Plant Hormones & TropismsPlant 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 growthPrimary growth

Apical meristemApical meristem

Leaf primordiaLeaf primordia

Forming axillary budForming axillary bud

Ground meristemGround meristem

protodermprotoderm

procambiumprocambium

Secondary growthSecondary growth

xylemxylem

Phloem withPhloem withbands of fibersbands of fibers

Vascular cambiumVascular cambium

Ray parenchymaRay parenchyma

corkcork

Plant Responses to Environment

Tropisms vs Taxisms +/-• Thigmotropism Physical Contact. • Chemotropism Chemicals • Thermotropism Temperature • Traumotropism Wounding • Electrotropism Electricity • Skototropism Dark • Aerotropism Oxygen • Gravitropism Gravity• Phototropismlight

Plants in MotionTropisms

Tropic responsesTropic responses

Directional movements in response to a directional

stimulus

ThigmotropismThigmotropism

Thigmotropic Response & Turgor Thigmotropic Response & Turgor movementmovement in Mimosa pudica in Mimosa pudica

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

Geotropism/GravitropismGeotropism/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 cell division differentiation

produced in roots

Gibberellins (GA)*

stem & internode elongation seed germination

produced in apical portion of root & shoot

Ethylene* abscission fruit ripeningproduced in leaves, stems & young fruits

Abscisic Acid

suppression of bud growth stomatal opening leaf senescence

mature leaves, fruits & root caps

*most horticultural/ agricultural applications

AuxinAuxins 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 experimentsAuxin associated with phototropism - early experimentsdemonstrate tip as receptor.demonstrate tip as receptor.

Additional responses to Additional responses to auxinauxin

Inhibits abscission - loss of leavesflower initiationsex determinationfruit 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 Control of abscission by auxinauxin

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

CytokininsCytokininsCytokinins 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 cytokininsFunction 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 Interaction of cytokinin and cytokinin and auxin in auxin in tobacco tobacco callus tissuecallus 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.

GibberellinGibberellin• 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 Seed Germination caused by Mobilization of reservesby Mobilization of reserves

Scarificationmechanicalchemicalheat

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

infecteduninfected

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 ethyleneFunctions of ethyleneGaseous 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 light2. 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

ResourcesPlant Hormones Info Plant Hormones, Nutrition & TransportTropism AnimationAuxin in Cell Walls Plant Responses to Environmental Challenges: Signaling

between Plants and Pathogens Growth/HormonesPlants in MotionAuxin Animation Transpiration LessonHow Hormones Protect Seed Development in Peas Virtual

lab Herbicide Mechanisms & Animations