Plant Reproduction Chapter 42. 2 Reproductive Development Angiosperms represent an evolutionary innovation with their production of flowers and fruits

Plant Reproduction

Chapter 42

2

Reproductive Development

Angiosperms represent an evolutionary innovation with their production of flowers and fruits

Plants go through developmental changes leading to reproductive maturity by adding structures to existing ones with meristems-A germinating seed becomes a vegetative plant through morphogenesis

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Gameteproduction and

pollination

Fertilization

Embryodevelopment

Fruitand seed

maturation

Developmentof plant body

Maturationand flowering

Dispersaland

germination

2n

2n

2n

2n

2n

2n

n

n

Zygote

CHAPTER 42

CHAPTER 37

CHAPTER 36

2n

2n

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Flowering is the default state

In Arabidopsis, the gene embryonic flower (EMF) prevents early flowering

-emf mutants lacking a functional EMF protein flower immediately

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The juvenile-to-adult transition can be induced by overexpressing a flowering gene

-LEAFY (LFY) was cloned in Arabidopsis

-Overexpression of LFY in aspen, causes flowering to occur in weeks instead of years

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Flower Production

Four genetically regulated pathways to flowering have been identified

1. The light-dependent pathway

2. The temperature-dependent pathway

3. The gibberellin-dependent pathway

4. The autonomous pathway

Plants can rely primarily on one pathway, but all four pathways can be present

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Let’s Take a Look at theAutonomous Pathway in Some

Detail First

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Autonomous Pathway

The autonomous pathway does not depend on external cues except for basic nutrition

It allows day-neutral plants to “count” nodes and “remember” node location

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Upper Axillary Bud Released from Apical Dominance Lower Axillary Bud Released from Apical Dominance

Intact plant Shoot removed Replacement shoot

Shootremovedhere

5 nodes*removed

5 nodes*replaced

Intact plant Shoot removed Replacement shoot

Shootremovedhere

13 nodes*removed

13 nodes*replaced

*nodes = leaf bearing node

Autonomous Pathway--Plants Can Count

-Tobacco plants produce a uniform number of nodes before flowering-Upper axillary buds of flowering tobacco remember their position if rooted or grafted

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Shootremovedhere

Shootremovedhere

Shoot Florally Determined Shoot Not Florally Determined

a. b.

Intact plant

Shootremoved

Rooted shoot Floweringrooted shoot

Intact plant

Shootremoved

Rooted shoot Floweringrooted shoot

Autonomous Pathway--Plants Can Remember

Not-Florally Determined Plants are said not to remember...Florally Determined plants are said to remember

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Control plant:no treatment

Experimental plant:pot-on-pot treatment

Experimental plant:Lower leaves were

continually removed


Roots Inhibit Flowering

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A Model of All the Flowering Pathways

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inhibitionactivation

Repression of Floral Inhibitors

Activation of Floral Meristem Identity GenesAdult meristem Floral meristem

Temperature-dependentpathway

Autonomouspathway

Flower-repressing

genes

Flower-promoting

genes

Vernalization

Autonomousgene expression

Cold

Gibberellin-dependentpathway

Light-dependentpathway

Gibberellin

CO

Light

LFY

AP1

ABCDEfloral organidentity genes

Floral organdevelopment

Gibberellin binds to the promoter of LFY

CO is a transcription factor that turns on other genes, resulting in the expression of LFY

-Phytochromes regulate CO transcription

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Flower Structure

Floral organs are thought to have evolved from leaves

A complete flower has four whorls-Calyx, corolla, androecium, and gynoecium

An incomplete flower lacks one or more of these whorls

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Stamen Anther Filament

CarpelStigmaStyleOvaryOvule

Petal

Receptacle

Sepal

all stamens = androeciumall carpels = gynoeciumall petals = corollaall sepals = calyx


Male structure

Female structure

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Gamete Production

Plant sexual life cycles are characterized by an alternation of generations-Diploid sporophyte haploid gametophyte

In angiosperms, the gametophyte generation is very small and is completely enclosed within the tissues of the parent sporophyte-Male gametophyte = Pollen grains-Female gametophyte = Embryo sac

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Gamete Production

Gametes are produced in separate, specialized structures of the flower

Reproductive organs of angiosperms differ from those of animals in two ways:

1. Both male and female structures usually occur together in the same individual

2. Reproductive structures are not permanent parts of the adult individual

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Anther Microsporemother cell

Pollen sac

Megaspores

Pollen grains(microgametophytes)

Tube cellnucleus

Generative cell

Ovule

Megasporemother cell

SurvivingmegasporeAntipodals

Polarnuclei

Degeneratedmegaspores

Eight-nucleate embryo sac(megagametophyte)

Synergids

Eggcell

diploid (2n)

haploid (n)

MEIOSIS

MEIOSIS

MITOSIS

MITOSIS


Microspores

Generative Cellsgo on to make 2 spermCells

Megaspore enlarges and

undergoes

repeated mitotic divisions

to produce eight haploid

nuclei...Egg cell is enclosed

within a seven-celled

embryo sac

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Pollination

Pollination is the process by which pollen is placed on the stigma

-Self-pollination = Pollen from a flower’s anther pollinates stigma of the same flower

-Cross-pollination = Pollen from anther of one flower pollinates another flower’s stigma

-Also termed outcrossing

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Pollination

Successful pollination in many angiosperms depends on regular attraction of pollinators

Flowers & animal pollinators have coevolved resulting in specialized relationships

-Bees are the most common insect pollinators

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Pollination

Flowers that are visited regularly by birds often have a red color

-Usually inconspicuous to insects

Hummingbirds obtain nectar from flowers that match the length and shape of their beaks

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Pollination

Self-pollinating plants usually have small, relatively inconspicuous flowers that shed pollen directly into the stigma

Self-pollination is favored in stable environments

1. Plants do not need to be visited by animals to produce seed

2. Offspring are more uniform and probably better adapted to their environment

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Pollination

Several evolutionary strategies promote outcrossing

1. Separation of male and female structures in space

-Dioecious plants produce only ovule or only pollen

-Monoecious plants produce male and female flowers on the same plant

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Pollination

2. Separation of male and female structures in time

-Even if functional stamens and pistils are both found in the same flower,

they may reach maturity at different times

-Plants in which this occurs are called dichogamous

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Pollination

3. Self-incompatibility

-Pollen and stigma recognize each other as self and so the pollen tube is blocked

-Controlled by alleles at the S locus

-Gametophytic self-incompatibility

-Block is after pollen tube germination

-Sporophytic self-incompatibility

-The pollen tube fails to germinate

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a. b.

Gametophytic Self-Incompatibility Sporophytic Self-Incompatibility

S1S2

pollen parent

S2

S1S2

S2

S1

S2

X

S1 S2

X X


S2S3

stigma ofpollen recipient

S1S2

pollen parent

S2S3

stigma ofpollen recipient

Pollination

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Fertilization

Angiosperms undergo a unique process called double fertilization

-A pollen grain that lands on a stigma forms a pollen tube that pierces the style

-While the pollen tube is growing, the generative cell divides to form 2 sperm cells

-When pollen tube reaches the ovule, it enters one of the synergids and

releases the two sperm cells

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Fertilization

-Then double-fertilization occurs

-One sperm cell nucleus fuses with the egg cell to form the diploid (2n) zygote

-Other sperm cell nucleus fuses with the two polar nuclei to form the triploid (3n) endosperm nucleus

-Eventually develops into the endosperm that nourishes

embryo

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Generative cellTube cell

Stigma

Style

Ovary

Ovule

Carpel

Pollination

Pollen grain

Embryosac

Tube cell

Sperm cells

Tube cellnucleus


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Growth of pollen tube

Pollen tube

Double fertilizationRelease of sperm cells

Zygote (2n)

Antipodals

Polar nuclei

Egg cell

Synergids

Endosperm nucleus (3n)


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Plant Life Spans

Once established, plants live for variable periods of time, depending on the species

Woody plants, which have extensive secondary growth, typically live longer than herbaceous plants, which don’t

-Bristlecone pine, for example, can live upward of 4,000 years

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Plant Life Spans

Perennial plants are able to flower and produce seeds and fruit for an indefinite number of growing seasons

-May be herbaceous or woody

-In deciduous plants all the leaves fall, and the tree is bare, at a particular time of year

-In evergreen plants, the leaves drop throughout the year, and so the plant is never completely bare

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Plant Life Spans

Annual plants grow, flower, and form fruits and seeds, and typically die within one growing season

-Are usually herbaceous

-The process that leads to the death of the plant is called senescence

Biennial plants have two-year life cycles

-They store energy the first year and flower the second year

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Plant Reproduction Chapter 42. 2 Reproductive Development Angiosperms represent an evolutionary innovation with their production of flowers and fruits