Plant Hormones & Tropisms

Controls of growth, development and movement

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.

Processes in growth

• Cell division.
• Cell enlargment.
• Cell differentiation.

Primary growth

Secondary growth

xylem

Phloem with

bands of fibers

Seed Germination

• Scarification
• mechanical
• chemical
• heat
• Mobilization of reserves

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Mobilization of reserves

Plant Responses to Environment

Tropisms vs Taxisms +/-

• Thigmotropism🡪 Physical Contact.
• Chemotropism🡪 Chemicals
• Thermotropism🡪 Temperature
• Traumotropism🡪 Wounding
• Electrotropism🡪 Electricity
• Skototropism🡪 Dark
• Aerotropism🡪 Oxygen
• Gravitropism🡪 Gravity
• Phototropism🡪 light

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Plants in Motion

Tropisms

Touch Me Not

Mimosa

Tendrils

Turgor movement�Mimosa pudica L. (sensitive plant)

Pulvinus of Mimosa pudica

Tropic responses

Directional movements in response to a directional stimulus

Growth movement

Phototropism

Photoperiodism, or the response to change in length of the night, that results in flowering in long-day and short-day plants

Geotropism

Thigmotropism

Plant hormones

• Five plant hormones known by the mid 1960s, new compounds called plant growth regulators
• Signal molecules produced at specific locations.
• Occur in low concentrations.
• Cause altered processes in target cells at other locations.

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• The five hormones
• Auxins
• Cytokinins
• Gibberellins
• Ethylene (ethene)
• Abscisic acid

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• Other plant growth regulators
• Brassinosteroids
• Salicylic acid
• Jasmonic acid
• Systemin

Plant hormones

• First plant hormone discovered was auxin, the chemical responsible for photo- and gravitropic responses
• The chemical itself was first isolated from horse urine, it is indoleacetic acid

Auxin

Auxins primarily stimulate cell elongation

Auxins also have many secondary actions: root initiation, vascular differentiation, tropic responses, apical dominance and the development of auxiliary buds, flowers and fruits.

Auxins are synthesized in the stem and root apices and transported through the plant axis.

Auxins are often most effective in eliciting their effects when combined with cytokinins. �

Auxin associated with phototropism - early experiments

demonstrate tip as receptor.

Additional responses to auxin

• Inhibits abscission - loss of leaves
• flower initiation
• sex determination
• fruit development

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Auxin Flavors:

• Indoleacetic Acid (IAA)
• Phenylacetic Acid (PAA)
• 4-chloroindoleacetic Acid (4-chloroIAA)
• Indolebutyric Acid (IBA)

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Loosening of cell wall

Auxin promotes rooting

Promotes Apical dominance

Auxin as a weed killer

• Many synthetic auxins are used as selective weed killers and herbicides. 2, 4 - D (2, 4 - dichloro phenoxy acetic acid) is used to destroy broad leaved weeds. It does not affect mature monocotyledonous plants. 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

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Parthenocarpy

• Auxin induces parthenocarpy, i.e., the formation of seedless fruits without the act of fertilization.

Control of abscission by auxin

Formation of an abscission layer at the base of petiole or pedicel results in shedding of leaves, flowers or fruits. But auxins inhibit abscission, as they prevent the formation of abscission layer.

Auxin Spray Prevents Premature Fruit Abscission and Increase in Yield.

a) Auxin Sprayed; b) Auxin not Sprayed

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

Cytokinins

Cytokinins are able to stimulate cell division and induce shoot bud formation in tissue culture.

They usually act as antagonists to auxins.

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Morphogenesis.

Lateral bud development.

Delay of senescence.

Stomatal opening.

Rapid transport in xylem stream.

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Function of cytokinins

• Promotes cell division.
• Morphogenesis.
• Lateral bud development.
• Delay of senescence.
• Stomatal opening.
• Rapid transport in xylem stream.

Other cytokinin facts

• Cytokinins delay and even reverse senescence
• Release buds from apical dominance

Cytokinins ↑

Auxin ↓

Interaction of cytokinin and auxin in tobacco callus tissue

High cytokinin to auxin ratio causes differentiation of shoots.

A low ratio of cytokinin to auxin causes root formation.

Intermediate cytokinin to auxin ratio causes formation of roots as well as shoots.

Intermediate cytokinin to low auxin causes growth of large amount of callus.

Gibberellin

Gibberellins are an extensive chemical family with over 80 different gibberellin compounds in plants but only giberrellic acid (GA3) and GA4+7 are often used in plant tissue culture

The main effect of gibberellins in plants is to cause stem elongation and flowering.

They are also prominently involved in mobilization of endosperm reserves during early embryo growth and seed germination.

Gibberellins

• Now known to be essential for stem elongation
• Dwarf plant varieties often lack gibberellins
• Gibberellins are involved in seed germination
• gibberellins will induce genes to make enzymes that break down starch
• Promotion of flowering.

Gibberellins are involved in bolting of rosette plants

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Gibberellin induces stem elongation in rosette plants. Cabbage is a rosette plant with profuse leaf growth and retarded internodal length. Just prior to flowering, internodes elongate enormously. This is called bolting. Bolting needs either long days or cold nights. When a cabbage head is kept under warm nights, it retains its rosette habit. Bolting can be induced artificially by the application of gibberellins under normal conditions.

Discovered in association with Foolish disease of rice (Gibberella fujikuroi)

infected

uninfected

Found as the toxin produced by some fungi that caused rice to grow too tall

Gibberellins are used to improve grapes

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Abscisic acid (ABA)

• Incorrectly named, not related to abscission, slows plant growth
• Important in drought stress and other stresses
• Causes stomatal closure
• Prevents premature germination of seeds (enhances dormancy)
• Changes gene expression patterns

Ethylene

• The smallest hormone
• A gas
• Important in seed germination, fruit ripening, epinasty, abscision of leaves
• Sex expression in cucurbits

Functions of ethylene

• Gaseous in form.
• Rapid diffusion.
• Affects adjacent individuals.
• Fruit ripening.
• Senescence and abscission.
• Interference with auxin transport.
• Inhibition of stem elongation
• Positive feedback mechanisms amplify responses in organisms. Amplification occurs when the stimulus is further activated which, initiates an additional response that produces system change.