Lecture 4: Leaf and flower development

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Lecture 4 Leaf and flower development

• Leaf development
• Origin of leaves at the shoot apex
• Stages of leaf development
• Leaf morphology
• Genetic regulation of leaf development
• Floral development
• Inflorescence meristem
• Flower organs and architecture
• Genes controlling flower development
• Summary

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Stages of leaf development
Stage 1 Organogenesis a few cells in L1 and L2
divide more rapidly than surrounding cells -gt
produce outgrowth leaf primordium, which
develops into leaf
Stage 2 Development of suborgan
domains primordia acquire identity as specific
parts of leaf this differentiation occurs along
three axes dorsiventral (adaxial abaxial)
proximodistal (basal apical)
lateral (margin blade midrib)
dorsal (adaxial)
ventral (abaxial)
midrib
margin
Stage 3 Cell and tissue differentiation
tissues and cells differentiate as leaf
grows L1 -gt epidermis with trichomes and
guard cells L2 -gt mesophyll cells L3
-gt vascular elements, bundle sheath cells
proximodistal
lateral
blade
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Leaf morphology
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Arrangement of leaf primordia, shapes and margins
is genetically programmed
Willow (alternate)
Arabidopsis (rosette)
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Arrangement of leaf primordia, shapes and margins
is genetically programmed
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Origin of leaves at the shoot apex
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How are shoot organs induced and positioned?
AUXIN!
Leaf primordium
Auxin
0.1 mM
0.3 mM
1 mM
Reinhardt et al. 2000 Plant Cell 12 507
3 mM
10 mM
30 mM
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Spatially regulated gene expression controls leaf
pattern
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KNOX genes - Regulation around leaf primordia
KNOX Knotted-like homeobox act as
transcription factors major role in regulating
developmental pathways in all eukaryotes

• Gain-of-function mutation kn1 causes
• cell proliferation after normal cell
• division ceases
• Division planes are abnormal, causing
• distortion of the blade surface ?
• knotted appearance

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KNOX genes regulation around leaf primordia
KNOX Knotted-like homeobox act as
transcription factors major role in regulating
developmental pathways in all eukaryotes Class I
KNOX genes expressed in shoot apical meristems,
not in leaf primordia STM SHOOT
MERISTEMLESS KNAT1 KNOTTED-LIKE
ARABIDOPSIS THALIANA 1 KNAT2
KNOTTED-LIKE ARABIDOPSIS THALIANA 2 Class II
KNOX genes more diverse patterns of expression
KNAT3 KNOTTED-LIKE ARABIDOPSIS
THALIANA 3 MYB transcription factors AS
ASYMMETRIC LEAVES expressed in leaf
primordia and developing cotyledons
35SKNAT1
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Model for gene action in SAM and leaf primordia
to control leaf development
lack of AS1 function ? ectopic expression of the
KNAT1 and KNAT2
WT
35SKNAT1
35SKNAT1
as mutant phenotype crinkled asymmetric
leaves abnormally arranged leaf veins serrations
in the leaf lamina multiple midveins on leaves
Lobed leaves of varying severity
Ectopic meristems form in nodes and initiate new
leaves
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Control of leaf size by AINTEGUMENTA (ANT)
Link to cell cycle if progress of cell cycle is
retarded or ceases earlier than normal in leaf
primordium, number of cells in leaf lamina
decreases and, at the same time, each leaf cell
tends to grow larger than the wild-type cells.
Mizukami and Fischer 2000 PNAS 97 942
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Floral meristem and floral organ development
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Floral meristem and floral organ development
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PIN1 is essential for the formation of lateral
organs from the inflorescence meristem
As plants initiate reproductive development, the
vegetative meristem is transformed into an
intermediate primary inflorescence meristem that
produces floral meristems on its flanks
In WT, inflorescence meristem generates a stem
bearing cauline leaves and floral buds
pin1 mutants produce inflorescence meristem (only
axial tissue), but no lateral organs
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pin1 may be rescued by application of auxin
Reinhardt et al. 2000 Plant Cell 12 507
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pin1 may be rescued by application of auxin
(D) Inflorescence apex of a pin1-1 mutant plant
devoid of flowers. The arrowhead indicates
the meristem. (E) to (H) Induction of flowers by
treatment with 1 mM IAA (red paste) at the flank.
Apices were analyzed 38 hr (arrow points to
local bulge) (E), 4 days (F), 7 days (G),
and 11 days (H) after treatment. Arrowheads in
(E) to (H) denote the meristem. (I) Induction
of a circular bulge (arrow) around the flank by
treatment for 2 days with 1 mM IAA at the
top of the meristem (arrowhead). (J) Circular
flower induced after 6 days by treatment as given
in (I). Note that the circular flower does
not correspond to a single normal flower but
rather is a ring of several flowers that
are laterally fused consequently, petaloid
organs are found on the adaxial side of
the circular carpel. In the center, the
indeterminate inflores- cence meristem is
maintained (not visible). (K) Flowers (arrows)
induced after 4 days by treatment as given in
(I) in this case, the initial ring bulge
broke into several individual flowers. Two of
these are partially fused (on the lower
left side). C, carpel Co, cotyledon F, flower
primordium P, petal Pa, papilla Pr, leaf
primordium. Bars in (A) to (C), (G), and (H)
500 µm bars in (D) to (F) and (I) 100 µm
bars in (J) and (K) 200 µm.
Reinhardt et al. 2000 Plant Cell 12 507
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Floral organs
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Floral architecture
B
A
C

• Floral organs are
• initiated sequentially by
• the floral meristem
• produced as successive
• whorls (concentric circles)

• ABC model explains determination
• of floral organ identity
• A alone ? sepals
• AB ? petals
• BC ? stamen
• C alone ? carpels

Floral organ identity is specified by five MADS
box genes APETALA1 (AP1) APETALA2
(AP2) PISTELLATA (PI) AGAMOUS (AG) APETALA3
(AP3)
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MADS box genes control flower development
Loss of A function -gt spread of C function
Loss of B function -gt only expression of A and C
Loss of C function -gt spread of A function
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Control of floral organ identity
Floral Organ Identity Genes
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ap1 ap2 ap3/pi ag quadruple mutant
Leaf-like structures in place of floral organs
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Meristem identity gene LFY regulates meristem
function
Inflorescence meristem producing floral
meristems br bracts subtending secondary
inflorescences, which replace early-arising
flowers (arrowhead)
LEAFY (LFY) transcription factor
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How are genes regulated in the floral meristem?
SAM
FM
Nakajimaa and Benfey 2002 Plant Cell 14(Suppl)
s265
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Summary
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Summary