Download Regulation of Plant Growth

Chapters 36, 37 & 38
Lecture 11
Regulation of Plant Growth
Dr. Chris Faulkes
Plant Growth
Aims:
•  To introduce the minerals required by
plants
•  To introduce plant hormones
•  To examine the life cycle of a plant
Plant Growth
Aims:
•  To introduce the minerals required by plants
•  To introduce plant hormones
•  To examine the life cycle of a plant
These lecture aims form part of the knowledge
required for learning outcomes 1 and 3:
Describe the structure, diversity and
reproductions of selected plants and animal
groups (LOC1) and Describe mechanisms for
the life processes (LOC3).
Plant Growth
Essential reading
•  page 780-782 (Chapter section 36.1)
•  pages 796-801 (Chapter section 37.1)
•  pages 818-824 (Chapter section 38.1)
• 36.1 How Do Plants Acquire Nutrients?
36.1 How Do Plants Acquire Nutrients?
Nutrients: the major ingredients for
macromolecules: carbon, hydrogen,
oxygen, nitrogen.
Carbon enters the living world from CO2
in the atmosphere through fixation by
photosynthesis. Hydrogen and oxygen
enter as water. These three elements
are in plentiful supply.
Nitrogen is in relatively short supply.
36.1 How Do Plants Acquire Nutrients?
Mineral nutrients required include sulfur
for proteins and phosphorus for nucleic
acids.
Chlorophyll has magnesium, many
compounds require iron.
In the soil, these and other minerals are
dissolved in the soil solution. Plants
take up most minerals from the soil
solution in ionic form.
36.1 How Do Plants Acquire Nutrients?
Autotrophs: make their own organic
molecules from simple inorganic
nutrients.
Plants, some protists, and some bacteria,
are autotrophs.
Heterotrophs required pre-formed
organic molecules and depend directly
or indirectly on autotrophs for food.
36.1 How Do Plants Acquire Nutrients?
Most autotrophs are photosynthesizers;
light is the source of energy to
synthesize organic compounds.
Some are chemolithotrophs, using
energy from reduced inorganic
compounds such as H2S. All are
bacteria.
36.1 How Do Plants Acquire Nutrients?
Plants cannot move around to obtain
nutrients, but they can extend
themselves by growth.
As roots grow through the soil, they mine
it for new sources of minerals and
water.
Growth of leaves helps plant obtain more
sunlight and CO2.
36.2 What Mineral Nutrients Do Plants Require?
Two categories of essential elements:
• Macronutrients: need at least 1 g per
kg of dry plant matter.
• Micronutrients: need less than 100 mg
per kg.
Table 36.1 Mineral Elements Required by Plants
37.1 How Does Plant Development Proceed?
• 37.1 How Does Plant Development
Proceed?
Plant development is regulated in complex
ways. Four factors regulate growth:
•  Environmental cues
•  Receptors such as photoreceptors
•  Hormones
•  The plant s genome
37.1 How Does Plant Development Proceed?
Much recent progress in
understanding plant growth and
development has come from
studies of Arabidopsis thaliana.
It is used as a model organism—
it is small, matures quickly, it s
genome is small and has been
fully sequenced.
Mutants provide insights into
mechanisms of hormones and
receptors.
37.1 How Does Plant Development Proceed?
Hormones: regulatory chemicals that act
at low concentrations at sites often quite
distant from where they were produced.
Each plant hormone is produced in many
cells, and has multiple roles.
Interactions can be complex.
Table 37.1 Plant Growth Hormones
37.1 How Does Plant Development Proceed?
Photoreceptors are involved in many
developmental processes.
They are pigments (molecules that
absorb light) associated with proteins.
Light acts directly on photoreceptors,
which regulate processes of
development.
37.1 How Does Plant Development Proceed?
Plants make use of signal transduction
pathways—sequences of biochemical
reactions by which a cell responds to a
stimulus.
Protein kinase cascades amplify
responses to signals just as they do in
other organisms.
37.1 How Does Plant Development Proceed?
Seeds are dormant—the cells do not
divide, expand, or differentiate.
As the seed begins to germinate, it takes
up water. The growing embryo obtains
chemical building blocks by digesting
the food stored in the seed.
Germination is completed when the
radicle (embryonic root) emerges. Now
called a seedling.
37.1 How Does Plant Development Proceed?
Imbibition, or uptake of water, is the first
step in seed germination.
A seed s water potential is very negative;
water will enter if the seed coat is
permeable. Expanding seeds exert
tremendous force.
Enzymes are activated with hydration,
RNA and proteins are synthesized and
respiration increases. Initial growth is by
expansion of pre-formed cells.
Figure 37.1 Patterns of Early Shoot Development
Early seedling development varies in monocots and eudicots
37.1 How Does Plant Development Proceed?
Formation of flowers may be initiated when plant
reaches a certain size or age.
Some plants flower at certain times of the year; plant
must be able to distinguish seasons.
Light absorption by photoreceptors is the first step in
measuring time. Hormone signals then trigger
flowering.
Hormones also control growth of pollen tube,
fertilization, and fruit and seed development.
37.1 How Does Plant Development Proceed?
Some plants are perennials; they continue to grow
year after year.
Annuals complete life cycle in a single year, then
senesce (deteriorate due to aging) and die.
Senescence is controlled by hormones such as
ethylene.
In some perennials, leaves senesce and fall at the
end of the growing season.
Leaf fall is regulated by interplay of ethylene and
auxin.
37.1 How Does Plant Development Proceed?
Annuals must cope with year to year
variation in rainfall and other factors.
Seeds may remain dormant in
unfavorable years.
Other seeds germinate at specific times
of year to ensure correct conditions.
Dormancy can also ensure that seeds
only germinate in specific environments.
37.1 How Does Plant Development Proceed?
Until the seedling can photosynthesize, it
depends on food reserves in the
cotyledons or endosperm.
Starch, lipids, and proteins must be
broken down by enzymes into
monomers that can enter the embryo s
cells.
38.1 How Do Angiosperms Reproduce Sexually?
• 38.1 How Do Angiosperms Reproduce
Sexually?
In angiosperms, flowers contain the sex organs.
Nearly all reproduce sexually, many reproduce
asexually as well.
Sexual reproduction produces new gene
combinations and diverse phenotypes.
Asexual reproduction produces clones of genetically
identical individuals.
38.1 How Do Angiosperms Reproduce Sexually?
In agriculture, both are important.
Many annual crops are grown from seeds (e.g.,
the grains (grasses), soybean, etc.
Other crops such as strawberries, potatoes,
and bananas, are produced asexually.
Navel oranges are seedless, and must be
propagated asexually. All descend from one
mutant tree.
38.1 How Do Angiosperms Reproduce Sexually?
A complete flower has four groups of
organs (modified leaves): carpels,
stamens, petals, sepals.
Carpels are female sex organs. A pistil is
composed of one or more carpels.
Base of pistil is the ovary, containing
ovules, each of which contain a
megasporangium. Female gametophyte
develops in the megasporangium.
38.1 How Do Angiosperms Reproduce Sexually?
Stamens are the male sex organs.
Each has a filament with a two-lobed
anther, containing four fused
microsporangia. The male gametophyte
develops here.
38.1 How Do Angiosperms Reproduce Sexually?
Petals make up the corolla. They are
often colored to attract pollinators.
Sepals make up the calyx.
All the flower parts are attached to the
stem tip or receptacle.
38.1 How Do Angiosperms Reproduce Sexually?
Angiosperms have alternation of
generations: multicellular diploid
generation alternates with a multicellular
haploid generation.
The diploid sporophyte generation
produces flowers. The flowers produce
haploid spores.
Spores develop into the haploid
gametophyte generation.
Figure 38.1 Development of Gametophytes and Nuclear Fusion
38.1 How Do Angiosperms Reproduce Sexually?
Megagametophytes (female) develop in
megasporangia, called embryo sacs.
Microgametophytes (male) develop in
microsproangia, called pollen grains.
38.1 How Do Angiosperms Reproduce Sexually?
Within an ovule, a megasporocyte
undergoes meiosis to produce four
haploid megaspores—all but one
degenerates.
The surviving megaspore undergoes
three mitotic divisions to produce eight
haploid nuclei. Cell wall formation leads
to a gametophyte with seven cells.
38.1 How Do Angiosperms Reproduce Sexually?
At one end of the gametophyte are three
cells—egg and two synergids.
Synergids attract the pollen tube and
receive the sperm nuclei.
Three antipodal cells at the opposite end
usually degenerate.
The central cell has two polar nuclei,
together they combine with one sperm
nucleus.
38.1 How Do Angiosperms Reproduce Sexually?
In the anther, a microsporocyte
undergoes meiosis to produce four
haploid microspores. Each develops a
spore wall and divides mitotically to
form two haploid cells in each pollen
grain (microgametophyte).
The two cells are the tube cell and
generative cell.
38.1 How Do Angiosperms Reproduce Sexually?
Pollination: transfer of pollen from
anther to stigma.
Many mechanisms have evolved for
pollen transport.
In some plants, such as peas, self
pollination occurs before the flower
opens, resulting in self-fertilization.
When pollen is transferred to a different
individual, it is called cross-pollination.
38.1 How Do Angiosperms Reproduce Sexually?
In many species, wind transports pollen.
The flowers have sticky or featherlike
stigmas, and pollen grains are produced
in great numbers.
In some aquatic plants, pollen is carried
by water.
Many species are pollinated by animals,
including insects, birds, and bats.
38.1 How Do Angiosperms Reproduce Sexually?
Some plants reject pollen from their own
flowers—self-incompatibility.
This promotes outcrossing between
different genotypes.
The S gene is responsible for selfincompatibility. It has many alleles. If S
allele in pollen matches either S allele in
pistil, the pollen grain fails to grow.
Figure 38.3 Self-Incompatibility
38.1 How Do Angiosperms Reproduce Sexually?
The stigmas of most plants are exposed
to pollen of many species.
Strong cell–cell signaling between pollen
and stigma from the same species
binds the pollen to the stigma.
Foreign pollen drops off or fails to
germinate.
38.1 How Do Angiosperms Reproduce Sexually?
Germination of the pollen grain involves the growth of
the pollen tube. The pollen tube grows through the
style to reach the ovule.
Downward growth is guided by a chemical signal
released by the synergids.
38.1 How Do Angiosperms Reproduce Sexually?
The pollen tube grows through the
megasporangium and reaches the
embryo sac.
The generative cell undergoes one
mitotic division to produce two haploid
sperm cells.
Both sperm cells enter a synergid, which
degenerates and releases the sperm
cells.
38.1 How Do Angiosperms Reproduce Sexually?
Double fertilization:
• One sperm cell fuses with the egg cell,
forming a diploid zygote. This divides
mitotically to produce the sporophyte
embryo.
• The other sperm cell fuses with the two
polar nuclei in the central cell, forming a
triploid nucleus. This divides by
mitosis to form the nutritive
endosperm.
Figure 38.5 Double Fertilization
38.1 How Do Angiosperms Reproduce Sexually?
First mitotic division of the zygote results
in asymmetric division of the cytoplasm.
One daughter cell becomes the embryo,
the other becomes a supporting
structure, the suspensor.
Asymmetric division establishes polarity
and the longitudinal axis of the new
plant.
Figure 38.6 Early Development of a Eudicot
38.1 How Do Angiosperms Reproduce Sexually?
In eudicots, the embryo develops into a
heart stage as the cotyledons begin to
form.
Further elongation gives rise to the
torpedo stage.
The shoot apex forms between the
cotyledons; the root apex forms at the
other end. They contain meristematic
cells.
38.1 How Do Angiosperms Reproduce Sexually?
The endosperm accumulates starch,
lipids, and proteins.
In some species the cotyledons absorb
nutrients from the endosperm and
become much larger than the embryo.
In other species, the cotyledons remain
thin, and use nutrients from the
endosperm during germination.
Figure 38.7 Variety in Angiosperm Seeds
38.1 How Do Angiosperms Reproduce Sexually?
In later stages of development, the seed
loses water and becomes dormant.
Integuments: tissues surrounding the
megasporangium—develop into the
seed coat.
The carpel becomes the wall of the fruit
that surrounds the seed.
Plant Growth
Check out
•  36.1 RECAP, page 782
•  36.1 CHAPTER SUMMARY, page 793
•  37.1 RECAP, page 801
•  37.1 CHAPTER SUMMARY, page 815, see web/CD activities
37.1 and 37.2
•  38.1 RECAP, page 825, question 1 only
•  38.1 CHAPTER SUMMARY, page 834, see web/CD activity
38.1
Plant Growth
Self quiz
•  page 794: Chapter 36 question 1
•  page 815: Chapter 37 questions 1-3
•  page 835: Chapter 38 questions 1-5
For Discussion
•  page 816: Chapter 37 questions 1-2
Plant Growth
Key terms:
absicic acid, annual, anther, antipodal cells, autotrophs, auxin,
calyx, carpels, chemolithotrophs, corolla, cotyledon, diploid,
dormancy, embryo, endosperm, ethylene, flower, fruit,
gametophyte, germination, gibberellin, haploid, heterotrophs,
hormones, imbibition, integuments, macronutrients,
megagametophtres, megasporangium (pl. megasporangia),
microenvironment, microgametophytes, micronutrients,
microsporangium (pl. microsporangania), ovule, perennial,
petals, photoreceptors, pistil, pollen, pollination, radicle, seed
coat, seedling, senescence, sepals, sessile, signal transduction
pathways, stamens, stigma, suspensor, synergids