1. Florigen is the systemic signal that initiates flowering in plants. It was discovered in 1936 by Mikhail Chailakhyan and its protein nature was revealed in 2007, encoded by the FLOWERING LOCUS T (FT) gene. 2. Florigen is transported through the phloem, is graft transmissible, and acts as a quantitative and nearly universal signal. It is produced in leaves in response to photoperiod, transported to the shoot apical meristem (SAM) where it forms a complex with FD to activate downstream genes controlling flowering. 3. At the SAM, meristem identity genes and floral organ identity genes are expressed to determine floral structure based on

1. 1
PRESENTED BY:
Shalini Sharma
SUBMITTED TO :
P. G. DEPARTMENT OF BOTANY
Sanmati Govt. College of Science Education and Research,
Jagraon (Ludhiana)

2. MEANING
• “Flora + genno”
Flora – flower, genno - to beget
• Systemic signal that initiates
flowering in plants.
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3. HISTORY
• Discovery: Mikhail Chailakhyan (1936)
postulated existence of flowering hormone
• Julius Von Sachs (1985) confirmed the
presence of Florigen in formation of flowers.
• Corbesier et al. and Tamaki et al. (2007)
revealed protein nature, encoded by
FLOWERING LOCUS T (FT) genes.
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Mikhail Kristoforovich Chailakhyan
(1902 – 1991)

4. MOLECULAR NATURE OF FLORIGEN
• Recent advances revealed the molecular
nature of Florigen as a protein (Corbesier et
al. 2007, Tamaki et al. 2007).
• Mol. Wt. : 20 – 25 kDa.
• FT in Arabidopsis and Hd3a in Rice
(orthologous) encodes for Florigen.
Crystal structure of Hd3a Florigen
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5. CHARACTERISTICS OF FLORIGEN
SYSTEMIC SIGNAL
TRANSPORT THROUGH PHLOEM
GRAFT TRANSMISSIBLE
A NEARLY UNIVERSAL SIGNAL
QUANTITATIVE
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6.  SYSTEMIC SIGNAL
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SDP SDP

7.  TRANSPORT THROUGH PHLOEM
• Enters translocation stream by
diffusion through plasmodesmata
(Yoo et al., 2013 in Cucurbita
moschata).
• A selective mechanism is thought
to regulate delivery of Florigen at
SAM.
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FTmRNA
FT Protein

8.  GRAFT TRANSMISSIBLE
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9.  A NEARLY UNIVERSAL SIGNAL
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10.  QUANTITATIVE
e.g.- Xanthium – 1 SD cycle, Impatiens balsamina- 3 SD cycles
Summative nature in Impatiens balsamina (SDP) (by K. K. Nanda)
PHOTOPERIOD INDUCTION
---- LD YES
LD LD NO
In most short day plants
16LD 16LD YES
In Impatiens balsamina
SD SD SD
SD SD SD
SD SD SD
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11. MECHANISM OF ACTION
The mechanism can be broken down into three steps:
1. PHOTOPERIOD - REGULATED PRODUCTION
2. SIGNAL TRANSLOCATION
3. INITIATION OF FLOWERING AT SHOOT APICAL
MERISTEM
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12. 1. PHOTOPERIOD - REGULATED PRODUCTION:
• Critical photoperiod varies from species to species.
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13. For Example:
SD PLANTS LIGHT DARK
Maryland Mammoth 12 hrs or less 12 hrs or more
Xanthium 15.5 hrs or less 8.5 hrs or more
LD PLANT LIGHT DARK
Hyoscyamus niger 11 hrs or more 13 hrs or less
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14. PERCEPTION OF PHOTOPERIODIC STIMULUS
• Perceived by leaves with the help of
phytochrome and cryptochrome
• Photoperiodic induction can also take place
in a leaf that has been separated from a
plant.
Flowers
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15. ROLE OF PHYTOCHROME IN FLOWER INITIATION
• A protein pigment that absorbs red and far-red light.
• Two interconvertible forms: Pr (660nm) and Pfr (730nm).
LDP
Short night critical/ High Pfr
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Low Pfr :Pr
High Pfr : Pr
red

16. ROLE OF CRYPTOCHROME IN FLOWER INITIATION
• A protein pigment that absorbs blue light.
• In Arabidopsis mutation in one of the cryptochrome gene,
CRY2, caused a delay in flowering and an inability to
perceive inductive photoperiods ( Guo et al. 1998).
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17. INITIATION OF SIGNAL IN LEAVES
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Red Far-red
Pfr Pr
Far-red Red Blue
Pr Pfr CRY

18. 2. SIGNAL TRANSLOCATION
• Via phloem.
• FTIP1 facilitates FT movement from
companion cell to sieve element.
• Along with sugars in the phloem.
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19. MOVEMENT OF FLORIGEN AT SAM
• Florigen binds to a phospholipid: phosphatidylcholine (PC)
(Nakamura et al., 2014). Two models have been proposed :
1. PC, a component of nuclear membrane, import Florigen into
nucleus from cytosol.
2. Vesicles containing PC function in trafficking of Florigen to FD.
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20. 3. INITIATION OF FLOWERING AT SAM
At SAM, two processes leads to floral transition:
I. Formation of Florigen Activation Complex
II. Downstream gene expression
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21. I. FORMATION OF FAC:
FLORIGEN ACTIVATION COMPLEX21

22. II. DOWNSTREAM GENE EXPRESSION
• FAC activates the meristem
identity genes.
• These genes are necessary for
initial induction of organ identity
genes, controls floral identity.
• In Arabidopsis, AGAMOUS-LIKE 20
(AGL20/ SOC1), APETLA1 (AP1)
and LEAFY (LFY)
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23. FLORAL ORGAN IDENTITY GENES
• In Arabidopsis, there are 5 floral organ identity genes: APETALA1
(AP1), APETALA2 (AP2), APETALA3 (AP3), PISTILLATA (PI) and
AGAMOUS (AG) (Bowman et al., 1989; Weigel and Meyerowitz,
1994) .
TYPE A
Sepal and Petal
TYPE B
Petal and Stamen
TYPE C
Stamen and Carpel
AP 1 , AP 2 AP 3 , PI AG
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24. DETERMINATION OF FLORAL ORGAN IDENTITY
• In 1991, the ABC
model was proposed
by Elliot Meyerowitz
and Enrico Coen, to
explain how
homeotic genes
control organ
identity.
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25. FORMATION OF INFLORESCENCE MERISTEM
• CONSTANS (CO) also suggested to
induce, directly or indirectly,
expression of TERMINAL FLOWER 1
(TFL1) gene.
• TFL1 controls inflorescence meristem
identity and delays the transition to
the reproductive phase (Simon et al.,
1996).
• It represses expression of several
genes downstream of Florigen such as
LEAFY (LFY) and APETALA 1 (AP1)
Inflorescence
Meristem
TFL1 Floral Meristem
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26. OTHER FUNCTIONS OF FLORIGEN
Formation of storage organs
e.g., Tuber formation in Potato (Navarro et al. 2011)
Bulb formation in Onion (Lee et al. 2013)
Fruit development in Pineapple (Lv et al. 2012)
Timing of reproduction
Plant Architecture
e.g., Leaf development in Rice (Tsuji et al. 2013)
Promotes lateral branching in the axillary meristem in Rice ( Tsuji et al.
2015).
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27. MECHANISM OF ACTION DURING OTHER FUNCTIONS
(StFDL 1a/1b)
(OsFD2 in Rice)
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28. ANTIFLORIGEN
• Transmissible inhibitors of flowering.
• Produced in leaves of LDPs during non-inductive photoperiod.
• Graft transmissible.
Experiment : by Lang et al. 1977
Grafting of uninduced LDP Nicotiana sylvestris onto a DNP
Nicotiana tabacum cultivar Trapezond
During short days ---------- No flowering in DNP
During long days ---------- Flowering
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29. CONCLUSION
When discovered Florigen was considered as a
flowering hormone. Recent studies show its nature as a
globular protein. But its exact molecular nature is still a
mystery as all attempts to isolate and identify Florigen
have been largely unsuccessful.
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30. REFERENCES
Malik, C. P. and Shrivastava, A. K. (2005). Textbook of Plant Physiology.
Kalyani Publishers, New Delhi-Ludhiana
Salisbury, F.B. and Ross, C. (1995). Plant Physiology ed 1st. CBS
Publishers and Distributors, Shahdara, Delhi
Taiz, L. and Zeiger, E. (2003). Plant Physiology ed 3rd. Panima Publishing
Corporation, New Delhi-110002
https://www.researchgate.net/publication/318667245_Molecular_f
unction_of_florigen
https://www.cell.com/molecular-plant/pdf/S1674-2052(15)00094-
5.pdf
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31. 31THANK YOU