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Plant Reproduction

Chapter 42

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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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Reproductive Development

Before flowers can form, plants must undergo a phase change to prepare a plant to respond to internal and external signals

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Reproductive Development

Phase change can be morphologically obvious or very subtle -In oak trees, lower branches (juvenile phase) cling to their leaves in the fall -Juvenile ivy makes adventitious roots and has alternating leaf phyllotaxy -Mature ivy lacks adventitious roots, has

spiral phyllotaxy, and can make flowers

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Reproductive Development

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Reproductive Development

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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Reproductive Development

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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Light-Dependent Pathway

Also termed the photoperiodic pathway -Sensitive to the amount of darkness a plant receives in each 24-hour period -Short-day plants flower when daylight becomes shorter than a critical length -Long-day plants flower when daylight becomes longer -Day-neutral plants flower when mature regardless of day length

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Light-Dependent Pathway

In obligate long- or short-day plants there is a sharp distinction between short and long nights, respectively

In facultative long- or short-day plants, the photoperiodic requirement is not absolute -Flowering occurs more rapidly or slowly depending on the length of day

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Light-Dependent Pathway

Using light as cue allows plants to flower when environmental conditions are favorable

-Manipulation of photoperiod in greenhouses ensures that short-day poinsettias flower in time for the winter holidays

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Light-Dependent Pathway

Flowering is regulated by phytochromes (red-light receptors) and cryptochrome (blue light receptor) via the gene CONSTANS (CO) -Phytochromes regulate CO transcription -CO mRNA is low at night and increase

at daybreak -Cryptochrome modulates CO protein level -Stabilizes CO and protects it from

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Light-Dependent Pathway

CO is a transcription factor that turns on other genes, resulting in the expression of LFY -LFY is a key gene that “tells” a meristem to switch over to flowering

One intriguing possibility is that CO is the elusive flowering hormone -Or that it affects such a flowering signal

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Temperature-Dependent Pathway

Some plants require a period of chilling before flowering called vernalization -It is necessary for some seeds or plants in later stages of development

Analysis of plant mutants reveals that vernalization is a separate flowering pathway

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Gibberellin-Dependent Pathway

Gibberellin binds to the promoter of LFY -Enhances its expression, thereby promoting flowering

In Arabidopsis and other species, decreased levels of gibberellins delay flowering

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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 -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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Autonomous Pathway

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Autonomous Pathway

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Autonomous Pathway

How do shoots “count” and “remember”? -Experiments using bottomless pots have shown that it is the addition of roots, and not the loss of leaves, that inhibits flowering

A balance between floral promoting and inhibiting signals may regulate flowering

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Model for Flowering

The four flowering pathways lead to an adult meristem becoming a floral meristem -They activate or repress the inhibition of floral meristem identity genes -Key genes: LFY and AP1 (APETALA1) -These two genes turn on floral

organ identity genes -Define the four concentric whorls

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Model for Flowering

The ABC model proposes that three organ identity gene classes specify the four whorls 1. Class A genes alone – Sepals 2. Class A and B genes together – Petals 3. Class B and C genes together – Stamens 4. Class C genes alone – Carpels

When any one class is missing, aberrant floral organs occur in predictable positions

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Model for Flowering

Recently, two other classes were identified -Class D genes are essential for carpel formation -Class E genes (SEPALATA) -SEP proteins interact with class A, B

and C proteins that are needed for the development of floral organs

Thus, a modified ABC model was proposed 29 30

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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Flower Structure

Calyx = Consists of flattened sepals Corolla = Consists of fused petals Androecium = Collective term for stamens

-A stamen consists of a filament and an anther

Gynoecium = Collective term for carpels -A carpel consists of an ovule, ovary, style, and stigma

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Male structure

Female structure

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Trends in Floral Evolution

Floral specialization

1. Separate floral parts have been grouped

2. Floral parts have been lost or reduced -Wild geranium

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Trends in Floral Evolution

Floral symmetry

-Primitive flowers are radially symmetrical

-Advanced flowers are bilaterally symmetrical -Orchid

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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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Pollen Formation

Anthers contain four microsporangia which produce microspore mother cells (2n) -Each microspore mother cell produces four haploid (n) microspores through meiosis -Each microspore develops by mitosis into a pollen grain (microgametophyte) -The generative cell in the pollen grain

will later divide to form two sperm cells

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Embryo Sac Formation

Within each ovule, a diploid microspore mother cell undergoes meiosis to produce four haploid megaspores -Usually only one survives -Enlarges and undergoes repeated

mitotic divisions to produce eight haploid nuclei

-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

Bees typically visit yellow or blue flowers -Yellow flowers are marked in distinctive ways that are normally invisible to us -Bull’s eye or landing strip

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Pollination

Flowers that are visited regularly by butterflies often have flat “landing platforms”

Flowers that are visited regularly by moths are often white, or pale in color

-They also tend to be heavily scented

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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

Other animals, including bats and small rodents, may aid in pollination -The signals here are also species-specific

Monkeys are attracted to orange and yellow -They can thus disperse fruits of this color in their habitat

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Pollination

Some angiosperms are wind-pollinated -A characteristic of early seed plants

Flowers of these plants are small, green, and odorless, with reduced or absent corollas -Often grouped and hanging down in tassels

Stamen- and carpel-containing flowers are usually separated between individuals -Strategy that greatly promotes outcrossing

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Pollination

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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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Pollination

Determined by the genotype of the haploid pollen itself

Determined by the genotype of the diploid pollen parent

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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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Asexual Reproduction

Produces genetically identical individuals because only mitosis occurs -Far more common in harsh environments where there is little leeway for variation

It is used in agriculture and horticulture to propagate a particularly desirable plant

Apoximis refers to the asexual development of a diploid embryo in the ovule

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Asexual Reproduction

In vegetative reproduction new plant individuals are cloned from parts of adults -Comes in many and varied forms -Runners or

stolons -Rhizomes -Suckers -Adventitious

plantlets

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Asexual Reproduction

Whole plants can be cloned by regenerating plant cells or tissues on nutrient medium

A protoplast is a plant cell enclosed only by a plasma membrane -When single plant protoplasts are cultured, cell wall regeneration takes place -Cell division follows to form a callus,

an undifferentiated mass of cells -Whole plants are then produced

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Asexual Reproduction

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Asexual Reproduction (Cont.)

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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 Life Spans