Download Alternation of generations: a review

Plant Reproduction
Level 1 Biological Diversity
Jim Provan
Campbell: Chapter 38
Alternation of generations: a review
Angiosperm life cycle includes alternation of generations:
haploid gametophyte generations alternate with diploid
sporophyte generation:
Sporophyte is recognisable “plant” - produces haploid spores by
meiosis in sporangia
Spores undergo mitotic division and develop into multicellular
male or female gametophyte
Gametophytes produce gametes (sperm and eggs) by mitosis:
gametes fuse to form zygote which develops into multicellular
sporophyte
Sporophyte is dominant in angiosperm life cycle: gametophyte
stage is reduced and is totally dependent on sporophyte
Alternation of generations: a review
Variations on the basic flower structure
Complete: has sepals, petals,
stamens and carpels
Incomplete: missing one or
more organs (e.g. grasses)
Perfect: has both stamens
and carpels
Imperfect: either staminate
or carpellate - unisex
Monoecious: has both
staminate and carpellate
flowers on same plant
Dioecious: has staminate
and carpellate flowers on
separate individual plants
Floral diversity
Development of male gametophyte
(pollen)
Within sporangial chamber of anther,
diploid microsporocytes undergo meiosis
to form four haploid microspores
Haploid microspore nucleus undergoes
mitotic division to give rise to a generative
cell and a tube cell
Wall of microspore thickens
Development of female gametophyte
(embryo sac)
Megasporocyte in sporangium of each ovule
grows and goes through meiosis to form four
haploid megaspores (only one usually survives)
Remaining megaspore grows and its nucleus
undergoes three mitotic divisions, forming
one large cell with eight haploid nucleii
Membranes partition this into a multicellular
embryo sac
Pollination brings male and female
gametophytes together
Pollination: the placement of pollen onto the stigma
of a carpel:
Some plants use wind to disperse pollen
Others interact with animals that transfer pollen directly
Some plants self pollinate, but most cross-pollinate
Most monoecious angiosperms have mechanisms to
prevent selfing - maximises genetic variation:
Stamens and carpels may mature at different times
Structural arrangement of flower reduces chance that
pollinator will transfer pollen between anthers and stigma of
same plant
Some plants are self-incompatible
Genetic basis of self-incompatibility
Based on S genes
Many alleles in plant
population gene pool
Pollen landing on stigma
with same allele at S-locus
is self-incompatible:
Pollen grain will not initiate
or complete formation of
pollen tube
Prevents fertilisation
between plants with similar
S-alleles
Multiple mechanisms at S-loci
Mechanism underlying
inhibition of pollen tube
varies:
Block occurs in pollen grain
(gametophyte selfincompatibility): RNAses
from carpel enter pollen and
destroy RNA
Block occurs in stigma
(sporophyte selfincompatibility) e.g. signal
transduction systems in
mustards
Double fertilisation gives rise to the
zygote and the endosperm
Double fertilisation: union of
two sperm cells with two cells
of the embryo sac
Pollen grain germinates and
extends pollen tube
Generative cell undergoes
mitosis, forming two sperm
Pollen tube enters through
micropyle and discharges sperm
One sperm unites with egg
Other sperm unites with polar
nuclei forming endosperm (3n)
Endosperm development
Begins before embryo development
Triploid nucleus divides to form multinucleate
“supercell”
This undergoes cytokinesis, forming cell membranes
and walls and thus becoming multicellular:
Endosperm is rich in nutrients, which it provides to the
developing embryo
In most monocots, endosperm stocks nutrients that can be
used by the seedling after germination
In many dicots, food reserves of the endosperm are
exported to the cotyledons
Embryo development
First mitotic division transverse:
Large basal cell forms suspensor
Terminal cell divides several times
to form spherical proembryo
Cotyledons appear at either side
of apical meristem
Suspensor attaches at apex of
embryonic root and meristem
After germination, apical and
root meristems sustain growth
Structure of the mature seed
In dicot seeds:
Hypocotyl terminates in the
radicle (embryonic root)
Epicotyl terminates in the
plumule (shoot tip)
Monocot seeds have a special
cotyledon called a scutellum:
Large surface area - absorbs
nutrients from endosperm during
germination
Embryo enclosed in sheath:
- Coleoptile protects the shoot
- Coleorhiza protects the root
The ovary develops into a fruit
adapted for seed dispersal
A true fruit is a ripened
ovary
Fruits can be classified by
their origin:
Simple fruits: derived from
a single ovary e.g. cherry
Aggregate fruits: derived
from a single flower with
several carpels e.g.
blackberry
Multiple fruits: develop
from an inflorescence
Seed dormancy
Prevents germination when conditions for seedling
growth are unfavourable
Conditions for breaking dormancy vary depending on
type of environment plant occupies:
Seeds of desert plants will not germinate until there has
been a heavy rainfall and not after a light shower
In chaparral regions where bushfires are common, seeds
may not germinate until exposed to heat of fire which clears
away older, competing vegetation
Other seeds require cold, sunlight or passage through an
animal’s digestive system
Viability ranges from a few days to decades
Seed germination
Imbibition causes seed to
swell, rupturing seed coat
Metabolic changes restart
growth of the embryo
Storage materials are
digested by enzymes and
nutrients transferred to
growing parts of embryo
Radicle (embryonic root)
emerges from seed
Seed germination (continued)
Next stage involves shoot tip
breaking through soil surface:
In many dicots, a hook forms in
the hypocotyl
Light stimulates the hypocotyl to
straighten, raising the cotyledons
Other plant species follow
different germination methods:
In peas, hook forms in epicotyl
which straightens and leaves
cotyledons in ground
In monocots, shoot grows straight
up through coleoptile tube
Many plants can clone themselves by
asexual reproduction
Asexual reproduction: production of offspring from a
single parent without recombination  clones
Two natural mechanisms of vegetative reproduction:
Fragmentation: separation of parent plant into parts that
reform whole plants:
- Most common form of vegetative reproduction
- Some species of dicots develop adventitious shoots that become
separate shoot systems
Apomixis: production of seeds without meiosis and fertilisation:
- Diploid cell in ovule gives rise to an embryo
- Ovules mature into seeds which are dispersed (e.g. dandelions)
Sexual and asexual reproduction are
complementary in many plants
Both have had featured roles in adaptation of plant
populations to their environments
Benefits of sexual reproduction:
Generates genetic variation
Produces seeds, which can disperse to new locations
Benefits of asexual reproduction:
In a stable environment, plants can clone many copies of
themselves in a short period
Progeny are mature fragments of the parental plant, as
opposed to small, fragile seedlings produced by sexual
reproduction