For the First Time in Forever: Vernalization

Frozen

Next up in the Frozen series… we’re talking about flowering for the first time in ‘forever’ (really, just after winter). Here’s the Disney version for your reference.

FRIGIDA’s gone, so’s FLC

Winter is over? Can it be?

We need to transcribe a thousand different genes

For months we’ve waited in dormancy

Halted all pluripotency

Finally, it’s time to make some scenes!

There’ll be actual real live flowers

It’ll be totally great

I’m so ready for this activated state!

‘Cause for the first time in forever

There’ll be flow’ring, there’ll be growth

For the first time in forever

Translating AP1 and LFY both!

It must be the gibberellic acid

There’s active meristematic zones

‘Cause for the first time in forever

FLC is gone

I can’t wait to bloom, everyone!

What if I meet… the one?

This Spring imagine me petals and all,

Fetchingly vining up the wall

My flowers a delicately shaped vase

Ooh! I suddenly see him buzzing by

A striped pollinator that can fly

I’m gonna shove some pollen in his face!

But then we trade pollen and nectar

Which is completely strange

Not like when FLC is in detectable range

For the first time in forever

There’ll be flowers, there’ll be sun

For the first time in forever

I’ll be making AP1

And I know it is totally crazy

Excitement over meristems

But for the first time in forever

There’s activity in them

[FLC:]

Don’t let them out, don’t turn them on

Winter could still go on and on and on

Repress, turn off, it’s not time to grow,

Make one wrong move and get killed by snow

[FLC:] Is it only for today?

[FT:] It seems like Spring today!

[FLC:] It’s agony to wait

[FT:] It’s agony to wait

[FLC:] The VRNs are saying open up the gate

[FT:] The gate

[FT:] For the first time in forever

[FLC:] Don’t turn them on, don’t let them out

[FT:] I’m getting what I’ve waited for

[FLC:] Be the regulator the plant needs about

[FT:] A chance to change my stagnant world

[FLC:] Repress

[FT:] A chance to pollinate!

[FLC:] Repress, turn off, don’t let them out

I know it all ends in autumn,

So it has to start today

‘Cause for the first time in forever

For the first time in forever

Nothing’s in my way!

Flowering is important for plants because it’s how they reproduce. So, they need to have ways of ensuring that this process occurs under conditions that are most favorable for seed production. Plant species solve this problem in a variety of ways. Some annual plants avoid harsh conditions altogether and complete their life cycle within a single warm growing season. Others must find ways of going dormant during the freezing winter months and wait out the season until spring comes back. Plants integrate information on both day-length and duration of cold temperatures so they are not fooled into flowering on a random warm day in winter.

Under any conditions, the transition to flowering requires an elaborate set of gene expression changes so that cells achieve the proper developmental fate. The perennials and winter annual plants that require a cold period of dormancy prior to the flowering transition have an extra layer of regulation to ensure the transition occurs at the right time. Plants accomplish this with a set of gene regulators, some promoting the floral transition and others repressing it. These tiered layers of control allow for seemingly binary (on/off) switches to become a way of fine-tuning the development of flowers in these plants.

It’s not unlike the opposing personalities of Anna and Elsa in the movie Frozen. The gene FLC functions to keep plants from flowering, while genes like AP1, SOC1, FT, and LEAFY turn on genes to promote flower production. In perennials and winter annual plants, FRIGIDA serves to keep plants from flowering through the work of FLC until a certain amount of winter temperatures have passed. The genes VIN3, VRN1 and VRN2 act to let the plants know that enough winter weather has occurred and the hold on flower development (via FLC and FRI) can be let go. The way that the repression system works is also an interesting case. It’s not that these proteins physically block transcription of certain genes; these repressors leave covalent marks on the genome and the histone proteins whose job it is to organize the DNA within the cell nucleus. It takes some effort on the part of the activators to undo these marks to subsequently turn on the necessary genes.

Major players controlling flowering in plants. Pointed arrows mean activation or promotion, while blocked lines indicate repression.

Major players controlling flowering in plants. Pointed arrows mean activation or promotion of the next downstream thing, while blocked lines indicate repression of the next step in the pathway.

It’s interesting to note that all of the genes mentioned in the above paragraph are gene transcription regulators of one variety of another. This highlights a continuing theme in how pathways for development or environmental acclimation are controlled. There are a small set of regulators which control sets of other genes that are also regulators or that actually do the necessary biochemical work. These regulators work together to integrate environmental information (in this case-temperature, light, day-length) and keep developmental programs in check. Scientists tend to identify these factors pretty early in their investigations of these big questions, usually because mutants in these important regulators have obvious defects in the process of interest.

However, I have to admit, this drives me crazy as a biochemist. I want to know what’s going on beyond the nucleus in these processes. Sure, we may know a lot about what conditions turn these regulators and other genes on and off, but we know very little about the physical mechanisms by which plants sense the winter temperatures and time how long they’ve persisted. What is the actual ‘winter sensor’ in plants? How is it different from the cold sensor that plants use for cold acclimation on the timescale of hours or just a few days? How does this winter sensor differ among varieties with different ‘chill requirements’ before they flower and make fruit in the spring?* These components remain a mystery, but one that plant scientists are furtively working to solve.

Vernalization (the plant biology word of the day) is the acquired ability of a plant to flower after a cold season. For our perennials and winter annual plants, the passing of sufficient winter (as measured by the mysterious ‘winter sensor’ and regulated by VIN3, VRN1, VRN2, and FRI) gives these plants the ability to burst forth in flowers in the spring. Even though these regulators have a protective role for the plants in keeping them from trying to flower in the freezing winter, once their control is finally released, the flurry of activity associated with flower development is akin to a grand celebration after what seems like a lifetime of seclusion. Upon flowering, it’s all about reproduction. When it comes to plants, there’s not even a pretense of modesty; they have no problems trading pollen grains with a pollinator they have just met.

A number of economically important agricultural crops, like wheat, cabbage, carrots and most fruit trees, have vernalization requirements for flowering. Understanding how this process works is critical for getting the most out of these plants where they are grown and expanding the arable ranges for these crops. It may be some time before plant biologists make a Honeycrisp apple tree I can grow in Louisiana, but even modest adjustments to vernalization requirements to staple crops like wheat may be needed to accommodate predicted climate changes.

For more on how plants remember winter, check out this post over at The Quiet Branches.

Johnna

*This is why apple trees (with varieties requiring 400 – 1000 chill hours) don’t flower efficiently and produce fruit in a place as far south as Baton Rouge, LA (~200 – 300 chill hours), but figs (requiring only 100 – 200 chill hours) do.

References and Links:

http://www.azlyrics.com/lyrics/idinamenzel/forthefirsttimeinforever.html

http://en.wikipedia.org/wiki/Vernalization

http://www.newphytologist.org/app/webroot/img/upload/files/amasino.pdf

http://www.plantcell.org/content/16/10/2553.full

https://www.jic.ac.uk/staff/caroline-dean/vernalization.htm

http://plantcellbiology.masters.grkraj.org/html/Plant_Growth_And_Development11-Physiology_Of_Vernalization.htm

http://www.weigelworld.org/research/projects/floweringtimes

http://www.pnas.org.libezp.lib.lsu.edu/content/111/27/10001.abstract

http://www.ndsu.edu/pubweb/~mcclean/plsc731/homework/kim-et-al-vernalization-winter-%20and-the-timing-of-flowering-in-plants.pdf

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4 thoughts on “For the First Time in Forever: Vernalization

  1. Pingback: Frozen: A Plant Science Parody | New Under The Sun Blog

  2. Ian Street

    Reblogged this on The Quiet Branches and commented:
    A great post on flowering that complements the one I wrote this week. It is true, plant scientists don’t know exactly how cold is perceived by plants with respect to flowering. They just know it ends up silencing FLC.

    Reply

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