BIOLOGY. Plant Responses to Internal and External Signals CAMPBELL. Reece Urry Cain Wasserman Minorsky Jackson

Transcription

1 CAMPBELL BIOLOGY TENTH EDITION Reece Urry Cain Wasserman Minorsky Jackson 39 Plant Responses to Internal and External Signals Lecture Presentation by Nicole Tunbridge and Kathleen Fitzpatrick

2 Stimuli and a Stationary Life Plants receive signals from the environment and respond by altering growth and development For example, the bending of a dodder seedling toward a host plant occurs in response to chemicals released by the host

3 Figure 39.1

4 Figure 39.1a

5 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 After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally

6 Figure 39.2 (a) Before exposure to light (b) After a week s exposure to natural daylight

7 Figure 39.2a (a) Before exposure to light

8 Figure 39.2b (b) After a week s exposure to natural daylight

9 A potato s response to light is an example of cell signal processing The stages are reception, transduction, and response

10 Figure 39.3 CELL WALL CYTOPLASM Reception Transduction Response Receptor Relay proteins and second messengers Activation of cellular responses Hormone or environmental stimulus Plasma membrane

11 Reception Internal and external signals are detected by receptors, proteins that change in response to specific stimuli In de-etiolation, the receptor is a phytochrome capable of detecting light

12 Transduction Second messengers transfer and amplify signals from receptors to proteins that cause responses Two types of second messengers play an important role in de-etiolation: Ca 2+ ions and cyclic GMP (cgmp) The phytochrome receptor responds to light by Opening Ca 2+ channels, which increases Ca 2+ levels in the cytosol Activating an enzyme that produces cgmp

13 Figure Reception CYTOPLASM Plasma membrane Cell wall Phytochrome activated by light Light

14 Figure Reception 2 Transduction Cell wall Light Plasma membrane CYTOPLASM Phytochrome activated by light cgmp Second messenger produced Protein kinase 1 Protein kinase 2 Ca 2+ channel opened Ca 2+

15 Figure Reception 2 Transduction 3 Response Cell wall Plasma membrane CYTOPLASM Phytochrome activated by light cgmp Second messenger produced Protein kinase 1 Protein kinase 2 Transcription factor 1 Transcription factor 2 P NUCLEUS P Transcription Light Translation Ca 2+ channel opened De-etiolation (greening) response proteins Ca 2+

16 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 This can occur by transcriptional regulation or post-translational modification

17 Post-Translational Modification of Preexisting Proteins Post-translational modification involves modification of existing proteins in the signal response Modification often involves the phosphorylation of specific amino acids The second messengers cgmp and Ca 2+ activate protein kinases directly Protein phosphatases switch off the signal transduction pathways by dephosphorylating proteins

18 Transcriptional Regulation Specific transcription factors bind directly to specific regions of DNA and control transcription of genes Some transcription factors are activators that increase the transcription of specific genes Other transcription factors are repressors that decrease the transcription of specific genes

19 De-Etiolation ( Greening ) Proteins De-etiolation activates enzymes that Function in photosynthesis directly Supply the chemical precursors for chlorophyll production Affect the levels of plant hormones that regulate growth

20 Figure 39.2 (a) Before exposure to light (b) After a week s exposure to natural daylight

21 Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli Plant hormones are chemical signals that modify or control one or more specific physiological processes within a plant Plant hormones are produced in very low concentrations, but can have profound effects on growth and development

22 Each hormone has multiple effects, but multiple hormones can influence a single process Plant responses depend on amount and concentration of specific hormones and often on the combination of hormones present

23 A Survey of Plant Hormones The major plant hormones include Auxin Cytokinins Gibberellins Abscisic acid Ethylene Brassinosteroids Jasmonates Strigolactones

24 Table 39.1

25 Auxin Any response resulting in curvature of organs toward or away from a stimulus is called a tropism In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant s response to light They observed that a grass seedling could bend toward light only if the tip of the coleoptile was present

26

27 They postulated that a signal was transmitted from the tip to the elongating region In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance

28 Figure 39.5 Results Control Shaded side Light Illuminated side Darwin and Darwin Boysen-Jensen Light Light Tip removed Opaque cap Transparent cap Opaque shield over curvature Gelatin (permeable) Mica (impermeable)

29 Figure 39.5a Control Shaded side Light Illuminated side

30 Figure 39.5b Darwin and Darwin Light Tip removed Opaque cap Transparent cap Opaque shield over curvature

31 Figure 39.5c Boysen-Jensen Light Gelatin (permeable) Mica (impermeable)

32 Video: Phototropism

33 The term auxin refers to any chemical that promotes elongation of coleoptiles Indoleacetic acid (IAA) is a common auxin in plants; in this lecture the term auxin refers specifically to IAA Auxin is produced in shoot tips and is transported down the stem Auxin transporter proteins move the hormone from the basal end of one cell into the apical end of the neighboring cell

34 Figure 39.6 Results Cell µm Cell 2 Epidermis Cortex Phloem Xylem Pith 25 µm Basal end of cell

35 Figure 39.6a 100 µm Epidermis Cortex Phloem Xylem Pith

36 Figure 39.6b Cell 1 Cell 2 25 µm Basal end of cell

37 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

38 Figure Low ph activates expansins. CELL WALL 4 Cell wall-loosening enzymes cleave cross-linking polysaccharides. H 2 O 2 Acidity increases. H + H + H + H + H + H + H + H + Plasma membrane Cell wall 1 Proton pump activity increases. ATP H + Plasma membrane CYTOPLASM Nucleus 5 Vacuole Cytoplasm Sliding cellulose microfibrils allow cell to elongate.

39 Figure 39.7a 4 Cell wall-loosening enzymes cleave cross-linking polysaccharides. CELL WALL 3 Low ph activates expansins. H + H + 2 Acidity increases. H + H + H + H + H + H + 1 Proton pump activity increases. ATP H + Plasma membrane CYTOPLASM

40 Figure 39.7b H 2 O Plasma membrane Cell wall Nucleus Vacuole Cytoplasm 5 Sliding cellulose microfibrils allow cell to elongate.

41 Auxin also alters gene expression and stimulates a sustained growth response

42 Auxin s Role in Plant Development Polar transport of auxin plays a role in pattern formation of the developing plant Reduced auxin flow from the shoot of a branch stimulates growth in lower branches Auxin transport plays a role in phyllotaxy, the arrangement of leaves on the stem Polar transport of auxin from leaf margins directs leaf venation pattern The activity of the vascular cambium is under control of auxin transport

43 Practical Uses for Auxins The auxin indolbutyric acid (IBA) stimulates adventitious roots and is used in vegetative propagation of plants by cuttings An overdose of synthetic auxins can kill plants For example 2,4-D is used as an herbicide on eudicots

44 Cytokinins Cytokinins are so named because they stimulate cytokinesis (cell division)

45 Control of Cell Division and Differentiation Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits Cytokinins work together with auxin to control cell division and differentiation

46

47 Control of Apical Dominance Cytokinins, auxin, and strigolactone 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

48 Figure 39.8 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

49 Figure 39.8a Axillary buds (a) Apical bud intact (not shown in photo)

50 Figure 39.8b Lateral branches Stump after removal of apical bud (b) Apical bud removed

51 Figure 39.8c (c) Auxin added to decapitated stem

52 Anti-Aging Effects Cytokinins slow the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues

53 Gibberellins Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination

54 Stem Elongation Gibberellins are produced in young roots and leaves Gibberellins stimulate growth of leaves and stems In stems, they stimulate cell elongation and cell division

55 Figure 39.9 (b) Grapes from control vine (left) and gibberellintreated vine (right) (a) Rosette form (left) and gibberellin-induced bolting (right)

56 Figure 39.9a (a) Rosette form (left) and gibberellin-induced bolting (right)

57 Figure 39.9b (b) Grapes from control vine (left) and gibberellintreated vine (right)

58 Fruit Growth In many plants, both auxin and gibberellins must be present for fruit to develop Gibberellins are used in spraying of Thompson seedless grapes

59 Germination After water is imbibed, release of gibberellins from the embryo signals seeds to germinate

60 Figure Aleurone Endosperm GA α-amylase Sugar Water GA Scutellum (cotyledon) Radicle

61 Abscisic Acid Abscisic acid (ABA) slows growth Two of the many effects of ABA Seed dormancy Drought tolerance

62 Seed Dormancy Seed dormancy ensures that the seed will germinate only in optimal conditions In some seeds, dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold Precocious (early) germination can be caused by inactive or low levels of ABA

63 Figure Red mangrove (Rhizophora mangle) seeds Coleoptile Maize mutant

64 Figure 39.11a Red mangrove (Rhizophora mangle) seeds

65 Figure 39.11b Coleoptile Maize mutant

66 Drought Tolerance ABA is the primary internal signal that enables plants to withstand drought ABA accumulation causes stomata to close rapidly

67 Ethylene Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection The effects of ethylene include response to mechanical stress, senescence, leaf abscission, and fruit ripening

68 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

69 Figure Ethylene concentration (parts per million)

70 Ethylene-insensitive mutants fail to undergo the triple response after exposure to ethylene Other mutants undergo the triple response in air but do not respond to inhibitors of ethylene synthesis

71 Figure ein mutant ctr mutant (a) ein mutant (b) ctr mutant

72 Figure 39.13a ein mutant (a) ein mutant

73 Figure 39.13b ctr mutant (b) ctr mutant

74 Senescence Senescence is the programmed death of cells or organs A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants

75 Leaf Abscission A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls

76 Figure mm Protective layer Stem Abscission layer Petiole

77 Figure 39.14a 0.5 mm Protective layer Stem Abscission layer Petiole

78 Fruit Ripening A burst of ethylene production in a fruit triggers the ripening process Ethylene triggers ripening, and ripening triggers release of more ethylene Fruit producers can control ripening by picking green fruit and controlling ethylene levels

79 More Recently Discovered Plant Hormones Brassinosteroids are chemically similar to the sex hormones of animals They induce cell elongation and division in stem segments They slow leaf abscission and promote xylem differentiation

80 Jasmonates, including jasmonate (JA) and methyl jasmonate (MeJA) play important roles in plant defense and development They are produced in response to wounding and involved in controlling plant defenses

81 Jasmonates also regulate many other physiological processes, including Nectar secretion Fruit ripening Pollen production Flowering time Seed germination Root growth Tuber formation Mycorrhizal symbiosis Tendril coiling

82 Strigolactones are xylem-mobile chemicals that Stimulate seed germination Suppress adventitious root formation Help establish mycorrhizal associations Help control apical dominance Strigolactones are named for parasitic Striga plants Striga seeds germinate when host plants exude strigolactones through their roots

83 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

84 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

85 Different plant responses can be mediated by the same or different photoreceptors There are two major classes of light receptors: blue-light photoreceptors and phytochromes

86 Blue-Light Photoreceptors Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism

87 Figure Phototropic effectiveness nm 0.8 Refracting prism White light Wavelength (nm) (a) Light wavelengths below 500 nm induce curvature. (b) Blue light induces the most curvature of coleoptiles.

88 Figure 39.15a Phototropic effectiveness nm Wavelength (nm) (a) Light wavelengths below 500 nm induce curvature.

89 Figure 39.15b Refracting prism White light (b) Blue light induces the most curvature of coleoptiles.

90 Phytochrome Photoreceptors Phytochromes are pigments that regulate many of a plant s responses to light throughout its life These responses include seed germination and shade avoidance

91 Phytochromes and Seed Germination Many seeds remain dormant until light and other conditions are near optimal In the 1930s, scientists at the U.S. Department of Agriculture determined the action spectrum for light-induced germination of lettuce seeds

92 Red light increased germination, while far-red light inhibited germination The effects of red and far-red light are reversible; the final light exposure determines the response The photoreceptors responsible for the opposing effects of red and far-red light are phytochromes

93 Figure Results Red Dark Red Far-red Dark Dark (control) Red Far-red Red Dark Red Far-red Red Far-red

94 Figure 39.16a Dark (control)

95 Figure 39.16b Red Dark

96 Figure 39.16c Red Far-red Dark

97 Figure 39.16d Red Far-red Red Dark

98 Figure 39.16e Red Far-red Red Far-red

99 Phytochromes exist in two photoreversible states, with conversion of P r to P fr triggering many developmental responses Red light triggers the conversion of P r to P fr Far-red light triggers the conversion of P fr to P r The conversion of P r to P fr is faster than the reverse process Sunlight, containing both red and far-red light, increases the ratio of P fr to P r and triggers germination

100 Figure Synthesis P r Red light Far-red light P fr Responses to P fr : Seed germination Inhibition of vertical growth and stimulation of branching Setting internal clocks Control of flowering Slow conversion in darkness (some species) Enzymatic destruction

101 Phytochromes and Shade Avoidance The phytochrome system also provides the plant with information about the quality of light Leaves in the canopy absorb red light Shaded plants receive more far-red than red light In the shade avoidance response, the phytochrome ratio shifts in favor of P r when a tree is shaded This shift induces vertical growth

102 Biological Clocks and Circadian Rhythms Many plant processes oscillate during the day in response to light and temperature changes Many legumes lower their leaves in the evening and raise them in the morning, even when kept under constant light or dark conditions

103 Figure Noon 10:00 PM

104 Figure 39.18a Noon

105 Figure 39.18b 10:00 PM

106 Circadian rhythms are cycles that are about 24 hours long and are governed by an internal clock Circadian rhythms can be set to exactly 24 hours by the day/night cycle The clock may depend on synthesis of a protein regulated through feedback control and may be common to all eukaryotes

107 The Effect of Light on the Biological Clock Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues

108 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

109 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 Flowering in day-neutral plants is controlled by plant maturity, not photoperiod

110 Critical Night Length In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length

111 Short-day plants are governed by whether the critical night length sets a minimum number of hours of darkness Long-day plants are governed by whether the critical night length sets a maximum number of hours of darkness

112 Figure hours (a) Short-day (long-night) plant Light Critical dark period Darkness Flash of light (b) Long-day (short-night) plant Flash of light

113 Red light can interrupt the nighttime portion of the photoperiod A flash of red light followed by a flash of far-red light does not disrupt night length Action spectra and photoreversibility experiments show that phytochrome is the pigment that detects the red light

114 Figure hours R R FR R FR R R FR R FR Critical dark period Short-day (long-night) plant Long-day (short-night) plant

115 Some plants flower after only a single exposure to the required photoperiod Other plants need several successive days of the required photoperiod Still others need an environmental stimulus in addition to the required photoperiod For example, vernalization is a pretreatment with cold to induce flowering

116 A Flowering Hormone? Photoperiod is detected by leaves, which cue buds to develop as flowers The flowering signal molecule is called florigen Florigen may be a protein governed by the FLOWERING LOCUS T (FT) gene

117 Figure hours 24 hours 24 hours Graft Short-day plant Long-day plant grafted to short-day plant Long-day plant

118 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

119 Gravity Response to gravity is known as gravitropism Roots show positive gravitropism; shoots show negative gravitropism Plants may detect gravity by the settling of statoliths, dense cytoplasmic components

120 Figure Statoliths 20 µm (a) Primary root of maize bending gravitropically (LMs) (b) Statoliths settling to the lowest sides of root cap cells (LMs)

121 Figure 39.22a

122 Figure 39.22b

123 Figure 39.22c Statoliths 20 µm

124 Figure 39.22d Statoliths 20 µm

125 Video: Gravitropism

126 Some mutants that lack statoliths are still capable of gravitropism Dense organelles, in addition to starch granules, may contribute to gravity detection

127 Mechanical Stimuli The term thigmomorphogenesis refers to changes in form that result from mechanical disturbance Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls

128 Figure 39.23

129 Thigmotropism is growth in response to touch It occurs in vines and other climbing plants Another example of a touch specialist is the sensitive plant Mimosa pudica, which folds its leaflets and collapses in response to touch Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses called action potentials

130 Figure (a) Unstimulated state (leaflets spread apart) (b) Stimulated state (leaflets folded)

131 Figure 39.24a (a) Unstimulated state (leaflets spread apart)

132 Figure 39.24b (b) Stimulated state (leaflets folded)

133 Video: Mimosa Leaf

134 Environmental Stresses Environmental stresses have a potentially adverse effect on survival, growth, and reproduction Stresses can be abiotic (nonliving) or biotic (living) Abiotic stresses include drought, flooding, salt stress, heat stress, and cold stress Biotic stresses include herbivores and pathogens

135 Drought During drought, plants reduce transpiration by closing stomata, reducing exposed surface area, and in some species, shedding leaves

136 Flooding Enzymatic destruction of root cortex cells creates air tubes that help plants survive oxygen deprivation during flooding

137 Figure Vascular cylinder Air tubes Epidermis (a) Control root (aerated) 100 µm (b) Experimental root (nonaerated) 100 µm

138 Figure 39.25a Vascular cylinder Epidermis (a) Control root (aerated) 100 µm

139 Figure 39.25b Vascular cylinder Air tubes Epidermis (b) Experimental root (nonaerated) 100 µm

140 Salt Stress Salt can lower the water potential of the soil solution and reduce water uptake 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

141 Heat Stress Excessive heat can denature a plant s enzymes Transpiration helps cool leaves by evaporative cooling Heat-shock proteins help protect other proteins from heat stress

142 Cold Stress Cold temperatures decrease membrane fluidity Altering lipid composition of membranes is a response to cold stress Freezing causes ice to form in a plant s cell walls and intercellular spaces Many plants, as well as other organisms, have antifreeze proteins that prevent ice crystals from growing and damaging cells

143 Concept 39.5: Plants respond to attacks by pathogens and herbivores Plants use defense systems to deter herbivory, prevent infection, and combat pathogens

144 Defenses Against Pathogens A plant s first line of defense against infection is the barrier presented by the epidermis and periderm Pathogens can enter through wounds or natural openings, such as stomata

145 Immune Responses of Plants The first line of immune defense depends on the plant s ability to recognize pathogen-associated molecular patterns (PAMPs) These molecular sequences are specific to certain pathogens PAMP recognition starts a chain of signaling events leading to the production of antimicrobial chemicals and toughening of the cell wall

146 Plants and pathogens have engaged in an evolutionary arms race Pathogens that have evolved the ability to deliver effectors into plant cells can suppress PAMPtriggered plant immunity Effectors are pathogen-encoded proteins that cripple the host s innate immune system A second level of plant immune defense evolved in response to these pathogens

147 Effector-triggered immunity results from the action of hundreds of disease resistance (R) genes Each R protein is activated by a specific effector R proteins activate plant defenses by triggering signal transduction pathways These defenses include the hypersensitive response and systemic acquired resistance

148 The Hypersensitive Response The hypersensitive response Causes cell and tissue death near the infection site Induces production of enzymes that attack the pathogen Stimulates changes in the cell wall that confine the pathogen

149 Figure Infected tobacco leaf with lesions 4 Signal Hypersensitive response Signal transduction pathway Signal transduction pathway 6 7 Acquired resistance 1 R protein Pathogen Effector protein Hypersensitive response Systemic acquired resistance

150 Figure 39.26a 4 Signal Hypersensitive response Signal transduction pathway Signal transduction pathway Acquired resistance Pathogen R protein Effector protein Hypersensitive response Systemic acquired resistance

151 Figure 39.26b Infected tobacco leaf with lesions

152 Systemic Acquired Resistance Systemic acquired resistance causes systemic expression of defense genes and is a long-lasting response Methylsalicylic acid is synthesized around the infection site and carried in the phloem to other remote sites where it is converted to salicylic acid Salicylic acid triggers the defense system to respond rapidly to another infection

153 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 trichomes, and chemical defenses, such as distasteful or toxic compounds

154 Molecular-Level Defenses Chemical compounds including terpenoids, phenolics, and alkaloids can be produced to deter attackers

155 Figure 39.27a MAKE CONNECTIONS: Levels of Plant Defenses Against Herbivores Molecular-Level Cellular-Level Tissue-Level Organ-Level

156 Figure 39.27aa Molecular-Level Defenses Opium poppy fruit

157 Cellular-Level Defenses Cells may be specialized to form trichomes, store chemical deterrents, or produce irritants

158 Figure 39.27ab Cellular-Level Defenses Raphide crystals from taro plant

159 Tissue-Level Defenses Leaves may be toughened with sclerenchyma tissue

160 Figure 39.27ac Tissue-Level Defenses Cross section through the major vein of an olive leaf

161 Organ-Level Defenses Leaves can be modified into spines and bristles; stems into thorns Some species have leaves that appear partially eaten, others have structures that mimic insect eggs

162 Figure 39.27ad Organ-Level Defenses Bristles on cactus spines

163 Figure 39.27ae Organ-Level Defenses Leaf of snowflake plant

164 Figure 39.27af Organ-Level Defenses Egg mimicry on leaf of passion flower plant

165 Organismal-Level Defenses Plants may respond to attack by altering flowering time

166 Figure 39.27b MAKE CONNECTIONS: Levels of Plant Defenses Against Herbivores Organismal-Level Community-Level Population-Level

167 Figure 39.27ba Organismal-Level Defenses Hummingbird pollinating wild tobacco plant

168 Population-Level Defenses Some plants release chemicals in response to herbivore attack that trigger defense responses in nearby members of the population Others populations synchronously produce massive amounts of seeds after long intervals

169 Figure 39.27bb Population-Level Defenses Flowering bamboo plants

170 Community-Level Defenses Some plants recruit predatory animals that help defend against specific herbivores

171 Figure 39.27bc Community-Level Defenses Parasitoid wasp cocoons on caterpillar host

172 Figure 39.27bd Community-Level Defenses Adult wasp emerging from a cocoon

173 Figure 39.UN01a Roots not connected Roots connected Osmotic shock

174 Stomatal opening (μm) Figure 39.UN01b Control 15 minutes after plant 6 osmotically shocked Plant number

175 Figure 39.UN01c Pea plant (Pisum sativum)

176 Figure 39.UN02 CELL WALL Plasma membrane CYTOPLASM 1 Reception 2 Transduction 3 Response Hormone or environmental stimulus Relay proteins and second messengers Activation of cellular responses Receptor

177 Figure 39.UN03 Plant Hormone Auxin Cytokinins Gibberellins Abscisic acid Ethylene Brassinosteroids Jasmonates Strigolactones Major Responses Stimulates cell elongation; regulates branching and organ bending Stimulate plant cell division; promote later bud growth; slow organ death Promote stem elongation; help seeds break dormancy and use stored reserves Promotes stomatal closure in response to drought; promotes seed dormancy Mediates fruit ripening and the triple response Chemically similar to the sex hormones of animals; induce cell elongation and division Mediate plant defenses against insect herbivores; regulate a wide range of physiological processes Regulate apical dominance, seed germination, and mycorrhizal associations

178 Figure 39.UN04 Red light P r P fr Responses Far-red light

179 Figure 39.UN05 Environmental Stress Drought Flooding Salt Heat Cold Major Response ABA production, reducing water loss by closing stomata Formation of air tubes that help roots survive oxygen deprivation Avoiding osmotic water loss by producing solutes tolerated at high concentrations Synthesis of heat-shock proteins, which reduce protein denaturation at high temperatures Adjusting membrane fluidity; avoiding osmotic water loss; producing antifreeze proteins

180 Figure 39.UN06 Control Ethylene added Ethylene synthesis inhibitor Wild-type Ethylene insensitive (ein) Ethylene overproducing (eto) Constitutive triple response (ctr)

181 Figure 39.UN07