Chapter 39 Plant Responses to Internal and External Signals
Overview: Stimuli and a Stationary Life
• At every stage in the life of a plant, sensitivity to the environment and
coordination of responses are evident.
• One part of a plant can send signals to other parts.
• Plants can sense gravity and the direction of light.
• A plant’s morphology and physiology are constantly tuned to its
variable surroundings by complex interactions between
environmental stimuli and internal signals.
• At the organismal level, plants and animals respond to environmental
stimuli by very different means.
• Animals, being mobile, respond mainly by behavioral
mechanisms, moving toward positive stimuli and away from
• Rooted in one location for life, a plant generally responds to
environmental cues by adjusting its pattern of growth and
• As a result, plants of the same species vary in body form
much more than do animals of the same species.
• At the cellular level, plants and all other eukaryotes are
surprisingly similar in their signaling mechanisms.
Concept 39.1 Signal transduction pathways link signal reception to
• All organisms, including plants, have the ability to receive specific
environmental and internal signals and respond to them in ways that
enhance survival and reproductive success.
• Like animals, plants have cellular receptors that they use to
detect important changes in their environment. • These changes may be an increase in the concentration
of a growth hormone, an injury from a caterpillar
munching on leaves, or a decrease in day length as
• In order for an internal or external stimulus to elicit a physiological
response, certain cells in the organism must possess an appropriate
receptor, a molecule that is sensitive to and affected by the specific
• Upon receiving a stimulus, a receptor initiates a specific series
of biochemical steps, a signal transduction pathway.
• This couples reception of the stimulus to the response of
• Plants are sensitive to a wide range of internal and external stimuli,
and each of these initiates a specific signal transduction pathway.
• Plant growth patterns vary dramatically in the presence versus the
absence of light.
• For example, a potato (a modified underground stem) can
sprout shoots from its “eyes” (axillary buds).
• These shoots are ghostly pale and have long, thin stems;
unexpanded leaves; and reduced roots.
• These morphological adaptations, called etiolation, are seen also in
seedlings germinated in the dark and make sense for plants sprouting
• The shoot is supported by the surrounding soil and does not
need a thick stem.
• Expanded leaves would hinder soil penetration and be
damaged as the shoot pushes upward.
• Because little water is lost in transpiration, an extensive root
system is not required.
• The production of chlorophyll is unnecessary in the absence of
light. • A plant growing in the dark allocates as much energy as
possible to the elongation of stems to break ground before the
nutrient reserves in the tuber are exhausted.
• Once a shoot reaches the sunlight, its morphology and biochemistry
undergo profound changes, collectively called de-etiolation, or
• The elongation rate of the stems slows.
• The leaves expand, and the roots start to elongate.
• The entire shoot begins to produce chlorophyll.
• The de-etiolation response is an example of how a plant receives a
signal—in this case, light—and how this reception is transduced into
a response (de-etiolation).
• Studies of mutants have provided valuable insights into the
roles played by various molecules in the three stages of cell-
signal processing: reception, transduction, and response.
• Signals, whether internal or external, are first detected by receptors,
proteins that change shape in response to a specific stimulus.
• The receptor for de-etiolation in plants is called a phytochrome,
which consists of a light-absorbing pigment attached to a
• Unlike many receptors, which are in the plasma
membrane, this phytochrome is in the cytoplasm.
• The importance of this phytochrome was confirmed through
investigations of a tomato mutant, called aurea, which greens
less when exposed to light.
• Injecting additional phytochrome into aurea leaf cells and
exposing them to light produced a normal de-etiolation
• Receptors such as phytochrome are sensitive to very weak
environmental and chemical signals. • For example, just a few seconds of moonlight slow stem
elongation in dark-grown oak seedlings.
• These weak signals are amplified by second messengers—
small, internally produced chemicals that transfer and amplify
the signal from the receptor to proteins that cause the specific
• In the de-etiolation response, each activated phytochrome may
give rise to hundreds of molecules of a second messenger,
each of which may lead to the activation of hundreds of
molecules of a specific enzyme.
• Light causes phytochrome to undergo a conformational change that
leads to increases in levels of the second messengers’ cyclic GMP
(cGMP) and Ca2+.
• Changes in cGMP levels can lead to ionic changes within the cell by
influencing properties of ion channels.
• Cyclic GMP also activates specific protein kinases, enzymes
that phosphorylate and activate other proteins.
• The microinjection of cyclic GMP into aurea tomato cells
induces a partial de-etiolation response, even without the
addition of phytochrome.
• Changes in cytosolic Ca2+ levels also play an important role in
phytochrome signal transduction.
• The concentration of Ca2+ is generally very low in the
• Phytochrome activation can open Ca2+ channels and lead to
transient 100-fold increases in cytosolic Ca2+.
• Ultimately, a signal transduction pathway leads to the regulation of
one or more cellular activities.
• In most cases, these responses to stimulation involve the
increased activity of certain enzymes. • This occurs through two mechanisms: by stimulating
transcription of mRNA for the enzyme or by activating existing
enzyme molecules (post-translational modification).
• In transcriptional regulation, transcription factors bind directly to
specific regions of DNA and control the transcription of specific
• In the case of phytochrome-induced de-etiolation, several
transcription factors are activated by phosphorylation, some
through the cyclic GMP pathway, while activation of others
• The mechanism by which a signal promotes a new
developmental course may depend on the activation of positive
transcription factors (proteins that increase transcription of
specific genes) or negative transcription factors (proteins that
• During post-translational modifications of proteins, the activities of
existing proteins are modified.
• In most cases, these modifications involve phosphorylation, the
addition of a phosphate group onto the protein by a protein
• Many second messengers, such as cyclic GMP, and some
receptors, including some phytochromes, activate protein
• One protein kinase can phosphorylate other protein kinases,
creating a kinase cascade, finally leading to phosphorylation of
transcription factors and impacting gene expression.
• Thus, they regulate the synthesis of new proteins, usually
by turning specific genes on and off.
• Signal pathways must also have a means for turning off once the
initial signal is no longer present.
• Protein phosphatases, enzymes that dephosphorylate specific
proteins, are involved in these “switch off” processes. • At any given moment, the activities of a cell depend on the
balance of activity of many types of protein kinases and protein
• During the de-etiolation response, a variety of proteins are either
synthesized or activated.
• These include enzymes that function in photosynthesis directly
or that supply the chemical precursors for chlorophyll
• Others affect the levels of plant hormones that regulate growth.
• For example, the levels of two hormones (auxin and
brassinosteroids) that enhance stem elongation will
decrease following phytochrome activation—hence, the
reduction in stem elongation that accompanies de-
Concept 39.2 Plant hormones help coordinate growth, development,
and responses to stimuli
• The word hormone is derived from a Greek verb meaning “to excite.”
• Found in all multicellular organisms, hormones are chemical signals
that are produced in one part of the body, transported to other parts,
bind to specific receptors, and trigger responses in target cells and
• Only minute quantities of hormones are necessary to induce
substantial change in an organism.
• Hormone concentration or rate of transport can change in
response to environmental stimuli.
• Often the response of a plant is governed by the interaction of
two or more hormones.
Research on how plants grow toward light led to the discovery of
• The concept of chemical messengers in plants emerged from a series
of classic experiments on how stems respond to light. • Plants grow toward light, and if you rotate a plant, it will reorient
its growth until its leaves again face the light.
• Any growth response that results in curvature of whole plant
organs toward or away from stimuli is called a tropism.
• The growth of a shoot toward light is called positive
phototropism; growth away from light is negative phototropism.
• Much of what is known about phototropism has been learned from
studies of grass seedlings, particularly oats.
• The shoot of a grass seedling is enclosed in a sheath called the
coleoptile, which grows straight upward if kept in the dark or
illuminated uniformly from all sides.
• If it is illuminated from one side, it will curve toward the light as
a result of differential growth of cells on opposite sides of the
• The cells on the darker side elongate faster than the cells
on the brighter side.
• In the late 19th century, Charles Darwin and his son Francis
observed that a grass seedling bent toward light only if the tip of the
coleoptile was present.
• This response stopped if the tip was removed or covered with
an opaque cap (but not a transparent cap).
• While the tip was responsible for sensing light, the actual
growth response occurred some distance below the tip, leading
the Darwins to postulate that some signal was transmitted from
the tip downward.
• Later, Peter Boysen-Jensen demonstrated that the signal was a
mobile chemical substance.
• He separated the tip from the remainder of the coleoptile by a
block of gelatin, preventing cellular contact, but allowing
chemicals to pass.
• These seedlings were phototropic. • However, if the tip was segregated from the lower coleoptile by
an impermeable barrier, no phototropic response occurred.
• In 1926, Frits Went extracted the chemical messenger for
phototropism, naming it auxin.
• Modifying the Boysen-Jensen experiment, he placed excised tips on
agar blocks, collecting the hormone.
• If an agar block with this substance was centered on a
coleoptile without a tip, the plant grew straight upward.
• If the block was placed on one side, the plant began to bend
away from the agar block.
• The classical hypothesis for what causes grass coleoptiles to grow
toward light, based on the previous research, is that an asymmetrical
distribution of auxin moving down from the coleoptile tip causes cells
on the dark side to elongate faster than cells on the brighter side.
• However, studies of phototropism by organs other than grass
coleoptiles provide less support for this idea.
• There is, however, an asymmetrical distribution of certain
substances that may act as growth inhibitors, with these
substances more concentrated on the lighted side of a stem.
Plant hormones help coordinate growth, development, and responses
to environmental stimuli.
• In general, plant hormones control plant growth and development by
affecting the division, elongation, and differentiation of cells.
• Some hormones also mediate shorter-term physiological
responses of plants to environmental stimuli.
• Each hormone has multiple effects, depending on its site of
action, its concentration, and the developmental stage of the
• Some of the major classes of plant hormones include auxin,
cytokinins, gibberellins, brassinosteroids, abscisic acid, and ethylene. • Many molecules that function in plant defenses against
pathogens are probably plant hormones as well.
• Plant hormones tend to be relatively small molecules that are
transported from cell to cell across cell walls, a pathway that
blocks the movement of large molecules.
• 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, affecting the
activity of existing enzymes, or changing the properties of
• Response to a hormone usually depends not so much on its absolute
concentration as on its relative concentration compared to other
• It is hormonal balance, rather than hormones acting in isolation,
that control growth and development of the plants.
• The term auxin is used for any chemical substance that promotes the
elongation of coleoptiles, although auxins actually have multiple
functions in both monocots and dicots.
• The natural auxin occurring in plants is indoleacetic acid, or
• In growing shoots, auxin is transported unidirectionally, from the
shoot apex down to the base.
• The speed at which auxin is transported down the stem from
the shoot apex is about 10 mm/hr, a rate that is too fast for
diffusion, but slower than translocation in the phloem.
• Auxin seems to be transported directly through parenchyma
tissue, from one cell to the next.
• This unidirectional transport of auxin is called polar transport,
and has nothing to do with gravity. • Auxin travels upward if a stem or coleoptile is placed
• The polarity of auxin transport is due to the polar distribution of
auxin transport protein in the cells.
• Concentrated at the basal end of the cells, auxin transporters
move the hormone out of the cell and into the apical end of the
• Although auxin affects several aspects of plant development, one of
its chief functions is to stimulate the elongation of cells in young
• The apical meristem of a shoot is a major site of auxin
• As auxin moves from the apex down to the region of cell
elongation, the hormone stimulates cell growth, binding to a
receptor in the plasma membrane.
• Auxin stimulates cell growth only over a certain concentration
range, from about 10?8 to 10?4 M.
• At higher concentrations, auxins may inhibit cell elongation,
probably by inducing production of ethylene, a hormone that
generally acts as an inhibitor of elongation.
• 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 and other cell wall
constituents, loosening the wall.
• Increasing the membrane potential enhances ion uptake into
the cell, which causes the osmotic uptake of water.
• Uptake of water increases turgor and elongates the loose-
walled cell. • Auxin also alters gene expression rapidly, causing cells in the region
of elongation to produce new proteins within minutes.
• Some of these proteins are short-lived transcription factors that
repress or activate the expression of other genes.
• Auxin stimulates a sustained growth response of making the
additional cytoplasm and wall material required by elongation.
• Auxins are used commercially in the vegetative propagation of plants
• Treating a detached leaf or stem with rooting powder containing
auxin often causes adventitious roots to form near the cut
• Auxin is also involved in the branching of roots.
• One Arabidopsis mutant that exhibits extreme
proliferation of lateral roots has an auxin concentration
17-fold higher than normal.
• 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.
• Auxin also affects secondary growth by inducing cell division in the
vascular cambium and by influencing the growth of secondary xylem.
• Developing seeds synthesize auxin, which promotes the growth of
• Synthetic auxins sprayed on tomato vines induce development
of seedless tomatoes because the synthetic auxins substitute
for the auxin normally synthesized by the developing seeds.
• Cytokinins stimulate cytokinesis, or cell division. • They were originally discovered in the 1940s by Johannes van
Overbeek, who found that he could stimulate the growth of
plant embryos by adding coconut milk to his culture medium.
• A decade later, Folke Skoog and Carlos O. Miller induced
cultured tobacco cells to divide by adding degraded samples of
• The active ingredients in both were modified forms of adenine,
one of the components of nucleic acids.
• These growth regulators were named cytokinins because they
• The most common naturally occurring cytokinin is zeatin, named from
the maize (Zea mays) in which it was found.
• Much remains to be learned about cytokinin synthesis and signal
• Cytokinins are produced in actively growing tissues, particularly in
roots, embryos, and fruits.
• Cytokinins produced in the root reach their target tissues by
moving up the plant in the xylem sap.
• Cytokinins interact with auxins to stimulate cell division and
• In the absence of cytokinins, a piece of parenchyma tissue
grows large, but the cells do not divide.
• In the presence of cytokinins and auxins, the cells divide, while
cytokinins alone have no effect.
• If the ratio of cytokinins and auxins is at a specific level,
then the mass of growing cells, called a callus, remains
• If cytokinin levels are raised, shoot buds form from the
• If auxin levels are raised, roots form. • Cytokinins, auxins, and other factors interact in the control of apical
dominance, the ability of the terminal bud to suppress the
development of axillary buds.
• Until recently, the leading hypothesis for the role of hormones in
apical dominance—the direct inhibition hypothesis—proposed
that auxin and cytokinin act antagonistically in regulating
axillary bud growth.
• Auxin levels would inhibit axillary bud growth, while cytokinins
would stimulate growth.
• Many observations are consistent with the direct inhibition
• If the terminal bud, the primary source of auxin, is removed, the
inhibition of axillary buds is removed and the plant becomes
• This can be inhibited by adding auxins to the cut surface.
• The direct inhibition hypothesis predicts that removing the primary
source of auxin should lead to a decrease in auxin levels in the
• However, experimental removal of the terminal shoot (decapitation)
has not demonstrated this.
• In fact, auxin levels actually increase in the axillary buds of
• Further research is necessary to uncover all pieces of this
• 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.
• A century ago, farmers in Asia noticed that some rice seedlings grew
so tall and spindly that they toppled over before they could mature
• 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.
• In the 1950s, researchers discovered that plants also make
gibberellins. Researchers have identified more than 100 different
• Typically each plant produces a much smaller number.
• Foolish rice seedlings, it seems, suffer from an overdose of
growth regulators normally found in lower concentrations.
• 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.
• 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 plants grow to
normal height. • However, if gibberellins are applied to normal plants, there is
often no response, perhaps because these plants are already
producing the optimal dose of the hormone.
• The most dramatic example of gibberellin-induced stem elongation is
bolting, the rapid formation of the floral stalk.
• In their vegetative state, some plants develop in a rosette form
with a body low to the ground with short internodes.
• As the plant switches to reproductive growth, a surge of
gibberellins induces internodes to elongate rapidly, which
elevates the floral buds that develop at the tips of the stems.
• In many plants, both auxin and gibberellins must be present for fruit
• 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.
• The embryo of a seed is a rich source of gibberellins.
• After hydration of the seed, the release of gibberellins from the
embryo signals the seed to break dormancy and germinate.
• Some seeds that require special environmental conditions to
germinate, such as exposure to light or cold temperatures, will
break dormancy if they are treated with gibberellins.
• Gibberellins support the growth of cereal seedlings by
stimulating the synthesis of digestive enzymes that mobilize
• First isolated from Brassica pollen in 1979, brassinosteroids are
steroids chemically similar to cholesterol and the sex hormones of
animals. • Brassinosteroids induce cell elongation and division in stem
segments and seedlings at concentrations as low as 10?12 M.
• They also retard leaf abscission and promote xylem
• Their effects are so qualitatively similar to those of auxin that it
took several years for plant physiologists to accept
brassinosteroids as nonauxin hormones.
• Joann Chory and her colleagues provided evidence from molecular
biology that brassinosteroids were plant hormones.
• An Arabidopsis mutant that has morphological features similar
to light-grown plants even when grown in the dark lacks
• This mutation affects a gene that normally codes for an enzyme
similar to one involved in steroid synthesis in mammalian cells.
• 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.
• 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 will germinate only when there are optimal
conditions of light, temperature, and moisture.
• Many types of dormant seeds will germinate when ABA is removed or
• 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.
• ABA is the primary internal signal that enables plants to withstand
• 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.”
• Mutants that are prone to wilting are often deficient in ABA
• In 1901, Dimitry Neljubow demonstrated that the gas ethylene was
the active factor that 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.
• Ethylene instigates a seedling to perform a growth maneuver called
the triple response that enables a seedling to circumvent an obstacle
as it grows through soil.
• Ethylene production is induced by mechanical stress on the stem tip.
• In the triple response, stem elongation slows, the stem thickens, and
curvature causes the stem to start growing horizontally.
• As the stem continues to grow horizontally, its tip touches upward
• If the probes continue to detect a solid object above, then
another pulse of ethylene is generated, and the stem continues
its horizontal progress.
• If upward probes detect no solid object, then ethylene
production decreases, and the stem resumes its normal upward
• It is ethylene, not the physical obstruction per se, that induces the
stem to grow horizontally.
• Normal seedlings growing free of all physical impediments will
undergo the triple response if ethylene is applied.
• Arabidopsis mutants with abnormal triple responses have been used
to investigate the signal transduction pathways leading to this
• Ethylene-insensitive (ein) mutants fail to undergo the triple
response after exposure to ethylene.
• Some lack a functional ethylene receptor. • Other mutants undergo the triple response in the absence of physical
• Some mutants (eto) produce ethylene at 20 times the normal
• Other mutants, called constitutive triple-response (ctr) mutants,
undergo the triple response in air but do not respond to
inhibitors of ethylene synthesis.
• Ethylene signal transduction is permanently turned on
even though there is no ethylene present.
• The various ethylene signal-transduction mutants can be
distinguished by their different responses to experimental treatments.
• The affected gene in ctr mutants codes for a protein kinase.
• Because this mutation activates the ethylene response, this
suggests that the normal kinase product of the wild-type allele
is a negative regulator of ethylene signal transduction.
• One hypothesis proposes that binding of the hormone ethylene
to a receptor leads to inactivation of the kinase and inactivation
of this negative regulator allows synthesis of the proteins
required for the triple response.
• The cells, organs, and plants that are genetically programmed to die
on a particular schedule do not simply shut down their cellular
machinery and await death.
• Rather, during programmed cell death, called apoptosis, there
is active expression of new genes, which produce enzymes that
break down many chemical components, including chlorophyll,
DNA, RNA, proteins, and membrane lipids.
• A burst of ethylene productions is associated with apoptosis
whether it occurs during the shedding of leaves in autumn, the
death of an annual plant after flowering, or as the final step in
the differentiation of a xylem vessel element. • 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
• When an autumn leaf falls, the breaking point is an abscission layer
near the base of the petiole.
• The parenchyma cells here have very thin walls, and there are
no fiber cells around the vascular tissue.
• The abscission layer is further weakened when enzymes
hydrolyze polysaccharides in the cell walls.
• The weight of the leaf, with the help of the wind, causes a
separation within the abscission layer.
• A change in the balance of ethylene and auxin controls abscission.
• An aged leaf produces less and less auxin, and this makes the
cells of the abscission layer more sensitive to ethylene.
• As the influence of ethylene prevails, the cells in the abscission
layer produce enzymes that digest the cellulose and other
components of cell walls.
• The consumption of ripe fruits by animals helps disperse the seeds of
• Immature fruits are tart, hard, and green but become edible at
the time of seed maturation, triggered by a burst of ethylene
• Enzymatic breakdown of cell wall components softens the fruit,
and conversion of starches and acids to sugars makes the fruit
sweet. • The production of new scents and colors helps advertise fruits’
ripeness to animals, which eat the fruits and disperse the
• A chain reaction occurs during ripening: ethylene triggers ripening
and ripening, in turn, triggers even more ethylene production—a rare
example of positive feedback on physiology.
• Because ethylene is a gas, the signal to ripen even spreads
from fruit to fruit.
• Fruits can be ripened quickly by storing the fruit in a plastic bag,
accumulating ethylene gas, or by enhancing ethylene levels in
• Alternatively, to prevent premature ripening, apples are stored
in bins flushed with carbon dioxide, which prevents ethylene
from accumulating and inhibits the synthesis of new ethylene.
• Genetic engineering of ethylene signal transduction pathways has
potentially important commercial applications after harvest.
• For example, molecular biologists have blocked the
transcription of one of the genes required for ethylene synthesis
in tomato plants.
• These tomato fruits are picked while green and are induced to
ripen on demand when ethylene gas is added.
• Plant responses often involve interactions of many hormones and
their signal transduction pathways.
• The study of hormone interactions can be a complex problem.
• For example, flooding of deepwater rice leads to a 50-fold
increase in internal ethylene and a rapid increase in stem
• Flooding also leads to an increase in sensitivity to GA that
is mediated by a decrease in ABA levels.
• Thus, stem elongation is the result of interaction among
three hormones and their signal transduction chains. • Imagine that you are a molecular biologist assigned the task of
genetically engineering a rice plant that will grow faster when
• What is the best molecular target for genetic
manipulation? Is it an enzyme that inactivates ABA, an
ethylene receptor, or an enzyme that produces more GA?
• Many plant biologists are promoting a systems-b