Angiosperm Reproduction and

BiotechnologyCh. 38

Orchid deception…Male wasps attempt to mate with flower b/c of its color and scent that is similar to a female wasp…all to transfer pollen

Flowers, Double Fertilization, Flowers, Double Fertilization, and Fruits…Unique to and Fruits…Unique to Angiosperm Life CycleAngiosperm Life Cycle

Sporophyte and gametophyte generations alternate in the life cycle of plants

• Multicellular haploid gametophyte generations alternate with diploid sporophyte generation

Male and female gametophytes develop within anthers and ovaries, respectively

Angiosperm - Alternation of Angiosperm - Alternation of GenerationsGenerations

Sporophyte• produces haploid spores by meiosis in sporangiaSpores undergo mitosis• develop into a multicellular male or female

gametophyteGametophytes produce gametes• by mitosis• pollen or eggs

Gametes fuse• zygote forms and develops into a multicellular

sporophyteSporophyte is dominant in the life cycle of the

angiosperm

Flower Structure and FunctionSepals

• bottom whorl; protects developing petals

• Calyx – collection of sepalsPetals• top whorl; important in

pollination• Corolla – collection of

petalsStamens• anther + filamentCarpels• stigma + style + ovary• Ovules within ovaries; #

dependent on species

Variations of Basic Flower Structure

Complete flower• sepals, petals,

stamens, carpelsIncomplete flower• missing 1 or more

complete flower parts

Inflorescences• Showy clusters of

flowers

Variations of Basic Flower Structure

Perfect flower• both stamens and carpels

present• may also be incomplete if

missing petals or sepalsImperfect flower• staminate or carpellate• unisex

– Monoecious• Flowers are either male or

female but on the same plant– Dioecious

• Male and female flowers are on separate plants

Development of Male Gametophytes in Pollen GrainsAnther

• Contains 4 microsporangia (pollen sacs)– Contain microspore mother cells that

undergo meiosis producing 4 microspores

– Each microspore gives rise to a male gametophyte by mitosis•Generative cell and tube cell

Pollen Grain• Spore, generative cell and tube cellTube cell• Produces pollen tube which can grow very

quickly to deliver the sperm cells to the female gametophyte

Generative cell• Divides producing 2 sperm cells

Development of Female Gametophytes Embryo Sacs

Megasporangium• Site of female gametophyte

development• Is surrounded by integuments except

at the micropyleMegasporocyte• Enlarges and undergoes meiosis

producing 4 haploid megaspores• One megaspore survives; others dieMegaspore• Divides 3 times by mitosis to form

the embryo sacEmbryo sac• 3 antipodal cells, 2 polar nuclei, 1

egg, 2 synergids

Pollination

Pollination

• Placement of pollen from the anther onto the stigma of a carpel

• Wind, water, animals– Bees, moths,

butterflies, flies, bats, birds

• Self or cross pollinate

Pollination

Double fertilization• union of 2 sperm cells with 2 cells within the

embryo sac• gives rise to endosperm (sperm + 2 polar

nuclei)

Pollination1. Pollen grain germinates and pollen tube grows

down the style2. Tube discharges two sperm into the embryo sac3. One sperm fertilizes the egg zygote; other sperm

combines with 2 polar nuclei triploid endosperm

Coevolution of Flower and Pollinator

Coevolution

• Joint evolution of two interacting species in response to selection imposed by the other

Seed Development, Form, and FunctionEndosperm Development

• Develops prior to embryo• Multinucleate “supercell” with a

milky consistency• Stores nutrients that can be used

by the seedling after germinationEmbryo Development• First mitotic division forms the

basal cell and terminal cell• Terminal cell suspensor• Basal cell proembryo• Cotyledons begin to appear

Structure of a Mature Eudicot Seed

Embryo is quiescent until germination• dehydrates; embryo surrounded by endosperm; seed coat formed• Enters dormancyHypocotyl• embryonic axis below the cotyledon(s)Radicle• end of the hypocotyl• embryonic rootEpicotyl• embryonic axis above the cotyledon(s)Plumule• end of the epicotyl• shoot tip with a tiny pair of leaves

Seed

Structure of a Mature Monocot Seed

Scutellum• Specialized cotyledon in

maize and wheat• Large SA and pressed

against the endospermColeoptile• Encloses the young shoot in

a grass seed• Aid in soil penetrationColeorhiza• Encloses the young root in a

grass seed• Aid in soil penetration

Seed Dormancy: An Adaptation for Hard Times

•Prevents germination during unfavorable conditions

•Increases chance that germination will occur at a time and place most advantageous to the survival of the seedling

Seed Germination and Seedling Development

• Imbibition– First step in germination– Uptake of water due to low water potential

• Radicle emerges• Shoot tip breaks through soil• Hypocotyl straightens

– Cotyledons and epicotyl raised• Epicotyl spreads first leaves

– Photosynthesis begins– Cotyledons shrivel and fall off

Germination

Germination

Fruit Form and FunctionFertilization necessary for fruit to form

True fruit = ripened ovary• fruit develops from the ovary• ovary wall becomes the pericarp• seed develops from the ovuleDry fruits• fruit ages and pods open to release seedsFleshy fruits• Ripens through hormonal changes• Fruit becomes sweeter and color change

Types of Fruits

Simple fruits

•derived from a single ovary•cherries, soybeans

Types of FruitsAggregate fruits• from a single flower

w/ separate carpels• Strawberries,

raspberry, blackberry

Types of Fruits

Multiple fruits• from separate

tightly clustered flowers

• PineappleAccessory fruit• Other floral parts

contribute to the fruit

Fruit and Seed Dispersal

Mechanisms of Asexual Reproduction

•Production of offspring from a single parent

•Occurs without genetic recombination resulting in a clone

•Extension of indeterminate growth•2 major natural mechanisms

– fragmentation– apomixis

Mechanisms of Asexual Reproduction

Fragmentation• Separation of parent plant into

parts that reform new whole plants• Kelanchoe, aspen treeApomixis• Production of seeds without

meiosis and fertilization• Diploid cell in the ovule gives rise

to the embryo• Dandelions

Advantages and Disadvantages of Asexual vs. Sexual Reproduction

Asexual advantages• No need for a pollinator• Pass on entire genome to it

progeny• Stronger seedlings• Vegetative reproduction

offspring arise from mature vegetative fragments

Sexual advantages• Genetic variation• Seeds allow for great dispersal• Seed dormancy

Mechanisms that Prevent Self Pollination

Dioecious plants can’t self pollinate

Self-incompatibility• rejection of pollen from

the same or closely related plant by the stigma (biochemical block)

Maturity• stamens and carpels

mature at different timesStructural arrangement• anthers are below the

stigma

Vegetative Propagation and Agriculture

Clones from cuttings• Callus forms,

adventitious roots develop from the callus

Grafting• Two different species

that share each others benefits

• Use root stock to grow another species

• Stock plant that provides the roots

• Scion twig that is grafted onto the stock

Vegetative Propagation and Agriculture

Test tube cloning• Transgenic

genetically modified organisms

Protoplast fusion• Protoplasts lack cell

walls

Humans Modify Crops by Breeding and Genetic

Engineering• Plant Breeding

– Artificial selection

• Plant Biotechnology– Innovations in the use of plants to make

products of use to humans• Reducing world hunger and malnutrition

– bt gene– Golden Rice

• Reducing Fossil Fuel Dependency– biofuels

Debate Over Plant Biotechnology

• Issues of human health– Inadvertent transfer of allergens to food

crops• Possible effects on nontarget organisms

– Monarch butterfly caterpillars and pollen from transgenic bt maize

• Addressing the problem of transgene escape– Genes escaping into weeds through crop to

weed hybridization