Transcript Lec-2 Plant Growth & Development

Plant Growth and Development
Plant Physiology 3(2-1)
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?
Angiosperms: Flowering plants whose ovules are produced within ovary and whose seeds
occur within a fruit that develops from the ovary
Gymnosperms: ovules not enclosed in ovary and seeds not enclosed in fruits
Monocots: Embryo with single cotyledons
Dicots: Embryo with two cotyledons
The outline of a mature plant!
Arabidopsis thaliana
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
Wheat
endosperm?
Small
Egg
Surrounds embryo and
provides nutrition in the
form of starch
Embryogenesis and plant development:
Axial patterning
Radial patterning
Primary meristems
Shoot apical meristem
Ovule
Root apical meristem
Axial
Patterning
Embryo development in Arabidopsis
Embryo goes
through divisions,
generating an
eight-cell (octant)
embryo after 30
hrs of fertilization
Cell elongation
throughout
embryo axis and
further
development of
cotyledons
Seed Dormancy: growth,
development and
metabolic activities stop..
Why?
Globular
stage
Heart
stage
Cell division in
apical regions that
later form
cotyledons
Torpedo
stage
Maturation
stage
Last stage, embryo
and seed lose
water to enter
dormancy
First division of zygote
Apical cell: receives more cytoplasm
•Divides vertically
•Generates globular (octant) embryo
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
meristem
Middle Region: gives rise to hypocotyl, root and most of
the root meristem
Hypophysis: gives rise to the rest of root meristem
Radial Patterning
•Visible at Globular Stage
•Radially arranged three regions
•Protoderm:
•Cortex:
•Endodermis:
•Vascular tissues:
•Pericycle:
Seed Dormancy
Arrested plant growth
Survival strategy against different external threats
Controlled by biological clock that tells plant when to
produce soft tissues to survive against harsh winters or other
factors-----Interesting????
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.
Genes involved in Embryogenesis
Plays role in Axial Patterning
No root and cotyledons
GNOM gene
MONOPTEROS gene
No hypocotyl and root
SHORT ROOT and SCARECROW genes
HOBBIT gene
Both take part in Radial Patterning
Defective root meristem development
SHOOTMERISTEMLESS gene
Mutants fail to form shoot meristem
HOBBIT gene
Columella (COL):
Lateral Root Cap (LRC):
Quiscent Center (QC):
Slowly dividing root
meristematic cells that
regulate the
differentiation of
neighboring cells
Role of HOBBIT gene in root meristem development
•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
Meristems in Plant Development
•Small isodiametric cells with embryonic characteristics
•Retain their embryonic character indefinitely
•Some differentiate while others retain capacity for cell division
Stem cells: cells that retain their
capacity for cell division
indefinitely
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 roots
2-Phloem distributes food from the leaves
to other parts of the plant
Shoot Apical Meristem
Stem
Leaves and lateral buds
•Shoot apical meristem can contain a few hundred to a
thousand cells but Arabidopsis SAM has about 60 cells
•Small thin-walled cells, dense cytoplasm, lacks large
central vacuole
•Grows rapidly in spring-slow growth during summerdormant in winter
Shoot apex: apical meristem+leaf primordia
Shoot Apical Meristem Structure
Cytohistological
Zonation
Like
Quiescent
center in
roots
internal
tissues of
stem
Preembryonic Meristems
Primary meristems
1. Root meristem
2. Shoot meristem
Postembryonic Meristems
Secondary meristems
1. Axillary
2. Inflorescence
3. Floral
4. Intercalary
5. lateral
Branch root
Axillary
•Formed from pericycle cells in mature root
•Formed in the leaf axils
•Derivative of shoot apical merstem regions
Cork Cambium (Lateral meristem)
•Produce branches
•Develops within mature cortex cells and
Intercalary
•Found within organs, near their bases secondary phloem
•Enables grasses to continue to grow •Periderm or Bark are its derivative layers that
form outer protective surface in woody trees
despite mowing or grazing
Fusiform Stem Cells
Highly elongated, vacuolate
cells that differentiate into
the conducting cells of
xylem and phloem
Vascular Cambium (Lateral meristem)
Woody tissues of stems and roots
Ray Stem Cells
Small cells whose
derivatives include the
radially oriented files of
parenchyma cells within
wood known as Rays
Floral meristems
•Produce floral organs such as sepals, petals, stamens and carpals
•Determinate
Inflorescence meristem
Determinate
•Produces bracts and floral meristems in the axils of bracts
meristems:
•Could be determinate or indeterminate
Genetically
programmed limit
to their growth
Indeterminate:
No predetermined
limit to growth
Consists of one or more leaves, the
node to which leaves are attached,
internode and one or more axillary
buds
Could also be apical meristems
provided they get the
developmental potential
Bracts:
A leaf from the
axils of which a
flower or floral
axil arise
Leaf Development
Axil Development
• leaves are lateral organs.
• leaves display consistent orientation and
polarity relative to the shoot i.e. axial
information in the leaf does not arise de novo
but depends on existing axial information.
• Angiosperm leaf is almost always a
determinate organ.
Stages of leaf development
1- Organogenesis:
•Leaf founder cells formed by L1 and L2 layers of
apical meristem, produce leaf primordium that
ultimately develops into leaves
2- Development of suborgan domains
•Primordium differentiates into specific leaf parts
•Dorsiventral (abaxial-adaxial)
•Proximodistal (apical-basal)
•Lateral (margin-blade-midrib)
3- cell and tissue differentiation
•L1 layer forms epidermis
•L2 layer forms photosynthetic mesophyll cells
•L3 layers forms vascular elements and bundle
sheath cells
Structural symmetry in the leaf
Simple leaves have three axes of symmetry.
• proximodistal axis from base of the leaf to the tip.
• adaxial-abaxial axis from the upper to the lower
epidermis.
• centrolateral axis from the midrib to the margin.
Leaf Primordia Arrangement
Phyllotaxy: The arrangement of leaves around the stem
Single leaf
Paired leaf
Opposite
leaves per
node at right
angle to each
other
More than
two leaves
per node
Spiral
arrangeme
nt of leaves
Further Readings
• Growth and Development, Chapter 16, Plant Physiology by Taiz
and Zeiger