Control Systems in Plants Notes
AP Biology
Mrs. Laux
Control systems in plants are adaptations that evolved over time in
response to interactions with their environment
-plants respond to their environment by:
1. sending signals between different parts of plant
2. tracking time of day and year
3. sensing and responding to gravity, light, and touch
4. adjusting growth pattern and development
Hormones
substances that are produced by specialized cells in one part
of an organism that influence functioning of cells elsewhere in the
organism
-able to move through cell walls
-small amounts alter functions
Hormone functions:
1. control plant growth and development by affecting division,
elongation, and differentiation of cells
-effects depend on:
a. site of action
b. stage of plant growth
c. hormone concentration
2. hormone signal is amplified, perhaps by affecting gene
expression, enzyme activity, or membrane properties
3. reaction to hormones depends on hormonal balance (relative
concentrations of one hormone compared with others
Five classes of plant hormones have been identified:
A. Auxins
-promote elongation of young developing shoots
-natural auxin in plants is indoleacetic acid (IAA)
-Facts
1. apical meristem is major site of auxin production
-stimulates growth here only at certain
concentration
-moves down stem affecting elongation and
growth of stem cells under given conditions
2. transport of IAA is active-called polar transport
-transported down stem via auxin carriers in cells
-energy provided by chemiosmosis
-ATP-driven pumps maintain proton
gradient across a plasma membrane
-as auxin passes through acidic cell wall, it picks
up a proton
-now it is neutral
therefore, can pass
through cell membrane
Control Systems in Plants Notes
AP Biology
Mrs. Laux
-auxin is ionized inside cell and is only permitted
to leave through specialized carrier proteins at
end of cell
-how auxin molecules carried cell to cell
3. auxin affects:
a. cell elongation
-bonding to cell wall causes it to loosen by
breaking cellulose microfibrils
-increases cell wall plasticity
-when influx of water, turgor pressure
causes cell to elongate because of new
plasticity in walls
b. affects secondary growth by inducing vascular
cambium cell division and differentiation of
secondary xylem
c. promotes formation of adventitious roots
d. promotes fruit growth
4. structure
auxin is a modified version of tryptophan
(amino acid)
B. Cytokinins
-structure
modified form of adenine
-stimulates cytokinesis
-many types, 2:
-natural
zeatin
-artificial
kinetin
-produced in roots and move up plant in xylem sap
-Functions:
1. Cell division and differentiation:
-works in conjunction with auxin
-cells with no cytokinins grow very large
and do not divide
-cells with equal concentrations of
cytokinins and auxins
grow and divide,
but remain undifferentiated
-cells, more cytokinins than auxins
shoot
buds develop
-cells, more auxins than cytokinins
roots
develop
2. Apical dominance
-again, cytokinins work with auxins, but
antagonistically
-auxin from terminal bud restrains axillary bud
growth
-cytokinins from roots stimulate axillary bud
growth
Control Systems in Plants Notes
AP Biology
Mrs. Laux
-in same way auxins stimulate lateral root growth
while cytokinins restrain it
-this relationship may help balance plant growth
since an increase in root system would signal a
plant to produce more shoots
-more roots
more cytokinins, makes
axillary buds bloom
-more branches, more auxins, apical
dominance returns
3. can retard senescence-aging of leaves and other
organs
-possibly by inhibiting protein breakdown
-sprayed on leaves of plants to help them stay
longer
C. Gibberellins
-~70 different hormones
-abbreviated: GA1, GA2, GA3, etc.
-are produced primarily in young leaves and roots and seeds
-Functions:
1. stimulate growth in leaves and stems-little effect on
roots
2. stimulates cell division and elongation in stems,
possibly in conjunction with auxin
-ex: fungus Gibberella
secretes gibberellin that
causes hyperelongation of rice stems-killed plant
-high concentrations can cause bolting
rapid
elongation of stems
-usually elevates flowers
-ex: Gibberella
3. fruit development caused by gibberellins and auxins
-artificial
spraying of gibberellins on
Thompson’s seedless grapes
-causes grapes to grow larger and farther
apart
4. in seeds, signals germination
-high concentration found in embryo of seeds
-imbibed water stimulates gibberellin release
-environmental cues may also cause release of
GA in seeds that require special conditions to
germinate
-in breaking both seed dormancy and apical bud dominance,
gibberellins act antagonistically with abscisic acid (inhibits
plant growth)
D. Abscisic Acid (ABA)
1. produced in terminal bud and helps prepare plants for
winter by suspending primary and secondary growth
Control Systems in Plants Notes
AP Biology
Mrs. Laux
-directs formation of scales to protect buds
-inhibits cell division in vascular cambium
2. onset of dormancy in seed also suspends growth
-increased gibberellins
germination
-increased ABA
dormancy
-in desert plants, ABA must be completely washed out
by a rainfall for germination to occur
-why germination only occurs after heavy rainfall
-ensures survival
3. also acts as a stress hormone
closing stomata in times of
stress to reduce transpiration
E. Ethylene
-a gaseous hormone
-increased auxins induce release of ethylene-a growth
inhibitor
-Functions:
-functions mainly in senescence
-plays a part at cellular, organ, or whole plant
level
-ex: leaf fall in autumn (leaf abscission)
withering of flowers
death of flowers
fruit ripening
1. fruit ripening
-ethylene in air spaces within fruit stimulates its
ripening
2. leaf abscission
-adaptation that prevents deciduous trees from
desiccating during the winter when roots cannot absorb
water from frozen ground
-before abscission, leaves essential elements are sent
to and stored in stem for use by new leaves in the
spring
-environmental stimuli
shorter days and lower
temperatures
-decrease in auxin makes cells in abscission layer more
sensitive to ethylene
-increased ethylene, decreased auxin
-increased ethylene induces synthesis of
enzymes that digest polysaccharides in the cell
wall, further weakening abscission layer
-wind and weight cause leaf to fall
-layer of cork forms a protective scar over abscission
layer
-prevents pathogens from entering plant
Control Systems in Plants Notes
AP Biology
Mrs. Laux
Plant Responses to Stimuli
-Tropisms
growth pattern in response to certain environmental
changes
-much
differential rates of cell elongation
-3 primary stimuli
light
gravity
touch
A. Phototropism
-growth toward or away from light
-achieved via auxin
1. auxin produced in apical meristem
-moves via polar transport to zone of elongation where
elongation is stimulated
2. when all sides of apical meristem are equally illuminated, stem
grows straight
3. unequal illumination
auxin concentrates on shady side of plant
4. this causes differential growth
shady side grows more than
sunny side
-causes plant to bend towards the light
B. Gravitropism (Geotropism)
-orientation of a plant in response to gravity
-roots
positive geotropism (down)
-stems
negative geotropism (up)
1. in stems:
-lay horizontal, auxin will move down the stem and cause cell
elongation on lower part of stem
-this causes stem to curve upward
2. in roots: (horizontal)
-auxin produced in root tip (apical meristem of root) moves
upward and concentrates in lower half of root
-in roots, auxins inhibit growth; therefore, root will curve
downward
-no ions or hormones respond directly to gravity
-starch; however, which is insoluble in water, does respond to
gravity
-it is believed that starch accumulates at low points of plant organs,
and somehow attracts the auxins
-this starch is stored in specialized plastids called statoliths
-plastids with dense starch grains
C. Thigmotropism
-directional growth in response to touch
Control Systems in Plants Notes
AP Biology
Mrs. Laux
-ex: vines and other climbing plants “wrap around” objects that they
come in contact with
Biological Clocks control circadian rhythms in plants and other eukaryotes
internal “clocks” that are very accurate
-common in all eukaryotes
-control rhythmic phenomena
-many human features fluctuate with time of day
-ex: blood pressure, temperature, metabolic rate
-certain fungi produce spores only for certain hours of the day
-plants
rhythmic pattern of opening and closing stomata
-circadian rhythm
physiological cycle with a frequency of ~24 hours
-persists even when organism is shielded from environmental
cues (endogenous)
-most biological clocks are cued to the light-dark cycle
resulting from Earth’s rotation
-may take days to reset once cues change
-ex: jet lag
human condition resulting from a lack of
synchronization of internal clock to a time zone
Photoperiodism
a physiological response to day length
-seasonal events (seed germination, flowering) are important to life
cycles
-plants detect the time of year by the photoperiod (relative to night
and day)
In Flowering Plants
-flowering is initiated in response to changes in the photoperiod
-can be divided into 3 groups:
1. Long-day plants
-flower in spring and early summer
-daylight is increasing
-flower when day length is longer and night length is shorter
than a critical length
2. Short-day plants
-flower in late summer, fall, and winter
-daylight is decreasing
-flower when daylength is less than critical length; night is
greater than critical length
3. Day-neutral plants
do not flower in response to environmental
changes
-in response to some other cue:
temperature, water
Control Systems in Plants Notes
AP Biology
Mrs. Laux
1940s
discovered that a critical night length, not day length, actually
controls flowering and other responses to the photoperiod
-observations:
1. if daylength is broken by a brief exposure to darkness, there is no
effect on flowering
2. if night time period is interrupted by short exposure to light,
photoperiod responses are disrupted and plants do not flower
3. therefore, short-day plants flower if night is longer than critical
length, and long day plants need a night shorter than a critical length
Evidence that a flowering hormone is present in plants since leaves detect
the photoperiod while buds produce flowers
-message from one area of plant transferred to another section
-hormone in leaves
bud
-for buds to develop, only one leaf needs to be present
-the hormone (S) is believed to be the same in long and short day
plants
-hormone
florigen, has yet to be isolated
How do plants detect differences in day length?
-pigment
phytochrome measures length of darkness in a photoperiod
-absorbs light via a light-absorbing component called chromophore
-2 forms of phytochrome
1. Pr or P660
-red light, λ=660 nm
2. Pfr or P730
-far-red light, λ=730 nm
-2 forms of light are photoreversible
-when Pr is exposed to red light, it is converted to Pfr
-when Pfr is exposed to far-red light, it is converted to Pr
Phytochrome functions as a photodetector that tells if light is present:
1. Plants synthesize Pr (only)- if kept in the dark, it remains as Pr , but
if illuminated, some Pr is converted to Pfr
2. Pfr triggers many plant responses to light
-ex: seed germination, glowering
3. Shifts in the Pr ↔ Pfr ratio may cause changes which would adjust
a plant’s growth and development in response to some environmental
changes
4. At night, Pfr gradually reverts to Pr
-every day after sunset
-plants synthesized as Pr and degradative enzymes destroy
more Pfr than Pr
-at sunrise, the Pfr level increases due to conversion of Pr
-by measuring Pr to Pfr ratio, phytochrome system evaluates the
quality of light reaching the plant
-presence of Pfr will stimulate processes, but internal circadian clock
measures night length