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In Your Future: Springtime In February?

May 28th, 2012 by

Flowers snowFast-Forward Flowering

Do many plants seem to you to be flowering earlier in the spring? You may be correct.

One of the most interesting scientific articles I’ve recently read reports that plants are indeed flowering earlier, and, more significantly, faster than most experimental models have predicted.

(Excellent summaries of this report, including photos, can be found here, here and here.)

In summary, these investigators state that “…we have shown that warming experiments underpredict the advance of spring events observed over recent decades. Furthermore, when sampling the same species, the experiments failed to predict both the magnitude and the direction of plant responses to warming. Such differences between observed and experimentally estimated temperature sensitivities indicate that experimental results alone should not be used for parameterizing species distribution and ecosystem models.” (from Ref 1 below)

Briefly, these scientists used millions of individual observations of the timing of life cycles (phenology) of over a thousand plant species spanning four continents to show that current scientific models are failing to accurately predict the impact of global warming on plants.

Want to get involved and be a citizen scientist?

Then check out the USA Phenology Network (or other such networks worldwide) to learn how to be an observer and report your results.

Bottom Line: Projections of how plants will change with increasing global warming are very uncertain, and these changes may be greater than most scientific models have predicted.

Reference
1. Wolkovich, E. M., et al. (2012) “Warming experiments underpredict plant phenological responses to climate change.” Nature, Vol. 485, pp. 494–497. (Abstract)

Posted in Ecological Physiology, Flowering, Plant Development | No Comments »

What’s New in Plant Biology (May 25, 2012)

May 25th, 2012 by

2942400127 4e4d06be38 zSuper weeds and early flowers.
(from: Nature, May 25, 2012).

The weeds are winning, at least for the time being. So-called “superweeds” have evolved in response to massive increases in the use of the herbicide Roundup®. (See previous post about this.)

What’s a farmer to do?

The answer from the chemical companies is to switch to other herbicides and to genetically engineer crops to withstand them. Many weed scientists disagree. They think a more multifaceted approach to weed control should be used. Who’s right? (Read about the controversy here.)

How will climate change, or what I call “global weirding”, affect flowering in the future? A paper published in this week’s Nature asserts that most of the experiments aimed at answering this question are wrong, significantly under-estimating plant phenological responses to climate change. (Read summary of article here.)

Photoreceptors galore.

Long-day afternoons may actually help some plants to flower. See how in the May 25, 2012 issue of Science.
If this isn’t enough to whet your plant photoreceptor appetite, the May 3, 2012 issue of Molecular Plant has a banquet of photoreceptor treats. Start with a summary by Winslow Briggs, and then dine on full-text articles. Thanks, Molecular Plant!

Plant-symbiotic fungi and bacteria may infect plants in similar ways.

Both fungal mycorrhizae and the bacteria Rhizobium form symbiotic relationships with plants via the roots. Though the endpoint of these partnerships is different, according to a recent report in PNAS, both of these very different microorganisms may establish their symbioses with similar subcellular mechanisms.

The essence of a tasty tomato.

The plant story that’s making the rounds this week has to do with the biochemical deconstruction of what makes homegrown tomatoes taste good. Read about it here and here, and listen to an interview with the teller of this tale (Prof. Harry Klee) on NPR’s Science Friday program.

HowPlantsWork © 2008-2012 All Rights Reserved.

Posted in Plant Science | No Comments »

“Farmaceuticals” and “Plantibodies” – Using Genetically-Engineered Plants to Produce Drugs and Vaccines

May 20th, 2012 by

Placebo New Meanings For “Medicinal Plants”?

Plants have been used by humans for thousands of years as a source of medicines, some effective, many not so much (except perhaps as placebos).

The first botanists were likely shaman herbalists who possessed the knowledge of which plants would kill and which plants would cure.

Contemporary shamans can now perhaps be found at the local food co-op or health food store in the herbal supplements section. Some may even be found ensconced in universities or drug companies. These PhD botanists, microbiologists, chemists and pharmacists are combing the planet, from oceans to jungles, in search of exotic plants that may contain compounds that cure cancer, AIDS, or you name it.

Such bioprospecting for plant-derived medicines is an active field and is a risky, but potentially lucrative, endeavor.

But instead of searching for natural plant compounds with medicinal properties, some scientists are using well-known crops, even tobacco plants, as solar-powered bioreactors to synthesize medicines and vaccines using plant genetic engineering.

PharmingPlant Molecular Pharming

With the blossoming of plant genetic engineering in the 1980s came a new way of looking at plants as potential sources of medicines.

Would it be possible, for instance, to use genetically-engineered (GE) plants to bio-synthesize drugs such as insulin and human growth hormone on a commercial scale?

Could we also use such plants as sources of vaccines? For example, by eating a couple of specially-GE bananas, could your child be effectively vaccinated against whooping cough or polio?

Much was made of so-called molecular pharming in the 1990s, but for various technical and regulatory reasons, the actual impact of plant biotechnology on the pharmaceutical industry has, to date, fallen way short of most of the predictions.

Recent approval by the FDA of a drug produced in plant cells may, however, herald a renaissance in plant molecular pharming.

“Drug-Making Plant Blooms”

As reported in the May 10, 2012 issue of Nature, the first biological drug for human use that is manufactured inside modified plant cells was approved by the US Food and Drug Administration (FDA).

Professor Charles Arntzen, a plant biotechnologist at Arizona State University in Tempe and one of the early proponents of edible plant vaccines, says that this approval “…sends a clear and positive signal to investors and companies that plant-manufactured drugs are worth pursuing.”

Farmaceutical: “A medically valuable compound produced from modified agricultural crops or animals (usually through biotechnology).”

Plantibody: “A plantibody (a portmanteau derived from Plant and antibody) is an antibody produced by genetically modified crops. Antibodies are part of animal immune systems, and are produced in plants by transforming them with antibody genes from animals.”

HowPlantsWork © 2008-2012 All Rights Reserved.

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Posted in Plant Biotechnology, Plant Cells, plant genes, Plant Metabolism, Plant Science | No Comments »

Weed Control Using “Agent Orange Corn”: A Really Bad Idea?

May 5th, 2012 by

BackToTheFutureLogo“Back To The Future.”

A while back on this blog, I spent a bit of time exploring how some herbicides kill plants.

The focus was primarily on auxin-based herbicides such as 2,4-D (one of the herbicides in the notorious Agent Orange) and glyphosate, commonly known as Roundup®. The former kills primarily broadleaf plants, and the latter basically kills all plants.

In these discussions, I briefly mentioned the topic of so-called “superweeds”, which have developed a natural resistance to Roundup®. Most agree that this is a consequence of a large increase in the use of this herbicide in the past 15 years mainly because of the widespread cultivation of genetically-modified (GM) soybeans, alfalfa, corn, cotton, canola, and sugarbeets resistant to Roundup®, a.k.a., Roundup-Ready® crop plants.

In the years since I wrote these blog posts, these superweeds have become even more of a problem.

In response, farmers and ranchers are turning back to killing weeds with the old auxin-based herbicides such as 2,4-D and dicamba, which have been used for nearly 50 years. (These herbicides are the active ingredients in such products as “Weed B Gon®” that have been used for many years mainly to kill dandelions on lawns.)

Some seed and agrichemical companies are also responding to the new Roundup®-resistant superweeds by developing new GM cultivars of crop plants such as soybean and corn that can tolerate relatively high levels of 2,4-D and dicamba. (These GM plants contain bacterial enzymes that break down these herbicides.)

If these new herbicide-resistant GM crops are widely adopted, then the old auxin-based herbicides will likely be used much more heavily over vastly expanded areas of the US.

Not everybody thinks this is such a great idea.

Here Comes “Agent Orange Corn” (Or Not).

What brought me back to this subject were two recent online articles, one in the New York Times and one in Wired magazine.

The New York Times article chiefly discussed the opposition to the application to the USDA by Dow Chemical for approval of their 2,4-D resistant corn cultivars called Enlist®.

(Detractors have deemed this so-called “Agent Orange corn” because, as mentioned above, 2,4-D was one of the components of this defoliant used in the Vietnam War. This moniker is somewhat inaccurate because the other auxin-based herbicide 2,4,5-T used in Agent Orange was the more toxic of the two. Indeed, the EPA has recently ruled that 2,4-D is not seriously toxic.)

Enlist® corn is opposed not only by environmentalists concerned with potential health risks of 2,4-D, but also by farmers concerned with the deleterious “drift effects” of auxin-based herbicides. Drift effects happen when these herbicides sprayed on one field drift through the air into adjacent fields harming susceptible crop plants such as sugar beets, sunflowers, soybeans, and fruit trees.

Despite the opposition, it looks as though these new herbicide resistant GM plants will likely be officially approved by the government.

4984458563 600768b297 nSuper Superweeds – The Next Generation?

Perhaps the best argument against the adoption and widespread use of these new herbicide resistant GM crops (HRGMC) is evolution.

As pointed out in an excellent article in the January 2012 issue of Bioscience (see Ref. 1 below), greatly increased use of auxin-based herbicides anticipated as result of the widespread adoption of HMGMC’s will have intense evolutionary selection pressure on weeds. This, as with the glyphosate-resistant superweeds, will probably lead to the emergence of new generation of superweeds resistant to 2,4-D, dicamba, or both. Indeed, we may even see the emergence of super superweeds that resist not only these auxin-based herbicides but also Roundup®. Woo hoo!

As also described by the authors of Ref. 1, there are at least two other reasons to oppose this new generation of HRGMC’s. The first is the degradation of environmental quality due to a large increase in the total amount of herbicide used to manage weed control in this country. Secondly, by relying so heavily on herbicides to control weeds as a short-term fix will negatively affect smarter solutions to the problem, namely, integrated weed management (IWM).

IWM “… is characterized by reliance on multiple weed management approaches that are firmly underpinned by ecological principles.” (from Ref. 1 below)

Update: Are professional weed scientists choosing to ignore true integrated solutions to the herbicide resistance problem? (Please see Ref. 2 below for an answer to this question.)

Bottom Line: Despite the opposition, we’re probably going to see a new generation of HRGMC’s in the not-too-distant future, instead of more IWM. Seems to me this is a really bad way to go.

References
1. Mortensen, D. A., J. F. Egan, B. D. Maxwell, M. R. Ryan, and R. G. Smith (2012) “Navigating a critical juncture for sustainable weed management.” Bioscience, Vol. 62, pp. 75–84. (Full Text)
2. Harker, K. N., J. T. O’Donovan, R. E. Blackshaw, H. J. Beckie, C. Mallory-Smith, and B. D. Maxwell (2012) “Our View.” Weed Science, Vol. 60, pp. 143-144. (Full Text)

HowPlantsWork © 2008-2012 All Rights Reserved.

Posted in Plant Evolution, Plant Hormones | No Comments »

The Genetics of Flowering as a Play in Three “Acts”

Apr 28th, 2012 by

240px Tile young man flowers Louvre D27813The 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, and it is likely a small protein called FT.

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

A specialized TF called FD protein (gene product of Flowering Locus D gene) is a so-called bZip TF. It turns out that FD partners with FT to promote flowering

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.

SOC1 (Suppressor of Overexpression of Constans1), a gene coding for a TF in the MADS-box family that plays a pivotal role in the story of flower initiation, at least in Arabidopsis.

The Stage

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

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.

320px HepaticaNobilisSLO flowerAct 1 -Floral Initiation (From Vegetative To Inflorescence Meristem)

At center stage is SOC1. This TF protein plays the main roll in the great leap from vegetative meristem to inflorescence meristem (IM).

The expression of SOC1 is affected, both directly and indirectly, by factors known to induce flowering, such as the plant hormone gibberellin and the FT protein (a.k.a., florigen).

FT gets into the act by first binding to FD (see above). 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. (We’ve previously seen 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 new “players” as such TF genes called 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, stamen, and carpel – 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, which will be discussed at length in a future post. (For an excellent review on flower development, see reference below.)

GOF
A “Simplified” Model of the Genetic Pathways Involved in Flowering, Plus Some of Their Effectors.
(For a YouTube video of this figure, just click on it.)

Reference

Irish, V. F. (2010) “The flowering of Arabidopsis flower development.” The Plant Journal, Vol. 61, pp. 1014-1028. (Full Text)

HowPlantsWork © 2008-2012 All Rights Reserved.

Posted in Flowering, Plant Development, plant genes | No Comments »

A Light-Sensitive Flowering “Alarm Clock”

Apr 24th, 2012 by

4587529742 c20d03b56bWe’ve seen that some plant species flower “autonomously” , that is, with little or no regard to environmental signals.

However, most of what is known about how plants make flowers comes from research on plants that do rely on environmental guidance for flower initiation.

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.)

The latest scientific evidence supports the hypothesis that the flower-inducing hormone florigen is actually a protein called FT. (The name “FT” comes from the finding that this protein is genetically coded for by the gene “Flowering Locus T” in plant molecular biologists’ favorite experimental plant Arabidopsis thaliana.)

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 (more on this to come).

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 is night length more important than day length? I don’t have the room to present the evidence here, but for a great summary, please see an animated explanation.)

ArabidopsisFirst, 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, which is very similar to the Arabidopsis 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, and 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.

Florigen cycling
A Simplified Model of Florigen Cycling and Transport in a “short-night” (Arabidopsis) versus a “long-night” (rice) plant. Proteins (red) include: FT (Flowering Locus T), Hd3a (Heading date 3a), CO (Constans) , HD1 (Heading date 1), and Ehd1 (Early heading date 1). Proposed florigens in Arabidopsis (FT) and rice (Hd3a) cycle in leaves with circadian rhythm, but with different phases in response to different photoperiods. At dawn, when photosynthesis resumes in leaves, phloem transport from leaves to SAMs also resumes, carrying florigen. If florigen is above a threshold level, then it may trigger the floral transition in the SAM.

References

1. 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. (Full Text)

2. Mach, J. (2011) “Will the real florigen please stand up? Sorting FT homlogs in maize.” The Plant Cell, Vol. 23, p. 843. (Full Text)

HowPlantsWork © 2008-2012 All Rights Reserved.

Posted in Flowering, Plant Development, plant genes | No Comments »

Plant “Puberty” – Acquiring A “License To Flower”

Apr 15th, 2012 by

Is There a Single Flower-Inducing Pathway?

It’s well known that “florigen” is the signal that triggers the transition from vegetative to reproductive development in plants that flower in response to photoperiod.

But some plants, for example the “Night-Neutral” (a.k.a., “Day-Neutral”) plants, apparently initiate flowering because of factors other than night length.

In plants that flower independently of environmental factors, the flower-inducing pathway is currently called “autonomous”.

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.

Plant Puberty?Seedling

The stage of human life called “puberty” is quite well-known to most people, especially to those currently experiencing it.

But do plants go through puberty, too?

Strictly speaking, no. But most flowering plants do progress through developmental stages somewhat analogous to puberty.

After germination, many flowering plant species enter a juvenile phase in which they are not “competent” to flower. That is, even when experiencing conditions favorable to flowering, they lack the ability to flower.

This may be because 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 juvenile phase to adult phase transition, which affects many aspects of plant development, is the classic example of an “autonomous” flowering pathway.

The plant will become competent to flower after it makes a developmental transition to its adult phase, which may be determined primarily by the size of the plant. 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, Ref. 1 below provides an exhaustive review of “node counting”.)

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.

5641308012 4640074fdd How Do Plants “Know” How Old They Are?

Aspen trees my not become competent to flower until they are over ten years old. But how does a tree “know” how many years have passed since it was a seedling?

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. (For an excellent review, please see Ref. 2 below)

From these genetic studies (mainly using the plant Arabidopsis thaliana) scientists have discovered the identity of florigen (much more on this here). 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 genetic study published in 2009 reported a 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.

Slide1 copy

References

1. Sachs, T. (1999) “‘Node counting’: an internal control of balanced vegetative and reproductive development.” Plant, Cell & Environment, Vol. 22, pp. 757-766. (Full Text)

2. Amasino, R. (2010) “Seasonal and developmental timing of flowering.” The Plant Journal, Vol. 61, pp. 1001-1013.(Full Text)

HowPlantsWork © 2008-2012 All Rights Reserved.

Posted in Flowering, Plant Development, Plant Signaling | No Comments »

How Plants Make Flowers – Environmental Cues

Apr 7th, 2012 by

2679000774 9c952dc3c1Many Plants Flower in Response to Night Length

For nearly 100 years scientists tried 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, soybean, 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 shoot apical meristems (SAM).

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?

Another important environmental cue that regulates flowering time is temperature.

In temperate regions of the world, flowering plants may use temperature as a significant seasonal indicator. Indeed, it’s well known that warmer conditions can accelerate flowering in spring.

And there is some recent information from scientific research that suggests that lower temperatures may actually inhibit the production of florigen. (For more on this see Ref. 1 below.)

364664434 5cacbe2022But we all know how variable spring-time temperatures can be from year to year, due to fluctuating weather patterns. So, the photoperiod is likely a much more reliable cue for plants with regard to the correct time to flower.

Another effect of temperature on flowering in plants has to do with sensing the passage of winter. Many plants from temperate regions flower only after they experience an extended period of relatively cold weather, or vernalization. Specifically, vernalization results in “…the acquisition or acceleration of the ability to flower by a chilling treatment.” (from Ref. 2 below) This cold exposure does not necessarily cause flowering but rather renders the plant competent to do so. (Please see Ref. 3 below for an excellent review on the subject.)

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.)

A requirement for vernalization permits biennials to become established during the fall without the risk of flowering as winter begins. During the winter, these plants experience and “remember” a cold treatment, which enables them to flower during the favorable conditions of spring.

Unlike photoperiod, which is perceived in leaves, cells of the SAM directly sense cold and become “vernalized”, that is, competent to flower. This is because a protein that blocks flowering is removed by vernalization, which does so by causing the gene for this protein to be chemically “pad-locked” (DNA methylation) so that is can’t be transcribed.

This, by the way, is how the plant “remembers” it has experienced winter, weeks later on in the spring. The story is a complex one, however. (For more about this, please see here).

Bottom Line: Plants may respond to a combination of photoperiod, temperature, and vernalization to ensure optimal timing of flowering.
But by discovering a way to systematically induce flowering primarily via photoperiod, Garner and Allard took the first major steps toward the identifying a flowering hormone in plants.

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

References

1. . 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. (Full Text)

2. Chouard, P. (1960) “Vernalization and its relations to dormancy.” Annual Review of Plant Physiology, Vol. 11, pp 191-238.

3. Amasino, R. (2004) “Vernalization, competence, and the epigenetic memory of winter.” The Plant Cell, Vol. 16, pp. 2553-2559. (Full Text)

HowPlantsWork © 2008-2012 All Rights Reserved.

Posted in Flowering | No Comments »

Hothouse Flowers: Why Does A Warm Spring Promote Flowering?

Mar 23rd, 2012 by

Warming Temps –> “Fast-Forward” Flowering
Unusually warm temperatures often cause some plants to flower early.

For example, this spring in eastern North America unseasonably high temperatures have induced some plants to put flowering on “fast forward”.

Coincidentally, a recently-published research report (see Ref. 1 below) has revealed a genetic “switch” that may trigger the flowering process in some plants as temperatures increase. (An excellent description of this research and its potential implications can be found here.)

How does this so-called “genetic switch” work?

Simply put, it promotes the production of the flowering hormone called florigen, which triggers plants to make the transition from a vegetative state to a flowering state.

I won’t go into what florigen is and how it works here, because this has been discussed in several previous posts (see here, for example). (Or you can read all about it in my e-book “How Plants Make Flowers”.)

Suffice it to say that florigen is a protein produced in the leaves that travels via the phloem to the apical meristems where it activates genes involved in floral induction. (Florigen protein can do so because it is a transcription factor.)

A Genetic Thermostat For Flowering

This newly reported research indicates that warmer temperatures somehow induce the production of a protein called PIF4, which, in turn, promotes the production of florigen.

Apparently, the gene coding for PIF4 is only active when it is warm. So, when plants are competent to flower, and they experience unseasonably warm temperatures, PIF4 may likely be the primary reason that some plants will flower early.

Now, since we have better information about the genetics of how warm temperatures induce flowering, this may allow scientists to modify plants’ responses to temperature changes through genetic modification or plant breeding. Being able to do this may help produce crop plants that are more resilient when faced with unusually high temperatures likely to occur as a result of “global weirding”.

References
1. Kumar, S. V., et al. (2012) “Transcription factor PIF4 controls the thermosensory activation of flowering.” Nature, published online 21 March 2012. (Abstract)

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How To Make A Decaf Coffee Plant

Mar 15th, 2012 by

540515222 tpWhy decaf coffee?

Have you ever had a cup of really good decaf coffee?

Me neither.

(Probably because the decaf coffee results from chemical processing of normal coffee beans.)

But why would anyone want coffee without caffeine?

As illustrated by the old Gary Larson cartoon on the left, sometimes you can get too much caffeine. Or maybe you just prefer coffee without the caffeine buzz. Or maybe you are a Mormon.

For whatever reason, there certainly is a very large potential market for coffee beans that naturally have very low or no caffeine.

3720001337 3dee0e6a02Indeed, coffee plant breeders have been trying to develop such varieties for decades, with very little success.

The subject was brought to my attention by an excellent article in the March 15, 2012 issue of Nature magazine. (See reference 1 below.)

It’s a fascinating story about “the enduring quest for a coffee bean without the buzz” and about why it’s been so difficult to produce coffee plants that don’t make caffeine.

Briefly, since it involves a complex metabolic pathway, the biosynthesis of caffeine in coffee plants has been a challenge for plant genetic engineers to effectively eliminate. (Anyway, many people would reject “genetically-modified” coffee.)

Breeding commercially-valuable coffee varieties with other low-caffeine species has also been frustrating.

Perhaps the best chances at obtaining commercially-viable decaf coffee plants will come from mutagenesis. This involves treating coffee seeds with a chemical that increases the probability of genetic mutations occurring and selecting any resulting decaf “mutant” plants. Indeed, about a half-dozen such plants – preliminarily trademarked “Decaffito” – have been found after screening nearly 30,000 potential mutants.

By the way, you can enjoy the complete story, along with informative graphics and a pertinent audio excerpt from the Nature podcast, without having a subscription to Nature. So I highly recommend that you head on over to the Nature website and check it out. (Thank you, Nature!)

449721832 04a59ee1d1But why do some plants make caffeine?

Simply put, it’s a chemical defensive compound that helps the plant to deter herbivory.

Caffeine is one of many plant “secondary compounds”, including nicotine and morphine, called alkaloids that can sometimes have dramatic effects on animals. Some alkaloids are even deadly. (For example, see my previous post regarding “wicked” plants.)

Plants are marvelous biochemists. They not only can produce sugars from CO2, water, and light (photosynthesis) but also can metabolize sugars to make energy or cellulose. All plants can perform such so-called “primary” metabolic pathways.

Secondary metabolic pathways are so-called because only some plants do them, not because they are necessarily secondary in importance to the plant. (“Secondary compounds” is perhaps an unfortunate name for them because many of these compounds are critical to plant survival, especially against herbivores.)

Such natural plant compounds range from vanilla to oil of Wintergreen, from aspirin to cocaine – so many thousands of amazing compounds that it’s way beyond the scope of this blog post.

But I’ll leave you with this parting shot – There is good evidence to suspect that some of these so-called “plant” compounds are actually synthesized by microorganisms, such as fungi or bacteria, called endophytes that live, literally, inside the plant (or “epiphytes” that live in the phyllospere, i.e., on the surface of leaves).

Bottom line: Although many people would surely want to have naturally-decaffeinated coffee, would this leave such coffee plants susceptible to some insect pests?

Reference

1. Borrell, B. (2012) “Plant biotechnology: Make it a decaf.” Nature, Vol. 483, pp. 264–266. (Full Text + Extras)

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