Transcript Plants

Plants
Basic
Structures
Apical meristems enable the plant
to grow in length.
-located in tips of roots and
in the buds.
Plants, being rooted to the ground
Must respond to whatever
environmental change comes their
way
Plant Hormones
•Auxin
•Cytokins
•Gibberelins
•Abscisic Acid
•Ethylene
• Plant hormones help
coordinate growth,
development, and
responses to stimuli
• Hormones
– Are chemical signals
that coordinate the
different parts of an
organism
• Plants have cellular receptors
– That they use to detect important changes in their
environment
• For a stimulus to elicit a response
– Certain cells must have an appropriate receptor
The Discovery of Plant Hormones
• Any growth response
– Is often caused by hormones
• EX: results in curvatures of whole plant organs toward or
away from a stimulus =tropism
– Shoots growing towards the light is positive phototropism
– Shoots growing up is negative geotropism
– Roots growing down is positive geotropism
Auxin
– Is used for any chemical substance that promotes
cell elongation in different target tissues
• Auxin transporters
– Move the hormone from
shoot tip to base and never
in the reverse even if the
plant is turned upside
down.
• Auxin
– Is involved in the formation and branching of roots
– Inhibiting lateral bud growth
Other Effects of Auxin
• Auxin affects secondary growth
– By inducing cell division in the vascular cambium and
influencing differentiation of secondary xylem
• Developing seeds synthesize auxin
• tomatoes grown in greenhouse conditions sprayed with auxin
induce fruit development without a need for pollination
• This allows for seedless tomatoes
• Charles Darwin and his son Francis
– Conducted some of the earliest experiments on phototropism, a plant’s
response to light, in the late 19th century
EXPERIMENT
In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the
coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for
phototropism is transmitted.
RESULTS
Control
Boysen-Jensen (1913)
Darwin and Darwin (1880)
Shaded
side of
coleoptile
Light
Light
Light
Illuminated
side of
coleoptile
CONCLUSION
Tip
removed
Tip covered
by opaque
cap
Tip
covered
by transparent
cap
Base covered
by opaque
shield
Tip separated
by gelatin
block
Tip separated
by mica
In the Darwins’ experiment, a phototropic response occurred only when light could
reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. BoysenJensen observed that a phototropic response occurred if the tip was separated by a
permeable barrier (gelatin) but not if separated by an impermeable solid barrier (a mineral
called mica). These results suggested that the signal is a light-activated mobile chemical.
• In 1926, Frits Went
– Extracted the
chemical
messenger for
phototropism,
auxin, by
removing the
coleoptile tip &
placed it on a
block of agar. This
allowed the
chemical to travel
through.
EXPERIMENT
In 1926, Frits Went’s experiment identified how a growth-promoting chemical
causes a coleoptile to grow toward light. He placed coleoptiles in the dark and removed their tips,
putting some tips on agar blocks that he predicted would absorb the chemical. On a control
coleoptile, he placed a block that lacked the chemical. On others,
he placed blocks containing the chemical, either centered on top of the coleoptile to distribute the
chemical evenly or offset to increase the concentration on one side.
RESULTS
The coleoptile grew straight if the chemical was distributed evenly.
If the chemical was distributed unevenly, the coleoptile curved away from the side with
the block, as if growing toward light, even though it was grown in the dark.
Excised tip placed
on agar block
Growth-promoting
chemical diffuses
into agar block
Control
Control
(agar block
lacking
chemical)
has no
effect
Agar block
with chemical
stimulates growth
Offset blocks
cause curvature
CONCLUSION
Went concluded that a coleoptile curved toward light because its dark
side had a higher concentration of the growth-promoting chemical, which he named auxin.
Cytokinins
• Cytokinins (like cytokinesis)
– Stimulate cell division
– Are produced in
actively growing tissues
such as roots, embryos,
and fruits
– Makes the plant branch
out.
Control of Apical Dominance
• Cytokinins, auxin, and other factors interact in
the control of apical dominance
– The ability of a terminal bud to suppress
development of axillary buds
Axillary buds
Figure 39.9a
Anti-Aging Effects
• Cytokinins retard the aging of some plant organs
– By inhibiting protein breakdown, stimulating RNA
and protein synthesis, and mobilizing nutrients from
surrounding tissues
Florists use cytokinins
on their cut flowers
to keep them fresh.
Gibberellins
• Control the plant’s yearly cycle
– Tells plant when to go dormant, when to flower…
• Gibberellins stimulate growth of both leaves
and stems
• In stems
– Gibberellins stimulate cell elongation and cell
division
The release of gibberellins from the embryo of a seed signals
the seed to break dormancy and germinate.
Fruit Growth
• In many plants
– Both auxin and gibberellins must be present for fruit
to set
• Gibberellins are
used commercially
– In the spraying of
Thompson
seedless grapes
making them grow
larger.
Abscisic Acid (opposite of gibberelins)
• In the fall, when leaves change color. ABA
basically cuts the leaves off the tree.
• Suppresses fruit formation
• Stops transpiration
• Basically causes plants to go into dormancy
– Preparing for winter
– Drought tolerance
Ethylene
• It’s a gas and travels
outside the plant
• Is an example of positive
feedback loop
The Triple Response to Mechanical Stress
• Ethylene induces the triple response
– Which allows a growing shoot to avoid obstacles
EXPERIMENT
Germinating pea seedlings were placed in the
dark and exposed to varying ethylene concentrations. Their growth
was compared with a control seedling not treated with ethylene.
1. Slowing of stem elongation
2. Thickening of the stem
3. Curvature causing stem to grow
horizontally.
RESULTS
All the treated seedlings exhibited the triple
response. Response was greater with increased concentration.
0.00
0.10
0.20
0.40
0.80
Ethylene concentration (parts per million)
Figure 39.13
CONCLUSION
Ethylene induces the triple response in pea seedlings,
with increased ethylene concentration causing increased response.
Apoptosis: Programmed Cell Death
• A burst of ethylene
– Is associated with the programmed destruction of
cells, organs, or whole plants
Fruit Ripening
• A burst of ethylene production in the fruit
– Triggers the ripening process
In animals, internal & external
signals regulate a variety of
physiological responses that
synchronize with environmental
cycles and cues.
Circadian rhythms
Resets every day; is influence by internal & external signals
such as light and dark; meal times; stress; exercise…
Plants are influenced by external
factors as well.
Such as night length.
This is called photoperiodism.
Phytochrome is an important
macromolecule.
From the results above these plants can be more appropriately called “long
night plants” and “short night plants”. Photoperiodism is controlled by the
length of the night= critical night length
It is the leaves that give the cue that the night is long enough or short enough
and signals the buds to flower.
During the day, PR is converted to PFR
During the night the PFR is converted back to PR
PR
PFR
IN SHORT DAY PLANTS
PR triggers flowering and PFR inhibits flowering
So the long nights give the plant plenty of time to get rid
of the PFR that built up during the day.
IN LONG DAY PLANTS
PR inhibits flowering and PFR triggers flowering
So the long days give the plant plenty of time to build up
the PFR & the nights are too short to get rid of all the PFR
Photoperiodism
• Day-neutral plants: light has no effect on their
growth
Tomatoes
Rice
Dandelions
Photoperiodism
• Vernalization:
– Some plants (winter wheat) need to be exposed to
cold temperatures for several weeks
Plants respond to a wide variety
of stimuli other than light
Geotropism/Gravitropism
• Auxin plays a key role
Underground how does a seedling know which way to grow?
Thigmomorphogenesis
• Plants growing on a windy range
– Have thicker trunks
• Touching leaves can alter plant growth
– Climbing vines
5 ways plants defend themselves