Biology:Flower induction

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Short description: Physiological process in the plant


Flower induction is the developmental switch by which the shoot apical meristem of a plant stops producing vegetative structures and begins to produce flowers. The right timing of this switch has a large effect on reproductive success and has been a central concern of plant physiology and crop breeding [1]. In flowering plants, the transition integrates environmental signals - photoperiod, vernalization, ambient temperature, and quality light, with internal cues such as the age of the plant, its sugar status and hormones [1]. These inputs converge on a small number of genes called floral pathway integrators which trigger flowering once their activity rises above a threshold [1]. Arabidopsis thaliana has been serving as the model plant for resolving much of the underlying molecular network of flower-promoting signals [1].

Genetic Pathways

In A. thaliana, the signals that promote flowering are channelled through several overlapping pathways that converge on a small set of floral pathway integrators which commit the meristem to flowering once their activity passes a threshold [1]:

  • FLOWERING LOCUS T (FT): the mobile florigen signal produced in the leaf.
  • SUPPRESSOR OF OVERPRESSION OF CONSTANS 1 (SOC1): a MADs-box gene that integrates inputs from the gibberellin, age, and vernalization pathways.
  • LEAFY (LFY), together with the floral meristem identity genes APETALA1 (AP1) and CAULIFLOWER (CAL), which lock the meristem into a floral fate.

The Age Pathway

The age pathway ensures that a plant becomes reproductively competent only after a juvenile vegetative stage. It is controlled by two opposing microRNAs :[2][3]

  • miR156 which is abundant in young plants and represses flowering by degrading floral activators such as SQUAMOSA PROMOTER BINDLING-LIKE (SPLs) transcription factors. As the plant ages, the level of miRNA156 drops, and hence the levels of SPL proteins increase, activating floral regulators such as AP1, LFY, and SOC1 [2].
  • miR172 which increases when miR156 drops promotes flowering by degrading repressors such as AP2-like, allowing floral regulators such as FT and SOC1 to turn on and induce flowering [3].
  • Sugar, specifically glucose and sucrose, regulates miR156 by acting as an endogenous cue that reduces its abundance, accelerating the juvenile-to-adult phase transition in plants. Sugar represses miR156 through a two-pronged mechanism: reducing the transcription of MIR156 genes and accelerating the degradation of its primary transcripts, primarily mediated by HEXOKINASE1 (HXK1). Sugars ensure that the plant does not attempt to flower until it has enough carbon energy to support the next generation [4].

The Photoperiod Pathway and Florigen

Photoperiodic flowering allows a plant to flower at a favorable time of year. A. thalinana is a facultative long-day plant in which the B-box transcription factor CONSTANS (CO) accumulates in the leaves only when its expression coincides with daylight [5]. CO transcription is controlled by the circadian clock, and CO protein is stabilized by light but degraded in darkness, so the protein accumulates only under long days [6][7]. Therefore, stabilized CO protein under long days is required for flowering because it then activates the transcription of FT in the leaf [8].

The product of FT is a small phosphatidylethanolamine-binding protein which moves through the phloem from the leaves to the shoot apex, where it forms a transcription-activating complex with the bZIP transcription factor FD to turn on floral identity genes such as AP1 and SOC1 [1]. The transcription of AP1 and SOC1 transform the vegetative meristem into a floral meristem, leading to the activation of the ABCDE genes and the physical development of the flower [1]. FT is therefore widely regarded as a major component of florigen, the movement of which from leaf to shoot apex has been demonstrated in both Arabidopsis and rice [9][10]. Notably, a closely-related protein, TERMINAL FLOWER 1 (TFL1) competes with FT for binding FD protein, and the TFL1-FD complex blocks flower induction [11].

The Vernalization Pathway

Vernalization is the acquisition of competence to flower only after a prolonged period of winter cold [12]. Vernalization ensures that flowering does not take place prematurely before winter has passed, and is a common requirement for winter-annuals [12]. In A. thalinana, the MADS-box transcription factor FLOWERING LOCUS C (FLC) represses FT and SOC1, thus blocking flowering [13]. It works together with related repressor SHORT VEGETATIVE PHASE (SVP), and its expression is increased by the protein FRIGIDA (FRI) [14]. During winter, FLC is silenced by two parallel processes [15][16]:

  • The onset of cold triggers the expression of COOLAIR which is the antisense RNA transcribed from the opposite strand of the FLC gene; the transcripts physically interfere with transcription of the sense FLC mRNA, inhibiting the transcription of FLC [15].
  • As the cold persists, VERNALIZATION INSENSITIVE 3 (VIN3) is induced and binds VERNALIZATION 5 (VRN5) to form a complex that guides the Polycomb Repressive Complex 2 PRC2 to modify the chromatin structure surrounding the FLC gene, leaving repressive histone marks (H3K27me3) [16]. These marks cluster the Transcription Start Site and Transcription Terminal Site, silencing transcription while the gene is attempting to be read (co-transcriptional) [16]. When the weather gets warm, plants stop making VIN3, but FLC must stay repressed. VERNALIZATION 1 (VRN1) recognizes the silenced, methylated state of the FLC locus, and makes sure that the repressive H3K27me3 marks are copied onto the newly synthesized DNA strands in cell division for growth in warmer springs [16]. The histone modification (epigenetics) prevents FLC gene turning back on when weather gets warm. The response of FLC mRNA to cold is gradual as the plant is able to measure how long it has been cold [17]. Once FLC is reduced, plants are free to respond to other signals such as photoperiod; the silenced FLC allows FT to turn on and induce flowering [17].

The Ambient Temperature Pathway

Even without vernalization, small changes in ambient temperature can alter flowering. The transcription factor PIF4 activates FT at warmer temperatures [18], while the MADS-box repressor FLM is subject to temperature-dependent alternative splicing that produces more of an active, flowering-repressing isoform in the cold [19][20]. The loading and membrane association of FT protein are themselves temperature-sensitive, adding a further layer of thermal regulation [21].

The Gibberellin Pathway

Gibberellins promote flowering, especially under short days and in rosette plants, in part by relieving the repression that DELLA proteins exert on the SPL transcription factors [1]. Gibberellin biosynthesis responds to photoperiod and temperature, and the precursor GA12 can act as a long-distance, phloem-mobile signal [22].

The Autonomous Pathway

The autonomous pathway promotes flowering independently of the environment - photoperiod or vernalization - by suppressing FLC expression [1]. By silencing or repressing FLC, the plant's flowering genes (like SOC1, LFY, and AP1) can switch on, allowing the plant to transition from vegetative to reproductive growth [23]. The suppression of FLC expression in the autonomous pathway can be achieved through a complex network of proteins and genes, in two parallel mechanisms [24]: 1) RNA processing and polyadenylation and 2) epigenetic modification [24]. Some of its components, such as FCA, are RNA-binding and RNA-processing proteins which act through a transcription-coupled chromatin-silencing mechanism at FLC [25]. FLC is unusually sensitive to the activity of these components because many of them are general regulators of transcription [1].

References

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 Robert, Maple; Pan, Zhu; Jo, Hepworth; Jia-Wei, Wang; Caroline, Dean (2024-04-30). "Flowering time: From physiology, through genetics to mechanism" (in en). Plant Physiology 195 (1). doi:10.1093/plphy. ISSN 0032-0889. https://academic.oup.com/plphys/article/195/1/190/7615930. 
  2. ↑ 2.0 2.1 Wang, Jia-Wei; Czech, Benjamin; Weigel, Detlef (2009-08-21). "miR156-Regulated SPL Transcription Factors Define an Endogenous Flowering Pathway in Arabidopsis thaliana" (in English). Cell 138 (4): 738–749. doi:10.1016/j.cell.2009.06.014. ISSN 0092-8674. PMID 19703399. https://www.cell.com/cell/abstract/S0092-8674(09)00713-2. 
  3. ↑ 3.0 3.1 Wu, Gang; Park, Mee Yeon; Conway, Susan R.; Wang, Jia-Wei; Weigel, Detlef; Poethig, R. Scott (2009-08-21). "The Sequential Action of miR156 and miR172 Regulates Developmental Timing in Arabidopsis" (in English). Cell 138 (4): 750–759. doi:10.1016/j.cell.2009.06.031. ISSN 0092-8674. PMID 19703400. 
  4. ↑ Yu, Sha; Cao, Li; Zhou, Chuan-Miao; Zhang, Tian-Qi; Lian, Heng; Sun, Yue; Wu, Jianqiang; Huang, Jirong et al. (2013-03-26). "Sugar is an endogenous cue for juvenile-to-adult phase transition in plants" (in en). eLife 2. doi:10.7554/eLife.00269. ISSN 2050-084X. PMID 23543845. 
  5. ↑ Putterill, Joanna; Robson, Frances; Lee, Karen; Simon, Rüdiger; Coupland, George (March 1995). "The CONSTANS gene of arabidopsis promotes flowering and encodes a protein showing similarities to zinc finger transcription factors" (in en). Cell 80 (6): 847–857. doi:10.1016/0092-8674(95)90288-0. PMID 7697715. https://linkinghub.elsevier.com/retrieve/pii/0092867495902880. 
  6. ↑ Suárez-López, Paula; Wheatley, Kay; Robson, Frances; Onouchi, Hitoshi; Valverde, Federico; Coupland, George (April 2001). "CONSTANS mediates between the circadian clock and the control of flowering in Arabidopsis" (in en). Nature 410 (6832): 1116–1120. doi:10.1038/35074138. ISSN 0028-0836. PMID 11323677. Bibcode: 2001Natur.410.1116S. https://www.nature.com/articles/35074138. 
  7. ↑ Valverde, Federico; Mouradov, Aidyn; Soppe, Wim; Ravenscroft, Dean; Samach, Alon; Coupland, George (2004-02-13). "Photoreceptor Regulation of CONSTANS Protein in Photoperiodic Flowering" (in en). Science 303 (5660): 1003–1006. doi:10.1126/science.1091761. ISSN 0036-8075. PMID 14963328. Bibcode: 2004Sci...303.1003V. https://www.science.org/doi/10.1126/science.1091761. 
  8. ↑ Kardailsky, Igor; Shukla, Vipula K.; Ahn, Ji Hoon; Dagenais, Nicole; Christensen, Sioux K.; Nguyen, Jasmine T.; Chory, Joanne; Harrison, Maria J. et al. (1999-12-03). "Activation Tagging of the Floral Inducer FT" (in en). Science 286 (5446): 1962–1965. doi:10.1126/science.286.5446.1962. ISSN 0036-8075. PMID 10583961. https://www.science.org/doi/10.1126/science.286.5446.1962. 
  9. ↑ Corbesier, Laurent; Vincent, Coral; Jang, Seonghoe; Fornara, Fabio; Fan, Qingzhi; Searle, Iain; Giakountis, Antonis; Farrona, Sara et al. (2007-05-18). "FT Protein Movement Contributes to Long-Distance Signaling in Floral Induction of Arabidopsis" (in en). Science 316 (5827): 1030–1033. doi:10.1126/science.1141752. ISSN 0036-8075. PMID 17446353. Bibcode: 2007Sci...316.1030C. https://www.science.org/doi/10.1126/science.1141752. 
  10. ↑ Tamaki, Shojiro; Matsuo, Shoichi; Wong, Hann Ling; Yokoi, Shuji; Shimamoto, Ko (2007-05-18). "Hd3a Protein Is a Mobile Flowering Signal in Rice" (in en). Science 316 (5827): 1033–1036. doi:10.1126/science.1141753. ISSN 0036-8075. PMID 17446351. Bibcode: 2007Sci...316.1033T. https://www.science.org/doi/10.1126/science.1141753. 
  11. ↑ Zhu, Yang; Klasfeld, Samantha; Jeong, Cheol Woong; Jin, Run; Goto, Koji; Yamaguchi, Nobutoshi; Wagner, Doris (2020-10-12). "TERMINAL FLOWER 1-FD complex target genes and competition with FLOWERING LOCUS T" (in en). Nature Communications 11 (1): 5118. doi:10.1038/s41467-020-18782-1. ISSN 2041-1723. PMID 33046692. Bibcode: 2020NatCo..11.5118Z. 
  12. ↑ 12.0 12.1 Amasino, Richard (October 2004). "Vernalization, competence, and the epigenetic memory of winter". The Plant Cell 16 (10): 2553–2559. doi:10.1105/tpc.104.161070. ISSN 1040-4651. PMID 15466409. Bibcode: 2004PlanC..16.2553A. 
  13. ↑ Michaels, Scott D.; Amasino, Richard M. (May 1999). "FLOWERING LOCUS C Encodes a Novel MADS Domain Protein That Acts as a Repressor of Flowering" (in en). The Plant Cell 11 (5): 949–956. doi:10.1105/tpc.11.5.949. ISSN 1040-4651. PMID 10330478. 
  14. ↑ Searle, Iain; He, Yuehui; Turck, Franziska; Vincent, Coral; Fornara, Fabio; Kröber, Sandra; Amasino, Richard A.; Coupland, George (2006-04-01). "The transcription factor FLC confers a flowering response to vernalization by repressing meristem competence and systemic signaling in Arabidopsis" (in en). Genes & Development 20 (7): 898–912. doi:10.1101/gad.373506. ISSN 0890-9369. PMID 16600915. 
  15. ↑ 15.0 15.1 Swiezewski, Szymon; Liu, Fuquan; Magusin, Andreas; Dean, Caroline (December 2009). "Cold-induced silencing by long antisense transcripts of an Arabidopsis Polycomb target" (in en). Nature 462 (7274): 799–802. doi:10.1038/nature08618. ISSN 0028-0836. PMID 20010688. Bibcode: 2009Natur.462..799S. https://www.nature.com/articles/nature08618. 
  16. ↑ 16.0 16.1 16.2 16.3 Bastow, Ruth; Mylne, Joshua S.; Lister, Clare; Lippman, Zachary; Martienssen, Robert A.; Dean, Caroline (January 2004). "Vernalization requires epigenetic silencing of FLC by histone methylation" (in en). Nature 427 (6970): 164–167. doi:10.1038/nature02269. ISSN 0028-0836. PMID 14712277. Bibcode: 2004Natur.427..164B. https://www.nature.com/articles/nature02269. 
  17. ↑ 17.0 17.1 Hepworth, Jo; Dean, Caroline (2015-08-05). "Flowering Locus C's Lessons: Conserved Chromatin Switches Underpinning Developmental Timing and Adaptation" (in en). Plant Physiology 168 (4): 1237–1245. doi:10.1104/pp.15.00496. ISSN 1532-2548. PMID 26149571. Bibcode: 2015PlanP.168.1237H. 
  18. ↑ Kumar, S. Vinod; Lucyshyn, Doris; Jaeger, Katja E.; Alós, Enriqueta; Alvey, Elizabeth; Harberd, Nicholas P.; Wigge, Philip A. (April 2012). "Transcription factor PIF4 controls the thermosensory activation of flowering" (in en). Nature 484 (7393): 242–245. doi:10.1038/nature10928. ISSN 0028-0836. PMID 22437497. Bibcode: 2012Natur.484..242K. 
  19. ↑ Posé, David; Verhage, Leonie; Ott, Felix; Yant, Levi; Mathieu, Johannes; Angenent, Gerco C.; Immink, Richard G. H.; Schmid, Markus (November 2013). "Temperature-dependent regulation of flowering by antagonistic FLM variants" (in en). Nature 503 (7476): 414–417. doi:10.1038/nature12633. ISSN 0028-0836. PMID 24067612. Bibcode: 2013Natur.503..414P. https://www.nature.com/articles/nature12633. 
  20. ↑ Lee, Jeong Hwan; Ryu, Hak-Seung; Chung, Kyung Sook; Posé, David; Kim, Soonkap; Schmid, Markus; Ahn, Ji Hoon (November 2013). "Regulation of Temperature-Responsive Flowering by MADS-Box Transcription Factor Repressors" (in en). Science 342 (6158): 628–632. doi:10.1126/science.1241097. ISSN 0036-8075. PMID 24030492. Bibcode: 2013Sci...342..628L. https://www.science.org/doi/10.1126/science.1241097. 
  21. ↑ Susila, Hendry; Jurić, Snježana; Liu, Lu; Gawarecka, Katarzyna; Chung, Kyung Sook; Jin, Suhyun; Kim, Soo-Jin; Nasim, Zeeshan et al. (2021-09-03). "Florigen sequestration in cellular membranes modulates temperature-responsive flowering" (in en). Science 373 (6559): 1137–1142. doi:10.1126/science.abh4054. ISSN 0036-8075. PMID 34516842. Bibcode: 2021Sci...373.1137S. https://www.science.org/doi/10.1126/science.abh4054. 
  22. ↑ Regnault, Thomas; Davière, Jean-Michel; Wild, Michael; Sakvarelidze-Achard, Lali; Heintz, Dimitri; Carrera Bergua, Esther; Lopez Diaz, Isabel; Gong, Fan et al. (2015-06-01). "The gibberellin precursor GA12 acts as a long-distance growth signal in Arabidopsis" (in en). Nature Plants 1 (6). doi:10.1038/nplants.2015.73. ISSN 2055-0278. PMID 27250008. https://www.nature.com/articles/nplants201573. 
  23. ↑ Cheng, Jing-Zhi; Zhou, Yu-Ping; Lv, Tian-Xiao; Xie, Chu-Ping; Tian, Chang-En (July 2017). "Research progress on the autonomous flowering time pathway in Arabidopsis" (in en). Physiology and Molecular Biology of Plants 23 (3): 477–485. doi:10.1007/s12298-017-0458-3. ISSN 0971-5894. PMID 28878488. Bibcode: 2017PMBP...23..477C. 
  24. ↑ 24.0 24.1 Wu, Zhe; Fang, Xiaofeng; Zhu, Danling; Dean, Caroline (January 2020). "Autonomous Pathway: FLOWERING LOCUS C Repression through an Antisense-Mediated Chromatin-Silencing Mechanism" (in en). Plant Physiology 182 (1): 27–37. doi:10.1104/pp.19.01009. ISSN 0032-0889. PMID 31740502. Bibcode: 2020PlanP.182...27W. 
  25. ↑ Macknight, Richard; Bancroft, Ian; Page, Tania; Lister, Clare; Schmidt, Renate; Love, Karina; Westphal, Lore; Murphy, George et al. (May 1997). "FCA, a Gene Controlling Flowering Time in Arabidopsis, Encodes a Protein Containing RNA-Binding Domains" (in en). Cell 89 (5): 737–745. doi:10.1016/S0092-8674(00)80256-1. PMID 9182761. https://linkinghub.elsevier.com/retrieve/pii/S0092867400802561. 



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