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Archive for the ‘Flowering’ Category

646px Satyrium pumilum 160403Some Pollinators Attracted By The Scent Of Death

Floral odors, produced by the vast majority of flowering plants, play important roles in plant–pollinator interactions.

A recent report of an orchid that attracts pollinators with the smell of carrion (see reference 1 below) reminded me of the infamous Voodoo lily, which was the subject of one of the professors in my department when I was a grad student.

Before I get to the Voodoo lily (a.k.a. “corpse flower”), what’s the story about this orchid?

From the abstract of ref 1:
Although pollination of plants that attract flies by resembling their carrion brood and food sites has been reported in several angiosperm families, there has been very little work done on the level of specificity in carrion mimicry systems and the importance of plant cues in mediating such specialization.

The authors, who are at the University of KwaZulu-Natal, South Africa, studied the orchid Satyrium pumilum, native to the dry inland regions of the southwest cape of South Africa, and a local assemblage of carrion flies that pollinated this plant.

Briefly, from the conclusion of this paper:
“Satyrium pumilum selectively attracts flesh flies, probably because its relatively weak scent resembles that of the small carrion on which these flies predominate.

I’ve previously posted about the Voodoo lily (Sauromatum guttatum) with regard to thermogenesis in plants. But I didn’t tell much about the awful smell this flower produces.

The odor of the flowering Voodoo lily is somewhat infamous. Imagine what hamburger would smell like if a package of it sat inside a car in the summer, for about a week. It’s really that bad.

Turns out that some of the volatile chemical constituents of the odor produced by the Voodoo lily are also produced by the stinkhorn mushroom (see ref 2 below).

Botanical Term of the Day: “Sapromyiophily”

Hundreds of individual plant species from at least eight plant families “…emit odours reminiscent of rotting fish, carrion or dung. These odours mimic the substrate to which insects within the orders Coleoptera and Diptera (Wiens, 1978; Faegri & Van der Pijl, 1979) are usually attracted in order to oviposit or feed. The attraction of flies to brood-site and food mimics has given this distinct, deceptive pollination syndrome its common name: sapromyiophily. Sapromyiophilous flowers present adaptations to their special method of pollinator attraction involving situation, shape, colour, pattern, texture, scent, thermogenesis, motile appendages and changes of posture (Proctor et al., 1996). The plant families involved are diverse, yet they show both clear parallels between families and, nevertheless, a high variation within families.” (from ref 3 below, which is, by the way, a good reference source)

Bottom line: What’s in a name? That which we call a “corpse flower”, by any other name would smell as sweet…to a carrion fly. (apologies to W. Shakespeare)

References

1. Timotheüs van der Niet, Dennis M. Hansen and Steven D. Johnson (2011) “Carrion mimicry in a South African orchid: flowers attract a narrow subset of the fly assemblage on animal carcasses.” Annals of Botany, doi: 10.1093/aob/mcr048.
(Abstract PDF)

To see photos related to this paper, click here.

2. Anna-Karin Borg-Karlson, Finn O. Englund and C. Rikard Unelius (1994) “Dimethyl oligosulphides, major volatiles released from Sauromatum guttatum and Phallus impudicus.” Phytochemistry, vol. 35, pp. 321-323. (Abstract PDF)

3. Andreas Jürgens, Stefan Dötterl and Ulrich Meve (2006) “The chemical nature of fetid floral odours in stapeliads (Apocynaceae-Asclepiadoideae-Ceropegieae).” New Phytologist, vol. 172, pp. 452-468. (Full Text PDF)

HowPlantsWork © 2008-2011 All Rights Reserved.

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SauromGutt.jpg“Hot Plants?”

In the previous post, the topic was how plants survive the cold. Although some perennial plants can withstand winter temperatures well below zero (F), plants certainly don’t generate body heat like mammals do in order to warm themselves.

Or do they?

There are a few plants in nature, like the remarkable Voodoo Lily (Sauromatum guttatum), that produce extraordinary heat when they flower. What actually warms up when the plant flowers is part of the inflorescence, called a spadix.

Typically, the plants do this to attract insect pollinators. But some, such as the Eastern skunk cabbage may actually use this mechanism against the cold.

In the case of the Voodoo Lily, flies are lured by chemical attractants, which are volatilized by the heat of the spadix. (The chemicals smell to us like putrid, rotting meat.)

The process of heat production by living organisms is called thermogenesis. And though it’s far from common in the plant kingdom, thermogenic plants occur in several plant families, especially the Araceae. Members of this plant family include the Eastern skunk cabbage and the giant carrion flower.


(from: Giant stinking flower reveals a hot secret)

Much fewer plants, however, are able to thermoregulate, that is, they actually regulate the temperature of thermogenesis within narrow limits. For an excellent slide-show about plant thermoregulation, see here (PDF).

How Do Plants Generate Heat?

meeuse.jpgMuch about what we know about how the Voodoo Lily spadix, for example, generates heat came from the research of Professor Bastiaan J. D. Meeuse.

Among his discoveries about heat production in plants, Dr. Meeuse and co-workers showed that a compound related to aspirin triggers pronounced heat production in the flowers and inflorescences of some thermogenic plants.

Briefly, heat generation in these plants is due to the massive activation of the alternative oxidase metabolic pathway in the mitochondria inside the plant cells.

Simply put, when this happens, instead of generating ATP as result of metabolizing sugars via oxidative phosphorylation, the mitochondria generate heat.

Bottom line: Though some plants can generate heat to promote flower pollination, it’s unlikely that they do so just to survive cold temperatures.

References

1. Meeuse B.J.D. (1966) The Voodoo Lily. Scientific American vol. 218, pp. 80-88.

2. Meeuse, B.J.D. (1975) Thermogenic Respiration in Aroids. Ann. Rev. Plant Physiology vol. 26, pp. 117-126. (Abstract)

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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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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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bud1.jpgIs There a Single Flower-Inducing Hormone?

Florigen is the signal that triggers the transition from vegetative to reproductive development in plants that flower in response to photoperiod.

But some plants, that I’ll refer to “Night-Neutral” (a.k.a., “Day-Neutral”), apparently initiate flowering because of factors other than night length.

Such plants may flower after attaining a certain size or age, for example. Thus, floral induction in these plants may happen mainly in response to internal (endogenous) conditions rather than to environmental (external) conditions.

Some plants may not produce flowers until they are sufficiently robust enough to support the drain on resources required by flowering. In other words, a plant may not flower until it has enough leaves (photosynthetic sugar production) to build and support flowers.

This size-related competency to flower may also be gauged by the plant’s age, presuming that the older a plant is, the bigger it is.

But if one proposes that some plants flower in response to size or age, important questions arise, such as:

How does a plant “know” how big or how old it is?

In plants that flower in response to internal cues (such as size or age), does florigen still play a primary role?

How Do Plants “Know” How Big They Are?

hourglass.jpgOne way plants may be able to determine their relative size is by “node counting”. That is, the more nodes (stem buds/leaves) the plant has, the bigger (more productive) it is. (For all you scholars out there, an exhaustive review of “node counting” can be found here.)

A plant may also gauge its size by how far the shoot apical meristem (SAM) is from the roots. Or a plant may determine its overall size by how big a root system it has.

There is scientific evidence for all of these possibilities. However, the key to all of them is that the nodes, the roots, or both produce chemical signals (likely one or more of the common plant hormones) that travel via the phloem to the SAM. (The SAM is where the floral transition will take place.)

Thus, flowering may be triggered at the SAM by a threshold amount of – or ratio of – one or more plant hormones.

How Do Plants “Know” How Old They Are?

It’s conceivable that a plant can obtain relative age info from the same ways it may estimate its size mentioned above.

It’s also been proposed that certain substances in plants (likely specific proteins) may start out at high levels in young seedlings, but then slowly decrease over the life of the plant (think sand through an hour-glass). Once the substance drops below a certain level in the SAM, the floral transition may then proceed.

interwoven.jpgMultiple Pathways Lead to Flowering

This, of course, is a big old subject in plant biology, with countless studies published over its hundred years of history. The past few years, however, have yielded much genetic insight into how plants make flowers.

From these genetic studies (mainly using the plant Arabidopsis thaliana) scientists have discovered the identity of florigen (much more on this later). These studies have revealed that the genetic mechanisms involved in floral induction are complex and are affected not only by florigen but by other plant chemical signals, such as gibberellins, as well as by environmental factors, such as temperature.

Indeed, a recently published genetic study has reported a newly discovered signaling pathway that ensures that a plant flowers, no matter what.

Bottom Line: There is likely a central genetic mechanism, common to all flowering plants, that initiates flowering. This mechanism is triggered not only by florigen but is also affected by other endogenous and environmental factors.

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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?

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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.

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