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Chap 39
Control Systems in Plants
Table 39.1
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Table 39.1, continued
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Plant hormones are produced at very low
concentrations.
– Signal transduction pathways amplify the hormonal
signal many fold and connect it to a cell’s specific
responses.
– These include altering the expression of genes, by
affecting the activity of existing enzymes, or
changing the properties of membranes.
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Elongation of cells along
shaded side causes
bending towards the light
Went
Experiments
Auxin causes
elongation of
cells
Cryptochromes or light receptors in the
tip of the plant may cause it to pump the
auxin to the opposite side
Unidirectional downwards transport of auxin
Charged Auxin can
only exit at basal end
of cell
Auxin when neutral,
can diffuse through
top of cell
• According to the acid growth hypothesis, in a
shoot’s region of elongation, auxin stimulates
plasma membrane proton pumps, increasing the
voltage across the membrane and lowering the
pH in the cell wall.
– Lowering the pH activates expansin enzymes that
break the cross-links between cellulose microfibrils.
– Increasing the voltage enhances ion uptake into
the cell, which causes the osmotic uptake of water
– Uptake of water with looser walls elongates the cell.
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Expansins
• Synthetic auxins, such as 2,4-dinitrophenol
(2,4-D), are widely used as selective herbicides.
– Monocots, such as maize or turfgrass, can rapidly
inactivate these synthetic auxins.
– However, dicots cannot, and die from a hormonal
overdose.
• Spraying cereal fields or turf with 2,4-D eliminates dicot
(broadleaf) weeds such as dandelions.
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Apical Dominance
the terminal bud releases auxin which
suppresses the lateral axillary buds
When the terminal bud is removed the
inhibition of the axillary buds are
removed and lateral growth starts
• Cytokinins retard the aging of some plant
organs.
– They inhibit protein breakdown by stimulating
RNA and protein synthesis, and by mobilizing
nutrients from surrounding tissues.
– Leaves removed from a plant and dipped in a
cytokinin solution stay green much longer than
otherwise.
– Cytokinins also slow deterioration of leaves on
intact plants.
– Florists use cytokinin sprays to keep cut flowers
fresh.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• A century ago, farmers in Asia notices that
some rice seedlings grew so tall and spindly
that they toppled over before they could mature
and flower.
– In 1926, E. Kurosawa discovered that a fungus in
the genus Gibberella causes this “foolish seedling
disease.”
– The fungus induced hyperelongation of rice stems
by secreting a chemical, given the name
gibberellin.
Fig. 39.9
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Promotes stem
elongation in dwarf
plants like cabbage
• Roots and leaves are major sites of gibberellin
production.
– Gibberellins stimulate growth in both leaves and
stems but have little effect on root growth.
– In stems, gibberellins stimulate cell elongation and
cell division.
– One hypothesis proposes that gibberellins stimulate
cell wall loosening enzymes that facilitate the
penetration of expansin proteins into the cell well.
– Thus, in a growing stem, auxin, by acidifying the
cell wall and activating expansins, and gibberellins,
by facilitating the penetration of expansins, act in
concert to promote elongation.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The effects of gibberellins in enhancing stem
elongation are evident when certain dwarf
varieties of plants are treated with gibberellins.
– After treatment with gibberellins, dwarf pea plant
grow to normal height.
– However, if applied to
normal plants, there is
often no response, perhaps
because these plants are
already producing the
optimal dose of the
hormone.
Fig. 39.10
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• In many plants, both auxin and gibberellins
must be present for fruit to set.
– Spraying of gibberellin during fruit development is
used to make the individual grapes grow larger and
to make the internodes of the grape bunch elongate.
• This enhances air circulation between the grapes and
makes it harder for yeast and other microorganisms to
infect the fruits.
Fig. 39.11
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• Abscisic acid (ABA) was discovered
independently in the 1960s by one research
group studying bud dormancy and another
investigating leaf abscission (the dropping of
autumn leaves).
– Ironically, ABA is no longer thought to play a
primary role in either bud dormancy or leaf
abscission, but it is an important plant hormone
with a variety of functions.
– ABA generally slows down growth.
– Often ABA antagonizes the actions of the growth
hormones - auxins, cytokinins, and gibberellins.
– It is the ratio of ABA to one or more growth
hormones that determines the final physiological
outcome.
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• One major affect of ABA on plants is seed
dormancy.
– The levels of ABA may increase 100-fold during
seed maturation, leading to inhibition of
germination and the production of special proteins
that help seeds withstand the extreme dehydration
that accompanies maturation.
– Seed dormancy has great survival value because it
ensures that the seed with germinate only when
there are optimal conditions of light, temperature,
and moisture.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Many types of dormant seeds will germinate
when ABA is removed or inactivated.
– For example, the seeds of some desert plants break
dormancy only when heavy rains wash ABA out
of the seed.
– Other seeds require light or
prolonged exposure to cold to
trigger the inactivation of ABA.
– A maize mutant that has seeds
that germinate while still on
the cob lacks a functional
transcription factor required
for ABA to induce expression
of certain genes.
Fig. 39.12
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• ABA is the primary internal signal that enables
plants to withstand drought.
– When a plant begins to wilt, ABA accumulates in
leaves and causes stomata to close rapidly,
reducing transpiration and preventing further water
loss.
– ABA causes an increase in the opening of
outwardly directed potassium channels in the
plasma membrane of guard cells, leading to a
massive loss of potassium.
– The accompanying osmotic loss of water leads to a
reduction in guard cell turgor and the stomata close.
– In some cases, water shortages in the root system
can lead to the transport of ABA from roots to
leaves, functioning as an “early warning system.”
Abscisic acid (ABA)
Unlike animals, plants cannot flee from potentially harmful conditions like
drought
the approach of winter
They must adapt or die.
The plant hormone abscisic acid (ABA) is the major player in mediating the adaptation of the plant to stress.
Here are a few examples.
1. Bud dormancy
ABA mediates the conversion of the apical meristem into a dormant bud. The newly developing leaves growing above the meristem become converted into stiff bud scales
that wrap the meristem closely and will protect it from mechanical damage and drying out during the winter.
ABA in the bud also acts to enforce dormancy so if an unseasonably warm spell occurs before winter is over, the buds will not sprout prematurely. Only after a prolonged
period of cold or the lengthening days of spring (photoperiodism) will bud dormancy be lifted.
2. Seed maturation and dormancy
Seeds are not only important agents of reproduction and dispersal, but they are also essential to the survival of annual and biennial plants. These angiosperms die after
flowering and seed formation is complete. ABA plays a role in seed maturation, at least in some species, and also enforces a period of seed dormancy. As we saw for buds, it
is important the seeds not germinate prematurely during unseasonably mild conditions prior to the onset of winter or a dry season. ABA in the seed enforces this dormancy.
Not until the seed has been exposed to a prolonged cold spell and/or sufficient water to support germination is dormancy lifted.
3. Abscission
ABA also promotes abscission of leaves and fruits (in contrast to auxin, which inhibits abscission). It is, in fact, this action that gave rise to the name abscisic acid.
The dropping of leaves in the autumn is a vital response to the onset of winter when ground water is frozen — and thus cannot support transpiration — and snow load would
threaten to break any branches still in leaf.
Most nondeciduous species in cold climates (e.g., pines) have "needles" for leaves. These are very narrow and have a heavy waterproof cuticle. The shape aids in shedding
snow, and the cuticle cuts down on water loss.
4. Seedling Growth
ABA inhibits the growth of seedlings.
5. Apical Dominance
ABA — moving up from the roots to the stem — synergizes with auxin — moving down from the apical meristem to the stem — in suppressing the development of lateral
buds. The result is inhibition of branching or apical dominance.
6. Closing of stomata
Some 90% of the water taken up by a plant is lost in transpiration. Most of this leaves the plant through the pores — called stomata — in the leaf. Each stoma is flanked by a
pair of guard cells. When the guard cells are turgid, the stoma is open. When turgor is lost, the stoma closes. Discussion of gas exchange in the leaf.
ABA is the hormone that triggers closing of the stomata when soil water is insufficient to keep up with transpiration.
The mechanism:
ABA binds to G-protein-coupled receptors at the surface of the plasma membrane of the guard cells as well as to other receptors in the cytosol.
Receptor activation produces
–
–
a rise in pH in the cytosol
transfer of Ca2+ from the vacuole and endoplasmic reticulum to the cytosol
These changes cause ion channels in the plasma membrane to open allowing the release of ions (Cl−, organic [e.g., malate2−], and K+) from the cell.
The loss of these solutes from the cytosol reduces the osmotic pressure of the cell and thus turgor.
The stomata close.
• The loss of leaves each autumn is an adaptation
that keeps deciduous trees from desiccating
during winter when roots cannot absorb water
from the frozen ground.
– Before leaves abscise, many essential elements are
salvaged from the dying leaves and stored in stem
parenchyma cells.
– These nutrients are recycled back to developing
leaves the following spring.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Abscission layer
that forms at the
base of a leaf is
controlled by a
rise in ethylene
and a lowering of
auxin
concentration
• In 1901, Dimitry Neljubow demonstrated that
the gas ethylene was the active factor which
caused leaves to drop from trees that were near
leaking gas mains.
– Plants produce ethylene in response to stresses such
as drought, flooding, mechanical pressure, injury,
and infection.
– Ethylene production also occurs during fruit
ripening and during programmed cell death.
– Ethylene is also produced in response to high
concentrations of externally applied auxins.
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• Ethylene instigates a seedling to perform a
growth maneuver called the triple response
that enables a seedling to circumvent an
obstacle. Thigmotropism – response to
mechanical stress
• Ethylene production is
induced by mechanical
stress on the stem tip.
• In the triple response, stem
1. elongation slows, the stem
2. thickens, and 3. curvature
causes the stem to start
growing horizontally. Fig. 39.13
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Adding antisense RNA
blocks the transcription
of Ethylene
Ethylene added
at the market to ripen
the tomato
Gravitropism
Root tip
Fig. 39.25
Statoliths may
redistribute the auxin
to the lower part
causing differential
rates of elongation on
either side of the
plant
• Plants may tell up from down by the settling of
statoliths, specialized plastids containing dense
starch grains, to the lower portions of cells.
– In one hypothesis, the aggregation of statoliths at
low points in cells of the root cap triggers the
redistribution of calcium, which in turn causes
lateral transport of auxin within the root.
– The high concentrations of auxin on the lower side
of the zone of elongation inhibits cell elongation,
slowing growth on that side and curving the root
downward.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The innerside cells of the pulvinus lose turgor pressure
causing the Mimosa leaves to close when stimulated
Fig. 39.27
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
If Red light is
flashed last the
plant will act like
it’s a long day
If FR light is
flashed last
the plant will
act like it’s a
short day
PHYTOCHROME
a photodector
homodimer -made of
twin protein molecules
each protein has two
domains
Regulates
cellular
responses
Strong sunlight favors
the production of Pfr
Sunlight or
During the night the
amount of Pfr decreases
and Pr increases
A strong spike of
Pfr in the morning
synchronizes the
plant’s biological
clock
The caterpillar
triggers the plant to
release an
attractant for the
parasitoid wasp
Hypersentive Response (HR)
enhances the production of
antimicrobial phytoalexins and
pathogenesis related proteins (PR)
which ‘seals’ the cell wall to set up a
barricade to slow infection and then
the cells kill themselves
A signal molecule
like salicylic acid
will trigger
Systemic
Acquired
Resistance (SAR)
in healthy leaves