LECTURE PRESENTATIONS For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson Chapter 39 Plant Responses to Internal and External Signals Lectures by Erin Barley Kathleen Fitzpatrick
Concept 39.1: Signal transduction pathways link signal reception to response A potato left growing in darkness produces shoots that look unhealthy, and it lacks elongated roots. These are morphological adaptations for growing in darkness, collectively called etiolation (a) Before exposure to light After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally (b) After a week s exposure to natural daylight
A potato s response to light is an example of cell-signal processing The stages are reception, transduction, and response CELL WALL CYTOPLASM 1 Reception 2 Transduction 3 Response Relay proteins and second messengers Activation of cellular responses Receptor Hormone or environmental stimulus Plasma membrane
Figure 39.3 CELL WALL CYTOPLASM 1 Reception 2 Transduction 3 Response Relay proteins and second messengers Activation of cellular responses Receptor Hormone or environmental stimulus Plasma membrane
Figure 39.4-1 1 Reception CYTOPLASM Plasma membrane Cell wall Light Phytochrome - capable of detecting light - responds to light by: Opens Ca 2+ channels, which increases Ca 2+ levels in the cytosol. - Activates an enzyme that produces cgmp
Figure 39.4-2 Reception 2 1 Transduction Cell wall Light CYTOPLASM Plasma membrane Phytochrome cgmp Second messenger Responds to light by: Activating an enzyme that produces cgmp - Opens Ca 2+ channels, which increases Ca 2+ levels in the cytosol. Protein kinase 1 Protein kinase 2 Ca 2 channel Ca 2
Figure 39.4-3 Cell wall 1 Reception 2 Transduction 3 Response Light CYTOPLASM Plasma membrane Phytochrome cgmp Second messenger Protein kinase 1 Stimulation involve increased activity of enzymes. This can occur by transcriptional regulation Protein kinase 2 Transcription factor 1 NUCLEUS Transcription factor 2 P P Transcription Translation Ca 2 channel De-etiolation (greening) response proteins Ca 2
Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli Tropism - Any response resulting in curvature of organs toward or away from a stimulus. Phototropism- A plant bending toward light only if the tip of the coleoptile was present. That means a signal must transmitted from the tip to the elongating region of the plant.
Figure 39.5 RESULTS Control Shaded side Light Illuminated side Boysen-Jensen Light Darwin and Darwin Light Gelatin (permeable) Mica (impermeable) Tip removed Opaque cap Transparent cap Opaque shield over curvature
In 1913, Peter Boysen- Jensen demonstrated that the signal was a mobile chemical substance. RESULTS Excised tip on agar cube Growth-promoting chemical diffuses into agar cube In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments Control Control (agar cube lacking chemical) Offset cubes
Auxin Refers to any chemical that promotes elongation of coleoptiles. Indoleacetic acid (IAA) is a common auxin in plants; Auxin is produced in shoot tips and is transported down the stem The Role of Auxin in Cell Elongation - Acid Growth Hypothesis - stimulates proton pumps in the plasma membrane Plasma membrane Cell wall H 2 O 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 Nucleus Vacuole Cytoplasm
Auxin s Role in Plant Development Pattern formation of the developing plant Reduced auxin flow from the shoot of a branch stimulates growth in lower branches Plays a role in phyllotaxy, the arrangement of leaves on the stem Directs leaf venation pattern Activity of the vascular cambium is under control of auxin transport An overdose of synthetic auxins can kill plants For example 2,4-D is used as an herbicide on eudicots
Cytokinins stimulate cytokinesis (cell division) Cytokinins work together with auxin to control cell division and differentiation Control of Apical Dominance Terminal bud suppresses development of axillary buds If the terminal bud is removed, plants become bushier
Figure 39.9 Lateral branches Stump after removal of apical bud (b) Apical bud removed Axillary buds (a) Apical bud intact (not shown in photo) (c) Auxin added to decapitated stem
Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination Produced in young roots and leaves Stimulate growth of leaves and stems Stimulate cell elongation and cell division (a) Rosette form (left) and gibberellin-induced Bolting (right) Fruit Growth Gibberellins are used in spraying of Thompson seedless grapes (b) Grapes from control vine (left) and gibberellintreated vine (right)
Germination After water is imbibed, release of gibberellins from the embryo signals seeds to germinate Aleurone Endosperm 1 2 3 GA -amylase Sugar Water GA Scutellum (cotyledon) Radicle
Abscisic Acid Abscisic acid (ABA) slows growth Two of the many effects of ABA Seed dormancy Drought tolerance Seed dormancy Ensures that the seed will germinate only in optimal conditions Dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold Drought Tolerance ABA accumulation causes stomata to close rapidly Primary internal signal that enables plants to withstand drought
Ethylene Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection Effects of ethylene include response to mechanical stress Senescence leaf abscission fruit ripening
Senescence is the programmed death of cells (apoptosis) or organs Leaf Abscission A change in the balance of auxin and ethylene controls- abcission layer grows and cuts off nutrients to leaf. Protective layer Stem Petiole 0.5 mm Abscission layer
Fruit Ripening Ethylene triggers ripening, and ripening triggers release of more ethylene Fruit producers can control ripening by picking green fruit and controlling ethylene levels
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
Phototropic effectiveness 1.0 436 nm Plants detect light s direction, intensity, and wavelength (color) An action spectrum depicts relative response of a process to different wavelengths 0.8 0.6 0.4 0.2 0 400 450 500 550 600 650 700 Wavelength (nm) (a) Phototropism action spectrum Light Time 0 min Time 90 min (b) Coleoptiles before and after light exposures
There are two major classes of light receptors: blue-light photoreceptors - control hypocotyl elongation, stomatal opening, and phototropism phytochromes - responses include seed germination and shade avoidance
Red light increased germination, while far-red light inhibited germination The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome RESULTS Red Dark Red Far-red Dark Dark (control) germination Red Far-red Red Dark Red Far-red Red Far-red Figure 39.17
Biological Clocks and Circadian Rhythms Many plant processes oscillate during the day Circadian rhythms cycles that are about 24 hours long and are governed by an internal clock Noon Midnight
The Effect of Light on the Biological Clock Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues Photoperiodism the relative lengths of night and day the environmental stimulus plants use most often to detect the time of year
Photoperiodism and Control of Flowering Short-day plants flower when a light period is shorter than a critical length Critical Night Length responses to photoperiod are actually controlled by night length Needs a minimum number of hours of darkness Long-day plants flower when a light period is longer than a certain number of hours Needs a maximum number of hours of darkness Day-neutral plants Flowering controlled by plant maturity, not photoperiod
Figure 39.21 24 hours (a) Short day (long-night) plant Cool season bloomers Light Critical dark period Flash of light Darkness (b) Long-day (short-night) plant spring time bloomers Flash of light
A Flowering Hormone? Photoperiod is detected by leaves, which cue buds to develop as flowers The flowering signal is called florigen 24 hours 24 hours 24 hours Graft Short-day plant Long-day plant grafted to short-day plant Long-day plant
Gravity Gravitropism - response to gravity Roots show positive gravitropism; Shoots show negative gravitropism Plants may detect gravity by the settling of statoliths, dense cytoplasmic components Primary root of maize bending gravitropically (LMs)
Mechanical Stimuli Thigmomorphogenesis - changes in growth that result from touch - results in clingy/wrapping growth occurs in vines and other climbing plants (Cudzu, Ivy) How do you grow Cudzu! It s easy. You throw the seeds to the left and you run to the right!
Defenses Against Herbivores Plants counter excessive herbivory with: physical defenses - thorns and trichomes chemical defenses - distasteful or toxic compounds Some plants even recruit predatory animals that help defend against specific herbivores 4 Recruitment of parasitoid wasps that lay their eggs within caterpillars 1 Wounding 1 Chemical in saliva 3 Synthesis and release of volatile attractants 2 Signal transduction pathway
Figure 39.28 4 Recruitment of parasitoid wasps that lay their eggs within caterpillars 1 Wounding 1 Chemical in saliva 3 Synthesis and release of volatile attractants 2 Signal transduction pathway
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