Angiosperms VIIInternal Control of

Development: The Plant Growth Regulators

The Growth Regulators

• Often referred to as “hormones”– adapted from animal physiology– animal hormones are produced in one

place (gland) and exert an effect some other location

– plant “hormones” don’t always work like that

– “growth regulator” better descriptive term

Five Basic Groups• Auxins• Cytokinins• Ethylene• Abscisic Acid• Gibberellins

Early Work on Phototropism• Charles Darwin and his son Francis

published early work on the problem:– “Power of Movement in Plants” (1880)–Worked with oat (Avena) Coleoptiles–Demonstrated importance of the “tip”

in the plant response

Early Darwin Experiments

Discovery of Auxin

• Fritz Went (1928)–Found that he could collect the

substance responsible for the bending response

– Developed the Avena Coleoptile Curvature Test (first bioassay)

–Went called the unknown substance AUXIN

Coleoptile Curvature Test

What is Auxin?

• German workers in the 1930’s identified Went’s auxin as indole-acetic acid (IAA)–Lucky discovery based on study of

urine (so the story goes) in pregnant women

Formula of IAA

Effects of IAA In Phototropism

• Causes local elongation of cells on the shaded side

• This unequal elongation causes the bending of the stem (coleoptile) toward the light

• Mechanism of action is called the acid growth hypothesis

Acid Growth Hypothesis• IAA triggers H+ ion transport which

lowers the pH• This drop in pH loosens cell wall

structure probably via proteins (expansins) which “break” bonds holding the cellulose microfibrils

• This creates a “loosening” of the cell wall structure so turgor pressure can “expand” the cells

• The effect is quite rapid

Cell Wall Expansion

Natural vs. Synthetic Auxins

• IAA (indole-acetic acid) is the naturally occurring growth regulator

• Synthetic auxins– substances which will cause bending the

Avena Coleoptile Curvature Test– examples include: indole-butyric acid (IBA),

naphthalene acid acid (NAA), 2,4-D and 2,4,5-T

AUXIN RESPONSES

• Apical Dominance– IAA produced in the shoot apex inhibits the

development of lateral (axillary) buds– However, it may be that high IAA

concentrations stimulate ethylene production which actually inhibits the bud development

– Concept used frequently in horticulture (creation of “bushy” shrubs)

Other Auxin Responses (cont.)

• Abscission– Actively growing leaves and fruits produce

large amounts of auxin (IAA) which is transported to the stem

– This inhibits abscission of leaves and fruits– Environmental or age changes stimulate

production of ethylene which stimulates production of abscission zone forming enzymes

Other Auxin Responses (cont.)

• Differentiation of Vascular Tissue– Auxin + gibberellins

and/or various concentrations of sucrose can stimulate development of xylem/phloem (either or both)

Other Auxin Responses (cont.)

• Fruit Development– Seeds (result of fertilization) are a source of auxin,

which in turn stimulates the formation (not ripening) of the fruit

– May form parthenocarpic fruits (tomato and cucumber)

Other Auxin Responses (cont.)

• Adventitious Root Formation– Several synthetic auxins (especially IBA)

are used commercially to stimulate root development in “cuttings”

– Some plants produce enough IAA in the shoot or leaves to stimulate the root formation in a cutting without additional hormone

Other Auxin Responses (cont.)

• Weed Killers and Defoliants– 2,4-D and 2,4,5-T

effective against dicots

– manufacture as part of “Agent Orange” produced toxic trace molecule dioxin (a carcinogen)

Cytokinins• Discovered in the 1940’s in attempting

plant tissue culture• Found that coconut milk stimulated cell

division• Trail led to “old herring sperm DNA”• Isolated “kinetin” and dubbed the group

of growth regulators cytokinins (after cytokinesis in cell division)

Cytokinins (cont.)• Zeatin the first naturally-occurring

cytokinin to be isolated• Most cytokinins are produced in roots,

but also in seeds, fruits and young leaves

• Effects include:– stimulation of cell division– retard senescence in leaf tissues (once

used as a bioassay)

Cytokinins (cont.)

– with IAA, stimulate formation of either roots or shoot• HIGH IAA, low

cytokinin = ROOTS• HIGH CYTOKININ,

low IAA = SHOOTS

Cytokinins (cont.)

–generally a “juvenile” hormone = keeps things young

–used commercially to keep cut foliage “green and fresh”, but NOT for human consumption (a suspected carcinogen because of its nucleotide structure)

Strange Observations?• Burning of “illuminating gas” in Europe

in 1800s caused trees near street lights to become defoliated on one side

• Oranges can cause rapid ripening of bananas (don’t store them together)

• The ancient Chinese burned incense in special huts to ripen fruit

• “One bad apple can spoil a whole bushel”

Ethylene• A gas (H2C=CH2), unusual for a growth

regulator• Produced in most CLIMATERIC fruits

like apples, oranges, tomatoes, bananas

• Used widely in the commercial fruit industry (here in Omaha)– The apples you buy in March were

probably picked in September

Other Ethylene Responses• Promotes flowering in some plants like

mangos and pineapples– Some growers may actually set fires near crops

• May induce senescence in some flowers (orchids)

• Generally promotes leaf and fruit abscission

• Also involved in in monocot sex expression flowers, stem elongation (shaking response inhibits normal elongation), waterlogging effects (epinasty)

Abscisic Acid (ABA)• First extracted from dormant buds and

called dormin• Later, found to be chemically identical

with another compound called abscisic acid (unfortunate choice since it is not involved in abscission)

• Involved in closure of stomata (guard cells) by stimulating loss of K+ ions (followed by water loss and closure)

Gibberellins• Discovered by E. Kurosawa studying

“foolish seedling” disease of rice• Fungus, Gibberella fujikuroi, found to be

disease agent• Could induce symptoms (stem

elongation) from fungus extract• Later, found same substances in plants

themselves

Effects of Gibberellins

• Growth of Intact Plants– elongation AND cell

division throughout the plant (unlike auxin)

– overcomes genetic dwarfing

Gibberellins and Mendel• One of the 7 pairs of traits that Mendel studied in

peas as he worked out the basic rules of inheritance was dwarf-tall.

• The recessive gene - today called le - turns out to encode an enzyme that is defective in enabling the plant to synthesize GA.

• The dominant gene, Le, encodes a functioning enzyme permitting normal GA synthesis and making the "tall" phenotype.

Effects ofGibberellins (cont.)• Seed

Germination (grasses)– produced by the

embryo and stimulates the aleurone layer to synthesize amylases

Starch Digestion in Seeds

Treated with water

Treated with 1 ppb

Treated with 1 ppm GA

Effects of Gibberellins (cont.)

–Used in production of sugarcane (increases biomass)

–Mechanism of action not involved in cell wall acidification

• May overcome light or cold requirements for seed germination

• used in brewing industry to help germinate barley and produce the “malt”

Effects of Gibberellins (cont.)

• Flowering/Bolting of Biennials– can substitute for

“winter” cold period for bolting (flowering) in rosette biennials

Effects of Gibberellins (cont.)

• Fruit Formation– used to

produce larger fruits in open clusters in Thompson seedless grapes