Internal Control of Development: The Plant Growth Regulators Angiosperms VII Internal 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 of Gibberellins (cont.) Seed Germination (grasses) produced by the embryo and stimulates the aleurone layer to synthesize amylases

Starch Digestion in Seeds Treated with 1 ppm GA Treated with 1 ppb Treated with water

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