Flower development BL5400

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, The plant cell vol. 17, pp

General developmental events during flower morphogenesis 1.The flower primordium is visible as a bulge on the inflorescence meristem 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 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 num- ber. 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 meris- tem. (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

(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 sta- men 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 gynoe- cium 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, 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, , 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

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, 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.

deficiens gene is involved in the flower morphogenesis in snapdragon Sommer et al, The EMBO Journal vol 9 no. 3 pp Phenotypic expression of defA-1 mutant in A. majus

(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, The Plant Cell, Vol. 2, A-group specific gene, APETALA1 control petal development

A-group specific gene, APETALA2 is required for formation of whorl 1 and whorl 2 Central role in floral meristem establishment Strong ap2 mutants, sepals are transformed into carpels and petal development is supressed Regulate floral organ development Wild type flower and ap2 mutant Jokufu et al, The plant cell vol 6 pp

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 Krizek and Meyerowitz, Development 122, Wild type and pistilata flower

-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. AGAMOUS, C-group specific gene

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, Nature 405, B and C floral organ identity functions require SEPALLATA MADS-box genes

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, Nature Genetics vol 33 pp

Summary