Plant Growth and Development

Plant Physiology 3(2-1)15-02-2012

From germination to senescence!!

Zygote Embryo Seedling

How do new plant structures arise from preexisting structures?

How do plant tissues grow in a particular pattern?

What are the basic principles that govern plant growth and development?

Embryogenesis

Sperm+Egg Zygote

During embryogenesis:•Single-celled zygote is transformed into multicellular, microscopic plant (embryo) that has the complete body plan of a mature plant present in a rudimentary form•It occurs within the Embryo sac of the ovule•Ovule and Endosperm are parts of a seed

Small Egg

Surrounds embryo and provides nutrition in the

form of starch

Wheat endosperm

?

The outline of a mature plant!

Angiosperms: Flowering plants whose ovules are produced within ovary and whose seeds occur within a fruit that develops from the ovaryGymnosperms: ovules not enclosed in ovary and seeds not enclosed in fruitsMonocots: Embryo with single cotyledonsDicots: Embryo with two cotyledons

Arabidopsis thaliana

Embryogenesis and plant development:Axial patterningRadial patterningPrimary meristems

Axial Patterning

Shoot apical meristem

Root apical meristem

Cytoplasmic

Large vacuole

Apical cell: receives more cytoplasm•Divides vertically•Generates globular (octant) embryo

First division of zygote

Basal cell: receives large vacuole•Horizontal division•Suspensor cells 6-9 cells that attach the embryo to the vascular system•Hypophysis derivative of basal cell that contributes to embryo development and forms Columella (central part of root cap)

Three axial regions develop before the embryo reaches the Heart stage;Apical region: gives rise to cotyledons and shoot apical meristemMiddle Region: gives rise to hypocotyl, root and most of the root meristemHypophysis: gives rise to the rest of root meristem

Radial Patterning

•Visible at Globular Stage•Radially arranged three regions• Protoderm:• Cortex:• Endodermis:• Vascular tissues:• Pericycle:

Primary meristems

Protoderm Procambium Ground meristem

Epidermis Primary vascular tissues and vascular cambium

Cortex and endodermis

Vascular Tissues: The tissue in vascular plants that circulates fluid and nutrients. Comprise of;1- Xylem conducts water and nutrients up from the roots2-Phloem distributes food from the leaves to other parts of the plant

Embryo development in Arabidopsis

Globular stage

Heart stage

Torpedo stage

Maturation stage

Cell division in apical regions that

later form cotyledonsCell elongation

throughout embryo axis and

further development of

cotyledonsLast stage, embryo

and seed lose water to enter

dormancy

Embryo goes through divisions, generating an eight-cell (octant) embryo after 30 hrs of fertilization

Seed Dormancy: growth, development and metabolic activities stop..

Why?

When a mature seed is placed under favorable conditions and fails to germinate, it is said to be dormant. Seed dormancy is referred to as embryo dormancy or internal dormancy and is caused by endogenous characteristics of the embryo that prevent germination. The oldest seed that has been germinated into a viable plant was an approximately 1,300-year-old lotus fruit recovered from a dry lakebed in northeastern China. Seed Coat Dormancy: External dormancy or hardseededness, which is caused by the presence of a hard seed covering or seed coat that prevents water and oxygen from reaching and activating the embryo. It is a physical barrier to germination, not a true form of dormancy.

Seed Dormancy

Arrested plant growthSurvival strategy against different external threatsControlled by biological clock that tells plant when to produce soft tissues to survive against harsh winters or other factros------ Intersting????

Genes involved in Embryogenesis

GNOM gene

MONOPTEROS gene

SHORT ROOT and SCARECROW genes

HOBBIT gene

SHOOTMERISTEMLESS gene

Plays role in Axial PatterningNo root and cotyledons

No hypocotyl and root

Both take part in Radial Patterning

Defective root meristem development

Mutants fail to form shoot meristem

HOBBIT gene

Role of HOBBIT gene in root meristem development

Columella (COL):Lateral Root Cap (LRC):Quiscent Center (QC): Slowly dividing root meristematic cells that regulate the differentiation of neighboring cells

•Marker of root meristem identity•hbt mutant shows abnormality in two- or four-cell stage•Hypophyseal precursor divides vertically instead of horizontally•Root without Hypophysis fails to form Quiescent Center and Columella•Consequently hbt mutants are unable to form lateral roots

Acknowledgement

•Plant Physiology by Taiz and Zeiger•Wikipedia