Flowering involves the sequential action of two groups of genes: those that switch the fate of the meristem from vegetative to floral (floral meristem identity genes) and those that direct the formation of the various flower parts (organ identity genes.)

1. CC06 PRESENTATION
ON
TRANSITION OF FLOWERING
NAME- SAKSHI PRIYA
CLASS- M.Sc
SEMSTER- II
SESSION- 2021-2023

2. Unlike some animal systems in which the germ line is set
aside during early embryogenesis, the germ line in plants is
established only after the transition from vegetative to
reproductive development—that is, flowering.
 The vegetative and reproductive structures of the shoot are all
derived from the shoot meristem formed during
embryogenesis.
The general body plan of plants is established during
embryogenesis, when the undifferentiated meristematic regions
of root and shoot are set aside

3. .
However, much of plant development occurs postembryonically,
through the reiterative production of organ primordial at the shoot
apical meristem (SAM).

4. In most species, the SAM initially gives rise to vegetative organs
such as leaves, but at some point the SAM makes the transition to
reproductive development and the production of flowers.
This change in the developmental fate of primordial initiated at the
SAM is controlled by environmental and endogenous signals

5. Flowering involves the sequential
action of two groups of genes:
those that switch the fate of the
meristem from vegetative to floral
(floral meristem identity genes),
 and those that direct the formation
of the various flower parts (organ
identity genes.

6. VEGETATIVE SHOOT REPRODUCTIVE SHOOT
 It is usually branched.  It is seldom branched.
 Growing point is not used up.  Growing point is used up in production
of floral organs
 Growth is usually unlimited.  Growth is limited.
 Nodes occur along the entire length  Nodes are borne only towards the tip
of thalamus.
 Internodes are quite distinct and long.
This is essential for proper placing of
leaves.
 Internodes are quite indistinct because
the nodes are crowded in order to
reduce space between adjacent
whorls.
 A vegetative shoot bears only one type
of dissimilar lateral outgrowths called
foliage leaves.
 A flower possesses four types of
dissimilar lateral outgrowths -sepals,
petals, stamens and carpels.
 Leaves usually possess stipules and
axillary buds.
 Floral leaves or organs do not bear
stipules and axillary buds.

7. VEGETATIVE APEX ANATOMY REPRODUCTIVE APEX

8. The flower is generally defined as a highly specialized
reproductive shoot, comparable to leaves- bearing shoot.
The sepals and petals may be regarded as modified leaves.
Stamens and carpels also show some resemblance to leaves in
position, arrangement, internal structure and development.

9. CHARACTERS VEGETATIVE
SHOOT
REPRODUCTIV
E SHOOT
• EMERGE IN TERMINAL OR AXILLARY
POSITION
 
• NODES AND INTERNODES
PRESENT
 
• IDENTICAL ARRANGEMENT ON THE
STEM OF PHYLLOTAXY
 
• Aestivation: PREFOLIAGE LEAF OF
LEAF APPENDAGE AND FLORAL
APPENDAGE
 
• THAT SEPAL IS A MODIFIED LEAF
CAN BE CLEARLY SEEN IN
Mussaenda.
 

10. Many species must reach a certain age or size before they can
flower, the vegetative meristem is thought to first pass through a
“juvenile” phase in which it is incompetent to respond to internal or
external signals that would trigger flowering in an “adult” meristem.
The acquisition of reproductive competence is often marked by
changes in the morphology or physiology of vegetative
structures—leaf shape is one example in a process known as
vegetative phase change.
It is likely that some of the genes identified as important in
controlling the transition from vegetative to reproductive
development are also involved in vegetative phase change.

12. 1) ENVIRONMENTAL FACTORS - The timing of flowering is
primarily influenced by environmental factors, which serve to
communicate the time of year and/or growth conditions favorable
for sexual reproduction and seed maturation
PHOTO
PERIOD
LIGHT
QUALITY
PHOTON
FLUX
DENSITY
VERNALI
ZATION
WATER
AVAILABILIT
Y
NUTRIEN
T

13. 2)INTERNAL FACTORS- Other species are less sensitive to
environmental variables and appear to flower in response to
internal cues such as plant size or number of vegetative nodes.
NUTRIENT
DEFICIENCY
OVERCROWDING
DRUGHT

14. MODELS FOR THE CONTROL OF
FLOWERING TIMING
1. FLORIGEN CONCEPT- It was proposed by Chailakhyan 1936
that florigen, a flower-promoting hormone, was based on the
transmissibility of substances or signals across grafts between
reproductive “donor” shoots and vegetative “recipients.” was
produced in leaves under favorable photoperiods and
transported to the shoot apex in the phloem.
 The identification of a graft-transmissible floral inhibitor also led
to the concept of a competing “antiflorigen.” Many research years
were consumed hunting for florigen in the phloem sap, but its
chemical nature has remained elusive.

15. 2)The nutrient diversion- The inability to separate the
hypothetical flowering hormones from assimilates led to a
hypothesis.
This model proposed that inductive treatments result in an increase
in the amount of assimilates moving to the apical meristem, which
in turn induces flowering

16. THE MULTIFACTORIAL
CONTROL MODEL
The view that assimilates are the only important component in
directing the transition to flowering was superseded by the
multifactorial control model, which proposed that a number of
promoters and inhibitors, including phytohormones and assimilates,
are involved in controlling the developmental transition .
According to this model, flowering can only occur when the limiting
factors are present at the apex in the appropriate concentrations
and at the right times. This model attempted to account for the
diversity of flowering responses by proposing that different factors
could be limiting for flowering in different genetic backgrounds
and/or under particular environmental conditions.

18. FIRST FORMULATED BY GEORGE HAUGN AND CHRIS SOMERVILLE IN 1988.
EXPERIMENTAL PLANTS- Arabidopsis thaliana AND Antirrhinum majus
 Both species have four verticils (sepals, petals, stamens and carpels), which are
defined by the differential expression of a number of homeotic genes present in each
verticil.
This means that the(i) sepals are solely characterized by the expression of A genes,
(ii) while the petals are characterized by the co-expression of A and B genes.
 (iii)The B and C genes establish the identity of the stamens and
(iv)the carpels only require C genes to be active.
ABC MODEL OF FLOWERING

19. Shoot vegetative
meristem FLOWER
FLORAL
MERISTEM
FLORAL ORGAN
IDENTITY GENES
FLORAL
MERISTEM
IDENTITY GENES
EX- LEAFY (LFY)
APETALA I(API)
CAULIFLOWER(CAL)
EX- IN Arabidopsis
APETALA I(AP1)
APETALA 2(AP 2)
APETALA 3(AP3)
PISTILLATA( PI)
AGAMOUS(AG)
FLOWERING GENE- THE FLORAL DEVELOPMENT INVOLOVES FIRST THE
TRANSITION OF VEGETATIVE MERISTEM INTO FLORAL MERISTEM WHICH
THEN FORMS THE FLOWER.

20. The ABC model of flower development in angiosperm
demonstrates the presence of three classes of genes that regulate
the development of floral organs.
The genes are referred to as class A genes, class B genes and
class C gene. These genes and the interaction between them
induce the development of floral organs.

21. The genes are referred to as-
Class A genes- APETALA I(AP1) and APETALA 2(AP 2)
Class B genes- APETALA 3(AP3) and PISTILLATA( PI)
Class C genes- AGAMOUS(AG)
Arabidopsis flower has four whorls- sepal, petal, carpel, stamen
IN Arabidopsis thaliana

22. Mutations that affect flower
development
Alter the organs formed in two adjacent whorls:
 A mutants : affect sepals and petals
B mutants : affect petals and stamens
C mutants : affect stamens and carpels
The function of class A genes is confined to whorls 1 and 2.
 Similarly the function of class C gene is restricted in whorls 3 and 4.
This can be interpreted in another way. In the whorls 1 and 2 the
function of class A genes prevents class C gene from functioning in the
same whorls. Similarly the function of class C gene prevents class A
genes from functioning in the whorls 3 and 4.

23. Any mutation in class A genes with defects in floral organ development will invite
class C gene to express in whorls 1 and 2.
The class C gene, in class A mutants, will express in whorls 1 and 2 in addition to
the normal whorls 3 and 4.

24. https://www.slideserve.com/langer/controls-on-flowering
https://www.mdpi.com/2223-7747/11/10/1318/htm
https://youtu.be/YUfPjqrLo0s
https://www.cell.com/abstract/S0092-8674%2806%2900571-X
https://www.slideshare.net/SnehaSahu20/transition-o-of-flowering-
plants
https://www.biologydiscussion.com/plants/flower-a-modified-shoot-
with-diagram/6271
https://slideplayer.com/slide/13050848/
https://www.ncbi.nlm.nih.gov/books/NBK10122/
https://www.biologydiscussion.com/flower/process-of-transition-in-
flowers-with-diagram-plants/69465
https://academic.oup.com/plcell/article/10/12/1973/5999400
https://www.biologydiscussion.com/plants/flower-a-modified-shoot-
with-diagram/6271