reproduction in flowering plantslrios/3052/life11e_ch37_lecture.pdfflowering in some plants. 37.2...

37Reproduction in

Flowering Plants

Chapter 37 Key Concepts

37.1 Most Angiosperms Reproduce

Sexually

37.2 Hormones and Signaling Determine

the Transition from the Vegetative to

the Reproductive State

37.3 Angiosperms Can Reproduce

Asexually

Investigating Life: What Signals Flowering?

By determining that poinsettias are short-

day plants, and developing a short,

compact variety, Albert and Paul Ecke

helped make them the most popular

potted plants in the United States.

How did an understanding of angiosperm

reproduction allow floriculturists to develop

a commercially successful poinsettia?

Key Concept 37.1 Focus Your Learning

• The embryo sac is the female

gametophyte and the pollen grain is the

male gametophyte.

• A pollen grain germinates to form a

pollen tube, which grows through the

style to the embryo sac.


• Angiosperms prevent self-fertilization

by physically separating male and

female gametophytes and by genetic

self-incompatibility.

• The embryo and endosperm develop

within the seed, enclosed within a fruit

derived from the ovary wall.

37.1 Most Angiosperms Reproduce Sexually

Most angiosperms reproduce sexually—

producing the genetic diversity that is the

raw material for evolution.

It involves mitosis, meiosis, and

alternation of haploid and diploid

generations.


Differences in sexual reproduction

between angiosperms and vertebrate

animals:

• Meiosis in plants produces spores, then

mitosis produces gametes.

• In animals, meiosis produces gametes

directly.


In most plants, multicellular diploid and

haploid life stages alternate; in animals,

there is no multicellular haploid stage.

Cells that will form gametes are

determined in the adult plant, usually in

response to environmental conditions; in

animals, the germline cells are

determined before birth.

In-Text Art, Ch. 37, p. 787 (1)


A complete flower has four concentric

groups of organs arising from modified

leaves:

In-Text Art, Ch. 37, p. 787 (2)


Carpels are female sex organs; contain

developing female gametophytes.

Stamens are male sex organs; contain

developing male gametophytes.

Most angiosperms are “perfect”—flowers

have both stamens and carpels.

Imperfect flowers have only stamens or

only carpels.

Figure 37.1 Perfect and Imperfect Flowers (Part 1)


Plants that bear both male and female

flowers on an individual plant:

monoecious (“one house”).

Plants that bear either male-only or

female-only flowers on an individual

plant: dioecious.


The haploid gametophytes develop from

haploid spores in the flower:

• Megagametophytes (female) are called

embryo sacs; develop in the ovules.

• Microgametophytes (male) are called

pollen grains; develop in anthers on

the stamens.

Figure 37.2 Sexual Reproduction in Angiosperms


A megasporocyte undergoes meiosis to

produce 4 haploid megaspores; 3

undergo apoptosis.

The surviving megaspore undergoes 3

mitotic divisions with no cytokinesis to

produce 8 haploid nuclei. Cell wall

formation leads to a gametophyte

(embryo sac) with 7 cells and 8 nuclei.


At one end of the gametophyte are 3

cells—the egg and 2 synergids.

Synergids attract the pollen tube.

Antipodal cells at the opposite end usually

degenerate.

The central cell has 2 polar nuclei.


Microsporocytes undergo meiosis to

produce 4 haploid microspores.

Each develops a cell wall and divides

mitotically to form 2 haploid cells in each

pollen grain:

• Tube cell forms the pollen tube that

delivers sperm to the embryo sac.

• After pollination, the generative cell

divides by mitosis to form 2 sperm cells.


Pollination: Transfer of pollen from

anther to stigma.

Germination of the pollen grain involves

uptake of water from the stigma and

growth of the pollen tube through the

style to reach the ovule.

Downward growth is guided by a chemical

signal released by the synergids.

Figure 37.3 Pollen Tubes Begin to Grow


In some plants, such as peas, self-

pollination occurs before the flower

opens, resulting in self-fertilization. This

leads to homozygosity, which reduces

genetic diversity.

Most plants have mechanisms to prevent

self-fertilization:


1. Physical separation of male and female

gametophytes:

In dioecious species, pollination occurs

only when one plant pollinates another.

In monoecious species, separation of

male and female flowers can reduce

self-fertilization. In some species the

two flower types bloom at different

times.


2. Genetic self-incompatibility:

Some plants are self-incompatible;

they reject pollen from their own

flowers.

The plant must determine whether the

pollen is genetically similar or not.

The S locus genes encode proteins in

the pollen and style that interact during

the recognition process.


The S locus has many alleles.

When pollen carries an allele that

matches an allele of the recipient pistil,

the pollen is rejected.

The rejected pollen either fails to

germinate or is prevented from growing

through the style.

Figure 37.4 Self-Incompatibility (Part 1)

Figure 37.4 Self-Incompatibility (Part 2)


Double fertilization:

• One synergid degenerates when the

pollen tube arrives and the 2 sperm

cells are released into its remains.

• One sperm cell fuses with the egg cell,

forming a diploid zygote.

• The other sperm cell fuses with the two

polar nuclei in the central cell, forming

a triploid (3n) cell.

Figure 37.5 Double Fertilization (Part 1)

Figure 37.5 Double Fertilization (Part 2)


The zygote nucleus begins mitotic division

to form the new sporophyte embryo.

The triploid nucleus undergoes mitosis to

form the endosperm. It will later be

digested by the developing embryo for

energy and building blocks.


Fertilization initiates growth and

development of the embryo, endosperm,

integuments, and carpel.

Integuments are tissues surrounding the

ovule that develop into the seed coat.

The carpel becomes the wall of the fruit

that surrounds the seed.


As seeds develop, they lose water and

become dormant.

The ovary and seeds develop into a fruit.

Fruits function to

• Protect the seed from damage and

infection

• Aid in seed dispersal


Some fruits consist only of ovary and

seeds, some include other flower parts.

Some species produce fleshy edible

fruits; some fruits are dry and inedible.

Figure 37.6 Angiosperm Fruits (Part 1)


Seeds of some species fall to the ground

near the parent plant.

If a plant has successfully reproduced, its

location is likely to be favorable for the

next generation.

But the offspring may then be competing

with the parent, and there is no

guarantee that conditions will remain

favorable in that spot.


Wider dispersal of seeds helps spread

genetic diversity and increases the

probability that some seeds will find

suitable conditions for growth.

Many fruits help seeds disperse over long

distances.


Some fruits have “wings” (e.g., maple) or

feathery structures (e.g., thistle) for wind

dispersal.

In-Text Art, Ch. 37, p. 791 (1)


Some fruits hitch rides on animals as

burs.

In-Text Art, Ch. 37, p. 791 (2)


Water disperses fruits such as coconuts;

they can float thousands of miles

between islands.

Seeds may travel through an animal’s

digestive tract and be deposited at some

distance from the parent plant.


Seed development is under control of the

hormone abscisic acid (ABA).

During early seed development ABA

levels are low, and increase as the seed

matures.

This stimulates the endosperm to

synthesize seed storage proteins and

proteins that prevent cell death as the

seeds dry.


ABA prevents developing seeds from

germinating prematurely on the parent

plant (vivipary).

• These seedlings are unlikely to survive

and cannot establish in the soil.

• In seed crops such as wheat, the grain

is damaged if it has started to sprout.

• ABA plays a key role in maintaining

seed dormancy.

Key Concept 37.1 Learning Outcomes

• Compare the processes of male and

female gamete formation.

• Describe the mechanisms that guide the

growth of a pollen tube.

• Describe and compare two methods for

preventing self-fertilization in

angiosperms.


• Analyze the relationship between the

diversity of fruits and their ability to

disperse seeds.

• Relate fruit development to seed

development.


• Although in terms of flowering, many

plants are classified as either short-day

plants (SDPs) or long-day plants (LDPs),

night length is actually the cue that

controls flowering.

• Receptors for the photoperiodic signal for

flowering are located in the leaf, and a

signal travels to the apical meristem.


• The protein florigen converts a vegetative

meristem into a reproductive meristem.

Several genes are involved in the

regulation and transport of florigen.

• Temperature or gibberellin can induce

flowering in some plants.

37.2 Hormones and Signaling Determine the Transition from the

Vegetative to the Reproductive State

Flowering is a major event in a plant’s life.

When a plant is old enough, it can

respond to internal or external signals

(such as light or temperature) to start

reproduction.

Or flowering occurs as part of a

predetermined developmental program.



Plants fall into three categories in terms of

maturation and flowering:

• Annuals complete life cycle in one

year; little or no secondary growth.

After flowering, most of their energy is

used to develop seeds and fruits and

the plant dies.



• Biennials take two years to complete

the life cycle.

They produce vegetative growth the

first year and store carbohydrates in

underground roots (carrots) or stems

(celery).

In the second year, stored

carbohydrates are used to produce

flowers and seeds.



• Perennials live three or more years.

They typically flower every year and

keep growing for another season.

In some species the reproductive

cycle repeats each year. Others grow

vegetatively for many years, flower

once, and die.



During vegetative growth, apical

meristems continuously produce leaves,

stems, and axillary buds (indeterminate

growth).

If an apical meristem becomes an

inflorescence meristem it produces

bracts and new meristems in the angle

between bract and stem.

Figure 37.7 Flowering and the Apical Meristem (Part 1)



The new meristems may be inflorescence

meristems, or floral meristems, which

give rise to a flower.

An inflorescence is an orderly cluster of

flowers.

Floral meristems produce 4 whorls of

organs—sepals, petals, stamens, and

carpels, with very short internodes

(determinate growth).



Genes for flowering have been studied in

Arabidopsis.

The expression of 2 meristem identity

genes starts a cascade of gene

expression.

The genes encode transcription factors

LEAFY and APETALA1, which are

necessary and sufficient for flowering.



Meristem identity gene products trigger

expression of floral organ identity

genes.

They encode transcription factors that

determine whether cells in the floral

meristem will be sepals, petals,

stamens, or carpels.

Figure 19.11 Organ Identity Genes in Arabidopsis Flowers (Part 1)

Figure 19.11 Organ Identity Genes in Arabidopsis Flowers (Part 2)



Floral organ identity genes are activated

in response to external cues (day length,

temperature) or internal cues

(hormones).



Photoperiod (day length)

• Studies began with two observations:

A tobacco variety grew to 5 meters, but

did not flower before frost killed it.

Soybean farmers planted seeds at

intervals to stagger the harvest, but the

plants all flowered at the same time.



• Greenhouse experiments measured

the day length required for different

plant species to flower.

Maryland Mammoth tobacco flowered

when day length became shorter than

14 hours, as it does in December.

Other plants (e.g., soybeans and

henbane) flowered only when days

were long.

Figure 37.8 Day Length and Flowering



Short-day plants (SDPs) flower when

the day is shorter than a critical

maximum.

• Coffee, morning glory,

chrysanthemums, poinsettias,

Maryland Mammoth tobacco

They flower in late summer or fall.



Long-day plants (LDPs) flower when the

day is longer than a critical maximum.

• Spinach, lettuce, clover

They flower in midsummer.



Photoperiodic plants actually measure

length of night, not day.

• Experiments with cocklebur, (SDP):

Day length was varied in one group,

night length in another. The critical

night length was 9 hours.

Figure 37.9 Night Length and Flowering



• Other experiments showed that

interruption of the dark period by light,

even briefly, nullified the effect of a long

night.

• Effects of red light interruptions could

be reversed with far-red light, indicating

that a phytochrome was the receptor.

Investigating Life: The Flowering Signal

Hypothesis: Red light participates in the

photoperiodic timing mechanism.

Method:

Grow plants under short-day conditions,

but interrupt the night with light of

different wavelengths.

Investigating Life: The Flowering Signal, Experiment

Investigating Life: The Flowering Signal

Conclusion:

When plants are exposed to red (R) and

far-red (FR) light in alternation, the final

treatment determines the effect.

Phytochrome is the photoreceptor.



Expression of the gene CONSTANS (CO)

follows a circadian rhythm.

Experiments with Arabidopsis, an LDP,

show that flowering is determined by

interactions between photoreceptors and

the CO protein.

Peak CO expression is late in the day—in

the afternoon on long days, but after

dark on short days.



On long days, the active forms of

phytochrome and blue-light receptors

activate pathways that stabilize the CO

protein, which promotes flowering.

This process does not occur on short

days.



Early experiments indicated that receptors

for photoperiod occur in the leaf.

• “Masking” experiments: Either buds or

leaves were covered to determine

which organ receives the light stimulus.

A diffusible chemical must travel from the

leaf to the bud meristem.

Figure 37.10A The Flowering Signal Moves from Leaf to Bud (Part 1)

Figure 37.10A The Flowering Signal Moves from Leaf to Bud (Part 2)



Other evidence:

• If a photoperiodically induced leaf is

immediately removed from a plant, the

plant does not flower.

• If cocklebur plants are grafted together

and only one plant is exposed to

inductive long nights, all the plants

flower.



• If an induced leaf from one species is

grafted onto a non-induced plant of a

different species, the recipient plant

flowers.

The diffusible signal was named florigen,

but the nature of the signal has only

recently been explained.



Three genes are involved in flowering:

1. FT (FLOWERING LOCUS T) codes for

florigen, which is small and can travel

through plasmodesmata.

FT is synthesized in phloem companion

cells of the leaf, diffuses into adjacent

sieve elements, and flows to the apical

meristem.



2. CO (CONSTANS) codes for a

transcription factor that activates

synthesis of FT. CO is also expressed

in phloem companion cells in the leaf.

3. FD (FLOWERING LOCUS D) codes

for a protein that binds to FT in the

apical meristem. The complex

activates promoters for meristem

identity genes.

Figure 37.11 Florigen and Its Molecular Biology (Part 1)

Figure 37.11 Florigen and Its Molecular Biology (Part 2)



The FT gene is involved in photoperiod

signaling in many species:

• Transgenic plants that express

Arabidopsis FT gene at high levels

flower regardless of day length.

• Transgenic Arabidopsis plants that

express high levels of FT homologs

from other species flower regardless of

day length.



In Arabidopsis (an LDP):

• At night, Pfr is gradually converted

back to Pr, which stimulates breakdown

of CO protein.

• In the morning and during the day, CO

protein levels go down.

• By the end of the day, Pr levels go

down, allowing CO to accumulate.



The key to flowering is a high level of CO,

which is related to a low level of Pr.

In long days, there is not a lot of dark time

for all the Pfr to be converted to Pr, and a

long day causes more conversion of Pr

to Pfr.

Low levels of Pr result in high levels of CO

and the transcription of genes for

flowering.



In some plants, flowering is signaled by

cold temperatures: vernalization.

• Example: Winter wheat is planted in the

fall, grows into a seedling, overwinters,

and flowers the next spring.

If it is not exposed to cold in its first year,

it will not flower normally the next year.



In strains of Arabidopsis that require

vernalization, FLC (FLOWERING

LOCUS C) encodes a transcription

factor that inhibits expression of FT and

FD in the florigen pathway.

Cold temperature inhibits synthesis of

FLC protein, allowing FT and FD to be

expressed.

Figure 37.12 Vernalization



Epigenetics plays a role in vernalization:

• Before vernalization, chromatin at the

promoter of the FLC gene is relaxed;

and DNA can be transcribed.

• During vernalization, chromatin

remodeling results in more compact

chromatin and reduced expression of

FLC.

Figure 37.13 Chromatin Remodeling during Vernalization (Part 1)

Figure 37.13 Chromatin Remodeling during Vernalization (Part 2)



Gibberellins are also involved in flowering.

Application of gibberellins to Arabidopsis

buds results in activation of the meristem

identity gene LEAFY, which in turn

promotes the transition to flowering.



Some plant species flower on cue from an

“internal clock.”

In some tobacco strains, flowering is

initiated in the terminal bud when the

stem has grown four phytomers in

length.

The position of the bud determines

transition to flowering.



A substance may form a gradient along

the length of the plant.

• The root may produce a flowering

inhibitor, so the plant must reach a

certain height before concentration of

the inhibitor is low enough for flowering.

• The inhibitor is unknown, but it may act

by decreasing the amount of FLC.



A positional gradient that acts on FLC is

consistent with other mechanisms that

converge on LEAFY and APETALA1.

In-Text Art, Ch. 37, p. 800 (1)


• Give evidence showing that night

length, rather than day length, is the

cue that triggers flowering.

• Describe evidence that a diffusible

chemical travels from the leaf to the

bud meristem to initiate flowering.


• Describe the three genes involved in

florigen production and action,

including where they are active, their

functions, and their interactions.

• List factors other than photoperiodism

and genetic triggering of florigen that

can initiate flowering, and give

evidence for each.


• Sexual and asexual reproduction

provide separate and distinct

advantages to plants.

• Asexual, or vegetative, reproduction in

plants occurs by changes in vegetative

(nonreproductive) organs.

• In apomixis, flowers produce clones;

the technique has the potential to

produce self-reproducing hybrids.

37.3 Angiosperms Can Reproduce Asexually

Asexual reproduction produces offspring

genetically identical to the parent.

If a plant is well adapted to its

environment, asexual reproduction can

preserve and spread that successful

genotype.


Stems, leaves, and roots are the

vegetative organs of a plant.

Asexual reproduction often occurs by

modification of vegetative organs; also

called vegetative reproduction.

Strawberries produce runners, or

stolons—horizontal stems that form

roots at intervals and can develop into

new plants.


Shoot tips that sag to the ground and

develop roots—new plant grows from the

branch tip (e.g., blackberry).

Potatoes form enlarged underground

stems called tubers that can produce

new plants from the “eyes.”

Rhizomes are horizontal underground

stems that give rise to new shoots (e.g.,

bamboo).

Figure 37.14 Vegetative Organs Modified for Reproduction (Part 1)


Bulbs and corms are short, vertical,

underground stems.

Bulbs have fleshy, modified leaves for

food storage—a large, underground bud.

These can give rise to new plants (e.g.,

lilies, onions, garlic).

Corms are mostly stem tissue and lack

modified leaves (e.g., crocuses, gladioli).


Leaves can be the source of new

plantlets, as in Kalanchoe.

Suckers are shoots produced by roots.

Many grasses and trees, such as

aspens, form interconnected stands of

genetically identical individuals.


Plants that commonly reproduce

asexually often live in unstable

environments.

Plants with stolons and rhizomes are

often pioneers on sand dunes, (e.g.,

beach grasses). Rapid reproduction

allows them to survive shifting sands,

and their roots help stabilize the dune.


Vegetative reproduction is efficient in an

unchanging environment, but can have

disadvantages if conditions change.

English elm (Ulmus procera) was

introduced by the ancient Romans as a

clone. When Dutch elm disease struck,

the clonal population lacked genetic

diversity and was wiped out.


Vegetative reproduction is used

extensively in agriculture.

Stem cuttings inserted into soil will often

grow into a new plant, especially if

treated with auxin.


Grafting is the process of attaching a bud

or piece of stem from one plant onto the

root or stem of another plant.

The root-bearing plant is the stock; the

part grafted onto it is the scion.

The vascular cambia of each must grow

together so that water and minerals can

be transported to the scion. Usually

closely related species are used.

Figure 37.15 Grafting


Meristem culture: Pieces of shoot apical

meristem are cultured to generate

plantlets, which can then be planted in

the field.

• Good when uniformity is desired, as in

forestry, or to produce virus-free plants,

as with strawberries and potatoes.


Apomixis: Asexual production of seeds.

• The megasporocyte fails to undergo

meiosis, resulting in a diploid egg cell,

which then forms an embryo and seed.

• Or, diploid cells from the integument

around the embryo sac form a diploid

embryo sac, and the sac forms an

embryo and seed.


Apomixis produces clones, and occurs

naturally in some crop plants such as

citrus and Kentucky bluegrass.

Some important crops, such as corn, are

grown as hybrids and cannot be selfed

to get more seeds.

If a hybrid had a gene for apomixis, its

offspring would be genetically identical

to itself.


An intensive search is on for genes for

apomixis that could be introduced into

desirable crops and allow them to be

propagated indefinitely.

Such a gene has been identified in corn,

but the yield of the variety that contains it

is low.


• Compare and contrast sexual and

asexual reproduction in plants,

including both the end result and the

advantages or disadvantages.

• List types of locations where

vegetatively reproducing plants might

occur, and give reasons for their

occurrence in these locations.

• Describe how apomixis occurs and how

this might be useful in agriculture.


Poinsettias are short-day plants. To

control photoperiod, the plants are

grown in greenhouses, and photoperiod

is carefully regulated.

How did an understanding of angiosperm

reproduction allow floriculturists to develop

a commercially successful poinsettia?


A shorter variety with more branching was

discovered, which was propagated

asexually by grafting to native plants.

New varieties have been generated by

conventional sexual reproduction.

reproduction in flowering plantslrios/3052/life11e_ch37_lecture.pdfflowering in some plants. 37.2...

Documents