Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Plant Responses to Internal and External Signals 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 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 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 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. 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 Plant hormones help coordinate growth, development, and responses to stimuli Hormones are chemical signals that coordinate different parts of an organism 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 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 LE 39-5c Boysen-Jensen (1913) Light Tip separated Tip separated by by mica gelatin block 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 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 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 LE 39-8a Cross-linking cell wall polysaccharides Cell wall enzymes Expansin CELL WALL Microfibril ATP Plasma membrane CYTOPLASM Lateral and Adventitious Root Formation Auxin is involved in root formation and branching An overdose of auxins can kill dicots Auxin affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of secondary xylem Cytokinins Cytokinins are so named because they stimulate cytokinesis (cell division) Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits Cytokinins work together with auxin 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 If the terminal bud is removed, plants become bushier LE 39-9 Axillary buds “Stump” after removal of apical bud Lateral branches Intact plant Plant with apical bud removed 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 Gibberellins Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination Gibberellins stimulate growth of leaves and stems In stems, they stimulate cell elongation and cell division 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 Germination After water is imbibed, release of gibberellins from the embryo signals seeds to germinate LE 39-11 Aleurone Endosperm a-amylase GA Water Scutellum (cotyledon) GA Radicle Sugar Abscisic Acid Two of the many effects of abscisic acid (ABA): Seed dormancy Drought tolerance 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 LE 39-12 Coleoptile Ethylene Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection 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 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. Apoptosis: Programmed Cell Death A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants Leaf Abscission A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls Fruit Ripening A burst of ethylene production in a fruit triggers the ripening process 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 Plants detect not only presence of light but also its direction, intensity, and wavelength (color) 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 Blue-Light Photoreceptors Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism Phytochromes as Photoreceptors Phytochromes regulate many of a plant’s responses to light throughout its life Studies of seed germination led to the discovery of phytochromes 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 Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses 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 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 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 The Effect of Light on the Biological Clock Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues 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 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 long-day plants In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length LE 39-22 Darkness Flash of light Critical dark period Light “Short-day” plants “Long-day” plants 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 LE 39-24 Graft Time (several weeks) Gravity Response to gravity is known as gravitropism Roots show positive gravitropism Stems show negative gravitropism Plants may detect gravity by the settling of statoliths, specialized plastids containing dense starch grainsVideo: Gravitropism 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 Thigmotropism is growth in response to touch It occurs in vines and other climbing plants Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses called action potentials NASTIC RESPONSE-DUE TO CHANGE IN TUGOR PRESSURE 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 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 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 A pathogen is avirulent if it has a specific Avr gene corresponding to an R allele in the host plant 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 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 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