Growth and Development & Plant Hormones Part 1 Flashcards
Growth results from:
– cell division
– cell enlargement
Development
coordination of growth and differentiation into
tissues, organs, and the whole organism
Plant hormone
Naturally occurring organic substance produced in minute
quantities in one part of the plant and then transported to
another part of the plant where it controls or affects growth and
development
Plant hormone
Present in nanogram/gram fresh weight
– (1 nanogram = 1.0×10-9 grams)
Plant growth regulators:
– Both natural and synthetic substances that affect plant
development as hormones do
– Includes plant hormones
– Also active at low concentrations
Major Groups of Known Plant Hormones
- Auxins
- Gibberellins
- Cytokinins
- Ethylene
- Abscisic Acid
- (Brassinosteroids)
Auxein (Greek) =
to increase
IAA (indole acetic acid)
- promotes cell enlargement
- sometimes promotes cell division
Charles Darwin and his son Francis provided
early evidence for
the existence of auxin
auxins have positive effects but at high concentration it can also
Auxins have growth promoting and other positive effects BUT at
high concentration auxins can inhibit growth
whats more sensitive to auxin roots or shoots
Roots more sensitive to auxin than shoots
adding auxin to roots can
inhibit growth
What is more sensitive to auxin
dicots or monocots
Dicots mores sensitive to auxin than monocots
Some herbicides contain
2,4D (synthetic auxin)
that kills dicot weeds
Auxin is synthesized in
meristematic regions
where specifically is auxin synthesized in meristematic regions
-shoot tips
-root tips
-germinating seeds
Auxin also synthesized in
– young, rapidly growing leaves
– developing flowers
– young embryos following fertilization.
Polar Transport of IAA
where
In the shoot, Polar Transport of IAA in
parenchyma cells in contact with vascular
bundles.
Polar Auxin Transport
As it is transported downwards, it causes:
– Root initiation of cuttings
– Vascular tissue differentiation
– Inhibition of axillary bud growth
Roles of Auxin: 1
1) Causes Root Initiation
1) Causes Root Initiation
-Occurs naturally when a
cutting is placed in
water
-Auxin can be added to
the base of cuttings to
enhance the process of
root initiation
Auxin is the active ingredient in rooting powders.
rooting powders
too much rooting powder can
inhibit
root growth.
role 2 of auxin
Auxin Causes Vascular
Tissue Differentiation
Auxin Causes Vascular
Tissue Differentiation
When the xylem of a plant stem is
cut, new xylem forms around the
wound.
evidence for a role of
auxin in xylem differentiation
-In this demonstration, the top of
the plant was cut off to remove the
tip of the plant, the natural source
of auxin.
* The plant was wounded and the
vascular tissue was cut
* IAA was added in a paste above the
wound, and the xylem regenerated
around the wound without the tip
of the plant
Roles of Auxin 3
Inhibits the growth of lateral buds, promoting
apical dominance (tip of plant (apex) is dominant
over lateral buds)
Auxin in an agar
block will also
substitute for
the tip
There is more auxin near the
tip of the plant
Growth of the axillary buds is
more inhibited closer to the
tip
If the tip is removed, the
axillary buds will grow
because the
main source of auxin has been removed
If the tip of a plant is cut off the
axillary bud will grow
If the tip is cut off and replaced with auxin
(axillary) buds do not grow
apical dominance
Strong in trees
with conical
shapes
(e.g. Pines, spruces, firs)
Norfolk Island Pine
Apical Dominance
Weak in trees that branch more
often (e.g. Elms, ashes, willows)
Weeping Willow
roles of Auxin 4
4) Involvement in tropisms
tropism
directional response of a plant part to an
external stimulus in which the direction of the
response depends on the direction of the stimulus
Positive phototropism of shoots
More auxins accumulate on the side in the shade, so the cells grow faster on the shaded side, making the shoot grow towards the sun.
b. Negative gravitropism of shoots
Shoots grow away from the direction of gravity
c. Positive gravitropism of roots
Shoots grow toward the direction of gravity
Roles of Auxin Continued 5
5) Inhibits leaf and fruit abscission.
Leaf Abscission in the Autumn
-Young leaves in spring and summer produce auxin
that travels through the petiole preventing
abscission
* Older leaves in autumn no longer produce enough
auxin and the abscission zone forms.
Roles of Auxin Continued 6
6) Promotes flowering in pineapples
G) Roles of Auxin Continued 7
7) Stimulates fruit development
Parthenocarpy:
seedless fruits
Some species will produce seedless fruits after
spraying the flowers with
auxin early in ovule
development:
– Tomatoes
– Eggplants
– Cucumbers
Apple Orchards auxin
- Spray with auxin:
– to promote uniform flowering
– to promote uniform fruiting
– to prevent early abscission of fruits
Roles of Auxin: 8)
controls weeds
2,4-D and 2,4,5-T herbicides (synthetic
auxins) in
Agent Orange (Defoliant in
Vietnam War)
2,4,5-T banned for most uses in 1979 due
to
dioxin contaminants
* Leukemia
* Miscarriages
* Birth Defects
* Lung & Liver Diseases
2,4-D is
still used
* Moderate toxicity for mammals and birds
Roles of Auxin Continued 9
9) Promotes cell division in tissue culture
Gibberellins =
GA’sa
over 135 diffrent kinds
Gibberellic acid effects
– Growth promoting and other positive effects
– Promotes cell division and cell enlargement
Discovery of Gibberellins (GA’s)
1) In Japan in the 1930’s (unknown in Western part of World)
2) Foolish seedling disease (Bakane)
– Gibberella fujikuroi
– Culture medium that fungus was in or fungal extract caused symptoms
-Infected seedlings exhibit abnormal elongation and in severe cases stunting leading to death. It led to the discovery of GA.
Roles of GA’s 1
1) Promote stem elongation of intact plants
Cabbage rosettes
bolt and flower
following treatment with gibberellins.
*Bolting typically occurs due to low
temperatures and longer days.
*GA’s substitute for environmental
cues.
Affects of Added GA’s on Stems
- Short bush beans become climbing pole beans
- Genetically dwarf rice, maize, peas become
phenotypically tall. - Genetically tall plants have little response to
added GA’s
C) Roles of GA’s continued 2
Promote germination of dormant seeds and
growth of buds.
C) Roles of GA’s continued 3
3) Can cause flowering.
C) Roles of GA’s continued 4
4) Promotes fruit development and formation
GA and promoting fruit gas use (grapes and its costs)
-Used to increase the size of Thompson seedless
grapes and makes less tightly packed bunches.
– Costly as Gibberella fujikuroi is still the source of
GA3
Roles of cytokinins
- Promote cell division (cytokinesis)
- Produced in roots
* Promotes shoot formation
* Promotes growth of lateral buds - Cause cell division in tissue culture
* also differentiation of shoots in tissue culture
Buds closer to tip
of plant have more
auxin and less
cytokinins from the
roots, so buds are
inhibited.
Buds at nodes
closer to base of
plant have more
cytokinins from the
roots and less auxin
from the tip, so
buds are less
inhibited and they
start to grow.
Buds closer to tip
of plant have more
auxin and less
cytokinins from the
roots, so buds are
inhibited.
Cytokinins from Roots also Affect
Growth of Axillary Buds
High cytokinin to
auxin ratio
promotes shoot
differentiation
promotes root
differentiation
