Plant Responses to Internal and External Signals Chapter 39 Plant Responses to Internal and External Signals

Overview: Stimuli and a Stationary Life Plants, being rooted to the ground Must respond to whatever environmental change comes their way

For example, the bending of a grass seedling toward light Begins with the plant sensing the direction, quantity, and color of the light Figure 39.1

Plants have cellular receptors Concept 39.1: Signal transduction pathways link signal reception to response Plants have cellular receptors That they use to detect important changes in their environment For a stimulus to elicit a response Certain cells must have an appropriate receptor

A potato left growing in darkness Will produce shoots that do not appear healthy, and will lack elongated roots These are morphological adaptations for growing in darkness Collectively referred to as etiolation (a) 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. Figure 39.2a

After the potato is exposed to light The plant undergoes profound changes called de-etiolation, in which shoots and roots grow normally Figure 39.2b (b) 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.

The potato’s response to light Is an example of cell-signal processing Figure 39.3 CELL WALL CYTOPLASM   1 Reception 2 Transduction 3 Response Receptor Relay molecules Activation of cellular responses Hormone or environmental stimulus Plasma membrane

Internal and external signals are detected by receptors Reception Internal and external signals are detected by receptors Proteins that change in response to specific stimuli

Transduction Second messengers Transfer and amplify signals from receptors to proteins that cause specific responses

An example of signal transduction in plants 1 Reception   2 Transduction 3 Response CYTOPLASM Plasma membrane Phytochrome activated by light Cell wall Light cGMP Second messenger produced Specific protein kinase 1 Transcription factor 1 NUCLEUS P Translation De-etiolation (greening) response proteins Ca2+ Ca2+ channel opened kinase 2 factor 2 2 One pathway uses cGMP as a second messenger that activates a specific protein kinase.The other pathway involves an increase in cytoplasmic Ca2+ that activates another specific protein kinase. 3 Both pathways lead to expression of genes for proteins that function in the de-etiolation (greening) response. 1 The light signal is detected by the phytochrome receptor, which then activates at least two signal transduction pathways. Figure 39.4

Ultimately, a signal transduction pathway Response Ultimately, a signal transduction pathway Leads to a regulation of one or more cellular activities In most cases These responses to stimulation involve the increased activity of certain enzymes

Transcriptional Regulation Transcription factors bind directly to specific regions of DNA And control the transcription of specific genes

Post-Translational Modification of Proteins Involves the activation of existing proteins involved in the signal response

De-Etioloation (“Greening”) Proteins Many enzymes that function in certain signal responses are involved in photosynthesis directly While others are involved in supplying the chemical precursors necessary for chlorophyll production

Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli Are chemical signals that coordinate the different parts of an organism

The Discovery of Plant Hormones Any growth response That results in curvatures of whole plant organs toward or away from a stimulus is called a tropism Is often caused by hormones

Charles Darwin and his son Francis Conducted some of the earliest experiments on phototropism, a plant’s response to light, in the late 19th century

EXPERIMENT CONCLUSION Figure 39.5 In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for phototropism is transmitted. EXPERIMENT In the Darwins’ experiment, a phototropic response occurred only when light could reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen-Jensen observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin) but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested that the signal is a light-activated mobile chemical. CONCLUSION RESULTS Control Darwin and Darwin (1880) Boysen-Jensen (1913) Light Shaded side of coleoptile Illuminated Tip removed Tip covered by opaque cap covered by trans- parent cap Base covered by opaque shield Tip separated by gelatin block by mica Figure 39.5

In 1926, Frits Went Went concluded that a coleoptile curved toward light because its dark side had a higher concentration of the growth-promoting chemical, which he named auxin. The coleoptile grew straight if the chemical was distributed evenly. If the chemical was distributed unevenly, the coleoptile curved away from the side with the block, as if growing toward light, even though it was grown in the dark. Excised tip placed on agar block Growth-promoting chemical diffuses into agar block Agar block with chemical stimulates growth Control (agar block lacking chemical) has no effect Control Offset blocks cause curvature RESULTS CONCLUSION In 1926, Frits Went’s experiment identified how a growth-promoting chemical causes a coleoptile to grow toward light. He placed coleoptiles in the dark and removed their tips, putting some tips on agar blocks that he predicted would absorb the chemical. On a control coleoptile, he placed a block that lacked the chemical. On others, he placed blocks containing the chemical, either centered on top of the coleoptile to distribute the chemical evenly or offset to increase the concentration on one side. EXPERIMENT Extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments Figure 39.6

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 concentrations But a minute amount can have a profound effect on the growth and development of a plant organ

Auxin The term auxin Is used for any chemical substance that promotes cell elongation in different target tissues

Auxin transporters Move the hormone out of the basal end of one cell, and into the apical end of neighboring cells Cell 1 Cell 2 100 m Epidermis Cortex Phloem Xylem Pith Basal end of cell 25 m To investigate how auxin is transported unidirectionally, researchers designed an experiment to identify the location of the auxin transport protein. They used a greenish-yellow fluorescent molecule to label antibodies that bind to the auxin transport protein. They applied the antibodies to longitudinally sectioned Arabidopsis stems. RESULTS The left micrograph shows that the auxin transport protein is not found in all tissues of the stem, but only in the xylem parenchyma. In the right micrograph, a higher magnification reveals that the auxin transport protein is primarily localized to the basal end of the cells. CONCLUSION The results support the hypothesis that concentration of the auxin transport protein at the basal ends of cells is responsible for polar transport of auxin. EXPERIMENT Figure 39.7

The Role of Auxin in Cell Elongation According to a model called the acid growth hypothesis Proton pumps play a major role in the growth response of cells to auxin

Cell elongation in response to auxin 3 Wedge-shaped expansins, activated by low pH, separate cellulose microfibrils from cross-linking polysaccharides. The exposed cross-linking polysaccharides are now more accessible to cell wall enzymes. Expansin CELL WALL Cell wall enzymes Cross-linking cell wall polysaccharides Microfibril H+ ATP Plasma membrane Plasma membrane Cell wall Nucleus Vacuole Cytoplasm H2O 4 The enzymatic cleaving of the cross-linking polysaccharides allows the microfibrils to slide. The extensibility of the cell wall is increased. Turgor causes the cell to expand. 2 The cell wall becomes more acidic. 1 Auxin increases the activity of proton pumps. 5 With the cellulose loosened, the cell can elongate. Figure 39.8

Lateral and Adventitious Root Formation Auxin Is involved in the formation and branching of roots

Auxins as Herbicides An overdose of auxins Can kill eudicots

Auxin affects secondary growth Other Effects of Auxin Auxin affects secondary growth By inducing cell division in the vascular cambium and influencing differentiation of secondary xylem

Cytokinins Cytokinins Stimulate cell division

Control of Cell Division and Differentiation Cytokinins Are produced in actively growing tissues such as roots, embryos, and fruits Work together with auxin

Control of Apical Dominance Cytokinins, auxin, and other factors interact in the control of apical dominance The ability of a terminal bud to suppress development of axillary buds Axillary buds Figure 39.9a

If the terminal bud is removed Plants become bushier “Stump” after removal of apical bud Lateral branches Figure 39.9b

Cytokinins retard the aging of some plant organs 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 have a variety of effects Such as stem elongation, fruit growth, and seed germination

Gibberellins stimulate growth of both leaves and stems In stems Stem Elongation Gibberellins stimulate growth of both leaves and stems In stems Gibberellins 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 commercially In the spraying of Thompson seedless grapes Figure 39.10

Germination After water is imbibed, the release of gibberellins from the embryo Signals the seeds to break dormancy and germinate 2 The aleurone responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. One example is -amylase, which hydrolyzes starch. (A similar enzyme in our saliva helps in digesting bread and other starchy foods.) Aleurone Endosperm Water Scutellum (cotyledon) GA -amylase Radicle Sugar 1 After a seed imbibes water, the embryo releases gibberellin (GA) as a signal to the aleurone, the thin outer layer of the endosperm. 3 Sugars and other nutrients absorbed from the endosperm by the scutellum (cotyledon) are consumed during growth of the embryo into a seedling. 2 Figure 39.11

2 The aleurone responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. One example is -amylase, which hydrolyzes starch. (A similar enzyme in our saliva helps in digesting bread and other starchy foods.) Aleurone Endosperm Water Scutellum (cotyledon) GA -amylase Radicle Sugar 2 1 After a seed imbibes water, the embryo releases gibberellin (GA) as a signal to the aleurone, the thin outer layer of the endosperm. 3 Sugars and other nutrients absorbed from the endosperm by the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.

Brassinosteroids Brassinosteroids Are similar to the sex hormones of animals Induce cell elongation and division

Two of the many effects of abscisic acid (ABA) are Seed dormancy Drought tolerance

Seed dormancy has great survival value Because it ensures that the seed will germinate only when there are optimal conditions

Precocious germination is observed in maize mutants That lack a functional transcription factor required for ABA to induce expression of certain genes Figure 39.12 Coleoptile

ABA is the primary internal signal Drought Tolerance ABA is the primary internal signal That enables plants to withstand drought

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 Ethylene induces the triple response in pea seedlings, with increased ethylene concentration causing increased response. CONCLUSION Germinating pea seedlings were placed in the dark and exposed to varying ethylene concentrations. Their growth was compared with a control seedling not treated with ethylene. EXPERIMENT All the treated seedlings exhibited the triple response. Response was greater with increased concentration. RESULTS 0.00 0.10 0.20 0.40 0.80 Ethylene concentration (parts per million) Figure 39.13

Ethylene-insensitive mutants Fail to undergo the triple response after exposure to ethylene Figure 39.14a ein mutant

Other types of mutants Undergo the triple response in air but do not respond to inhibitors of ethylene synthesis ctr mutant Figure 39.14b

A summary of ethylene signal transduction mutants Control Ethylene added Ethylene synthesis inhibitor Wild-type Ethylene insensitive (ein) Ethylene overproducing (eto) Constitutive triple response (ctr) Figure 39.15

Apoptosis: Programmed Cell Death A burst of ethylene Is associated with the programmed destruction of cells, organs, or whole plants

A change in the balance of auxin and ethylene controls leaf abscission The process that occurs in autumn when a leaf falls 0.5 mm Protective layer Abscission layer Stem Petiole Figure 39.16

A burst of ethylene production in the fruit Fruit Ripening A burst of ethylene production in the fruit Triggers the ripening process

Systems Biology and Hormone Interactions Interactions between hormones and their signal transduction pathways Make it difficult to predict what effect a genetic manipulation will have on a plant Systems biology seeks a comprehensive understanding of plants That will permit successful modeling of plant functions

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 what plant biologists call photomorphogenesis

Plants not only detect the presence of light But also its direction, intensity, and wavelength (color) A graph called an action spectrum Depicts the relative response of a process to different wavelengths of light

Phototropic effectiveness relative to 436 nm Action spectra Are useful in the study of any process that depends on light Researchers exposed maize (Zea mays) coleoptiles to violet, blue, green, yellow, orange, and red light to test which wavelengths stimulate the phototropic bending toward light. EXPERIMENT RESULTS The graph below shows phototropic effectiveness (curvature per photon) relative to effectiveness of light with a wavelength of 436 nm. The photo collages show coleoptiles before and after 90-minute exposure to side lighting of the indicated colors. Pronounced curvature occurred only with wavelengths below 500 nm and was greatest with blue light. Wavelength (nm) 1.0 0.8 0.6 0.2 450 500 550 600 650 700 Light Time = 0 min. Time = 90 min. 0.4 400 Phototropic effectiveness relative to 436 nm The phototropic bending toward light is caused by a photoreceptor that is sensitive to blue and violet light, particularly blue light. Figure 39.17 CONCLUSION

Research on action spectra and absorption spectra of pigments Led to the identification of 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 Regulate many of a plant’s responses to light throughout its life

Phytochromes and Seed Germination Studies of seed germination Led to the discovery of phytochromes

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

EXPERIMENT During the 1930s, USDA scientists briefly exposed batches of lettuce seeds to red light or far-red light to test the effects on germination. After the light exposure, the seeds were placed in the dark, and the results were compared with control seeds that were not exposed to light. The bar below each photo indicates the sequence of red-light exposure, far-red light exposure, and darkness. The germination rate increased greatly in groups of seeds that were last exposed to red light (left). Germination was inhibited in groups of seeds that were last exposed to far-red light (right). RESULTS Dark (control) Dark Red Far-red CONCLUSION Red light stimulated germination, and far-red light inhibited germination. The final exposure was the determining factor. The effects of red and far-red light were reversible. Figure 39.18

A phytochrome Is the photoreceptor responsible for the opposing effects of red and far-red light A phytochrome consists of two identical proteins joined to form one functional molecule. Each of these proteins has two domains. Chromophore Photoreceptor activity. One domain, which functions as the photoreceptor, is covalently bonded to a nonprotein pigment, or chromophore. Kinase activity. The other domain has protein kinase activity. The photoreceptor domains interact with the kinase domains to link light reception to cellular responses triggered by the kinase. Figure 39.19

Phytochromes exist in two photoreversible states With conversion of Pr to Pfr triggering many developmental responses Synthesis Far-red light Red light Slow conversion in darkness (some plants) Responses: seed germination, control of flowering, etc. Enzymatic destruction Pfr Pr Figure 39.20

Phytochromes and Shade Avoidance The phytochrome system Also provides the plant with information about the quality of light In the “shade avoidance” response of a tree 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 Noon Midnight Figure 39.21

Cyclical responses to environmental stimuli are called circadian rhythms And are approximately 24 hours long 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 developmental processes, including flowering in many species Requires a certain photoperiod

Critical Night Length In the 1940s, researchers discovered that flowering and other responses to photoperiod Are actually controlled by night length, not day length EXPERIMENT During the 1940s, researchers conducted experiments in which periods of darkness were interrupted with brief exposure to light to test how the light and dark portions of a photoperiod affected flowering in “short-day” and “long-day” plants. RESULTS Darkness Flash of light 24 hours Critical dark period Light (a) “Short-day” plants flowered only if a period of continuous darkness was longer than a critical dark period for that particular species (13 hours in this example). A period of darkness can be ended by a brief exposure to light. (b) “Long-day” plants flowered only if a period of continuous darkness was shorter than a critical dark period for that particular species (13 hours in this example). CONCLUSION The experiments indicated that flowering of each species was determined by a critical period of darkness (“critical night length”) for that species, not by a specific period of light. Therefore, “short-day” plants are more properly called “long-night” plants, and “long-day” plants are really “short-night” plants. Figure 39.22

Action spectra and photoreversibility experiments Show that phytochrome is the pigment that receives red light, which can interrupt the nighttime portion of the photoperiod Figure 39.23 A unique characteristic of phytochrome is reversibility in response to red and far-red light. To test whether phytochrome is the pigment measuring interruption of dark periods, researchers observed how flashes of red light and far-red light affected flowering in “short-day” and “long-day” plants. EXPERIMENT RESULTS CONCLUSION A flash of red light shortened the dark period. A subsequent flash of far-red light canceled the red light’s effect. If a red flash followed a far-red flash, the effect of the far-red light was canceled. This reversibility indicated that it is phytochrome that measures the interruption of dark periods. 24 20 16 12 8 4 Hours Short-day (long-night) plant Long-day (short-night) plant R FR Critical dark period

The flowering signal, not yet chemically identified A Flowering Hormone? The flowering signal, not yet chemically identified Is called florigen, and it may be a hormone or a change in relative concentrations of multiple hormones

Figure 39.24 To test whether there is a flowering hormone, researchers conducted an experiment in which a plant that had been induced to flower by photoperiod was grafted to a plant that had not been induced. EXPERIMENT RESULTS CONCLUSION Both plants flowered, indicating the transmission of a flower-inducing substance. In some cases, the transmission worked even if one was a short-day plant and the other was a long-day plant. Plant subjected to photoperiod that induces flowering that does not induce flowering Graft Time (several weeks)

Meristem Transition and Flowering Whatever combination of environmental cues and internal signals is necessary for flowering to occur The outcome is the transition of a bud’s meristem from a vegetative to a flowering state

Because of their immobility Concept 39.4: Plants respond to a wide variety of stimuli other than light Because of their immobility Plants must adjust to a wide range of environmental circumstances through developmental and physiological mechanisms

Roots show positive gravitropism Stems show negative gravitropism 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 grains Statoliths 20 m (a) (b) Figure 39.25a, b

The term thigmomorphogenesis Mechanical Stimuli The term thigmomorphogenesis Refers to the changes in form that result from mechanical perturbation

Rubbing the stems of young plants a couple of times daily Results in plants that are shorter than controls Figure 39.26

Growth in response to touch Is called thigmotropism 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 (a) Unstimulated (b) Stimulated Side of pulvinus with flaccid cells turgid cells Vein 0.5 m (c) Motor organs Leaflets after stimulation Pulvinus (motor organ) Figure 39.27a–c

Environmental Stresses Have a potentially adverse effect on a plant’s survival, growth, and reproduction Can have a devastating impact on crop yields in agriculture

Drought During drought Plants respond to water deficit by reducing transpiration Deeper roots continue to grow

Enzymatic destruction of cells Flooding Enzymatic destruction of cells Creates air tubes that help plants survive oxygen deprivation during flooding Vascular cylinder Air tubes Epidermis 100 m (a) Control root (aerated) (b) Experimental root (nonaerated) Figure 39.28a, b

Salt Stress Plants respond to salt stress by producing solutes tolerated at high concentrations Keeping the water potential of cells more negative than that of the soil solution

Heat Stress Heat-shock proteins Help plants survive heat stress

Altering lipid composition of membranes Cold Stress Altering lipid composition of membranes Is a response to cold stress

Plants counter external threats 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

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 With chemical defenses such as distasteful or toxic compounds

Some plants even “recruit” predatory animals That help defend the plant against specific herbivores Recruitment of parasitoid wasps that lay their eggs within caterpillars 4 3 Synthesis and release of volatile attractants 1 Chemical in saliva Wounding 2 Signal transduction pathway Figure 39.29

Defenses Against Pathogens A plant’s first line of defense against infection Is the physical barrier of the plant’s “skin,” the epidermis and the periderm Once a pathogen invades a plant The plant mounts a chemical attack as a second line of defense that kills the pathogen and prevents its spread

The second defense system Is enhanced by the plant’s 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 is a widespread form of plant disease resistance That involves recognition of pathogen-derived molecules by the protein products of specific plant disease resistance (R) genes

Plant cell is resistant A pathogen is avirulent If it has a specific Avr gene corresponding to a particular R allele in the host plant Figure 39.30a Receptor coded by R allele (a) 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. R alleles probably code for receptors in the plasma membranes of host plant cells. Avr alleles produce compounds that can act as ligands, binding to receptors in host plant cells. Signal molecule (ligand) from Avr gene product R Avr allele Avirulent pathogen Plant cell is resistant

If the plant host lacks the R gene that counteracts the pathogen’s Avr gene Then the pathogen can invade and kill the plant Figure 39.30b No Avr allele; virulent pathogen Plant cell becomes diseased Avr allele No R allele; plant cell becomes diseased Virulent pathogen (b) 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. R

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 4 Before they die, infected cells release a chemical signal, probably salicylic acid. 3 In a hypersensitive response (HR), plant cells produce anti- microbial molecules, seal off infected areas by modifying their walls, and then destroy themselves. This localized response produces lesions and protects other parts of an infected leaf. Signal 5 The signal is distributed to the rest of the plant. 4 5 Hypersensitive response Signal transduction pathway 6 3 6 In cells remote from the infection site, the chemical initiates a signal transduction pathway. Signal transduction pathway 2 Acquired resistance 7 2 This identification step triggers a signal transduction pathway. 1 7 Systemic acquired resistance is activated: the production of molecules that help protect the cell against a diversity of pathogens for several days. Avirulent pathogen 1 Specific resistance is based on the binding of ligands from the pathogen to receptors in plant cells. R-Avr recognition and hypersensitive response Systemic acquired resistance Figure 39.31

Systemic Acquired Resistance Systemic acquired resistance (SAR) Is a set of generalized defense responses in organs distant from the original site of infection Is triggered by the signal molecule salicylic acid