating I1 from SAM
•Radially symmetric leaf, with abaxialized epidermis, uniform
parencyhyma, core of vascular tissue
Signal from SAM is necessary to adaxialize tissue
Figure 5-2
Bowman et al., 2002
phantastica
•Most extreme phenotype is needle-like, radially symmetrical leaves
•Adaxial cells are replaced by abaxial
Phantastica
•PHAN is a MYB-transcription
factor
•Expressed uniformly
throughout the leaf primordia
•May respond to putative
adaxializing signal from the
SAM
Bowman et al., 2002
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•ago and phn
(ago1/ago1 pnh/+)
mutants unable to
maintain a meristem
•Mutants in AGO
show a reversal of
leaf polarity
Figure 5-4
Kidner and Martienssen, 2004
•DICER cuts primiRNA and premiRNA in plants to
generate a duplex
DNA
•Duplex DNA is
unwound and loaded
onto RISC,
including AGO
•RISC is guided to
mRNA
•mRNA cleaved
Kidner and
Martinssen, 2005
•AGO - PAZ and PIWI domains - common in eukaryotes
•Required for RNA interference (part of RNA interference complex)
that targets mRNA for degradation
•PNH (also PAZ and PIWI domains - overlaps AGO function)
expressed in leaves - first throughout, then in adaxial
•Coordination between regions of cell division and cell differentiation
Wild type
+ phb
phb phb
McConnell and Barton, 1998
phabulosa (dominant) plants are severely adaxialized
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Ectopic buds form on abaxial side of phb leaves
Wild type
Ad
ab
phb phb
adaxialized
phb phb
adaxialized
phb +
Seedling
Wild type
McConnell and Barton, 1998
phb +
Radially symmetrical
McConnell and Barton, 1998
Adaxial cell fate promotes axillary bud development
Kidner and Martienssen, 2004
•PHB encodes homeodomain, leucine zipper (HD-ZIPIII) containing protein
•Loss of function mutations have no phenotype (redundant with
PHAVOLUTA (PHV) and REVOLUTA (REV) - triple mutants are abaxialized)
•Also has a sterol/lipid-binding domain
•PHB is expressed throughout leaf
•May be activated by sterol/lipid ligand only in adaxial region (ligand identity
unknown) - could be a ligand secreted by SAM
•Dominant mutations result in constitutive activation
•In addition, PHB undergoes miRNA induced degradation on abaxial side,
dominant mutant alleles inhibit miRNA induced degradation
•miRNA specific to PHB, PHV and REV is localized initially in meristem,
then on leaf abaxial side - signal from meristem?
•PHB mutations disrupt the miRNA binding site - no degradation on abaxial
side
•Alleles of AGO that affect the PIWI domain look like PHB mutants
•In ago mutants, PHB is ectopically expressed
•ago mutants enhance the phb phenotype
•In ago mutants, miRNA is ectopically expressed, possibly because it never
gets into degradation pathway
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KANADI loss-of-function morphological phenotypes
KANADI genes (family of
3 transcription factors) are
redundant
•Double and triple mutants
have only adaxial tissue trichomes, ectopic axillary
meristems
•Genes are normally
expressed on abaxial side
of organs (e.g. cotyledons)
Figure 5-4
•KANADI genes necessary for YABBY expression(6 gene family =
transcription factors, includes FILAMENTOUS FLOWER (FIL))
•triple KAN mutants do not express YAB
•YABBY gene expression is on abaxial side
Eshed, Y. et al. Development 2004;131:2997-3006
•If express KAN throughout leaf via AS1 promoter, leaves are abaxialized
•Expression of YAB3 throughout leaf also results in abaxialization, suggesting ecotpic
KAN phentoype is the result of ectopic YAB
•Expression of YAB and KAN downregulate PHB and PHV
•Activated PHB downregulates YAB and KAN
+miRNA
Bowman et al., 2002
Bowman et al., 2002
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Figure 5.5
•lam1 tobacco mutants are
normal until P2
•In lamina, adaxial cell types
are replaced by abaxial
•Blade does not grow
•In lam1 L1 chimera, adaxial
epidermis develops normally,
suggesting LAM1 L2 signals
to mutant L1, telling it to be
adaxial
•In lam1 L1/L2 chimera,
palisade develops normally,
but epidermis does not normal L3 enough to inform
interior L2, but not L1,
subepidermal L2
Laminar outgrowth requires juxtaposition of ab- and adaxial cell types
•Leaves that are either adaxialized or abaxialized are needle-like (no
blade outgrowth
Figure 5-6
Phan mutants are
temperature sensitive:
•High temp - radially
symmetrical leaves
•Low temp - normal leaves
•Intermediate leaves, but
adaxial surface has patches
of abaxial cells within it
•Where patch of abaxial
meets adaxial, laminar
outgrowth occurs
•Similar phenomenon
occurs in leafbladeless
mutant of maize
D/V compartment
specification in
wing imaginal
disk
Adaxial cell fate
ad/ab
compartment
specification in
leaf primordium
Margin cells form between leaf and wing
dorsal and ventral compartments
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Sterol?
miRNA?
PHB
KAN,
YAB
•in phantastica mutants, ectopic abaxial cells in adaxial
compartment causes ectopic margin formation and
laminar outgrowth
•Signal from SAM to leaf
•Sterol ligand to activate
PHB, PHV and REV in
adaxial domain?
•miRNA may move from
SAM to abaxial domain,
where it degrades PHB,
PHV and REV mRNA
•At 15C, phan mutant SAM
arrests
•Ectopic expression of YAB
or FIL can arrest SAM
•Suggest that adaxial cell
fate is required to maintain
meristem
Proximal/distal axis - the role of KNOTTED
Figure 5-8
•In maize leaf, sheaf is
proximal, blade is distal
•Ligule and auricles
mark boundary
•Dominant kn1
mutations cause
mesophyll cells along
the lateral veins to
continue division
resulting in knots
•Cells between and over
knots develop into sheath
•KN1 - homeodomain
protein
•mRNA can go through
plasmodesmata
•Wild type expressed in
SAM
•Mutant ectopically
expressed in lateral veins
Figure 5-9
Models for leaf P/D patterning:
a) Morphogen gradient, produced from SAM is high proximally and low distally;
KN1 is required for morphogen activity, ectopic KN1 causes ectopic
morphogen and proximal fate
b) distal cells are recruited first into the leaf primordium, proximal cells last; cells
can measure time in leaf - older cells = distal, younger = proximal; KN1
(normally in SAM) resets timing
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Determinate vs. indeterminate development
Figure 5-10
•Lack of KN1 results in no
SAM
•Ectopic KN1 in leaves
results in meristem activity in
leaves (knots in maize,
shoots in tobacco)
•PHAN and its homologues
RS2 (maize) and AS1
(Arabidopsis) act to repress
KN1 in the leaf (determinate)
Figure 5-10
Compound leaves:
•In tomato, KN1 is expressed not only in SAM, but also in developing
leaves
•Constitutive expression of KN1 results in super-compound leaves
•Compound leaves in pea develop acropetally - requires the pea
homologue of LEAFY, UNIFOLIATA (UNI)
Figure 5-10
Figure 5-11
•Chimeras with albino L2 sectors
•In b), albino sector is also slowly dividing
•In this case, the L2 contribution to the edge is much less
•there must be competition between cells
•Leaf achieves normal size
•If cell size is altered (e.g. ABP1), cell division alters to compensate and vice versa
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