The document discusses regulation of floral development in plants. It describes how flowers are modified shoots consisting of four whorls of modified leaves called sepals, petals, stamens, and carpels. It explains that three classes of genes regulate floral development: meristem identity genes, floral organ identity genes, and boundary genes. The ABC model is described which explains how combinations of A, B, and C class floral organ identity genes specify sepals, petals, stamens and carpels. The document also discusses environmental and endogenous factors that influence the transition from vegetative to reproductive growth such as photoperiodism and phase changes in the shoot apical meristem.

1. Regulation of Floral
Development
ঠাস্ ঠাস্ দ্রুম দ্রাম, শুনে লানগ খটকা
ফু ল ফফানট ? তাই বল ! আমম ভামব পটকা !
Semester V
Botany Core Course XI
Dr. Riddhi Datta
Department of Botany
Dr. A.P.J. Abdul Kalam Government College

2. • The flower consists of several leaf-like structures attached to a specialized
region of the stem called the receptacle.
• Calyx (unit: sepal):
• It represent the outermost whorl and protect the inner whorls in buds.
• They can photosynthesize.
• Corolla (unit: petal):
• It primarily attracts insects to serve as pollinators and are often showy and
brightly-colored appearance.
• Androecium (unit: stamen):
• It is the male sexual structure.
• The stamen consists of a narrow stalk called the filament and a
chambered structure called the anther.
• The anther contains tissue that gives rise to pollen grains.
• Gynoecium (unit: carpel):
• It is the female sexual structure.
• The carpel consists of the stigma (the tip where pollen lands during
pollination), the style (an elongated structure), and the basal ovary.
• The ovary encloses one or more ovules.
• Each ovule, in turn, contains an embryo sac, the structure that gives rise
to the female gamete, the egg.
Parts of a flower
© Dr. Riddhi Datta

3. © Dr. Riddhi Datta
Flower is a modified determinate shoot
Homology of the floral bud with vegetative bud
• Floral and vegetative buds both emerge either in terminal or in
axillary position
• The floral buds may sometimes get modified in to vegetative buds
or bulbils (e.g. Agave, Allium). Thus proving that the two are
analogous structures.
Axis nature of receptacle
• The internodes in floral axis remain highly reduced, yet in a
number of plants such as Capparis, Passiflora etc. the receptacle
shows prominent nodes and internodes.
• In certain plants (eg., rose, pear, etc.), sometimes the receptacle of
the flower continues its growth even after producing all the four
types of floral appendages and then produces normal foliage
leaves.
• In Michelia champaca the thalamus can elongate like an ordinary
stem beyond carpels and bears aggregate fruit.

4. © Dr. Riddhi Datta
Flower is a modified determinate shoot
Foliage nature of floral appendages
• Foliage leaves (phyllotaxy) and floral appendages (aestivation)
have identical arrangement on the stem.
• Sometimes, sepal can be modified to an enlarged leaf-like
structure as seen in Mussaenda.
Transition of floral leaves
• In nature, in many cases, such as Nymphaea (water lily) all
degrees of transition from sepals to petals and from petals to
stamens can be seen.
• In Canna, the stamens and the style become petaloid.
• In Zinnia, some of the stamens and carpels become petaloid or
sepaloid.

5. Floral development
• Transition to flowering involves major changes in the pattern of morphogenesis and cell
differentiation at the shoot apical meristem leading to the formation of the floral organs.
• These events are collectively referred to as floral evocation.
• The developmental signals that bring about floral evocation include:
 Endogenous factors: circadian rhythms, phase change, and hormones
 External factors: day length (photoperiod) and temperature (vernalization)
• In the case of photoperiodism, transmissible signals from the leaves, collectively referred to as the
floral stimulus, are translocated to the shoot apical meristem.
• The interactions of these endogenous and external factors enable plants to synchronize their
reproductive development with the environment.
© Dr. Riddhi Datta

6. Floral meristems and floral organ development
• The transition from vegetative to reproductive development is
marked by an increase in the frequency of cell divisions within the
central zone of the shoot apical meristem.
• As reproductive development commences, the increase in the size of
the meristem is largely a result of the increased division rate of these
central cells.
LS through a vegetative (A) and a reproductive (B) shoot apical region of Arabidopsis
A B
LS through a vegetative shoot apical region
© Dr. Riddhi Datta

7. Four Different Types of Floral Organs Are Initiated as
Separate Whorls
• Floral meristems initiate four different types of floral organs
in concentric rings (called whorls): sepals, petals, stamens,
and carpels.
• In Arabidopsis flower, the whorls are arranged as follows:
 The first (outermost) whorl consists of four sepals, which
are green at maturity.
 The second whorl is composed of four petals, which are
white at maturity.
 The third whorl contains six stamens, two of which are
shorter than the other four.
 The fourth whorl is a single complex organ, the
gynoecium or pistil, which is composed of an ovary with
two fused carpels, each containing numerous ovules, and
a short style capped with a stigma.
© Dr. Riddhi Datta

8. Three Types of Genes Regulate Floral Development
• Three classes of genes regulate floral development:
• Meristem identity genes are necessary for the initial induction of the organ identity
genes. These genes are the positive regulators of floral organ identity.
• Floral organ identity genes directly control floral identity. The proteins encoded by
these genes are transcription factors that likely control the expression of other genes
whose products are involved in the formation and/or function of floral organs.
• Cadastral genes act as spatial regulators of the floral organ identity genes by setting
boundaries for their expression.
© Dr. Riddhi Datta

9. Meristem Identity Genes Regulate Meristem Function
• Meristem identity genes must be active for the proper floral meristem development.
• In Arabidopsis, three genes must be activated to establish floral meristem identity:
 AGAMOUS-LIKE 20, also called SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1
(AGL20/SOC1)
 APETALA1 (AP1)
 LEAFY (LFY)
• AGL20 serves as a master switch initiating floral development by integrating signals from both
environmental and internal cues.
• Once activated, AGL20 triggers the expression of LFY, and LFY turns on the expression of AP1.
• AP1 expression also stimulates the expression of LFY (positive feedback loop).
© Dr. Riddhi Datta

10. Homeotic Mutations Led to the Identification of Floral Organ
Identity Genes
• The genes that determine floral organ identity were discovered as floral homeotic
mutants.
• Mutations in these genes resulted in development of the floral organs in the wrong
place.
• Because mutations in these genes change floral organ identity without affecting the
initiation of flowers, they are homeotic genes.
• Five different genes are known to specify floral organ identity in Arabidopsis:
 APETALA1 (AP1)
 APETALA2 (AP2)
 APETALA3 (AP3)
 PISTILLATA (PI)
 AGAMOUS (AG)
© Dr. Riddhi Datta

11. Homeotic Mutations Led to the Identification of Floral Organ
Identity Genes
• The homeotic genes encode transcription factors—proteins that control the
expression of other genes.
• Most plant homeotic genes belong to MADS box genes.
• The MADS domain enables these transcription factors to bind to specific DNA
motifs.
• Not all genes containing the MADS box domain are homeotic genes.
• For example, AGL20 is a MADS box gene, but it functions as a meristem identity
gene.
© Dr. Riddhi Datta

12. Three Types of Homeotic Genes Control Floral Organ Identity
• The organ identity genes initially were identified through homeotic mutations:
 Plants with the ap2
mutation lack sepals and
petals.
 Plants bearing ap3 or pi
mutations produce sepals
instead of petals in the
second whorl, and carpels
instead of stamens in the
third whorl.
 And plants homozygous for
the ag mutation lack both
stamens and carpels.
© Dr. Riddhi Datta

13. The ABC model for floral development
• In 1991 the ABC model was proposed to explain how homeotic genes control organ identity.
• The ABC model for the acquisition of floral organ identity is based on the interactions of the
three different types of activities of floral homeotic genes: A, B, and C.
 Activity of type A alone specifies
sepals.
 Activities of both A and B are
required for the formation of petals.
 Activities of B and C form stamens.
 Activity of C alone specifies carpels.
© Dr. Riddhi Datta

14. Type A activity:
• Encoded by AP2
• Controls organ identity in the first
and second whorls.
• Loss of activity results in the
formation of carpels instead of
sepals in the first whorl, and of
stamens instead of petals in the
second whorl.
© Dr. Riddhi Datta

15. Type B activity:
• Encoded by AP3 and PI
• Controls organ determination in the
second and third whorls
• Loss of activity results in the
formation of sepals instead of petals
in the second whorl, and of carpels
instead of stamens in the third
whorl.
© Dr. Riddhi Datta

16. Type C activity:
• Encoded by AG
• Controls events in the third and fourth
whorls.
• Loss of activity results in the formation
of petals instead of stamens in the third
whorl, and replacement of the fourth
whorl by a new flower such that the
fourth whorl of the ag mutant flower is
occupied by sepals.
© Dr. Riddhi Datta

17. • Quadruple-mutant plants (ap1, ap2, ap3/pi, and ag) produce floral
meristems that develop as pseudoflowers. All the floral organs are replaced
with green leaflike structures, although these organs are produced with a
whorled phyllotaxy.
• This demonstrates that the floral organs are highly modified leaves.
© Dr. Riddhi Datta

18. The ABCE model (Quartet model) • A class genes specify sepals in the first whorl.
• A and B class genes specify petals within the
second whorl.
• B and C class genes specify stamens within the
third whorl
• C class gene function specifies carpel identity
within the fourth whorl.
• The E class genes (SEPALLATA 1-4) are active
within all four whorls.
• Combinatorial interactions of floral organ
identity factors within each whorl form
tetrameric complexes.
• These floral organ identity factors can act as
pioneer factors, influencing chromatin
accessibility throughout flower development.
• Additionally, class D genes regulate ovule
development.
Thomson et al. 2017 (Plant Physiology)

19. FLORAL EVOCATION: INTERNAL
AND EXTERNAL CUES
© Dr. Riddhi Datta

20. Flowering is regulated by:
 Internal factors controlling the switch to reproductive development:
• Age
• Size
• Number of leaves
 External cues controlling seasonal response:
• Photoperiodism (Light/dark cycle) and light quality (wavelength and intensity)
• Vernalization (Low temperature)
• Total light radiation
• Water availability
• External cues and internal developmental factors together fine tune flower evocation in most plants.
• When flowering occurs strictly in response to internal developmental factors and does not depend on
any environmental conditions, it is referred to as autonomous regulation.
© Dr. Riddhi Datta

21. THE SHOOT APEX AND PHASE CHANGES
• In higher plants, developmental changes occur in a single,
dynamic region, the shoot apical meristem.
• During postembryonic development, the shoot apical
meristem passes through three well-defined developmental
stages:
 The juvenile phase
 The adult vegetative phase
 The adult reproductive phase
• The transition from one phase to another is called phase
change.
• Juvenile tissues are produced first and are located at the base
of the shoot.
© Dr. Riddhi Datta

22. Competence and determination are two stages in floral evocation
• To initiate floral development, the cells of the meristem must first become competent i.e. capable
of responding to floral stimulus (induction).
• A competent vegetative meristem responds to a floral stimulus (induction) to becoming florally
determined (committed to producing a flower).
• Once determined, it will flower even after removal of the floral stimulus.
© Dr. Riddhi Datta

23. Competence and determination are two stages in floral evocation
• Competent vegetative shoot (scion) grafted onto a flowering stock will flower.
• Reason: It is capable of responding to floral stimulus present in the stock.
• The grafted scion will fail to flower before attaining competence.
• Reason: Its shoot apical meristem is not yet able to respond to the floral stimulus present in the
stock..
© Dr. Riddhi Datta

24. Competence and determination are two stages in floral evocation
• A bud is said to be determined if it progresses to the next developmental stage (flowering) even
after being removed from its normal context.
• Thus a florally determined bud will produce flowers even if it is grafted onto a vegetative plant that
is not producing any floral stimulus
© Dr. Riddhi Datta

25. Demonstration of the determined state of axillary buds in tobacco
• In a day-neutral tobacco, plants typically flower after producing about 41 leaves or nodes.
• If a flowering tobacco plant is decapitated just
above the 34th leaf, the axillary bud of the 34th leaf
grows out, and flowers after producing 7 more
leaves (for a total of 41).
• If the 34th bud is excised from the plant and either
rooted or grafted onto a stock without leaves near
the base, it produces a complete set of 41 leaves
before flowering.
• Reason: The 34th bud was not yet florally
determined.
© Dr. Riddhi Datta

26. Demonstration of the determined state of axillary buds in tobacco
• In a day-neutral tobacco, for example, plants typically flower after producing about 41 leaves or
nodes.
• If a flowering tobacco plant is decapitated just above
the 37th leaf, the 37th axillary bud flowered after
producing 4 more leaves in all three situations.
• Reason:
 The 37th bud was already florally determined.
 The number of nodes a meristem produces
before flowering is a function of two factors:
• Strength of the floral stimulus from the leaves
• Competence of the meristem to respond to the
signal
© Dr. Riddhi Datta

27. PHOTOPERIODISM: MONITORING DAY LENGTH
• Photoperiodism is the ability of an organism to detect day length which
makes it possible for an event to occur at a particular time of year, thus
allowing for a seasonal response.
• Circadian rhythms (Biological clock) and photoperiodism have the
common property of responding to cycles of light and darkness.
© Dr. Riddhi Datta

28. Plants can be classified by their photoperiodic responses
• The classification of plants according to their photoperiodic responses is
usually based on flowering.
• The two main photoperiodic response categories are:
 Long-day plant (LDPs)
 Short-day plants (SDPs)
© Dr. Riddhi Datta

29. Critical day length
• Flowering in LDPs is promoted only when the day length exceeds a certain duration,
called the critical day length, in every 24-hour cycle.
• Promotion of flowering in SDPs requires a day length that is less than the critical day
length.
• The absolute value of the critical day length widely varies among species.
© Dr. Riddhi Datta
LDPs flower:
Day length > critical day length
SDPs flower:
Day length < critical day length

30. Critical day length
• Both Xanthium (a SDP) and Arabidopsis (a LDP) flowers under 12 hours of
photoperiod.
• Critical day length of Xanthium is 15 hours and it flowers when photoperiod is less than
15 hours).
• Critical day length of Arabidopsis is 11 hours and it flowers when photoperiod is greater
than 11 hours.
© Dr. Riddhi Datta
Since, 12 hours of photoperiod
is less than 15 hours and
greater than 11 hours, both
Xanthium and Arabidopsis can
flower.

31. Plants can be classified by their photoperiodic responses
 Long-day plant (LDPs):
• Flower only in long days (qualitative LDPs) or flowering is accelerated
by long days (quantitative LDPs).
• Flowering promoted only when the day length exceeds critical day
length, in every 24-hour cycle.
• LDPs measure the lengthening days of spring or early summer and delay
flowering until the critical day length is reached.
• Ex: Triticum aestivum
© Dr. Riddhi Datta

32. Plants can be classified by their photoperiodic responses
 Short-day plants (SDPs):
• Flower only in short days (qualitative SDPs) or flowering is accelerated
by short days (quantitative SDPs)
• Promotion of flowering in SDPs requires a day length that is less than the
critical day length.
• SDPs flower in fall, when the days shorten below the critical day length.
• Ex: Chrysanthemum morifolium
© Dr. Riddhi Datta

33. Plants can be classified by their photoperiodic responses
• However, day length alone is an ambiguous signal because it cannot distinguish
between spring and fall.
• For avoiding the ambiguity of day length signal, plants often couple a
temperature requirement to a photoperiodic response.
• Other plants avoid seasonal ambiguity by distinguishing between shortening and
lengthening days and are called ―dual–day length plants‖.
© Dr. Riddhi Datta

34. Plants can be classified by their photoperiodic responses
The ―dual–day length plants‖ fall into two categories:
• Long-short-day plants (LSDPs): flower only after a sequence of long days
followed by short days. LSDPs,
• Ex: Bryophyllum, Kalanchoe, and Cestrum nocturnum flower in the late
summer and fall, when the days are shortening
• Short-long-day plants (SLDPs): flower only after a sequence of short days
followed by long days. SLDPs
• Ex: Trifolium repens, Campanula medium, and Echeveria harmsii, flower in
the early spring in response to lengthening days.
© Dr. Riddhi Datta

35. Plants can be classified by their photoperiodic responses
• Day neutral plants (DNPs): Species that flower under any photoperiodic
condition and are insensitive to day length.
• Flowering in DNPs is typically under autonomous regulation—that is, internal
developmental control.
• Ex: Phaseolus vulgaris, Castilleja chromosa, and Abronia villosa
© Dr. Riddhi Datta

36. Plants Monitor Day Length by Measuring the Length of the Night
© Dr. Riddhi Datta
SDPs
light periods longer than
critical value, followed by
sufficiently long nights
Flowering
short days were followed
by short nights
No flowering
LDPs
light periods shorter than
critical value, followed by
sufficiently short nights
Flowering
Long days were followed
by long nights
No flowering

37. Night breaks can cancel the effect of the dark period
© Dr. Riddhi Datta
• The dark period can be made ineffective
by interruption with a short exposure to
light, called a night break.
• But interrupting a long day with a brief
dark period does not cancel the effect of
the long day.
• When given during a long dark period,
a night break promotes flowering in
LDPs and inhibits flowering in SDPs.
• A night break was found to be most
effective when given near the middle of
a dark period.

38. Leaf Is the Site of Perception of the Photoperiodic Stimulus
© Dr. Riddhi Datta
• Treatment of a single leaf of a SDP with short photoperiods is sufficient to cause flowering, when the
rest of the plant is exposed to long days.
• But, treatment of the shoot apex with short photoperiods doesn‘t induce flowering if the rest of the plant
is exposed to long days
• In response to photoperiod, leaf transmits a signal that regulates the transition to flowering at the shoot
apex. The photoperiod-regulated processes that occur in the leaves resulting in the transmission of
a floral stimulus to the shoot apex are referred to collectively as photoperiodic induction.

39. Floral stimulus is transported via the phloem
© Dr. Riddhi Datta
• Once produced, the floral stimulus is transported to the meristem via phloem, and it appears to
be chemical in nature.
• Treatments that block phloem transport, such as girdling or localized heat-killing prevent
movement of the floral signal.
• The floral stimulus is translocated along with sugars in the phloem and it is subject to source–
sink relations.
• An induced leaf positioned close to the shoot apex is more likely to cause flowering than an
induced leaf at the base of a stem, which normally feeds the roots.

40. PHYTOCHROME AND PHOTOPERIODISM
© Dr. Riddhi Datta

41. Phytochrome Is the Primary Photoreceptor in Photoperiodism
© Dr. Riddhi Datta
• Phytochrome is a protein pigment that absorbs red and far-red light most
strongly, but that also absorbs very low amount of blue light.
• Phytochrome can interconvert between Pr and Pfr forms:
 Phytochrome is synthesized in a red light–absorbing form (Pr) which is
blue to the human eye.
 Pr is converted by red light to a far-red light–absorbing form (Pfr) which is
blue-green.
 Pfr, in turn, can be converted back to Pr by far-red light.
 This phenomenon is known as photoreversibility.
• Pfr is the physiologically active form of phytochrome
*Darkness also converts Pfr to Pr.

42. Phytochrome Is the Primary Photoreceptor in Photoperiodism
© Dr. Riddhi Datta
 Similarly, under far-red light irradiation, an
equilibrium of 97% Pr and 3% Pfr is achieved.
 Reason: Very small amount of far-red light
absorbed by Pr makes it impossible to convert
Pfr entirely to Pr by far-red light.
 This equilibrium is called photostationary state.
Photostationary state:
 The proportion of phytochrome in the Pfr form after saturating irradiation by red light is
only about 85%.
 Reason: Most of Pr absorb red light and are converted to Pfr, but some Pfr also absorbs
red light and are converted back to Pr

43. Phytochrome is a dimer composed of two polypeptides
© Dr. Riddhi Datta
• Native phytochrome occurs as a homo-dimer.
• Each subunit consists of:
• a light-absorbing pigment molecule called the chromophore
• a polypeptide chain called the apoprotein.
• The chromophore is attached to the apoprotein through a thioether linkage to a
cysteine residue
• Both the chromophore and protein undergo conformational changes during Pr-Pfr
interconversion.
Phytochrome Is the Primary Photoreceptor in Photoperiodism

44. Phytochrome Is the Primary Photoreceptor in Photoperiodism
© Dr. Riddhi Datta
PHY genes encode two types of phytochrome:
 Phytochrome is encoded by a multigene family consisting of five members: PHYA, PHYB, PHYC,
PHYD, and PHYE.
 Type I phytochrome:
• Encoded by PHYA gene
• Transcriptionally active in dark-grown seedlings
• Expression is strongly inhibited in light in monocots and less dramatically in dicots.
• PfrA is also unstable.
unstable

45. Phytochrome Is the Primary Photoreceptor in Photoperiodism
© Dr. Riddhi Datta
PHY genes encode two types of phytochrome:
 Type II phytochrome:
• Encoded by PHYB, PHYC, PHYD, PHYE genes
• Expression of their mRNAs is not significantly changed by light.
• Proteins are more stable in the Pfr form.

46. Phytochrome is the primary photoreceptor in photoperiodism
© Dr. Riddhi Datta
• In SDPs, a night break becomes
effective only when the supplied dose
of light (Red light) is sufficient to
saturate the photoconversion of Pr to
Pfr.
• A subsequent exposure to far-red light,
which photoconverts the pigment back
to the physiologically inactive Pr form,
restores the flowering response.

47. Phytochrome is the primary photoreceptor in photoperiodism
© Dr. Riddhi Datta
Reason:
• During night, Pfr gets converted to Pr.
• A long night length is required to convert
sufficient amount of Pfr to Pr.
• If a flash of red light is given, Pr absorbs
red light and gets converted back to Pfr.
• So, the effective night length gets
reduced and sufficient Pfr is not
generated.

48. Phytochrome is the primary photoreceptor in photoperiodism
© Dr. Riddhi Datta
Reason:
• If the flash of red light is followed by a
flash of far-red light, the Pfr again gets
converted to Pr and this nullifies the
effect of red light flash.
• Pfr/Pr > 1 …….Flowering in LDPs
• Pfr/Pr < 1 …….Flowering in SDPs

49. Phytochrome is the primary photoreceptor in photoperiodism
© Dr. Riddhi Datta
• In LDPs, a night break of red light
promoted flowering, and a
subsequent exposure to far-red light
prevented this response.

50. A Blue-Light Photoreceptor Also Regulates Flowering
© Dr. Riddhi Datta
• In some LDPs, such as Arabidopsis, blue light can promote flowering.
• Cryptochrome is the major blue light photoreceptor that regulates
flowering.

51. GENETIC AND MOLECULAR REGULATION OF
FLOWERING
© Dr. Riddhi Datta

52. © Dr. Riddhi Datta
Five genetically distinct developmental pathways control flowering. These are:
 Photoperiodic pathway
 Vernalization pathway
 Autonomous pathway
 Sucrose pathway
 Gibberellin pathway
Transition to flowering involves multiple factors
and pathways

53. © Dr. Riddhi Datta
Photoperiodic pathway
It involves phytochromes (PHYA and PHYB acting
antagonistically) and cryptochromes.
In Arabidopsis (LDP)
• In the morning, PHYB represses CONSTANS (CO)
gene expression.
• In the evening, PHYA and cryptochrome interacts
with circadian clock genes to induce CO
expression.
• CO is a transcription factor which induces
FLOWERING LOCUS T (FT).
• FT protein (‗Florigen‘) travels from the induced
leaf to the shoot apex via phloem.
• FT forms a complex with transcription factor FD.
• FT-FD complex activates downstream genes like
SOC1, LFY, etc which in turn activates the floral
homeotic genes for floral development.

54. © Dr. Riddhi Datta
Photoperiodic pathway
In rice (SDP)
• In the morning, PHYB represses Heading date
1 (Hd1) which is the homologue of CO.
• In the evening, PHYA interacts with circadian
clock genes to induce Hd1 expression.
• Here, Hd1 acts as an inhibitor of flowering.
• Absence of Hd1 stimulates expression of
Hd3a.
• Hd3a (‗Florigen‘) travels from the induced
leaf to the shoot apex via phloem for floral
development.

55. © Dr. Riddhi Datta
Coincidence model
• Transition to flowering occurs when the expression peak of CO coincides with the light phase.
• PHYA/PHYB perceive light signals and entrain the clock.
• The clock genes control the diurnal expression of CO and its expression peaks between 12 hours of light and dawn
(SuaÂrez-LoÂpez et al., 2001; Nature).
• In long days, the CO expression peak
coincides with the light phase (coincidence
phase).
• In light, the CO protein is stable and can
activate FT expression, promoting flowering.
• In short days, CO expression peaks in the
dark (no coincidence of light phase and CO
expression).
• In the dark, CO protein is degraded and thus
cannot induce FT expression and further
processes.
(Singh et al., 2016; New Phytologists)

56. Florigen concept
© Dr. Riddhi Datta
• During flowering, a biochemical signal is produced in photoperiodically induced
leaves and is then transmitted to the shoot apex where it stimulates flowering.
• This mode of action resembles a hormonal effect.
• Hence, Mikhail Chailakhyan postulated the existence of a universal flowering
hormone, florigen (1930s).
• Evidence: non-induced receptor plants were stimulated to flower by being grafted
onto a leaf or shoot from photoperiodically induced donor plants.
• Movement of the floral stimulus from a donor leaf to the stock across the graft union
correlated closely with the translocation of 14C-labeled assimilates via vascular
continuity across the graft union.

57. © Dr. Riddhi Datta
• Since 1930s, many attempts to isolate florigen have remained unsuccessful.
• A major breakthrough is the identification of FLOWERING LOCUS T (FT) in
Arabidopsis.
• FT is a downstream target of CO and is expressed in companion cells of leaves (like
CO).
• FT is a small globular protein induced by CO in response to long days.
• FT then travels from leaf to shoot apex via phloem to induce flowering.
• It can travel through grafting unions and induce flowering in competent recipient plants.
• Therefore, FT (or its homologue, Hd3a in rice) is the florigen.
Identification of florigen

58. © Dr. Riddhi Datta
• FT mRNA is synthesized in leaf companion cells.
• Endoplasmic reticulum protein, FT
INTERACTING PROTEIN 1 (FTIP1), helps the
transport of FT from companion cells to sieve
tubes.
• FT protein then moves via phloem translocation
stream to shoot apex.
• In the shoot apex (floral meristem), FT enters
nucleus to bind FLOWERING LOCUS D (FD)
which is a transcription factor.
• FT-FD complex activates floral identity genes like
SOC1 and AP1.
• SOC1 then activates LFY which further activates
the floral homeotic genes for floral development.
Regulation of floral induction by FT

59. VERNALIZATION: PROMOTING FLOWERING WITH COLD
© Dr. Riddhi Datta
• Vernalization is the process whereby flowering is promoted by a cold treatment given
to a fully hydrated seed (i.e., a seed that has imbibed water) or to a growing plant.
• Commonly, a combination of cold treatment followed by long day exposure is required.

60. Vernalization results in competence to flower at the shoot apical meristem
© Dr. Riddhi Datta
• The effective temperature range for
vernalization is from just below freezing to
about 10°C.
• The effect of cold increases with the duration
of the cold treatment until the response is
saturated.
• Vernalization can be lost as a result of
exposure to devernalizing conditions, such as
high temperature, but the longer the exposure
to low temperature, the more permanent the
vernalization effect.

61. Vernalization results in competence to flower at the shoot apical meristem
© Dr. Riddhi Datta
• Vernalization takes place primarily in the shoot apical meristem.
• Localized cooling causes flowering when only the stem apex is chilled and is largely
independent of the temperature experienced by the rest of the plant.
• In case of seed vernalization, fragments of embryos consisting essentially of the
shoot tip are sensitive to low temperature.
• Vernalization results in the acquisition of competence of the meristem to undergo the
floral transition.

62. Vernalization involves epigenetic changes in gene expression
© Dr. Riddhi Datta
• Changes in gene expression that are stable even after the signal that induced the
change (in this case cold) is removed are known as epigenetic regulation.
• The involvement of epigenetic regulation in the vernalization process has been
confirmed in the LDP Arabidopsis.

63. Vernalization involves epigenetic changes in gene expression
© Dr. Riddhi Datta
• In Arabidopsis, a gene, FLOWERING
LOCUS C (FLC), acts as a repressor of
flowering.
• FLC is highly expressed in non-vernalized
shoot apical meristems and negatively regulates
SOC1.
• After vernalization, FLC is epigenetically
(conversion from euchromatin to
heterochromatin by histone methylation)
switched off permitting flowering in response
to long days to occur.
• In the next generation, however, the gene is
switched on again, restoring the requirement for
cold.

64. © Dr. Riddhi Datta
Autonomous pathway:
• Here, flowering occurs in response to internal
signals—the production of a fixed number of leaves,
age, etc.
• The autonomous pathway acts by reducing the
expression of the flowering repressor gene
FLOWERING LOCUS C (FLC), an inhibitor of LFY.
Sucrose pathway:
• Sucrose stimulates flowering in Arabidopsis by
increasing LFY expression.
Gibberellin pathway:
• Required for early flowering and for flowering under
non-inductive short days.
• GA induces GA-MYB (transcription factor) mediated
activation of LFY.
GA
GA-MYB

65. Thank you…
© Dr. Riddhi Datta