Flower development in Arabidopsis thaliana is controlled by several key gene pathways. Flowering time genes determine how long the plant remains in a vegetative state before flowering. At least five pathways interact to control flowering time in response to factors like photoperiod and vernalization. Floral identity genes such as LFY, AP1, AP2, and CAL control the transition of shoot meristems into floral meristems and developing flowers. Organ identity genes including AP1, AP2, AP3, PI, and AG specify the development of floral organs into sepals, petals, stamens or carpels. Mutations in these genes disrupt normal flower development.

1. FLORAL INDUCTION AND DEVELOPMENT-GENETIC
AND MOLECULAR ANALYSIS

2. CONTENTS:-
Introduction
Flowering time genes influence the duration of Vegetative
growth
Floral-Identity Genes And Identity Genes Overlap In Time
And Function

3. Introduction:-
•Flower development provides a model system to study
mechanisms that govern pattern formation in plants. Most flowers
consist of four organ types that are present in specific order from the
periphery on the center of the flower. Hear are studies on flower
development in one model species. Arabidopsis thaliana
•Flower development is initiated when that signal arrives at the shoot
apical meristem. During the vegitative state the shoot apical
meristem is programmed to produce leafe primordial.
• When the floral signals arrives from the leaf the Meristem acquires
floral identity and secondary inflorescence meristems, or floral
primordia, arise in the axils of the uppermost leaf primordia.

4. FLOWERING-TIME GENES INFLUENCE THE
DURATION OF VEGETATIVE GROWTH
 Flowering time genes provide the connection between florigen, or
the floral induction signal , and the transition to the floral organs.
 flowering time mutants many therefore interfere with the
production of the signal in the leaf, translocation of the signal to the
apex, or its activity in the apex.
 Most flowering time mutants identified thus far causes plants to
flower later than normal, indicating that the mutant interfere with
pathways that normally promote flowering.
 Note that flowering time, as it is used here, refers to a
developmental time rather than chronological time. For ex.
Arabidopsis is a facultative long day plant with a critical
photoperiod of 8 to 10 hour.

5.  It is interesting to note that ,although a large number of late-
flowering mutants have been described, no single Arabidopsis
mutants that remains vegetative indefinitely has yet been
identified.
 This fits with the general assumption that there are multiple
pathways controlling flowering time with a certain amount of
built in redundancy.
 Redundancy provides that inactivation of genes in one
pathway is at least partially compensated for by other genes or
complmentary pathways.
 In Arabidopsis, at least five separate, but interacting,
pathways for controlling flowering time have been identified.

7.  Several flowering time mutants, including fac, ld, and fve, flower
later than wildtype plants under both LD and SD conditions but
remain sensitive to vernalization.
 Because fiowering in the mutans is equally affected under both
LD and SD conditions, the corresponding wildtype genes are
thought to be active in an autonomous pathway that monitors
developmental stage and initiates flowering in response to
internal developmental signals.
 Such a signal is commonly reflected in a minimum leaf number
that must be achieved before flowering can proceed.

8.  Flowering of a second group of mutans, including constans
(CO) and gigantea (GI),is delayed under LD conditions, but
not under SD response.
 The CO gene is believed to be a central component in the
photoperiodic or long day pathway and is responsible for
promoting the mobile floral stimulus FT.
 Both CO and GI have been cloned and studied in some detail.
 It appears that GI operates before CO in the same pathway
and that floral promotion under long days depends on the
amount of CO protein and, subsequently, FT protein that is
produced.
 The action of FT at the shoot apical meristem is at least in part
mediated by a transcription factor FD.

10. FLORAL-IDENTITY GENES AND IDENTITY
GENES OVERLAP IN TIME AND FUNCTION
 While the principal effect of flowering time mutants
is on the duration of vegetative development,
mutations in the floral-identity genes disrupt the
transition of the floral identity genes disrupt the
transition of the undifferentiated primordia to floral
meristem.At least four identity genes have been
isolated from Arabidopsis:
1. LEAFY(LFY)
2.APETALA1(AP1)
3.APETALA2(AP2)
4.CAULIFLOWER(CAL)

11.  The LEAFY gene appears to play a key role in floral
meristem identity .This can be demonstrated by placing
the gene under control of a strong promoter from the
cauliflower mosaic virus. LEAFY appears to have a
central role in flowering Process. It is probably the
principal target of the mobile stimulus FT when it arrives
in the meristem . LEAFY, in turn activates organ identity
genes such as APETALA1.
 The Arabidopsis flower is rather typical among
advanced flowering plants, consisting of four distinct
whorls of floral organs. The outermost whorl consists of
four sepals, Which are green and leaf like. The next
whorl consists of four yellow petals.

12.  The third whorl consists six stamen, or male
reproductive organs, and the inner most whorl
contains two fused carpels at the base of the female
reproductive structure, the pistil.

13.  Mutations in combination with studies of temporal and spatial
expression patterns have identified five genes that are
involved in the determination of organ identity:
APETALA1(AP1), APETALA2(AP2), APETALA3(AP3),
PISTILATA(P1) , and AGAMOUS(AG).
 Note that AP1Vand AP2 have both been previously identified
as floral identity genes as well.
 Mutations in the organ-identity genes generally result in the
modification, displacement, or total absence of floral organs.
 In addition , mutations in any one of these genes generally
influence the development of two adjacent floral organs.

14.  The influence of organ-identity genes on the development
of the Arabidopsis flower can best be understood by
viewing the floral meristrem as three overlapping
developmental fields or fields gene activity ;Designated
A,B and C field A includes the sepals and petals, field
B includes the petals and stamens, and field C includes
stamens and central carpels. This view is referred to as
the ABC model for floral organ specification.

16. REFERENCE:-
 Introduction to Plant Physiology.
William G. Hopkins
Norman P. A. Huner
 WWW.wiley.com/college/hopkins

17. Thank You