Chapter 39. Plant Response 1
Plant Reactions Stimuli & a Stationary Life u animals respond to stimuli by changing behavior move toward positive stimuli move away from negative stimuli u plants respond to stimuli by adjusting growth & development 2
What mechanism causes this response? grown in dark 1 week exposure to light 3
Signal Transduction Pathway model signal triggers receptor receptor triggers internal cellular messengers & then cellular response receptor signal pathway (2 messengers) response What kinds of molecules are the receptors? General model for signal transduction pathways: A hormone or another signal binding to a specific receptor stimulates the cell to produce relay molecules, such as second messengers. These relay molecules trigger the cell s various responses to the original signal. In this diagram, the receptor is on the surface of the target cell. In other cases, hormones enter cells and bind to specific receptors inside. 4
Signal Transduction Pathway example 1. Light signal is detected by the phytochrome receptor, which then activates 2. One at pathway least 2uses cgmp as a 2nd signal transduction messenger 3. Both pathways pathways to activate leada protein kinase. to expression of genes The for other proteins pathway that involves increases function in cytoplasmic greening Ca 2+ that activates response a different of plant. protein kinase. Signal Transduction Pathway controlling greening (de-etiolation) of plant cells, like in a sprouting potato tuber. 1. Light signal is detected by the phytochrome receptor, which then activates at least 2 signal transduction pathways 2. One pathway uses cgmp as a 2nd messenger to activate a protein kinase.the other pathway involves increases in cytoplasmic Ca 2+ that activates a different protein kinase. 3. Both pathways lead to expression of genes for proteins that function in greening response of plant. 5
Plant hormones Chemical signals that coordinate different parts of an organism u only minute amounts are required u produced by 1 part of body u transported to another part u binds to specific receptor u triggers response in target cells & tissues 6
Plant hormones auxins cytokinins gibberellins brassinosteroids abscisic acid ethylene 7
Phototropism Went 1926 Growth towards light 8
Auxin Indolacetic acid (IAA) u stimulates cell elongation near apical meristems u enhances apical dominance u classical explanation of phototropism asymmetrical distribution of auxin cells on darker side elongate faster than cells on brighter side 9
Zones of meristem growth shoot root 10
Polarity of auxin transport 1. Auxin picks up H+ between cells & is neutralized 2. Neutral auxin passes through membrane 3. Cellular ph 7 causes auxin to ionize & is trapped in cell 4. Auxin stimulates proton pump 5. Auxin leaves through carriers 11
Auxin response Acid growth hypothesis According to a model called the acid growth hypothesis, proton pumps play a major role in the growth response of cells to auxin. In a shoot s region of elongation, auxin stimulates the plasma membrane s proton pumps. This pumping of H+ increases the voltage across the membrane (membrane potential) and lowers the ph in the cell wall within minutes Acidification of the wall activates enzymes called expansins that break the cross links (hydrogen bonds) between cellulose microfibrils and other cell wall constituents, loosening the wall s fabric. (Expansins can even weaken the integrity of filter paper made of pure cellulose.) Increasing the membrane potential enhances ion uptake into the cell, which causes osmotic uptake of water and increased turgor. Increased turgor and increased cell wall plasticity enable the cell to elongate. 12
Cytokinins Family of hormones u modified forms of adenine u produced in roots, fruits & embryos Effects u control of cell division & differentiation u enhances apical dominance interaction of auxin & cytokinins 13
Gibberellins Family of hormones u over 100 different gibberellins identified Effects u stem elongation u fruit growth u seed germination 14
Brassinosteroids Steroids Effects u similar to auxins u cell elongation & division in shoots & seedlings 15
Abscisic acid (ABA) Effects u slows growth u seed dormancy high concentrations of ABA germination only after ABA is inactivated down or leeched out survival value: seed will germinate only under optimal conditions light, temperature, moisture u drought tolerance rapid stomate closing 16
Ethylene Ethylene is a gas released by plant cells Multiple effects u response to mechanical stress triple response slow stem elongation thickening of stem curvature to horizontal growth u apoptosis u leaf abscission u fruit ripening 17
Apoptosis & Leaf abscission Balance of ethylene & auxin u many events in plants involve pre-programmed cell death death of annual plant after flowering differentiation of xylem vessels loss of cytosol shedding of autumn leaves What is the evolutionary advantage of loss of leaves in autumn? The loss of leaves each autumn is an adaptation that keeps deciduous trees from desiccating during winter when the roots cannot absorb water from the frozen ground. Before leaves abscise, many essential elements are salvaged from the dying leaves and are stored in stem parenchyma cells. These nutrients are recycled back to developing leaves the following spring. Fall color is a combination of new red pigments made during autumn and yellow and orange carotenoids that were already present in the leaf but are rendered visible by the breakdown of the dark green chlorophyll in autumn. Photo: Abscission of a maple leaf. Abscission is controlled by a change in the balance of ethylene and auxin. The abscission layer can be seen here as a vertical band at the base of the petiole. After the leaf falls, a protective layer of cork becomes the leaf scar that helps prevent pathogens from invading the plant (LM). 18
Fruit ripening Hard, tart fruit protects developing seed from herbivores Ripe, sweet, soft fruit attracts animals to disperse seed u burst of ethylene triggers ripening process breakdown of cell wall = softening conversion of starch to sugar = sweetening u positive feedback system ethylene triggers ripening ripening stimulates more ethylene production 19
Applications Truth in folk wisdom! u one bad apple spoils 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 u high CO 2 storage = reduces ethylene production 20
Responses to light Photomorphogenesis u effect of light on plant growth Light detection u intensity u direction u wavelength blue-light receptors phytochromes (red-light receptors) Why does it make biological sense that red & blue light have greater effects on plants response that other wavelengths? 21
Phytochrome photoreceptors Molecular switch reaction to red light u conversion of P r Æ P fr in sunlight stimulates germination, flowering, branching u conversion of P fr Æ P r in dark inhibits response, & stimulates other responses: growth in height Chromophore Photorecptor Light induced Kinase activity Phytochrome Phytochrome Response: Vertical growth 22
Practical Application Why do you plant lettuce seed by scattering them on the ground instead of burying seed? What is the evolutionary advantage to lettuce seeds? 23
Flowering Response Triggered by photoperiod u relative lengths of day & night u night length critical period is trigger Plant is sensitive to red light exposure What is the evolutionary advantage of photoperiodism? Short-day plants Long-day plants Synchronizes plant responses to season 24
Is there a flowering hormone? Plant on left is induced to flower & then grafted onto plant on right u plant on right is triggered to flower What can you conclude? 25
Circadian rhythms Internal (endogenous) 24-hour cycles Morning glory 26
Responses to gravity How does a sprouting shoot know to grow towards the surface from underground? u environmental cues? roots = positive gravitropism shoots = negative gravitropism settling of statoliths (dense starch grains) may detect gravity 27
Responses to touch Thigmotropism Mimosa (Sensitive plant) closes leaves in response to touch Caused by changes in osmotic pressure = rapid loss of K + = rapid loss of H 2 O = loss of turgor in cells 28
Plant defenses Defenses against herbivores 29
Plant defenses Defenses against pathogens 30