Transcript The Plant Cell wall

The Plant Cell
Wall
Growth and
Development
From: Biochemistry and Molecular Biology of plants
Acid Growth hypothesis
This is dependent on the
growth hormone auxin.
Auxin activates a plasma membrane
proton pump, which acidifies the cell
wall.
Experimentally, H-ions have the same
effect as auxin – so lowering the pH is
a good substitute.
The lower pH, in turn, activates
growth-specific enzymes that
hydrolyze the bonds holding the
cellulose microfibrils to xyloglucan.
The cleavage of these bonds results
in the loosening of the cell wall and
Causes uptake of water – which leads
to a passive increase in cell size.
From: Biochemistry and Molecular Biology of plants
Wall-loosening enzymes
Expansins: Break hydrogen
bonds between cellulose and
xyloglucan.
They are the only proteins
shown to the active expansion
of cell walls in vitro. They
are always present in growing
tissue of all plant cell
material.
When expansins are added to
a heat-inactivated stem
sample at acidic pH, cell wall
extension is restored.
From: Biochemistry and Molecular Biology of plants
For a plant to grow:
•new wall material has to be laid down as the cell
expands
New cellulose microfibrils are
made as the cell expands.
Plasma
membrane
These line up perpendicular
to the direction of growth.
As this happens the
existing wall has to be
loosened.
From: Biochemistry and Molecular Biology of plants
Enzymes break the bonds holding
xyloglucan to cellulose.
Enzymes break the xyloglucan
molecules.
Other enzymes break the pectin
Molecules (NOT SHOWN).
The cell is now free to expand
in a given direction.
From: Biochemistry and Molecular Biology of plants
When expansion has stopped:
Enzymes form new xyloglucan
Molecules which re-attach to the
Cellulose microfibrils.
Also form New cross-links
with newly formed Cellulose
microfibrils.
Cell wall proteins lock the cells
new shape as these new wall
components are being made.
From: Biochemistry and Molecular Biology of plants
Plant Development.
Major Stages of Plant Development
Germination: Occurs in response toe to
the environment.
This is driven by food (sugars) that
are stored within the seed. This
continues until photosynthesis starts.
Roots extend downwards and the
leaf-bearing shoots extend upwards.
First leaves formed - cotyledons.
Meristematic development:
Apical meristems –
Special groups of
self-renewing cells. Located at the
tips of stems and roots. Makes a
large number of cells needed to
form leaves, flowers and roots.
Once the meristems begin to fully
function, the growth of the plant
begins in earnest.
Morphogenesis:
Plants can not move.
The form of the plant body is controlled
by the way plant cells expand and alter
in shape.
The direction in which plant cells divide,
and thus the direction in which the plant
itself grows, is ultimately governed by
the plant cell wall.
Many factors control the formation
of new cell walls
Plants are effected
by
Hormones too
Auxin
Increases the flexibility of the cell
wall. A more flexible wall will stretch
more as the cell is actively growing.
Auxin accumulates in the apical
meristem. Allows selective cell
elongation.
By interacting with other hormones,
Auxin also induces cell width.
Auxin
plant growth regulators that stimulate elongation of specific plant
cells (young developing shoots/coleoptiles)/inhibit growth of other
plant cells
• First plant hormone
discovered.
– Modified tryptophan amino acid
actively transported from cell to
cell in specific direction
(unidirectional from apex to
shoot-polar transport) by means
of chemiosmotic process.
Auxin
plant growth regulators that stimulate elongation of specific plant
cells (young developing shoots/coleoptiles)/inhibit growth of other
plant cells
• Active in leaves, fruits and
germinating seeds.
– Indoleacetic acid (IAA) is
naturally occurring.
– Humanmade auxin, 2,4-D, used as
weed killer (dicots).
– When used as rooting powder, it
causes adventitious roots to
develop quickly in plant cutting.
– Synthetic auxin sprayed on
tomato plants will induce fruit
production without pollination,
results in seedless tomatoes.
Gibberellins
Produced in apical meristems. Have
an important effect on elongation of
the stem.
Like auxin, it is involved with making
plant cell growth occur in one
direction
by causing the cellulose microfibrils
to line up in one direction.
Gibberellins
Has other effects:
Induces flower
development
Quickens seed
germination
Gibberellins
(think grow) group of hormones that promotes cell growth
(stem elongation), induce growth of dormant seeds, buds,
and flowers
Promotes stem/leaf
elongation.
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Synthesized in young
leaves, roots and seeds
but are often
transported to other
parts of plant.
Have identified 100
different naturally
occurring ones.
Gibberellins
(think grow) group of hormones that promotes cell growth
(stem elongation), induce growth of dormant seeds, buds,
and flowers
Work in concert with auxins to
promote cell growth.
Induce bolting, rapid growth of
floral stalk.
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To ensure pollination/seed
dispersal, plant growing low to
ground sends up tall shoot for
flower/fruit development.
Promotion of fruit
development/seed germination,
inhibition of aging of leaves.
In breaking both seed
dormancy/apical bud dormancy,
gibberellins act antagonistically with
abscisic acid, which inhibits plant
growth.
Cytokinins
Stimulates cell division in
plants.
As such, has no direct effect
on wall.
Mainly produced in apical
meristems of roots.
Stimulates growth of lateral
buds into branches. This
hormone stops wall expansion in
the shoot and induces a new
stem to form a branch.
Cytokinins
group of hormones that stimulate cytokinesis (cell division)
and leaf enlargement, as well as slowing down leaf aging
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Structurally are variations of
nitrogen base adenine.
Include naturally occurring zeatin
and artificially produced kinetin.
Stimulate cytokinesis/cell
division/organ development
(organogenesis).
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Stimulate growth of cells in tissue
culture, influence shedding of leaves
and fruit, seed germination and
pattern of branch growth.
Cytokinins
group of hormones that stimulate cytokinesis (cell division)
and leaf enlargement, as well as slowing down leaf aging
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Work in concert w/auxins to
promote growth/cell division
(determine whether roots or shoots
will develop).
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Work antagonistically against auxins
in relation to apical dominancestimulates growth of lateral buds.
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Delay senescence (aging) by
inhibiting protein breakdown (spray
on flowers to keep them fresh).
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Produced in roots and travel upward
in xylem sap.
Ethylene (H2C=CH2)
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gas that promotes ripening of fruit and
causes flowers and leaves to drop from
trees (positive feedback)
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Promotes fruit ripening:
During later stages of fruit development, it diffuses through and fills fruit’s intercellular air
spaces and stimulates its ripening.
Signal to ripen spreads from fruit to fruit because ethylene is a gas.
Large quantities produced in times of stress (drought, flooding, mechanical pressure,
injury, infection).
Facilitates apoptosis (programmed cell death)
Prior to death, cells salvage/reuse many of their chemical components.
Breakdown of cell walls/loss of chlorophyll-aging processes.
In combination w/auxin, it inhibits elongation of roots, stems, and leaves
and influences leaf abscission (aging & dropping of leaves)
Triggers senescence (aging-leaves fall, flowers wither, annuals die after flowering, xylem
vessel elements/cork cells die and become fully functional), then aging cells release more
ethylene.
Scar forms when leaf falls off to keep pathogens from entering.
Works in opposition to auxins-ethylene increases and auxin decreases.
Plant salvages usable compounds before leaf dies and falls off.
Involved in stimulating production of flowers
Tropisms
Positive or negative growth
responses of plants to external
stimuli that mainly come from one
direction.
As tropisms effect the growth
pattern of plants, they greatly
effect the plant cell wall.
Best known:
Phototropism
Induces cells AWAY from light to
elongate. Cell wall expands in a
specific direction.
Gravitropism:
Response of a plant to gravity. Causes roots to grow
downwards and stems to grow upwards. This response is
governed by Auxin.
Auxin builds up in the cells of the upper surface of root
This induces localized cell
elongation and re-orientation
of the cell walls to allow the
root to grow downwards.
Summary
•Plant cell growth is governed by the presence of a plant
cell wall
•Special groups of cells – apical meristems – produce new
cells
For a plant to grow:
New wall material has to be laid down as the cell
expands.
The primary cell wall is a site of metabolic activity.
Tightly controlled and coordinated process.
Summary
Hormones are the tools plants use to regulate their growth
Auxin: Produced in meristems.
Cell elongation.
Involved in TROPISMS
Gibberellins: Play a major role in cell elongation
Involved in germination
Induce flowering.
Cytokinins: Promote growth of lateral buds