Physiology of Seed Plants
Regulating Growth and
Development: The Plant Hormones
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Auxins
Cytokinins
Ethylene
Abscisic Acid
Gibberellins
Hormones
• Chemical signals that help both plants and
animals regulate and coordinate
metabolism, growth, and differentiation.
• Phytohormones- plant hormones
Three basic elements of Hormones
1. Synthesis of the hormone in one part of
the organism
2. Transport of the hormone to another part
(target tissue)
3. Induction of chemical response
Phytohormones
• Produced in tissues or glands
• Very active in small quantities
– Pineapple Ananas comosus for example are
only 6 micrograms of indoleacetic acid (IAA) a
common plant hormone per kg of plant
material. (analogous to a needle in 20 metric
tons)
– Can stimulate or inhibit depends on chemical
structure and how it is read by the target
tissue
Five classes of plant hormones
The “Classic Five”
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AuxinsCytokininsEthyleneAbscisic acidGibberellins-
Auxins
• Charles Darwin and Francis Darwin- The
Power of Movement in Plants 1881
In response to light
an “influence” that causes
bending is transmitted
from the tip to area below
the tip
The Principle naturally occuring Auxin- Indoleacetic Acid
In plants variety of pathways to produce
- tryptophan usually precursor
- All tissue produces IAA but typically
found in shoot apical meristems, young
leaves and developing fruit and seeds.
Mutants lacking either auxin or cytokinin
Have yet to be found- mutations eliminating
Them are lethal
Auxin synthesis- the site of auxin
synthesis along the margin of a
young leaf
Site corresponds to the location
of cells that will differentiate into
a hydothode (gland like structure)
GIS reporter gene detects auxin
synthesis
Auxin transport- experimental demo of polar auxin transport in
stems represented here by a segment of hypocotyl from a
seedling
In the root, nonpolar transport of IAA takes place in
the phloem of the vascular cylinder whereas the polar transport
occurs in the epidermis and cortical parenchyma cells
Arrowhead- vascular regeneration
Acropetal polar movement of auxin from
Above the arrow and then around the wound
IAA induced xylem regeneration around a wound
• - When the apical bud is cut off a plant, the development
of axillary buds in lateral branches is observed.
• - If the apical bud is replaced by cotton impregnated
with auxin, no axillary bud development is observed.
• - So the auxin replaces the apical bud.
- It can be deduced that this hormone is produced in the
apical part of the plant.
The inferior part of the plant including roots ( or root cap) is cut off and the plant is
put in a medium containing auxin or free of it.
Without auxin, adventitious roots can developed. This is the principle of cuttings.
However, with auxin, root development is much better.
Auxin promotes fruit
development
• Auxin is involved with the formation of fruit
• Parthenocarpic fuit- by treating a female
flower parts (carpels) of certain species
with auxin it is possible to produce a fruit
(without fertilization- a virgin fruit) i.e.
seedless tomatoes, cucumbers and
eggplants.
• Developing seed is a source of auxin
Auxin and fruit development- Normal strawberry
Strawberry with all seeds removed
Strawberry with horizontal band of seeds removed
Other characteristics of Auxin
• Auxin provides chemical signals that
communicate information over long
distances
• Promotes the formation of lateral and
adventitious roots
• Synthetic auxin (2,4 dichlorophenoxyacetic
acid) are used to kill weeds (broad leaf)
– Mechanism unk.
Cytokinins
• In 1941 Johannes van Overbeek found
that coconut milk (liquid endosperm)
contained potent growth factors
• Factors greatly accelerated the
development of plant embryos and
promoted the growth of isolated tissue and
cells in vitro (test tube)
Discovery had two affects
– It gave impetus to studies of isolated plant
tissues
– Launched the search for another major group
of growth regulators
Basic medium used for tissue
culture of plant cells
• Contained sugar
• Vitamins
• Various salts
• Grown in this culture, growth slowed or stopped
• Thus some growth stimulus declined and the
addition of IAA had no affects
• Adding coconut milk encouraged the cells to
divide and growth to resume
Growth factor from DNA
• Isolation of growth factor from DNA
identifying its chemical nature called
kinetin and the group of regulators called
cytokinins because its involvement with
cytokinesis
Kinetin
• Resembles purine- adenine (a nitrogenous
base)
• Probably does not occur naturally in plants
• Has relatively simple structure
• Chemist able to synthesize a number of
related compounds
– Zeatin- most active naturally occurring
cytokinin (maize)
Cytokinins
• Found in active dividing structures,
seeds, fruits, leaves and root tips
• Found also in SVP horsetail, fern
• Central to tissue culture methods
and extremely important in
biotech.
• Tx of lateral buds causes growth
even in the presence of auxin thus
modifying apical growth.
• The cytokinin/auxin ratio regulates the
production of roots and shoots in tissue cultures
• Undifferentiated plant cell has two courses open
to it
– It can enlarge, divide, enlarge and divide again
(undifferentiate)
– without undergoing cell division, it can differentiate for
example it can elongate then divide to form different
type of cells
In tobacco stem tissue
• Application of IAA causes rapid cell expansiongiant cells are formed
• Kinetin alone has little or no effect
• IAA + Kinetin results in rapid cell division, so that
large numbers of relatively small,
undifferentiated cells are formed.
• High IAA, callus tissue- a growth of undiff. Cells
in tissue culture frequently gives rise to roots.
Callus development- effects of increasing [IAA] at various kinetin
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- At the top, the apical dominance have been annulled by cutting the apical bud.
- At the centre, the terminal bud and the root cap have been cut. No futher
development of axillary buds on the explant can be observed. So the roots are
necessary for the development of buds.
- on the other hand (at the bottom), if one identical explant is introduiced in a
medium containing cytokinins, the development of axillary buds occurs. So the
cytokinins replace the roots and it can be deduced that these hormones are
produced in the roots.
It is observed also that there is less root development in the presence of cytokinins.
Summary
Auxins:
• - They are produced in the apical part of the
plant.
- They prevent the development of the axillary
buds.
- They favour the rhizogenesis ...(development
of roots).
Cytokinins:
• - They are produced in the roots.
- They prevent the development of roots.
- They favour the development of the axillary
buds.
Cytokinins delay leaf
senescence
• Yellowing (loss of chlorophyll) of leaf can
be delayed with the addition of cytokinins
• Xanthium strumarium Leaves turned
yellow in about ten days in plain water
• Add 10mg of kitenin help retain green
Ethylene
• Plays a role in fruit ripening
• Promotes abscission (shedding of leaves,
flowers and fruit)
– Triggers enzymes that promote fruit loosening
from trees
Auxin prevents abscission (prevents
preharvest)
High concentration does the opposite
Abscisic Acid (ABA)/Dormin
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First discovered as dormin and abscisin
Dormin ash
In other plants abscisin
Identical compounds now called Abscisic
acid
Abscisic Acis
• Levels increase during early seed
development
• Stimulates the production of seed storage
protein
• Prevents premature seed germination
• Decline in ABA leads to germination
Gibberellin
• Found in immature seeds- highest
concentration
• Stimulates cell division and cell elongation
• Application to dwarf mutants cause them
to grow tall
• Plays a role in breaking seed dormancy
and germination
Additional chemicals used by
plants
• Brassinosteroids- naturally occuring polyhydroxyl
steroids (tissue growth)
• Salicylic acid- phenolic compound similar structure to
aspirin implicated in defense responses
• Jasmonates- class of compounds known as oxylipins
plant growth regulation and defense
• Polyamines- strongly basic molecules found in all
organisms (bacteria, fungi, animal, plants) are essental
for growth and development and affects cell division
• Systemin- a polypeptide functions as a long distance
signal to activate chemical defenses against herbivores.
• Nitric oxide (NO) serves as a signal in hormonal defense
responses.