Basic attributes common to many forms of life

Cellular structure and intracellular compartmentalization; and

Metabolism and transfer of energyStorage and transmission of genetic informationReproduction

Development- each individual will go through certain changes of form and function during its life

A After a corn grain (seed) germinates, its radicle and coleoptile emerge. The radicle develops into the primary root. The coleoptile grows upward and opens a channel through the soil to the surface,

B The plumule develops into the seedling’s primary shoot, which pushes through the coleoptile and begins photosynthesis. In corn plants, adventitious roots that develop from the stem afford additional support for the rapidly growing plant.

Fig. 31-3, p. 525

Stepped Art

hypocotyl

radicle

branch root

branch root

primary root

primary root

adventitious (prop) root

primary leaf

coleoptile

coleoptile

coleoptile

Early Growth of a Bean (Eudicot)

Fig. 31-4a, p. 525

seed coat radicle

cotyledons (two)

hypocotyl

primary root

A After a bean seed germinates, its radicle emerges and bends in the shape of a hook. Sunlight causes the hypocotyl to straighten, which pulls the cotyledons up through the soil.

Fig. 31-4b, p. 525

primary leaf

primary leaf

withered cotyledon

branch rootprimary root

root nodule

B Photosynthetic cells in the cotyledons make food for several days, then the seedling’s leaves take over the task. The cotyledons wither and fall off.

Fig. 31-22, p. 535

germinationmature

sporophyte (2n)

zygote in seed (2n)

fertilizationmeiosis in anther

meiosis in ovary

DIPLOID

HAPLOID

microspores (n)

megaspores (n)

eggs (n) sperm (n)

male gametophyte (n)

female gametophyte (n)

Plant Development

• Plant development includes seed germination and all events of the life cycle, such as root and shoot development, flowering, fruit formation, and dormancy

• These activities have a genetic basis, but are also influenced by environmental factors

Pollination and fertilization

Development- process – complex, multicellular organism arises from a single cell, a gradual process, so the complexity of the embryo increases progressively.

Development –progressive-i.e. a simple embryo with few cell types organized in a crude pattern gradually refined to generate a complex organism with many cell types showing highly detailed organization.

Development- process by which an organism changes to acquire new structures and abilities. It occurs in response to various levels of control:

1. Genetic instructions2. Intercellular interaction3. Environmental factors

gametes

zygote

Differentiation Pattern formation

MorphogenesisCell division

Growth

Diversificationof cell types

OrganizationGenerationof shapes & structures

Increase incell number

Increase

in size

Adult

Gametes

Major overlapping processes

Growth : increase of size and weight,.Diameter, height, volume and weight can be measured. Biochemical properties can also be established:

Enzyme activity, pigment content, protein content, DNA and RNA content may be used to characterize the organism.

Single cells show 2 major components of growth: division and enlargement

After imbibition of water by seeds, the growth by whichthe embryo becomes a young seedling occurs by bothexpansion of cells originally present in the dormant embryoand mitotic divisions resulting in an increase in cell number

gametes

zygote

Differentiation Pattern formation

MorphogenesisCell division

Growth

Diversificationof cell types

OrganizationGenerationof shapes & structures

Increase incell number

Increase

in size

Adult

Gametes

Major overlapping processes

Cell division

The plane in which a cell divides is determined during late interphase.

First sign of this spatial orientation is rearrangement of the cytoskeleton In the cytoplasm into a ring called pre-prophase band. Band disappears before metaphase but it predicts the future plane of division. It predicts where the cell plate will be inserted (the division site).

The “imprint” consist of actinmicrofilaments that remainafter microtubules disperse.

Fig. 35-25

Plane ofcell division

(a) Planes of cell division

Developingguard cells

Guard cell“mother cell”

Unspecializedepidermal cell

(b) Asymmetrical cell division

If planes of division of the descendants are parallel to the plane of the ist cell division, single file of cells results.

Cell division in 3 planes gives rise to a cube. If planes vary randomly, will be a disorganized clump.

Guard cells form perpendicular to the first division

PLANES OF DIVISION VARY AS DEVELOPMENT OF CALLUS

Cell expansion contributes to plant form.Orientation of cell growth is in the plane perpendicular to the orientation of the cellulose microfibrils in the wall. Enzymes weaken cross-links in the wall, and allow it to expand as water diffuses into vacuole by osmosis.

The orientation of cellulose microfibrils (CMFs) is a determining factor in cell growth. Elongation is favored when CMFs are oriented transversely to the direction of growth while elongationis limited when CMFs are oriented in the oblique or longitudinal direction.

Orientation of cellulose microfibrils in growth- cell expansion. CA 80 CELLULOSE IN A MICROFIBRIL

Auxins in cell expansionENZYME THAT BREAKS CROSS LINKSOR HYDROGEN BONDS BETWEENCELLULOSE MICROFIBRILS

gametes

zygote

Differentiation Pattern formation

MorphogenesisCell division

Growth

Diversificationof cell types

OrganizationGenerationof shapes & structures

Increase incell number

Increase

in size

Adult

Gametes

Major overlapping processes

Cell DifferentiationCells become specialized in structure and function. A fertilized egg gives rise to many different kinds of cells, each with a different structure and function.

A program of differential gene expression (the expression of different sets of genes by cells with the same genome) leads to the different cell types in a multicellular organism

Gene expression Cells must continually turn genes on and off in response to signals from external and internal environment.

Regulation of gene expression is necessary for cell specialization in multicellular organisms.

The differences between cell types are not due to different genes being present but to differential gene expression, the expression of different sets of genes by cells with the same genome

DNA

Primary RNA transcript

proteininactive mRNA

Inactive protein

mRNA degradation control

Translational control by ribosome selection among mRNAs

Protein activity control

Transcriptional control

1

2 Processing control

3 Transport control

mRNA

mRNA

6

4 5

Steps at which gene expression can be controlled in eukaryotes

NUCLEUS

CYTOPLASM

Each stage is a potential control point at which gene expression can be turned on or off, accelerated or slowed down.

In all organisms, a common control point for gene expression is at transcription

. In this stage regulation is often in response to signals coming from outside cell e.g. hormones or other signaling molecules.

gametes

zygote

Differentiation Pattern formation

MorphogenesisCell division

Growth

Diversificationof cell types

OrganizationGenerationof shapes & structures

Increase incell number

Increase

in size

Adult

Gametes

Major overlapping processes

Pattern structure

Form –one of outstanding characteristics of living organisms.

Though complex,various parts bear predictable, repeated relations to one another.

Regularity or deviation from random distribution of various parts of cells or tissues

O X O

O X O

O X O

The distribution of the specialized cell is not random since the location of any given cell is at least predictable from the location of other cells

B

X O

X O

X O

X O

X X

X X O O

X x O O

O O

C

D

Non-random

Non-random

Non-random

O X O

X

X O

O X

O X O

O X O

O X O

A

B

X O

X O

X O

X O

X X

X X O O

X x O O

O O

C

D

random

Non-random

Non-random

Non-random

Pattern formation- Development of a spatial organization in which the tissues and

organs are all in their characteristic places.

It is the development of specific structures in specific locations. Cells must be organized into multicellular arrangements of tissue and organs.

Pattern formation is determined by positional information in the form of signals that continuously indicate to each cell its location within a developing structure

Each cell within a developing organ responds to positional information from neighbouring cells by differentiating into a particular cell type, oriented in a particular way.- gradients of specific molecules- hormones, proteins -mRNA provide positional information

Pattern formation

First patterning event in the embryo- axis specification. This reflects asymmetric division of the zygote:

apical cellbasal cell

Establishment of the principal body axis--ANTEROPOSTERIOR--DORSOVENTRAL

embryo

Suspensor filament

Arrangement of leaves

Leaf primordia flanking the apical meristem

Development of zygote into an embryo

Organ expansion and maturationGlobular-heart transition

Embryogenesis

X-section of a young root Epidermal tissue

Morphogenesis- creation of form

Physical process that give an organism its shape in each cell type

The different kinds of cells not randomly distributed but organized into tissues and organs in a particular three- dimensional arrangement.

Reflects different aspects of cell structure and behavior including: cell division, cell shape and size, interaction between cells , and cell death.

In animals, morphogenesis – many involve movement of cells relative to other cells

In plants, cells have cell walls and middle lamella which tightly cement cells together. No relative cell movement or migration. Morphogenesis reflects a restricted set of processes-such as:

1.differential rates and planes of cell division

2. changes in cell size due to the increasing volume of the vacuole.

Planes of cell division determine shape of particular tissues.

Terms to describe planes of cell division:anticlinal – SURFACE GROWTH.

occur in the plane of the sheet- expands the sheet WITHOUT INCREASING THE THICKNESS.

periclinal- occur at right angles to the plane of a sheet so results in its expansion into multiple layers.

Switching from anticlinal to periclinal cell division is critical for some morphogenetic processes, e.g. outgrowth of leaves.

Model organismsUses:• to gain comprehensive knowledge about a complete plant. • to further detailed understanding of mechanisms and processes in plants.• to understand particular biological phenomena with the expectation that discoveries made on the model organism will provide insight into the workings of other organisms Select model organism that lend themselves to study of a particular group and are representative of a larger group.

Arabidopsis thaliana

Small, ca 30 cm tall, with flat rossette of leaves.

Arabidopsis thaliana (wall cress)Small, less than 30 cmLife cycle-about 6-8 weeks, hermaphrodite flowers, self-fertilizing flowers.

Easy to grow large numbers in the lab. Under continuous light 25 degrees centigrade, up to 10,000 to 50,000 seeds. Plants can grow to form ripe seeds within 8 weeks

A single flower can produce 30-50 seeds. Whole plant can produce several thousands, up to 10,000 seeds per plant making study of genetics easier.

Ideal for isolating mutants and for genetic characterization of mutants

2n=10, have 26,700 protein-encoding genes but many are duplicates, ca 15,000 different types of genes,

It has one of the smallest genomes in the plant kingdom: 115,409,949 base pairs of DNA distributed in 5 chromosomes (2n

= 10).

Very little "junk" DNA

Transgenic plants can be made easily using Agrobacterium tumefaciens as the vector to introduce foreign genes.

Mutations can be easily generated (e.g., by irradiating the seeds or treating them with mutagenic chemicals).

It is normally self-pollinated so recessive mutations quickly become homozygous and is expressed

Aim is to to establish a blueprint for how plants develop

inflorescences

• 2n=20, 10 large chromosome pairs•Large no of progeny per cross ca 100 to 200 )•Facilitated discovery of transposons (jumping genes)-mobile genetic elements that disrupt the functions of some genes.

Levels of developmental control

1. Genetic and intracellular control of development. An individual mature cell in the vegetative body of the plant retains within nucleus all the genetic information to reproduce the dev. steps necessary to form the whole organism.Genetic constitution is expressed in terms of the biochemical events within the cell which lead to specific cell differentiation at specific times.

Transplantation experiments inAcetabularia mediterranea and A. crenulata

Shows importance of nucleusfor cell differentiation

Morphogenesis of the cap is dependent upon species-specific RNA molecules translated into proteins

Flow of genetic information

Fig. 14-4, p. 218

Stepped Art

DNA template

New DNA strand

DNA template

RNA transcript

Transcription

Genetic Information• From DNA to mRNA to amino acid

sequence

Levels of developmental control

2. Hormonal and intercellular control of developmentA hormone may act by altering gene expression affect activity of existing enzymes

changing properties of membrane

Any of the above could redirect the metabolism and development of a cell responding to small number of molecules

Lack abscissic acid

MAIN Factor that affects color is soil pH. Acidic= pink/red flowersAlkaline= blue flowers

Environmental factors

Etiolated shoot in potato –developing in the absence of light, turn green upon exposure to light. The plant is able to detect the light intensity and wavelength by using photoreceptors , but receptordoes not interact directly with the cell’s DNA but a signal transduction chain is involved:phytochrome, blue light/UV-A and UV-B receptors .