Embryogenesis of seed - University of Washingtonstaff.washington.edu/raista/Embryogenesi.pdfbreaks down abscisic acid and inhibitors of embryonic development. Seed Banking refers to

Cell DivisionsCell division is process by which a parent cell divides into two daughter cells. Opinion that microbes and worms are generated spontaneously from dust & would multiply by breaking apart during collisions.First cell division was observed by Lazarro Spallanzani – a catholic priest – & another of the Lord’s men for the sciences. S. spent months trying to divide drops with hairs so that he had only one microbe under the microscope. It divided by fission.

S.was first to recognize fertilization as unification of sperm and egg cell; proved by first artificial insemination in female dogs.

(1) For simple unicellular organisms such as bacteria, amoeba etc. cell division is equivalent to reproduction– a new organism is created. cell division in prokaryotes is known as binary fission.

(2) Cell division in multicellular eukaryotes is part of mitosis and cell cycle resulting in daughter cells that belong to the same, growing organism.Mitotic cell division can create progeny, such as plants grow from cuttings.

(3) A novel type of cell division developed only in eukaryotes: meiosis. A cell is permanently transformed into a gamete and cannot divide again until fertilization. This is the ontogenetic reflection of their prokaryotic origin.

Cell Division in animals vs plantsMitosis was discovered by German botanist Eduard Strasburger in 1875 in onion cells and later by German zoologist Walther Flemming in animal gill cells in 1879. Flemming named it ‘Mitosis’ in 1882.

Cell DivisionsCytokinesis usually occurs in conjunction with mitosis. However, there are many cells where mitosis and cytokinesis occur separately, forming single cells with multiple nuclei & sometimes multiple copies of chromosomes & DNA (polyploid). This occurs mostly among fungi, but is found also in animals. Errors in mitosis can kill a cell through apoptosis (1) or cause mutations (2).

Mitosis – chromosomes are divided

G = gap phases 1 & 2

S = synthesis phase doubles chromosomes back to diploid status

Cell GrowthFor a multicellular organism to grow, it takes more than cell division, which does not increase cell volume It needs cell ell expansion.

Cell expansion in animals is slow since based on the synthesis of more cytoplasm (s –phase). Cell expansion in plants is quick since it involves only water uptake into the vacuole (novel phase). This not only enlarges the organism quickly but is also the basis for plant movements (tropisms). Animal movement is based on musles = contractileactin/myosin protein chains.

expansion

vacu

ole

DevelopmentPlant development is an umbrella term for a broad spectrum of processes that include: the formation of a complete embryo from a zygote ; seed germination; the elaboration of a mature vegetative plant from the embryo; the formation of flowers, fruits, and seeds; and many of the plant's responses to its environment.

Development

Most higher plants start their adult life as germinating seedlings

Most higher plants start their life as an embryo contained in a seed or a vegetative prop unit.

Most embryos start with a fused cell called zygote- a fertilized ovule.

E. Haeckel’s biogenetic rule” “ Ontogenesis repeats phylogenesis” plant seedlings look very much alike in the monocots or dicots!

Cell division and expansion are particularly simple & fast in plants

Plants reach sexual maturity by flowering & subsequent seed production

How higher plants reproduce!

How higher plants reproduce, Pollen & Ovary

How Pollen germinates

Pollen germinates only in a highly osmotic medium (in water it bursts 7 is destroyed). It also germinates only on the stigma of its own species, in non-selfing species only on the stigma of a different plant individual.

Pollen grain has 3 nuclei

1. A bigger one that drives the tube growth

2. 2 smaller sexual nuclei that fertilize either the ovule or the polar cell to give rise to embryo and endosperm respect.

Pollen – flying male gametes that replace outdated spermsPollen germinates on stigma of only his own species, otherwise there would be chaos in the plant world.

Artificial fertilization in plants

Date palms and vanilla plants have been-hand pollinated ever since we know about them. Also Mendel strictly hand pollinated his peas in order to get pure-bred lines and then clean crosses.

Artificial pollination (i.e. by humans does not strike us as strange as artificial insemination in cows etc. since plant are pollinated by other creatures anyway.

Embryo development: double fertilization

Fertilization in higher plants is different from lower plants and animals. The pollen has 3 nuclei rather than one: one to drive pollen tube expansion, one to fertilize the egg cell and form the embryo and one to fertilize the polar nucleus to form the endosperm

Embryo development in dicot seeds

For the ovule in the ovary to become a seed in a fully developed fruit

there are seven simple steps it has to undergo

Embryo development in dicot seeds 1. Fertilization of egg cell

zygote

2. Unequal cell division

3. Terminal cell embryo Basal cell suspensor = umbilical cord

4. Embryo divides globular stage

5. Embryo heart stage (cotyledons develop)

6. Torpedo stage stretching of embryo axis stem & root meristem appear

7. Seed development takes a break dormancy induced

GerminationGermination is the process in which a seed or spore emerges from a period of dormancy. This includes the sprouting of seedling from a seed (angiosperm, gymnosperm) & growth of sporelings (felt-like protonema, heart-shaped prothalli ) from a spore in bryophytes & ferns..Seed germination requires:(1) Water. Dormant seeds have water content of approx. 10 %. Hydration increases internal pressure of a seed to burst the coat & activate preformed enzymes. (2) Oxygen is required for aerobic respiration, the sole source of energy. If a seed is buried in waterlogged soil or has impermeable seed coats the seed it will be oxygen starved. (3) Temperature. Seeds have a temperature range & will not germinate above or below this range. Most seeds germinate at temperatures slightly above room-temperature 60-75 F (16-24 C).. Some seeds require exposure to cold temperatures (vernalization) to break dormancy.(4 C ). (4) Light or darkness: dill, lettuce seeds need light – shallow planting

Embryo development in dicot seeds

Note that early dicot seed still has endosperm, which gets digested to feed the swelling cotyledons as the new storage organ for food.

Embryo development in monocot seeds

1. Fertilization & unequal division 2. Embryo undergoes cell division

1. Fertilization & unequal division 1. Fertilization & unequal division

1. Fertilization & unequal division

Embryo development in monocot & dicot seeds

Polarity – a basic feature of plant cells in stem & rootsPolarity of plants is well established during embryogenesis where it is set with the first division of the zygote and apparent in the torpedo stage. Polarity of plants is reflected in the polarity of its stem and root cells. At the level of the individual cell polarity is organized by asymmetric distribution of organelles& and Golginetwork of transport proteins, especially for the polar transport of auxin. Polar cells exist also in animals, but only at tissue level.Hermann von Vochting 1878: established term “Polarity “ for plants. Went 1974environmental factors regulate polar auxin flows to induce physiol. responses like reorientation, leaf abscission, new apical leaders 1974 Rubery & Sheldrakechemisosmotic model: PM with auxin uptake carriers and an efflux carrier confined to the lower side of cells 1999 PIN proteins are PM proteins acting as auxin efflux carriers. Their localization in the cell determines direction of auxin flow in the tissue!!

REF: E. Ferraru & Jiri Friml: PIN polar targeting, Plant Physiol 147, 1553-1559 (2008)

Embryo Size

Animal embryos at maturity coconut embryo at maturity

Bean Phaseolus vulgaris

Embryos of plants look different from those of animals. But like the later any similarity to the adult parents is only a matter of flattery.

Embryo Size - Dicots

Pea Pisum sativum Red Pepper Capsicum Onion Alium

Gingko biloba Phaseolus vulgaris Avocado Persea amer.

Dormancy – a state between life & deathDormancy is a period in an organism's life cycle when growth, development, and physical activity is temporarily suspended to conserve energy and survive adverse environmental conditions

Bacterial dormancy: bacteria produce metabolically inactive forms that survive intensely adverse conditions unharmed; cysts or endospores. To inactivate these resistant forms you need autoclave

Torpor = short-term reduction of body temperature to ambient level during periods of inactivity & occurs e.g. in hummingbirds & bats

Aestivation, also estivation, is dormancy in response to very hot or dry or winter conditions & occurs in invertebrates such as garden snail

Diapause is a strategy that is common in insects suspending developm. til next spring . Mammals like red deer delay attachm. of embryos to uterus, ensures offspring born in spring

During hibernation animals have decreased heart rate (95%) & body temperature: bats, hamsters, other rodents, European hedgehog, bears.

Plant dormancy – Seeds & BudsPlant embryos are always put into dormancy at the end of the embryogenesis to prevents germination on the mother plant viviparyDef.: Viable seeds that do not germinate are said to be dormant.When a mature, viable (embryo cells alive) seed fails to germinate under favorable conditions (water, temps), it is dormant. 2 types of dormancy1. seed coat dormancy or external dormancy is caused by the presence of a hard seed covering or seed coat that prevents water and oxygen from reaching and activating the embryo. 2. embryo dormancy or internal dormancy, is caused by inhibition of the embryo which prevents it to germinate under increased hydration

In the dehydrated (< 10 % water) state of dormancy, the embryo can survive extremely hot or cold temperatures (300 C for a short time), radioactive radiation is not causing genetic mutations.

Therefore annuals survive adverse environments like winter or drought as seeds

How to overcome seed dormancy!Wild plants have exceeding long periods of dormancy, domesticated plants do not. Dormant seeds were abolished during the selection process of harvesting and sowing the next year. There are several ways to overcome the dormancy of seeds.

Scarification is artificial damaging of seed coat to allow access of water. Use of file (mechanic sc.) or 10 min soak in con. Sulfuric acid (chem. S)

Stratification: cold treatment simulating the winter frost. Seeds are mixed together with moist sand or peat moss and stored for a few weeks in the refrigerator (4 C or 40 F). This treatment starts a process that breaks down abscisic acid and inhibitors of embryonic development.Seed Banking refers to the fact that some plant species (weeds, Scots broom) have many generations of seeds in the soil. A variable length of the dormancy create seed banks, which allow them to germinate even if the previous years did not set seeds or even if the plant was destroyed years before.

Plant dormancy –Trees & BudsPlant embryos are always put into dormancy at the end of the embryogenesis to prevents germination on the mother plant viviparyTree dormancy: Trees that have dormancy needs too. leaf abscission and dormant buds. Deciduous plants will lose their leaves; evergreens will curtail all new growth.

if a Japanese Maple (Acer palmatum) is given an "eternal summer" through exposure to additional daylight, it will grow continuously for as long as two years. Eventually, however, a temperate climate plant will automatically go dormant, no matter what the environmental conditions..

Going through an "eternal summer" and growth is stressful to the plant and usually fatal. The fatality rate increases to 100% if the plant does not receive the necessary period of cold temperatures required to break the dormancy. Most plants will require a certain number of hours of "chilling" at temperatures between about 0 °C and 10 °C to be able to break bud dormancy

How plants survive the winter!Annuals survive the winter as dormant seeds, biennials as buried storage roots, bulbs or shoots (rhizomes), perennial plants survive with their shoots in the open; have new buds ready in the fall.Seeds & buds form next generation of full plants or shoots only. Bud is embryonic shoot with short internodes & protective bud scales as leaves. Bud dormancy develops before the old leaves senesce and fall. Shortening of day length induces dormancy in buds.

How to overcome bud dormancy? .

1. Dormancy is broken in s leafless trees by rising day length: birch, , larch yellow poplar, red oak scales are light-sensitive, keep them indoors in constant light or try method 3 below :Molisch

2. Bud dormancy can be overcome by low tempsapples need 1000 hours at 7C, lilac branch inside warm greenhouse will stay dormant, branches outside house will overcome dormancy

3. Immersing branches in warm water baths at 40-55 C for a minute