Plant Responses to Internal and External Signals 31 Plant Responses to Internal and External Signals

Overview: The Race to Live Young seedlings must outcompete their neighbors in the race for resources in order to survive Unlike animals, which respond through movement, plants must respond to environmental challenges by altering their growth and development 2

Figure 31.1 Figure 31.1 How do plants detect light? 3

Concept 31.1: 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 4

Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ Most aspects of plant growth and development are under hormonal control 5

The Discovery of Plant Hormones 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 6

They postulated that a signal was transmitted from the tip to the elongating region 7

Video: Phototropism

Results Shaded side Control Light Illuminated side Boysen-Jensen Light Figure 31.2 Results Shaded side Control Light Illuminated side Boysen-Jensen Light Darwin and Darwin Gelatin (permeable) Mica (impermeable) Figure 31.2 Inquiry: What part of a grass coleoptile senses light, and how is the signal transmitted? Light Opaque shield over curvature Trans- parent cap Tip removed Opaque cap 9

Shaded side Control Light Illuminated side Figure 31.2a Figure 31.2a Inquiry: What part of a grass coleoptile senses light, and how is the signal transmitted? (part 1: control) 10

Darwin and Darwin: Phototropism occurs only Figure 31.2b Darwin and Darwin: Phototropism occurs only when the tip is illuminated. Light Opaque shield over curvature Trans- parent cap Figure 31.2b Inquiry: What part of a grass coleoptile senses light, and how is the signal transmitted? (part 2: Darwin and Darwin) Tip removed Opaque cap 11

Boysen-Jensen: Phototropism occurs Figure 31.2c Boysen-Jensen: Phototropism occurs when the tip is separated by a permeable barrier but not an impermeable barrier. Light Figure 31.2c Inquiry: What part of a grass coleoptile senses light, and how is the signal transmitted? (part 3: Boysen-Jensen) Gelatin (permeable) Mica (impermeable) 12

In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance

In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments 14

Results Excised tip on agar cube Growth-promoting chemical diffuses Figure 31.3 Results Excised tip on agar cube Growth-promoting chemical diffuses into agar cube Control (agar cube lacking chemical) Offset cubes Control Figure 31.3 Inquiry: Does asymmetric distribution of a growth-promoting chemical cause a coleoptile to grow toward the light? 15

A Survey of Plant Hormones The major classes of plant hormones include Auxin Cytokinins Gibberellins Brassinosteroids Abscisic acid Ethylene 16

Table 31.1 Table 31.1 Overview of plant hormones 17

Auxin 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 18

Results Cell 1 100 m Cell 2 Epidermis Cortex Phloem 25 m Xylem Figure 31.4 Results Cell 1 100 m Cell 2 Epidermis Cortex Phloem Figure 31.4 Inquiry: What causes polar movement of auxin from shoot tip to base? Xylem 25 m Basal end of cell Pith 19

100 m Epidermis Cortex Phloem Xylem Pith Figure 31.4a Figure 31.4a Inquiry: What causes polar movement of auxin from shoot tip to base? (part 1: LM, lower magnification) Pith 20

Cell 1 Cell 2 25 m Basal end of cell Figure 31.4b Figure 31.4b Inquiry: What causes polar movement of auxin from shoot tip to base? (part 2: LM, higher magnification) 25 m Basal end of cell 21

With the cellulose loosened, the cell can elongate The role of auxin in cell elongation: Polar transport of auxin stimulates proton pumps in the plasma membrane According to the acid growth hypothesis, 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 22

4 3 H 2 H H H H H H H 1 ATP H 5 Figure 31.5 CELL WALL 4 Cell wall-loosening enzymes cleave cross-linking polysaccharides. 3 Low pH activates expansins. H2O Cell wall H Plasma membrane 2 Acidity increases. H H H H H H H Figure 31.5 Cell elongation in response to auxin: the acid growth hypothesis 1 Proton pump activity increases. Nucleus Cytoplasm Plasma membrane Vacuole ATP H 5 Sliding cellulose microfibrils allow cell to elongate. CYTOPLASM 23

4 3 H 2 H H H H H H H 1 Plasma membrane H Figure 31.5a 4 Cell wall-loosening enzymes cleave cross-linking polysaccharides. CELL WALL 3 Low pH activates expansins. H 2 Acidity increases. H H H H H H Figure 31.5a Cell elongation in response to auxin: the acid growth hypothesis (part 1: mechanism) H 1 Proton pump activity increases. Plasma membrane ATP H CYTOPLASM 24

Sliding cellulose microfibrils allow cell to elongate. Figure 31.5b H2O Cell wall Plasma membrane Figure 31.5b Cell elongation in response to auxin: the acid growth hypothesis (part 2: cell elongation) Nucleus Cytoplasm Vacuole 5 Sliding cellulose microfibrils allow cell to elongate. 25

Auxin also alters gene expression and stimulates a sustained growth response 26

Auxin’s role in plant development: Polar transport of auxin controls the spatial organization 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 27

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 Tomato growers spray their plants with synthetic auxins to stimulate fruit growth 28

Cytokinins Cytokinins are so named because they stimulate cytokinesis (cell division) 29

Control of cell division and differentiation: Cytokinins work together with auxin to control cell division and differentiation Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits 30

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 31

Gibberellins Gibberellins (GAs) have a variety of effects, such as stem elongation, fruit growth, and seed germination 32

Gibberellins are produced in young roots and leaves Stem elongation: Gibberellins stimulate stem and leaf growth by enhancing cell elongation and cell division Gibberellins are produced in young roots and leaves They can induce bolting, rabid growth of the floral stalk 33

vine (left) and gibberellin- treated vine (right) Figure 31.6 Grapes from control vine (left) and gibberellin- treated vine (right) Figure 31.6 Effects of gibberellins on stem elongation and fruit growth (a) Rosette form (left) and gibberellin-induced bolting (right) 34

(a) Rosette form (left) and gibberellin-induced bolting (right) Figure 31.6a Figure 31.6a Effects of gibberellins on stem elongation and fruit growth (part 1: bolting) (a) Rosette form (left) and gibberellin-induced bolting (right) 35

vine (left) and gibberellin- treated vine (right) Figure 31.6b Grapes from control vine (left) and gibberellin- treated vine (right) Figure 31.6b Effects of gibberellins on stem elongation and fruit growth (part 2: fruits) 36

Gibberellins are used in spraying of Thompson seedless grapes 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 37

Germination: After water is imbibed, release of gibberellins from the embryo signals seeds to germinate 38

Aleurone Endosperm Water Radicle Scutellum (cotyledon) GA GA -amylase Figure 31.7 Aleurone Endosperm -amylase Sugar GA GA Water Figure 31.7 Mobilization of nutrients by gibberellins during the germination of grain seeds such as barley Radicle Scutellum (cotyledon) 39

Brassinosteroids Brassinosteroids are chemically similar to cholesterol and the sex hormones of animals They induce cell elongation and division in stem segments and seedlings They slow leaf abscission (leaf drop) and promote xylem differentiation 40

Abscisic Acid Abscisic acid (ABA) slows growth Two of the many effects of ABA include Seed dormancy Drought tolerance 41

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 42

Red mangrove (Rhizophora mangle) seeds Coleoptile Maize mutant Figure 31.8 Red mangrove (Rhizophora mangle) seeds Coleoptile Figure 31.8 Precocious germination of wild-type mangrove and mutant maize seeds Maize mutant 43

Red mangrove (Rhizophora mangle) seeds Figure 31.8a Figure 31.8a Precocious germination of wild-type mangrove and mutant maize seeds (part 1: wild-type mangrove seeds) 44

Coleoptile Maize mutant Figure 31.8b Figure 31.8b Precocious germination of wild-type mangrove and mutant maize seeds (part 2: mutant maize seeds) Maize mutant 45

ABA accumulation causes stomata to close rapidly Drought tolerance: ABA is the primary internal signal that enables plants to withstand drought ABA accumulation causes stomata to close rapidly 46

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 47

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 48

Other mutants are not responsive to inhibitors of ethylene synthesis Ethylene-insensitive mutants fail to undergo the triple response after exposure to ethylene Some ethylene-overproducing mutants undergo the triple response even in air but are returned to normal growth when treated with ethylene synthesis inhibitors Other mutants are not responsive to inhibitors of ethylene synthesis 49

ein mutant ctr mutant (a) ein mutant (b) ctr mutant Figure 31.9 Figure 31.9 Ethylene triple-response Arabidopsis mutants (a) ein mutant (b) ctr mutant 50

ein mutant (a) ein mutant Figure 31.9a Figure 31.9a Ethylene triple-response Arabidopsis mutants (part 1: ein mutant) (a) ein mutant 51

ctr mutant (b) ctr mutant Figure 31.9b Figure 31.9b Ethylene triple-response Arabidopsis mutants (part 2: ctr mutant) (b) ctr mutant 52

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 53

Leaf abscission: A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls 54

0.5 mm Protective layer Abscission layer Stem Petiole Figure 31.10 0.5 mm Figure 31.10 Abscission of a maple leaf Protective layer Abscission layer Stem Petiole 55

0.5 mm Protective layer Abscission layer Stem Petiole Figure 31.10a 0.5 mm Figure 31.10a Abscission of a maple leaf (part 1: LM) Protective layer Abscission layer Stem Petiole 56

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 57

Concept 31.2: Responses to light are critical for plant success Light triggers many key events in plant growth and development, collectively known as photomorphogenesis 58

Photomorphogenesis 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 59

(a) Before exposure to light (b) After a week’s exposure Figure 31.11 Figure 31.11 Light-induced de-etiolation (greening) of dark-grown potatoes (a) Before exposure to light (b) After a week’s exposure to natural daylight 60

(a) Before exposure to light Figure 31.11a Figure 31.11a Light-induced de-etiolation (greening) of dark-grown potatoes (part 1: before) (a) Before exposure to light 61

(b) After a week’s exposure to natural daylight Figure 31.11b Figure 31.11b Light-induced de-etiolation (greening) of dark-grown potatoes (part 2: after) (b) After a week’s exposure to natural daylight 62

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 63

Phototropic effectiveness Figure 31.12 1.0 436 nm 0.8 Refracting prism 0.6 Phototropic effectiveness 0.4 White light 0.2 400 450 500 550 600 650 700 Figure 31.12 Action spectrum for blue-light-stimulated phototropism in maize coleoptiles Wavelength (nm) Light wavelengths below 500nm induce curvature. Blue light induces the most curvature of coleoptiles. 64

Phototropic effectiveness Figure 31.12a 1.0 436 nm 0.8 0.6 Phototropic effectiveness 0.4 0.2 Figure 31.12a Action spectrum for blue-light-stimulated phototropism in maize coleoptiles (part 1: action spectrum) 400 450 500 550 600 650 700 Wavelength (nm) Light wavelengths below 500nm induce curvature. 65

Blue light induces the most curvature of coleoptiles. Figure 31.12b Refracting prism White light Figure 31.12b Action spectrum for blue-light-stimulated phototropism in maize coleoptiles (part 2: prism) Blue light induces the most curvature of coleoptiles. 66

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, photoreceptors that absorb mostly red light 67

Blue-Light Photoreceptors Various blue-light photoreceptors control phototropism (movement in response to light), stomatal opening, and hypocotyl elongation 68

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 69

Phytochromes and seed germination: Many seeds remain dormant until light conditions are optimal In the 1930s, scientists at the U.S. Department of Agriculture determined the action spectrum for light-induced germination of lettuce seeds 70

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 71

Results Red Dark Red Far-red Dark Dark (control) Red Far-red Red Dark Figure 31.13 Results Red Dark Red Far-red Dark Dark (control) Figure 31.13 Inquiry: How does the order of red and far-red illumination affect seed germination? Red Far-red Red Dark Red Far-red Red Far-red 72

Dark (control) Figure 31.13a Figure 31.13a Inquiry: How does the order of red and far-red illumination affect seed germination? (part 1: control) 73

Figure 31.13b Red Dark Figure 31.13b Inquiry: How does the order of red and far-red illumination affect seed germination? (part 2: red, dark) 74

Red Far-red Dark Figure 31.13c Figure 31.13c Inquiry: How does the order of red and far-red illumination affect seed germination? (part 3: red, far-red, dark) 75

Red Far-red Red Dark Figure 31.13d Figure 31.13d Inquiry: How does the order of red and far-red illumination affect seed germination? (part 4: red, far-red, red, dark) 76

Red Far-red Red Far-red Figure 31.13e Figure 31.13e Inquiry: How does the order of red and far-red illumination affect seed germination? (part 5: red, far-red, red, far-red, dark) 77

Red light triggers the conversion of Pr to Pfr Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses Red light triggers the conversion of Pr to Pfr Far-red light triggers the conversion of Pfr to Pr The conversion to Pfr is faster than the conversion to Pr Sunlight increases the ratio of Pfr to Pr and triggers germination 78

• Inhibition of vertical growth and stimu- lation of branching Figure 31.14 Responses to Pfr: • Seed germination • Inhibition of vertical growth and stimu- lation of branching • Setting internal clocks • Control of flowering Red light Synthesis Pr Pfr Far-red light Slow conversion in darkness (some species) Enzymatic destruction Figure 31.14 Phytochrome: a molecular switching mechanism 79

Leaves in the canopy absorb red light 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 Pr when a tree is shaded This shift induces the vertical growth of the plant 80

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, even when kept under constant light or dark conditions 81

Figure 31.15 Figure 31.15 Sleep movements of a bean plant (Phaseolus vulgaris) Noon 10:00 PM 82

Figure 31.15a Figure 31.15a Sleep movements of a bean plant (Phaseolus vulgaris) (part 1: noon) Noon 83

Figure 31.15b Figure 31.15b Sleep movements of a bean plant (Phaseolus vulgaris) (part 2: 10 PM) 10:00 PM 84

Circadian rhythms are cycles that are about 24 hours long and are governed by an internal “clock” Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle The clock may depend on synthesis of a protein regulated through feedback control 85

The Effect of Light on the Biological Clock Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues 86

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 87

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 Flowering in day-neutral plants is controlled by plant maturity, not photoperiod 88

Critical night length: In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length 89

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 90

24 hours (a) Short-day (long-night) plant Light Flash of light Figure 31.16 24 hours (a) Short-day (long-night) plant Light Flash of light Darkness Critical dark period (b) Long-day (short-night) plant Figure 31.16 Photoperiodic control of flowering Flash of light 91

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 receives red light 92

Short-day (long-night) plant Long-day (short-night) plant Figure 31.17 24 hours R R F R Figure 31.17 Reversible effects of red and far-red light on photoperiodic response R F R R R F R R F R Short-day (long-night) plant Long-day (short-night) plant Critical dark period 93

Other plants need several successive days of the required photoperiod 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 94

A Flowering Hormone? Photoperiod is detected by leaves, which cue buds to develop as flowers The flowering signal is called florigen Florigen may be a protein governed by the FLOWERING LOCUS T (FT) gene 95

24 hours 24 hours 24 hours Graft Short-day plant Long-day plant Figure 31.18 24 hours 24 hours 24 hours Graft Figure 31.18 Experimental evidence for a flowering hormone Short-day plant Long-day plant grafted to short-day plant Long-day plant 96

Concept 31.3: 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 97

Gravity Response to gravity is known as gravitropism Roots show positive gravitropism by growing downward; shoots show negative gravitropism by growing upward Plants may detect gravity by the settling of statoliths, dense cytoplasmic components 98

Video: Gravitropism

(a) Primary root of maize bending gravitropically (LMs) Figure 31.19 Statoliths 20 m Figure 31.19 Positive gravitropism in roots: the statolith hypothesis (a) Primary root of maize bending gravitropically (LMs) (b) Statoliths settling to the lowest sides of root cap cells (LMs) 100

Figure 31.19a Figure 31.19a Positive gravitropism in roots: the statolith hypothesis (part 1: horizontal root) 101

Figure 31.19b Figure 31.19b Positive gravitropism in roots: the statolith hypothesis (part 2: bent root) 102

Figure 31.19c Statoliths Figure 31.19c Positive gravitropism in roots: the statolith hypothesis (part 3: statoliths at start) 20 m 103

Figure 31.19d Statoliths 20 m Figure 31.19d Positive gravitropism in roots: the statolith hypothesis (part 4: settled statoliths) 104

Some mutants that lack statoliths are still capable of gravitropism Mechanical pulling on proteins that connect the protoplast to the cell wall may aid in gravity detection Dense organelles and starch granules may also contribute to gravity detection 105

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 106

Figure 31.20 Figure 31.20 Altering gene expression by touch in Arabidopsis 107

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 108

Video: Mimosa Leaf

(a) Unstimulated state (leaflets spread apart) Figure 31.21 Figure 31.21 Rapid turgor movements by the sensitive plant (Mimosa pudica) (a) Unstimulated state (leaflets spread apart) (b) Stimulated state (leaflets folded) 110

(a) Unstimulated state (leaflets spread apart) Figure 31.21a Figure 31.21a Rapid turgor movements by the sensitive plant (Mimosa pudica) (part 1: unstimulated) (a) Unstimulated state (leaflets spread apart) 111

(b) Stimulated state (leaflets folded) Figure 31.21b Figure 31.21b Rapid turgor movements by the sensitive plant (Mimosa pudica) (part 2: stimulated) (b) Stimulated state (leaflets folded) 112

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 113

Drought During drought, plants reduce transpiration by closing stomata, reducing exposed surface area, or even shedding their leaves 114

Flooding Enzymatic destruction of root cortex cells creates air tubes that help plants survive oxygen deprivation during flooding 115

(a) Control root (aerated) (b) Experimental root (nonaerated) Figure 31.22 Vascular cylinder Air tubes Epidermis Figure 31.22 A developmental response of maize roots to flooding and oxygen deprivation 100 m 100 m (a) Control root (aerated) (b) Experimental root (nonaerated) 116

(a) Control root (aerated) Figure 31.22a Vascular cylinder Figure 31.22a A developmental response of maize roots to flooding and oxygen deprivation (part 1: aerated) Epidermis 100 m (a) Control root (aerated) 117

Vascular cylinder Air tubes Epidermis 100 m (b) Experimental root Figure 31.22b Vascular cylinder Air tubes Epidermis Figure 31.22b A developmental response of maize roots to flooding and oxygen deprivation (part 2: nonaerated) 100 m (b) Experimental root (nonaerated) 118

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 119

Heat Stress Excessive heat can denature a plant’s enzymes Heat-shock proteins, which help protect other proteins from heat stress, are produced at high temperatures 120

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 Water leaves the cell in response to freezing, leading to toxic solute concentrations in the cytoplasm 121

Many plants, as well as other organisms, have antifreeze proteins that prevent ice crystals from growing and damaging cells The five classes of antifreeze proteins have markedly different amino acid sequences but similar structure, indicating they arose through convergent evolution 122

Concept 31.4: Plants respond to attacks by herbivores and pathogens Through natural selection, plants have evolved defense systems to deter herbivory, prevent infection, and combat pathogens 123

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 Some plants even “recruit” predatory animals that help defend against specific herbivores 124

4 Recruitment of parasitoid wasps that lay their eggs Figure 31.23 4 Recruitment of parasitoid wasps that lay their eggs within caterpillars 3 Synthesis and release of volatile attractants 1 Wounding 1 Chemical in saliva Figure 31.23 A maize leaf “recruiting” a parasitoid wasp as a defensive response to an armyworm caterpillar, an herbivore 2 Signal transduction pathway 125

Plants damaged by insects can release volatile chemicals to warn other plants of the same species Arabidopsis can be genetically engineered to produce volatile components that attract predatory mites 126

Defenses Against Pathogens A plant’s first line of defense against infection is the barrier presented by the epidermis and 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 plant’s ability to recognize certain pathogens 127

Host-Pathogen Coevolution A virulent pathogen is one that a plant has little specific defense against An avirulent pathogen is one that may harm but does not kill the host plant 128

Gene-for-gene recognition involves recognition of effector molecules by the protein products of specific plant disease resistance (R) genes An R protein recognizes a corresponding molecule made by the pathogen’s Avr gene R proteins activate plant defenses by triggering signal transduction pathways These defenses include the hypersensitive response and systemic acquired resistance 129

The Hypersensitive Response Causes localized cell and tissue death near the infection site Induces production of phytoalexins and PR proteins, which attack the specific pathogen Stimulates changes in the cell wall that confine the pathogen 130

Infected tobacco leaf with lesions Figure 31.24 Infected tobacco leaf with lesions Signal 4 5 6 Hypersensitive response Signal transduction pathway 3 2 Signal transduction pathway Figure 31.24 Defense responses against an avirulent pathogen 7 Acquired resistance 1 R protein Avirulent pathogen Avr effector protein R-Avr recognition and hypersensitive response Systemic acquired resistance 131

Signal transduction pathway Figure 31.24a Signal 4 5 6 Hypersensitive response Signal transduction pathway 3 2 Signal transduction pathway 7 Acquired resistance 1 Figure 31.24a Defense responses against an avirulent pathogen (part 1: art) R protein Avirulent pathogen Avr effector protein R-Avr recognition and hypersensitive response Systemic acquired resistance 132

Infected tobacco leaf with lesions Figure 31.24b Figure 31.24b Defense responses against an avirulent pathogen (part 2: photo) Infected tobacco leaf with lesions 133

Systemic Acquired Resistance Causes plant-wide expression of defense genes Protects against a diversity of pathogens Provides a long-lasting response Methylsalicylic acid travels from an infection site to remote areas of the plant where it is converted to salicylic acid, which initiates pathogen resistance 134

Roots not connected Roots connected Osmotic shock Figure 31.UN01a Figure 31.UN01a Skills exercise: interpreting experimental results from a bar graph (part 1) 135

15 minutes after osmotic shock 13 Figure 31.UN01b 14 Control 15 minutes after osmotic shock 13 12 11 Stomatal opening (M) 10 9 8 Figure 31.UN01b Skills exercise: interpreting experimental results from a bar graph (part 2) 7 6 1 2 3 4 5 6 7 8 9 10 11 Plant number 136

Figure 31.UN02 Figure 31.UN02 Summary of key concepts: plant hormones 137

Figure 31.UN04 Figure 31.UN04 Summary of key concepts: environmental stress 138

Pea plant (Pisum sativum) Figure 31.UN01c Figure 31.UN01c Skills exercise: interpreting experimental results from a bar graph (part 3) Pea plant (Pisum sativum) 139

Red light Pr Pfr Responses Far-red light Figure 31.UN03 Figure 31.UN03 Summary of key concepts: responses to light 140

Ethylene synthesis inhibitor Ethylene added Figure 31.UN05 Ethylene synthesis inhibitor Ethylene added Control Wild-type Ethylene insensitive (ein) Ethylene overproducing (eto) Figure 31.UN05 Test your understanding, question 7 (ethylene mutants) Constitutive triple response (ctr) 141