Transcript Chapter 39: Plant Communication

Chapter 39:
Plant Communication
Plants and their Environments
Plants are living and respond to envir.
They detect light and gravity especially.
Plants respond by altering growth.
Receptors receive signals and initiate a
cellular response.
–Signal Transduction Pathway
–Ex: Potatoes bud in dark not light
b/c they grow in absence of light
(underground)
Hormones Chemical Signals
Sent by one cell to initiate responses in
others
Can be introduced to soil or water to
affect plant
Minimal concentrations needed for big
effect
–Triggers a cascade response
Relative, not absolute levels of
hormones regulate plant responses
Tropisms
Whole body response of plant to stimuli.
Phototropism – Plant grows towards light.
Coleoptile tip must be exposed to light. If
removed or covered, no response.
Detection at coleoptile, but growth occurs
further down shoot.
–Chemical response sent from tip to
shoot.
Gravi-. Photo- and thigmo-tropism
–Gravity, light, touch
Phototropism
Plants detect the intensity,
direction, and wavelength of light.
They will bend towards the area
with the best light
(intensity/quality)
Phytochromes = receptors
Phototropism is a Hormonal Response
Auxins – Plant Growth Hormone
Responsible for
tropisms.
2 Theories:
–Auxin concentrated
on dark side of plant
 traditional theory
–An auxin inhibitor
concentrated on
light side of plant
 more recent
research
Auxins
Main function is to promote shoot
elongation at low concentrations
Loosen cell wall and allow increase H2O
uptake (elongation)
Also induce division in cambium
Too high of a [auxin] will cause production
of ethylene which inhibits elongation
Causes rapid translation of proteins
–Gene regulation
Cytokinins
Induce cytokinesis (cell division)
Operate in balance with auxins
–Equal: No differentiation, just
division
–More cytokinin: Shoot buds form
–More auxin: roots form
–Auxins inhibit auxiliary growth,
cytokinins promote it
–Slow aging
Gibberellins
Produced in roots
and leaves
Stimulate growth of
leaves and stems
Elongation and
division
Works in cooperation
with auxin to loosen
cell walls (more H2O
in = elongation)
Effect of Gibberellins
Seed Dormancy and Gibberellins
When conditions are appropriate,
the release of gibberellins signals
the seed to germinate
May stimulate digestive enzymes
to mobilize stored nutrients
Abscisic Acid (ABA)
Slows growth
Works antagonistically against the
other growth hormones
(gibberellins, auxins, cytokinins)
Ratio b/t ABA and others
Seed dormancy
Allows plants to withstand droughts
(close stomata)
Ethylene
Leaf abscission (fall off)
Fruit ripening
Programmed cell death
High [auxins] stimulate production
Involved in thigmotropism (touch)
Apoptosis
Apoptosis
Systematic, preprogrammed (DNA)
cell death.
Enzymes break down organics (DNA,
chlorophyll, RNA, proteins)
Bursts of ethylene during autumn
cause plants to lose their leaves and
prepare for winter. First, they break
down the organic molecules.
–Stored in stem parenchyma cells
(reused by new spring leaves)
Seed Dispersal  Fruits
When a bear eats a
piece of fruit, he eats
the seeds.
He doesn’t digest
the seeds.
He poops the seeds.
The poop is fertilizer
for the seeds.
Thanks Fozzy.
Ripening of Fruit
Ethylene production causes
ripening
Storing fruits in a paper bag
accelerates ripening (traps
ethylene)
Storing orange juice in the cabinet
accelerates ripening
Circadian Rhythms
Plants go through processes in
a 24-hour (approx.) cycle
When isolated, they lose sync
with the external ques and
regain when replaced
Think Jetlag
Photoperiodism
Plant leaves monitor the relative length of
day and night
Allows seasonal changes in plants (ie:
flowering)
Short-day plants require light period
shorter than a critical length to flower.
Long-day plants flower when the light
period is longer than a critical number of
hours
Day-neutral plants flower at maturity,
regardless of day length
Short/Long Day Plants
Short day =
long night
Long Day =
short night
Many plants
require
uninterrupted
darkness
hours to
flower
Water Deficit
Bright, sunny days  plants can lose
more H2O than they take in
As H2O dec. the guard cells close
stomata
Inc. ABA production (close stomata)
Inhibit growth of young leaves
Leaves roll to dec. surface area
All designed to decrease transpiration
Heat/Cold Stress
Denatures plant enzymes
Transpiration = evaporative cooling
Heat shock protein = produced when
temp. is raised beyond threshold
Often bind to enzymes to prevent
denaturization
Extreme cold also freezes plants
Many plants collect solutes (dec. fp)
Some “call”
other species
to fight
predators off
Plants release
signal
chemicals to
alert other
plants of an
infestation of
herbivores
Defense