Chapter 33 Control Systems in Plants

Chapter 33 Control Systems in Plants PowerPoint Lectures for Biology: Concepts & Connections, Sixth Edition Campbell, Reece, Taylor, Simon, and Dickey Copyright 2009 Pearson Education, Inc. Lecture by L. Brooke Stabler

Introduction: What Are the Health Benefits of Soy? Soy protein is one of the few plant proteins that provide all of the essential amino acids Benefits of soy include It reduces the risk of heart disease It is rich in antioxidants and fiber It is low in fat and helps increase good cholesterol (HDL) while reducing bad cholesterol (LDL) Soy contains phytoestrogens, hormones that can reduce the symptoms of menopause in women More research into the use of phytohormones to treat the symptoms of menopause is needed Copyright 2009 Pearson Education, Inc.

33.1 Experiments on how plants turn toward light led to the discovery of a plant hormone 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 Copyright 2009 Pearson Education, Inc.

Shaded side of shoot Light Illuminated side of shoot

33.1 Experiments on how plants turn toward light led to the discovery of a plant hormone 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 Copyright 2009 Pearson Education, Inc.

33.1 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 Copyright 2009 Pearson Education, Inc.

Light Control 1 2 3 4 5 6 Tip removed Tip covered by opaque cap Tip covered by transparent cap Base covered by opaque shield Tip separated by gelatin block Tip separated by mica Darwin and Darwin (1880) Boysen-Jensen (1913)

33.1 Experiments on how plants turn toward light led to the discovery of a plant hormone A graduate student named Frits Went isolated the chemical hormone responsible for phototropism Plant tips were placed on an agar block to allow the chemical signal molecules to diffuse from the plant tip to the agar When agar blocks containing chemical signals were centered on the ends of decapitated plants, they grew straight When agar blocks were offset to one side of the decapitated plants, they bent away from the side with the agar block Copyright 2009 Pearson Education, Inc.

33.1 Experiments on how plants turn toward light led to the discovery of a plant hormone Went concluded that a chemical produced in the shoot tip was transferred down through the plant, and high concentration of that chemical increased cell elongation on the dark side of the plant The chemical signal responsible for phototropism is a hormone that Went called auxin Copyright 2009 Pearson Education, Inc.

Shoot tip placed on agar block. Chemical diffuses from shoot tip into agar. Agar Control 1 Block with chemical stimulates growth. No light

Agar Control 1 Block with chemical stimulates growth. 2 Shoot tip placed on agar block. Chemical diffuses from shoot tip into agar. Offset blocks with chemical stimulate curved growth. No light

Agar Shoot tip placed on agar block. Chemical diffuses from shoot tip into agar. Control 1 Block with chemical stimulates growth. 2 Offset blocks with chemical stimulate curved growth. 3 Other controls: Blocks with no chemical have no effect. No light

33.2 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 Copyright 2009 Pearson Education, Inc.

33.2 Five major types of hormones regulate plant growth and development 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 Copyright 2009 Pearson Education, Inc.

33.3 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.

33.3 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.

Elongation Inhibition Promotion Stems 0 Roots 0.9 g/l 10 8 10 6 10 4 10 2 1 10 2 Increasing auxin concentration (g/l)

33.3 Auxin stimulates the elongation of cells in young shoots 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 Copyright 2009 Pearson Education, Inc.

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

33.4 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.

33.4 Cytokinins stimulate cell division 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 Copyright 2009 Pearson Education, Inc.

Terminal bud No terminal bud

33.5 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.

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

33.6 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.

33.6 Abscisic acid inhibits many plant processes 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 Copyright 2009 Pearson Education, Inc.

33.7 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.

33.7 Ethylene triggers fruit ripening and other aging processes 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 Copyright 2009 Pearson Education, Inc.

1 3 2

Leaf stalk Stem (twig) Protective layer Stem Abscission layer Leaf stalk

33.8 CONNECTION: Plant hormones have many agricultural uses Agricultural uses of plant hormones include Control of fruit production, ripening, and dropping Production of seedless fruits Use as weed killers Agricultural uses of plant hormones help keep food prices down and benefit the environment Some consumers are concerned that synthetic plant hormones may have dangerous side effects for humans Copyright 2009 Pearson Education, Inc.

33.9 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.

33.10 Plants have internal clocks Circadian rhythms are innate biological cycles of approximately 24 hours Both plants and animals have circadian rhythms Circadian rhythms are influenced by environmental cues such as light, but they are controlled by biological clocks The biological clocks of plants are likely the result of rhythmic production of proteins that influence gene expression Copyright 2009 Pearson Education, Inc.

Noon Midnight

33.11 Plants mark the seasons by measuring photoperiod Flowering, seed germination, and dormancy are all seasonal phenomena in plants Plants detect season by measuring photoperiod, the relative lengths of day and night Copyright 2009 Pearson Education, Inc.

33.11 Plants mark the seasons by measuring photoperiod Plant flowering signals are determined by night length Short-day plants flower when the dark period is greater than some critical length Long-day plants flower when the dark period is shorter than some critical length Experiments that altered light and dark periods were used to determine that it is night length and not day length that cues plants to flower Copyright 2009 Pearson Education, Inc.

Time (hr) Critical night length 24 1 2 3 4 5 6 Darkness Flash of light Light 0 Short-day (long-night) plants Long-day (short-night) plants

Time (hr) Critical night length 24 1 2 3 Darkness Flash of light 0 Light Short-day (long-night) plants

Time (hr) Critical night length 24 4 5 6 Darkness Flash of light 0 Light Long-day (short-night) plants

33.12 Phytochrome is a light detector that may help set the biological clock Phytochromes are proteins with a light-absorbing component Phytochromes detect light in the red and far-red wavelengths One form of phytochrome absorbs red light (P r ) One form detects far-red light (P fr ) When P r absorbs light, it is converted into P fr When P fr absorbs light, it is converted into P r Copyright 2009 Pearson Education, Inc.

33.12 Phytochrome is a light detector that may help set the biological clock P r is naturally produced during dark hours, while P fr is broken down The relative amounts of P r and P fr present in a plant change as day length changes Copyright 2009 Pearson Education, Inc.

Red light Rapid conversion in daylight P r P fr Slow conversion in darkness Far-red light

Time (hr) Critical night length Short-day (long-night) plant 24 20 R FR R R FR R FR R FR R 16 12 8 4 0 1 2 3 4 Long-day (short-night) plant

33.13 TALKING ABOUT SCIENCE: Joanne Chory studies the effects of light and hormones in the model plant Arabidopsis Scientists often use small and easily manipulated species as models to learn about biological processes Arabidopsis is a plant in the mustard family that has been used extensively to study plant genetics and physiology Dr. Joanne Chory has used Arabidopsis to study genes that control hormones and signal transduction pathways; her work has many applications in science and agriculture Copyright 2009 Pearson Education, Inc.

33.14 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.

33.14 EVOLUTION CONNECTION: Defenses against herbivores and infectious microbes have evolved in plants 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 Copyright 2009 Pearson Education, Inc.

1 Damage to plant and chemical in caterpillar saliva Plant cell

1 Damage to plant and chemical in caterpillar saliva 2 Plant cell Signal transduction pathway

1 Damage to plant and chemical in caterpillar saliva 2 3 Synthesis and release of chemical attractants Plant cell Signal transduction pathway

4 Recruitment of wasp 1 Damage to plant and chemical in caterpillar saliva 2 3 Synthesis and release of chemical attractants Plant cell Signal transduction pathway

5 Wasp lays eggs 4 Recruitment of wasp 1 Damage to plant and chemical in caterpillar saliva 2 3 Synthesis and release of chemical attractants Plant cell Signal transduction pathway

1 Binding of pathogen s signal molecule to plant s receptor molecule Avirulent pathogen

1 Binding of pathogen s signal molecule to plant s receptor molecule Avirulent pathogen 2 Signal transduction pathway

1 Binding of pathogen s signal molecule to plant s receptor molecule 3 Enhanced local response Avirulent pathogen 2 Signal transduction pathway R-Avr recognition leading to a strong local response

1 Binding of pathogen s signal molecule to plant s receptor molecule 3 Enhanced local response 4 Avirulent pathogen 2 Signal transduction pathway Hormones R-Avr recognition leading to a strong local response

1 Binding of pathogen s signal molecule to plant s receptor molecule 3 5 Enhanced Signal local transduction response pathway 4 Avirulent pathogen 2 Signal transduction pathway Hormones R-Avr recognition leading to a strong local response

1 Binding of pathogen s signal molecule to plant s receptor molecule Avirulent pathogen 2 3 5 Enhanced Signal local transduction response pathway 4 Signal transduction pathway Hormones 6 Additional defensive chemicals R-Avr recognition leading to a strong local response Systemic acquired resistance

33.15 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.

33.15 TALKING ABOUT SCIENCE: Plant biochemist Eloy Rodriguez studies how animals use defensive chemicals made by plants 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 Copyright 2009 Pearson Education, Inc.

Gravity Light Phototropism Gravitropism Thigmotropism

Critical night length Critical night length Short-day (long-night) plants Long-day (short-night) plants

(a) stimulates inhibits (c) enhances Cell elongation Axillary bud growth opposes (b) stimulates stimulates (d) (e) inhibits inhibits (g) opposes Leaf abscission Seed dormancy opposes (f) stimulates stimulates (h)

Chlorophyll fluorescence 6 5 4 3 2 1 0 25 30 35 40 45 50 55 Leaf temperature (ºC)

You should now be able to 1. Explain what hormones are and how they work 2. Describe the experiments that led to the discovery of auxins 3. Name the five general classes of plant hormones and describe the actions of each class 4. Explain what tropisms are and give examples of different kinds of plant tropisms Copyright 2009 Pearson Education, Inc.

You should now be able to 5. Describe circadian rhythms and biological clocks; recognize the innate basis of such rhythms and how they are affected by environmental cues 6. Explain the difference between short-day and longday plants 7. Describe the experiments that led to the discovery of the effects of night length on flowering 8. Explain how plants detect seasons using proteins Copyright 2009 Pearson Education, Inc.

You should now be able to 9. Give examples of plant defenses that have evolved to protect plants against herbivores and pathogens 10. Explain how scientists can help treat human diseases by studying the things that other animals eat Copyright 2009 Pearson Education, Inc.