Principles of Biology BIOL 100C: Introductory Biology III Plant Hormones & Plant Defenses Dr. P. Narguizian Fall 2012

Phototropism  Phototropism is a phenomenon by which plants grow toward a light source  Phototropism occurs when the cells on the dark side of a plant stem elongate faster than those on the light side  Charles Darwin and his son Francis conducted experiments that showed that the shoot tips of plants controlled their ability to grow toward light  Peter Boysen-Jensen later conducted experiments that showed that chemical signals produced in shoot tips were responsible for phototropism

Light Shaded side of shoot Illuminated side of shoot

 The Darwins’ experiment –When plant tips were removed, plants did not grow toward light –When plant tips were covered with an opaque cap, they did not grow toward light –When plant tips were covered with a clear tip, they did grow toward light Experiments on how plants turn toward light led to the discovery of a plant hormone

 Jensen’s experiment –When a gelatin block that allowed chemical diffusion was placed below the shoot tip, plants grew toward light –When a mica block that prevented chemical diffusion was placed below the shoot tip, plants did not grow toward light Experiments on how plants turn toward light led to the discovery of a plant hormone

Light Control Tip removed Tip covered by opaque cap Tip covered by trans- parent cap Base covered by opaque shield Tip sepa- rated by gelatin block Tip separated by mica Boysen-Jensen (1913) Darwin and Darwin (1880)

Five major types of hormones regulate plant growth and development  A hormone is a chemical signal that is produced in one part of the body and transported to another, where it triggers responses in target cells  Binding of hormones to specific cellular receptors triggers a signal transduction pathway  Tiny amounts of hormone can have a big effect

 All aspects of plant growth and development are affected by hormones  There are five classes of plant hormones and each class can have multiple effects on plant growth and development Five major types of hormones regulate plant growth and development

Auxin stimulates the elongation of cells in young shoots  Indoleacetic acid (IAA) is a naturally occurring auxin that promotes seedling elongation  Auxin is produced in shoot apical meristems and transported downward through a plant Copyright © 2009 Pearson Education, Inc.

Auxin stimulates the elongation of cells in young shoots  Concentration of auxin and site of activity are important to auxin’s effects –In moderate concentrations, auxin promotes cell elongation in stems –In high concentrations, auxin reduces cell elongation in stems –Auxins affects cell elongation in roots at lower concentrations Copyright © 2009 Pearson Education, Inc.

 A hypothesis for the action of auxin –Auxins stimulate plant cells to take up H + ions, lowering pH –Acidity causes separation of cross linkages in cellulose –As the cell takes up water, the cell elongates because of weakening of the cellulose cell wall –Auxins stimulate the plant to produce additional cell wall material –As pH decreases, the larger cell wall restabilizes Auxin stimulates the elongation of cells in young shoots Copyright © 2009 Pearson Education, Inc.

Plasma membrane Cell wall Vacuole Cytoplasm Proton pump (protein) H+H+ 1 H+H+ 2 Cell wall Cellulose molecule 3 Cell elongation H2OH2O Enzyme Cellulose loosens; cell can elongate Cellulose molecule Cross-linking molecule

Cytokinins stimulate cell division  Cytokinins promote cytokinesis, or cell division  Cytokinins –Are produced in actively growing organs such as roots, embryos, and fruits –Produced in roots move upward through the plant –Retard aging in leaves and flowers Copyright © 2009 Pearson Education, Inc.

 Cytokinins and auxins interact to control apical dominance –Auxins inhibit axillary bud growth, reducing lateral branching –Cytokinins counter the action of auxin by promoting axillary bud growth –The ratio of auxins to cytokinins controls axillary bud growth 33.4 Cytokinins stimulate cell division Copyright © 2009 Pearson Education, Inc.

Terminal bud No terminal bud

Gibberellins affect stem elongation and have numerous other effects  Gibberellins are plant hormones that promote stem elongation by increasing cell division and elongation  Gibberellins were named for a genus of fungi that produce the same chemical and cause “foolish seedling” disease  There are more than 100 distinct gibberellins produced primarily in roots and young leaves Copyright © 2009 Pearson Education, Inc.

 Gibberellins also promote fruit development and seed germination  Gibberellins act antagonistically against another plant hormone called abscisic acid Gibberellins affect stem elongation and have numerous other effects Copyright © 2009 Pearson Education, Inc.

Abscisic acid inhibits many plant processes  Abscisic acid (ABA) is a plant hormone that inhibits growth  High concentrations of ABA promote seed dormancy –ABA must be removed for germination to occur –The ratio of ABA to gibberellins controls germination Copyright © 2009 Pearson Education, Inc.

 ABA also influences plant water relations –Accumulation of ABA in wilted leaves promotes stomatal closure –ABA produced in roots can signal low soil moisture conditions and triggers plants to conserve water by closing stomata Abscisic acid inhibits many plant processes Copyright © 2009 Pearson Education, Inc.

Ethylene triggers fruit ripening and other aging processes  Ethylene is a gaseous by-product of natural gas combustion and a naturally occurring plant hormone  Plants produce ethylene in response to stresses such as mechanical pressure, injury, infection, and drought or flood Copyright © 2009 Pearson Education, Inc.

 Ethylene promotes aging processes such as fruit ripening and natural cell death –It is used commercially to ripen fruits –Growers inhibit ethylene production using CO 2 to inhibit ripening in stored fruit  Ethylene promotes leaf abscission in fall by breaking down cells at the base of the petiole Ethylene triggers fruit ripening and other aging processes Copyright © 2009 Pearson Education, Inc.

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GROWTH RESPONSES AND BIOLOGICAL RHYTHMS IN PLANTS Copyright © 2009 Pearson Education, Inc.

Tropisms orient plant growth toward or away from environmental stimuli  Tropisms are responses that cause plants to grow in response to environmental stimuli –Positive tropisms cause plants to grow toward a stimulus –Negative tropisms cause plants to grow away from a stimulus  Plants respond to various environmental stimuli –Phototropism—response to light –Gravitropism—response to gravity –Thigmotropism—response to touch Copyright © 2009 Pearson Education, Inc.

Light Phototropism Gravity Gravitropism Thigmotropism

PLANT DEFENSES Copyright © 2009 Pearson Education, Inc.

EVOLUTION CONNECTION: Defenses against herbivores and infectious microbes have evolved in plants  Herbivores are organisms that feed on plants; many plant adaptations have evolved to defend against herbivores –Production of distasteful or poisonous compounds –Symbioses with organisms that defend plants Copyright © 2009 Pearson Education, Inc.

 Plants have also evolved defenses against pathogens –The epidermis is the first line of defense against infection –Chemical defenses offer a way to fight pathogens that enter the plant EVOLUTION CONNECTION: Defenses against herbivores and infectious microbes have evolved in plants Copyright © 2009 Pearson Education, Inc.

Wasp lays eggs Damage to plant and chemical in caterpillar saliva Synthesis and release of chemical attractants Signal transduction pathway Plant cell Recruitment of wasp

TALKING ABOUT SCIENCE: Plant biochemist Eloy Rodriguez studies how animals use defensive chemicals made by plants  Animals may “medicate” themselves with chemicals produced by plants  Scientists observe which plants animals eat for “medicinal” purposes and how much of each plant they eat Copyright © 2009 Pearson Education, Inc.

 Observation of such animal behavior has led scientists to study how such chemicals might benefit humans –Plant chemicals can kill animal parasites –Some may be useful for treatment of tumors TALKING ABOUT SCIENCE: Plant biochemist Eloy Rodriguez studies how animals use defensive chemicals made by plants Copyright © 2009 Pearson Education, Inc.