Flower development

flower development from embryo

1. Flower development
Floral development begins with the conversion
of vegetative meristems to flowering
meristems

2. The switch to flowering occurs through the induction of floral developmental
genes in response to environmental signals. Different plants respond to either
a short photoperiod or a long photoperiod to initiate flowering. Specifically,
leaves detect photoperiod and flowering is induced by a transmissible
signalling molecule called florigen. This hormone-like molecule is produced in
the leaves and is transported to the apical vegetative meristem via the phloem.
Some plants require a vernalisation stage where exposure to cold
temperatures before the correct photoperiod is required to trigger flowering.
Hormones and the stage of maturity of the plant also play a part in flower
initiation. These requirements ensure flowers are developed at the right point
in the season to maximise the plants success.

4. Flower Evocation: The events occurring in the shoot apex that
specifically commit the apical meristem to produce flowers.
Internal factors
Phase change
Hormones
External factors
Light
Temperature
Total light radiation
Water availability

5. The ABC model of flower development

6. The proteins encoded by the ABC genes are MADS box transcription factors.
These transcription factors include a MADS box sequence that binds DNA and
a K-box sequence for dimerization. Therefore, these proteins form dimers on
DNA. ABC genes activate the expression of other genes that cause cells of the
meristem to form different parts of the flower.
Activity of each class of genes is required in the cells of two adjacent floral
whorls. Each floral organ type originates from a specific region in the floral
meristem due to the expression of ABC genes in developmental fields. Each
class of gene is expressed in specific parts of the meristem. However, there is
areas of overlap in gene expression

8. 1. Gene A activity controls the first and second whorls
2. Gene B activity controls the second and third whorls
3. Gene C activity controls the third and fourth whorls.
Three Types of Genes Control Floral Identify

9. ABC Model for Flower Development

10. Genetic analysis of floral development using homeotic mutants
Mutations that occur in hox (or homeotic) genes (genes which regulate the
development of anatomical structures) are called homeotic mutations. These
genes encode transcription factors that control development by regulating the
identity of body parts to eventually give rise to the body plan. Homeotic
mutants are displayed as transformation of one body part into another. These
mutations were used in the model plant Arabidosis thaliana to determine the
model for floral development. Plants with mutated ABC genes produce
homeotic mutant flowers.

11. Losing class A or C genes makes the remaining gene extend it's activity into
the absent genes’ developmental field. Thus class A and C genes are mutually
antagonistic; the functional proteins produced inhibit expression of the
opposing gene. Additionally, the class C genes terminate stem cell
maintenance which allows cells to differentiate into floral organ cells.
Class A mutant: sepals (Figure 1, green) and petals
(Figure 1, red) transformed into stamens (purple) and
carpels (blue). Class C gene activity was not
inhibited by class A genes, therefore it's activity
spread into the other whorls.

12. Class B mutant: petals (Figure 1, red) and
stamens (Figure 1, purple) transformed
into sepals (green) and carpels (blue). In
the absence of class B genes, ABC genes
cannot work together to activate petal
and stamen development.
Class C mutant: stamens (Figure 1, purple)
and carpels (Figure 1, blue) transformed into
petals (red) and sepals (green). Activity of class
A genes was not inhibited by class C genes,
therefore their activity spread into the other
whorls.

13. Double mutant plants helped to determine the inhibitory characteristics of
the class A and C genes.
Class B and C mutant: only sepals form in all four whorls.
Class A and B mutant: only carpels form in all four whorls.
Class A and C mutant: leaves form in whorls 1 and 4, petal/stamen
intermediates form in whorls 2 and 3.

17. The ABC model gets a bit more complicated…
A triple ABC mutant plant does not form flowers at all however ectopic
expression of the ABC genes does not form a flower either. Something else
was needed for flower development. Using multiple gene mutants, it was
found that four SEPALLATA genes (E genes), which are also MADS box
transcription factors, act redundantly together with ABC genes to specify
floral organ identity. Moreover, E genes are necessary for ovule
development along with D genes. Thus, the general ABC model expanded to
an ABCDE model (Figure 3), although other variations exist.

18. the respective ABC transcription factors work in combination with SEPALLATA
transcription factors by forming combinations of tetrameric complexes. These
four protein complexes act together to regulate genes involved in
development of parts of the flower. For instance, the development of carpels
requires class C genes, so the class C transcription factor AGAMOUS forms a
complex with three other proteins (in this case two SEPALLATAs and another
AGAMOUS) to promote transcription of genes for carpel development

19. But how do vegetative meristems convert to flowering meristems?
When leaves produce the chemical signal florigen, it is transmitted to apical
vegetative meristems to initiate the conversion process by turning on the
Flowering Locus T gene which then switches on the expression of LEAFY.
LEAFY is a transcription factor that binds as a dimer to the promoters of ABC
genes to turn on the floral organ identity genes to then cause undifferentiated
cells in the meristem to change fate and develop as flowers rather than shoots. A
LEAFY mutant makes a leafy plant because the developmental ground state of a
floral organ is a leaf since vegetative meristems first need to convert to floral
meristems. The vegetative meristem carries on making leaves as there are no ABC
transcription factors to switch on floral genes and so there is no transition from
vegetative growth to flowering.

20. Overexpressing LEAFY in Arabidopsis thaliana, results in a short terminal flower.
LEAFY activity is regulated by Terminal Flower 1 (TFL1) which represses LEAFY
activity. In a TFL1 mutant, LEAFY is no longer repressed and a single terminal
flower is developed. Conversely, when TFL1 is constitutively expressed,
flowering is delayed.
So how do asymmetrical flowers develop?
Snapdragon flowers (Antirrhinum majus) are not radially symmetrical but bilaterally
symmetrical. If floral developmental genes function in designated regions of the floral
meristem, how does asymmetry develop? There are two genes involved in the
asymmetrical growth of Antirrhinum flowers; CYCLOIDEA and DICHOTOMA. Double
mutants for these two genes result in radially symmetrical flowers.

21. Is this model conserved amongst flowering plants?
Evidence suggests that the ABC model is an ancient regulatory system and may
apply to most angiosperms. Homologues of ABC genes are found across many
model plant species including Petunia hybrida (petunia), Oryza saliva (Asian
rice) and Zea Mays (maize) to name a few. Separate genes may be involved in
specifying diverse flowers such as the bilaterally symmetrical flowers of
Antirrhinum but the ABC model can be applied to nearly all flowering plants.

22. Thanks