Plant Hormones & Tropisms

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.

• Affect division, elongation and differentiation

Primary growth

protoderm procambium Leaf primordia Apical meristem Ground meristem Forming axillary bud

Secondary growth

Vascular cambium xylem Ray parenchyma cork Phloem with bands of fibers

Plant Responses to Environment

• Tropisms vs Taxisms +/ Thigmotropism  • Chemotropism  Physical Contact. Chemicals • Thermotropism  • Traumotropism  Temperature Wounding • Electrotropism  • Skototropism  • Aerotropism  • Gravitropism  • Phototropism  Electricity Dark Oxygen Gravity light Plants in Motion Tropisms

Tropic responses

Directional movements in response to a directional stimulus

Thigmotropism

Thigmotropic Response & Turgor movement in Mimosa pudica

Phototropism

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

Geotropism/Gravitropism

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.

• The five hormones – Auxins – Cytokinins – Gibberellins – Ethylene (ethene) – Abscisic acid • Other plant growth regulators – Brassinosteroids – Salicylic acid – Jasmonic acid – Systemin

Summary of Functions of Major Plant Hormones Hormone Function Location Auxins (IAA)* stem elongation apical dominance root formation produced in shoot apical meristem

Cytokinins

Gibberellins

(GA)* cell division differentiation stem & internode elongation seed germination produced in roots produced in apical portion of root & shoot

Ethylene

* abscission fruit ripening produced in leaves, stems & young fruits Abscisic Acid suppression of bud growth stomatal opening leaf senescence mature leaves, fruits & root caps *most horticultural/ agricultural applications

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 Auxin Flavors:  Indoleacetic Acid (IAA)   Phenylacetic Acid (PAA) 4-chloroindoleacetic Acid (4-chloroIAA)  Indolebutyric Acid (IBA)

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

Parthenocarpy

• Auxin induces parthenocarpy, 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. Morphogenesis.

Lateral bud development.

Delay of senescence.

Stomatal opening.

Rapid transport in xylem stream.

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 Auxin  Cytokinins 

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 >80 compounds in plants The main effect of gibberellins in plants is to cause stem elongation and flowering. • Also prominently involved in mobilization of endosperm reserves during early embryo growth and seed germination.

Gibberellin Signal Transduction

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.

Seed Germination caused by Mobilization of reserves

 Scarification  mechanical  chemical  heat

Gibberellins are involved in bolting of rosette plants

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)

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

Gibberellins are used to improve grapes

EK2.E.1: Timing and coordination of specific events are necessary for the normal development of an organism, and these events are regulated by a variety of mechanisms.

b. Induction of transcription factors during development results in sequential gene expression.

3. Temperature and the availability of water determine seed germination in most plants.

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.

EK2.C.2: Organisms respond to changes in their external environments.

a. Organisms respond to changes in their environment through behavioral and physiological mechanisms.

• Photoperiodism and phototropism in plants

EK 2.E.2: Timing and coordination of physiological events are regulated by multiple mechanisms.

a. In plants, physiological events involve interactions between environmental stimuli and internal molecular signals. 1. Phototropism, or the response to the presence of light 2. Photoperiodism, or the response to change in length of the night, that results in flowering in long day and short-day plants

EK 2.E.2: Timing and coordination of physiological events are regulated by multiple mechanisms.

b. Responses to information and communication of information are vital to natural selection. 1. In phototropism in plants, changes in the light source lead to differential growth, resulting in maximum exposure of leaves to light for photosynthesis.

2. In photoperiodism in plants, changes in the length of night regulate flowering and preparation for winter.

Mammalian Circadian Rhythms

Resources

Plant Hormones Info Plant Hormones, Nutrition & Transport Tropism Animation Auxin in Cell Walls Plant Responses to Environmental Challenges: Signaling between Plants and Pathogens Growth/Hormones Plants in Motion Auxin Animation Transpiration Lesson How Hormones Protect Seed Development in Peas Virtual lab Herbicide Mechanisms & Animations