Transcript Development of the Seed in a Eudicot (cont.)

Essentials of Biology
Sylvia S. Mader
Chapter 21
Lecture Outline
Prepared by: Dr. Stephen Ebbs
Southern Illinois University Carbondale
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
21.1 Responses in Flowering
Plants
• Plants change their growth and physiology
in response to environmental stimuli.
– Light
– Gravity
– Seasonal changes
• Plants also have hormones that help
control plant responses to these stimuli.
Plant Hormones
• There are five recognized plant hormones.
– Auxins
– Gibberellins
– Cytokinins
– Abscisic acid
– Ethylene
• There are also man-made analogs of
these chemicals that can also regulate
plant responses to the environment.
Auxins
• Auxins such as indoleacetic acid (IAA) are
the most common plant hormone.
• Auxins are produced in the shoot apical
meristem.
• Auxins are found in young tissues,
including young leaves, flowers, and fruits.
Auxins (cont.)
• Auxins control aspects of plant growth and
development.
– Auxins play a key role in cell elongation.
– Auxins are responsible for apical dominance.
– Application of auxins can induce rooting of
woody plant cuttings.
– Auxin production by seeds enhances the
maturation of fruit.
Auxins (cont.)
Auxins (cont.)
Auxins (cont.)
Gibberellins
• The primary effect of gibberellins (GAs) is
a stimulation of stem elongation.
• GAs are involved in seed germination.
– GAs reverse the dormancy of the embryo.
– GAs also stimulate the production of amylase,
which helps breakdown the stored endosperm
to provide the embryo with sugars for energy.
Gibberellins (cont.)
Cytokinins
• Cytokinins are found in actively dividing
tissues where they promote cell division.
• Cytokinins can also slow the process of
senescence in leaves.
• Cytokinins interact with auxins to
coordinate the development of plant roots
and shoots.
Abscisic Acid
• Abscisic acid (ABA) is produced by all
green plant tissues.
• ABA has two main functions in plants.
– ABA is involved in seed and bud dormancy.
– ABA contributes to the closure of stomata.
Abscisic Acid (cont.)
Abscisic Acid (cont.)
Ethylene
• Ethylene is a gaseous
plant hormone.
– A primary effect of the
gaseous hormone
ethylene is the
abscission of leaves.
– Ethylene is also
involved in the
ripening of fruits like
apples and bananas.
Environmental Stimuli and Plant
Responses
• Plant response are strongly influenced by
environmental stimuli.
– Light intensity and wavelength
– Day length
– Gravity
– Touch
• The speed of the plant’s response to a
stimuli varies from very rapid to slow
changes in growth.
Plant Tropisms
• A tropism involves directional growth of a plant
towards or away from a stimulus.
– Phototropism involves movement in response to light.
– Gravitropism is movement in response to gravity.
– Thigmotropism is movement in response to touch.
• Tropisms typically involve differential patterns of
growth induced by auxins.
Plant Tropisms (cont.)
Photoperiodism
• Flowering in plants is a response to
seasonal changes in photoperiod.
– Short-day/long-night plants flower when the
night length is longer than the critical length.
– Long-day/short-night plants flower when the
night length is shorter than the critical length.
– The flowering of day-neutral plants is not
influenced by photoperiod.
Photoperiodism (cont.)
Photoperiodism (cont.)
Phytochrome and Plant
Flowering
• Plants sense the photoperiod with a
pigment called phytochrome.
• Besides its role in flowering, phytochrome
plays other roles in plants.
– Sensing light conditions during germination
– Influencing leaf expansion and stem
branching
21.2 Sexual Reproduction in
Flowering Plants
• The plant cycle is an alternation of
generation between different multicellular
forms of the plant.
– The diploid, spore-producing sporophyte
– The haploid, gamete-producing gametophyte
Overview of the Plant Life Cycle
• For flowering plants, the sporophyte is the
dominant, flowering-producing generation.
• Flowers produce two kinds of spores.
– Microspores that develop into the male gametophyte
(a pollen grain)
– Megaspores that develop into the female
gametophyte (an embryo sac)
• Flower pollen carries sperm to the flower egg in
the embryo sac.
Overview of the Plant Life Cycle
(cont.)
• After the sperm in the pollen has fertilized
the egg, an embryo develops within the
flower.
• The structure that houses the embryo
becomes the seed.
Overview of the Plant Life Cycle
(cont.)
Flowers
• Flowers serve several important functions.
– Production of spores
– Protection for gametophytes
– Attraction of pollinators
– Pollen dispersal
• Monocots have floral structures in
multiples of three while eudicots have
floral structures in multiples of four or five.
Flowers (cont.)
• Flowers are comprised of four whorls of
modified leaves attached to a receptacle.
– The sepals protect the developing flower.
– The petals are colored to attract pollinators.
– The male portion of the flower, consisting of
the stamen, anther, and filament.
– The female portion of the flower, or carpel,
consisting of the stigma, style, ovary, and
ovules.
Flowers (cont.)
From Spores to Fertilization
• The flower anther produces male
microspores, which divide mitotically to
form pollen.
• The pollen is released from the anther.
• Within the ovule, female megespores
undergo mitosis to produce the egg.
From Spores to Fertilization
(cont.)
• During pollination, a pollen grain is
transported to the stigma.
• The pollen tube germinates and extends a
pollen tube to the ovule.
• The pollen tube delivers two sperm to the
egg to carry out double fertilization.
From Spores to Fertilization
(cont.)
• The sperm are involved in two fusion
events.
– One sperm fuses with an egg to form a diploid
zygote.
– One sperm fuses with two other ovule cells to
form the triploid endosperm.
• The ovule develops into the seed, bearing
the embryo and the stored nutrients.
Development of the Seed in a
Eudicot
• Eudicot seeds have three main parts.
– The seed coat is a protective layer.
– The endosperm provides a food reserve.
– A plant embryo is present.
• As the seed matures, the embryo
undergoes a specific series of
developmental changes before the plant
axis develops.
Development of the Seed in a
Eudicot (cont.)
Development of the Seed in a
Eudicot (cont.)
Fruit Types and Seed Dispersal
• There is great diversity in the types of
fruits produced by plants.
– Fruits can be dry or fleshy.
– Fruits can be simple, as for cereal grains.
– Nuts can have a hard shell that surrounds a
single seed.
– Legumes are fruits with several seeds.
Dispersal of Seeds
• Once produced, seeds must be dispersed
in order to germinate.
– Some seeds have hooks that allow the seed
to cling to the fur of animals.
– Some seeds must pass through the digestive
tract of animals before they can germinate.
– Some seeds are dispersed by wind or water.
– Some seeds are dispersed in a projectile-like
fashion.
Germination of Seeds
• Seed germination is a programmed
developmental process during which the
embryo breaks dormancy and continues
its development.
• Seed germination only occurs when
sufficient moisture, temperature, and
oxygen is present to sustain growth.
Germination of Seeds (cont.)
Germination of Seeds (cont.)
21.3 Asexual Reproduction in
Flowering Plants
• Because plants have nondifferentiated meristem
tissues, they are totipotent, meaning that a
single cell can be used to regenerate an entire
plant in tissue culture.
• Specific combinations of plant hormones are
needed to control this development.
• These totipotent cells or fragments or the plant
can be used to vegetatively (asexually)
propagate plants.
21.3 Asexual Reproduction in
Flowering Plants (cont.)
21.3 Asexual Reproduction in
Flowering Plants (cont.)
21.3 Asexual Reproduction in
Flowering Plants (cont.)
Genetic Engineering of Plants
• Hybridization can occur if two plants from
different species are successfully crossed
to produce a hybrid.
• Genetic engineering can be used to
transfer genetic information from another
organism to plant cells, creating a
transgenic plant.
Agricultural Plants with
Improved Traits
• Genetic engineering has been used
extensively to improve agricultural plants.
– Increased pest resistance
– Resistance to certain herbicides
– Tolerance to toxic elements and salinity
– Increased disease resistance
– Increased nutritional content
– Increase yield and productivity
Commercial Products
• Genetically engineered plants have also
been created to produce a variety of
specific products.
– Human hormones
– Clotting factors
– Antibodies
– Vaccines