Florigen
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Florigen (or flowering hormone) is the hypothesized
hormone A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are require ...
-like molecule responsible for controlling and/or triggering
flowering A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechanism ...
in plants. Florigen is produced in the leaves, and acts in the
shoot apical meristem The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...
of
bud In botany, a bud is an undeveloped or embryonic shoot and normally occurs in the axil of a leaf or at the tip of a stem. Once formed, a bud may remain for some time in a dormant condition, or it may form a shoot immediately. Buds may be spec ...
s 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 disputed.


Mechanism

Essentially, to understand florigen, you must first understand how flowering works. For a plant to begin flowering, it must make its changes to the
shoot apical meristem The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...
(SAM). However, there are factors the plant must first consider before it begins this process such as the environment but even more specifically, 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." The way the plant determines this optimal time is through day-night periods through the use of
photoperiodism Photoperiodism is the physiological reaction of organisms to the length of night or a dark period. It occurs in plants and animals. Plant photoperiodism can also be defined as the developmental responses of plants to the relative lengths of light a ...
. 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. 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'')" # "Long-day plants (LDPs) flower only in long days (''qualitative LDPs''), or their flowering is accelerated by long days (''quantitative LDPs'')" 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. 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. Plants also have the ability to prevent the response of the photoperiodic stimulus until a certain temperature is reached. Species like winter wheat that rely on just that. The wheat require a cold period before being able to respond to the photoperiod. This is known as
vernalization Vernalization (from Latin ''vernus'', "of the spring") is the induction of a plant's flowering process by exposure to the prolonged cold of winter, or by an artificial equivalent. After vernalization, plants have acquired the ability to flower, ...
or overwintering. This ebb-and-flow of flowering in plants is essentially controlled by an internal clock known as the ''endogenous oscillator''. 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 nd thatlight may augment the amplitude of the oscillations by activating the morning and evening genes." 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. This process is known as photoperiodic induction and is a photoperiod-regulated process that is also dependent on the endogenous oscillator. 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. 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 In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The f ...
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 Translation is the communication of the meaning of a source-language text by means of an equivalent target-language text. The English language draws a terminological distinction (which does not exist in every language) between ''transla ...
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 some light. CO protein promotes
transcription Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including: Genetics * Transcription (biology), the copying of DNA into RNA, the fir ...
of another gene called (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 thus, the induction of flowering—relies on a comparison between the plant's perception of day/night and its own internal biological clock.


Translocation

The FT protein resulting from the short period of CO transcription factor activity is then transported via the
phloem Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is c ...
to the shoot apical meristem.


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. 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 Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a sp ...
. 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''. 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 The Basic Leucine Zipper Domain (bZIP domain) is found in many DNA binding eukaryotic proteins. One part of the domain contains a region that mediates sequence specific DNA binding properties and the leucine zipper that is required to hold tog ...
containing transcription factor, FD, is somehow interacting with FT to form a transcriptional complex that activates floral genes. Studies using rice found that there is an interaction between Hd3a and OsFD1,
homologs A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization. Homologs have the same genes in the same loci where they provide points alon ...
of FT and FD respectively, that is mediated by the 14-3-3 protein GF14c. 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. 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'').


Antiflorigen

Florigen is regulated by the action of an antiflorigen. Antiflorigens are hormones that are encoded by the same genes for florigen that work to counteract its function. The antiflorigen in ''Arabidopsis'' is ''TERMINAL FLOWER1'' (''TFL1'') and in tomato it is ''SELF PRUNING'' (''SP'').


Research history

Florigen was first described by Soviet Armenian plant physiologist
Mikhail Chailakhyan Mikhail Khristoforovich Chailakhyan ( hy, Միքայել Քրիստափորի Չայլախյան, russian: Михаи́л Христофо́рович Чайлахя́н) (1902–1991) was an Armenian-Soviet scientist who is widely known for pro ...
, who in 1937 demonstrated that floral induction can be transmitted through a
graft Graft or grafting may refer to: *Graft (politics), a form of political corruption * Graft, Netherlands, a village in the municipality of Graft-De Rijp Science and technology *Graft (surgery), a surgical procedure *Grafting, the joining of plant t ...
from an induced plant to one that has not been induced to flower. Anton Lang showed that several long-day plants and biennials could be made to flower by treatment with
gibberellin Gibberellins (GAs) are plant hormones that regulate various developmental processes, including stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence. GAs are one of the longest-known classes of plan ...
, 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. 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. 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. 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 Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is c ...
to the growing tip at the
shoot apical meristem The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...
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. 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". The initial article that described FT mRNA as flowering stimuli was retracted by the authors themselves.


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. 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, Ca2+ binding to calmodulin, a Ca2+-binding protein in animals and plants, controls gene transcriptions.


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 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 Ca2+/CaM signaling system triggers 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.


References


External links

* * * {{Plant hormones Plant hormones