{{Short description|Flowering hormone in plants}} {{cs1 config|name-list-style=vanc|display-authors=6}} '''Florigens''' (or '''flowering hormone''') are proteins capable of inducing flowering time in angiosperms.<ref name="Tsuji_2017" /> The prototypical florigen is encoded by the FT gene and its orthologs in ''Arabidopsis'' and other plants.<ref>{{cite journal | vauthors = Takeshima R, Nan H, Harigai K, Dong L, Zhu J, Lu S, Xu M, Yamagishi N, Yoshikawa N, Liu B, Yamada T, Kong F, Abe J | title = Functional divergence between soybean FLOWERING LOCUS T orthologues FT2a and FT5a in post-flowering stem growth | journal = Journal of Experimental Botany | volume = 70 | issue = 15 | pages = 3941–3953 | date = August 2019 | pmid = 31035293 | pmc = 6685666 | doi = 10.1093/jxb/erz199 | doi-access = free }}</ref> Florigens are produced in the leaves, and act in the shoot apical meristem of buds and growing tips.<ref name = "Corbesier_2007">{{cite journal | vauthors = Corbesier L, Vincent C, Jang S, Fornara F, Fan Q, Searle I, Giakountis A, Farrona S, Gissot L, Turnbull C, Coupland G | title = FT protein movement contributes to long-distance signaling in floral induction of Arabidopsis | journal = Science | volume = 316 | issue = 5827 | pages = 1030–1033 | date = May 2007 | pmid = 17446353 | doi = 10.1126/science.1141752 | hdl-access = free | bibcode = 2007Sci...316.1030C | hdl = 11858/00-001M-0000-0012-3874-C | doi-access = free }}</ref><ref name = "Jaeger_2007">{{cite journal | vauthors = Jaeger KE, Wigge PA | title = FT protein acts as a long-range signal in Arabidopsis | journal = Current Biology | volume = 17 | issue = 12 | pages = 1050–1054 | date = June 2007 | pmid = 17540569 | doi = 10.1016/j.cub.2007.05.008 | doi-access = free | bibcode = 2007CBio...17.1050J }}</ref>
== Mechanism ==
For a plant to begin flowering, it must undergo changes in its shoot apical meristem (SAM).<ref name="Taiz_2018">{{Cite book| vauthors = Taiz L, Zeiger E, Møller IM, Murphy AS |title=Fundamentals of plant physiology|year=2018|isbn=978-1-60535-790-4|location=New York, NY | publisher = Oxford University Press |chapter=17: Flowering and Fruit Development|oclc=1035316853}}</ref> However, there are multiple environmental factors affecting the plant even before it begins this process — in particular, light. It is through "the evolution of both internal and external control systems that enables plants to precisely regulate flowering so that it occurs at the optimal time for reproductive success."<ref name="Taiz_2018" /> The way the plant determines this optimal time is through day-night periods through the use of photoperiodism. Although it was originally thought that the accumulation of photosynthetic products controlled the flowering of plants, two men by the names of Wightman Garner and Henry Allard proved it was not.<ref name="Taiz_2018" /> They instead found that it was a matter of day length rather than the accumulation of the products within the plants that affected their flowering abilities.
Flowering plants fall into two main photoperiodic response categories:
# '''"Short-day plants (SDPs)''' flower only in short days (''qualitative SDPs''), or their flowering is accelerated by short days (''quantitative SDPs'')"<ref name="Taiz_2018" />{{rp|478}} # '''"Long-day plants (LDPs)''' flower only in long days (''qualitative LDPs''), or their flowering is accelerated by long days (''quantitative LDPs'')"<ref name="Taiz_2018"/>
These types of flowering plants are differentiated by the whether the day has exceeded some duration - usually calculated by 24-hour cycles - known as the critical day length.<ref name="Taiz_2018"/> It is also important to note that there is no absolute value for the minimum day length as it varies greatly amid species. Until the correct amount of day length is reached, the plants ensure no flowering results. They do so through adaptations like preventing immature plants from responding to inadequate day lengths.<ref name="Taiz_2018"/> Plants also have the ability to prevent the response of the photoperiodic stimulus until a certain temperature is reached.<ref name="Taiz_2018" /> Species like winter wheat that rely on just that.<ref name="Taiz_2018" /> The wheat require a cold period before being able to respond to the photoperiod.<ref name="Taiz_2018" /> This is known as vernalization or overwintering.<ref name="Taiz_2018" />
This ebb-and-flow of flowering in plants is essentially controlled by an internal clock known as the ''endogenous oscillator''.<ref name="Taiz_2018" />{{rp|479}} It is thought that these internal pacemakers "are regulated by the interaction of four sets of genes expressed in the dawn, morning, afternoon, and evening hours [and that] light may augment the amplitude of the oscillations by activating the morning and evening genes."<ref name="Taiz_2018" />{{rp|479}} The rhythms between these different genes are generated internally in the plants, starts with the leaves, but requires an environmental stimulus such as light. The light essentially stimulates the transmission of a floral stimulus (florigen) to the shoot apex when the correct amount of day-length is perceived.<ref name="Taiz_2018" />{{rp|482}} This process is known as photoperiodic induction and is a photoperiod-regulated process that is also dependent on the endogenous oscillator..<ref name="Taiz_2018" />{{rp|482}}
The current model suggests the involvement of multiple different factors. Research into florigen is predominately centred on the model organism and long day plant, ''Arabidopsis thaliana''. Whilst much of the florigen pathways appear to be well conserved in other studied species, variations do exist.<ref name="Turck2008">{{cite journal | vauthors = Turck F, Fornara F, Coupland G | title = Regulation and identity of florigen: FLOWERING LOCUS T moves center stage | journal = Annual Review of Plant Biology | volume = 59 | pages = 573–594 | year = 2008 | pmid = 18444908 | doi = 10.1146/annurev.arplant.59.032607.092755 | hdl-access = free | s2cid = 39798675 | hdl = 11858/00-001M-0000-0012-374F-8 }}</ref> The mechanism may be broken down into three stages: photoperiod-regulated ''initiation'', signal ''translocation'' via the phloem, and induction of ''flowering'' at the shoot apical meristem.
===Initiation===
In ''Arabidopsis thaliana'', the signal is initiated by the production of messenger RNA (mRNA) coding a transcription factor called CONSTANS (CO). CO mRNA is produced approximately 12 hours after dawn, a cycle regulated by the plant's circadian rhythms, and is then translated into CO protein.<ref name="Distinct roles of GIGANTEA">{{cite journal | vauthors = Mizoguchi T, Wright L, Fujiwara S, Cremer F, Lee K, Onouchi H, Mouradov A, Fowler S, Kamada H, Putterill J, Coupland G | title = Distinct roles of GIGANTEA in promoting flowering and regulating circadian rhythms in Arabidopsis | journal = The Plant Cell | volume = 17 | issue = 8 | pages = 2255–2270 | date = August 2005 | pmid = 16006578 | pmc = 1182487 | doi = 10.1105/tpc.105.033464 | bibcode = 2005PlanC..17.2255M }}</ref><ref name="Yanovsky2002" /> However CO protein is stable only in light, so levels stay low throughout short days and are only able to peak at dusk during long days when there is still some light.<ref name="Yanovsky2002">{{cite journal | vauthors = Yanovsky MJ, Kay SA | title = Molecular basis of seasonal time measurement in Arabidopsis | journal = Nature | volume = 419 | issue = 6904 | pages = 308–312 | date = September 2002 | pmid = 12239570 | doi = 10.1038/nature00996 | s2cid = 4407399 | bibcode = 2002Natur.419..308Y }}</ref><ref name="Valverde2004">{{cite journal | vauthors = Valverde F, Mouradov A, Soppe W, Ravenscroft D, Samach A, Coupland G | title = Photoreceptor regulation of CONSTANS protein in photoperiodic flowering | journal = Science | volume = 303 | issue = 5660 | pages = 1003–1006 | date = February 2004 | pmid = 14963328 | doi = 10.1126/science.1091761 | hdl-access = free | s2cid = 6622199 | bibcode = 2004Sci...303.1003V | hdl = 11858/00-001M-0000-0012-3C25-D }}</ref> CO protein promotes transcription of another gene called {{visible anchor|Flowering Locus T|FLOWERING LOCUS T|FT}} (FT).<ref>{{cite journal | vauthors = An H, Roussot C, Suárez-López P, Corbesier L, Vincent C, Piñeiro M, Hepworth S, Mouradov A, Justin S, Turnbull C, Coupland G | title = CONSTANS acts in the phloem to regulate a systemic signal that induces photoperiodic flowering of Arabidopsis | journal = Development | volume = 131 | issue = 15 | pages = 3615–3626 | date = August 2004 | pmid = 15229176 | doi = 10.1242/dev.01231 | doi-access = free }}</ref> By this mechanism, CO protein may only reach levels capable of promoting FT transcription when exposed to long days. Hence, the transmission of florigen—and thus, the induction of flowering—relies on a comparison between the plant's perception of day/night and its own internal biological clock.<ref name="Turck2008"/>
===Translocation===
The FT protein resulting from the short period of CO transcription factor activity is then transported via the phloem to the shoot apical meristem.<ref name = "Corbesier_2007" /><ref name="Mathieu2007">{{cite journal | vauthors = Mathieu J, Warthmann N, Küttner F, Schmid M | title = Export of FT protein from phloem companion cells is sufficient for floral induction in Arabidopsis | journal = Current Biology | volume = 17 | issue = 12 | pages = 1055–1060 | date = June 2007 | pmid = 17540570 | doi = 10.1016/j.cub.2007.05.009 | doi-access = free | bibcode = 2007CBio...17.1055M }}</ref><ref name = "Jaeger_2007" />
===Flowering=== Florigen is a systemically mobile signal that is synthesized in leaves and the transported via the phloem to the shoot apical meristem (SAM) where it initiates flowering.<ref name="Tsuji_2017">{{cite journal | vauthors = Tsuji H | title = Molecular function of florigen | journal = Breeding Science | volume = 67 | issue = 4 | pages = 327–332 | date = September 2017 | pmid = 29085241 | pmc = 5654465 | doi = 10.1270/jsbbs.17026 }}</ref><ref name = "Taiz_2018" />{{rp|488}} In ''Arabidopsis'', the ''FLOWERING LOCUS T'' (''FT'') genes encode for the flowering hormone and in rice the hormone is encoded by ''Hd3a'' genes thereby making these genes orthologs.<ref name="Tsuji_2017" /> It was found though the use of transgenic plants that the ''Hd3a'' promoter in rice is located in the phloem of the leaf along with the ''Hd3a'' mRNA. However, the Hd3a protein is found in neither of these places but instead accumulates in the SAM which shows that Hd3a protein is first translated in leaves and then transported to the SAM via the phloem where floral transition is initiated; the same results occurred when looked at ''Arabidopsis''.<ref name="Tsuji_2017" /> These results conclude that ''FT/Hd3a'' is the florigen signal that induces floral transition in plants.
Upon this conclusion, it became important to understand the process by which the FT protein causes floral transition once it reaches the SAM. The first clue came with looking at models from ''Arabidposis'' which suggested that a bZIP domain containing transcription factor, FD, is somehow interacting with FT to form a transcriptional complex that activates floral genes.<ref name="Tsuji_2017" /> Studies using rice found that there is an interaction between Hd3a and OsFD1, homologs of FT and FD respectively, that is mediated by the 14-3-3 protein GF14c.<ref name="Tsuji_2017" /><ref name="Nakamura_2014">{{cite journal | vauthors = Nakamura Y, Andrés F, Kanehara K, Liu YC, Dörmann P, Coupland G | title = Arabidopsis florigen FT binds to diurnally oscillating phospholipids that accelerate flowering | journal = Nature Communications | volume = 5 | issue = 1 | page = 3553 | date = April 2014 | pmid = 24698997 | pmc = 3988816 | doi = 10.1038/ncomms4553 | bibcode = 2014NatCo...5.3553N }}</ref> The 14-3-3 protein acts as intracellular florigen receptor that interacts directly with Hd3a and OsFD1 to form a tri-protein complex called the florigen activation complex (FAC) because it is essential for florigen function.<ref name="Tsuji_2017" /> The FAC works to activate genes needed to initiate flowering at the SAM; flowering genes in ''Arabidopsis'' include ''AP1, SOC1'' and several SPL genes, which are targeted by a ''microRNA'' and in rice the flowering gene is ''OsMADS15'' (a homolog of ''AP1'').<ref name="Nakamura_2014" /><ref>{{cite journal | vauthors = Taoka K, Ohki I, Tsuji H, Furuita K, Hayashi K, Yanase T, Yamaguchi M, Nakashima C, Purwestri YA, Tamaki S, Ogaki Y, Shimada C, Nakagawa A, Kojima C, Shimamoto K | title = 14-3-3 proteins act as intracellular receptors for rice Hd3a florigen | journal = Nature | volume = 476 | issue = 7360 | pages = 332–335 | date = July 2011 | pmid = 21804566 | doi = 10.1038/nature10272 | s2cid = 4401138 | bibcode = 2011Natur.476..332T }}</ref><ref name="Wang2009">{{cite journal | vauthors = Wang JW, Czech B, Weigel D | title = miR156-regulated SPL transcription factors define an endogenous flowering pathway in Arabidopsis thaliana | journal = Cell | volume = 138 | issue = 4 | pages = 738–749 | date = August 2009 | pmid = 19703399 | doi = 10.1016/j.cell.2009.06.014 | doi-access = free }}</ref>
===Antiflorigen===
Florigen is regulated by the action of an antiflorigen.<ref name="Eshed_2019">{{cite journal | vauthors = Eshed Y, Lippman ZB | title = Revolutions in agriculture chart a course for targeted breeding of old and new crops | journal = Science | volume = 366 | issue = 6466 | article-number = eaax0025 | date = November 2019 | pmid = 31488704 | doi = 10.1126/science.aax0025 | doi-access = free }}</ref> Antiflorigens are hormones that are encoded by the same genes for florigen that work to counteract its function.<ref name="Eshed_2019" /> The antiflorigen in ''Arabidopsis'' is ''TERMINAL FLOWER1'' (''TFL1'')<ref name="Turck2008"/> and in tomato it is ''SELF PRUNING'' (''SP'').<ref>{{cite journal | vauthors = Pnueli L, Carmel-Goren L, Hareven D, Gutfinger T, Alvarez J, Ganal M, Zamir D, Lifschitz E | title = The SELF-PRUNING gene of tomato regulates vegetative to reproductive switching of sympodial meristems and is the ortholog of CEN and TFL1 | journal = Development | volume = 125 | issue = 11 | pages = 1979–1989 | date = June 1998 | pmid = 9570763 | doi = 10.1242/dev.125.11.1979 }}</ref>
==Research history== Florigen was first described by Soviet Armenian plant physiologist Mikhail Chailakhyan, who in 1937 demonstrated that floral induction can be transmitted through a graft from an induced plant to one that has not been induced to flower.<ref name="Chailakhyan1985">{{cite journal | vauthors = Chaïlakhyan MK |title=Hormonal regulation of reproductive development in higher plants |journal=Biologia Plantarium |volume=27 |pages=292–302 |year=1985 |doi=10.1007/BF02879865 |issue=4–5}}</ref> Anton Lang showed that several long-day plants and biennials could be made to flower by treatment with gibberellin, even when grown under a non-flower-inducing (or non-inducing) photoperiod. This led to the suggestion that florigen may be made up of two classes of flowering hormones: Gibberellins and Anthesins.<ref name="Chailakhyan1975">{{cite journal | vauthors = Chaïlakhyan MK |title=Substances of plant flowering |journal=Biologia Plantarium |volume=17 |pages=1–11 |year=1975 |doi=10.1007/BF02921064|s2cid=11676305 }}</ref> It was later postulated that during non-inducing photoperiods, long-day plants produce anthesin, but no gibberellin, while short-day plants produce gibberellin, but no anthesin.<ref name="Chailakhyan1985"/> However, these findings did not account for the fact that short-day plants grown under non-inducing conditions (thus producing gibberellin) will not cause flowering of grafted long-day plants that are also under noninductive conditions (thus producing anthesin).
As a result of the problems with isolating florigen, and of the inconsistent results acquired, it has been suggested that florigen does not exist as an individual substance; rather, florigen's effect could be the result of a particular ratio of other hormones.<ref name="Zeevart1976">{{cite journal| vauthors = Zeevaart JA |year=1976|title=Physiology of flower formation |journal=Annual Review of Plant Physiology and Plant Molecular Biology |volume=27 |pages=321–348 |doi=10.1146/annurev.pp.27.060176.001541}}</ref><ref name="Bernier1993">{{cite journal | vauthors = Bernier G, Havelange A, Houssa C, Petitjean A, Lejeune P | title = Physiological Signals That Induce Flowering | journal = The Plant Cell | volume = 5 | issue = 10 | pages = 1147–1155 | date = October 1993 | pmid = 12271018 | pmc = 160348 | doi = 10.1105/tpc.5.10.1147 }}</ref> However, more recent findings indicate that florigen does exist and is produced, or at least activated, in the leaves of the plant and that this signal is then transported via the phloem to the growing tip at the shoot apical meristem where the signal acts by inducing flowering. In ''Arabidopsis thaliana'', some researchers have identified this signal as mRNA coded by the ''FLOWERING LOCUS T'' (''FT'') gene, others as the resulting ''FT'' protein.<ref name=notaguchi>{{cite journal | vauthors = Notaguchi M, Abe M, Kimura T, Daimon Y, Kobayashi T, Yamaguchi A, Tomita Y, Dohi K, Mori M, Araki T | title = Long-distance, graft-transmissible action of Arabidopsis FLOWERING LOCUS T protein to promote flowering | journal = Plant & Cell Physiology | volume = 49 | issue = 11 | pages = 1645–1658 | date = November 2008 | pmid = 18849573 | doi = 10.1093/pcp/pcn154 | doi-access = free }}</ref> First report of FT mRNA being the signal transducer that moves from leaf to shoot apex came from the publication in Science Magazine. However, in 2007 other group of scientists made a breakthrough saying that it is not the mRNA, but the FT Protein that is transmitted from leaves to shoot possibly acting as "Florigen".<ref name="Retraction">{{cite journal | vauthors = Böhlenius H, Eriksson S, Parcy F, Nilsson O | title = Retraction | journal = Science | volume = 316 | issue = 5823 | page = 367 | date = April 2007 | pmid = 17446370 | doi = 10.1126/science.316.5823.367b | doi-access = free }}</ref> The initial article<ref>{{cite journal | vauthors = Huang T, Böhlenius H, Eriksson S, Parcy F, Nilsson O | title = The mRNA of the Arabidopsis gene FT moves from leaf to shoot apex and induces flowering | journal = Science | volume = 309 | issue = 5741 | pages = 1694–1696 | date = September 2005 | pmid = 16099949 | doi = 10.1126/science.1117768 | s2cid = 9262331 | doi-access = free | bibcode = 2005Sci...309.1694H }}{{Retracted|doi=10.1126/science.316.5823.367b|pmid=17446370|intentional=yes}}</ref> that described FT mRNA as flowering stimuli was retracted by the authors themselves.<ref name="Retraction"/>
== Triggers of gene transcription == There are three genes involved in clock-controlled flowering pathway, GIGANTEA (GI), CONSTANS (CO), and FLOWERING LOCUS T (FT). Constant overexpression of GI from the ''Cauliflower mosaic virus'' 35S promoter causes early flowering under short day so an increase in GI mRNA expression induces flowering. Also, GI increases the expression of FT and CO mRNA, and FT and CO mutants showed later flowering time than GI mutant. In other words, functional FT and CO genes are required for flowering under short day. In addition, these flowering genes accumulate during light phase and decline during dark phase, which are measured by green fluorescent protein. Thus, their expressions oscillate during the 24-hour light-dark-cycle. In conclusion, the accumulation of GI mRNA alone or GI, FT, and CO mRNA promote flowering in ''Arabidopsis thaliana'' and these genes expressed in the temporal sequence GI-CO-FT.<ref name="Distinct roles of GIGANTEA"/>
Action potential triggers calcium flux into neurons in animal or root apex cells in plants. The intracellular calcium signals are responsible for regulation of many biological functions in organisms. For instance, Ca<sup>2+</sup> binding to calmodulin, a Ca<sup>2+</sup>-binding protein in animals and plants, controls gene transcriptions.<ref name="Experience teaches">{{cite journal | vauthors = Gagliano M, Renton M, Depczynski M, Mancuso S | title = Experience teaches plants to learn faster and forget slower in environments where it matters | journal = Oecologia | volume = 175 | issue = 1 | pages = 63–72 | date = May 2014 | pmid = 24390479 | doi = 10.1007/s00442-013-2873-7 | s2cid = 5038227 | bibcode = 2014Oecol.175...63G }}</ref>
== Flowering mechanism == A biological mechanism is proposed based on the information we have above. Light is the flowering signal of ''Arabidopsis thaliana''. Light activates photo-receptors<ref name="Distinct roles of GIGANTEA" /> and triggers signal cascades in plant cells of apical or lateral meristems. Action potential is spread via the phloem to the root and more voltage-gated calcium channels are opened along the stem. This causes an influx of calcium ions in the plant. These ions bind to calmodulin and the Ca<sup>2+</sup>/CaM signaling system triggers<ref name="Experience teaches" /> the expression of GI mRNA or FT and CO mRNA. The accumulation of GI mRNA or GI-CO-FT mRNA during the day causing the plant to flower.<ref name="Distinct roles of GIGANTEA" />
== References == {{reflist}}
== Further reading == {{refbegin}} * {{cite journal | vauthors = Zeevaart JA | title = Florigen coming of age after 70 years | journal = The Plant Cell | volume = 18 | issue = 8 | pages = 1783–1789 | date = August 2006 | pmid = 16905662 | pmc = 1533981 | doi = 10.1105/tpc.106.043513 | bibcode = 2006PlanC..18.1783Z }} * {{cite book |vauthors=Zeevaart JA |title=Plant Physiology Online |chapter=FT Protein, not mRNA, is the Phloem-Mobile Signal for Flowering |date=September 2007 |chapter-url=http://4e.plantphys.net/article.php?ch=&id=398 |access-date=2009-04-02 |archive-date=2008-12-28 |archive-url=https://web.archive.org/web/20081228144206/http://4e.plantphys.net/article.php?ch=&id=398 }} * {{cite journal | vauthors = | title = Breakthrough of the year: the runners-up | journal = Science | volume = 310 | issue = 5756 | pages = 1880–1885 | date = December 2005 | pmid = 16373539 | doi = 10.1126/science.310.5756.1880a | doi-access = free }} {{refend}}
{{Plant hormones}}
Category:Plant hormones