PLANT GROWTH REGULATORS
Plant Growth Regulators
AKA Plant Hormones
Plant Growth Regulators - control
growth, development and movement
Plant growth hormones and
regulators
Hormones: Naturally occuring
Regulators: Synthetic eg: 2,4-D (2,4Dichlorophenoxyacetic acid) and Kinetin
Growth:
It may be defined simply as an irreversible
increase in mass due to the division and enlargement of cells,
and may be applied to an organism as a whole or to any of its
parts.
Many plants, such as radishes and pumpkins, go through
a sequence of growth stages. They grow rapidly at first, then
for a while they show little, if any, increase in volume, and
eventually, they stop growing completely. Finally, tissues
break down, and the plant dies. Such growth is said to be
determinate. Parts of other plants may exhibit
indeterminate
growth and continue to be active for several to many years.
After cells have enlarged, differentiation
occurs; that is, the cells develop different forms
adapted to specific functions, such as
conduction, support, or secretion of special
substances
The coordination of growth and differentiation
of a single cell into multicellular tissues and
organs is called development.
What regulates development? What determines where and when growth and
differentiation occur?
The answers to these questions lie in the organism’s genes and its
environment.
In the 1950s, F. C. Steward and his colleagues
at Cornell University illustrated this concept with a tissue
culture experiment. They isolated cells from a carrot
root and placed them in jars containing nutrients. The cells
divided, grew, and developed into embryo-like structures.
The embryos eventually grew into new carrot plants, demonstrating
that a cell has all the genes necessary to form a
new individual.
Certain genes are programmed to synthesize specific enzymes
continually in every living cell, while other genes produce enzymes only in specific
tissues or at specific times during a plant’s life cycle. The environment determines
which genes will be expressed. In an organ or an organism, the environment
includes both external and internal factors. The internal environment includes
nutrients, vitamins, and hormones. The external environment includes water,
minerals, gases, light, and temperature, among many factors.
Nutrients are substances that furnish the
elements and energy for the organic
molecules that are the building blocks
by which an organism develops
Vitamins play an important role in reactions
catalyzed by enzymes: coenzymes or parts
of coenzymes
Genes also dictate the production of hormones, which influence
many developmental phenomena.
1 produced mostly in actively growing regions of plants
2 They are organic substances that differ from enzymes in structure.
3 active in far smalleramounts than vitamins and enzymes.
4 Hormones ordinarily are transported from their point of origin to
another part of the plant where they have specific effects, such as
causing stems to bend, initiating flowering, or even inhibiting
growth.
5 Because some of the effects of vitamins are similar to
those of hormones, they are sometimes difficult to distinguish
from one another.
6 The term growth regulator has been applied
to both natural and synthetic compounds
7 hormones act by chemically binding to specific receptors. The
hormone-receptor association triggers a series of biochemical
events, including turning genes on and off. The biochemical events
are called signal transduction
PLANT GROWTH REGULATORS
(PLANT HORMONES)
 Internal and external signals that regulate plant growth are
mediated, at least in part, by plant growth-regulating
substances, or hormones (from the Greek word hormaein,
meaning "to excite").
 Plant hormones differ from animal hormones in that:
 No evidence that the fundamental actions of plant and
animal hormones are the same.
 Unlike animal hormones, plant hormones are not made in
tissues specialized for hormone production. (e.g., sex
hormones made in the gonads, human growth hormone pituitary gland)
 Unlike animal hormones, plant hormones do not have
definite target areas (e.g., auxins can stimulate
adventitious root development in a cut shoot, or shoot
elongation or apical dominance, or differentiation of
vascular tissue, etc.).
PLANT GROWTH REGULATORS
 PLANT GROWTH REGULATORS ARE NECESSARY
FOR, BUT DO NOT CONTROL, MANY ASPECTS OF
PLANT GROWTH AND DEVELOPMENT. - BETTER
NAME IS GROWTH
REGULATOR.
 THE EFFECT ON PLANT PHYSIOLOGY IS DEPENDENT
ON THE AMOUNT OF
HORMONE PRESENT AND TISSUE SENSITIVITY TO
THE PLANT GROWTH REGULATOR
 substances produced in small quantities by a plant, and
then transported elsewhere for use
 have capacity to stimulate and/or inhibit physiological
processes
 at least five major plant hormones or plant growth
regulators:
 auxins, cytokinins, gibberellins, ethylene and abscisic acid
General plant hormones
 Auxins (cell elongation)
 Gibberellins (cell elongation + cell division translated into growth)
 Cytokinins (cell division + inhibits
senescence)
 Abscisic acid (abscission of leaves and
fruits + dormancy induction of buds and
seeds)
 Ethylene (promotes senescence, epinasty,
and fruit ripening)
EARLY EXPERIMENTS ON PHOTROPISM SHOWED
THAT A STIMULUS (LIGHT) RELEASED CHEMICALS
THAT INFLUENCED GROWTH
Results on growth of coleoptiles of canary grass and
oats suggested that the reception of light in the tip of the
shoot stimulated a bending toward light source.
Auxin
 Discovered as substance associated
with phototropic response.
 Occurs in very low concentrations.
Isolated from human urine, (40mg 33
gals-1)
In coleoptiles (1g 20,000 tons-1)
 Differential response depending on
dose.
Auxins
1881 Charles Darwin and his son Francis development of Coleoptile:
Metal foil blocked its growth towards light and the growth resumed after the
removal of the foil. Later it was found to be a water soluble influence.
1926 Frits went: placed cut coleoptile tips on agar, cut the agar into blocks at
that place and placed them onto cut tips of coleoptile. When placed in the
center the coleoptile grew straight and it bent when placed off center
Demonstration of transported chemical
Auxin
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Auxin increases the plasticity of plant cell walls and is involved in
stem elongation.
Arpad Paál (1919) - Asymmetrical placement of cut tips on
coleoptiles resulted in a bending of the coleoptile away from the side
onto which the tips were placed (response mimicked the response
seen in phototropism).
Frits Went (1926) determined auxin enhanced cell elongation.
Auxins
Auxins
1926 Frits went:His experiments showed that something moved out of the
Coleoptile tip into the agar. He called these substances as auxins (Greek to
increase)
1 First discovered hormones
2 At least three major groups apparently promote the growth of plants
(auxins, gibberellins, and cytokinins). Depending on the concentration, some
may also have an inhibitory effect.
3 Structure similar to tryptophan
4 Production of auxins occurs mainly in apical meristems,buds, young
leaves, and other active young parts of plants.
5 It is sometimes difficult to predict how cells will respond to auxins
because responses vary according to the concentration,location, and other
factors.
6 At appropriate concentrations, auxins normally stimulate the enlargement
of cells by increasing the plasticity (irreversible stretching) of cell walls.
7. Hormone concentrations can be measured by Gass chromatography
Auxins
Auxins also may have many other effects, including triggering the production
of other hormones or growth regulators (especially ethylene), causing the
dictyosomes to increase rates of secretion, playing a role in controlling some
phases of respiration, and influencing many developmental aspects of
growth. Auxins promote cell enlargement and stem growth, cell division in
the cambium, initiation of roots, and differentiation of cell types. Auxins
delay developmental processes such as fruit and leaf abscission, as well
as fruit ripening, and inhibit lateral branching.
Sensitivity
to auxins is less in many monocots than it is in dicots, and
shoots are less sensitive than roots.
Auxin
• Auxin promotes activity of the vascular
cambium and vascular tissues.
– plays key role in fruit development
• Cell Elongation: Acid growth hypothesis
– auxin works by causing responsive cells
to actively transport hydrogen ions from
the cytoplasm into the cell wall space
Signal-transduction pathways
in plants
Auxin interacts with calcium ions which in turn calmodulin, a
protein, which regulates many processes in plants, animals, and
microbes.
Loosening of cell wall
Auxins
Movement of auxins from the cells where they
originate requires the expenditure of energy stored in
ATP molecules. It is slow, polar i.e. from stem tip
(source) toward the base even when the stem is
inverted. Movement is from cell to cell of the
parenchyma cells of the vascular bundles and not
phloem sieve tubes
Polar transport of Auxin
Auxin
• Synthetic auxins
widely used in agriculture and horticulture
prevent leaf abscission
prevent fruit drop
promote flowering and fruiting
control weeds
Agent Orange - 1:1 ratio of 2,4-D and 2,4,5T used to defoliate trees in Vietnam War.
Dioxin usually contaminates 2,4,5-T, which is linked to
miscarriages, birth defects,leukemia, and other types of
cancer.
Auxins
indoleacetic acid (IAA)
phenylacetic acid (PAA)
4-chloroindoleacetic acid (4-chloroIAA), is found in germinating seeds of legumes.
indolebutyric acid (IBA), occurs in the leaves of corn and various dicots.
Synthetic growth regulators presently in use include NAA (naphthalene acetic acid), 2,4-D (2,4dichlorophenoxyacetic acid), and MCPA (2-methyl-4-chlorophenoxyacetic acid).
Additional responses to auxin
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abscission - loss of leaves
flower initiation
sex determination
fruit development
apical dominance
Control of abscission by auxin
Apical Dominance
Lateral branch
growth are inhibited
near the shoot apex,
but less so farther
from the tip.
Apical dominance is
disrupted in some
plants by removing
the shoot tip, causing
the plant to become
bushy.
Apical dominance is the suppression of the growth of the lateral buds (also called
axillary buds), each of which can form a branch with its own terminal bud. Apical
dominance is believed to be brought about by an auxinlike inhibitor in a terminal
bud. It is strong in trees with conical shapes and little branching toward the top
(e.g., many pines, spruces, firs) (Fig. 11.6) and weak in trees that branch more
often (e.g., elms, ashes, willows).
Senescence The breakdown of cell components and membranes that eventually
leads to the death of the cell is called senescence.The leaves of deciduous trees
and shrubs senesce and drop through a process of abscission every year.
both ABA and ethylene promote senescence. On the other hand, auxins,
gibberellins, and cytokinins delay senescence in a number of plants that have
been studied. Other factors, such as nitrogen deficiency and drought, also hasten
senescence.
Orchardists spray fruit trees with auxins to promote
uniform flowering and fruit set, and they later spray
the fruit to prevent the formation of abscission layers
and subsequent premature fruit drop.
If auxins are applied to flowers before pollination
occurs, seedless fruit can be formed and developed.
Some orchardists use auxins for controlling the
number of fruits that will mature in order not to have
too many small ones, and some even control the
shapes of plants for sales appeal.
synthetic auxins, including 2,4-D, 2,4,5-T, 2,4,5-TP,
NAA, and MCPA, when sprayed at low
concentrations, kill some weeds.
Gibberellin
Gibberellins
• Gibberellins are named after the
fungus Gibberella fujikuroi which
causes rice plants to grow abnormally
tall.
– synthesized in apical portions of stems
and roots
– important effects on stem elongation
– in some cases, hastens seed germination
Gibberellins
1926, Eiichi Kurosawa of Japan reported the
discoveryof a new substance that was causing what
was referred to as bakanae or “foolish seedling,” a
serious disease of rice. Rice grains twice in size but
weakened stems. Substances extracted and
crystallized from this fungus (Gibberella fujikuroi) was
,
called as gibberellin.
There are now more than 110 known gibberellins,
customarily abbreviated to GA. Each form of GA is
identified by a subscript (e.g., GA6). They have been
isolated from immature seeds (especially those of dicots),
root and shoot tips, young leaves, and fungi.
Discovered in association with In 1930's, bakanae
or foolish seedling disease of rice (Gibberella
fujikuroi)
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•
•
In 1930's, Ewiti Kurosawa and
colleagues were studying plants
suffering from bakanae, or "foolish
seedling" disease in rice.
Disease caused by fungus called,
Gibberella fujikuroi, which was
stimulating cell elongation and
division.
Compound secreted by fungus could
cause bakanae disease in
uninfected plants. Kurosawa named
this compound gibberellin.
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–
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Gibberella fujikuroi also causes stalk
rot in corn, sorghum and other
plants.
Secondary metabolites produced by
the fungus include mycotoxins, like
fumonisin, which when ingested by
horses can cause equine
leukoencephalomalacia - necrotic
brain or crazy horse or hole in the
head disease.
Fumonisin is considered to be a
carcinogen.
Gibberellins
No single plant species has thus far been found to
have more than 15 kinds of GA, which probably also occur
in algae, mosses, and ferns.
Acetyl coenzyme A, which is vital to the process of
respiration, functions as a precursor in the synthesis of GA.
Interest in the hormonal properties of GA is high, for their
ability to increase growth in plants is far more impressive
than that of auxin alone, although traces of auxin apparently
need to be present for GA to produce its maximum effects.
e.g. monocots and dicots
Gibberellins
Gibberellins not only dramatically increase stem
growth, but they are also involved in nearly all of the same
regulatory processes in plant development as auxins. Flowering, the
dormancy of buds and seeds
can be broken, lower the threshold of growth; that is, plants may start
growing at lower temperatures
In some varieties of lettuce and cereals, the seeds normally
germinate only after being subjected to a period of
cold temperatures. An application of GA can eliminate the
cold requirement for germination. It is common for seedlings to
produce juvenile foliage at first and then different adult foliage as the
plant matures. In such plants, GA treatment of buds that would
normally develop into adult branches causes the foliage to develop in
juvenile form.
Effects of Gibberellins
• Cell elongation.
• GA induces cellular division and cellular elongation; auxin
induces cellular elongation alone.
• GA-stimulated elongation does not involve the cell wall
acidification characteristic of auxin-induced elongation
• Breaking of dormancy in buds and seeds.
• Seed Germination - Especially in cereal grasses, like
barley. Not necessarily as critical in dicot seeds.
• Promotion of flowering.
• Transport is non-polar, bidirectional producing general
responses.
Gibberellins
Several growth retardants available commercially
inhibit or block the synthesis of GA. Applications of growth
retardants result in stunted plants because stem elongation
is inhibited. When applied to certain commercially grown
crops such as chrysanthemums, flowers with thicker,
stronger stalks are produced.
GA has been used experimentally to increase yields of
sugar cane and hops
In seedless grapes: by increasing the size of the fruit and
lengthening fruit internodes, which results in slightly wider
spaces between grapes in the bunches. Better air circulation
between grapes reduces their susceptibility to fungal
diseases,and the need for hand thinning is eliminated.
Gibberellins:
GA has been used in navel orange orchards to delay the aging
of the fruit’s skin and in celery to increase the length and
crispness of petioles (the parts that are most in demand as a
raw food). GA is now used to increase seed production in
conifers and to increase the rate of malting in breweries by
enhancing starch digestion.
Gibberellins and Fruit Size
• Fruit Formation - "Thompson Seedless"
grapes grown in California are treated with
GA to increase size and decrease packing.
Wild Radish – Rosette & Bolt
A FLOWERING ANNUAL
YEAR ONE
YEAR ONE
Common Mullen – Rosette & Bolt
A FLOWERING BIENNIAL
YEAR ONE
YEAR TWO
Mobilization of reserves
Cytokinins
Cytokinins:
1913, Gottlieb Haberlandt of Germany discovered an
unidentified chemical in the phloem of various plants. This
chemical stimulated cell division and initiated the production
of cork cambium. In 1941, Johannes van Overbeek discovered
that coconut “milk” (a liquid endosperm) contained something
that increased thegrowth rate of tissues and embryos.
In 1955, a substance named kinetin (isolated from heat
killed sperms) was found, in the presence of auxin, to
stimulate the proliferation of parenchyma cells in tobacco pith.
It demonstrated that a simple chemical could promote cell
Division.
In 1964, the identity of a substance very similar to
kinetin was also determined in kernels of corn. These various
stimulants to cell division came to be known as cytokinins.
Cytokinins:
They are similar to adenine. Have been found only in RNA.
Cytokinins are synthesized in root tips and germinating seeds.
If auxin is present during the cell cycle, cytokinins promote
cell division by speeding up the progression from the
G phase to the mitosis phase, but no such effect takes place
in the absence of auxin.
Cytokinins also play a role in the enlarging of cells, the
differentiation of tissues, the development of chloroplasts, the
stimulation of cotyledon growth, the delay of aging in leaves,
and in many of the growth phenomena also brought about by
auxins and gibberellins. Cytokinins move throughout plants
via the xylem, phloem, and parenchyma cells.
2
Cytokinins:
Experiments have shown that cytokinins prolong the life
of vegetables in storage. Related synthetic compounds have
been used extensively to regulate the height of ornamental
shrubs and to keep harvested lettuce and mushrooms fresh.
They also have been used to shorten the straw length in
wheat, so as to minimize the chances of the plants blowing
over in the wind, and to lengthen the life of cut flowers. Many
have not yet been approved for general agricultural use.
Discovery of cytokinins
•
Gottlieb Haberlandt in 1913 reported an unknown compound that stimulated cellular
division.
•
In the 1940s, Johannes van Overbeek, noted that plant embryos grew faster when they
were supplied with coconut milk (liquid endosperm), which is rich in nucleic acids.
•
In the 1950s, Folke Skoog and Carlos Miller studying the influence of auxin on the growth
of tobacco in tissue culture. When auxin was added to artificial medium, the cells
enlarged but did not divide. Miller took herring-sperm DNA. Miller knew of Overbeek's
work, and decided to add this to the culture medium, the tobacco cells started dividing.
He repeated this experiment with fresh herring-sperm DNA, but the results were not
repeated. Only old DNA seemed to work. Miller later discovered that adding the purine
base of DNA (adenine) would cause the cells to divide.
•
Adenine or adenine-like compounds induce cell division in plant tissue culture. Miller,
Skoog and their coworkers isolated the growth facto responsible for cellular division from
a DNA preparation calling it kinetin which belongs to a class of compounds called
cytokinins.
•
In 1964, the first naturally occurring cytokinin was isolated from corn called zeatin. Zeatin
and zeatin riboside are found in coconut milk. All cytokinins (artificial or natural) are
chemically similar to adenine.
•
•
Cytokinins move nonpolarly in xylem, phloem, and parenchyma cells.
Cytokinins are found in angiosperms, gymnosperms, mosses, and ferns. In angiosperms,
cytokinins are produced in the roots, seeds, fruits, and young leaves
Function of cytokinins
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Promotes cell division.
Morphogenesis.
Lateral bud development.
Delay of senescence.
Cytokinins
• Cytokinins, in combination with auxin,
stimulate cell division and
differentiation.
– most cytokinin produced in root apical
meristems and transported throughout
plant
• inhibit formation of lateral roots
– auxins promote their formation
Cytokinins
Interaction of cytokinin and auxin in tobacco callus
(undifferentiated plant cells) tissue
Organogenesis: Cytokinins and auxin affect organogenesis
High cytokinin/auxin ratios favor the formation of shoots
Low cytokinin/auxin ratios favor the formation of roots.
Abscisic acid
In 1940s, scientists started searching for hormones that would inhibit growth
and development, what Hemberg called dormins.
In the early 1960s, Philip Wareing confirmed that application of a dormin to
a bud would induce dormancy.
F.T. Addicott discovered that this substance stimulated abscission of cotton
fruit. he named this substance abscisin. (Subsequent research showed that
ethylene and not abscisin controls abscission).
Abscisin is made from carotenoids and moves nonpolarly through plant
tissue.
Functions of abscisic acid
 General growth inhibitor.
 Causes stomatal closure.
 Produced in response to stress.
Abscisic Acid
• Abscisic acid is produced chiefly in
mature green leaves and in fruits.
– suppresses bud growth and promotes
leaf senescence
– also plays important role in controlling
stomatal opening and closing
Abscisic Acid:
Torsten Hemberg, a Swedish botanist, showed that
substances produced in dormant buds (i.e., buds whose
development is temporarily arrested) blocked the effects
of auxins. He called these growth inhibitors dormins.
In 1963, three groups of investigators working
independently
in the United States, Great Britain, and New Zealand
discovered a growth-inhibiting hormone, which was in
1967
officially called abscisic acid (ABA). Later, it was shown
that ABA and dormins were the same compound.
Abscisic Acid:
ABA is synthesized in plastids, apparently from
carotenoid pigments.
Found in fleshy fruits.
Inhibits/prevents seeds from germinating
other hormones are inhibited by ABA
ABA was originally believed to promote the formation of
abscission layers in leaves and fruits. However, the
evidence suggesting that ethylene is far more important
than ABA in abscission
Abscisic Acid:
ABA apparently helps leaves respond to excessive
waterloss. When the leaves wilt, ABA is produced in
amounts several times greater than usual. This
interferes with the transportor retention of potassium
ions in the guard cells, causing the
stomata to close. When the uptake of water again
becomes sufficient for the leaf’s needs, the ABA breaks
down, and the stomata reopen. ABA produced in times
of drought is transported from the shoot to the root and
causes an increase in both root growth and water
uptake.
Discovery of ethylene
 In the 1800s, it was recognized that street lights that
burned gas, could cause neighboring plants to
develop short, thick stems and cause the leaves to
fall off. In 1901, Dimitry Neljubow identified that a
byproduct of gas combustion was ethylene gas and
that this gas could affect plant growth.
 In R. Gane showed that this same gas was naturally
produced by plants and that it caused faster ripening
of many fruits.
 Synthesis of ethylene is inhibited by carbon dioxide
and requires oxygen.
Ethylene
H
H
\
/
C = C
/
\
H
H
Ethylene:
In 1901, Dimitry Neljubow, a Russian student at the
St. Petersburg Botanical Institute, discovered that pea seedlings
grown in the dark in a laboratory showed reduced
stem elongation, increased swelling of stems, and abnormal
horizontal growth. When the seedlings were placed outside
in fresh air, however, they resumed normal growth due to gas
ethylene, produced by gas lamps in the laboratory.
In 1934, R. Gane discovered that ethylene was produced
naturally by fruits, although it had been known for some
time that the ripening of green fruits could be accelerated
artificially by placing them in ethylene. It is now known that
ethylene is produced not only by fruits, flowers,
seeds, leaves, and even roots.
The production of ethylene by plant tissues varies considerably
under different conditions
Ethylene:
In 1901, Dimitry Neljubow, a Russian student at the
St. Petersburg Botanical Institute, discovered that pea seedlings
grown in the dark in a laboratory showed reduced
stem elongation, increased swelling of stems, and abnormal
horizontal growth. When the seedlings were placed outside
in fresh air, however, they resumed normal growth due to gas
ethylene, produced by gas lamps in the laboratory.
In 1934, R. Gane discovered that ethylene was produced
naturally by fruits, although it had been known for some
time that the ripening of green fruits could be accelerated
artificially by placing them in ethylene. It is now known that
ethylene is produced not only by fruits, flowers,
seeds, leaves, and even roots.
The production of ethylene by plant tissues varies considerably
under different conditions
Ethylene:
When a storm blows across a green field or animals pass across it,
plants in the field respond to these and other mechan- ical
stresses with an increase in the production of fibers and
collenchyma tissue. Cell elongation is also inhibited, resulting in
shorter, sturdier plants. The responses, called thigmomorphogenesis, are under the control of genes that are activated by
touch. Several substances, including enzymes, ethylene, and a
protein called calmodulin, are involved.
Ethylene
Today, the commercial use of ethylene is extensive. It has been used for many
years to ripen harvested green fruits, such as bananas, mangoes, and honeydew
melons, and to cause citrus fruits to color up before marketing. Since ethylene
pro- duction almost ceases in the absence of oxygen, apple and other fruit
growers have found that if they place unripe fruit in sealed warehouses after
harvest, reduce temperatures to just above freezing, and replace the air with inert
nitrogen gas or carbon dioxide, the fruit will remain metabolically inactive for long
periods. The growers can then remove the fruit in batches throughout the year,
add as little as one part per million of eth- ylene, and have ripe fruit at any time
there is a demand. This is why apples are always available in supermarkets, even
though harvesting is usually confined to a few weeks in the fall.
A number of compounds called oligosaccharins, which are released from cell
walls by enzymes, influence cell d ifferentiation, reproduction, and growth in
plants and there- fore must be considered hormones. However, oligosaccharins
are 1000 times less effective than other growth regulators
Functions of ethylene
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Gaseous in form and rapidly diffusing.
Gas produced by one plant will affect nearby plants.
Fruit ripening.
Epinasty – downward curvature of leaves.
Encourages senescence and abscission.
Initiation of stem elongation and bud development.
Flowering - Ethylene inhibits flowering in most
species, but promotes it in a few plants such as
pineapple, bromeliads, and mango.
 Sex Expression - Cucumber buds treated with ethylene become carpellate
(female) flowers, whereas those treated with gibberellins become staminate
(male) flowers.
HOW PLANTS RESPOND TO
ENVIRONMENTAL STIMULI
• Tropisms - plant growth toward or
away from a stimulus such as light
or gravity.
• Nastic Movements - response to
environmental stimuli that are
independent of the direction of the
stimulus. Pre-determined response.
Tropic responses
Directional movements by growth in
response to a directional stimulus
Phototropism
Growth movement
Phototropisms
• Phototropic responses involve bending
of growing stems toward light sources.
– Individual leaves may also display
phototrophic responses.
• auxin most likely involved
Plants Respond to Gravity
• Gravitropism is the response of a
plant to the earth’s gravitational field.
– present at germination
• auxins play primary role
– Four steps
•
•
•
•
gravity perceived by cell
signal formed that perceives gravity
signal transduced intra- and intercellularly
differential cell elongation
Gravitropism
• Increased auxin concentration on the lower side in
stems causes those cells to grow more than cells
on the upper side.
– stem bends up against the force of gravity
• negative gravitropism
• Upper side of roots oriented horizontally grow
more rapidly than the lower side
– roots ultimately grow downward
• positive gravitropism
Gravitropism = Geotropism
Statoliths
Plants Respond to Touch
• Thigmotropism is directional growth
response to contact with an object.
– tendrils
Thigmotropism
SEISMONASTY - a nastic response resulting
from contact or mechanical shaking
Mimosa pudica L. (sensitive plant)
Pulvinus of Mimosa pudica
Plants Response to Light
•
•
Photomorphogenesis
– nondirectional, light-mediated changes in plant growth and development
• red light changes the shape of phytochrome and can trigger
photomorphogenesis
• Stems go from etiolated (in dark or Pfr) to unetiolated (in light with
Pr).
Photoperiodism
– Regulates when seeds of lettue and some weeds. Presence of Pr
inhibits germination, while its conversion to Pfr in red light induces
germination
Red light ===> germination
Far-red light ===> no germination
Red ===> far-red ===> red ===> germination
Red ===> far-red ===> red ===> far-red ===> no germination
Those seeds not buried deep in the ground get exposed to red light, and
this signals germination.
– Regulates when plants flower; either in the Spring or later in the Summer
and Fall.
How Phytochrome Works
NYCTINASTY
• sleep movements
• prayer plant - lower
leaves during the
day and raises
leaves at night
• shamrock (Oxalis)
• legumes
Credit:(http://employees.csbsju.edu/ssa
upe/biol327/Lab/movie/movies.htm)
Circadian Clocks
• Circadian clocks are endogenous
timekeepers that keep plant
responses synchronized with the
environment.
– circadian rhythm characteristics
• must continue to run in absence of external inputs
• must be about 24 hours in duration
• can be reset or entrained (to determine or modify the
phase or period of <circadian rhythms entrained by a
light cycle>)
• can compensate for temperature differences