Apomixis
Apomixis is the production of
an embryo that bypasses the
usual process of meiosis and
fertilization.
The genotype of the embryo
and resulting plant will be the
same as the seed parent.
This is clonal seed production !
Apomixis
Apomixis occurs in over
300 species from at
least 35 plant families.
It is most common in the
Asteraceae
Graminaceae
Rosaceae
Rutaceae
Goat’s beard (Tragopogon)
Apomixis
Only a few economically
important food crops exhibit
apomixis.
These include Citrus, Mango
(Mangifera), and Mangosteen
(Garcinia).
Several grass species and
cultivars are apomicts
including Kentucky blue grass.
Orange (Citrus)
Apomixis
Types of apomixis:
Gametophytic
Diplospory
Apospory
Sporphytic
Adventive embryony
Apple (Malus)
Apomixis
Gametophytic apomixis
Embryo derived from a nonreduced gametophyte cell.
Diplospory
Apospory
Dandelion
(Taraxacum)
Apomixis
Normal sexual development from a reduced gamete.
Apomixis
Diplospory
Megaspore mother cell does not finish or initiate meiosis.
Egg sac organizes normally but cells are diploid.
Apomixis
Diplospory
Embryo initiates development without being fertilized.
Endosperm develops either
with or without fertilization.
In some cases (Poa),
fertilization is required for
endosperm formation.
The egg does not get
fertilized because it is
already growing.
Common in grasses and
Asteraceae.
Eastern gamagrass
(Tripsacum dactyloides)
Apomixis
Apospory
The megaspore mother cell undergoes normal meiosis, but
the resulting cells degrade before forming an embryo sac.
Apomixis
Apospory
Additional cell(s) in the nucellus become densely
cytoplasmic and take on the role of apomictic initials.
These cells bypass meiosis to form their own embryo
sac within the same ovule.
Apomixis
Apospory
Endosperm must be
fertilized (pseudogamy).
Aposporous apomicts are
found in the Rosaceae,
Asteraceae and in some
grasses.
Garden rose
Apomixis
Adventive embryony
The megaspore
mother cell
undergoes normal
meiosis and forms
a normal sexual
embryo sac.
These cells are
fertilized by male
sperm cells as in
normal
embryogenesis.
Normal
Megaspore
Mother Cell
Nucellus
Adventive
embryony
Meiosis
Egg Sac
Apomixis
Adventive embryony
However, cells in
the nucellus become
embryogenic and
form multiple
embryos in the
micropylar region
of the ovule.
Adventive
embryony
Normal
Megaspore
Mother Cell
Nucellus
Meiosis
Egg Sac
Apomixis
Adventive embryony
Cells in the nucellus
become embryogenic.
These develop into
somatic embryos.
Apomixis
Adventive embryony
These asexual embryos do not
produce their own embryo sac
but grow into the embryo sac
of the sexual embryo.
Endosperm must be fertilized.
Common in tropical fruit trees
like citrus and mango.
Apomixis
Adventive embryony
Multiple seedlings
(polyembryony) from a
single seed in Citrus.
One of the seedlings will
be sexual and the others
will be clones.
Multiple
embryos
Single seed
Apomixis
Adventive embryony
Single seed
Pachira contains up to
5 seedlings per seed.
The large seedling is
sexual and the smaller
are nucellar derived.
Multiple
embryos
Apomixis
Somatic embryogenesis
Somatic embryogenesis is also the formation of embryos from
vegetative tissue that does not involve gamete fusion.
Seed Development
Plant Hormones and Seed Development
Auxin
Cytokinin
Gibberellin
Ethylene
Abscisic acid
Seed Development
Plant Hormones and Seed Development
Growth and differentiation of the
embryo.
Accumulation of food reserves.
Storage for use during germination.
Growth and development of fruit.
Seed Development
Plant Hormones and Seed Development
Auxin
Abundant in developing seeds.
Relatively high levels throughout
seed development before declining
during maturation drying.
Required for normal embryogenesis.
High during cell division activity
including the endosperm.
Seed Development
Plant Hormones and Seed Development
Auxin
This is critical for orienting
the eight nuclei into their
proper positions.
Chalazal
Low
Auxin gradient
Early in embryo sac
development an auxin
gradient is established from
the micropylar to chalazal
regions of the embryo sac.
High
Micropylar
Seed Development
Plant Hormones and Seed Development
Auxin
The gradient is from the
apical to basal portions of
the embryo and is required
for the appropriate bipolar
(shoot and root) symmetry
of the embryo.
Auxin gradient
An auxin gradient is also
important for normal embryo
formation.
High
Low
Seed Development
Plant Hormones and Seed Development
Auxin
Auxin from seeds
can be the signal for
fruit to develop.
Seed Development
Plant Hormones and Seed Development
Cytokinins
Abundant in developing seeds.
The highest concentration of
cytokinins is found during the cell
division stages of embryo and
endosperm development.
Key role is in cell division.
Seed Development
Plant Hormones and Seed Development
Cytokinins
The major role for
cytokinins during seed
development is the control
of endosperm development.
Cytokinin determines the
number of cells in the
endosperm and the timing
for cellularization.
Endosperm
Seed Development
Plant Hormones and Seed Development
Gibberellins
Abundant in developing seeds.
Various forms high in Stages I and II.
GA mutants form normal embryos but
seed size is reduced.
Seed Development
Plant Hormones and Seed Development
Gibberellins
That gibberellins are important in the development of
cotyledons as storage reserve organs.
Cotyledons
Endosperm
Oak (Quercus)
Seed Development
Parthenocarpy
Parthenocarpy is fruit production
without seed formation. Both auxins
and gibberellins can be important to
induce parthenocarpy.
Seed Development
Parthenocarpy
Vegetative parthenocarpy does not require pollination.
Examples - fig and pear.
Stimulative parthenocarpy requires pollination.
Example – grapes.
Seed Development
Plant Hormones and Seed Development
Abscisic Acid
Abundant in developing seeds.
ABA high in Stage II.
Facilitates storage reserve
accumulation.
Prevents precocious germination.
Seed Development
Plant Hormones and Seed Development
Abscisic Acid
ABA induces
expression of genes
for storage proteins.
Redrawn from Finkelstein and Crouch, 1987
Seed Development
Plant Hormones and Seed Development
Abscisic Acid
Seed Development
Precocious germination (Vivipary)
Seeds by-pass maturation
drying and there is premature
germination while seeds are
still on the mother plant.
Tomato
Seed Development
Precocious germination (Vivipary)
Premature germination is
generally undesirable.
Genetic mutation in
response to ABA.
Rhipsalis
Seed Development
Precocious germination
(Vivipary)
For a few species, vivipary is the
normal type of seedling development.
Mangrove species are important
plants that provide stabilization and
habitat along the edge of waterways.
Mangrove (Rhizophora)
Seed Development
Precocious germination
(Vivipary)
In mangrove, the seed germinates
while still on the plant.
Black Mangrove
(Avicennia gerinans)
Mangrove (Rhizophora mangle)
Seed Development
Precocious germination
(Vivipary)
Seed Development
Plant Hormones and Seed Development
Ethylene
Ethylene is produced during
seed development.
Ethylene mutants produce
normal seeds.
Seed Development
Plant Hormones and Seed Development
Ethylene
Appears to be important in de-greening prior to maturation drying.
Seed Development
Plant Hormones and Seed Development
There is an interesting interaction between ethylene, gibberellin and
ABA in controlling programmed cell death in endosperm in cereals.
Endosperm
Corn
Seed Development
Plant Hormones and Seed Development
There must be a mechanism in place that programs endosperm cells to die,
while adjacent aleurone and embryo cells continue the maturation process.
Aleurone
Endosperm
Wheat