Florigen
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Florigen (or
flowering hormone) is the term used to describe the hypothesized
hormone-like molecules responsible for controlling and/or triggering
flowering in plants. Florigen is produced in the leaves and acts in the
shoot apical meristem of
buds and growing tips. It is known to be graft-transmissible and even functions between species. However despite having been sought since the 1930s, the exact nature of florigen is still a mystery.
Contents
[hide]
[edit] Mechanism
Central to the hunt for florigen is an understanding of how plants use seasonal changes in day length to mediate flowering, a mechanism known as
photoperiodism. Plants which exhibit photoperiodism may be either 'short day' or 'long day' plants, which in order to flower require short days or long days respectively. Although plants in fact determine day length from night length.
[1]
The current model suggests the involvement of multiple different factors. Research into florigen is predominately centred around 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.
[2] 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.
[edit] Initiation
In
Arabidopsis, 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 biological clock.
[3] This mRNA is then
translated into CO protein. 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 a little light.
[4][5] CO protein promotes
transcription of another gene called Flowering Locus T (FT). 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 so the induction of flowering, relies on a comparison between the plant's perception of day/night and its own internal biological clock.
[2]
[edit] 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.
[6]
[edit] Flowering
At the shoot apical meristem the FT protein is thought to interact with another transcription factor, FD protein, to activate floral identity genes, thus inducing flowering.
[7][8] Specifically, arrival of FT at the shoot apical meristem and formation of this FT/FD heterodimer is followed by the increased expression of: SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1),
[9] LEAFY (LFY),
[10] APETALA 1 (AP1),
[7] SEPALLATA 3 (SEP3) and FRUITFUL (FUL).
[11]
[edit] Research history
Please help improve this section by expanding it. Further information might be found on the
talk page.
(April 2009) Florigen was first described by Russian plant physiologist
Mikhail Chailakhyan in 1937, who demonstrated that floral induction can be transmitted through a
graft from an induced plant to one that has not been induced to flower.
[12] Anton Lang showed that several long-day plants and biennials could be made to flower by treatment with
gibberellin, 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.
[13] 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.
[12] 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).
Problems with isolating florigen and the inconsistent results acquired led to the suggestion that florigen does not exist; rather, a particular ratio of other hormones must be achieved for the plant to flower.
[14][15] 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.
[edit] References
- ^ Garner W.W., Allard H.A. (1920). "Effect of the relative length of day and night and other factors of the environment on growth and reproduction in plants". Journal of Agricultural Research 18: 553–606. doi:10.1175/1520-0493(1920)48<415b:EOTRLO>2.0.CO;2.
- ^ a b Turck, F., Fornara, F., Coupland, G. (200. "Regulation and Identity of Florigen: FLOWERING LOCUS T Moves Centre Stage". Annual Review of Plant Biology 59: 573–594. doi:10.1146/annurev.arplant.59.032607.092755.
- ^ Mizoguchi, T., Wright, L., Fujiwara, S., Cremer, F., Lee, K., et al. (2005). "Distinct roles of GIGANTEA in promoting flowering and regulating circadian rhythms in Arabidopsis". Plant Cell 17: 2255–2270. doi:10.1105/tpc.105.033464.
- ^ Yanovsky, M.J., Kay, S.A. (2002). "Molecular basis of seasonal time measurement in Arabidopsis". Nature 419: 308–312. doi:10.1038/nature00996.
- ^ Valverde, F., Mouradov, A., Soppe, W., Ravenscroft, D., Samach, A., Coupland, G. (2004). "Photoreceptor regulation of CONSTANS protein in photoperiodic flowering". Science 303: 1003–1006. doi:10.1126/science.1091761.
- ^ Corbesier, L., Vincent, C., Jang, S., Fornara, F., Fan, Q., et al. (2007). "FT protein movement contributes to long distance signalling in floral induction of Arabidopsis". Science 316: 1030–1033. doi:10.1126/science.1141752.
- ^ a b Abe, M., Kobayashi, Y., Yamamoto, S., Daimon, Y., Yamaguchi, A., et al. (2005). "FD, a bZIP protein mediating signals from the floral pathway integrator FT at the shoot apex". Science 309: 1052–1056. doi:10.1126/science.1115983.
- ^ Samach, A., Onouchi, H., Gold, S.E., Ditta, G.S., Schwarz-Sommer, Z., et al. (2000). "Distinct roles of CONSTANS target genes in reproductive development of Arabidopsis". Science 288: 1613–1616. doi:10.1126/science.288.5471.1613.
- ^ Yoo, S.K., Chung, K.S., Kim, J., Lee, J.H., Hong, S.M., et al. (2005). "CONSTANS activates SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 through FLOWERING LOCUS T to promote flowering in Arabidopsis". Plant Physiology 139: 770–778. doi:10.1104/pp.105.066928.
- ^ Eckardt, N.A. (2007). "Phloem-Borne FT Signals Flowering in Cucurbits journal=Plant Cell". The Plant Cell Online 19: 1435–1438. doi:10.1105/tpc.107.053447.
- ^ Teper-Bamnolker, P., Samach, A. (2001). "The flowering integrator FT regulates SEPALLATA3 and FRUITFULL accumulation in Arabidopsis leaves". Cell 17: 2661–2675. doi:10.1105/tpc.105.035766.
- ^ a b Chaïlakhyan, M.K. (1985). "Hormonal regulation of reproductive development in higher plants". Biologia Plantarium 27: 292–302. doi:10.1007/BF02879865.
- ^ Chaïlakhyan, M.K. (1975). "Substances of plant flowering". Biologia Plantarium 17: 1–11. doi:10.1007/BF02921064.
- ^ Zeevaart, J.A.D. (1976). "Physiology of flower formation". Annual Reviews of Plant Physiology and Plant Molecular Biology 27: 321–348. doi:10.1146/annurev.pp.27.060176.001541.
- ^ Bernier, G., Havelange, A., Houssa, C., Petitjean, A., and Lejeune, P. (1993). "Physiological signals that induce flowering". Plant Cell 5: 1147–1155. doi:10.1105/tpc.5.10.1147.
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