the plant cell wall growth and development. from: biochemistry and molecular biology of plants
TRANSCRIPT
The Plant Cell WallGrowth and
Development
From: Biochemistry and Molecular Biology of plants
Acid Growth hypothesis
This is dependent on the growth hormone auxin.
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. Causes uptake of water – which leads to a passive increase in cell size.
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.
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 aremade as the cell expands.
Plasma membrane
These line up perpendicular to the direction of growth.
As this happens the existing wallhas to be loosened.
From: Biochemistry and Molecular Biology of plants
Enzymes break the bonds holdingxyloglucan to cellulose.
Enzymes break the xyloglucanmolecules.
Other enzymes break the pectinMolecules (NOT SHOWN).
From: Biochemistry and Molecular Biology of plants
The cell is now free to expandin a given direction.
When expansion has stopped:
From: Biochemistry and Molecular Biology of plants
Cell wall proteins lock the cells new shape as these new wallcomponents are being made.
Enzymes form new xyloglucanMolecules which re-attach to theCellulose microfibrils.
Also form New cross-links with newly formed Cellulose microfibrils.
Plant Development.
Germination: Occurs in response to the environment.
Major Stages of Plant Development
This is driven by food (sugars) thatare stored within the seed. This continues until photosynthesis starts.
Roots extend downwards and the leaf-bearing shoots extend upwards.
First leaves formed - cotyledons.
Apical meristems – Special groups of
self-renewing cells. Located at the tips of stems and roots. Makes alarge number of cells needed to form leaves, flowers and roots.
Once the meristems begin to fullyfunction, the growth of the plantbegins in earnest.
Meristematic development:
Morphogenesis: Plants can not move.
The form of the plant body is controlledby the way plant cells expand and alterin shape.
Many factors control the formation of new cell walls
The direction in which plant cells divide,and thus the direction in which the plantitself grows, is ultimately governed bythe plant cell wall.
Plants are effected by
Hormones too
Auxin
Increases the flexibility of the cellwall. A more flexible wall will stretchmore as the cell is actively growing.
Auxin accumulates in the apicalmeristem. 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. Havean important effect on elongation ofthe stem.
Like auxin, it is involved with makingplant cell growth occur in one directionby 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.
– 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.
– 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.
Cytokininsgroup of hormones that stimulate cytokinesis (cell
division) and leaf enlargement, as well as slowing down leaf aging
• Structurally are variations of nitrogen base adenine.
– Include naturally occurring zeatin and artificially produced kinetin.
• Stimulate cytokinesis/cell division/organ development (organogenesis).
– Stimulate growth of cells in tissue culture, influence shedding of leaves and fruit, seed germination and pattern of branch growth.
Cytokininsgroup of hormones that stimulate cytokinesis (cell
division) and leaf enlargement, as well as slowing down leaf aging
• Work in concert w/auxins to promote growth/cell division (determine whether roots or shoots will develop).
• Work antagonistically against auxins in relation to apical dominance-stimulates growth of lateral buds.
• Delay senescence (aging) by inhibiting protein breakdown (spray on flowers to keep them fresh).
• Produced in roots and travel upward in xylem sap.
Ethylene (H2C=CH2) gas that promotes ripening of fruit and causes flowers and leaves to drop from
trees (positive feedback)• 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
Best known:Phototropism
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.
Induces cells AWAY from light toelongate. 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 cellelongation and re-orientationof the cell walls to allow the root to grow downwards.
•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
Summary