CHAPTER 38 PLANT REPRODUCTION

Sexual Reproduction & Biotechnology

Floral Organs

• Sepals and petals are nonreproductive organs.• Sepals: enclose and protect the floral bud

before it opens; usually green and more leaf-like in appearance.

• In many angiosperms, the petals are brightly colored to attract pollinators.

Stamens: male reproductive organs

• Stalk: the filament

• Anther: pollen sacs.

- The pollen sacs produce pollen.

Carpels: female reproductive organs• Ovary- base of the carpel

- Ovules- Egg cell

- Embryo Sac (female gametophyte), i.e., seed

• Stigma- platform for pollen grain

• Style- slender neck, connects ovary and stigma

• The stamens and carpels of flowers contain sporangia, within which the spores and then gametophytes develop.• The male gametophytes are sperm-producing

structures called pollen grains, which form within the pollen sacs of anthers.

• The female gametophytes are egg-producing structures called embryo sacs, which form within the ovules in ovaries.

• Pollination begins the process by which the male and female gametophytes are brought together so that their gametes can unite.• Pollination- when pollen released from anthers

lands on a stigma.

• Each pollen grain produces a pollen tube, which grows down into the ovary via the style and discharges sperm into the embryo sac, fertilizing the egg.

• The zygote gives rise to an embryo.

• The ovule develops into a seed and the entire ovary develops into a fruit containing one or more seeds.

• Fruits disperse seeds away from the source plant where the seed germinates.

Function of Flowers

Classification of Flowers• Complete Versus Incomplete Flowers

• Complete: possess sepals, petals, stamens, and carpels

• Incomplete: lack one or more of these components

• Perfect Versus Imperfect Flowers• Perfect: possess both stamens and carpels

• Imperfect: possess either stamens (staminate) or carpels (carpelate), but not both

Complete Flower

Monoecious Versus Dioecious

• Monoecious: both staminate and carpellate flowers are found together on the same plant (e.g., corn).

• Dioecious: staminate flowers occur on separate plants from those that carry carpellate flowers (e.g., date palms).

Monoecious

Dioecious

Angiosperm Life Cycle

Important Things to Note About Angiosperm Life Cycles

• Mature pollen grains are entire haploid male gametophyte plants.• Microsporangia in anthers produce

microsporocytes that undergo meiosis and become haploid microspores.

• Each microspore undergoes mitosis to produce two-celled male gametophyte plants (pollen grains).

• One cell is the generative cell while the other is the tube cell.

Important Things to Note About Angiosperm Life Cycles

• Entire mature female gametophyte plants spend their entire lives supported by the parent sporophyte.• Megasporangia in ovules produce

megasporocytes that each undergo meiosis and become four haploid megaspores.

• Only one of these four cells become functional megaspores, the remaining three degenerating.

Important Things to Note About Angiosperm Life Cycles

• The haploid nucleus of the megaspore undergoes three mitotic divisions to produce a multinucleate cell with eight haploid nuclei.

• Cytokinesis divides these nuclei into seven cells: three antipodal cells, two synergids, one egg cell, and one binucleate central cell.

• Together these cells form the mature female gametophyte or embryo sac.

Important Things to Note About Angiosperm Life Cycles

• Fertilization involves a double fertilization event.• After attachment to the stigma, the haploid

generative cell of a pollen grain undergoes mitosis to produce two sperm nuclei.

• The two sperm nuclei migrate down the pollen tube as it elongates through the style to the ovary containing the ovules.

• One sperm nucleus enters the egg cell; the other enters the binucleate central cell of the female gametophyte.

Important Things to Note About Angiosperm Life Cycles

• The central cell is trinucleate for a while.

• Fusion of all three haploid nucleus yield one triploid nucleus.

• Mitosis of the triploid central cell produces the multinucleate triploid endosperm tissue.

• This endosperm tissue represents a source of stored organic energy to be used by the developing sporophyte embryo (derived from the zygote) and to be used during seed germination.

• The development of angiosperm gametophytes involves meiosis and mitosis.

• The male gametophyte begins its development within the sporangia (pollen sacs) of the anther.• Within the sporangia are microsporocytes,

each of which will from four haploid microspores through meiosis.

• Each microspore can eventually give rise to a haploid male gametophyte.

• A microspore divides once by mitosis and produces a generative cell and a tube cell.• The generative cell forms sperm.

• The tube cell, enclosing the generative cell, produces the pollen tube, which delivers sperm to the egg.

Pollen Tubes

Pollen Grains• This is a pollen grain, an immature male gametophyte.

Barriers to Self-Fertilization• Stamens and carpels may mature at different times.

• Self-incompatibility- plant rejects its own pollen

• Plant design prevents an animal pollinator from transferring pollen from the anthers to the stigma of the same flower.

The Genetic Basis for the Inhibition of Self-Fertilization

S-genes: self-incompatibility gene• If a pollen grain and the carpel’s stigma have matching

alleles at the S-locus, then the pollen grain fails to initiate or complete the formation of a pollen tube.

Pollen Tube Formation and Double Fertilization

Seed Development

Release of Sugars from the Endosperm During Germination

Fate of the Endosperm• Typical Monocot (e.g., corn)

• endosperm present in substantial quantities in mature seed.

• cotyledon absorbs nutrients from endosperm during seed germination.

• Typical Dicot (e.g, garden bean)• endosperm completely absorbed into cotyledons during seed

maturation.

• Other Dicots (e.g., castor bean)• endosperm only partially absorbed by cotyledons during seed

maturation.

• remainder of endosperm absorbed by cotyledons during germination.

Seed Structure

Relationship of the Flower to the Fruit

• As the seeds are developing from ovules, the ovary of the flower is developing into a fruit, which protects the enclosed seeds and aids in their dispersal by wind or animals.• Pollination triggers hormonal changes that cause the

ovary to begin its transformation into a fruit.

• If a flower has not been pollinated, fruit usually does not develop, and the entire flower withers and falls away.

The ovary develops into a fruit adapted for seed dispersal

Fruit Formation• The ovary wall becomes the pericarp, the

thickened wall of the fruit

• Other flower parts wither and are shed.• However, in some angiosperms, other floral parts

contribute to what we call a fruit.

Development of a pea fruit (pod)

Functions of the Fruit

• Protection of the enclosed seed (e.g., pea pods).

• Facilitating dispersal.• wings for wind dispersal (e.g., maple).

• hocks and barbs for attachment to animal fur or avian feathers (e.g., cocklebur).

• sweet, fleshy fruit encouraging ingestion and dispersal of seeds by animals (e.g., cherry).

Fruits

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Types of FruitsSimple Fruits

Aggregate Fruits

Multiple Fruits

Seed Dormancy

• Function: allows seeds to germinate at the most optimal time.

• Length of dormancy• Signals triggering the end of dormancy.

• occurrence of water

• period of cold temperature

• fire

• light

• scarification

Germination of Bean

Germination of a Pea

Germination of Corn

Genet Concept

Asexual and Sexual Reproduction in the Life

Histories of Plants

Types of Asexual Reproduction in Plants

• Vegetative Reproduction• consequence of the existence of meristematic

tissues and indeterminate growth in plants

• typically involves fragmentation

• Apomixis• =production of seeds without fertilization

• diploid cell in ovule develops into embryo

Asexual Propagation

Asexual Propagation of Plants in Agriculture

• Shoot or stem cuttings generate roots.• Cloning from single leaves.• Potato eyes used to generate whole potato

plants.• Grafting.• Plant tissue culture.

Plant Tissue Culture: Cloning from Individual Cells

Plant Tissue Culture: Plant biotechnologists have adopted in vitro methods to create and clone novel plants varieties.

Genetic Engineering Applications of Plant Tissue Culture

• Injecting foreign DNA into host cells

• Protoplast fusion

A DNA Gun

Protoplasts

MonocultureRisks and Benefits

Humans as Genetic Engineers

Some Definitions Related to Plant Development

development• the sum of all of the changes that progressively elaborate an

organism’s body; involves growth, morphogenesis, and cellular differentiation

growth• an irreversible increase in size resulting from increases in cell

number and size

morphogenesis• the development of form

cellular differentiation• the specialization of cells into different types with different

functions

Plant and Animal Development

Lifelong Morphogenesis

• Indeterminant growth in plants means that morphogenesis is a continuous, never-ending process.

• Plant morphogenesis involves oriented cell division and growth but not the migration of cells to different parts of the plant body.

Role of the Cytoskeleton in the Orientation of Cell Division

• Ring of microtubles (preprophase band) in the cortex of the cell determines the division plane.

• Preprophase band of microtubules disappears, leaving an imprint of actin microfilaments.• hold nucleus in place until spindle apparatus

forms.

• directs the movements of cell plate vesicles.

Orientation of Mitosis

Cell Growth Depends upon the Orientation of Cellulose Microfibrils in the Cell Wall

Hypothetical Mechanism for the Orientation of Cellulose Microfibrils

Cellular Differentiation in Plants

• Involves changes in gene expression, not in the genomic information of the cells.

• Pattern Formation.• development of specific structures in specific locations

• depends upon positional information of the cells

• also related to positional consequences of cell division and elongation

• Clonal analysis.

Genetic Basis for Pattern Formation in Flower Development

Genetic Basis for Pattern

Formation in Flower

Development