Vegetative Plant Development Chapter 37. 2 Embryo Development Begins once the egg cell is fertilized -The growing pollen tube enters angiosperm embryo

Vegetative Plant Development

Chapter 37

2

Embryo Development

Begins once the egg cell is fertilized

-The growing pollen tube enters angiosperm embryo sac and releases two sperm cells

-One sperm fertilizes central cell and initiates endosperm development

-Other sperm fertilizes the egg to produce a zygote

-Cell division soon follows, creating the embryo

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Polar nuclei

Egg celljust beforefertilization

Integuments(ovule wall)

Micropyle

Pollen tube

Sperm celljust beforefertilizingcentral cell

Sperm celljust beforefertilizingegg cell

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Embryo Development

The first zygote division is asymmetrical, resulting in two unequal daughter cells

-Small cell divides repeatedly forming a ball of cells, which will form the embryo

-Large cell divides repeatedly forming an elongated structure called a suspensor

-Transports nutrients to embryo

The root-shoot axis also forms at this time

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Polar nuclei

Egg

Micropyle

Sperm

Pollen tube

3n endosperm 2n zygote

Embryo Development

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First celldivision

EndospermSuspensor

Basal cell

Cotyledon

ProcambiumGround

meristem

ProtodermRoot apex (radicle)

Globularproembryo

Hypocotyl

Root apical meristem

Cotyledons

Shoot apicalmeristem

Shoot apical meristem

Endosperm

Cotyledons

Embryo Development

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Embryo Development

Asymmetrical cell division is also observed in the zygote of the brown alga Fucus -Unequal material distribution forms a bulge-Cell division occurs there, resulting in:

-A smaller cell that develops into a rhizoid that anchors the alga

-A larger cell that develops into the thallus, or main algal body

Fate of two cells is held “in memory” by cell wall components

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Embryo Development

Light

Zygote

Bulge

RhizoidRhizoid cell

ThallusThallus cell

Young algaAdult alga

First celldivision

(asymmetrical)

Gravity

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Embryo Development

Arabidopsis mutants have revealed the normal developmental mechanisms

-Suspensor mutants undergo aberrant development in the embryo followed by embryo-like development of the suspensor

-Thus, the embryo normally prevents embryo development in suspensor

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Development of Body Plan

In plants, three-dimensional shape and form arise by regulating cell divisions

-The vertical axis (root-shoot axis) becomes established at a very early stage

-Cells soon begin dividing in different directions producing a solid ball of

cells

-Apical meristems establish the root-shoot axis in the globular stage

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The radial axis (inner-outer axis) is created when cells alternate between synchronous cell divisions

-Produce cells walls parallel to and perpendicular to the embryo’s surface

The 3 basic tissue systems arise at this stage

-Dermal, ground and vascular

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Embryo

Suspensor

Root–shoot axisRadial axis

Cell wall forming parallelto embryo surface


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Cell wallforming

perpendicularto embryo

surface

Multiple paralleland perpendicular

divisions, accompaniedby apical growth divisions

lengthening the root–shoot axis

Vascular tissue system(procambium)

Ground tissue system(ground meristem)

Dermal tissue system(protoderm)


Root apical meristem

Root–shootaxis


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Both shoot and root meristems are apical meristems, but are independently controlled

-Shootmeristemless (STM) is necessary for shoot formation, but not root development

STM wild type

stm mutant

-STM encodes a transcription factor with homeobox region

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The HOBBIT gene is required for root meristem, but not shoot meristem formation

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One way that auxin induces gene expression is by activating the MONOPTEROS (MP) protein

-Auxin releases the repressor from MP

-MP then activates the transcription of a root development gene

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Formation of Tissue Systems

Primary meristems differentiate while the plant embryo is still at the globular stage

-No cell movements are involved

The outer protoderm develops into dermal tissue that protects the plant

The ground meristem develops into ground tissue that stores food and water

The inner procambium develops into vascular tissue that transports water & nutrients

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Morphogenesis

The heart-shaped globular stage gives rise to bulges called cotyledons

-Two in eudicots and one in monocots

These bulges are produced by embryonic cells, and not by the shoot apical meristem

-This process is called morphogenesis

-Results from changes in planes and rates of cell division

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Morphogenesis

The form of a plant body is largely determined by the plane in which its cells divide

-Based on the position of the cell plate

-Determined by microtubule orientation

Microtubules also guide cellulose deposition as the cell wall forms around the new cell

-Cells expand in the directions of the two sides with the least cellulose reinforcement

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Nucleus Microtubules

Cell division

Forming cellplate

Cell division

Cellulosefiber

Water uptake

Expansion

a.

b.


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Morphogenesis

Early in embryonic development, most cells can give rise to a wide range of cell and organ types, including leaves

-As development proceeds, the cells with multiple potentials are restricted to the meristem regions

-Many meristems have been established by the time embryogenesis ends and the seed becomes dormant

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Morphogenesis

During embryogenesis, angiosperms undergo three other critical events:

-Storage of food in the cotyledons or endosperm

-Differentiation of ovule tissue to form a seed coat

-Development of carpel wall into a fruit

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Morphogenesis

Endosperm varies between plants

-In coconuts it is liquid

-In corn it is solid

-In peas and beans it is used up during embryogenesis

-Nutrients are stored in thick, fleshy cotyledons

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Seeds

In many angiosperms, development of the embryo is arrested soon after meristems and cotyledons differentiate

-The integuments develop into a relatively impermeable seed coat

-Encloses the seed with its dormant embryo and stored food

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Seed coat(integuments)

Cotyledons

Root cap

Root apicalmeristem

Procambium

Endosperm


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Seeds

Seeds are an important adaptation because:

1. They maintain dormancy under unfavorable conditions

2. They protect the young plant when it is most vulnerable

3. They provide food for the embryo until it can produce its own food

4. They facilitate dispersal of the embryo

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Seeds

Once a seed coat forms, most of the embryo’s metabolic activities cease

Germination cannot take place until water and oxygen reach the embryo

-Seeds of some plants have been known to remain viable for thousands of years

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Seeds

Specific adaptations ensure that seeds will germinate only under appropriate conditions

-Some seeds lie within tough cones that do not open until exposed to fire

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Seeds

-Some seeds only germinate when sufficient water is available to leach inhibitory chemicals from the seed coat

-Still other seeds germinate only after they pass through the intestines of birds or mammals

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Fruits

Fruits are most simply defined as mature ovaries (carpels)

-During seed formation, the flower ovary begins to develop into fruit

-It is possible, however, for fruits to develop without seed development

-Bananas are propagated asexually

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prior sporophyte generationdegenerating gametophyte generationnext sporophyte generation

Carpel(developing

fruit)

Stigma

Style

Pericarp(ovary wall)

Exocarp

Mesocarp

Endocarp

Developingseed coat

Embryo

Ovary Part ofovarydevelopinginto seed

Endosperm (3n)

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Fruits

The ovary wall is termed the pericarp

-Has three layers: exocarp, mesocarp and endocarp

-Their fate determines the fruit type

Fruits can be:

-Dry or fleshy

-Simple (single carpel), aggregate (multiple carpels), or multiple (multiple flowers)

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Split along two carpel edges (sutures) with seeds attachedto edges; peas, beans. Unlike fleshy fruits, the three tissuelayers of the ovary do not thicken extensively. The entire pericarp is dry at maturity.

The entire pericarpis fleshy, althoughthere may be a thinskin. Berries havemultiple seeds ineither one or moreovaries. Thetomato flower hadfour carpels thatfused. Each carpelcontains multipleovules that developinto seeds.

Legumes

Stigma

StyleSeed

Outer pericarp

Seed

Fusedcarpels


Pericarp

True Berries

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Seed

Single seed enclosedin a hardpit; peaches, plums, cherries.Each layer of the pericarp has a different structureand function, with the endocarp forming the pit.

Not split and with a wing formed from the outer tissues; maples, elms, ashes.

PericarpExocarp (skin)

Drupes

Seed

Pericarp

Endocarp (pit)Mesocarp

Samaras

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Ovary

Sepals of asingle flower

Derived from many ovaries of a single flower; strawberries, blackberries. Unlike tomato, these ovaries are not fused and covered by a continuous pericarp.

Individual flowers form fruits around a single stem. The fruits fuse as seenwith pineapple.

Seed

Aggregate Fruits

Main stem

Pericarp ofindividual flower

Multiple Fruits

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Fruits

Developmentally, fruits are fascinating organs that contain 3 genotypes in one package:

-The fruit and seed coat are from the prior sporophyte generation

-The developing seed contains remnants of the gametophyte generation

-The embryo represents the next sporophyte generation

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Fruit Dispersal

Occurs through a wide array of methods

-Ingestion and transportation by birds or other vertebrates

-Hitching a ride with hooked spines on birds and mammals

-Burial in caches by herbivores

-Blowing in the wind

-Floating and drifting on water

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Germination

Germination is defined as the emergence of the radicle (first root) from the seed coat

Germination begins when a seed absorbs water & oxygen is available for metabolism

-Often requires an additional environmental signal such as specific wavelength of light

-Or appropriate temperature

-Or stratification (period of low temperature exposure

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Germination

Germination can occur over a wide temperature range (5o-30oC)

Some seeds will not germinate even under he best conditions

-The presence of ungerminated seeds in the soil of an area is termed the seed bank

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Germination

Germination requires energy sources such as:

-Starch stored in amyloplasts, proteins, or fats and oils

In cereal grain kernels, the single cotyledon is modified into a massive scutellum

-Its abundant food is used first during germination

-Later it serves as a conduit from the endosperm to the rest of the embryo

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Germination

Embryo produces gibberellic acid

-This hormone signals the aleurone (outer endosperm layer) to produce -amylase

-Breaks down the endosperm’s starch into sugars that are passed to embryo

Abscisic acid, another hormone, can inhibit starch breakdown

-Establishes dormancy

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1. Gibberellic acid (GA) binds to cell membrane receptors on the cells of the aleurone layer. This triggers a signal transduction pathway.

Pericarp

Aleurone

Endosperm

Scutellum(cotyledon)

Embryo

Starch

Sugars

Gibberellicacid

-amylase

Aleurone cell

Signaling pathway

GA receptor

GA

DNA

Myb protein

Transcriptionand translation

-amylase

Transcription and translation


2. The signaling pathway leads to the transcription of a Myb gene in the nucleus and translation of the Myb RNA into Myb protein in the cytoplasm.

3. The Myb protein then enters the nucleus and activates the promoter for the -amylase gene, resulting in the production and release of -amylase.

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Germination

As the sporophyte pushes through the seed coat, it orients with the environment such that the root grows down & shoot grows up

-Usually, the root emerges before the shoot

-The shoot becomes photosynthetic, and the postembryonic phase is under way

Cotyledons may be held above or below the ground

-May become photosynthetic or shrivel

50a. b.

Hypocotyl

Secondary rootsPrimary roots

Seed coat

CotyledonWitheredcotyledons

Hypocotyl

Plumule

Scutellum

Primary root

Adventi-tious root

RadicleColeorhiza

First leaf

Coleoptile

Epicotyl

First leaves


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Vegetative Plant Development Chapter 37. 2 Embryo Development Begins once the egg cell is fertilized -The growing pollen tube enters angiosperm embryo