BIOSCI 152 1st Edition Lecture 20Plant Sensory Systems and Movement Chapter 36Learning Objectives. After suitable revision, you should be able to-• Identify key environmental factors to which plants respond • Define – phototropism, gravitropism, thigmotropism • Know the light wavelengths active for phytochrome • Know how turgor pressure is changed in the sensitive plant and venusfly trap • Give an example of a circadian rhythm in plants • Define the term photoperiodism • Explain how long day (short night) and short day (long night) plants controlflowering • Recognize roles of auxins, ethylene and GAA in plants VocabularyEnvironmental Positive/negative Long day plantsstimulus/stimuli gravitropism Short day plantsTropism Amyloplast Day neutral plantsPhototropism Statolith HormoneGravitropism Pressure receptors AuxinThigmotropism Turgor pressure Cell elongationGrowth responses Turgor movement IAAStem elongation Membrane potential GibberellinPhytochrome Charge separation GARed light Proton pump CytokininFar red light Membrane EthyleneGermination polarization/depolariz Abscisic acidPfr ation RipeningPr Circadian rhythms Cell divisionConformation change Clock genes Seed dormancyPhotomorphogenesis PhotoperiodismStomatal regulationGreening Physiological Guard cellsChloroplast response CellDay lengthHow do plants sense environmental cues and know how to respond?Tropisms• Growth responses resulting in curvature or growth towards or away from astimulus • Caused by differential elongation of cells on each side of the stem, leaf, root etc. – Phototropism – Gravitropism – Thigmotropism - Response to mechanical stimuli Light is a major factor influencing plant movement, growth and development Phototropism• Cells on darker side of shoot elongate more than cells of lighter side • Illuminating one side causes auxin to migrate across the tip to theun-illuminated side • Sensing light – specific to light spectrum • How do plants ‘see’ the light?http://www.youtube.com/watch?v=Ze8NV7cvW8k http://www.youtube.com/watch?v=zctM_TWg5Ik&NR=1&feature=fvwpSearch for plant hormones(and the scientific method)• Oats – monocots with coleoptile • Coleoptile grows straight in light from above • Grows towards a directional light source Charles Darwin (1809 -1882)• Geologist, biologist • Voyage of the Beagle 1830’s (in his early 20s!) • 1859 – The Origin of Species - modern ideas about evolutionary principles • Also – earthworms, insectivorous plants, plant reproduction and movement inplants… Charles & Francis Darwin’s experiments in the 1880’sWhat part of coleoptile is responsible for phototropism?• A number of other scientists contributed ideas, then Went’s ExperimentsConcluded – • The chemical is produced by the coleoptile tip • The chemical stimulates growth as it passes through the coleoptile • The coleoptile curves to the light because of higher concentrations (and moregrowth) on the shaded sideGravitropism• Roots display positive gravitropism (grow down) • Shoots display negative gravitropism (grow up) Responses to mechanical stimuli• Thigmotropism • Thigmorphogenesis • Turgor movements ThigmotropismGk. Thigmo - touch• Trophic response to touch • Vines – wind around objects • Touch causes cells on opposite side of stem to elongate and shoot bendstowards support object Thigmorphogenesis – thig – touch, morpho – form, genesis – origin/developmentTurgor movementsMimosa pudicaSensitive plant responds to touch by folding/dropping leaf Turgor movementshttp://www.youtube.com/watch?v=g0LFBM3hOLs&feature=related• Changes in turgor pressure in specialized cells allows plants to orient (ormove) their leaves • Loss of turgor pressure in the leaf pulvini results in wilting turgor movements are stimulated by rapid ion fluxes across membranesMembrane action potential (like nerves but slower)– Uses energy (ATP) – Involves membrane ion channels – Movement of ions across plasmamembrane causes charge separation -voltage – Ion fluxes cause water to enter or leave a cell – 2 – 5 seconds in plants (<millisec in animals)Venus fly-traps use action potentials to stimulate rapid turgor-inducedmovement• Mechanical stimulation on receptor hairs• Electrical signal propagated – action potentials due to ion movement• Effector cells on outside of trap swell (increased turgor pressure) forcing trapshut Photoactive compound in plants which senses light and darkPhytochrome discovered in 1959Responses of plants to red and ‘far-red’ light (shade)How phytochrome influences seed germination (Fig. 26.11)Red light promotes germination Far-red light inhibits germination Phytochrome undergoes conformation change Fig. 26.12Can plants tell time? circadian ‘endogenous’ rhythm• Plants can ‘tell the time’ independent of light - internal ‘clock’ proteins‘The Biological Clock’• Physiological cycle ~24h: first evidence was from plants • Bean plant will continue sleep movements even in continuous light or dark • Without outside stimuli, rhythm weakens • ‘Clock’ genes and proteins investigated PhotoperiodismPhotoperiodism• Physiological responses to day length – seed germination, flowering, leafabscission • Short-day plants – require long night period (Chrysanthemums, poinsettias –flower in fall, winter) • Long day plants – require short dark period (spinach, radish, iris, grasses –late spring or summer flowering) • Day neutral plants – unaffected by day length, depends on plant maturity(tomatoes, peas, rice, dandelions)How do plants know when it is spring???http://rspb.royalsocietypublishing.org/content/early/2010/04/01/rspb.2010.0291 https://www.usanpn.org/data/springCritical night length controls flowering Fig. 27.7, 27.8Plant measures length of darkness in each 24 h (using phytochrome)In class exercise on photoperiodism – predicting flowering from photoperiodPlant Hormones• Produced in one part of the plant, transported to other parts where it triggersresponses in target cells and tissues • Very small concentrations needed for an effect – gene expression – enzyme activity – membrane properties However, higher concentration may have opposite effectsAuxin• e.g. Indoleacetic Acid – natural hormone found in plants (one of several auxins)• Stimulates cell elongation• Produced in apical