Section 1: Pollination enables gametes to come together within a flower

Chapter 38: Angiosperm Reproduction and

Biotechnology

Review: The Angiosperm Life CycleRecall that:

-The life cycles of plants have an alteration of alteration of generationsgenerations between haploid and diploid states.

-In angiosperms, the sporophytesporophyte is the dominate state.

-PollinationPollination is achieved by wind, water, or animals.

Flower Structure

-Flowers are the reproductive shoots of the angiosperm sporophyte.

-They are typically composed of four whorls of highly modified leaves called floral organs, which are separated by short internodes.

-Flowers are determinate shoots, meaning they cease growing after the flower and fruit are formed

The Floral Organs• Consist of the sepalssepals, petalspetals, stamensstamens, and carpels carpels that are attached

to a part of the stem called the receptaclereceptacle.

• Reproductive organs: stamensstamens and carpelscarpels

• Sterile organs: sepalssepals and petalspetals

• Sepals Sepals : a modified leaf that closes and protects the floral bud before it opens

• PetalsPetals : modified leaf of a flowering plant; often colorful parts of a flower that advertise it to insects and other pollinators

Subparts of Floral Organs• Stamen Stamen : consists of a stalk called a filament and a terminal structure

called the antheranther

• anther anther : contains chambers called pollen sacs in which pollen is produced

• CarpelCarpel : consists of an ovaryovary at its base, and a long slender neck called the stylestyle

• stylestyle : has a sticky structure called a stigmastigma at the top that serves as a platform for pollen to land

• ovaryovary : contains one or more ovules• pistilpistil : refers to a group of fused carpels

Floral Variations• Complete flowersComplete flowers have all the basic floral organs.

• Incomplete flowersIncomplete flowers lack one or more of these floral organs.

-Flowers can differ in symmetry.

-They can have bilateral symmetry, in which they can be divided into two equal parts by an imaginary line.

-They can also have radical symmetry, in which any imaginary line through the central axis divides the flower into two equal parts

SymmetrySymmetry Ovary LocationOvary Location Floral Distribution Floral Distribution

-The location of a flower’s ovary may vary in relation to the stamens, petals, and sepals.

-An ovary is called superior if those parts are attached below it; semi-inferior if they are attached alongside it; and inferior if they are attached above it.

-Floral distribution can also differ.

-Some species have individual flowers, while others have clusters called inflorescencesinflorescences..

Floral Variations (cont’d)

Reproductive VariationsReproductive Variations-In most incomplete flowers, stamens and carpels are wither absent or nonfunctional.

-Incomplete flowers that have only function stamen are called staminatestaminate, and those with only functional carpels are called carpellatecarpellate.

-If staminate and carpellate flowers are on the same species, the species is said to be monoeciousmonoecious.

--A dioeciousdioecious species has staminate flowers and carpellate flowers on separate plants.

Terms: Gametophyte Development and Pollination

• MicrosporesMicrospores : spores from a heterosporous plant species that develops into a male gametophyte

• MegasporesMegaspores : spore from a heterosporous plant species that develops into a female gametophyte

Gametophyte Development and Pollination (cont’d)

Development of a male gametophyte (pollen grain)Development of a male gametophyte (pollen grain). Pollen sacs develop within the microsporangia (pollen sacs) of anthers at the

tips of the stamens.

1. Each of the microsporangia contains diploid microsporocytes (microspore mother cells).

2. Each microsporocyte divides by meiosis, producing four haploid microspores, each of which develops into a pollen grain.

3. A pollen grain becomes a mature male gametophyte when it generative nucleus divides and forms two sperm. This usually occurs after a pollen

gain lands on the stigma of a carpel and the pollen tube begins to grow.

Gametophyte Development and Pollination (cont’d)

Development of a female gametophyteDevelopment of a female gametophyte (embryo sac)(embryo sac). The embryo sac develops within an ovule, itself enclosed by the ovary at the base

of a carpel.

1. Within the ovule’s megasporangium is a large diploid cell called the megasporocyte.

2. The megasporocyte divides by meiosis and gives rise to four haploid cells, but in most species only one of these survives as

the megaspore.3. Three mitotic divisions of the megaspore form the embryo sac, a

multicellular female gametophyte. The ovule now consists of the embryo sac along with the surrounding integuments (protective

tissue).

Gametophyte Development and Pollination (cont’d)

• PollinationPollination, the transfer of pollen from anther to stigma, is the first step in a chain of events that can lead to fertilization.

• Plants can rely on: air, water, or animals for pollination.

Mechanisms That Prevent Self-Fertilization

• Many angiosperms have mechanisms that make it difficult or impossible for self-fertilization or “selfing” to occur.

• This contributes to genetic variety by ensuring that the sperm and eggs come from different parents.

• The most common anti-selfing mechanism is self-self-incompatibilityincompatibility, the ability of a plant to reject its own pollen and sometimes the pollen of a closely related relative.

Mechanisms That Prevent Self-Fertilization (cont’d)

• Recognition of “self” pollen is based on genes for self-incompatibility, called S genes.

• If a pollen grain has an allele that matches an allele of the stigma on which it lands, self-recognition blocks growth by either : gametophytic self-compatibility or sporophytic self-compatibility.

• Gametophytic self-compatibility: The S –allele in the pollen genome governs

the blocking of fertilization.

– Ex.) S1 pollen grain from an S1S2 parental sporophyte will fail to fertilize eggs of an S1S2 flower but will fertilize an S2S3 flower. An S2 pollen grain would not fertilize either flower

– Involves enzymatic destruction of RNA within a rudimentary pollen tube

Mechanisms That Prevent Self-Fertilization (cont’d)

• Sporophytic self-compatibility: fertilization is blocked by S –allele gene products in tissues of the parental sporophyte that adhere to the pollen wall.

• Ex.) neither an S1 nor S2pollen grain from an S1S2 parental sporophyte will fertilize eggs of an S1S2 flower or S2S3 flower

• Involves a signal transduction pathway in epidermal cells of the stigma

38.2After fertilization, ovules develop into seeds and ovaries into fruits.

A look at fertilization and its products: seeds and fruits.

Double Fertilization

• After landing on a receptive stigma, a pollen grain absorbs moisture and germinates– Produces a pollen tube that extends down

between the cells of the style towards the ovary

• Nucleus of generative cell divides by mitosis and forms two sperm

• Tip of pollen tube enters ovary, discharges two sperm near embryo sac

Double Fertilization

• Angiosperm life cycle:– One sperm fertilizes egg and forms zygote– Other sperm combines with two polar nuclei to

form triploid nucleus– Triploid nucleus in large central cell, cell gives rise

to endosperm (food-storage)– Two sperm cells joined with different nuclei of the

embryo sac is double fertilization– Prevents angiosperms from squandering nutrients

Double Fertilization

• First cellular event after gamete fusion is increase in cytoplasmic calcium levels of the egg – Also occurs during animal gamete fusion

• Establishment of a block to polyspermy (fertilization of an egg by more than one sperm cell) also similar to animals

From Ovule to Seed

• After double fertilization each ovule becomes a seed and the ovary becomes a fruit enclosing the seed

• Seeds stockpile proteins, oils, and starch and become sugar sinks

• Storage function of endosperm is eventually taken over by swelling cotyledons of embryo

Endosperm Development

• Precedes embryo development• After double fertilization, triploid nucleus

divides forming multinucleate supercell• This liquid mass (endosperm) becomes

multicellular when cytokinesis forms membranes between nuclei

• Naked cells produce cell walls and endosperm becomes solid – Ex: coconut milk vs. coconut meat

Embryo Development• First mitotic division of the zygote is transverse,

splitting the fertilized egg into a basal cell and a terminal cell

• Terminal cell gives rise to most of embryo• Basal cell continues to divide transversely producing

thread of cells (suspensor) which anchors embryo to parent

• Suspensor functions in transfer of nutrients and in protection

• Terminal cell divides several times, forms proembryo attached to suspensor– Cotyledon begins to form as bumps on proembryo

Structure of the Mature Seed• Seed dehydrates in final stages, making water content 5-

15% of weight• Embryo becomes dormant• Embryo and food supply are enclosed by seed coat• Hypocotyl (embryonic axis) terminates in the radicle

(embryonic root)– Portion above where cotyledons attach is the epicotyl

• Embryo of a monocot has a single cotyledon– Grass family has scutellum (specialized cotyledon) which is thin

with large surface area for absorption• Embryo of grass seed enclosed by two sheaths

– Coleoptile covers young shoot, coleorhiza covers young root

From Ovary to Fruit• Fruit protects enclosed seeds and aids in their dispersal• Fertilization triggers hormonal changes that cause ovary-

fruit transformation• Ovary wall becomes pericarp (thickened fruit wall)• Most fruits, derived from a single carpel or several fused

carpels, are simple fruits– Ex: peach, pea pod, nut

• Aggregate fruit results from single flower with more than one separate carpel; clustered fruitlets– Ex: raspberry

• Multiple fruit develops from inflorescence, group of flowers clustered tightly together; walls thicken and fuse– Ex: pineapple

From Ovary to Fruit• Sometimes other floral parts contribute to fruit

(accessory fruits)• Apple flowers: fruit embedded in receptacle and the

fleshy part is derived mainly from receptacle; only core comes from ovary

• Strawberries: aggregate fruit consisting of enlarged receptacle embedded with tiny one-seeded fruits

• Fruit ripens when seeds complete development• Ripening of dry fruit involves aging & drying out of

tissues• Complex hormone interactions enable ripening of

fleshy fruits

Seed Germination

• As seed matures, it dehydrates and enters dormancy– Extremely low metabolic rate– Suspension of growth and development

• Some seeds germinate as soon as they enter suitable environment

• Others remain dormant until receiving specific environmental cue

Seed Dormancy• Seed dormancy is for increased chance of

germination at advantageous time and place• Breaking dormancy requires specific conditions

– Ex: desert plants may germinate after substantial rainfall, and not mild drizzle

– Many seeds require intense heat to break dormancy, or extended exposure to cold

– Some require light and some have coats that must be weakened by chemical attack (in digestion)

• Soil has bank of ungerminated seeds that accumulate and reappear after disaster

From Seed to Seedling

• Germination of seeds depends on imbibition (uptake of water due to the low water potential of dry seed)

• Imbibing water causes seed to expand and rupture coat; triggers metabolic changes in embryo to enable resumed growth

• First organ to emerge from germinating seed is radicle (embryonic root) followed by shoot tip

• Followed by hypocotyl, cotyledons, epicotyl

From Seed to Seedling

• Monocots (maize and other grasses) use different method– Coleoptile (sheath enclosing shoot) pushes upward

followed by shoot tip through tubular coleoptile and out coleoptile’s tip

• In germination, tough seed gives way to fragile seedling exposed to harsh conditions

• Only a fraction of seedlings survive in the wild• Production of seeds in enormous numbers

compensates for this

Many flowering plants clone themselves by asexual reproduction

Asexual Reproduction

• Exact copies are derived from one parent– Has no genetic recombination

• Many plants produce sexually and asexually– Ex: seedless, gymnosperms and some

angiosperms

• Vegetative reproduction- asexual plants are more mature then when sexually reproduced– Beneficial when in an unstable environment

Mechanisms of Asexual Reproduction

• Asexual reproduction is an extension of the capacity for indeterminate growth– Plants have undifferentiated cells which can

divide, sustaining or renewing growth indefinitely

• Detached vegetative fragments can develop into whole offspring– Fragmentation-separation of plant parts which

grow into individual entire plants• Common form of asexual reproduction

Mechanisms of Asexual Reproduction

• Seeds produced without fertilization or pollination is a process called apomixis– Only one parent, no sperm and egg– Diploid cell in ovule matures into seeds-clones of

parent– Seeds are dispersed, although seed dispersal us

typically a sexual process

Vegetative Propagation and Agriculture

• Genetic modification occurs by incorporating foreign genes to get better characteristics– Clones from cuttings asexual reproduction

using plant fragments• After a cut at end of shoot or stem mass of

undifferentiated cells called callus forms– Cells differentiate and can become whole plant

– Grafting-modification that is a result of cuttings combines best qualities of plants in vivo

• stock: root system• scion: twig grafted onto stock

Vegetative Propagation and Agriculture

• Plants can be created or cloned in vitro– Cells cultured and then begin to divide so placed

in soil to grow into a plant– Whole plants can grow from culturing small pieces

of parent tissue– Genetic modification also occurs in vitro

• Transgenic-organisms engineered to express a gene from another species

Vegetative Propagation and Agriculture

• Protoplast fusion– Protoplasts-plant cells with cell walls removed

• Screened for mutations looking to improve the agricultural value of the plant

• Different species protoplasts can be fused– Otherwise reproductively incompatible– Now are “hybrids”

• ex- potato and wild potato combined to acquire resistance to a weed killer

Plant Biotechnology is Transforming Agriculture!!!

“Biotechnology has the potential to bring tremendous benefits …”

-John Prescott

Artificial Selection

Artificial Selection

• Maize, cont.• Using hybridization and artificial selection, scientists

created a “super maize” with a hard endosperm and sufficient protein nutrition

• Using modern genetic engineering techniques, scientists can transfer genes from completely unrelated species

Reducing World Hunger and Malnutrition

Reducing World Hunger and Malnutrition

The Debate Over Plant Biotechnology• Issues of Human Health

– Many people are concerned that genetic engineering may pass allergens from gene source to plant

– In fact, there is no evidence that genetically modified organisms (GMOs) have adverse effects on human health usually positive effects!

– Ex.) Bt Maize has less cancer-causing agents, fumonisin

– Those against GMOs want:– 1- labeling of GMO products– 2- Separate transport, processing, and storage of GMO

products

The Debate Over Plant Biotechnology• Possible Effects on Non-target Organisms

– STUDY: Caterpillars of monarch butterflies died after consumption of milkweed leaves heavily dusted with pollen from Bt maize

• It was eventually found that other plant parts with Bt toxin caused death

– Despite the negative effects of Bt maize, the alternative to insect control would be insecticide spraying, having an even greater impact on non-target organisms

The Debate Over Plant BiotechnologyAddressing the Problem of Transgene Escape◦ transgene escape – the escape of genes from

transgenic crops into related weeds via crop-to-weed hybridization The major fear of biotechnologists is that a “super weed” will

developBecause of transgene escape, scientists are trying

to: 1- breed male sterility into transgenic crops no viable pollen

would be produced 2- engineer transgene into chloroplast DNA of crop

Would work because chloroplast DNA is often inherited strictly from maternal plants, so transgenes in chloroplasts cannot be transferred by pollen

The Debate Over Plant Biotechnology

• “Terminator technology” is another possible solution for transgene escape

• GMOs that undergo the terminator process grow normally until last stages of seed or pollen maturation

• Then, “terminator” protein, toxic to plants, is activated in nearly mature seed or pollen, making the seeds or pollen unviable

• These proteins are produced only if original seeds are pretreated with a specific chemical