Angiosperm (seed formation and development)

Daisy S. CaponPhD 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.

http://www.yourarticlelibrary.com/wp-content/uploads/2013/10/image32.png

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)

Stages of Plant Embryo Development (Dicot)

Stages of Plant Embryo Development

Plant Embryo Development (Plant Embryogenesis)

Embryogenesis – is defined as the formation and development of an embryo from zygote

Steps1. 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 EmbryoEmbryo 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 StructureSeed 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 Developmenta. 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 Seeda. 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) CotyledonsTheir 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.

Seed Structure of Different Kinds of Plants1. 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.


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.

Seed Structure of Different Kinds of Plants2. 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.

Seed Structure of Different Kinds of Plants3. 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.

Seed Structure of Different Kinds of Plants3. 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.

Seed Structure of Different Kinds of Plants3. 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.


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 fusedwith seed coat

Endosperm

Epicotyl

Hypocotyl

Coleoptile

RadicleColeorhiza

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 Familyex: soybean, peanut, mungbean other beans –

important source of proteins and oils Other Species

ex: sunflower, palm, canola , cotton seeds – important source of edible oils


composition of seed is essential. 2. They are important source of pharmaceuticals(medicine and

drugs) either active ingredients or components of the formulationsex: 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.


composition of seed is essential. 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)


composition of seed is essential. 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

THE CHEMISTRY OF SEEDSForms 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.

THE CHEMISTRY OF SEEDS Structure of Glucose and how it branches to form starch

O | | C —H | H—C —OH | H—C —OH | H—C —OH | H—C —OH | H

Glucose molecule

glucose glucose

Glucose-glucose-glucose - gluc glucose- glucose

Amylopectin

Glucose—glucose—glucose--glu Amylose

MAIN CHEMICAL STORED IN SEEDS Hemicellulose

Forms of Hemicellulose reserve food materials1) Xylans2) Mannans3) Galactans

Raffinose series oligosaccharides- are composed of sucrose plus a variable number of galactose units.

MAIN CHEMICAL STORED IN SEEDS

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.

MAIN CHEMICAL STORED IN SEEDSLIPID STORAGE IN SEEDS

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).

MAIN CHEMICAL STORED IN SEEDSLIPID STORAGE IN SEEDS Classification of Lipids1. 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. Phospholipids3. 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

MAIN CHEMICAL STORED IN SEEDS 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 SeedsProtein- 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 Cereals1. 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.

MAIN CHEMICAL STORED IN SEEDS 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


2. Environmental Factors 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


2. Environmental Factors 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

Thank you for listening…