Transcript 15-20 Pollination in flowering plants

Part 3: Plant form and function
Chapter 15: Reproduction and
development of flowering plants
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The life cycle of a homosporous plant
Copyright © Professor Pauline Ladiges, University of Melbourne
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Fig. 15.1: The life cycle of a heterosporous plant
Copyright © Professor Pauline Ladiges, University of Melbourne
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Alternation of generations
• Many mosses and ferns are homosporous, in that
their sporophytes produce only one type of haploid
spore
• Flowering plants are heterosporous as they
produce separate male and female spores, by
meiosis, each of which undergoes mitosis to
produce male and female gametophytes
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The angiosperm flower
• In angiosperms, the sporophyte is the dominant
generation
• The sporophyte produces flowers, which are the
sites of sexual reproduction
• A flower is a specialised shoot composed of four
whorls of leaves, grouped around the tip of the
flower stalk or receptacle
• These whorls, beginning with the outermost, are
the sepals, petals, stamens and carpel
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Fig. 15.2a: Longitudinal section of a
flower of oilseed rape, Brassica napus
Copyright © E Evans
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Fig. 15.2b: Top view and longitudinal
section of a typical flower
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The flower is the site of sexual
reproduction
• Stamens are the male reproductive organs
• A carpel is the female reproductive organ
• A single carpel consists of a stigma, style and
ovary
• In some species of flowering plants, a number of
carpels are fused together to form a gynoecium
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Fig. 15.3 (top): Development of pollen and
embryo sac
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Fig. 15.3 (middle): Development of pollen and
embryo sac
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Fig. 15.3 (bottom): Development of pollen and
embryo sac
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The anther is the male reproductive
organ
• An anther consists of two pollen sacs, each
containing a large number of multicellular pollen
grains
• Pollen grains are the sperm-producing male
gametophytes
• Pollen forms when a unicellular microspore
undergoes mitosis to produce a small generative
cell and a larger vegetative cell
• When pollen lands on a stigma, it germinates to
produce a pollen tube
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The carpel is the female reproductive
organ
• The stigma may be wet or dry and either smooth
or covered in elongated cells known as papillae,
which trap pollen
• The pollen tube of a germinating grain grows down
through the style into the ovary
• An ovary contains ovules, within each of which is
an embryo sac
• A pollen tube enters the ovule via the micropyle,
and releases sperm into the embryo sac, fertilising
the egg
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Fig. 15.4: Pollen tube of oilseed rape
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Double fertilisation
• Each pollen tube contains a tube nucleus and two
sperm nuclei
• The egg sac of an ovule contains an egg cell,
situated near the micropyle, and two polar nuclei
contained within a large central cell
• At fertilisation, one sperm fuses with the egg cell to
form a diploid zygote
• The other sperm fuses with the polar nuclei to form
triploid endosperm, which will support the growth
of the embryo and in some cases the growth of a
germinating seedling
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15-15
Fig. 15.9: Typical double fertilisation event
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Apomixis
• Some plants have the capacity to reproduce
without fertilisation e.g. apomictic species
• Apomicts produce a diploid megaspore that does
not undergo meiosis, but instead divides by mitosis
to produce an embryo, which then develops in the
same way as sexually-produced embryos
• The absence of meiosis means that apomictic
plants are identical to one another  lack of
genetic variation
• Apomixis is common among successful species
and provides a means of rapid reproduction
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Question 1:
Consider a plant that reproduces without
fertilisation (apomixis). What are the possible
evolutionary advantages to plants in utilising this
method of reproduction?
a) It would be good in areas with little variability
b) It would not be good in areas with little variability
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Pollination in flowering plants
• Pollination, the first step in the chain of events
leading to fertilisation, unites male and female
gametophytes
• Most flowering plants have close interactions with
insects, birds or other animals that convey pollen
directly between flowers
• Plants (e.g. grasses, she-oaks) that lack such
mutualisms may be wind-pollinated, compensating
for the randomness of this form of dispersal by
releasing large quantities of pollen
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Pollination in flowering plants (cont.)
• Plants, which are essentially fixed in place, have
evolved mechanisms that prevent cross-species
pollination
• The stigma recognises pollen belonging to the
same species and either prevents the pollen from
other species from germinating, or blocks pollen
tube growth down the style
• The flowers of most species contain both male and
female reproductive organs—they are bisexual—
and some of these may self-fertilise
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Pollination in flowering plants (cont.)
• Many plants have evolved mechanisms that
encourage fertilisation between separate
individuals of the same species (cross-fertilisation)
• Cross-fertilisation maintains genetic variation in
offspring, which is advantageous in unpredictable
or changing environments
– monoecious species: male and female organs occur on
separate flowers of the same plant
– dioecious species: male and female flowers occur on
separate plants
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Enhancing cross-pollination
• In plants with bisexual flowers, a variety of
mechanisms may prevent self-fertilisation
– some species produce flowers that go through separate
male and female phases
– others have flower structures that inhibit self-pollination
e.g. ‘pin’ and ‘thrum’ flowers on separate primrose
plants
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Question 2:
Consider a brightly coloured flower with a strong
aroma. What type of pollen dispersal method do
you think this plant utilises?
a) birds
b) mammals
c) insects
d) humans
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Self-incompatibility
• Self-incompatibility is the most common means by
which plants prevent self-fertilisation
• This genetically-controlled recognition system
stops eggs from being fertilised by pollen from the
same plant
• If pollen is deposited on the stigma of a flower on
the same plant, a biochemical block prevents the
pollen from forming a pollen tube and fertilising an
egg
• Recognition of ‘self’ pollen is based on genes for
self-incompatibility, called S-genes
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Self-incompatibility (cont.)
•
As a pollen grain is haploid, it will be recognised as
‘self’ if its S allele is the same as either of the two
S alleles of the diploid stigma
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15-25
Fig. 15.13: Genetics of self-incompatibility
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Seed development
• After fertilisation, a zygote undergoes a series of rapid
cell divisions to form an embryo
• In dicotyledons (e.g. beans) the embryo continues to
develop and generates two seed leaves (cotyledons),
between which is situated the shoot apical meristem
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Fig. 15.16a: The zygote divides into a two-celled
proembryo
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Seed development (cont.)
• Mitotic divisions of the triploid endosperm nucleus
ultimately generate liquid endosperm, which, as it
forms cell walls, solidifies and expands
• In this state, endosperm is the major nutritive
tissue of the seed, rich in lipids or carbohydrates
• As a seed matures, it enters dormancy, a state of
extremely low metabolic rate with deferral of
growth and development
• Dormancy increases the likelihood that when the
seed germinates, it will be under conditions (light,
temp. etc.) that are most advantageous to the
seedling
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Fruit development
• As seeds develop from ovules, other changes
occur in the flower, including swelling of the ovary
to form a fruit, which protects the seeds and
assists in their dispersal
• Fruits normally ripen at about the same time as its
seeds are completing their development
• In cereals and grasses, the fruit contains a single
fertilised ovule and develops into a grain
• If a flower is not pollinated, fruit will not normally
develop and the flower will shrivel and drop
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Seed germination
• Germination of seeds relies on imbibition, which is
the uptake of water resulting from the low water
potential of the dry seed
• As the seed expands, it ruptures the seed coat,
providing oxygen to the embryo and triggering
metabolic changes that enable growth to restart
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Fig. 15.18b: Seed structure, germination and
development in a dicot (green bean)
Copyright © Ed Reschke
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Organogenesis
• Plant growth describes the irreversible increase in
mass that results from cell division and expansion
• Development, on the other hand, is the sum of all
the changes that together define the plant body
• Most plants demonstrate indeterminate growth,
growing for as long as they remain alive
• In contrast, most animals and certain plant organs,
such as leaves and flowers, undergo determinate
growth, ceasing to grow after they attain a certain
size
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Organogenesis (cont.)
• Growth involves the production of new cells by
repeated mitotic division, together with
enlargement of existing cells
• These cells will differentiate into a range of cell
types, each of which will assemble into the threedimensional structures characteristic of mature
organs
• Growth and development of new organs begins in
specialised regions of cells found at the tips of
shoots and roots—the apical meristems
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The shoot apex
• The shoot apical meristem produces stems and
leaves, and also flowers when the plant enters its
reproductive phase
• The apex of the shoot is a spherical dome of
meristematic cells that divide to produce leaf
primordia, structures that develop into leaves
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Fig. 15.19: Shoot apical meristem (dicot plant)
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The root apex
• The root apical meristem of flowering plants
contains a zone of rapidly dividing cells that gives
rise to the mature tissues of the root
• Including root hairs, which arise by elongation of an
epidermal cell, and lateral roots, they arise deep
within the tissues of more mature parts of the root
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Fig. 15.20: Barley root tip
Copyright © Professor S Y Zee, University of Hong Kong
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Asexual reproduction in flowering
plants
• Many plants have the ability to clone themselves
by asexual, or vegetative, reproduction
• Some plants, such as strawberries and Spinifex
grass, have stolons, long stems that grow
horizontally along the soil surface, forming roots
and leaves that eventually form independent units
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Fig. 15.21: Spinifex grass (Spinifex hirsutus)
Copyright © Susan Gehrig
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Asexual reproduction in flowering
plants (cont.)
• Some species have the ability to form shoots from
underground storage organs such as corms and
bulbs, or from root tubers
• Rhizome-producing species such as bracken, and
those that have horizontal roots, such as wattle,
also have the capacity to reproduce vegetatively
• These clones are genetically identical to the parent
• Plants have the ability, under suitable conditions,
to generate an entire plant from a single cell—a
property known as totipotency
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Tissue culture
• Plant biotechnologists use a number of in vitro
methods to generate new plant varieties
• Tissue culture is a propagation technique in which
one or a few cells are grown on artificial media,
containing nutrients and hormones, to generate
large numbers of plants
• Via manipulation of the hormonal balance, the
callus (a mass of dividing undifferentiated cells)
that forms can be induced to develop shoots and
roots with fully differentiated cells
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Genetic engineering of plants
• Tissue culture techniques are now used to
produce genetically-modified (transgenic) plant
species
• Desirable plant traits can be introduced into crop
plants to increase disease resistance, improve
nutritional value and increase crop survival in
adverse environments
• The gene that codes for the plant trait is identified
and isolated, and then incorporated into the
nuclear DNA of a host cell
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Genetic engineering of plants (cont.)
• Transformation is the process by which the
genetic makeup of a single cell is altered
• This process uses vectors such as bacterial
plasmids to transfer the gene
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Transferring cloned genes
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Summary
• Life cycles of sexually reproducing organisms
show a pattern of alternation between diploid and
haploid stages
• In flowering plants, the sexual organs are within
the flower, the male organ being the anther and
the female organ the carpel, consisting of stigma,
style and ovary
• Flowering plants are characterised by double
fertilisation
• Some plants use cross-fertilisation and others use
self-fertilisation
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Summary (cont.)
• Seeds are the result of fertilised ovules
• Plant cells are totipotent—individual cells possess
the potential to generate an entire organism
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