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winding_road.jpgThe Long and Winding Road

So far, this journey through the subject of how plants make flowers has consisted of three parts:

Part 1, an introduction to the flowering hormone florigen,

Part 2, how environmental cues affect flowering,

..and, Part 3, how the size and age of the plant itself may trigger flowering.

The 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.

The latest scientific evidence supports the hypothesis that florigen is actually a protein called FT coded for by the gene Flowering Locus T.

Most of the other key genetic “players” turn out to be proteins called transcription factors, 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.

flower(genes).jpgAn 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.

The Genetics of Flowering (A Story in Three “Acts”)

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

Act 1 – Floral Initiation (From Vegetative To Inflorescence Meristem)

At center stage (currently), is SOC1 (Suppressor of Overexpression of Constans 1), a gene coding for a TF in the MADS-box family. SOC1 protein plays a pivotal roll in the great leap from vegetative meristem to inflorescence meristem (IM). The expression of SOC1 is effected, directly and indirectly, by factors known to induce flowering, such as the plant hormone gibberellin and FT protein (a.k.a., florigen).

FT gets into the act by first binding to a bZip TF called FD protein (gene product of Flowering Locus D). 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. (Further on down the trail, we’ll see how vernalization knocks off FLC and thus promotes flowering.)

flower2(genes).jpgAct 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 such TF genes as 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, carpel, and stamen – 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. (Though I think it’s the ABCD model now, but that’s for a later date.)

Bottom Line: For a visual summary of the above feel free to download and play this PowerPoint file: Flower_Genetics.ppt or see the corresponding YouTube video here.

Next Up: Making a Flower – Part 5: How does photoperiod induce florigen (FT protein) synthesis in leaves?

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