Plant Hormones & Plant Hormones & TropismsTropisms

Controls of growth, Controls of growth, development and development and

movementmovement

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 growthProcesses in growth

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

Seed GerminationSeed Germination

Scarificationmechanicalchemicalheat

Mobilization of reserves

Mobilization of Mobilization of reservesreserves

Plant Responses to EnvironmentTropisms vs Taxisms +/-

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

Plants in MotionTropisms

Touch Me Not

Mimosa

Tendrils

Turgor movementTurgor movementMimosa pudicaMimosa pudica L. (sensitive L. (sensitive

plant)plant)

Pulvinus of Pulvinus of Mimosa Mimosa pudicapudica

Tropic responsesTropic responses

Directional movements in response to a directional

stimulus

Growth movementGrowth movement

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

GeotropismGeotropism

ThigmotropismThigmotropism

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

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

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, i.e., 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.

GibberellinGibberellinGibberellins 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

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

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.

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