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Chapter 39 Plant Responses to Internal and External Signals 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 (꽃시계) • 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 © 2011 Pearson Education, Inc. Figure 39.1 Concept 39.1: Signal transduction pathways link signal reception to response • 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(황화현상: 엽록소 형성없이 카로티노이드의 황색이 나타냄) • After exposure to light, a potato undergoes changes called de-etiolation(greening), in which shoots and roots grow normally © 2011 Pearson Education, Inc. Figure 39.2 Dark condition (a) Before exposure to light Light condition (b) After a week’s exposure to natural daylight What happen to potato? • A potato’s response to light is an example of cell-signal processing • The stages are reception, transduction, and response © 2011 Pearson Education, Inc. Figure 39.3 Review of a general model for signal transduction pathways CELL WALL 1 Reception CYTOPLASM 2 Transduction 3 Response Relay proteins and second messengers Receptor Hormone or environmental stimulus Plasma membrane Activation of cellular responses Reception • Internal and external signals are detected by receptors, proteins that change in response to specific stimuli • In de-etiolation, the receptor is a phytochrome capable of detecting light © 2011 Pearson Education, Inc. Transduction • Second messengers transfer and amplify signals from receptors to proteins that cause responses • Two types of second messengers play an important role in de-etiolation: Ca2+ ions and cyclic GMP (cGMP) • The phytochrome receptor responds to light by – Opening Ca2+ channels, which increases Ca2+ levels in the cytosol – Activating an enzyme that produces cGMP © 2011 Pearson Education, Inc. Figure 39.4-1 An example of signal transduction in plants: the role of phytochrome in the de-etiolation response 1 Reception CYTOPLASM Plasma membrane Phytochrome Cell wall Light Figure 39.4-2 2 Transduction 1 Reception CYTOPLASM Plasma membrane cGMP Second messenger Phytochrome Protein kinase 1 Cell wall Protein kinase 2 Light Ca2+ channel Ca2+ Figure 39.4-3 2 Transduction 1 Reception 3 Response Transcription factor 1 NUCLEUS CYTOPLASM Plasma membrane cGMP Second messenger Phytochrome P Protein kinase 1 Transcription factor 2 P Cell wall Protein kinase 2 Transcription Light Translation Ca2+ channel 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 © 2011 Pearson Education, Inc. Post-Translational Modification of Preexisting Proteins • Post-translational modification involves modification of existing proteins in the signal response • Modification often involves the phosphorylation of specific amino acids • The second messengers cGMP and Ca2+ activate protein kinases directly © 2011 Pearson Education, Inc. Transcriptional Regulation • Specific transcription factors bind directly to specific regions of DNA and control transcription of genes • Some transcription factors are activators that increase the transcription of specific genes • Other transcription factors are repressors that decrease the transcription of specific genes © 2011 Pearson Education, Inc. De-Etiolation (“Greening”) Proteins • De-etiolation activates enzymes that – Function in photosynthesis directly – Supply the chemical precursors for chlorophyll production – Affect the levels of plant hormones that regulate growth © 2011 Pearson Education, Inc. Concept 39.2: 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 © 2011 Pearson Education, Inc. The Discovery of Plant Hormones • Any response resulting in curvature of organs toward or away from a stimulus is called a tropism(굴성) • In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism(굴광성), a plant’s response to light • They observed that a grass seedling could bend toward light only if the tip of the coleoptile(자엽초, 떡잎) was present • They postulated that a signal was transmitted from the tip to the elongating region (see next slide) © 2011 Pearson Education, Inc. Figure 39.5 What part of a grass coleoptile senses light, and how is the signal transmitted? RESULTS Shaded side Control (1) 정단부위에서만 빛 인지함 (2) 굴광성을 유도하는 신호는 빛에 의해 활성화되어 이동 가능한 화학물질임 Light Boysen-Jensen(1913년) Illuminated side Light Phototropism occurs only when the tip is illuminated The tip senses light Darwin and Darwin (1880년) Light Gelatin (permeable) Tip removed Opaque cap Transparent cap Opaque shield over curvature Mica(운모) (impermeable) Phototropism occurs when the tip is separated by a permeable barrier but not an impermeable barrier The signal was a mobile chemical substance • In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments Figure 39.6 RESULTS Excised tip on agar cube Does asymmetrical distribution of a growth-promoting chemical cause a coleoptile to grow toward the light? Growth-promoting chemical diffuses into agar cube Control (agar cube lacking chemical) Control has no effect 자엽초의 굴광성은 어두운 부분은 고농도의 옥신이 분포해서 발생하는 것임 © 2011 Pearson Education, Inc. Agar cube with chemical stimulates growth Offset cubes cause curvature 화학물질을 균일하게 전달되는 경우 직선성장 경사지게 놓아 둔 조각은 식물을 휘어지게 함 A Survey of Plant Hormones • Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ • In general, hormones control plant growth and development by affecting the division, elongation, and differentiation of cells © 2011 Pearson Education, Inc. Table 39.1 종자의 배, 줄기 정단분열조직, 어린잎 뿌리에서 합성되어 다른 기관으로 이동 줄기정단분열조직, 뿌리분열조직, 어린잎, 배 성장촉진, 발아촉진, 노화지연 종자, 눈의 발아, 줄기신장, 잎생장 촉진, 뿌리생장과 분화 조절 뿌리생장 억제, 잎의 탈리 지연, 물관형성 촉진 생장억제, 종자휴면 유도, 기공 폐쇄 과일성숙 촉진 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 © 2011 Pearson Education, Inc. Figure 39.7 • Q: What causes polar movement of auxin from shoot tip to base? Auxin transporters (옥신 수송체는 한 세포의 기저부에 위치하여 극성수송을 가능하게 함) – Move the hormone out of the basal end of one cell, and into the apical end of neighboring cells 100 µm RESULTS Cell 1 Cell 2 Epidermis Cortex Conclusion: The results Phloem support the hypothesis that concentration of the auxin Xylem transport protein at the basal ends of cells Pith mediates the polar transport of auxin 25 µm Basal end of cell 옥신 수송단백질의 항체를 녹색형광물질로 표지한 후 위치추적함 The Role of Auxin in Cell Elongation • According to the acid growth hypothesis, auxin stimulates proton pumps in the plasma membrane • 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 © 2011 Pearson Education, Inc. Figure 39.8 Cell elongation in response to auxin: the acid growth hypothesis Cross-linking polysaccharides Cell wall–loosening enzymes (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 Cellulose microfibril (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. H2O H+ (2) The cell wall becomes more acidic. H+ H+ (1) Auxin increases the activity of proton pumps. Plasma membrane H+ ATP H+ H+ H+ Cell wall H+ H+ Plasma membrane CYTOPLASM Nucleus Cytoplasm Vacuole (5) With the cellulose loosened, the cell can elongate. • Auxin also alters gene expression and stimulates a sustained growth response © 2011 Pearson Education, Inc. Auxin’s Role in Plant Development • Polar transport of auxin plays a role in pattern formation of the developing plant • Auxin is synthesized in shoot tips • Reduced auxin flow from the shoot of a branch stimulates growth in lower branches • Auxin transport plays a role in phyllotaxy(잎차례), the arrangement of leaves on the stem(줄기에 잎이 달리는 배열상태) • Polar transport of auxin from leaf margins(잎둘레) directs the patterns of leaf veins (극성옥신수송을 저해하면 잎자루의 관다발 연속성이 결핍한 잎사귀 초래) • The activity of the vascular cambium is under control of auxin transport (식물체가 휴면기에 들어갈때 옥신수송능력 감소와 옥신수송체의 유전자발현의 감소와 관련됨) © 2011 Pearson Education, Inc. Practical Uses for Auxins • The auxin indolbutyric acid (IBA) stimulates adventitious roots and is used in vegetative propagation of plants by cuttings : treating a detached leaf or stem with powder containing IBA causes adventitious roots to form near the cut surface • An overdose of synthetic auxins can kill plants – For example 2,4-D is used as an herbicide on eudicots (Monocots such as maize and turfgrass can rapidly inactivate such synthetic auxins 2,4-D spray를 이용하면 쌍떡잎 씨앗을 제거할 수 있음) © 2011 Pearson Education, Inc. Cytokinins • Cytokinins are so named because they stimulate cytokinesis (cell division) • • • • 1940s Johannes van Overbeek에 의해 발견 코코넛 밀크를 첨가하면 식물성장 촉진 핵산인 아데닌의 변형물질임이 밝혀짐 (Skoog & Miller) 대표적인 시토키닌은 옥수수에서 발견된 zeatin임 © 2011 Pearson Education, Inc. (1) Control of Cell Division and Differentiation • Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits • Cytokinins work together with auxin to control cell division and differentiation © 2011 Pearson Education, Inc. (2) Control of Apical Dominance (정단우성조절) • Cytokinins, auxin, and strigolactone interact in the control of apical dominance, a terminal bud’s ability to suppress development of axillary buds • If the terminal bud is removed, plants become bushier (숱이 많아짐) © 2011 Pearson Education, Inc. Figure 39.9a (1) 끝눈의 옥신이 곁눈의 생장을 억제하며 줄기의 주축성장을 촉진함 (2) 뿌리에서 이동되어 온 시토키닌은 옥신과는 반대로 곁눈생장을 촉진함 Axillary buds (a) Apical bud intact (not shown in photo) Figure 39.9b 끝눈의 제거시 옥신의 주요생성장소인 끝눈이 없으므로 상대적으로 시토키닌의 기능이 우세하게 되어 곁눈생장을 촉진되며 따라서 곁가지가 자람 Lateral branches “Stump (잘리고 난 남은 부분)” after removal of apical bud (b) Apical bud removed Figure 39.9c 끝눈의 제거후 옥신 첨가시 곁눈의 성장 억제 (c) Auxin added to decapitated stem (3) 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 • Cytokinins slow the progress of apoptosis • 시토키닌 용액에 잎을 담가두면 오랫동안 싱싱한 상태를 유지할 수 있다 • 화초재배자들은 시토키닌 스프레이를 꽃의 신선도 유지를 위해 사용함 © 2011 Pearson Education, Inc. Gibberellins • 1926년, 일본 식물병리학자인 Kurosawa에 의해 발견 • 키만 멀대처럼 큰 허약한 벼의 원인균인 Gibberella속의 곰팡이에서 특정 화학물질이 분비되어 벼 줄기의 길이신장을 유도함을 알게 되었고 이를 지베렐린으로 명명함 • Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination © 2011 Pearson Education, Inc. Stem Elongation • Gibberellins are produced in young roots and leaves • Gibberellins stimulate growth of leaves and stems • In stems, they stimulate cell elongation and cell division: 세포벽을 느슨하게 해 주는 효소의 활성화를 유도하여 expansin의 세포벽 안쪽으로의 유입을 촉진하여 세포신장 발생함 • Bolting(추대): 꽃자루의 급격한 생장현상; 배추는 영양생장기에는 로제트형태(키가 낮고 땅에 붙은)로 자라고, 생식생장기에는 지베렐린의 농도증가로 인해 줄기 마디사이의 급격한 신장으로 꽃눈의 위치가 위로 올라감 © 2011 Pearson Education, Inc. Figure 39.10a (a) Rosette form (left) and gibberellin-induced bolting (추대: (right) 꽃줄기 형성) 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 Figure 39.10b (b) Grapes from control vine (left) and gibberellin-treated vine (right) © 2011 Pearson Education, Inc. Germination • After water is imbibed(absorbed), release of gibberellins from the embryo signals seeds to break dormancy and germinate © 2011 Pearson Education, Inc. Figure 39.11 Mobilization of nutrients by GA during the germination of grain seeds such as barley Aleurone(호분층) Endosperm 1 2 3 (배젖) α-amylase Sugar GA Water GA Scutellum (cotyledon) Radicle (어린뿌리, 유근) (떡잎) 종자에 물이 흡수되면 배로부터 GA방출되어 호분층에 신호전달 호분층에서는 소화효소 (알파 아밀라제) 분비하여 배젖의 영양분을 가수분해함 떡잎에 의해 배젖으로부터 흡수된 영양분은 배발생동안 소비됨 Brassinosteroids • Brassinosteroids are chemically similar to the sex hormones of animals • They induce cell elongation and division in stem segments and seedlings • They slow leaf abscission(leaf drop, 낙옆) and promote xylem differentiation © 2011 Pearson Education, Inc. Abscisic Acid(앱시스산) • Abscisic acid (ABA) slows growth • ABA often antagonizes the actions of growth hormones, and the ratio of ABA to one or more growth hormones determines the final physiological outcome • Two of the many effects of ABA – Seed dormancy(종자 휴면) – Drought tolerance(내건성) © 2011 Pearson Education, Inc. Seed Dormancy • 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 • ABA와 GA간의 농도비가 휴면 또는 발아의 결정에 중요함 (see next slide) © 2011 Pearson Education, Inc. Figure 39.12a Red mangrove (Rhizophora mangle) seeds • Precocious germination is observed in maize mutants That lack a functional transcription factor required for ABA to induce expression of certain genes (앱시스산에 의해 조절되는 전사조절인자의 돌연변이로 인해 휴면관련 유전자발현에 이상이 생김) Coleoptile Figure 39.12b Maize mutant Drought Tolerance • ABA is the primary internal signal that enables plants to withstand drought • ABA accumulation causes stomata to close rapidly © 2011 Pearson Education, Inc. Strigolactones • The hormones called strigolactones – Stimulate seed germination – Help establish mycorrhizal associations (균근공생) – Help control apical dominance • Strigolactones are named for parasitic Striga plants (악마의 풀) • Striga seeds germinate when host plants exude strigolactones through their roots © 2011 Pearson Education, Inc. Ethylene • A gaseous plant hormone • 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 © 2011 Pearson Education, Inc. (1) 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 © 2011 Pearson Education, Inc. Figure 39.13 0.00 0.10 0.20 0.40 0.80 Ethylene concentration (parts per million) • Ethylene-insensitive mutants fail to undergo the triple response after exposure to ethylene • Other mutants undergo the triple response in air but do not respond to inhibitors of ethylene synthesis © 2011 Pearson Education, Inc. Figure 39.14a ein mutant Ethylene-insensitive mutants (ein mutant): Fail to undergo the triple response after exposure to ethylene (a) ein mutant Figure 39.14b ctr mutant Other types of mutants (ctr mutant): Undergo the triple response in air but do not respond to inhibitors of ethylene synthesis (에틸렌 신호전달 과정이 지속적인 가동으로 인해 항상 3중반응을 보이며, 에틸렌 합성 저해제 처리에도 3중반응 보임) ctr돌연변이 유전자는 protein kinase을 암호화하며, 이 효소는 에틸렌 신호전달과정의 negative regulator로 추정됨 (b) ctr mutant (2) 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 (3) Leaf Abscission • A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls © 2011 Pearson Education, Inc. Figure 39.15 옥신과 에틸렌의 상대적인 농도변화에 따른 잎의 탈리 조절반응 0.5 mm 오랜된 잎에서는 옥신의 합성 감소로 탈리층 세포의 에틸렌에 대한 민감성 증가 초래 에틸렌의 효과 증대로 세포들은 셀룰로오스 등의 세포벽 구성물질을 분해하는 효소의 합성을 촉진하여 탈리를 유도함 Protective layer Abscission layer Stem Petiole 단풍나무 잎의 탈리 (4) 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 © 2011 Pearson Education, Inc. 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 called photomorphogenesis(광형태형성) © 2011 Pearson Education, Inc. • Plants detect not only presence of light but also its direction, intensity, and wavelength (color) • A graph called an action spectrum(작용스펙트럼) depicts relative response of a process to different wavelengths • Action spectra are useful in studying any process that depends on light – 식물체에서 두 종류의 빛수용체가 있음 – 광형태형성과정(photomorphogenesis)에는 적색광과 청색광이 가장 중요함 – 청색광은 blue-light photoreceptor, 적색광은 phytochrome(피토크롬)에 의해 흡수됨 © 2011 Pearson Education, Inc. Figure 39.16 Phototropic effectiveness What wavelengths stimulate phototropic bending toward light? (어떤 파장의 빛이 굴광성을 유도하나?) 1.0 0.8 0.6 0.4 0.2 0 굴광성은 청색과 보라색 빛에 민감한 광수용체에 의해 발생함 436 nm 400 450 500 550 600 650 700 Wavelength (nm) (a) Phototropism action spectrum Light Time = 0 min Time = 90 min (b) Coleoptiles before and after light exposures Blue-light photoreceptors와 phytochromes의 발견 • 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 (1) Blue-Light Photoreceptors • Various blue-light photoreceptors – Control hypocotyl elongation, stomatal opening, and phototropism (자엽하축 신장, 기공열림, 굴광성의 조절) (2) Phytochromes as Photoreceptors • Phytochromes – Regulate many of a plant’s responses to light throughout its life – 종자발아(seed germination), 음지회피반응(shade avoidance) © 2011 Pearson Education, Inc. Phytochromes and Seed Germination • Many seeds remain dormant until light conditions change • In the 1930s, scientists at the U.S. Department of Agriculture determined the action spectrum for light-induced germination of lettuce seeds © 2011 Pearson Education, Inc. Figure 39.17 How does the order of red and far-red illumination affect seed germination? (적색광과 원적외선의 조사순서가 종자발아 미치는 영향은?) RESULTS 미발아 발아 Red Dark Red Far-red Dark 미발아 발아 Dark (control) Red Far-red Red Dark Red Far-red Red Far-red (1) 적색광은 발아촉진, 원적외선은 발아억제함 (2) 최종조사한 빛의 종류가 결정인자임 (3) 적색광과 원적외선의 효과는 가역적임 • The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome Figure 39.18 : Structure of phytochrome Two identical subunits (1) 피토크롬은 두개의 동일한 단백질이 결합되어 기능을 나타냄 (2) 광수용체 활성부위는 비단백질성 색소인 색소포와 공유결합을 함 (3) 인산화 활성부위는 빛수용반응과 세포내 신호전달과정을 연결하는 기능 © 2011 Pearson Education, Inc. Chromophore (색소포: nonprotein pigment) Photoreceptor activity Kinase activity Figure 39.UN01 The chromophore of phytochrome is photoreversible Red light Pr Pfr Far-red light Phytochrome: a molecular switching mechanism • 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 (see next slide) © 2011 Pearson Education, Inc. Figure 39.19 Sunlight Pr Pfr Red light Synthesis Responses: seed germination, control of flowering, etc. Far-red light Slow conversion in darkness (some plants) Enzymatic destruction Phytochromes and Shade Avoidance • The phytochrome system also provides the plant with information about the quality of light • Leaves in the canopy(차양막) 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 나무가 직접 햇빛을 받으면 Pfr의 농도가 증가하여 수직성장은 억제되고 곁가지의 생장이 촉진되어 옆으로 퍼지게 됨 키 큰 나무에 의해 음지에 놓이게 되면 잎을 통과한 원적외선을 조사받게 되므로 Pr의 농도가 증가하여 수직성장이 촉진됨 햇빛을 많이 받게 위한 전략임 © 2011 Pearson Education, Inc. Biological Clocks and Circadian Rhythms • Many plant processes oscillate during the day • 일정 환경조건 속에서도 기공개폐, 광합성관련 효소의 합성 등 식물체 내의 많은 생리작용은 24시간의 주기에 따라 조절됨 • Many legumes(콩과식물) lower their leaves in the evening and raise them in the morning, even when kept under constant light or dark conditions (see next slide) © 2011 Pearson Education, Inc. Figure 39.20 Sleep movements of a bean plant Noon Midnight The movements are caused by reversible changes in the tugor pressure of cells on opposing sides of the pulvini(옆침), motor organs of the leaf • 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 and may be common to all eukaryotes © 2011 Pearson Education, Inc. The Effect of Light on the Biological Clock • Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues 어두운 곳: 피토크롬의 비율은 Pr이 높아짐. 즉, Pr형태로 합성되며 Pfr이 Pr보다 쉽게 분해됨 빛이 있으면 Pr은 Pfr로 재빨리 전환되어 생체시계를 다시 맞추어 줌 © 2011 Pearson Education, Inc. Photoperiodism(광주기성) 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 © 2011 Pearson Education, Inc. Photoperiodism and Control of Flowering(개화) • 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 longday plants • Flowering in day-neutral plants is controlled by plant maturity, not photoperiod © 2011 Pearson Education, Inc. – 단일식물(short-day plants): 낮의 길이가 개화에 필요한 특정길이보다 짧으면 개화하는 식물 (늦여름, 가을, 겨울에 개화.); 국화(chrysanthemums), 포인세티아스, 콩과 식물(soybean) – 장일식물(long-day plants): 광주기가 특정 시간보다 길어야 개화함 (늦봄이나 초여름에 개화. 즉, 낮이 14시간 이상일 때); 시금치, 무우, 상치, 붓꽃 (iris), 곡물류 – 중일식물(Day-neutral plants): 개화가 광주기와 상관없는 식물로서 maturity에 따라 개화; 민들레 토마토, 벼 Critical Night Length • In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length © 2011 Pearson Education, Inc. How to control flowering by photoperiodic system • 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 © 2011 Pearson Education, Inc. Figure 39.21 24 hours How does interrupting the dark peroid with a brief exposure to light affect flowering? (암기 중 빛의 조사가 개화에 미치는 영향은?) (a) Short day (long-night) plant Light 각 식물 종의 개화는 밤길이에 의해 결정됨. 따라서 short-day plant는 long-night plant로, longday plant는 short-night plant로 부르는 게 타당함 Flowers when night exceeds a critical dark period. A flash of light interrupting the dark period prevents Flash Darkness flowering of Critical dark period light (b) Long-day (short-night) plant Flowers only if the night is shorter than a critical dark period. A brief flash of light interrupts a long dark period, thereby inducing flowering Flash of light Reversible effect of red and far-red light on photoperiodic response • 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 (see next slide) © 2011 Pearson Education, Inc. Figure 39.22 24 hours Is phytochrome the pigment that measures the interruption of dark peroids in photoperiodic response? (피토크롬이 광주기 반응에서 암기의 중단을 인지하는 색소인가?) R (1) 적색광 조사는 암기를 짧게 하며, 근적외선의 연이은 조사는 적색광이 효과를 반전시킴. 그 역반응도 가능함 (2) 이런 가역성은 피토크롬이 dark period의 중단을 인식하는 색소임을 입증함 R FR RFR R R FRR FR Critical dark period Long-day Short-day (long-night) (short-night) plant plant • 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(춘화처리) is a pretreatment with cold to induce flowering © 2011 Pearson Education, Inc. 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 macromolecule governed by the FLOWERING LOCUS T (FT) gene © 2011 Pearson Education, Inc. Figure 39.23 Experimental evidence for a flowering hormone 24 hours 24 hours Long-day plant grafted to short-day plant Long-day plant 24 hours Graft Short-day plant Under short-day conditions, a short-day plant will flower and a long-day one will not. However, both flower if grafted together and exposed to short days Concept 39.4: 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 (1) Gravity (2) Mechanical Stimuli (3) Environmental stresses Drought Flooding Salt Stress Heat Stress Cold stress © 2011 Pearson Education, Inc. Gravity • Response to gravity is known as gravitropism(굴중성) • Roots show positive gravitropism; shoots show negative gravitropism • Plants may detect gravity by the settling of statoliths, dense cytoplasmic components Statoliths [stǽtəlìθ] : 평형립, specialized plastids containing dense starch grains © 2011 Pearson Education, Inc. Figure 39.24 Positive gravitropism in roots: the statolith hypothesis 뿌리가 수평으로 놓인 후 수분내로 평형립은 골무세포의 기저부로 모이게 됨 평형립 가설에 의하면 이렇게 평형립이 놓이게 되는 것이 중력감지기작이며 칼슘의 재분포를 유도하여 옥신의 측면수송을 가능게 함으로써 뿌리 아래쪽은 옥신의 농도가 높아져 세포신장이 억제되며, 따라서 뿌리 안팎의 세포신장률을 다르게 되고 뿌리가 굽어지게 되는 것임 Statoliths (a) Primary root of maize bending gravitropically (LMs) 20 µm (b) Statoliths settling to the lowest sides of root cap cells (LMs) • Some mutants that lack statoliths are still capable of gravitropism • Dense organelles, in addition to starch granules, may contribute to gravity detection © 2011 Pearson Education, Inc. Mechanical Stimuli • The term thigmomorphogenesis(접촉형태형성) refers to changes in form that result from mechanical disturbance (thigma=touch) • Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls (see next slide) © 2011 Pearson Education, Inc. Figure 39.25 Altering gene expression by touch in Arabidopsis 기계적인 자극으로 세포내 칼슘농도 변화가 유도되고 세포벽의 성질을 변화시킬 수 있는 유전자발현을 촉진하게 됨 • Thigmotropism(굴촉성) is growth in response to touch • It occurs in vines and other climbing plants • 포도나무를 포함하는 덩굴식물들은 덩굴손을 가지고 있으며 접촉이 없으면 곧게 자라지만 접촉이 있으면 덩굴손 양쪽 세포의 생장 차이가 생겨 동그랗게 말리는 현상(coiling response)이 유도됨. • 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 Ex: Rapid tugor movements by the sensitive plant (see next slide) © 2011 Pearson Education, Inc. Figure 39.26 (a) Unstimulated state (b) Stimulated state Side of pulvinus with flaccid cells Pulvinus (motor organ) (c) Cross section of a leaflet pair in the stimulated state (LM) (흐늘흐늘해진 엽침의 측면) Side of pulvinus with turgid cells (팽팽해진 엽침 측면) Vein 0.5 µm Leaflets after stimulation 미모사의 잎은 자극에 반응하여 소엽의 안쪽은 K+ 이온손실이 유도되고 이로인해 삼투압에 인한 물손실로 쭈글쭈글해지고 바깥쪽은 팽팽한 상태로 유지됨 따라서 운동기관인 엽침의 굽어지는 현상이 유발됨 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. Flooding • Enzymatic destruction of root cortex cells creates air tubes that help plants survive oxygen deprivation during flooding (홍수로 인한 산소부족을 극복하도록 예정된 세포죽음을 통해 공기통로를 생성함) © 2011 Pearson Education, Inc. Figure 39.27 A developmental response of maize roots to flooding and oxygen deprivation (홍수시의 옥수수뿌리의 발달 반응) 공기공급한 대조군 뿌리 공기공급 중단한 뿌리 Vascular cylinder Air tubes Epidermis 100 µm (a) Control root (aerated) 100 µm (b) Experimental root (nonaerated) 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 염류 농도가 높아지면 토양의 수분포텐셜이 감소하여 식물은 수분부족을 겪게 됨 유기물 용질을 합성하여 세포의 수분포텐셜을 토양의 그것보다 낮게 유지함으로써 해로운 염류의 유입을 방지함 © 2011 Pearson Education, Inc. Heat Stress • Excessive heat can denature a plant’s enzymes • Heat-shock proteins help protect other proteins from heat stress © 2011 Pearson Education, Inc. Cold Stress • Cold temperatures decrease membrane fluidity • Altering lipid composition of membranes is a response to cold stress – 불포화지방산의 비중을 높여 저온에서 지질이 고형화되는 것을 방지 막의 유동성 유지 – 동결스트레스를 견디기 위해 겨울이 오기 전에 당분과 같은 특정 용질의 농도를 증가시켜 세포바깥이 얼더라도 수분손실을 최소화함 • Freezing causes ice to form in a plant’s cell walls and intercellular spaces • Many plants, as well as other organisms, have antifreeze proteins that prevent ice crystals from growing and damaging cells © 2011 Pearson Education, Inc. Concept 39.5: Plants respond to attacks by herbivores and pathogens (Biotic stresses) • Plants use defense systems to deter herbivory, prevent infection, and combat pathogens © 2011 Pearson Education, Inc. 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 • 특정 식물에 의한 Canavanine 생성 (아르기닌 유사아미노산) 곤충섭취시 canavanine은 곤충의 단백질 속으로 들어가게 되고 단백질 구조 또는 기능 이상을 유도하여 곤충을 죽게 함 • Some plants even “recruit” predatory animals that help defend against specific herbivores (see next slide) © 2011 Pearson Education, Inc. Figure 39.28 A maize leaf “recruiting” a parasitoid wasp as a defensive response to an armyworm caterpillar, an herbivore (초식동물의 포식자 유인용 휘발성물질 방출) 4 Recruitment of parasitoid wasps that lay their eggs within caterpillars 1 Wounding 1 Chemical in saliva 2 Signal transduction pathway 3 Synthesis and release of volatile attractants 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 inherited ability to recognize certain pathogens © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. • Gene-for-gene recognition involves recognition of elicitor 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 © 2011 Pearson Education, Inc. The Hypersensitive Response • The hypersensitive response – Causes cell and tissue death near the infection site – Induces production of phytoalexins and PR (pathogenesis-related proteins) proteins, which attack the pathogen – Stimulates changes in the cell wall that confine the pathogen © 2011 Pearson Education, Inc. Defense responses against an avirulent pathogen (fig. 39.29) (1) Specific resistance is based on the binding of pathogen effector molecules to specific plant resistance R proteins (2) The identification in step 1 triggers a signal transduction pathway (3) In a hypersensitive response, plant cells produce antimicrobial 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 (4) Before they die, infected cells release the signaling molecule methylsalicylic acid (5) The signaling molecule is distributed to the rest of the plant (6) In cells remote from the infection site, methylsalicylic acid is converted to salicylic acid, which initiates a signal transduction pathway (7)Systemic acquired resistance is activated: the production of molecules that help protect the cell against a diversity of pathogens for several days Figure 39.29 Infected tobacco leaf with lesions 4 3 Signal 5 Hypersensitive response Signal transduction pathway 6 2 Signal transduction pathway 7 Acquired resistance 1 R protein Avirulent pathogen Avr effector protein R-Avr recognition and hypersensitive response Systemic acquired resistance Systemic Acquired Resistance • 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 © 2011 Pearson Education, Inc.