Physiological and Molecular Mechanisms Underlying Flowering

Physiological and Molecular Mechanisms Underlying Flowering

1. DOCTROAL SEMINAR ON Physiological and Molecular Mechanisms Underlying Flowering Department of Plant Physiology, AB & MAPs Indira Gandhi Krishi Vishwavidyalaya Raipur, Chhattisgarh-492012 Presented by- Ujjwal kumar Ph.D 1 st year Doctoral Seminar I

2. Contents

3. It is the reproductive structure found in flowering plants. A flower is a metamorphosed shoot which is consist of sepal, petal, stamen, pistil & receptacle. The biological function of a flower is to facilitate reproduction, usually by providing a mechanism for the union of gametes. This can be of two types: a) Female Flower: A flower that lacks stamens is said to female flower or pistillate flower. b) Male Flower: A flower that does not possess pistil is called a male flower or staminate flower. What is a Flower ?

4. Flowering is a crucial event in the life cycle of flowering plants, as it enables them to reproduce and produce new generations of plants. It is a transitional phase in the life cycle of a plant. It takes place by the transformation of vegetative apex into a reproductive structure . Shoot meristem is reduced to develop sepals, petals, etc . The plant must attain the specific state of “ Ripeness to respond ” before it flowers. Once the stage is reached, then it can induce to flowering. What is flowering? Izawa 2021

5. Why flowering is important? Flowering is an essential process in the life cycle of flowering plants. It plays a vital role in the reproductive success and survival of these plants . some reasons why flowering is important : Reproduction: Genetic Diversity : flowers attracts promote cross-pollination and facilitating the mixing of genetic material and this is essential for the adaptation and evolution of plant species , enhancing their ability to survive changing environmental conditions . Food Production: Ecosystem Services: it provides nectar as a food source for pollinators, such as bees and butterflies, which are crucial for the pollination of other plants. This interaction between flowers and pollinators contributes to the stability and functioning of ecosystems, including the maintenance of biodiversity. Aesthetic and Cultural Value: Flowers are often admired for their beauty and aesthetic appeal. They have cultural and symbolic significance in many societies and are used in various rituals, ceremonies, and celebrations. Flowers also have economic value in the form of the floral industry, which includes activities such as flower cultivation, floristry, and horticulture.

6. Onset of flowering can be considered under three steps 1. Plant must acquire competence to respond to inductive signals . 2. Perception of the signal resulting it’s transition from vegetative to reproductive phase . 3. Differentiation of shoot apex into floral organ. Juvenile Phase Vegetative Phase Reproductive Phase/ Floral Evocation Phase transition Gradual Phase transition Abrupt Floral Initiation Kalra et al. 2018

7. Plant Growth Stages A plant pass through its growth on different phases Juvenile stage: in which it will not flower Transitional stage: have both juvenile and mature tissue Mature stage : in which appropriate environmental stimuli will evoke flowering Reproductive stage : in which flowering actually takes place Senescence stage: the final stage in plant life cycle Juvenile stage Transitional stage Mature stage Reproductive stage Senescence stage Kalra et al. 2018

8. Flower initiation It is a process in which shoot meristem is specifically committed to produce flowers. Factors – 1. External factors- eg. Light, Temperature 2. Internal factor- eg. Leaf number, Plant height Carbohydrate content, Hormone level Self regulatory & Internal factor play role in flowering Depend on external factor like temperature & light Based on both, internal and external factor Floral Evocation Izawa 2021

9. Four genetically regulated pathways leading to flowering have been identified: ( i ) Vernalization pathway (ii) Gibberellic acid pathway (iii) Autonomous pathway (iv) Photoperiodic pathway Pathway Regulating Flowering All these pathways converge to activate Floral Meristematic Identity genes such as LYF and AP1 genes Matsoukas et al. 2012; Liu et al. 2021

10. Vernalization Pathway Regulating Flowering Liu et al. 2021

11. Gibberellic acid Pathway Regulating Flowering Izawa 2021

12. Autonomous Pathway Regulating Flowering Goslin et al. 2017

13. Photoperiodic Pathway Regulating Flowering Goslin et al. 2017

14. The plants in order to flower require a certain day length i.e. the relative length of day and night which is called a photoperiod. The response of plants to the photoperiod expressed in the form of flowering is called as photoperiodism. There are five types of plants on the basis of photoperiodism. Short day plants (SDP)- eg. Chrysanthemum Long day plants (LDP)- eg. Wheat, Arabidopsis Short long day plants (SLDP)- eg. Trifolium repens Long short day plants (LSDP)- eg. Bryophyllum Day neutral plants (DNP)- eg. Phaseolus vulgaris Garner Allard Photoperiodism

15. Florigen Concept A mobile molecule that is synthesized in leaves in response to favorable photoperiod and migrates through the vascular system to the apical meristem to promote floral initiation W.W. Garner and H.A. Allard In 1936, Chailakhyan Sachs (1985) also confirmed the presence of florigen in formation of flowers. Identity of florigen remains one of the major mysteries in plant biology. florigen Matsoukas et al. 2012

16. Mechanism of Florigen Concept Photoperiodism CO gene CO protein T.F for F.T gene m-RNA F.T protein Move from leaf to SAM through Pholem SAM F.T protein F.T protein+ FD Compex LYF gene AP1 & SOC1 vegetative flowering F.T properties similar to florigen Matsoukas et al. 2012

17. Shoot Apical Meristem is the site of cold perception during vernalization and that vernalization causes the meristem to become competent to flower. Vernalin compound is responsible for vernalization stimulus & get activated at low temperature. Vernalin is hypothetical plant growth compound as it has not been isolated yet . The Induction of Flowering through Cold Exposure Vernalization Pearce et al. 2017

18. FLC genes (Flower locus C)- genes that acts as a repressor of flowering . FLC is highly expressed in the non-vernalized shoot apical meristem . After vernalization, this gene is epigenetically switched off by an unknown mechanism for the remainder of the plant’s life cycle, permitting flowering in response to long days to occur. FT genes (Flower locus T)- Gene in leaves, florigenic factor (flowering gene). SOC1 (SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1)- gene in shoot apical meristem (flowering gene) FD (Flower locus D)- gene in shoot apical meristem (flowering gene). VRN1 AND VRN3 is activated by vernalization and help in flowering. FRIGIDA(FRI) serves to prevent plants from flowering through the work of FLC . Mechanism Of Floral Induction In Vernalized Plant Goslin et al. 2017; Pearce et al. 2017

19. If FLC Active No flowering, only vegetative phase If FLC Inactive Flowering Some Vernalization genes: VRN1 Vernalization 1 VIN3 Vernalization Insensitive 3 Case I: FT gene (In leaves) FLC Active FD gene (In SAM) No Flowering SOC 1 gene (In SAM) Case II: Vernalization VIN3, VRN1 FLC Flower Results Mechanism Of Floral Induction In Vernalized Plant Goslin et al. 2017; Pearce et al. 2017

20. Floral meristem identity genes Floral organ identity genes Transition of shoot vegetative meristem to floral meristem Control the floral identity Convert shoot into flower Floral meristem identity genes:- LEAFY (LFY) APETALA 1 (AP1) CAULIFLOWER (CAL) Floral organ identity genes are:- APETALA 1 (AP1) APETALA 2 (AP2) APETALA 3 (AP3) PISTILLATA (PI) AGAMOUS (AG) Transition of floral meristem to flower Floral Development Goslin et al. 2017

21. 4 3 2 1 Ca St Pe Se This 4 whorls formation depends on 5 homeotic genes- AP1, AP2, AP3, PI and AG APETALA 1 (AP1) APETALA 2 (AP2) APETALA 3 (AP3) PISTILATA (PI) AGAMOUS (AG) CLASS A GENES CLASS B GENES CLASS C GENES Anatomy of Flower Goslin et al. 2017

22. Introduction- ABC Model The ABC model of flowering is a widely accepted genetic model that explains the development and formation of floral organs in flowering plants. floral organ identity is determined by the combined expression of three classes of genes: A, B, and C. These genes control the expression of other genes involved in flower development. first proposed by scientists Elliot Meyerowitz and Enrico Coen in 1991 in Arabidopsis thaliana Se………A Pe………A + B St……….A + C Ca………C A C C A Mutually antagonistic B: always associative Either B + C or B + A Robles et al. 2004

23. APETALA 1 (AP1) APETALA 2 (AP2) APETALA 3 (AP3) PISTILATA (PI) AGAMOUS (AG) CLASS A GENES CLASS B GENES CLASS C GENES Wild type Genes Sepal Petal Stamen Carpel B- Class genes A- Class genes C- Class genes ABC Model of Flower Robles et al. 2004

24. Normal (WT) A class mutant B class mutant C class mutant 1. Se — A 1. C — Ca 1. A — Se 1. A — Se 2. Pe — A+B 2. C+B — St 2. A — Se 2. A+B — Pe 3. St — B+C 3. B+C — St 3. C — Ca 3. B+A — Pe 4. Ca — C 4. C — Ca 4. C — Ca 4. A — Se ABC Model: Mutation Case Sepals absent Petals absent Stamens absent Robles et al. 2004

25. Robles et al. 2004

26. The ABCE model, also known as the ABCDE model, is an extension of the original ABC model of flowering. This model suggests that the combination of A, B, C, D, and E gene activities determines the identity of all floral organs , including sepals, petals, stamens, carpels, and additional structures such as bracts or floral meristems . In Arabidopsis- A- function genes are APETALA1 (AP1) and APETALA2 (AP2). B- function is provided by APETALA3 and PISTILLATA (PI). C- function by AGAMOUS(AG ) E- function by multiple SEPALLATA genes (SEP1-4) Class D- genes: SHATTERPROOF1 or SHATTERPROOF2 (SHP1,SHP2) or SEEDSTICK (STK)- responsible for ovule ABCE Model E- function plays a major role in the formation of floral organs and is closely allied with ABC- function Goslin et al. 2017

27. As per this model, floral organ identities are specified through the action of three key gene functions such that- A function alone specifies sepal. A and B functions together determine petals. Combined B and C functions specify stamens. C function alone determines carpels. D controlling aspects of ovule development. E interacting with A, B and C functions to specify organ identify. ABCE Model Goslin et al. 2017

28. Conclusion Physiological mechanisms , such as photoperiodic responses, temperature sensitivity, and hormonal regulation, play crucial roles in determining when and how flowering occurs. At the molecular level, a complex network of genes and their interactions governs the progression of flowering. Floral integrators, transcription factors, and flowering time genes coordinate the expression of downstream genes involved in floral organ development and the precise timing of flowering. Manipulating these mechanisms can enable the control and synchronization of flowering in crops, leading to improved yields and productivity.

29. Future S cope of Research The study of physiological and molecular mechanisms underlying flowering is a dynamic field with several promising avenues for future research . By exploring novel genetic regulators , investigating epigenetic modifications, deciphering signaling pathway integration , studying environmental adaptations , employing systems biology approaches, and conducting comparative studies , researchers can continue to unravel the intricate mechanisms that control flowering . Such advancements will not only deepen our understanding of plant development but also have practical implications for crop improvement and environmental resilience .

30. Objective:- To study the impact of prolonged vernalisation treatment in the floral transition mechanism of germinating seed of strong winter mustard ( Brassica rapa ). Case Study- 01

31. Germinated seeds vernilization treatment, and effects on floral time of plants. A: seeding growth during vernilization at 4 ℃; B: Effects of seeds vernalization days on floral time of plants from seeds that sowed at the same time after vernalization treatment.

32. The germinating seeds of Brassica rapa can sense the low temperature and complete vernalization, which requires 56.5 days of low temperature with 100% vernalization rate for quality and quantity production. The main effect of low temperature for 0d to 50d on germinating seeds is transition to flowering of winter rapeseed, and the main effect of low temperature for 50d to 60d on germinating seeds is earlier showing budding and flowering in plant. The prolongation of vernalization treatment time significantly and positively increased the fruiting ability of plants and variations were also observed in the membrane lipid oxidation and physiological characteristics. The results demonstrated that the SA (Salicylic acid ) acting as a floral repressor was involved in regulating the vernalization process during germination, and content was found to be in decreased level in compared to control . Perhaps the expression SA-related biosynthesis, SA signal transduction, the flowering key negative regulators were suppressed and positive regulators were promoted. Research Findings

33. Objective:- To have a deeper understanding of the temperature control of flowering in this species and the impact of heat, vernalization , and their interaction on flowering induction and reproductive development. Case Study- 02

34. Research Findings

35. (A) A ‘ concave ’ apex that corresponds to the vegetative state (B) a ‘ domed ’ apex that corresponds to the start of bolting process (C) the ‘flowering’ apex with cauline leaves and the start of floral stack development Research Findings 1. The number of rosette leaves increased along the experiment in all of the experimental groups except for the vernalized plants exposed to the control temperature. 2. It was concluded that, heat had contrasting effects on flowering induction in root chicory: it induced bolting and flowering in non-vernalised plants , whereas it suppressed flowering of vernalized plants. 3. The heat had a mainly negative impact on flower development by altering floral stalk development , decreasing the number of inflorescences and flower fertility .

36. Goslin K, Zheng B, Serrano- Mislata A, Rae L, Ryan P T, Kwaśniewska K, Graciet E (2017). Transcription factor interplay between LEAFY and APETALA1/CAULIFLOWER during floral initiation. Plant physiology 174(2):1097-1109. Izawa T (2021). What is going on with the hormonal control of flowering in plants? The Plant Journal, 105(2): 431-445. Kalra , G, and A Lal, M (2018). Physiology of flowering. In Plant Physiology, Development and Metabolism (pp. 797-819). Springer, Singapore. Liu Z, Zou Y, Dong X, Wei J, Xu C, Mi W, Mi C (2021). Germinating seed can sense low temperature for the floral transition and vernalization of winter rapeseed ( Brassica rapa ). Plant Science, 307: 110900. Mathieu AS, Périlleux C, Jacquemin G, Renard ME, Lutts S, Quinet M (2020). Impact of vernalization and heat on flowering induction, development and fertility in root chicory ( Cichorium intybus L. var. sativum ). Journal of Plant Physiology, 254: 153272. Matsoukas IG, Massiah AJ, Thomas B (2012). Florigenic and antiflorigenic signaling in plants. Plant and Cell Physiology, 53(11): 1827-1842. Pearce S, Shaw LM, Lin H, Cotter JD, Li C, Dubcovsky J (2017). Night-break experiments shed light on the Photoperiod1-mediated flowering. Plant Physiology 174(2):1139-1150. Robles P, and Pelaz S (2004). Flower and fruit development in Arabidopsis thaliana. International Journal of Developmental Biology 49(5-6):633-643. References

37. REFERENCES Goslin , K., Zheng, B., Serrano- Mislata , A., Rae, L., Ryan, P. T., Kwaśniewska , K., ... & Graciet , E. (2017). Transcription factor interplay between LEAFY and APETALA1/CAULIFLOWER during floral initiation.  Plant physiology ,  174 (2), 1097-1109. Matsoukas , I. G., Massiah , A. J., & Thomas, B. (2012). Florigenic and antiflorigenic signaling in plants.  Plant and Cell Physiology ,  53 (11), 1827-1842. Izawa, T. (2021). What is going on with the hormonal control of flowering in plants?.  The Plant Journal ,  105 (2), 431-445. Liu, Z., Zou, Y., Dong, X., Wei, J., Xu, C., Mi, W., ... & Mi, C. (2021). Germinating seed can sense low temperature for the floral transition and vernalization of winter rapeseed (Brassica rapa ).  Plant Science ,  307 , 110900. Mathieu, A. S., Périlleux , C., Jacquemin , G., Renard, M. E., Lutts , S., & Quinet, M. (2020). Impact of vernalization and heat on flowering induction, development and fertility in root chicory (Cichorium intybus L. var. sativum).  Journal of Plant Physiology ,  254 , 153272.

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