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Flower_alrm_clock.jpgIt’s Time to Flower!

The correct timing of flowering is essential to maximize reproductive success in angiosperms.

And many flowering plants rely on the photoperiod (specifically, the relative night length) as an environmental signal to tell seasonal time. (To see how, please see previous posts about How Plants Tell Time and Why Plants Tell Time.)

As mentioned in the previous post, the so-called “flowering hormone”, historically known as florigen, is likely a small protein called FT.

Briefly, FT is produced in the leaves and is transported via the phloem to the shoot apical meristem (SAM). Here FT acts like a molecular “alarm-clock”, evoking a complex genetic scenario, which culminates in flower formation.

But what sets off this “alarm-clock”, i.e. the production of FT in the leaves?

Turns out the story involves red, far-red, and blue light, the length of the night, and the plant’s biological clock. (Please note: Why night length is more important than day length: animated explanation.)

First some caveats:arabidopsis.jpg

1. Most of this information is based on genetic research using the plant Arabidopsis thaliana. (Although specific genes and proteins vary, depending on plant species, it appears that the basic story presented below holds for most photoperiodic flowering plants.)

2. Arabidopsis is a so-called long-day (LD) flowering plant (in reality, a “short-night” plant, but don’t get me started). So, adjustments in the story need to be made for so-called short-day (SD) plants. (Yes, they really are “long-night” plants.)

3. In Arabidopsis florigen is likely the FT protein. In some SD cereals (such as rice), florigen is likely a protein called Hd3a, an ortholog of FT protein.

A Light-Sensitive, Flowering Alarm-Clock

The so-called biological clock in plants is set primarily in the leaves by phytochromes, which are sensitive to red and far-red light. They get help from blue-light-sensitive cryptochrome. These photoreceptors interact with “clock-genes” that cause some proteins in plant cells to cycle with a circadian rhythm.

One of these proteins regulates the gene that codes for florigen (FT in Arabidopsis and Hd3a in rice, for instance).

Thus, florigen cycles in the leaves also with a circadian rhythm.

Briefly, in LD (“short-night”) plants florigen apparently peaks not long after sundown, then slowly degrades during the night. If the nights are too long, the florigen level is below the threshold level to induce flowering at dawn, when the leaves begin to transport material to the SAM via the phloem. (Please note: florigen appears to be synthesized primarily by leaf vein cells adjacent to the phloem.)

Conversely, in SD (“long-night”) plants, the florigen apparently peaks long after sundown. So, if the night is too short, at dawn, the florigen hasn’t exceeded the threshold level to trigger flowering.

For more information, click on image below:

photo_flowering.jpg

References:

1. Zeevaart, J.A.D. (2007) FT Protein, not mRNA, is the Phloem-Mobile Signal for Flowering. (see here)

2. Bäurle, I. and Dean, C. (2006) The Timing of Developmental Transitions in Plants. Cell, vol. 125, pp. 655-664 (see here)

3. Greenup, A., et al. (2009) The molecular biology of seasonal flowering-responses in Arabidopsis and the cereals. Annals of Botany, vol. 103, pp. 1165-1172. (see here)

HowPlantsWork © 2008-2011 All Rights Reserved.

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2679000774_9c952dc3c1.jpgMany Plants Flower in Response to Night Length

For nearly 100 years scientists have been trying to identify the elusive flowering hormone called florigen.

Early in the last century two USDA researchers took a major step toward this by discovering how to induce flowering in plants under controlled conditions. In 1920, these two scientists, W.W. Garner and A.H. Allard, first published (PDF) their work on the effect of photoperiod on flowering in tobacco, soy bean, and many other plants. (Their findings are summarized here and nicely described with an historical perspective at a USDA webpage.)

At first, scientists thought that the day-length was the controlling factor in inducing flowering. Hence, plants were divided into three groups with regard to photoperiodic effects on flowering.

We now know that the night-length is more important than the day-length in inducing flowering in responsive plants. So, we can divide flowering plants into three groups – “Short-Night” plants, “Long-Night” plants, and “Night-Neutral” plants. (Unfortunately, most textbooks persist in using the old – and incorrect! – nomenclature. Sigh.)

Thus, many plants make the flowering transition from vegetative growth in response to a very dependable environmental cue, namely, the photoperiod.

But What Does This Have To Do With Florigen?

Firstly, by finding a way to induce many plants to flower at will by adjusting the photoperiod in the laboratory, Garner and Allard set the experimental stage for the eventual discovery of florigen.

In other words, this finding allowed other scientists to artificially induce the floral transition in some plants. Thus, by enabling them to initiate flowering at will, scientists began to study the sequence of events in how plants make flowers.

364664434_5cacbe2022.jpg Secondly, it was discovered that plants sense the photoperiod in their leaves. (We’ll see how they do this later on.)

But the flower transition occurs, not in the leaves, but at the apical meristems.

Therefore, in plants that flower in response to photoperiod, some sort of flower-inducing signal must be sent from the leaves to the shoot apex.

This signal turned out to be florigen.

Are There Other Environmental Cues That Induce Flowering?

The short answer is: Yes.

The long answer is: Some biennial plants, such cabbage and carrots, require a long period (weeks) of “cold” (below 35o to 40o F) to become competent to flower. (Please note that this does not induce flowering but allows flowering to be induced.)

The story is a complex one, however. (See more about this here).

Bottom Line: By discovering a way to induce flowering via photoperiod, the first steps were taken toward the identifying a flowering hormone in plants.

Next-Time: Are there endogenous signals, other than florigen, that induce flowering in plants?

HowPlantsWork © 2009-2011 All Rights Reserved.

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2699470516_bbc1ca00fd.jpgThe Mystery of the Flowering Hormone

What if you discovered a chemical that, when sprayed onto the leaves of plants, would induce them to flower?

How much do you think the patent on such a chemical would be worth? Especially to the agricultural and horticultural industries.

And what if I told you that scientific evidence for the existence of such a flower-inducing chemical has been known for nearly 100 years? And that whole scientific careers have been devoted to discovering this chemical…mostly in vain.

The story is true….and the hypothetical flowering hormone was even given a name in 1936 by the Russian scientist Mikhail Chailakhyan. He called it florigen* (derived from Latin for “flower-former”).

When did the story of the elusive flowering hormone florigen begin?

What Causes Plants to Flower?3030351845_4eec0308f1.jpg

As mentioned in a previous post, unlike animals, plants don’t start out with their “naughty bits” – they have no sexual organs, a.k.a., flowers.

Before flowering, plants grow “vegetatively”, that is, they produce just stems, leaves, and roots.

It’s a very big deal when the transition from a vegetative plant to a flowering plant occurs. This involves the “flipping” of some major genetic “switches”, that is, major changes in gene regulation.

Florigen is apparently the signal that “flips the switch”, that is, it’s the internal chemical signal that triggers the floral transition in plants.

But to understand the physiology of the floral transition, scientists first needed a way to be able to induce flowering in vegetative plants under controlled conditions.

A major breakthrough toward this goal was reported in 1920…and not long after, scientific evidence for the existence of a flowering-inducing signal emerged.

Next-Time: What environmental factors induce the flowering transition in plants?

*More information on florigen can be found at Wikipedia. And for a more scientific discussion of florigen, please see a 2007 review by Jan Zeevaart.

HowPlantsWork © 2008-2011 All Rights Reserved.

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