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Biology 102 Week 7 Angiosperm Reproduction and Biotechnology PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: To Seed or Not to Seed • The parasitic plant Rafflesia arnoldi produces huge flowers that produce up to 4 million seeds • Many angiosperms reproduce sexually and asexually • Since the beginning of agriculture, plant breeders have genetically manipulated traits of wild angiosperm species by artificial selection Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 38.1: Pollination enables gametes to come together within a flower • In angiosperms, the sporophyte is the dominant generation, the large plant that we see • The gametophytes are reduced in size and depend on the sporophyte for nutrients • Male gametophytes (pollen grains) and female gametophytes (embryo sacs) develop within flowers Video: Flower Blooming (time lapse) Video: Time Lapse of Flowering Plant Life Cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 38-2a Anther Stamen Filament Stigma Carpel Style Ovary Sepal Petal Key Haploid (n) Diploid (2n) Receptacle An idealized flower LE 38-2b Germinated pollen grain (n) (male gametophyte) Anther Ovary Ovule Embryo sac (n) (female gametophyte) Pollen tube FERTILIZATION Egg (n) Mature Sperm (n) sporophyte plant (2n) Zygote (2n) Seed Key Seed Haploid (n) Diploid (2n) Germinating seed Simplified angiosperm life cycle Embryo (2n) (sporophyte) Simple fruit Flower Structure • Flowers are the reproductive shoots of the angiosperm sporophyte • They consist of four floral organs: sepals, petals, stamens, and carpels • Many flower variations have evolved during the 140 million years of angiosperm history Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 38-3a SYMMETRY OVARY LOCATION FLORAL DISTRIBUTION Bilateral symmetry (orchid) Lupine inflorescence Superior ovary Radial symmetry (daffodil) Sepal Fused petals Semi-inferior ovary Inferior ovary Sunflower inflorescence LE 38-3b REPRODUCTIVE VARIATIONS Maize, a monoecious species Dioecious Sagittaria latifolia (common arrowhead) Gametophyte Development and Pollination • In angiosperms, pollination is the transfer of pollen from an anther to a stigma • If pollination succeeds, a pollen grain produces a pollen tube that grows down into the ovary and discharges sperm near the embryo sac • Pollen develops from microspores within the sporangia of anthers Video: Bat Pollinating Agave Plant Video: Bee Pollinating Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 38-4 Development of a male gametophyte (pollen grain) Development of a female gametophyte (embryo sac) Pollen sac (microsporangium) Megasporangium Microsporocyte Ovule MEIOSIS Megasporocyte Integuments Micropyle Microspores (4) Surviving megaspore Each of 4 microspores Female gametophyte (embryo sac) MITOSIS Ovule Generative cell (will form 2 sperm) Male gametophyte (pollen grain) Antipodal cells (3) Polar nuclei (2) Egg (1) Integuments Nucleus of tube cell Synergids (2) 20 µm Key to labels Haploid (n) Diploid (2n) 100 µm 75 µm (LM) Ragweed pollen grain (colorized SEM) Embryo sac (LM) • Embryo sacs develop from megaspores within ovules Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mechanisms That Prevent Self-Fertilization • Many angiosperms have mechanisms that make it difficult or impossible for a flower to self-fertilize • The most common is self-incompatibility, a plant’s ability to reject its own pollen • Researchers are unraveling the molecular mechanisms involved in self-incompatibility Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 38-5 Stigma Stigma Anther with pollen Pin flower Thrum flower • Some plants reject pollen that has an S-gene matching an allele in the stigma cells • Recognition of self pollen triggers a signal transduction pathway leading to a block in growth of a pollen tube Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 38.2: After fertilization, ovules develop into seeds and ovaries into fruits • In angiosperms, the products of fertilization are seeds and fruits Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Double Fertilization • After landing on a receptive stigma, a pollen grain produces a pollen tube that extends between the cells of the style toward the ovary • The pollen tube then discharges two sperm into the embryo sac • One sperm fertilizes the egg, and the other combines with the polar nuclei, giving rise to the food-storing endosperm Animation: Plant Fertilization Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 38-6 Pollen grain Stigma Pollen tube If a pollen grain germinates, a pollen tube grows down the style toward the ovary. 2 sperm Style Ovary Polar nuclei Ovule (containing female gametophyte, or embryo sac) Egg Micropyle Ovule Polar nuclei The pollen tube discharges two sperm into the female gametophyte (embryo sac) within an ovule. One sperm fertilizes the egg, forming the zygote. The other sperm combines with the two polar nuclei of the embryo sac’s large central cell, forming a triploid cell that develops into the nutritive tissue called endosperm. Egg Two sperm about to be discharged Endosperm nucleus (3n) (2 polar nuclei plus sperm) Zygote (2n) (egg plus sperm) From Ovule to Seed • After double fertilization, each ovule develops into a seed • The ovary develops into a fruit enclosing the seed(s) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Endosperm Development • Endosperm development usually precedes embryo development • In most monocots and some eudicots, endosperm stores nutrients that can be used by the seedling • In other eudicots, the food reserves of the endosperm are exported to the cotyledons Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Embryo Development • The first mitotic division of the zygote is transverse, splitting the fertilized egg into a basal cell and a terminal cell Animation: Seed Development Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 38-7 Ovule Endosperm nucleus Integuments Zygote Zygote Terminal cell Basal cell Proembryo Suspensor Basal cell Cotyledons Shoot apex Root apex Suspensor Seed coat Endosperm Structure of the Mature Seed • The embryo and its food supply are enclosed by a hard, protective seed coat Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In some eudicots, such as the common garden bean, the embryo consists of the hypocotyl, radicle (embryonic root), and thick cotyledons (seed leaves) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 38-8a Seed coat Epicotyl Hypocotyl Radicle Cotyledons Common garden bean, a eudicot with thick cotyledons • The seeds of some eudicots, such as castor beans, have thin cotyledons Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 38-8b Seed coat Endosperm Cotyledons Epicotyl Hypocotyl Radicle Castor bean, a eudicot with thin cotyledons • A monocot embryo has one cotyledon • Grasses, such as maize and wheat, have a special cotyledon called a scutellum • Two sheathes enclose the embryo of a grass seed: a coleoptile covering the young shoot and a coleorhiza covering the young root Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 38-8c Scutellum (cotyledon) Coleoptile Pericarp fused with seed coat Endosperm Epicotyl Hypocotyl Coleorhiza Maize, a monocot Radicle From Ovary to Fruit • A fruit develops from the ovary • It protects the enclosed seeds and aids in seed dispersal by wind or animals • Depending on developmental origin, fruits are classified as simple, aggregate, or multiple Animation: Fruit Development Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 38-9a Ovary Stamen Stigma Ovule Pea flower Seed Pea fruit Simple fruit LE 38-9b Carpels Stamen Raspberry flower Carpel (fruitlet) Stigma Ovary Stamen Raspberry fruit Aggregate fruit LE 38-9c Flower Pineapple inflorescence Each segment develops from the carpel of one flower Pineapple fruit Multiple fruit Seed Germination • As a seed matures, it dehydrates and enters a phase called dormancy Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Seed Dormancy: Adaptation for Tough Times • Seed dormancy increases the chances that germination will occur at a time and place most advantageous to the seedling • The breaking of seed dormancy often requires environmental cues, such as temperature or lighting changes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings From Seed to Seedling • Germination depends on imbibition, the uptake of water due to low water potential of the dry seed • The radicle (embryonic root) emerges first • Next, the shoot tip breaks through the soil surface • In many eudicots, a hook forms in the hypocotyl, and growth pushes the hook above ground Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 38-10a Foliage leaves Cotyledon Epicotyl Hypocotyl Cotyledon Cotyledon Hypocotyl Hypocotyl Radicle Seed coat Common garden bean • In maize and other grasses, which are monocots, the coleoptile pushes up through the soil Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 38-10b Foliage leaves Coleoptile Coleoptile Radicle Maize Concept 38.3: Many flowering plants clone themselves by asexual reproduction • Many angiosperm species reproduce both asexually and sexually • Sexual reproduction generates genetic variation that makes evolutionary adaptation possible • Asexual reproduction in plants is also called vegetative reproduction Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mechanisms of Asexual Reproduction • Fragmentation, separation of a parent plant into parts that develop into whole plants, is a very common type of asexual reproduction • In some species, a parent plant’s root system gives rise to adventitious shoots that become separate shoot systems Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Vegetative Propagation and Agriculture • Humans have devised methods for asexual propagation of angiosperms • Most methods are based on the ability of plants to form adventitious roots or shoots Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Clones from Cuttings • Many kinds of plants are asexually reproduced from plant fragments called cuttings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Grafting • A twig or bud can be grafted onto a plant of a closely related species or variety Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Test-Tube Cloning and Related Techniques • Plant biologists have adopted in vitro methods to create and clone novel plant varieties • Protoplast fusion is used to create hybrid plants by fusing protoplasts, plant cells with their cell walls removed Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 38-12 Just a few parenchyma cells from a carrot gave rise to this callus, a mass of undifferentiated cells. The callus differentiates into an entire plant, with leaves, stems, and roots. Concept 38.4: Plant biotechnology is transforming agriculture • Plant biotechnology has two meanings: – In a general sense, it refers to innovations in use of plants to make useful products – In a specific sense, it refers to use of genetically modified (GM) organisms in agriculture and industry Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Artificial Selection • Humans have intervened in the reproduction and genetic makeup of plants for thousands of years • Hybridization is common in nature and has been used by breeders to introduce new genes • Maize, a product of artificial selection, is a staple in many developing countries Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Modern plant biotechnology is not limited to transfer of genes between closely related species or varieties of the same species Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reducing World Hunger and Malnutrition • Genetically modified plants may increase the quality and quantity of food worldwide • Progress has been made in developing transgenic plants that tolerate herbicides Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Nutritional quality of plants is being improved • “Golden Rice” is a transgenic variety being developed to address vitamin A deficiencies among the world’s poor Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 38-16 Genetically modified rice Ordinary rice The Debate over Plant Biotechnology • Some biologists are concerned about risks of releasing GM organisms into the environment Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Issues of Human Health • One concern is that genetic engineering may transfer allergens from a gene source to a plant used for food Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Possible Effects on Nontarget Organisms • Many ecologists are concerned that the growing of GM crops might have unforeseen effects on nontarget organisms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Addressing the Problem of Transgene Escape • Perhaps the most serious concern is the possibility of introduced genes escaping into related weeds through crop-to-weed hybridization • Efforts are underway to breed male sterility into transgenic crops Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 39 Plant Responses to Internal and External Signals PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: Stimuli and a Stationary Life • Plants, being rooted to the ground, must respond to environmental changes that come their way • For example, the bending of a seedling toward light begins with sensing the direction, quantity, and color of the light Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.1: Signal transduction pathways link signal reception to response • Plants have cellular receptors that detect changes in their environment • For a stimulus to elicit a response, certain cells must have an appropriate receptor Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A potato left growing in darkness produces shoots that look unhealthy and lacks elongated roots • These are morphological adaptations for growing in darkness, collectively called etiolation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-2 Before exposure to light. A dark-grown potato has tall, spindly stems and nonexpanded leaves— morphological adaptations that enable the shoots to penetrate the soil. The roots are short, but there is little need for water absorption because little water is lost by the shoots. After a week’s exposure to natural daylight. The potato plant begins to resemble a typical plant with broad green leaves, short sturdy stems, and long roots. This transformation begins with the reception of light by a specific pigment, phytochrome. • After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A potato’s response to light is an example of cellsignal processing • The stages are reception, transduction, and response Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-3 CELL WALL Reception CYTOPLASM Transduction Relay molecules Receptor Hormone or environmental stimulus Plasma membrane Response Activation of cellular responses Reception • Internal and external signals are detected by receptors, proteins that change in response to specific stimuli Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transduction • Second messengers transfer and amplify signals from receptors to proteins that cause responses Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-4_3 Reception Response Transduction Transcription NUCLEUS factor 1 CYTOPLASM Plasma membrane cGMP Second messenger produced Specific protein kinase 1 activated Transcription factor 2 Phytochrome activated by light Cell wall Specific protein kinase 2 activated Transcription Light Translation Ca2+ channel opened Ca2+ De-etiolation (greening) response proteins Response • A signal transduction pathway leads to regulation of one or more cellular activities • In most cases, these responses to stimulation involve increased activity of enzymes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transcriptional Regulation • Transcription factors bind directly to specific regions of DNA and control transcription of genes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Post-Translational Modification of Proteins • Post-translational modification involves activation of existing proteins in the signal response Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings De-Etioloation (“Greening”) Proteins • Many enzymes that function in certain signal responses are directly involved in photosynthesis • Other enzymes are involved in supplying chemical precursors for chlorophyll production Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli • Hormones are chemical signals that coordinate different parts of an organism Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Discovery of Plant Hormones • Any response resulting in curvature of organs toward or away from a stimulus is called a tropism • Tropisms are often caused by hormones Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light • They observed that a seedling could bend toward light only if the tip of the coleoptile was present • They postulated that a signal was transmitted from the tip to the elongating region Video: Phototropism Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-5a Shaded side of coleoptile Control Light Illuminated side of coleoptile LE 39-5b Darwin and Darwin (1880) Light Tip Tip removed covered by opaque cap Base covered Tip covered by opaque by trans- shield parent cap • In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-5c Boysen-Jensen (1913) Light Tip separated Tip separated by by mica gelatin block • In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-6 Excised tip placed on agar block Growth-promoting chemical diffuses into agar block Control Control (agar block lacking chemical) has no effect Agar block with chemical stimulates growth Offset blocks cause curvature A Survey of Plant Hormones • In general, hormones control plant growth and development by affecting the division, elongation, and differentiation of cells • Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Auxin • The term auxin refers to any chemical that promotes cell elongation in target tissues • Auxin transporters move the hormone from the basal end of one cell into the apical end of the neighboring cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-7 Cell 1 100 µm Cell 2 Epidermis Cortex Phloem Xylem Pith Basal end of cell 25 µm The Role of Auxin in Cell Elongation • According to the acid growth hypothesis, auxin stimulates proton pumps in the plasma membrane • The proton pumps lower the pH in the cell wall, activating expansins, enzymes that loosen the wall’s fabric • With the cellulose loosened, the cell can elongate Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-8a Cross-linking cell wall polysaccharides Cell wall enzymes Expansin CELL WALL Microfibril ATP Plasma membrane CYTOPLASM LE 39-8b H 2O Plasma membrane Cell wall Nucleus Cytoplasm Vacuole Lateral and Adventitious Root Formation • Auxin is involved in root formation and branching Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Auxins as Herbicides • An overdose of auxins can kill eudicots Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Other Effects of Auxin • Auxin affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of secondary xylem Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cytokinins • Cytokinins are so named because they stimulate cytokinesis (cell division) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Control of Cell Division and Differentiation • Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits • Cytokinins work together with auxin Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Control of Apical Dominance • Cytokinins, auxin, and other factors interact in the control of apical dominance, a terminal bud’s ability to suppress development of axillary buds Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-9 Axillary buds “Stump” after removal of apical bud Lateral branches Intact plant Plant with apical bud removed • If the terminal bud is removed, plants become bushier Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Anti-Aging Effects • Cytokinins retard the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gibberellins • Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Stem Elongation • Gibberellins stimulate growth of leaves and stems • In stems, they stimulate cell elongation and cell division Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fruit Growth • In many plants, both auxin and gibberellins must be present for fruit to set • Gibberellins are used in spraying of Thompson seedless grapes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Germination • After water is imbibed, release of gibberellins from the embryo signals seeds to germinate Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-11 Aleurone Endosperm a-amylase GA Water Scutellum (cotyledon) GA Radicle Sugar Brassinosteroids • Brassinosteroids are similar to the sex hormones of animals • They induce cell elongation and division Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Abscisic Acid • Two of the many effects of abscisic acid (ABA): – Seed dormancy – Drought tolerance Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Seed Dormancy • Seed dormancy ensures that the seed will germinate only in optimal conditions • Precocious germination is observed in maize mutants that lack a transcription factor required for ABA to induce expression of certain genes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-12 Coleoptile Drought Tolerance • ABA is the primary internal signal that enables plants to withstand drought Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ethylene • Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Triple Response to Mechanical Stress • Ethylene induces the triple response, which allows a growing shoot to avoid obstacles • The triple response consists of a slowing of stem elongation, a thickening of the stem, and horizontal growth Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-13 0.00 0.10 0.20 0.40 Ethylene concentration (parts per million) 0.80 • Ethylene-insensitive mutants fail to undergo the triple response after exposure to ethylene Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-14 ein mutant ctr mutant ein mutant. An ethylene-insensitive (ein) mutant fails to undergo the triple response in the presence of ethylene. ctr mutant. A constitutive triple-response (ctr) mutant undergoes the triple response even in the absence of ethylene. • Other mutants undergo the triple response in air but do not respond to inhibitors of ethylene synthesis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-15 Control Wild-type Ethylene insensitive (ein) Ethylene overproducing (eto) Constitutive triple response (ctr) Ethylene added Ethylene synthesis inhibitor Apoptosis: Programmed Cell Death • A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Leaf Abscission • A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-16 0.5 mm Protective layer Stem Abscission layer Petiole Fruit Ripening • A burst of ethylene production in a fruit triggers the ripening process Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Systems Biology and Hormone Interactions • Interactions between hormones and signal transduction pathways make it hard to predict how genetic manipulation will affect a plant • Systems biology seeks a comprehensive understanding that permits modeling of plant functions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.3: Responses to light are critical for plant success • Light cues many key events in plant growth and development • Effects of light on plant morphology are called photomorphogenesis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Plants detect not only presence of light but also its direction, intensity, and wavelength (color) • A graph called an action spectrum depicts relative response of a process to different wavelengths • Action spectra are useful in studying any process that depends on light Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phototropic effectiveness relative to 436 nm LE 39-17 1.0 0.8 0.6 0.4 0.2 0 400 450 500 550 600 Wavelength (nm) Light Time = 0 min. Time = 90 min. 650 700 • There are two major classes of light receptors: blue-light photoreceptors and phytochromes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Blue-Light Photoreceptors • Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phytochromes as Photoreceptors • Phytochromes regulate many of a plant’s responses to light throughout its life Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phytochromes and Seed Germination • Studies of seed germination led to the discovery of phytochromes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In the 1930s, scientists at the U.S. Department of Agriculture determined the action spectrum for light-induced germination of lettuce seeds Dark (control) Dark Dark Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-18 Dark (control) Red Dark Red Far-red Red Red Far-red Dark Dark Red Far-red Red Far-red • The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-19 A phytochrome consists of two identical proteins joined to form one functional molecule. Each of these proteins has two domains. Chromophore Photoreceptor activity Kinase activity • Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-20 Pr Pfr Red light Responses: seed germination, control of flowering, etc. Synthesis Far-red light Slow conversion in darkness (some plants) Enzymatic destruction Phytochromes and Shade Avoidance • The phytochrome system also provides the plant with information about the quality of light • In the “shade avoidance” response, the phytochrome ratio shifts in favor of Pr when a tree is shaded Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Biological Clocks and Circadian Rhythms • Many plant processes oscillate during the day • Many legumes lower their leaves in the evening and raise them in the morning Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-21 Noon Midnight • Cyclical responses to environmental stimuli are called circadian rhythms and are about 24 hours long • Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Effect of Light on the Biological Clock • Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photoperiodism and Responses to Seasons • Photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year • Photoperiodism is a physiological response to photoperiod Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photoperiodism and Control of Flowering • Some processes, including flowering in many species, require a certain photoperiod • Plants that flower when a light period is shorter than a critical length are called short-day plants • Plants that flower when a light period is longer than a certain number of hours are called longday plants • In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-22 Darkness Flash of light Critical dark period Light “Short-day” plants “Long-day” plants • Action spectra and photoreversibility experiments show that phytochrome is the pigment that receives red light • Red light can interrupt the nighttime portion of the photoperiod Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-23 24 20 R FR R 16 12 8 4 0 Short-day (long-night) plant Long-day (short-night) plant R FR R FR R FR R A Flowering Hormone? • The flowering signal, not yet chemically identified, is called florigen • Florigen may be a hormone or a change in relative concentrations of multiple hormones Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-24 Graft Time (several weeks) Meristem Transition and Flowering • For a bud to form a flower instead of a vegetative shoot, meristem identity genes must first be switched on • Researchers seek to identify the signal transduction pathways that link cues such as photoperiod and hormonal changes to the gene expression required for flowering Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.4: Plants respond to a wide variety of stimuli other than light • Because of immobility, plants must adjust to a range of environmental circumstances through developmental and physiological mechanisms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gravity • Response to gravity is known as gravitropism • Roots show positive gravitropism • Stems show negative gravitropism Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Plants may detect gravity by the settling of statoliths, specialized plastids containing dense starch grains Video: Gravitropism Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-25 Statoliths 20 µm Mechanical Stimuli • The term thigmomorphogenesis refers to changes in form that result from mechanical perturbation • Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Thigmotropism is growth in response to touch • It occurs in vines and other climbing plants Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses called action potentials Video: Mimosa Leaf Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-27 Unstimulated Stimulated Side of pulvinus with flaccid cells Leaflets after stimulation Side of pulvinus with turgid cells Vein Pulvinus (motor organ) Motor organs 0.5 mm Environmental Stresses • Environmental stresses have a potentially adverse effect on survival, growth, and reproduction • They can have a devastating impact on crop yields in agriculture Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Drought • During drought, plants respond to water deficit by reducing transpiration • Deeper roots continue to grow Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Flooding • Enzymatic destruction of cells creates air tubes that help plants survive oxygen deprivation during flooding Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-28 Vascular cylinder Air tubes Epidermis 100 µm Control root (aerated) 100 µm Experimental root (nonaerated) Salt Stress • Plants respond to salt stress by producing solutes tolerated at high concentrations • This process keeps the water potential of cells more negative than that of the soil solution Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Heat Stress • Heat-shock proteins help plants survive heat stress Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cold Stress • Altering lipid composition of membranes is a response to cold stress Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.5: Plants defend themselves against herbivores and pathogens • Plants counter external threats with defense systems that deter herbivory and prevent infection or combat pathogens Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Defenses Against Herbivores • Herbivory, animals eating plants, is a stress that plants face in any ecosystem • Plants counter excessive herbivory with physical defenses such as thorns and chemical defenses such as distasteful or toxic compounds Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Some plants even “recruit” predatory animals that help defend against specific herbivores Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-29 Recruitment of parasitoid wasps that lay their eggs within caterpillars Synthesis and release of volatile attractants Wounding Chemical in saliva Signal transduction pathway Defenses Against Pathogens • A plant’s first line of defense against infection is its “skin,” the epidermis or periderm • If a pathogen penetrates the dermal tissue, the second line of defense is a chemical attack that kills the pathogen and prevents its spread • This second defense system is enhanced by the inherited ability to recognize certain pathogens Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene-for-Gene Recognition • A virulent pathogen is one that a plant has little specific defense against • An avirulent pathogen is one that may harm but not kill the host plant • Gene-for-gene recognition involves recognition of pathogen-derived molecules by protein products of specific plant disease resistance (R) genes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A pathogen is avirulent if it has a specific Avr gene corresponding to an R allele in the host plant Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-30a Signal molecule (ligand) from Avr gene product Receptor coded by R allele R Avr allele Avirulent pathogen Plant cell is resistant If an Avr allele in the pathogen corresponds to an R allele in the host plant, the host plant will have resistance, making the pathogen avirulent. • If the plant host lacks the R gene that counteracts the pathogen’s Avr gene, then the pathogen can invade and kill the plant Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-30b R No Avr allele; virulent pathogen R allele; plant cell becomes diseased Avr allele Avr allele; virulent pathogen No R allele; plant cell becomes diseased No Avr allele; virulent pathogen No R allele; plant cell becomes diseased If there is no gene-for-gene recognition because of one of the above three conditions, the pathogen will be virulent, causing disease to develop. Plant Responses to Pathogen Invasions • A hypersensitive response against an avirulent pathogen seals off the infection and kills both pathogen and host cells in the region of the infection Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 39-31 Signal Hypersensitive response Signal transduction pathway Signal transduction pathway Acquired resistance Avirulent pathogen R-Avr recognition and hypersensitive response Systemic acquired resistance Systemic Acquired Resistance • Systemic acquired resistance (SAR) is a set of generalized defense responses in organs distant from the original site of infection • Salicylic acid is a good candidate for one of the hormones that activates SAR Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings