Plant
Growth Notes
Introduction
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 targets cells
and tissues.
Introduction
Only minute quantities of hormones are
necessary to induce substantial change in an organism.
Often the response of a plant is governed
by the interaction of two or more hormones.
Plant hormones
Hormone was first used to describe
substances in animals
a substance produced in a gland that
circulates in the blood and has an effect far away from the site of production
In plants used to mean a compound that
acts at low concentrations to affect growth and development.
Plant hormones
Five plant hormones known by the mid
1960s, new compounds called plant growth regulators
Plant hormones
First plant hormone discovered was auxin,
the chemical responsible for photo- and gravitropic responses
The fact that a chemical was involved
demonstrated by Frits Went
The chemical itself was first isolated
from horse urine, it is indole acetic acid
Introduction
Hormones: organic molecules synthesized in one part of
an organism, transported to another part (target tissue), and exert an effect
on the target tissue.
Plant Hormones
Hormone action
Chemically bind to receptor
Hormone-receptor complex initiates a
series of biochemical events
Turning genes on and off
Ί
signal transduction
Plant Hormones
Categories of hormones
Auxins
Gibberellins
Cytokinins
Abscisic
acid
Ethylene
Auxins
Charles and Francis Darwin
Phototropism Experiments
Oat coleoptiles bent toward light
Covered coleoptile tip no bending
Uncovered tip, continued bending
Auxins
Fritz Went
Repeated coleoptile experiments
Cut tips off, placed upon agar for few
hours, cut agar
Placed agar on entire coleoptile ΰ upward
growth
Placed agar on one side of coleoptile ΰ
bending response
Frits Wents experiments
Frits Wents experiments
Frits Wents experiments
Fritz Went Experiment
Auxins
Went named the substance, auxin
Production
Apical meristems, buds, young leaves,
other metabolically active plants
Auxins
Auxin
Activity
Stimulatory
Activities
Phototropism
bending response to light
Production of other hormones
Cell enlargement
Stem growth
Root
initiation
Auxins
Inhibitory
Activities
Delay fruit and leaf abscission
Fruit ripening
Lateral branching
Auxins
Auxin applied to stem base
Note root development from stem base
Auxins
Auxins
(naturally occurring)
IAA
indoleacetic acid
PAA
phenylacetic acid
4-chloroIAA
found in germinating legume seeds
IBA
indolebutyric acid
Leaves of corn and other dicots
Auxins
Auxins
(synthetic)
Used in agriculture and horticulture
Stimulate root formation on any plant
tissue
Orchardists
spray auxins
uniform flowering and fruit set
Prevent abscission layer formation ΰ
premature fruit drop
Auxin
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.
Loosening of cell wall
In growing shoots auxin is transported
unidirectionally, from the apex down to the shoot.
Auxin enters a cell at its apical end as
a small neutral molecule, travels through the cell as an anion, and exits the
basal end via specific carrier proteins.
Outside the cell, auxin becomes neutral
again, diffuses across the wall, and enters the apex of the next cell.
Auxin movement is facilitated by
chemiosmotic gradients established by proton pumps in the cell membrane.
Polar transport of Auxin
Model for polar auxin transport.
According
to the chemiosmotic hypothesis, the difference in pH between the relatively
acidic cell wall and the cytoplasm (maintained by plasma membrane H+-ATPases)
promotes the accumulation of IAA (entering by diffusion) inside the cell.
At
the more basic pH of the cytoplasm, the majority of IAA H undergoes
deprotonation and become "trapped" inside the cell.
Deprotonated*
IAA− can only leave the cell via
active efflux, mediated by the specific efflux carriers.
*Deprotonation
is a chemistry term that refers to the removal of a proton (hydrogen cation H+)
The asymmetric
distribution of efflux carriers within each cell, promotes unidirectional
(polar) auxin transport from cell to cell.
Auxin efflux is presumably facilitated by a
multi-component complex consisting of the efflux carrier (family of PIN
proteins), the NPA-binding protein (NBP) and third unstable component.
Auxin influx carrier (AUX1) facilitate auxin
uptake into the cell.
ABC transporters of the
PGP family mediate additional efflux and influx.
Additional responses to auxin
§
abscission - loss of leaves
§
flower initiation
§
sex determination
§
fruit development
Control of abscission by auxin
Auxins
Influence plant growth found in leaves
and stems growth regulators and hormones
Cell enlargement or elongation located
in meristems and shoot tips (terminal & lateral buds). Auxins move mainly
from apex (top) down.
Lengthening of the internodes and
influence the developing embryos in the seed.
Auxins
Apical dominance high levels of auxin
in the stem just above lateral buds block their growth (blockage of growth of
lateral buds by presence of terminal buds). If shoot tip is removed. The auxin
level behind the lateral buds is reduced and the lateral buds begin to grow.
(the auxin which formed the blockage to keep lateral buds small is reduced so
they can grow)
Auxins
Photo (light) and geotropism (gravity)
involved in tropism responses positive responses
Flower initiation and development
Root initiation and development (rootone)
used on cuttings to help stimulate root growth
Wild Type
Auxin Transport and Differential Growth
Auxins (IAA)
Plant Growth Regulators - Indobutyric
acid (IBA)(synthetic), napthaleneacetic acid (NAA)(synthetic),
2,4-dichlorophenoxyacetic acid (2-4D)(synthetic)
Hormone - indoleactic acid
(IAA)(naturally occurring).
Auxin
Indole acetic acid and related molecules
Photo-and gravitropism
The shoot hormone, made in the shoot apex
Travels down the stem
Polar Auxin Transport
Auxin promotes rooting
Other Effects of Auxin
Apical dominance
Prevents leaf abscission
Enhances fruit growth
Auxin from the developing seeds results
in fruit growth
The infamous side of auxin
Active ingredient in Agent Orange
Chemicals with auxin activity sprayed
(together with kerosene) on forests in Viet Nam to cause leaf drop (and fire)
The chemical process used to make the
auxins also made dioxin, an extremely toxic compound
Auxin as a weed killer
2,4-D 2,4 dichlorophenoxy acetic acid
Causes a plant to grow itself to death
More readily absorbed by broad-leaved
plants
Most often the weed of Weed and Feed
lawn fertilizers
Although auxin affects several aspects of
plant development, one of its chief functions is to stimulate the elongation of
cells in young shoots.
The apical meristem of a shoot is a major
site of auxin synthesis.
As auxin moves from the apex down to the
region of cell elongation, the hormone stimulates cell growth.
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.
Plant Hormones
Auxins
Auxin production occurs mainly in apical
meristems, buds, and young leaves.
Plant response varies according to
concentration, location, and other factors.
Promotes cell enlargement, stem growth,
and delays development processes such as fruit and leaf abscission and fruit
ripening.
Plant Hormones
Movement of auxins from the cells where
they originate requires energy expenditure.
Movement is polar.
Several Forms
Indoleacetic Acid (IAA)
Phenylacetic Acid (PAA)
4-chloroindoleacetic Acid (4-chloroIAA)
Indolebutyric Acid (IBA)
According to the acid growth hypothesis,
in a shoots 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.
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 the sustained growth
response of more cytoplasm and wall material required by elongation.
Auxins
are used commercially in the vegetative propagation of plants by cuttings.
Treating a detached leaf or stem with
rooting powder containing auxin often causes adventitious roots to form near
the cut surface.
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.
Gibberellins
Eiichi Kurosawa (1926, Japan)
Rice fields, some plants grew abnormally
tall and fell over; rice seed loss
Foolish seedling disease
Caused by fungus
Gibberella
fugikuroi
Substance called gibberellin (GA)
Gibberellins
Kinds
of Gibberellins
110 different gibberellins isolated
No more than 15 GAs in any one species
GA
Source in plants
Immature seeds (esp. dicots)
Young leaves
Fungi
Gibberellins
GA Activity in Plants
Increased overall growth in monocots and
dicots but not in gymnosperms
Growth of genetically dwarf plants to
normal height (internode elongation)
Breaking dormancy in seeds of many
species
GA applied to dwarf plants -- cabbage
Notice difference in internodes
Normal cabbage plants
Gibberellins (GA)
Gibberellic Acid
Have a regulatory function
Are produced in the shoot apex primarily
in the leaf primordial (leaf bud) and root system
Stimulates stem growth dramatically
Gibberellins (GA)
Stimulates cell division, cell
elongation (or both) and controls enzyme
secretions. Ex: dwarf cultivars can be treated with GA and grow to normal
heights indicates dwarf species lack normal levels of GA
Gibberellins
Involved in overcoming dormancy in seeds
and buds.
GA translocates easily in the plant (able
to move freely) in both directions because produced in not only shoot apex
but also in the root structure.
Gibberellins
Used commercially in:
Increasing fruit size of seedless grapes
Stimulating seed germination &
seedling growth
Gibberellins
Promoting male flowers in cucumbers for
seed production.
Overcoming cold requirements for some
seed, application of GA foregoes the cold requirements (some seed require to be
frozen or placed in the refrigerator for a period of time before they will
germinate).
Gibberellins
Uses in Agriculture and Horticulture
Increase yields in sugarcane and hops
(experimentally)
Seedless
grapes
Seeds (plant embryos) secrete hormone
that stimulate mesocarp development
GA supplements this activity
Gibberellins
Naval oranges
delay in aging of fruits skin
Celery petioles
length and crispness
Gibberellin
Discovered in association with Foolish disease of rice (Gibberella
fujikuroi)
Effects of Gibberellins
§
General cell elongation.
§
Breaking of dormancy.
§
Promotion of flowering.
§
Transport is non-polar, bidirectional
producing general responses.
Plant Hormones
Gibberellins (GA)
Named after the fungus that produced it (Gibberella
fujikuroi).
Most GA produced by plants are inactive,
apparently functioning as precursors to active forms.
Most dicots and a few monocots grow
faster with an application of GA.
Plant Hormones
Gibberellins are involved in nearly all
the same regulatory processes in plant development as auxins.
Appears to lower the threshold of growth.
Several commercial growth retardants can
be used to block GA synthesis.
Gibberellins
A large family of compounds with a few
biologically active members
Found as the toxin produced by some fungi
that caused rice to grow too tall
Now known to be essential for stem
elongation
Dwarf plant varieties often lack
gibberellins
Gibberellins
Gibberellins are involved in bolting of
rosette plants
Gibberellins are used to improve grapes
Gibberellins are involved in seed
germination
gibberellins will induce genes to make
enzymes that break down starch
Cytokinins
Haberlandt (Germany, 1913)
Chemical found in phloem that stimulated
cell division
Later found in coconut milk (endosperm)
Cytokinins:
substances that stimulate cell division
Chemically similar to adenine
(nitrogenous base in nucleic acids)
Cytokinins
Activities
Work with auxin to stimulate cell
division past the G2 phase
Cell enlargement
Tissue differentiation
Development of chloroplasts
Stimulate cotyledon growth in embryos
Cytokinins
Function of cytokinins
§
Promotes cell division.
§
Morphogenesis.
§
Lateral bud development.
§
Delay of senescence.
§
Stomatal opening.
§
Rapid transport in xylem stream.
Cytokinins are a major factor in
regulating cell division.
Overproduction of cytokinins and auxin
together stimulate hyperplastic growth following infection by Agrobacterium.
Cytokinins initiate cell division by
controlling the cell cycle at two points.
In
one case, cytokinins catalyze the transition from the G2 phase to mitosis by
generating an active cyclin dependent kinase complex.
The second control point is the G1 to S phase
transition, where cytokinins appear to be responsible for inducing the
synthesis of a D-type cyclin.
The control of cell enlargement by auxin
(indole-3-acetic acid) can be explained by the acidgrowth hypothesis.
According to this hypothesis, auxin causes the
cell to excrete protons by stimulating the activity of a plasma membrane-bound
ATPase-proton pump.
The resulting acidification of the cell
wall space stimulates expansion activity which, in turn, increases wall
extensibility and allows for turgor-induced cell expansion.
The signal transduction chain for auxin
is not yet clear, but probably involves a receptor protein, ABP1 (auxin-binding
protein 1) and phospholipase A2 as a second messenger.
Maintenance of auxin-induced growth
requires gene activation and a number of auxin-regulated genes have been
identified.
Cytokinins
Promotes cell division
Found in all tissues with considerable
cell division.
Ex: embryos (seeds) and germinating
seeds, young developing fruits
Cytokinins
Roots supply cytokinins upward to the
shoots.
Interact with auxins to influence
differentiation of tissues (may be used to stimulate bud formation).
Cytokinins
As roots begin to grow actively in the
spring, they produce large amounts of cytokinins that are transported to the
shoot, where they cause the dormant buds to become active and expand.
Cytokinins
Tissue cultures use cytokinins to induce
shoot development
Cytokinins may slow or prevent leaf
senescence (leaf ageing or leaf fall).
Cytokinin
Cytokinins the root hormone
Discovered by Folke Skoog (in this
department) as the last unknown compound needed to get plant cells to grow
undifferentiated
Cytokinin
Cytokinins allowed the growth of callus
cultures
The mix of auxin and cytokinin determine
root, shoot, or callus
Other cytokinin facts
Cytokinins delay and even reverse
senescence
Release buds from apical dominance
Abscissic Acid
Hemberg (Sweden, 1941)
Dormins:
substance that kept plant parts dormant buds
Abscissic acid
(ABA) characterized (1967) by researchers in US, Great Britain, New
Zealand
ABA is inhibitory hormone; opposite
effect of auxins
Abscissic Acid
Source
Synthesized in plastids from carotenoid
pigments
Fleshy fruits:
keep seeds dormant while inside fruit
Leaves:
ABA helps respond to excessive water loss
Abscisic acid
Functions of abscisic acid
§
General growth inhibitor.
§
Causes stomatal closure.
§
Readily translocated.
§
Produced in response to stress.
Inhibitors
Abscisic Acid (ABA)
Widespread in plant body moves readily
through plant
ABA appears to be synthesized (made) by
the leaves.
Interacts with other hormones in the
plant, counteracting the growth promoting the effects of auxins &
gibberellins.
Abscisic acid
Incorrectly named, not related to
abscission
Important in water stress and other
stresses
Causes stomatal closure
Prevents premature germination of seeds
Changes gene expression patterns
Abscisic Acid
Involved with leaf and fruit abscission
(fall), onset of dormancy in seeds and onset of dormancy (rest period) in
perennial flowers and shrubs
ABA is effective in inducing closure of
stomata in leaves, indicating a role in the stress physiology in plants. (ex:
increases in ABA following water, heat and high salinity stress to the plant)
Plant Hormones
Abscisic Acid
Has inhibitory effect on the stimulatory
effects of other hormones, and thus on plant growth.
Synthesized in plastids.
Particularly common in fleshy fruits.
Has little influence on abscission.
Plant Hormones
Cytokinins
Regulate cell division.
Synthesized in root tips and germinating
seeds.
If present during the cell cycle,
cytokinins promote cell division by speeding up the progression from the G2
phase to the mitosis phase.
Can prolong the life of vegetables in
storage.
Plant Hormones
Other Compounds
Oligosaccharides
Released from cell walls by enzymes -
influence cell differentiation, reproduction, and growth in plants.
Brassinosteroids
Have gibberellin-like effects on plant
stem elongation.
Hormonal Interactions
Apical Dominance
Apical dominance is the suppression of
the growth of lateral or axillary buds.
Believed to be brought about by an
auxin-like inhibitor in a terminal bud.
If cytokinins are applied in appropriate
concentration to axillary buds, they will begin to grow, even in the presence
of a terminal bud.
Hormonal Interactions
Senescence
Senescence is the breakdown of cell
components and membranes, eventually leading to the death of the cell.
Some studies have suggested certain
plants produce a senescence factor.
Other Interactions
Root and shoot development regulated by
auxins and cytokinins.
Seed germination regulated by
gibberellins.
Ethylene
Neljubow (1901, St. Petersburg Botanical Inst.)
Ethylene gas produced by gas lamps
reduced growth and stem swelling in pea seedlings
Gane (1934)
Ethylene gas produced naturally by fruits
Ethylene is a strong promoter of
senescence and abscission, particularly of leaves, floral parts, and ripening
fruits.
In
the case of ripening fruits, ethylene release is keyed to a dramatic rise in
respiration, the climacteric rise.
Ethylene is autocatalytic and its release
will stimulate a climacteric rise and further ethylene release in other fruits
nearby.
Senescence of intact and detached leaves
is delayed by exogenous cytokinins.
If a detached leaf is stimulated to
produce roots, senescence of the leaf will be delayed due to the cytokinins synthesized
in the roots, and when the mature plant enters senescence there is a sharp
decline in the level of cytokinins exported from the roots.
Ethylene
Ethylene now known to be produced by all
plant organs
Ethylene produced from methionine; O2 required
Commercial
uses
Hastens green fruit ripening
Growers harvest green fruit, expose to
ethylene gas ΰ
ripening
Ethylene
Ethylene
Gaseous hormone
Produced in the actively growing
meristems of the plant, in senescing ripening or ageing fruits, in senescing
(ageing or dying) flowers, in germinating seeds and in certain plant tissues as
a response to bending, wounding or bruising.
Ethylene as a gas, diffuses readily
throughout the plant.
Ethylene
May promote leaf senescing and abscission
(leaf fall).
Increases female flowers in cucumbers
(economically - will increase fruit production).
Degreening of oranges, lemons and
grapefruit ethylene gas breaks down chlorophyll and lets colors show through.
Functions of ethylene
§
Gaseous in form.
§
Rapid diffusion.
§
Affects adjacent individuals.
§
Fruit ripening.
§
Senescence and abscission.
§
Interference with auxin transport.
§
Initiation of stem elongation and bud
development.
Plant Hormones
Ethylene
Produced by fruits, flowers, seeds,
leaves, and roots.
Produced from amino acid methionine.
Used to ripen green fruits.
Production almost ceases in the presence
of oxygen.
Ethylene
Harvested
fruit
Stored in warehouses
Pump out air
Reduce temperature to just above freezing
Replace removed air with CO2
or nitrogen ΰ
Fruit metabolically inactive, no ripening
Ethylene
Hormonal Interactions
Apical Dominance
Def:
suppression of the growth of axillary buds
Plants have a long slender appearance
Removal of apical meristem releases
lateral bud for growth
Ethylene
The smallest hormone
A gas
Important in seed germination, fruit
ripening, epinasty, abscision of leaves
Sex expression in cucurbits
Florigen, the Hormone that Cannot Be Found
A signal travels from leaves to the
apical meristem to cause flowering
Some effects of salycilic acid in
duckweed
How Do Hormones Work?
Ion channels
acid growth of auxin
stomatal closure by ABA
Plant Growth Regulators
Chemical
Messengers
Hormones
In plants, many behavioral patterns and
functions are controlled by hormones. These are chemical messengers
influencing many patterns of plant development.
Plant hormones a natural substance
(produced by plant) that acts to control plant activities. Chemical messengers.
Hormones
Are produced in one part of a plant and
then transported to other parts, where they initiate a response.
They are stored in regions where stimulus
are and then released for transport through either phloem or mesophyll when the
appropriate stimulus occurs.
Growth Regulators
Plant growth regulators include plant
hormones (natural & synthetic), but also include non-nutrient chemicals not
found naturally in plants that when applied to plants, influence their growth
and development.
Growth Regulators
5 recognized groups of natural plant
hormones and growth regulators.
1. Auxins
2. Gibberellins
3. Cytokinins
4. Ethylene
5. Abscisic
acid
Apical Dominance
Note two stems
Result from apical meristem removal
SUMMARY
Plant hormones are notable for the
multiplicity of their effects and the extent to which they interact with each
other.
The complexity of hormone interactions
has, in the past, made it difficult to study hormonal control of development.
However, modern genomic research and the
use of mutants that either lack a hormone or are insensitive to it are shedding
new light on hormones and the way they work.
Many developmental effects of auxin can
be attributed to polarity of auxin transport.
The principal direction of auxin transport is
basipetal, or from the apex to the base.
Transport polarity in stems is due to the
presence of an auxin efflux carrier, the PIN1 protein, that is preferentially
located along the basal membranes of transporting cells.
A similar basal efflux carrier, PIN2,
operates in cortex and epidermis of roots.
Auxins stimulate vascular differentiation
around wounds in Coleus stems and in plant tissue cultures.
While cytokinins do not alone stimulate
vascular differentiation, they do enhance the effects of auxin.
The level and activities of various
hormones change dramatically during the development of seeds.
Cytokinins are highest and gibberellins
and auxins low during the early stages of development when active cell division
is required.
Auxin and gibberellin levels increase during
the period of active cell enlargement.
ABA levels are highest late in seed
development, presumably to prevent precocious germination.
During germination of cereal grains,
gibberellins move from the scutellum into the aleurone, where they stimulate
the de novo synthesis of amylases and other enzymes required to digest the
endosperm and mobilize the products to support the nutritional needs of the
developing embryo.
Auxins, gibberellins, and
brassinosteroids all have the capacity to stimulate stem elongation.
Gibberellins cause hyperelongation of green stems, especially of rosette
species and in cold-requiring species.
Chemicals that specifically inhibit
gibberellin biosynthesis are used in horticulture to produce more compact
plants.
The growth of axillary buds is inhibited
by the polar flow of auxin out of the shoot apex, a phenomenon known as apical
dominance.
This effect of auxin is antagonized by
cytokinin.
The initiation of secondary and
adventitious roots is controlled by auxin, although high ratios of cytokinin to
auxin will suppress this effect.
Cytokinins may delay senescence by
controlling nutrient mobilization and retention.
Abscission, or the shedding of leaves and
other organs, is regulated by the balance of auxin concentration on either side
of the abscission zone, a layer of cells near the base of the petiole.
Introduction
Growth
Def: irreversible
increase in mass due to the division and enlargement of cells
Differentiation
Def: cells
develop into different kinds of cells with different structure and different
function
Introduction
Essential Components of Growth
Nutrients: inorganic
elements and organics molecules that enable growth
Vitamins:
coenzymes or parts of coenzymes (organic molecules) that participate in enzyme
catalyzed reactions
CELL DIVISION, ENLARGEMENT, AND
DIFFERENTIATION
Cell division regularly occurs in meristematic
regions such as root and shoot apical meristems and the vascular cambium, as
well as during the early stages of leaf and fruit development.
Depending on the tissue or stage in
development, the daughter cells may continue to divide, thus increasing cell
mass of that particular organ.
Alternatively, one or both cells may
proceed to enlarge, mature, and differentiate.
Unlike animal cells, however, postmitotic
plant cells do not permanently lose the capacity to divide.
Mature plant cells may, when provided
with the appropriate stimuli, be encouraged to resume division and growth.
CYTOKININS ARE A SIGNIFICANT FACTOR IN
REGULATING CELL DIVISION
The role of cytokinins in regulating cell
division first became apparent as a result of attempts to culture isolated
carrot and tobacco tissues on defined media (see
The Plant Hormones: Control of
Development
Cell proliferation occurred only when
auxin plus some "cell-division factor" was present in the medium.
The "cell division factor"
turned out, of course, to be kinetin.
It was soon learned that kinetin and
other cytokinins, in the presence of auxin, stimulated cell division in a wide
variety of tissues..
CYTOKININS REGULATE PROGRESSION THROUGH THE
CELL CYCLE
Although much remains to be learned about
how cytokinins regulate cell division, studies of tobacco suspension-cultured
cells and Arabidopsis indicate a direct role for cytokinins in regulating
progression through the cell cycle .
Freshly established tobacco cell cultures
require both auxin and cytokinin for continued cell division.
The absence of either hormone causes the
cells to be arrested in either the G1 or G2 phase of the cell cycle.
Following addition of hormone, the onset
of cell division can be detected within 12 to 24 hours.
In
1996, K Zhang and co-workers reported that cultured cells arrested in G2 by the
absence of cytokinin contained cyclin-dependent kinases (CDK) with reduced
activity, due to a high level of phosphorylation on a tyrosine residue.
When
such cultures were resupplied with cytokinin, the tyrosine was
dephosphorylated, the enzyme reactivated, and cell division resumed
AUXINS
STIMULATE CELL ENLARGEMENT IN EXCISED TISSUES
Control
of cell enlargement was the first hormonal regulated response to be
demonstrated in plants.
Auxin-regulated
cell enlargement in Avena coleoptiles was the basis for its discovery and this
action has been demonstrated repeatedly with excised plant tissues such as
subapical coleoptile tissues and stem segments cut from dark-grown pea
seedlings.
Auxin
concentration-response curves typically show an increasing response with
increasing concentrations of auxin until an optimum concentration is reached
(Fig. 16.4).
Concentrations
exceeding the optimum characteristically result in reduced growth.
If the auxin concentration is high enough,
growth may be inhibited compared with controls.
Growth
responses such as these are often used to assay for unknown hormone
concentrations, a technique known as bioassay.
Another
very characteristic feature of auxin physiology is that intact stems and
coleoptiles do not show a significant response to exogenous application of the
hormone.
Apparently
the endogenous auxin content of intact tissues is high enough to support
maximum elongation and added auxin has little or no additional effect.
Thus,
it is a general rule that the effect of exogenously supplied auxin can be
demonstrated only in tissues that have been removed from the normal auxin
supply, such as excised segments of stems and coleoptiles.
This
rule may be subject to change, however, since a sustained growth of intact pea
internodes following exogenous applications of IAA has been reported.
Although
it is commonly assumed that auxin is essential for cell enlargement and growth
of leaves, flowers, and other organs as well as stems, there is little direct
evidence available.
Auxin-induced
cell enlargement is also the basis for the capacity of auxins to initiate and
sustain growth of undifferentiated cells when plant tissues are cultured on
artificial media containing cytokinin