Transcript pistilata

Flower development
Steps of flower development
• Apical meristem
• Inflorescence meristem
• Flower meristem on which flowers develop
Genes, genes, genes!!!
• Floral meristem genes
• Oragan identity genes
• Organ boundry genes
• Organ polarity genes
Flower meristem genes
• LEAFY from Arabidopsis and its ortholog from Antirrhinum,
FLORICAULA influence flower meristem and replaced by shoots
with inflorescence like properties
• Floral meristem identity is promoted by a MADS box
transcription factor gene orthologs SQUAMOSA and APETALA1
• The homeobox TF WUSCHEL promotes the production of
CLAVATA3 ligand in the overlying stem cells and this ligand
moves sideways and downwards and likely to interact
CLAVATA1 receptor, which apparently control the number of
stem cells
• This regulation is important in terminating the meristem growth
for floral meristem determination.
Smith, 2005. The plant cell vol. 17, pp. 330-341
General developmental events during flower
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Flower development: stages 1-7. (A) SEM image of the inflorescence meristem (i) and floral stages 1-5. The floral stages are labeled with the corresponding number. The medial (m) and lateral (l) axes are labeled on a stage 5 flower. The abaxial (ab) and adaxial (ad) sides of a stage 4 flower are labeled relative to the inflorescence meristem. (B) SEM of a late stage 5 floral meristem that has formed a flattened oval where the gynoecium will arise. Arrowheads point to the petal primordia and two of the medial
stamens are labeled
The flower primordium is visible as a bulge on the inflorescence
2. The floer primordium enlarges and becomes seperated from the
inflorescence meristem
3. Sepal primordia form
4. Sepal primordia partially cover the floral meristem
5. Petal and stamen primordia are initiated. The floral meristem expands to
form a platform on which gynoecium will develop
The Arabidopsis book
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(C) SEM of a stage 6 flower showing the beginning of formation of the gynoecium as a ridge of raised cells around a central cleft (arrow). A lateral stamen is labeled l. (D) Longitudinal section of a stage 6 gynoecium. The arrow points to the central cleft. (E) Cross section of a stage 6 gynoecium. (F) SEM of a stage 7 gynoecium showing the vertical growth of the tube. (G) Longitudinal section of a stage 7 gynoecium. (H) Transverse section of a late-stage 7 or early-stage 8 gynoecium. Scale bar
in D represents 22 μm. A-D: from Sessions, 1997. E-G: Reprinted from Current Topics in Developmental Biology, 45, Bowman, J.L., Baum, S.F., Eshed, Y., Putterill, J., and
Alvarez, J., Molecular genetics of gynoecium development in Arabidopsis, 155-205, Copyright (1999), with permission from Elsevier. H: from Hill and Lord, 1989.
Organ identity genes
ABC model
Model describes the interactions of different genes that control floral
organ identity
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ABC model overview
• Simple rule that underlie the whorl specifications using floral
homeotic mutants
• Class A mutants have carpels in the first whorl instead of sepals,
and stamens in the second whorl in place of petals
• Class B mutants have sepals rather than petals in the second
whorl and carpels in the rather than stamens in the third whorl
• Class C mutants have petals instead of stamens in the third
whorl and sepals instead of carpels in the fourth
Theien, 2001. Nature vol. 414 p.491
What are MADS box genes?
The MADS box is a highly conserved sequence motif found in a family of
transcription factors. The conserved domain was recognized after the first
four members of the family, which were MCM1, AGAMOUS, DEFICIENS
and SRF (serum response factor). The name MADS was constructed form
the "initials" of these four "founders".
The MADS box genes in flowering plants are the "molecular architects" of
flower morphogenesis.
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deficiens gene is involved in
the flower morphogenesis in
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Phenotypic expression of defA-1 mutant in A. majus
Sommer et al, 1990. The EMBO Journal vol 9 no. 3 pp. 605-613
A-group specific gene, APETALA1 control petal development
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(A) Wild type. Bar = 100 ^m.
(B) Diagram of wild type. The adaxial sepal is adjacent to
the inflorescence axis (indicated by small circle).
(C) Homozygous ap1-1 mutant flower. Bar = 200 ^m.
(D) Diagram of ap1-1 flower.
Irish and Sussex, 1990. The Plant Cell, Vol. 2, 741-753
A-group specific gene, APETALA2 is required for
formation of whorl 1 and whorl 2
Wild type flower and ap2 mutant
Central role in floral meristem establishment
Strong ap2 mutants, sepals are transformed into carpels and petal development is supressed
Regulate floral organ development
Jokufu et al, 1994. The plant cell vol 6 pp.1211-1225
B class genes APELATA 3 and PISTILATA controls
development of whorl 2 and 3
* APETALA and PISTILATA genes encode MADS domain and are necessary and
sufficient to specify petal and stamen identity in the flower
•AP3/PI hetero-dimer binds to the sequence in the AP3 promoter that are necessary
for AP3t expression and can activate transcription in absence of protein synthesis
*study was carried out by combination of class A and C genes
*Phenotypic analysis of AP3/PI over-expression lines indicated their additional roles
in proliferation of floral meristem
Wild type and pistilata flower
Krizek and Meyerowitz, 1996. Development 122, 11-22
AGAMOUS, C-group specific gene
-Flowers with this mutation have petals in whorl 3 instead of stamens, and
sepals in whorl 4 instead of carpels.
-the floral meristem is not determinate - flowers continue to form within the
flowers, so the pattern of organs (from outside to inside) is: sepal, petal,
petal; sepal, petal, petal; sepal, petal, petal, etc.
B and C floral organ identity functions require SEPALLATA MADS-box genes
a, Wild-type flower consisting of four sepals, four petals, six stamens and two fused carpels. b, sep1 sep2 sep3 triple
mutant flower in which the four petals and six stamens are replaced by sepaloid organs and carpels are replaced by
a new flower that repeats this same phenotype. In addition, there is internode elongation between internal flowers,
presumably because of a functional ERECTA gene. c, Dissected sep1 sep2 sep3 triple mutant flower with first-whorl
sepals (top), second and third whorl sepaloid organs (middle), and a new flower (bottom) that replaces the carpels.
d, pi ag (bc) double mutant that reiterates the same sepal, sepal, sepal phenotype. e, Abaxial surface of wild-type sepal.
f, Abaxial surface of wild-type petal. g, Abaxial surface of sep1 sep2 sep3 second-whorl sepaloid organ. h, Abaxial
surface of sep1 sep2 sep3 third-whorl sepaloid organ. Arrows indicate several stomata. Scale bar in e–h: 50 microm.
Pelaz et al, 2000. Nature 405, 200-203
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A, proposed model on the basis of homeotic mutants such as ag and genetic interactions among such mutants
b. Cloning and gain of function mutants in various combinations
c. Current understanding of the activation of floral identity genes
Pruitt et al, 2003. Nature Genetics vol 33 pp. 294-304
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