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240px Tile young man flowers Louvre D27813The Players

Because the genetic story of how plants flower turns out to involve many cellular “players”, as well as an intricate plot, perhaps it would be a good idea to first introduce the main “cast of characters”.

Let’s start with florigen.

As previously described, this is the so-called flowering hormone that can trigger the floral transition in plants, and it is likely a small protein called FT.

Most of the other key genetic “players” turn out to be proteins called transcription factors (TF), which bind to specific DNA sequences and affect gene transcription.

Many of the flowering-related transcription factors (TFs) are members of a “family” called MADS-box TFs.

A specialized TF called FD protein (gene product of Flowering Locus D gene) is a so-called bZip TF. It turns out that FD partners with FT to promote flowering

An especially interesting member of this MADS-box family with regard to flowering is the FLC protein. FLC (the product of a gene called Flowering Locus C) actually represses flowering.

SOC1 (Suppressor of Overexpression of Constans1), a gene coding for a TF in the MADS-box family that plays a pivotal role in the story of flower initiation, at least in Arabidopsis.

The Stage

Since flowering takes place in the shoot apical meristem (SAM), let’s set the stage there.

Please keep in mind (1) that this is a very simplified version of a very complex, and as yet incomplete, story and (2) that most of this story is based on a single plant – Arabidopsis thaliana – though the basic storyline is likely the same for most flowering plants.

320px HepaticaNobilisSLO flowerAct 1 -Floral Initiation (From Vegetative To Inflorescence Meristem)

At center stage is SOC1. This TF protein plays the main roll in the great leap from vegetative meristem to inflorescence meristem (IM).

The expression of SOC1 is affected, both directly and indirectly, by factors known to induce flowering, such as the plant hormone gibberellin and the FT protein (a.k.a., florigen).

FT gets into the act by first binding to FD (see above). Together FT/FD promote SOC1 gene expression.
(Though FT is not a transcription factor, it acts as a “key” to activate FD protein, which is a TF.)

Finally, the antagonist in “Act 1” is the FLC protein (see above). It inhibits flowering by suppressing the expression of the SOC1 gene. (We’ve previously seen how vernalization knocks off FLC and thus promotes flowering.)

Act 2 -”Arranging the Chairs” (From Inflorescence to Floral Meristem – Part 1)

The second act of the story involves the first step in the transition from the inflorescence meristem (IM) to the floral meristem (FM).

What’s the difference?

Well, think of the transition from vegetative to IM as “making the decision” to flower, without any overt signs of flowering. And the IM –> FM transition is actually starting to build a flower.

The first step in building a flower involves the spatial arrangement of the flower parts, sort of analogous
to arranging the chairs in a room for a meeting. This involves new “players” as such TF genes called LEAFY (LFY) and APETAL1 (AP1), which are both activated by SOC1 and FT/FD.

Act 3 -”Seating the Guests” (From Inflorescence to Floral Meristem -Part 2)

There are four “guests” to be seated at the end of our story – sepal, petal, stamen, and carpel – the four basic floral organs.

The genes involved in floral organ identity are called homeotic genes. Together they are responsible for the so-called “ABC model” of floral organ development, which will be discussed at length in a future post. (For an excellent review on flower development, see reference below.)

GOF
A “Simplified” Model of the Genetic Pathways Involved in Flowering, Plus Some of Their Effectors.
(For a YouTube video of this figure, just click on it.)

Reference

Irish, V. F. (2010) “The flowering of Arabidopsis flower development.” The Plant Journal, Vol. 61, pp. 1014-1028. (Full Text)

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