Angiosperm (seed formation and development)

Daisy S. Capon
PhD Student (Crop Science)
1st
Semester 2014
ANGIOSPERM:
Seed Formation and Development

Topical Outline
 Development of male and female
gametophyte;
 Plant embryo development; and
 Chemical composition of seed and
factors affecting on it.

Development of Male Gametophyte
(Microsporogenesis and Microgametogenesis)
 Anthesis-is the period of flower development when the
stigma is ready to receive pollen. It is the
time when pollen begins to blow.
 Microsporangium- structure made of four sacs where
pollen is usually produced.
 Within the sporangia, certain cells become the
microspore mother cells and undergo a two-step
reduction division (meiosis), or microsporogenesis, to
yield four microspores, each of which is haploid (1n).
Each of the 4 microspores is normally functional and
undergoes two divisions, known as microgametogenesis,
or mature pollen grain.
 Sporangia- are enclosure or structure in which spores
are formed.

C. SEED FORMATION: Female
 Megasporogenesis- the formation and maturing of megaspores
 Ovule Primordia- is a meristematic tissue of the ovary wall
where seeds of angiosperms originate
 Diploid- possessing two matched sets of chromosomes in the
cell nucleus, one set from each parent.
 There is a characteristic diploid number of chromosomes for
each species.
 Haploid- having a single set of unpaired chromosomes
 The megaspore mother cell is diploid (2N), having the same number
of chromosomes as the parent plant. It undergoes a two-step cell
division (Meiosis I & Meiosis II). This process gives rise to 4
megaspores haploid cells (1N). Normally, only one megaspore is
functional, the other 3 degenerate.
Development of Female Gametophyte
(Megasporogenesis and Megagametogenesis)

Megagametogenesis- is the development of the female
gametophyte, or embryo sac , from the functional megaspore.

 It is a process of successive nuclear division within an
enlarging cell which becomes the embryo sac. Three
successive free nuclear division (mitosis) occur, culminating in
8 haploid (1N) nuclei. Soon these nuclei arrange themselves
within the enlarging embryo sac and formation of cell walls
occur, resulting in 3 antipodal cells at one end, 2 polar nuclei
at the center with the egg apparatus at the other end.
Development of Female Gametophyte
(Megasporogenesis and Megagametogenesis)

Megagametogenesis: A. three normal mitotic nuclear divisions leading to one large cell
enclosing eight nuclei. Later, cell walls enclose the nuclei and the entire structure becomes
the female gametophyte, or embryo sac; B, mature female gametophyte.

Megagametophyte formation of the genera Polygonum and Lilium. Triploid nuclei are shown
as ellipses with three white dots. The first three columns show the meiosis of the
megaspore, followed by 1-2 mitoses.

Ovule with megagametophyte: egg cell (yellow), synergids (orange),
central cell with two polar nuclei (bright green), and antipodals (dark
green)

THE DEVELOPING OVULE
 Ovule development occurs within the ovary. The
developing ovule is commonly attached to the
placenta by the funiculus.

 Hilum- is the scar on the ovule made where the
funiculus detaches at maturity.

 Micropyle- is the point where the integuments meet
at the nucellar apex.

 Chalaza- the region of integumentary origin and
attachment, usually opposite the micropyle.

 Raphe- the area between the chalaza and the hilum.

THE DEVELOPING OVULE
 Nucellus- provides tissue for the origin and
nurture of the female gametophyte, from
archesporial cell to the mature
megagametophyte.
Integuments
 The integuments is absorbed and consumed
by the developing embryo, leaving it naked
inside the pericarp.
Aril- a third integument that may arise either
from the base of the nucellus or may split
off from the outer integument.
Micropyle- is an integumentary pore or
opening in the ovule through which the pollen
tube grows to fertilize the egg cells of the
female gametophyte.
 Epistase- is the development of well-
defined nucellar or integumentary tissue in
the micropylar region of the seed of certain
species.

Stages of Plant Embryo Development
(Monocot)

(Dicot)

Plant Embryo Development
(Plant Embryogenesis)
 Embryogenesis – is defined as the formation and
development of an embryo from zygote
 Steps
1. Asymmetric cell division resulting in a smaller apical
(terminal) cell and a larger basal cell (plant embryo
develops)
2. Suspensor develops from the basal cell (serves as a
nutrient for the developing embryo)
3. Further cell division leads to globular stage. The three
basic tissue systems (dermal, ground and vascular) can be
recognized. The globular shape of the embryo is then lost
as the cotyledons in dicots (embryonic leaves) begin to
form. The formation of two cotyledons in dicots gives the
embryo a heart-shaped appearance.

Plant Embryo Development
(Plant Embryogenesis)
Embryo formation begins with cell division that establishes the apical-basal (top-bottom)
axis. Further divisions elaborate on this basic plan, finally forming the cotyledons (seed
leaves), as well as the apical meristems of root and shoot.

Plant Embryogenesis
Fertilization Two-Celled Embryo Young Embryo
Cotyledon Stage Bending Embryo Mature Embryo
Embryo Development in Capsella, Shepard’s Purse, a Dicot

The Origin of a Seed and Fruit from
Immature Stage
Flower Parts Mature Structure
 Egg + sperm = zygote Embryo
 2 Polar nuclei + Sperm Endosperm
 Integument Testa (Seedcoat)
 Nucellus Perisperm
 Micropyle Micropyle
 Funiculus Hilum
 Ovary Wall Pericarp (fruit)
 Ovule Seed
 Ovary Fruit

Seed Structure
Seed vary in structure in different kinds of plants.
Seed is defined as a mature, integumented,
megasporangium. All flowering plants bears seeds which
encloses an inactive embryo. Under suitable conditions the
embryo becomes active and germinates to give rise to adult
plant.

Parts of a Typical Seed
 1. Seed coat- is made up of two layers: (a) outer-called
testa (developed from ovule integuments after
fertilization) which is usually hard, and (b) inner-called
tegmen which is thin and papery.

Seed Coat
 There is a small opening at one end of the seed coat, called
micropyle through which water enters the seed. The stalk
of the speed with which the seed is attached to fruit wall
is called funiculus. A large scar is located near the middle
of one edge, where the seed breaks from the stalk of
funiculus, this is called hilum. There is a ridge beyond the
hilum opposite the micropyle. It represents the base of
the funiculus which is fused with the integuments and is
called raphe.
rapheraphe

Seed Coat
 Functions of Seed Coat in Embryo Development
a. Pathway for transport and conversion of amino acid and
carbohydrates from the pericarp into the ovule for
development of the embryo.
b. Temporary storage of compounds for later use by seed
coat cells.
c. Involvement in gas exchange.
d. Possible supply of growth compounds to the growing
embryo and maternal organs.
e. Protection of the embryo and endosperm from desiccation
and mechanical injury

Seed Coat
 Functions of Seed Coat in Mature Seed
a. Protective covering of the seed (biotic and mechanical
injury)
b. Regulation of water uptake and gas exchange with the
surrounding ambient environment.
c. Regulation of germination and influencing the intensity of
dormancy expression (hard seed coat)
d. Control of seed dispersal (wings, hairs mucilages etc.)

Embryo (2n)
It is a young plant enclosed in a seed coat and has two
parts
(i) Cotyledons
Their number is either one or two and they are the leaves of
embryo. Sometimes they store food materials and become
fleshy. When they do not store food they remain thin and
papery. The cotyledons are hinged to an axis (tigellum) at a
point called cotyledonary node.

Embryo (2n)
 It is a young plant enclosed in a seed coat and has two
parts
(ii) Tigellum:
The main axis of the embryo is known as tigellum, one end of
which is pointed and protrudes out of cotyledons. This lies
next to micropyle and is called radicle (rudimentary root).
The other end of the tigellum is the plumule (first apical
bud of shoot). The portion of the axis above the point of
attachment of cotyledons is called epicotyl and that below
the cotyledonary node is called hypocotyl.

Endosperm (3n)
Endosperm is develops from union of 2 polar nuclei and 1 sperm
nucleus. It is a food-laden tissue, surrounding the embryo on all
sides or either present on one side of the embryo. Depending on
its presence or absence, seeds are of two types-
(i) Non endospermic or exalbuminous seeds:
In these seeds like gram, pea, groundnut, the endosperm is
completely consumed by the embryo.
(ii) Endospermic or albuminous seeds:
In monocots and castor bean (dicots) embryo does not consume
all endosperm. So it persists in the mature seed. Such seeds are
called endospermic or albuminous seeds. In these seeds, food is
stored in endosperm. In monocot seeds, the
membranouscovering present around radicle is called coleorrhiza
and around plumule is called coleoptile.

Seed Structure of Different Kinds of
Plants
1. Bean Seed:
It is kidney-shaped
brownish non endospermic
dicotyledonous seed. The surface
is smooth. Concave surface is
darker. It has a whitish scar or
hilum, a small pore or micropyle and
a faint ridge or raphe. A bulge of
underlying radicle is observed on
the opposite side of raphe. The
seed is covered by a thick, tough,
brownish seed coat or testa. A thin
papery transparent tegmen lies
below the testa.

Plants
1. Bean Seed:
Seed coats enclose the embryo. There is no other
structure. Embryo axis or tigellum is curved. It is covered by
two massive cotyledons borne over it in the region called
cotyledonary node. One end of embryo axis called plumule lies
embedded in between the two cotyledons. It bears two small
folded leaves.
The other end of embryo axis is radicle. It protrudes out of
the cotyledons. Part of the embryo axis lying between radicle
and cotyle donary node is called hypocotyl while the part
between the cotyledonary node and plumule is known as
epicotyl. Food is stored in the cotyledons.

Plants
2. Castor Seed:
It is oblong, mottled
brown endospermic and
dicotyledonous seed. The
narrow end bears a bilobed
white spongy caruncle. Both
hilum and micropyle occur in
this area. Raphe develops
from this part and proceeds
towards the broad end
where it bifurcates. A thick
hard but brittle testa
covers the seed.

Plants
2. Castor Seed:
A thin perisperm lies below it and around the kernel. A
white oily endosperm lies below the perisperm. It stores food
reserve as oil drops and proteins. Endosperm is source of
castor oil. Embryo lies in the center of seed. It consists of a
short embryo axis bearing two thin papery semitransparent
oval cotyledons, a small indistinct plumule and a knob-shaped
radicle. Palmate venation occurs over the cotyledons.

Plants
3. Maize Grain:
It is a monocotyledonous,
endospermic, single seeded dry
fruit called caryopsis. The grain is
conical and flattened. Shallow husk
occurs over the pointed end. On one
side the broader end bears a papilla
representing remains of the style.
The same side has a depression in
which a ridge indicates the position
of underlying embryo. Hilum and
micropyle are absent since grain is
a fruit and the seed is internal.

Plants
3. Maize Grain:
Color is variable. Surface is nearly smooth. The covering of
the grain is made of fused pericarp and testa. 2/3 of the
grain interior has food storage tissue of endosperm. It is
rich in starch. A protein rich aleurone layer lies on the
outside of endosperm. Embryo lies on one side towards the
upper pointed part. A single large cotyledon lies lateral and
parallel to the embryo axis. It is called scutellum. Scutellum
is attached to the middle part of embryo axis. Its outer
layer in contact with endosperm is called epithelial layer.

Plants
3. Maize Grain:
The layer secretes GA for formation of amylase during
germination. Embryo axis ends in plumule towards broader
side and radicle towards pointed side. Radicle has a root cap.
Plumule bears a few small leaves. Sheaths derived from
scutellum cover the two ends of embryo axis,
undifferentiated coleorhiza over the radicle root cap region
and hollow folial coleoptile over the plumule. Area of embryo
axis is between plumule and cotyledonary node is epicotyl
while the area between cotyledonary node and radicle is
called hypocotyl.

4. Onion Seed:
It is a small blackish endospermic monocotyledonous seed
with wrinkled surface. Seed coat is quite tough. It is
coloured. Endosperm or food storage tissue is also tough. It
is semitransparent. Embryo is curved. It is embedded in the
endosperm.
Embryo axis is small as compared to single cotyledon called
scutellum. Epicotyl is inconspicuous. Plumule is not
distinguishable. Instead shoot apical meristem is present. A
notch occurs in the area of origin of single cotyledon.
Hypocotyl is larger. It bears radicle or root tip.
Plants

Differences in Seed Structure of Different
Kinds of Plant

Epicotyl
Hypocotyl
Cotyledons
Radicle
Seed coat
Seed coat
Endosperm
(a) Common garden bean, a eudicot with thick cotyledons
Cotyledons
Epicotyl
Hypocotyl
Radicle
(b) Castor bean, a eudicot with thin cotyledons
(c) Maize, a monocot
Scutellum
(cotyledon)
Pericarp fused
with seed coat
Endosperm
Epicotyl
Hypocotyl
Coleoptile
Radicle
Coleorhiza

II. THE CHEMISTRY OF SEEDS
 Important consideration why knowledge on chemical
composition of seed is essential.
1. Seeds are basic source of food for both man and animals.
 Poaceae/Grasses Family
ex: wheat. rice, corn – contribute more to human
nutrition (provide carbohydrates to diet) source of
proteins and other essential nutrients
 Fabaceae/Legume Family
ex: soybean, peanut, mungbean other beans –
important source of proteins and oils
 Other Species
ex: sunflower, palm, canola , cotton seeds – important
source of edible oils

2. They are important source of pharmaceuticals(medicine and
drugs) either active ingredients or components of the
formulations
ex: alkaloid
amino acid, amines, protein
glycosides
phenolics, volatile oils and polysaccharides
* Oils extracted from seeds improve the delivery of the active
ingredients (massage oil, ointments and creams) These oils
(coconut, castor bean, peanut, almond) help the respective
penetration through the skin.

3. They contain various antimetabolites which adversely
affect human and animal nutrition.
ex: alkaloids, lectins, proteinase inhibitors, phytin,
raffinose oligosaccharides
* These biologically active compounds present in seeds may
be harmful/toxic to humans and animals depending of the
dose (Jatropha curcas)

4. They contain reserve food supplies and growth substances
that influence seed germination, and seedling vigor, seed
storage and longevity, as well as industrial and agricultural
uses of seeds.

 Major Classes of Chemical Compounds
Stored in Seeds
 1. Carbohydrates (sugar)
 2. Lipids (fats and oils)
 3. Proteins (amino acids)

Carbohydrate Storage in Seeds
 Carbohydrates- are the major storage in
seeds of most cultivated plants.
 Cereals and grasses are specifically rich in
carbohydrates and low in fats and proteins.
THE CHEMISTRY OF SEEDS

Forms of carbohydrates in seeds
 a) Starch
 b) Hemicellulose
 c) Raffinose series oligosaccharides

Starch
 Starch is stored in seeds in two related forms
 1. Amylose- composed of about 300-400 glucose
residues in straight chain.
 2. Amylopectin- with much larger molecules than
amylase up to a thousand times larger, with
multiple branches, rather than straight chain.

MAIN CHEMICAL STORED IN SEEDS
 Hemicellulose
Forms of Hemicellulose reserve food materials
1) Xylans
2) Mannans
3) Galactans
 Raffinose series oligosaccharides- are composed of
sucrose plus a variable number of galactose units.

LIPID STORAGE IN SEEDS
 Lipids- are plant or animal substances that are
insoluble in water but soluble in ether, chloroform,
benzene, or other fat solvents.
 Lipids are a diverse group of organic compounds
such as fats, oils, waxes, sterols, glycolipids,
phospholipids, fat-soluble vitamins (e.g., vitamins A,
D, E and K), etc.
 Biological functions of Lipids:
 1) energy storage
 2) structural components of cell membranes,
 3) participating in signaling pathways.

 Fatty acids are classified as:
 Saturated fatty acids do not have carbon-to-
carbon double bonds (bad fats).
 Unsaturated fatty acids have one or more carbon-
to-carbon double bonds (good fats).

Classification of Lipids
1. Simple- include esters of fatty acids and glycerols or various
other alcohols. Ex. Fats & fatty oils.
2. Compound- are esters of fatty acids containing additional
chemical groups. Ex. Phospholipids
3. Derived- derived from simple lipid and compound lipids by
hydrolysis and are soluble in fat solvents. Ex. Cholesterol

* Majority of seed lipid are simple lipids which include fats,
fatty oils, and waxes.
* All seed waxes are solids.
* High lipid content is usually associated with high protein
content. Ex. Soybeans, Peanut, Cotton seeds.

Percentage of Fats and Oils in Dry Matter of Seeds of Plant Species
Species % fat or oil Species % fat or oil

Coconut 65 Sesame seed 50-55
Sunflower seed 45-50 Corn seed 2.1
Cotton Seed 15-20 Wheat seed 1.8
Cacao bean 40-50 Bean 2.8
Soybeans 15-20 Rice seed 2.5

 Fatty acid- is a constituents of natural fats, and in
the free state they resemble fats in physical properties.
 Only found in germinating and deteriorating seeds as a
result of fat hy6drolysis.
 Glycerol and Other Alcohol
 They combine with fatty acids to form different kinds
of lipids.
 Fat Hydrolysis- the breakdown of fats during seed
germination or seed deterioration occurs by action of
lipase enzymes yielding glycerol and free fatty acids.

Protein Storage in Seeds
Protein- are nitrogen-containing molecules of large size and
exceedingly complex structure which yield amino acids upon
hydrolysis.
 Majority of seed proteins are metabolically inactive and
serves are food reserves for the growing embryo during
germination.
Proteins Found in most Cereals
1. Glubulin- soluble in water at neutral or slightly acid
reaction and coagulated by heat. Ex. Leucolins
for cereals, Legumelin for pulse seeds, Recin
for rice.

 Four categories are used to describe protein structure:
 Primary structure is the sequence of amino acids from
the amino group to the carboxyl end.
 Secondary structure represents the interactions
between amino acids from some regions in the
same polypeptide chain by hydrogen-oxygen bonds,
which results in helical- shaped (α-helices) or
sheet shaped (β-sheet) configurations.
 Tertiary structure describes the shape of the fully
folded polypeptide chain.
 Quaternary structure refers to the arrangement of
two or more polypeptide chains into a multi-subunit or
oligomeric protein.

 Seed proteins have been classified in four groups
according to their solubility:
1) Albumins, soluble in water at neutral or slightly acid pH.
This fraction is primarily enzymes.
2) Globulins, soluble in saline solution, but insoluble in water
(dicots).
3) Glutelins, soluble in acid or alkali solutions(cereals)
4) Prolamins, soluble in 70-90% ethanol (cereal grains)

 Storage Proteins in Cereal Seeds
 Zein (corn) is relatively rich in alanine and
leucine, with low levels of lysine and almost no
tryptophan;
 Gluten- major storage in wheat and abundant in rye
and barley seeds, has elastic properties that make it
valuable for baking products such as bread.
 Gliadin
 Glutenin -
 Storage Proteins in Dicotyledonous Seeds
 Predominantly globulins
 Very little prolamin and glutelin

OTHER CHEMICAL COMPOUNDS
FOUND IN SEEDS
 Alkaloids- physiologically active substance and, in many
cases, poisonous. Alkaloids may also serve as protective
mechanisms of seeds against pests and pathogens
because of their bitter flavor.
Ex. Caffeine
 Tannins- is a group of complex astringent polyphenolic
compounds occurring widely in plants.
 Glycosides- are compounds formed from the reaction
of a sugar (glucose) with one or more nonsugar
compounds, which are called aglycones (benzylaldehyde).

OTHER CHEMICAL COMPOUNDS
FOUND IN SEEDS
 Phytin- is the insoluble mixed potassium (K+), magnesium
(Mg++), and calcium (Ca++) salt of myo-inositol
hexaphosphoric acid (phytic acid).
 Vitamins- are a heterogeneous group of chemical
compounds synthesized by plants. They function principally
as enzyme cofactors.
 Hormones-are organic compounds that, in small
concentrations, have important regulatory effects on plant
and animal metabolism.

FACTORS AFFECTING THE CHEMICAL
COMPOSITION OF SEEDS
1. Genetic Factors
 Position of seed in the mother plant
wheat - 1st
and 2nd
seeds higher N content than 3rd
and 4th
seeds
Abutilon theophrastii (Malvaceae) – 1st
2 fruits high
N content than the last 2 fruits
Oilseed rape – the proportioned of 7 fatty acids
varies depending on the position of the pod in
the inflorescence

2. Environmental Factors
 Season effects
Soybean seeds – higher protein content at late
season
Sunflower seeds – oil quality tends to improve
gradually over the season (late summer)
* Differences in oil composition are differences in
temperature or photoperiod throughout the growing
season

 Temperature
Higher temperature during seed growth – produced
small seeds
Smaller seeds – lower accumulation of storage
reserves (carbohydrates)
Cooler temperature – higher linolenic: oleic acid ratio
(better quality oil), wild sunflower, flax, cacao
Higher temperatures – high protein content in wheat
and soybean seeds is the result of lower
carbohydrate accumulation during development

 Nutrients or Soil Fertility
addition of N fertilizer to soil – higher protein
content in wheat, rice, wheat, cotton seeds
addition of phosphate – higher phosphorus content of
pea, soybean, wheat, common bean seeds
increase presence of trace elements in the soil – it can
be also observed in soybean (manganese, boron,
zinc), wheat (copper), lupin (cobalt), lettuce
(selenium and cadmium)
low sulphur availability – methionine and cysteine is
lower content
increase N availability – higher seed protein content
and less oil content

 Environmental Factors
 Water Stress
plant subjected to water stress during seed filling is
33% higher protein content and 18% oil
concentration

Angiosperm (seed formation and development)

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