How Plants Make Flowers

By Richard Stout

How, When, and Where Did Flowers Originate?Introduction

How Plants Tell TimeWhy Plants Tell TimeEnvironmental Cues

Endogenous CuesPhotoperiod, Biological Clocks, and FlorigenThe Genetic Pathways of Flower Formation

How Plants Make FlowersA Tale of Two Theories: The Mossy Earth and The Conquering Flowers

GlossaryAttributions

About the Author

Copyright © 2011 by Richard G. Stout All Rights Reserved.

Notice of Rights: All rights reserved. No part of this book may be reproduced or transmitted in any form by any means, electronic, mechanical, photocopying,

recording, or otherwise, without the prior written permission of the author. For in-formation on getting permission for republication and excepts, contact Richard by

sending email to: howplantswork@gmail.com

How, When, and Where Did Flowers Originate?

Before getting into the subject of how plants make flowers, perhaps it would be a good idea to

briefly explore what’s known about how plants invented flowers in the first place.

The First Flower?

How did flowering plants (angiosperms) evolve from non-flowering seed plants (gymnosperms)? Or

did they? When did the first flower appear on this planet? And where on Earth did it occur?

These are some of the most hotly debated questions among botanists today, partly because some of

the fossil data is at odds with some of the DNA data. So, what’s the story?

Goodbye Naked Seeds

Non-flowering gymnosperms, such as conifers, bear naked seeds on scales. Angiosperms have seeds

encased in remnants of the flower. Gymnosperms arose about 370 million years ago (MYA) and domi-

nated the Earth for 250 million years.

Then within a few tens of millions of years, angiosperms appeared, and their species greatly prolif-

erated. (Currently almost 9 out of 10 land plant species are angiosperms.) This is the “abominable mys-

tery” that confounded Charles Darwin. That is, how did flowering plants diversify and spread so rapidly

across the planet. (Still, today, 130 years after Darwin’s lament, this remains a perplexing topic among

botanists….for example, see reference R1 below.)

The Missing Link?

Where are the intermediates between the gymnosperm to angiosperm transition?

In 2002, there was much excitement over the fossil discovery of Archaefructus. This aquatic seed

plant fossil was initially dated to the late Jurassic, about 145 million years ago (MYA), making it the earli-

est example of an angiosperm. But since 2002 this fossil has been found to be not as old as originally

thought, only about 125 MYA, and some scientists think it may be a member of the water-lily family.

This would render Archaefructus less primitive than Amborella, which currently sits at the bottom of the

angiosperm family tree. (For more on Archaefructus, see Links below.)

Amborella (see photo below) is a small shrub with tiny

greenish-yellow flowers and red fruits that grows only in the

understory in the rain forests of New Caledonia.

Thus, the simple answer to the question “Where is the

missing link?” is: currently, there isn’t any.

The fossil data are incomplete and difficult to interpret.

The molecular (DNA analysis) data from living plants group

the gymnosperms all together and the angiosperms all to-

gether, with no plant species in between. It’s quite possible

that he missing link may have gone extinct.

And, short of a very fortuitous fossil discovery, it may

never be found.

Bottom line: Where and how flowering plants arose on Earth about 130 MYA is still very much an

unsolved mystery.

Links:

Science magazine published an essay

on the origin of flowering plants by Elizabeth

Pennisi in April, 2009. Click here to listen

to an interview with Pennisi regarding this

article.

Nova (PBS) program on the first flower.

The Smithsonian on Archaefructus.

The Deep Time Project (featuring Ar-

chaefructus)

For some more recent info on the pos-

Fig. 1A - Fossil of Archaefructus from

China

Fig. 1B - Amborella trichopoda (Werthheim Conser-

vatory, Florida International University, Miami, FL, USA)

sible origins of flowering plants jump here.

There has also been recently reported another fossil of a plant called Leefructus, which is about the

same age as Archaefructus.

Reference

R1. Friedman, William E. (2009) The meaning of Darwin’s “abominable mystery”. American Journal

of Botany, vol. 96, pp. 5-21. (online: full text)

Introduction 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 exis-

tence 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 “flow-

er-former”).

When did the story of the elusive flowering hormone florigen

begin? The first to propose that the leaves of some plants pro-

duced a flower-inducing substance under certain conditions was

Julius Sachs in 1865 (see reference R2 below). But why did it take over 70 years from Sach’s discovery

for this flowering-forming substance to even be given a name?

What Causes Plants to Flower?

Unlike animals, plants don’t start out with their “naughty bits” - that is, they have no sexual organs,

a.k.a., flowers.

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

Over and over again.

So, it’s a very big deal for the plant when the transition from the vegetative stage to the flowering

stage occurs. (Indeed, we now know that this “floral transition” involves the “flipping” of some major

genetic “switches”, that is, major changes in gene regulation.*)

But what triggers this floral transition?

The answer is that both internal and environmental factors - not even considering differences among

flowering plant species - may affect the floral transition. And this perhaps is the answer to the question of

why it took so long to come up with solid evidence for the existence of a flowering hormone.

Figure 2 - Still Life with Flowers by Balthasar van der Ast (1593– 1657)

To truly understand the physiology of the

floral transition, scientists first needed a way to

be able to induce flowering in vegetative plants

under controlled and reproducible conditions.

A major breakthrough toward this goal was

reported nearly 100 years ago. And not long after,

scientific evidence for the existence of a flower-

ing-inducing signal emerged.

However, before we see what this break-

through was, let’s see how plants measure time,

and why.

Reference:

R2. Zeevaart, J.A.D. (2006) Florigen coming of age after 70 years. The Plant Cell, Vol. 18, pp. 1783-

1789. (online: full text)

*It is now generally accepted that in most, if not all, angiosperms, florigen is the signal that “flips

the switch”. Florigen is apparently the plant’s internal chemical signal that triggers the floral transition.

Figure 2B - Factors that may affect the floral transi-tion

How Plants Tell Time

How Do We Know Plants Can Tell Time?

The daily opening and closing of flowers and the rhythmic leaf movement of some plants suggests,

even to the casual observer, that plants have an internal clock.

To more careful observers, such as Carl Linnaeus and Charles Darwin, the evidence was clear that

plants can tell time.

For example, in 1751 Linnaeus published Philosophia Botanica in which he noted what time of day

flowers of various species opened and closed.

And also in this book, Linnaeus conceived the idea of a floral clock (“horologium florae”) garden by

which one could estimate the time of day by observing which flowers were open and which had closed.

(More information on floral clocks can be found at Wikipedia.)

Darwin, assisted by his son Francis, studied

the diurnal movement of leaves (sometimes called

“sleep movements”, a.k.a., nyctinasty). In his book

The Power of Movement in Plants Darwin argued

that the plants had an internal clock that gener-

ated the observed rhythms, rather than them being

solely imprinted by the diurnal cycle.

Of course, we now know that these “sleep”

movements in plants are manifestations of the

circadian rhythm, which is evident in most living

organisms.

What Sets the Clock?

Think about it...what happens during the course of a typical 24-hr period on Earth?

In simplest terms, it cycles between light/warm and dark/cool.

So, what sets (entrains) the biological clock of plants are mainly light/dark transitions, augmented or

reinforced by diurnal cycles in temperature. In other words, light (dawn/dusk) acts to reset the clock, but

temperature also has an effect, albeit a not very well defined one.

Figure 3A - An example of a floral clock

It turns out that, in most plants, the leaves play a central role in sensing the light that entrains the

biological clock. But it’s not chlorophyll that is the light-sensing pigment, but two other non-photosyn-

thetic pigments called phytochrome and cryptochrome. (Much more will be revealed about these two

photoreceptors later on.)

How Does the Clock Work?

Research on the cellular mechanisms of circadian (“about a day”) rhythms in plants has greatly ad-

vanced our understanding of how the clock works at the molecular level. (For an excellent review from

an historical perspective see reference R3 below.)

Briefly, the clock works at the individual cell level and

consists of three basic components as shown in Figure 3C

below.

It turns out that plants likely have three such mecha-

nisms, all interlocked in a complex system, working inside

leaf cells. As mentioned above, phytochrome and crypto-

chrome are the photoreceptors. These modify other proteins

involved in a transcription/translation feedback loop that

serves as the central oscillator.

The collective output consists chiefly of proteins, and

maybe even RNA, that serve to modify the plant’s metabo-

lism and development. These output signals may even travel

from the leaves through the phloem to other parts of the

plant.

Some Recent News About Plant Circadian Rhythms

Leaves may have three interlocking clocks, but there may be only one root clock, and it’s apparently

a slave of the leaf clocks.

The circadian rhythm also apparently results in the rhythmic growth of plants.

Researchers at the University of Texas at Austin have shown that modifying the internal clock may

result in bigger plants.

Much has been learned about clock genes in plants and how they relate to clock genes in fungi and

animals.

Figure 3B - A so-called “Clock Flower”, genus Passiflora

Bottom line: For hundreds of years people have recognized that plants have an internal clock, but

only recently have plant molecular biologists discovered the complex inner workings of this timepiece.

Reference:

R3. McClung, C. R. (2006) Plant circadian rhythms. The Plant Cell, Vol. 18, pp. 792-803. (online:

full text)

Figure 3C - A conceptual model of the biological clock in plants

Why Plants Tell Time

Time to Flower?

Now that we’ve had a taste of HOW

plants tell time, what, if any, are the adaptive

advantages to plants for doing so?

It has long been presumed that the abil-

ity to anticipate day/night cycles gives organ-

isms a fitness advantage. For example, this

would allow plants to anticipate daylight and

adjust their photosynthetic metabolism ac-

cordingly, perhaps getting a “head-start” on

plants that didn’t.

Also, the ability of plants to tell time al-

lows them to adjust their development - such

as when to flower - to seasonal variations in

the environment.

As previously mentioned, the transition

from vegetative growth to flowering is a fun-

damental stage in angiosperm development.

What factors influence this flowering

transition in plants? This has been one of the

most interesting, and confounding, questions regarding the nature of flowering plants.

The Length of the Night

It turns out that one of the most important factors involved in the transition to flowering in many

plants is photoperiodism.

By definition, the photoperiod is the duration of an organism’s daily exposure to light.

For plants - particularly in temperate zones - it’s conceivably a way for them to reliably tell the time

of year.

A major discovery regarding photoperiodism and flowering in plants is attributed to two USDA

Figure 4A

researchers, Garner and Allard. In 1920 they published their findings on how day length affected flower-

ing in certain varieties tobacco. From their research, they determined that the plants could be divided

into three general groups by how they flowered in response to relative day lengths: Short-Day Plants

(SDP) flowered after exposure to relatively short days; Long-Day Plants (LDP) that flowered after relative-

ly long days; and Day-Neutral Plants (DNP) that didn’t seem to flower in response to photoperiod.

From experiments that interrupted the night with a brief period of light, we now know that it’s the

night length that is critical in the photoperiodic control of flowering.

Photoperiod + Circadian Rhythm

To further complicate our attempts at understanding of the photoperiodic control of flowering, it’s

clear that the photoperiodic time-keeping mechanism is coupled with the plant’s internal circadian

clock. Though this complex mechanism is currently not fully understood, I offer you a simplified expla-

nation here.

In plants that flower in response to photoperiod, a flowering signal (called florigen) may fluctuate

in the leaves with a circadian rhythm. (LDP and SDP may differ in how the level of florigen is coupled to

the circadian rhythm.) When the external photoperiod (sensed by the leaves) is coincident with a certain

phase of the internal clock (i.e., level of florigen in the leaves), then leaves send enough florigen to the

apical meristem (via the phloem) to trigger the floral transition. (For a more thorough explanation see

reference R4 below.) This means that SDP are really Long-Night Plants!

Much recent progress has been made in identifying florigen and how it triggers the massive changes

in gene regulation that lead to the floral transition, as we’ll see later on.

Bottom line: The ability of plants to tell time allows them to adjust their metabolism and develop-

ment on a daily, as well as seasonal, basis.

Reference:

R4. Hayama, R. and G. Coupland (2004) The molecular basis of diversity in the photoperiodic flow-

ering responses of Arabidopsis and Rice. Plant Physiology, Vol.135, pp. 677–684. (online: full text)

R4B. Gardner, M. J., K. E. Hubbard, C. T. Hotta, A. N. Dodd, and A. A. R. Webb (2006) How plants

tell time. Biochem Journal, Vol. 397, pp. 15–24. (online: full text)

Environmental Cues

Many Plants Flower in Response to Relative Night Length

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

florigen.

As previously mentioned, early in the last century W. W. Garner and A. H. Allard took a major step

toward this goal by discovering how to induce flowering in plants under controlled conditions. They

first published their work (see reference R5 below) on the effect of photoperiod on flowering in tobacco,

soybean, and many other plants. (Their findings are 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. (Unfor-

tunately, most textbooks persist in using the

old - and incorrect - nomenclature.)

Thus, many plants make the flowering

transition from vegetative growth in re-

sponse to a very dependable environmental

cue, namely, the photoperiod.

But What Does This Have To Do With Florigen? Figure 5A

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.

Temperature Effects on Floral Induction

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. (Fore more on this see reference R8)

But we all know how variable spring-time tem-

peratures 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 ex-

perience an extended period of cold, or vernalization.

Specifically, vernalization results in “...the acquisition or

acceleration of the ability to flower by a chilling treat-

ment.” (from reference 5B below) This cold exposure

does not necessarily cause flowering but rather renders

the plant competent to do so.

For example, some biennial plants, such as cab-

bage or carrots or winter wheat, require a long period

(weeks) of “cold” (below 35o to 40o F) before they be-

come “competent” to flower.

Figure 5B

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. (More on this protein is later on.) This, by the way, is how

the plant “remembers” it has experienced winter, weeks later on in the spring.

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.

Reference:

R5. Garner, W.W. and H.A. Allard (1920) “Effect of the realtive length of day and night and other

factors of the environment on growth and reproduction in plants.” (PDF)

R5B. Chouard, P. (1960) Vernalization and its relations to dormancy. Annual Review of Plant Physi-

ology, Vol. 11, pp 191-238.

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

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 historically have been referred to as “Day-Neutral”, apparently initiate flower-

ing because of factors other than photoperiod. Such plants may flower after attaining a certain size or

age, for example. Thus, floral induction in these plants occurs mainly or entirely in response to internal

(endogenous) signals rather than to environmental (external) conditions. In plants that apparently flower

independently of environmental sensing, the flower-inducing pathway is currently called “autonomous”.

The classic example of an “autonomous” flowering

pathway is the juvenile phase to adult phase transition,

which affects many aspects of plant development. After

germination, some plant species enter a juvenile phase

in which they are not “competent” to flower. That is,

even when experiencing conditions favorable to flower-

ing, they lack the ability to flower.

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 re-

sponse to size or age, important questions arise, such as:

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

Also, in plants that flower “autonomously” in

response to internal cues (such as size or age), does

florigen still play a role?

How Do Plants “Know” How Big They Are?

Figure 6A - Juvenile plant (incompetent to flower)

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 an exhaustive review of

“node counting”, see reference R6 below.)

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?

Aspen trees my not become competent to

flower until the are over ten years old. But how

does the tree know how many years have passed

since it was a seedling?

It’s conceivable that a plant can obtain rela-

tive age info from the same ways it may estimate

its size mentioned above.

It’s also been proposed that certain substanc-

es 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 transi-

tion may then proceed.

Multiple Pathways Lead to Flowering

Flowering, 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 reference R7 below)

From these genetic studies (mainly using the plant Arabidopsis thaliana) scientists have discov-

Figure 6B - Aspen trees

ered the identity of florigen (much more on this later). These studies have also 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 the plant hormone gibberellin, as well as by environmental factors such

as temperature, as we’ve seen with vernalization.

Recent research has also implicated microRNAs, small sequences of RNA, as playing roles in plant

development, including flower induction. (See reference R7 below, for examples.)

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 most, if not all, flowering

plants, that initiates flowering. This mechanism is triggered not only by florigen but is also affected by

other endogenous factors.

References:

R6. Sachs, T. (1999) ‘Node counting’: an internal control of balanced vegetative and reproductive

development. Plant, Cell & Environment, Vol. 22, pp. 757-766. (online: full text)

R7. Amasino, R. (2010) Seasonal and developmental timing of flowering. The Plant Journal, Vol. 61,

pp. 1001-1013. (online: full text)

Photoperiod, Biological Clocks, and Flo-rigen

We’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 en-

vironmental signal to tell seasonal time. (To see how, please see previous chapters about How Plants Tell

Time and Why Plants Tell Time.)

The latest scientific evidence supports the hypothesis that

florigen is actually a protein called FT coded for by the gene

Flowering Locus T in Arabidopsis.

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 sce-

nario, which culminates in flower formation (see next chapter).

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. (Al-

though specific genes and proteins vary, depending on plant species, it appears that the basic story pre-

Figure 7A - Arabidopsis thaliana

sented 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 photo-

receptors 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.

Please see Figure 7B on following page.

References:

R8. 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)

R9. Mach, J. (2011) Will the real florigen please stand up? Sorting FT homlogs in maize. The Plant

Cell Vol. 23, p. 843. (Full Text)

Figure 7B - 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 (Con-stans) , 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 photope-riods. 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.

The Genetic Pathways of Flower Formation

The Long and Winding Roads

So far, this journey through the subject of how plants

make flowers has consisted of four parts:

Part 1, an introduction to the flowering hormone flori-

gen; Part 2, how environmental cues affect flowering; Part 3,

how the size and age of the plant itself may trigger flowering;

and Part 4, how the photoperiod and the plant’s biological

clock affect the production of florigen.

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.

As mentioned in the previous chapter, the so-called “flowering hormone”, historically known as

florigen, 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) is a so-called bZip TF.

An especially interesting member of this MADS-box family with regard to flowering is the FLC pro-

tein. FLC (the product of a gene called Flowering Locus C) actually represses flowering.

Figure 8A

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 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 is SOC1. This TF protein plays the main roll in the great leap from vegetative meri-

stem 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 expres-

sion. (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 Meri-stem -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 analo-

gous 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, 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, which is discussed at length in the next

chapter. (For an excellent review on flower development, see reference below.)

Bottom Line: For a visual summary of this subject, please see my corresponding YouTube video.

References:

R10. Irish, V. F. (2010) The flowering of Arabidopsis flower development. The Plant Journal, Vol. 61,

pp. 1014-1028. (online: full text)

Figure 8B - A “Simplified” Model of the Genetic Pathways Involved in Flowering, Plus Some of Their Effec-tors.

How Plants Make Flowers

Most of what’s been presented so far has had to do with floral initiation, that is, how the plant’s SAM

makes the transition from the vegetative state to the flowering state. But how are flowers actually con-

structed? And how does this relate to the huge variety of floral forms displayed by the angiosperms?

There’s No Turning Back Now

Once a plant’s SAM commits to making a flower, it usu-

ally can not be reversed. It’s committed. Sort of like a sky-

diver jumping out of an airplane.

Also, the transition from a vegetative meristem to a flo-

ral meristem is a developmental “dead end”.

What I mean by this is that making a flower is a one-

time event: a story with a beginning, a middle and an end. In

contrast, vegetative meristems repeatedly make shoots and

leaves indeterminately, that is, over and over again, indefi-

nitely.

Another way of thinking about this is that a vegetative

meristem is “immortal”. Whereas a floral meristem is “termi-

nal”. With rare exceptions, floral meristems don’t revert to

vegetative meristems.

Shifting Into Flowering Mode

What happens in the shoot apical meristem after flow-

ering is initiated, either by florigen or by an autonomous

pathway?

In other words, how does a plant build a flower once is

committed to doing so?

As briefly mentioned in the previous chapter, this hap-

pens in two general phases. In the first phase, which I called

“arranging the chairs”, the general architecture of the flower

Figure 9A

is established. The vegetative shoot and leaf primordia transform into floral primordia, forming the basic

flower structure, which consists of four types of organs arranged in concentric whorls: sepals, petals,

stamens, and carpels.

In the second phase, which I called “seating the guests”, each of the four “guests” -- sepal, petal,

stamen, and carpel – develops in its proper place, depending on the flowering plant species.

It’s worth mentioning at this point that each of the four flower organs is basically a highly-modified

leaf. So, simply put, the genes expressed to make a sepal, for example, are likely the “let’s make a leaf”

set of genes, plus the “let’s make a sepal” set of genes. But how are all of these genes turned on (ex-

pressed) in the flower primordia at the appropriate time in order to grow a sepal, petal, stamen or car-

pel?

The answer is that there are so-called “master control genes” that regulate the expression of a hier-

archy of genes. These master control genes are like army generals issuing orders to captains who, in turn,

issue orders to lieutenants, etc. About twenty years ago such master control genes (homeotic genes con-

taining a MADS-box sequence) that determined floral organ identity were first discovered and integrated

in a model for flower development called the ABC model, described below.

The ABC’s of Flowering

Floral organ identity genes are divided into three classes,

depending on which organs they affect. Class A genes affect

sepals and petals. Class B genes affect petals and stamens,

while those in class C affect stamens and carpels. All three

classes of genes are homeotic genes, which are translated into

TF proteins.

The ABC model proposes that class A genes alone are

responsible for the development of sepals, but act together

with class B genes to effect petal development. Class C genes

alone are responsible for initiating the development of car-

pels, but act together with class B genes to determine the

development of stamens.

Summary:

The expression of A genes induces the development of

sepals.

The expression of B genes together with A genes induces

the development of petals.

The expression of B genes together with C genes induces the development of stamens.

The expression of C genes induces the development of carpels.

Figure 9B - The ABC Model

Flower Diversity

Angiosperms display a huge variety of floral forms. How has this enormous diversity in the shape,

color and size of flowers emerged over the course of flower evolution? And how does the ABC model

contribute to the story of the evolutionary developmental (evo-devo) biology of floral diversity?

Briefly, it appears that the ABC model provides a good framework for studies of comparative floral

development. Although the ABC model serves as a basis for flower development in many plant species,

it may only provide part of the story in others. For example, monocots, such as grasses, orchids and tu-

lips, and the primitive flowering plants, such as Amborella, don’t possess the “standard” four-whorl flow-

er structure of sepal-petal-stamen-carpel. Thus, the ABC model must be significantly modified, including

changes in some of the floral identity genes and also their expression patterns in the flowering SAM.

Although the ABC model provides a good starting point, there are still many questions that need

to be answered with regard to the evo-devo of the remarkable floral diversity of angiosperms. For more

information about this, check out the Floral Genome Project.

The Mystery of the Flowering Hormone Solved?

Coming full circle from the beginning of this story: Can we now induce plants to flower at will by

simply spraying a “flowering hormone” on them?

The answer is: Not yet, but maybe someday real soon.

Recent experiments indicate that the first commercially-available plants you’ll likely see will be

genetically-engineered to over-express the FT gene under special conditions. (see reference R11 below,

for example)

The FT gene codes for the FT protein, which most currently agree is indeed florigen. Presuming

the plant is competent to flower, then the sudden appearance of lots of florigen would probably “tip the

scales” to induce flowering in responsive SAMs.

To “turn on” the FT genes, one could engineer them to be activated by alcohol (ethanol), for ex-

ample. So that to induce such an engineered plant to flower, one would merely need to spray a dilute

solution of alcohol (wine?) on the leaves.

References:

R11. C. C. Yeoh, M. Balcerowicz, R. Laurie, R. Macknight and J. Putterill (2011) Developing a

method for customized induction of flowering. BMC Biotechnology 11:36. (Full Text)

A Tale of Two Theories: The Mossy Earth and The Conquering

Flowers

And, finally, here is a some recent news regarding the origins of flowering plants and why they be-

came so successful, a.k.a., “the abominable mystery’.

A Long, Long Time Ago on Planet Not So Far Away

What were the first plants to colonize the land on Earth? And when did this occur in the history of

the biosphere?

Why did a burgeoning of flowering plant species come to dominate their gymnosperm and fern

predecessors so quickly?

The Mossy (Algal and Fungal) Earth

Most biology textbooks state that plant life emerged on land about

450 million years ago.

However, a new study suggests that plants colonized land much

earlier than this.

As summarized here, the authors think that their geochemical data

suggests that photosynthetic life forms (largely mats composed of mosses

and algae, accompanied by fungi) carpeted the land over 800 million

years ago. Their evidence, albeit indirect, may help to explain the in-

crease in atmospheric oxygen levels that allowed for the evolution of

relatively large respiring animals about 600 MYA.

This green “welcome mat” may have set the stage for animal coloni-

zation of the land.

The “Abominable Mystery” of the Conquering Flowers

Figure 10A - Sphagnum moss

It’s been over 100 years since Charles Darwin described it as an “abominable mystery”.

What was perplexing Darwin was the fossil evidence that flowering plants (angiosperms) rapidly

diversified and spread across the planet. (This was at odds with his belief that evolution was a gradual

process.)

A new theory has been proposed in an attempt

to solve this “mystery”.

As brilliantly summarized here, flowering

plants may have taken advantage of changes in

soil fertility, which were due largely to the higher

growth and turnover rates of angiosperms com-

pared to gymnosperms.

Thus, a sort of positive feedback loop was cre-

ated that allowed for the rapid proliferation of flow-

ering plant species. The originators of this theory,

Frank Berendse and Marten Scheffer, published

this ecological explanation of Darwin’s “abomi-

nable mystery” in Ecology Letters.

Bottom Line: Looks like studying the soil can

provide answers to botanical questions.

Figure 10B

Glossary

Please note: Here are some concise definitions. For more information, click on the word in question

to be taken to more thorough, online definition.

angiosperm - Generally speaking, a flowering plant. More precisely, a seed-bearing flowering plant

that produces its seeds enclosed inside a fruit. (angio = vessel, container + sperm = seed)

apical meristem - see meristem below

Arabidopsis = Arabidopsis thaliana - A small flowering plant (a.k.a., “mouse-ear cress” and “thale

cress”) that has been the darling of plant molecular biologists since the 1980’s primarily because

of its relatively small genome and short life cycle. These characteristics greatly facilitate genetic

analyses.

autonomous flowering - Floral transition occurs in response to internal factors within the plant, not

to environmental factors, such as photoperiod. See “Day-Neutral plant”.

biological clock - A complex cellular mechanism that uses oscillations of biochemical reactions

to measure the passage of time. This “biological clock” governs various rhythmic physiological

processes, such as circadian rhythms.

circadian rhythm - A cellular or physiological process the activity of which fluctuates with about a

24-hour period.

cryptochrome - A blue-light-absorbing protein (photoreceptor) found in both plants and animals

that is likley involved in setting the ”biological clock” (see above).

Day-Neutral plant = “Night-Neutral” plant - A plant that flowers apparently independent of photo-

period.

DNA = Deoxyribonucleic acid - It is the genetic information used in the development and func-

tioning of all known living organisms.

FLC = Flowering Locus C - A gene, originally characterized in Arabidopsis, that codes for a protein

that somehow blocks flowering in Arabidopsis, as well as in other plants.

floral transition - Shoot apical meristem goes from being a vegetative meristem to a floral meri-

stem.

florigen - The so-called “flowering hormone”. Since the 1930’s, most scientific evidence supported

that this hypothetical substance was produced in the leaves and traveled via the phloem to the

shoot apical meristem, where it induced flower initiation. Despite more than 50 years of active

research, the identity of florigen remained a mystery, until recently. See FT protein below.

clock.

primordia - Tissues or organs in their earliest recognizable stage of development.

protein - A polymer of amino acids that act primarily as enzymes, that is, they catalyze or speed up

chemical reactions. There are proteins that also serve structural, as well as hormonal, functions in

organisms.

RNA = Ribonucleic acid - A nucleic acid molecule similar to DNA but containing ribose rather

than deoxyribose. RNA, typically a single-stranded molecule, is formed upon a DNA template.

There are several classes of RNA, which play critical roles in protein synthesis.

SAM = shoot apical meristem - see meristem above

Short-Day plant = “Long-Night” plant - A plant that typically flowers in response to a specific pho-

toperiod: in this case, relatively long night lengths (e.g., an uninterrupted 12 hours of darkness or

more).

transcription - Process whereby RNA is synthesized from a DNA template. In plant cells, this so-

called messenger RNA (mRNA) is complementary to the DNA sequence and is used as an infor-

mation template to synthesize a specific protein via “translation” (see below).

TF = transcription factor - A protein that serves to regulate the transcription of DNA.

translation - Process whereby mRNA template is used to synthesize specific sequence of amino ac-

ids into a polymer (polypeptide), a.k.a., protein. A single strand of mRNA can be translated many

times to synthesize many copies of a specific protein.

UV = ultraviolet light - Part of the electromagnetic spectrum of sunlight with shorter wavelengths

than the light spectrum typically visible to humans. Though it is not visible to humans, UV light

is visible to some insects, such as bees.

vernalization - An extended cold period required by some plants before they will flower. The pe-

riod required may be from weeks to months, depending on the plant species. The effective tem-

peratures are typically around freezing, that is, about 33 to 45 degrees Fahrenheit (1 to 7 degrees

Celsius), again, depending on plant species. Most agree that vernalization is a way for plants to

“know” that they’ve experienced winter, so as to prevent precocious flowering in the fall.

FT protein - The gene product (i.e., protein) coded by the gene in Arabidopsis called “Flowering

Locus T”. Currently, this protein is the generally-accepted identity of “florigen” in Arabidopsis, as

well as in most other flowering plants.

gene - A gene is a unit of heredity in a living organism. Typically, in plant cells, it is the name given

to a portion of the DNA that codes for an individual protein, which usually has a specific function

in the organism.

gymnosperm - A non-flowering, seed-bearing plant that produces “naked seeds”, in contrast to

angiosperm (see above). For example, a conifer such as a pine tree is a gymnosperm.

homeotic genes - Developmental “master” genes that affect the placement of organs in growing

plants and animals.

hormone - A natural chemical substance that regulates plant development, metabolism, or both.

Hormones, by definition, are typically effective at extremely low concentrations.

Long-Day plant = “Short-Night” plant - A plant that typically flowers in response to a specific pho-

toperiod: in this case, relatively short night lengths (e.g., 8 hours or less).

MADS-box - A specific part of a gene that is characteristic of a special family of transcription fac-

tors (see below). These MADS-box TFs typically play important roles in developmental processes.

The name “MADS” is an acronym that refers to the four genes in which this sequence element or

“box” was first identified.

master control genes - Master control genes encode the first transcription factor in a hierarchy.

This master control gene product (TF) activates the next set of genes that encodes the next set of

transcription factors, and the cascade of gene expression has been set in motion.

meristems - Growth regions in plants where new, unspecialized cells are produced through very

active cell division. Apical meristems usually occur at the tip (apex) of roots and shoots. The shoot

apical meristem (SAM) gives rise to new shoots, leaves, and flowers.

phloem - In vascular plants, phloem is the living, conduit-like tissue that serves as a passive plumb-

ing system carrying mainly sugar (sucrose) from regions of relatively high sucrose concentrations

(think leaves) to areas of relatively low sucrose concentrations (think roots, flowers, and develop-

ing parts of the plant). Besides sucrose, the phloem also serves as a conduit for many other organ-

ic substances (even proteins), as well as some mineral elements. Please note that entry into, and

exit from, the phloem by certain substances may be specifically regulated by the phloem cells.

photoperiod - The relative duration of an organism’s daily exposure to day (light) versus night

(dark), typically within a 24-hour period. This term is usually used within the context the phe-

nomena of photoperiodism. That is, the nature of the responses (such as flowering, jet-lag, etc.) of

living organisms to different photoperiods.

phytochrome - A red- and far-red-light-absorbing protein (photoreceptor) found in all plants that

affects many aspects of plant development and physiology, including setting the plant’s biological

Attributions

Thanks to the following photographers and artists who have offered their photo-

graphs and illustrations via Creative Commons.

Cover - By Karva Javi, via Flickr

http://www.flickr.com/photos/karvajavi/2171002389/in/photostream/

Figure 1A - By Shizhao (Own work) [CC-BY-SA-2.5 (www.creativecommons.org/

licenses/by-sa/2.5)], via Wikimedia Commons

http://commons.wikimedia.org/wiki/File:Archaefructus_liaoningensis.jpg

Figure 1B – By scott.zona, via Flickr

http://www.flickr.com/photos/scottzona/3065132233/

Figure 2A - By Balthasar van der Ast (1593/1594, Middelburg – 1657, Delft)

(WebMuseum : Home : Info : Pic) [Public domain], via Wikimedia Commons

http://commons.wikimedia.org/wiki/File:Flowers-rijks.jpg

Figure 3A – By Corey Leopold, via Flickr

http://www.flickr.com/photos/cleopold73/184567535/

Figure 3B – By Hamed Saber, via Flickr

http://www.flickr.com/photos/hamed/175262601/

Figure 4A – By RL Hyde, via Flickr

http://www.flickr.com/photos/breatheindigital/4366907400/

Figure 5A – By L. Marie, via Flickr

http://www.flickr.com/photos/lenore-m/510420532/sizes/o/

Figure 5B – By OliBac, via Flickr

http://www.flickr.com/photos/olibac/2983779842/

Figure 6A – By Rev Stan, via Flickr

http://www.flickr.com/photos/revstan/3668053095/

Figure 6B – By Kaibab National Forest, via Flickr

http://www.flickr.com/photos/kaibabnationalforest/4446695252/

Figure 7A – By Brona at en.wikipedia. User:Roepers at nl.wikipedia [GFDL

(www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons

http://upload.wikimedia.org/wikipedia/commons/6/6f/Arabidopsis_thaliana.jpg

Figure 8A – By Roger Cornfoot [CC-BY-SA-2.0 (www.creativecommons.org/li-

censes/by-sa/2.0)], via Wikimedia Commons

http://upload.wikimedia.org/wikipedia/commons/f/fd/The_long_and_winding_

road_-_geograph.org.uk_-_1124953.jpg

Figure 9B – By Madprime (Own work) [GFDL (www.gnu.org/copyleft/fdl.html) or

CC-BY-SA-3.0-2.5-2.0-1.0 (www.creativecommons.org/licenses/by-sa/3.0)], via Wiki-

media Commons

http://commons.wikimedia.org/wiki/File:ABC_flower_development.svg

Figure 10A - By External Affairs [CC-BY-2.0 (www.creativecommons.org/licenses/

by/2.0)], via Wikimedia Commons

http://commons.wikimedia.org/wiki/File:Sphagnum_moss_in_May_2010.jpg

Figure 10B - By Jim_Sneddon [CC-BY-2.0 (www.creativecommons.org/licenses/

by/2.0)], via Wikimedia Commons

http://commons.wikimedia.org/wiki/File:Lake_elsinore_wildflowers.jpg

All other figures are by the author.

About the Author

The author received a Ph.D. in Plant Physiology from the University of Wash-

ington, Seattle, in 1980. Since then, he has taught biology, biochemistry, botany, and

plant physiology as a faculty member at Williams College (Williamstown, MA) and

Montana State University (Bozeman, MT). His research interests mainly encompass

plant growth and development, but he has also done research on plants growing in

geothermal environments in Yellowstone National Park. He has received research

funding from NSF, NASA, USDA and DOE and has published over 25 refereed papers

in scientific journals and several book chapters on plant biology. He currently resides

in Bellingham, Washington, and writes the weblog HowPlantsWork.com.