Transcript Biology
Patterns of Plant Growth Plants grow in response to environmental factaors such as light, moisture, temperature, and gravity. Specific chemicals direct, control, and regulate plant growth. Slide 1 of 32 Copyright Pearson Prentice Hall Plant Hormones What are plant hormones? Slide 2 of 32 Copyright Pearson Prentice Hall Plant Hormones Plant Hormones A hormone is a substance that is produced in one part of an organism and affects another part of the same individual. Slide 3 of 32 Copyright Pearson Prentice Hall Plant Hormones Plant hormones are chemical substances that control a plant's patterns of growth and development and its responses to environmental conditions. Slide 4 of 32 Copyright Pearson Prentice Hall Overview: Stimuli and a Stationary Life Linnaeus noted that flowers of different species opened at different times of day and could be used as a horologium florae, or floral clock Plants, being rooted to the ground, must respond to environmental changes that come their way For example, the bending of a seedling toward light begins with sensing the direction, quantity, and color of the light Fig. 39-1 Signal transduction pathways link signal reception to response Plants have cellular receptors that detect changes in their environment For a stimulus to elicit a response, certain cells must have an appropriate receptor Stimulation of the receptor initiates a specific signal transduction pathway A potato left growing in darkness produces shoots that look unhealthy and 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 Fig. 39-2 (a) Before exposure to light (b) After a week’s exposure to natural daylight A potato’s response to light is an example of cell-signal processing The stages are reception, transduction, and response Fig. 39-3 CELL WALL 1 Reception CYTOPLASM 2 Transduction Relay proteins and second messengers Receptor Hormone or environmental stimulus Plasma membrane 3 Response Activation of cellular responses 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 responses Fig. 39-4-1 1 Reception 2 Transduction CYTOPLASM Plasma membrane cGMP Second messenger produced Phytochrome activated by light Cell wall Light Specific protein kinase 1 activated NUCLEUS Fig. 39-4-2 1 Reception 2 Transduction CYTOPLASM Plasma membrane cGMP Second messenger produced Specific protein kinase 1 activated Phytochrome activated by light Cell wall Specific protein kinase 2 activated Light Ca2+ channel opened Ca2+ NUCLEUS Fig. 39-4-3 1 Reception 2 Transduction 3 Response Transcription factor 1 CYTOPLASM Plasma membrane cGMP Second messenger produced Specific protein kinase 1 activated NUCLEUS P Transcription factor 2 Phytochrome activated by light P Cell wall Specific protein kinase 2 activated Transcription Light Translation Ca2+ channel opened Ca2+ De-etiolation (greening) response proteins 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 Transcriptional Regulation Specific transcription factors bind directly to specific regions of DNA and control transcription of genes Positive transcription factors are proteins that increase the transcription of specific genes, while negative transcription factors are proteins that decrease the transcription of specific genes Plant Hormones The portion of an organism affected by a particular hormone is known as its target cell or target tissue. Slide 19 of 32 Copyright Pearson Prentice Hall Plant Hormones To respond to a hormone, the target cell must contain a receptor to which the hormone binds. If the receptor is present, the hormone can influence the target cell by: changing its metabolism affecting its growth rate activating the transcription of certain genes Slide 20 of 32 Copyright Pearson Prentice Hall Plant Hormones Cells that do not contain receptors are generally unaffected by hormones. Different kinds of cells may have different receptors for the same hormone. As a result, a single hormone may affect two different tissues in different ways. For example, a particular hormone may stimulate growth in stem tissues but inhibit growth in root tissues. Slide 21 of 32 Copyright Pearson Prentice Hall Plant Hormones How do auxins, cytokinins, gibberellins, and ethylene affect plant growth? Slide 22 of 32 Copyright Pearson Prentice Hall Auxins Auxins Charles Darwin and his son Francis carried out the experiment that led to the discovery of the first plant hormone. They described an experiment in which oat seedlings demonstrated a response known as phototropism—the tendency of a plant to grow toward a source of light. Slide 23 of 32 Copyright Pearson Prentice Hall Auxins In the experiment, they placed an opaque cap over the tip of one of the oat seedlings. This plant did not bend toward the light, even though the rest of the plant was uncovered. Slide 24 of 32 Copyright Pearson Prentice Hall Auxins However, if an opaque shield was placed a few centimeters below the tip, the plant would bend toward the light as if the shield were not there. Slide 25 of 32 Copyright Pearson Prentice Hall Auxins The Darwins suspected that the tip of each seedling produced substances that regulated cell growth. Forty years later, these substances were identified and named auxins. Slide 26 of 32 Copyright Pearson Prentice Hall Auxins Auxins are produced in the apical meristem and are transported downward into the rest of the plant. They stimulate cell elongation. Slide 27 of 32 Copyright Pearson Prentice Hall Auxins When light hits one side of the stem, the shaded part develops a higher concentration of auxins. This change in concentration stimulates cells on the dark side to elongate. Slide 28 of 32 Copyright Pearson Prentice Hall Auxins As a result, the stem bends away from the shaded side and toward the light. Recent experiments have shown that auxins migrate toward the shaded side of the stem. Slide 29 of 32 Copyright Pearson Prentice Hall Auxins Auxins and Gravitropism Auxins are also responsible for gravitropism— the response of a plant to the force of gravity. Slide 30 of 32 Copyright Pearson Prentice Hall Auxins Auxins build up on the lower sides of roots and stems. In stems, auxins stimulate cell elongation, helping turn the trunk upright. Slide 31 of 32 Copyright Pearson Prentice Hall Auxins In roots, their effects are exactly the opposite. There, auxins inhibit cell growth and elongation, causing the roots to grow downward. Slide 32 of 32 Copyright Pearson Prentice Hall Auxins Auxins also influence how roots grow around objects in the soil. If a growing root is forced sideways by an obstacle, auxins accumulate on the lower side of the root. Slide 33 of 32 Copyright Pearson Prentice Hall Auxins High concentrations of auxins inhibit the elongation of root cells. Uninhibited cells on the top elongate more than auxininhibited cells on the bottom and the root grows downward. Slide 34 of 32 Copyright Pearson Prentice Hall Auxins Auxins and Branching Auxins also regulate cell division in meristems. Slide 35 of 32 Copyright Pearson Prentice Hall Auxins As a stem grows in length, it produces lateral buds. A lateral bud is a meristematic area on the side of a stem that gives rise to side branches. Slide 36 of 32 Copyright Pearson Prentice Hall Auxins Most lateral buds do not start growing right away. The reason for this delay is that growth at the lateral buds is inhibited by auxins. Slide 37 of 32 Copyright Pearson Prentice Hall Auxins Because auxins move out from the apical meristem, the closer a bud is to the stem's tip, the more it is inhibited. This phenomenon is called apical dominance. Slide 38 of 32 Copyright Pearson Prentice Hall Auxins Apical meristem removed When the apical meristem is removed, the concentration of auxin is reduced and the side branches begin to grow more rapidly. Slide 39 of 32 Copyright Pearson Prentice Hall Auxins Auxinlike Weed Killers Chemists have produced compounds that mimic the effects of auxins. Since high concentrations of auxins inhibit growth, many of these are used as herbicides—compounds toxic to plants. Slide 40 of 32 Copyright Pearson Prentice Hall Cytokinins Cytokinins Cytokinins are plant hormones produced in growing roots and developing fruits and seeds. Cytokinins delay the aging of leaves and play important roles in early stages of plant growth. Slide 41 of 32 Copyright Pearson Prentice Hall Cytokinins In plants, cytokinins stimulate cell division and the growth of lateral buds, and cause dormant seeds to sprout. Slide 42 of 32 Copyright Pearson Prentice Hall Cytokinins Cytokinins and auxins often produce opposite effects. Auxins stimulate cell elongation. Cytokinins inhibit cell elongation and cause cells to grow thicker. Auxins inhibit the growth of lateral buds. Cytokinins stimulate lateral bud growth. Slide 43 of 32 Copyright Pearson Prentice Hall Cytokinins Recent experiments show that the rate of cell growth in most plants is determined by the ratio of the concentration of auxins to cytokinins. In growing plants, therefore, the relative concentrations of auxins, cytokinins and other hormones determine how the plant grows. Slide 44 of 32 Copyright Pearson Prentice Hall Gibberellins Gibberellins A gibberellin is a growth-promoting substance in plants. Gibberellins produce dramatic increases in size, particularly in stems and fruit. Gibberellins are also produced by seed tissue and are responsible for the rapid early growth of many plants. Slide 45 of 32 Copyright Pearson Prentice Hall Ethylene Ethylene In response to auxins, fruit tissues release small amounts of the hormone ethylene. Ethylene is a plant hormone that causes fruits to ripen. Commercial producers of fruit sometimes use this hormone to control the ripening process. Slide 46 of 32 Copyright Pearson Prentice Hall 25–2 Plant Responses Slide 47 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Tropisms Tropisms What are plant tropisms? Slide 48 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Tropisms Plants change their patterns and directions of growth in response to a multitude of cues. The responses of plants to external stimuli are called tropisms. Slide 49 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Tropisms Plant tropisms include: • gravitropism, • phototropism, and thigmotropism. Each of these responses demonstrates the ability of plants to respond effectively to external stimuli, such as gravity, light, and touch. Slide 50 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Tropisms Gravitropism Gravitropism, the response of a plant to gravity, is controlled by auxins. Gravitropism causes the shoot of a germinating seed to grow out of the soil—against the force of gravity. It also causes the roots of a plant to grow with the force of gravity and into the soil. Slide 51 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Tropisms Phototropism Phototropism, the response of a plant to light, is also controlled by auxins. This response can be so quick that young seedlings reorient themselves in a matter of hours. Slide 52 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Tropisms Thigmotropism Thigmotropism is the response of plants to touch. An example of thigmotropism is the growth of vines and climbing plants. Slide 53 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Tropisms The stems of these plants do not grow straight up. The growing tip of each stem points sideways and twists in circles as the shoot grows. When the tip encounters an object, it quickly wraps around it. Slide 54 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Tropisms Some climbing plants have long, twisting leaf tips or petioles that wrap tightly around small objects. Other plants, such as grapes, have extra growths called tendrils that emerge near the base of the leaf and wrap tightly around any object they encounter. Slide 55 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Rapid Responses Rapid Responses Not all plant responses involve growth. One example is the rapid closing of leaflets that occurs in the Mimosa pudica. If you touch the leaves of a mimosa plant, within seconds, the leaves snap shut. Slide 56 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Rapid Responses The secret to this movement is changes in osmotic pressure. The leaves are held apart due to osmotic pressure where the two leaflets join. When the leaf is touched, cells near the center of the leaflet pump out ions and lose water due to osmosis. Pressure from cells on the underside of the leaf, which do not lose water, forces the leaflets together. Slide 57 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Photoperiodism Photoperiodism What is photoperiodism? Slide 58 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Photoperiodism Plants such as chrysanthemums and poinsettias flower when days are short and are therefore called short-day plants. Spinach and irises flower when days are long and are therefore known as long-day plants. Photoperiodism is the response to periods of light and darkness. Slide 59 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Photoperiodism Photoperiodism in plants is responsible for the timing of seasonal activities such as flowering and growth. Slide 60 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Photoperiodism Slide 61 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Photoperiodism It was later discovered that a plant pigment called phytochrome is responsible for photoperiodism. Phytochrome absorbs red light and activates a number of signaling pathways within plant cells. Plants respond to regular changes in these pathways and these changes determine the patterns of a variety of plant responses. Slide 62 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Winter Dormancy Winter Dormancy Phytochrome also regulates the changes in activity that prepares many plants for dormancy as winter approaches. Dormancy is the period during which an organism's growth and activity decreases or stops. Slide 63 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Winter Dormancy How do deciduous plants prepare for winter? Slide 64 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Winter Dormancy As cold weather approaches, deciduous plants turn off photosynthetic pathways, transport materials from leaves to roots, and seal leaves off from the rest of the plant. Slide 65 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Winter Dormancy Leaf Abscission At summer’s end, the phytochrome in leaves absorbs less light as days shorten and nights become longer. Auxin production drops, but the production of ethylene increases. Slide 66 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Winter Dormancy The change in the relative amounts of auxin and ethylene hormones starts a series of events that gradually shut down the leaf. First chlorophyll synthesis stops. Light destroys the remaining green pigment. Other pigments—including yellow and orange carotenoids—become visible for the first time. Slide 67 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Winter Dormancy Production of new plant pigments—the reddish anthocyanins—begins in the autumn. Every available carbohydrate is transported out of the leaf, and much of the leaf’s water is extracted. Slide 68 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Winter Dormancy Finally, an abscission layer of cells at the petiole seals the leaf off from the plant’s vascular system. Before long, the leaf falls to the ground, a sign that the tree is fully prepared for winter. Slide 69 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Winter Dormancy Overwintering of Meristems Hormones also produce important changes in apical meristems. Instead of continuing to produce leaves, meristems produce thick, waxy scales that form a protective layer around new leaf buds. Enclosed in its coat of scales, a terminal bud can survive the coldest winter days. Slide 70 of 32 Copyright Pearson Prentice Hall 25-2 Plant Responses Winter Dormancy At the onset of winter, xylem and phloem tissues pump themselves full of ions and organic compounds. These molecules act like antifreeze in a car, preventing the tree’s sap from freezing, thus making it possible to survive the bitter cold. Slide 71 of 32 Copyright Pearson Prentice Hall