The Ripe Stuff – Part 3: How Plants Respond to Ethylene

Sniffing Out Ethylene

In the previous post I summarized the latest information on how the plant hormone ethylene is made (at least in the model plant Arabidopsis).

Though most people are familiar with the plant hormone ethylene mainly because of its effects on fruit ripening, the list of plant responses to ethylene is long.

How do plants manage to detect (“sniff out”) minute quantities of this gaseous plant hormone?

And how do minute quantities of ethylene, once detected by the plant, elicit such a wide array plant responses, from stimulating seed germination in some plants to promoting aging in others, for example?

Detecting the Ethylene Signal

In order to listen to FM radio, you have to have a FM receiver.

That is, in order to detect FM radio signals (at frequencies invisible to our eyes), you have to have a way to detect such weak signals. (The rest of the radio is for amplifying the signal so you can hear it.)

How do plants detect the weak ethylene signal?

They have ethylene receivers, or in biological terms, ethylene “receptors” that can very specifically discern ethylene from other small molecules. (Hormone receptors are typically proteins because they have the ability to form very complex and precise 3D structures. This allows for their high specificity in recognizing distinct molecular structures.)

It turns out that each ethylene molecule interacts with two receptors, which are paired together to form a pair or “dimer”.

In the photo on the right, the two rings are like the two individual ethylene receptors, and the gymnast is like ethylene.

Interestingly, these ethylene receptors are transmembrane proteins located inside plant cells in the endoplasmic reticulum (ER). (Since ethylene is a relatively small gas molecule, it can easily pass through the plant cell membrane from outside to inside the cells.)

In Arabidopsis there apparently is a 5-member family of these individual ethylene receptors. Each member of the family can pair with another copy of itself to form a “homodimer” functional ethylene receptor or with other members of the family to form a “heterodimer”. (The term “homodimer” is used when the two molecules are identical, e.g. A-A, and “heterodimer” when they are not, e.g. A-B.)

Thus, the ethylene serves as a “key” in the receptors’ “lock”, setting in motion a cascade of events…….

Amplifying the Ethylene Signal
You text a secret to two friends. Then each of your friends texts the secret to two more people. Then each of these people text the secret to two of their friends. And on, and on, and pretty soon it’s no longer a secret.

This “cascade” of events effectively “amplifies” the original text message. The ethylene signal is similarly amplified via a cascade of cellular events. For example, when ethylene binds to its receptors, this causes conformational changes in these receptor proteins, which then “activates” them…somewhat analogous to you using your car key (ethylene) to start your car (receptor proteins).

The “activated” receptor proteins, in turn, initiate an intracellular signal cascade ultimately leading to the expression of an array of genes, which collectively result in the “display” we observe, whether it be inhibition of shoot growth or fruit ripening.

Which genes ethylene “turns on” depends on the biological “context”, that is, what part of the plant?, what stage of the plant’s development?, what plant species?, etc.

Qiagen offers an illustration of “The Big Picture” of the ethylene signaling pathway in Arabidopsis, though it’s somewhat technical.

Bottom line: Decades of scientific research has revealed not only the receptors for ethylene in several plant species but also a complex cascade of cellular events leading to the plants’ responses to ethylene that we can observe in nature.

The Ripe Stuff – Part 2: How Plants Make Ethylene

C’est un gaz. Es un gas. Es ist ein gas. It’s a Gas!

If you are familiar with the gaseous plant hormone ethylene, it’s most likely because of its ability to promote fruit ripening in apples, bananas, tomatoes, etc..

Ethylene also affects many other aspects of a plant’s life cycle, including seed germination, growth and development, responses to physical and biological stress, leaf abscission, and senescence.

It may even be responsible for how some herbicides kill plants.

But how does ethylene work?

That is, how can this simple gas (CH₂-CH₂) mediate so many different processes in plants? Where and how is this chemical signal produced in plants? And how is this signal received by plant cells and then amplified into the responses we observe?

Making the Signal

Virtually all the cells in a plant have the potential to make ethylene. Plant cells make ethylene starting from the amino acid methionine (see below).

And it turns out that during the course of a plant’s life – from seed germination to senescence – ethylene may be made not only as part of the developmental program of the plant, but also in response to random events, such as wounding and pathogen attack.

The primary way plants (at least Arabidopsis) regulate the amount of ethylene is through the expression of a family of genes that encode enzymes (the ACS protein family) that catalyze the synthesis of the key metabolic precursor to ethylene, namely, ACC*.

Methionine –> SAM* –ACS proteins –> ACC* + O₂ –> Ethylene
* SAM = S-Adenosyl-L-Methionine, ACC = 1-AmminoCyclopropane-Carboxylate

(Please see here for diagram of ethylene biosynthetic pathway in plants.)

Quoting part of a recent report (ref. 1 below) on the genetics of ethylene biosynthesis:

“We view the ACS protein family as a “Symphony Orchestra” (45-member when all nine genes are expressed in a cell) that regulates ethylene-mediated processes by generating appropriate amounts of ACC in the proper spatial and temporal manner through their harmonious interplay. At any given moment, the orchestra is tuned by various inducers to produce ACC sufficient to mediate myriad ethylene responses…”

So, this may explain why the biosynthesis of ethylene in plants can be affected by so many different factors.

Next-time: The Ripe Stuff – Part 3: How Plants Respond to Ethylene

References:

1. Tsuchisaka, A., et al. (November 2009) “A Combinatorial Interplay Among the 1-Aminocyclopropane-1-Carboxylate Isoforms Regulates Ethylene Biosynthesis in Arabidopsis thaliana.” Genetics vol. 183, pp. 979-1003. (PDF)

The Ripe Stuff: Green Tomatoes, Bananas & Ethylene

Fried Green Tomatoes

About this time of year in the North Temperate Zone people may be getting very tired of fried green tomatoes and green tomato pie.

As a plant physiologist, I was often asked how to ripen green tomatoes. (Rather than go into this here, I’ll refer you to this recent blogpost.)

You probably already know that tomato ripening is promoted by the gaseous plant hormone ethylene.

Regarding ethylene, one of the best online resources I’ve found provides comprehensive information about this gas, including a table listing the relative production of, and sensitivity to, ethylene in a large number of fruits and vegetables.

And if you want to inhibit fruit ripening, there’s even a commercial website for reducing ethylene.

Going Green to Red (or Yellow)

But what’s happening to the tomato when it ripens from green to red? And to the banana when it goes green to yellow (and then brown)?

The change in color is primarily the result in the active conversion of chloroplasts to chromoplasts. The chlorophyll is actively (enzymatically) broken down, and the red and yellow colors are due mainly to carotenoid pigments in the chromoplasts.

Since the fruits are there primarily to promote seed dispersal by animals, red colors apparently tend to attract birds, for example. Interestingly, as reported here, at tropical temperatures (about 90^o F), the Cavendish banana skin tends to stay green, though the fruit ripens.

“Over-ripe” fruits (the brown banana, for example) tend to be primarily the result of senescence.

Fruit Softening

The “softening” of the fruit is the result of the breakdown of plant cell walls by cell wall-digesting enzymes such as cellulases, pectinases, and expansins.

Indeed, scientists were able to extend the shelf-life of the Flavr Savr tomato by effectively turning off the gene coding for a pectinase.

Getting Sweeter

Another characteristic of fruits that tends to attract some animals for seed dispersal is the presence of sugars. Increased sugars in the process of fruit ripening is due mainly to the production of enzymes that break down starch.

Interestingly, the process of fruit ripening involves many enzymes that digest polymers of sugar molecules (i.e., cell walls and starch).

How does ethylene promote ripening?

The expression of many of the above enzymes is promoted by ethylene in plant species that have so-called climacteric fruits, such as tomatoes and bananas. (Please see here for a list of climacteric and non-climacteric fruits.)

Climacteric fruits are characterized by a burst of cellular respiration that is often immediately preceded by, or happens simultaneously with, a sharp increase in ethylene. (Ethylene also may promote its own biosynthesis.)

Ethylene is an essential component of climacteric fruit ripening. Blocking ethylene biosynthesis or action prevents ripening.

Ethylene apparently regulates the expression of many ripening-associated genes, including those coding for enzymes involved in color change, fruit softening, and starch breakdown. (How ethylene activates such genes is a topic for another day.)

How “One Bad Apple Spoils the Bunch” (and ripens your green tomatoes).

As mentioned above, ethylene may promote its own biosynthesis in climacteric fruits.

So an over-ripe apple producing a lot of ethylene gas – which diffuses throughout the bunch – triggers the biosynthesis of ethylene in the rest of the apples (and also in your green tomatoes), thus promoting the ripening process.

Bottom line: For climacteric fruits, the gaseous hormone ethylene is indeed “the ripe stuff’.

How Plants Make Flowers – Part 5: Photoperiod, Biological Clocks, and Florigen

It’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:

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:

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)

How Plants Make Flowers – Part 4: Genetics of Flowering

The 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.

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.

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.)

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 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?

Life in the Phyllosphere: What Microbes Commonly Dwell on the Surface of Leaves?

Leaf Surfaces = Microbial Habitats

Imagine all the leaves of all the plants currently living on planet Earth.

Now, add up all the surface areas of all of those leaves.

And your answer is?… No idea?….

Luckily, some microbiologists have made an estimate, and it’s an astounding number.

According to a current review (see ref 1 below), the terrestrial leaf surface area that might be colonized by microbes is approximately 640,000,000 square kilometers or about 250 million square miles! (For perspective, consider that the total surface area of the Earth is 197 million square miles.)

From this, microbiologists have estimated that the planetary leaf-surface bacterial population on Earth may be as large as 10²⁶ cells! (Yes, that’s the numeral 1 with 26 zeros behind it.)

“Clearly, in aggregate, these bacteria are sufficiently numerous to contribute in many processes of importance to global processes, as well as to the behavior of the individual plants on which they live.”-(from ref 1 below).

In other words, the bacteria that live on the leaf surface are so numerous that they not only likely affect the plant on which they dwell but, collectively – on a planetary scale – they are so numerous as to significantly affect the global carbon and nitrogen cycles on Earth.

The Phyllosphere

The term “phyllosphere” was first published by Dr. Jakoba Ruinen in 1961 (see ref 2 below), who studied tropical ecology beginning in the 1950’s. She called the interface between leaves and air the “phyllosphere”, and said that this was a much neglected milieu, compared to studies of the rhizosphere.

“Under the microscope, aerial plant leaves resemble eerie landscapes, with deep gorges, tall peaks and gaping pits that riddle the waxy surface. [e.g., see photo below] Add to this scenery a climate that features temperature highs of 50 °C [122 °F] or more, exposure to harmful ultraviolet rays, erratic periods of drought and limited access to nutrients, and one gets the picture that this is a hostile environment. Still, many bacteria, fungi, yeast and other microorganisms dwell in great abundance in this ‘phyllosphere’…” (from ref 3 below).

Who Lives in the Phyllosphere?

“The microbial communities of leaves are diverse and include many different genera of bacteria, filamentous fungi, yeasts, algae, and, less frequently, protozoa and nematodes.” (from ref 1 below)

Most of what we know about these leaf “epiphytes” come from culturing (or trying to culture) representative isolates in the lab.

In a recent report (ref 4 below), the investigators have taken a 21^st-century approach to the question: “What bacteria live in the phyllosphere?”.

Using a new “metaproteogenomic” approach, they analyzed the bacteria associated with leaves of field-grown soybean and clover plants and wild populations of Arabidopsis thaliana plants. (This technique is related to proteogenomics.)

This study not only revealed “who” was there, but also provided clues to how such bacteria survive in the phyllosphere. Briefly, they discovered “…a high consistency of the communities on the 3 different plant species, both with respect to the predominant community members…” and with respect to the proteins that these bacteria apparently use to survive such a relatively hostile environment.

Photo Credit: Top two photos above by Koolpix/ Jay D., who has been awarded the Nature Photo of the Week by the Nature Conservancy. Twice! (please see here and here). Congratulations! And thank you!

Bottom Line: Using new molecular biology techniques, scientists have discovered important new information regarding not only what bacteria live in the phyllosphere but also how they do it.

References

1. Lindow, S. and Brandl, M.T. (2006) “Microbiology of the Phyllosphere”, Applied and Environmental Microbiology vol. 69, pp. 1875-1883. (full text)

2. Ruinen, J. (1961) “The Phylloshere. I. An Ecologically Neglected Mileau”, Plant and Soil vol. 15, pp. 81-106. (preview PDF)

3. Leveau, J. (2009) “Life on leaves.” Nature vol. 461, p. 741. (Abstract)

4. Delmotte, N., et al. (2009) “Community proteogenomics reveals insights into the physiology of phyllosphere bacteria” Proceedings of the National Academy of Sciences (USA) vol. 106, pp. 16428-16433. (full text).

How Plants Make Flowers – Part 3: Endogenous Cues

Is 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?

One 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.

Multiple 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.

How Plants Make Flowers – Part 2: Environmental Cues

Many 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.

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 35^o to 40^o 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?

How Plants Make Flowers

The 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?

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.

Does the Moon Affect Plants? Part 3: Gravitational Effects?

Does the Moon’s Gravity Affect Trees?

To me this sounds like a silly question.

Why?

Because I’d no more expect the moon’s gravity to affect trees than it affects the water in a pond or swimming pool.

If the moon doesn’t elicit tidal effects in such small bodies of water, then why would it’s gravity affect trees?

But, largely due to so-called “peasant beliefs” passed down through the ages, there are some who are convinced that the moon’s gravity does indeed have measurable effects on trees.

Do the Tides Correlate with Tree Rhythms?

In 1998, Ernst Zurcher and colleagues published this paper in the scientific journal Nature claiming that they had evidence that there was a correlation of tree stem diameters and the tides.

This, and other lunar effects on trees, was discussed by Zurcher in a subsequent publication (see ref. 1 below).

In 2000, a another team of scientists provided evidence against Zurcher’s conclusions (see ref. 2 below).

You Decide

From the physics of gravity explained by a fellow by the name of Isaac Newton, it seems obvious that the moon’s gravity is too weak, the distance from Earth to moon too large, and even a giant Sequoia’s mass is too small for the moon to have any significant gravitational effect on trees.

But the notion simply won’t die.

So you, dear reader, weigh the evidence – even do the calculation – and be the judge.

References
1. Zurcher, E. (1999) “Lunar Rhythms In Forestry Traditions – Lunar-Correlated Phenomena In Tree Biology And Wood Properties .” Earth, Moon, and Planets vol. 85-86, pp. 463-478. (PDF)

2. Vesala, T., et al. (2000) “Do tree rings shrink and swell with the tides?” Tree Physiology vol. 20, pp. 633-635. (PDF)

Bottom Line: Despite what we know about the laws of gravity and even direct scientific evidence to the contrary, some still believe that the moon affects trees as it does the tides. Oh well. Sigh.