Plant Response

Plant reactions Stimuli & a Stationary Life Animals respond to stimuli by changing behavior Move toward positive stimuli Move away from negative stimuli Plants respond to stimuli by adjusting growth & development

Signal Transduction Pathways in Plants Plants have cellular receptors that detect changes in their environment For a stimulus to elicit a response, certain cells must have an appropriate receptor Signal triggers receptor Receptor triggers internal cellular messengers which transfer and amplify signals from receptors to proteins that cause responses Cellular response may involve increased activity of enzymes by: Stimulating transcription of mRNA for an enzyme (Transcriptional regulation) Activating an existing enzyme (Post-translational modification)

Transcriptional Regulation Specific transcription factors bind directly to specific regions of DNA and control transcription of genes Positive transcription factors increase the transcription of specific genes Negative transcription factors decrease the transcription of specific genes

Post-Translational Modification of Proteins Involves modification of existing proteins in the signal response Ex. phosphorylation of enzymes for activation

Signal Transduction Pathway Example Both pathways lead to expression of genes for proteins that functions in greening response of plants

Greening Response in Potato Plants Grown in dark 1 week exposure to light

Discovery of Plant Hormones In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light Curvatures of whole plant organs toward or away from stimuli is called a tropism Grass seedling could bend toward light only if the tip of the coleoptile was present They postulated that a signal was transmitted from the tip to the elongating region

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

Discovery of Plant Hormones In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments

Plant hormones Chemical signals that coordinate different parts of an organism Only minute amounts are required Produced in one part of the body and is transported to another part Binds to specific receptor Triggers response in target cells & tissues

Plant hormones Auxins Cytokinins Gibberellins Brassinosteroids Abscisic acid ethylene

Table 39-1

Auxin Indoleacetic acid (IAA) Stimulates cell elongation Involved in lateral root formation and branching Enhances apical dominance An overdose of synthetic auxins can kill eudicots (used as herbicide) Classical explanation of phototropism Asymmetrical distribution of auxin Cells on darker side elongate faster than cells on brighter side

Produced in actively growing tissues: roots, fruits & embryos Effects Control of cell division & differentiation Enhances apical dominance Terminal bud suppresses development of axillary buds If terminal bud is removed, plants become bushier Retard aging of some plant organs by inhibiting protein breakdown and stimulating RNA and protein synthesis auxin & cytokinins interact to control cell division and differentiation Cytokinins

Gibberellins Over 100 different gibberellins identified Effects Stem elongation Stimulate cell elongation and cell division Fruit growth Auxin and gibberellins must be present for fruit to set Seed germination After seed is imbibed, release of gibberellins from embryo signals seeds to germinate

α-amylase and other enzymes. Sugars and other nutrients are consumed. Fig. 39-11 Gibberellins (GA) send signal to aleurone. 1 Aleurone secretes α-amylase and other enzymes. 2 Sugars and other nutrients are consumed. 3 Aleurone Endosperm -amylase Sugar GA GA Figure 39.11 Mobilization of nutrients by gibberellins during the germination of grain seeds such as barley Water Radicle Scutellum (cotyledon)

Brassinosteroids Similar to sex hormones of animals Effects Similar to auxins Cell elongation & division in shoots & seedlings

Abscisic acid (ABA) Effects Slows growth High concentration of ABA promotes seed dormancy Germination occurs only after ABA is inactivated or leached out Ensures that seeds will germinate only under optimal conditions Drought tolerance Rapid stomata closure to allow plant to withstand drought

Ethylene Ethylene is a gas released by plant cells in response to stresses such as drought, flooding, injury, infection Multiple effects Response to mechanical stress Such as a seedling growing around a stone (an obstacle) Apoptosis Ex. shedding leaves in autumn

Ethylene Effects: Fruit Ripening Hard, tart fruit protects developing seeds from herbivores Ripe, sweet, soft fruit attracts animals to disperse seed Burst of ethylene triggers ripening process Breakdown of cell wall = softening Conversion of starch to sugar = sweetening Positive feedback system Ethylene triggers ripening Ripening stimulates more ethylene production

Applications “One bad apple DOES spoil the whole bunch” Ripening apple releases ethylene to speed ripening of fruit nearby Ripen green bananas by bagging them with an apple Climate control storage of apples Air is circulated to prevent ethylene buildup Stored in high amounts of CO2 which inhibits the release of ethylene

Responses to light Photomorphogenesis Plants can detect Effect of light on plant growth Plants can detect Presence of light Intensity of light Direction of light Wavelength (color) Blue-light receptors Phytochromes (red-light receptors) An action spectrum depicts responses of a plant process to different wavelengths

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

Phytochromes as 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 Photoreceptor activity In each subunit, one domain, which functions as a photoreceptor, is covalently bonded to a nonprotein pigment, chromophore Kinase activity The other domain has protein kinase activity. The two domains interact linking light reception to cellular responses triggered by the kinase

Phytochromes as Photoreceptors The chromophore of a phytochrome is photoreversible, reverting back and forth between two isomeric forms, depending on the color of light Pr absorbs red (r) light maximally Pfr absorbs far-red (fr) light The conversion triggers many developmental responses such as germination

Phytochrome photoreceptors Molecular switch reaction to red light Conversion of Pr Pfr in sunlight stimulates germination, flowering, branching… Conversion of Pfr Pr in dark inhibits response & stimulates other responses: growth in height “Shade avoidance” response

Circadian Rhythms Internal 24-hour cycles Morning Glory

The Effect of Light on the Biological Clock The clock may depend on synthesis of a protein regulated through feedback control 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 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 Chrysanthemums, soybeans Plants that flower when a light period is longer than a certain number of hours are called long-day plants Spinach, lettuce, iris Flowering in day-neutral plants is controlled by plant maturity, not photoperiod Tomatoes, rice, dandelions

Flowering Response Controlled by night length – “critical period” Short-day plants (long-night) flower when night exceeds a minimum number of hours of darkness Long-day plants (short-night) flower only if the night is shorter than a critical dark period Flash of light can interrupt the nighttime portion of the photoperiod

Flowering Response Red light can interrupt the nighttime portion of the photoperiod Action spectra and photoreversibility experiments show that phytochrome is the pigment that detects red light If a flash of R light during dark period is followed by a flash of FR light, the plant detects no interruption of the night length

Flowering Response 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

Is there a flowering hormone? A flowering signal, not yet chemically identified is called florigen

Responses to stimuli: gravity How does a sprouting shoot “know” to grow towards the surface from underground? Environmental cues Roots = positive gravitropism Shoots = negative gravitropism Settling of statoliths (dense starch grains in plastids) may detect gravity

Responses to stimuli: touch Thigmotropism Growth in response to touch Caused by changes in osmotic pressure = rapid loss of K+ = rapid loss of H2O = loss of turgor in cells Example Mimosa closes leaves in response to touch

Responses to Stimuli: Touch Thigmomorphogenesis Changes in form resulting from mechanical disturbance Ex. Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls

Responses to Stimuli Environmental Stresses have a potentially adverse effect on survival, growth, and reproduction Stresses can be abiotic or biotic Abiotic stresses include drought, flooding, salt stress, heat stress, and cold stress Drought During drought, plants reduce transpiration by closing stomata, slowing leaf growth, and reducing exposed surface area Growth of shallow roots is inhibited, while deeper roots continue to grow Flooding Enzymatic destruction of root cortex cells creates air tubes that help plants survive oxygen deprivation during flooding

Responses to Stimuli Salt Stress Heat Stress Cold 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 water potential of cells becomes more negative than that of the soil solution Heat Stress Excessive heat can denature a plant’s enzymes Heat-shock proteins help protect other proteins from heat stress Cold Stress Cold temperatures decrease membrane fluidity Plants can alter lipid composition of membranes Freezing causes ice to form in a plant’s cell walls and intercellular spaces

Plant defenses Defenses against herbivores Thorns Chemicals Recruitment of predatory animal to defend against specific herbivores Volatile chemicals to warn other plants of same species Methyljasmonic acid can activate expression of genes involved in plant defenses

Plant Defenses Defenses against pathogens First line of defense Epidermis and periderm Second line of defense Chemical attack that kills pathogen Enhanced by ability to recognize pathogens

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

Signal Transduction Pathways The hypersensitive response Causes cell and tissue death near the infection site Induces production of phytoalexins and PR (pathogenesis-related) proteins which attack the pathogen Stimulates changes in the cell wall that confine the pathogen Systemic acquired resistance causes systemic expression of defense genes and is a long-lasting response Salicylic acid is synthesized around the infection site and is likely the signal that triggers systemic acquired resistance

Plant defenses: Signal Transduction Pathways Specific resistance is based on pathogen-receptor binding Signal transduction pathway is triggered Antimicrobial molecules that seal off infected areas are produced Infected cells release methylsalicylic acid before they die The signaling molecule is distributed to rest of the plant In cells away from infection site, methylsalicylic acid is converted to salicylic acid which initiates a signal transduction pathway Molecules that help protect the cell against a diversity of pathogens for several days are produced