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Chapter 26 Control of Plant Growth Responses in Flowering Plants Read Ch 26 in Textbook Read pg 210-214 in Cliffs AP Book Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses in Flowering Plants Overview • Topics – 26.1 Plant Hormones – 26.2 Plant Responses Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses 26.1 Plant Hormones • Mechanism of action – hormones – Receptor – Transduction pathway – Cellular response Mills AP Bio 2003/2013 Second Messenger Animation Mills AP Bio 2003/2013 • Groups of Plant Hormones – – – – Auxins Gibberellins Cytokinins Inhibitory hormones – Hormones work together to cause various effects Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses 26.1 Plant Hormones • Auxins – Most common auxin is indoleacetic acid(IAA) – Prevent growth of axillary buds supports apical dominance – Promotes adventitious root growth (roots from stem) – Promotes root branching – Auxins produced by seeds promote growth of fruit. If auxins high in fruits and low in stems and leaves, fruit won’t fall off. – Promote phototropism and gravitropism Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses 26.1 Plant Hormones • How Auxins work –Auxin binds to receptors on cell membrane –Activates ATP driven proton (H+) pump –Pumps H ions into space between cell membrane and cell wall. –H ions (acid) weaken cell wall. –Gradient set up that pulls solutes then water into cell causing it to swell –Pushes on cell wall –Cell elongates Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses 26.1 Plant Hormones • Gibberellins – About 70 different gibberellins, GA3 the most common – Promote stem elongation especially in dwarf plants – Can break dormancy in seeds and buds – Sources: young leaves, roots, embryos, seeds, fruits Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses 26.1 Plant Hormones • How Gibberellins work – Gibberellin binds to a receptor on the cell membrane – Activates a second messenger inside the cell – Ca++ – Ca++ combines with a protein called calmodulin – This complex activates the gene that codes for amylase – Amylase acts on starch to release sugars Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses 26.1 Plant Hormones • Cytokinins(remember cytokinesis?) – Derivatives of adenine In tissue culture: • Zeatin Auxin and cytokonins in • –Kinetin usual amounts tobacco strip develops into undifferentiated callous. If ratio of auxin to cytokinins is appropriate, the callous produces shoots. A different ratio – Promote cell division and differentiation promotes vegetative shoots and leaves. – Prevent senescence (aging process) A different ratio promotes floral shoots. • In senescence, large molecules in the plant are broken down • Can lead to leaf loss etc • Cytokinins inhibit leaf death and can promote leaf growth – Varying ratios of cytokinins and auxins can effect how plant differentiates Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses 26.1 Plant Hormones • Inhibitory Hormones – Abscisic acid (stress hormone) • Promotes bud and seed dormancy • Closes stomates(when water supply low) by causing K+ to leave guard cells • Inhibits gibberellins (which break dormancy) • Produced by any “green tissue” with chloroplasts, by endosperm and by roots. • Abscission=dropping of leaves, fruit and flowers from plants – Originally thought that abscisic acid caused this – now believe it is ethylene – Ethylene • Induces leaf, flower and fruit abscission (falling off) • Promotes fruit ripening-increases activity of enzymes that soften fruit (ex. cellulase) • Gas released from fruit (wound?) Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses 26.1 Plant Hormones Functions of Plant Hormones Hormone Function – all of these hormones work together Auxins Stimulate growth Involved in stem and root cell elongation in phototropism and gravitropism Gibberellins Stimulate growth especially stems, especially in dwarf plants. Can break seed and bud dormancy Cytokines Stimulate growth by causing cell division Abscisic Acid Inhibit growth Causes stomates to close Maintains seed and bud dormancy Ethylene Inhibit growth Causes fruit to ripen and fall Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses 26.2 Plant Responses • 38.1 Plant responses – Tropisms – Nastic movements • Tropisms – Plant growth toward (positive) or away (negative) from a stimulus – Receptiontransductionresponse – Common tropisms • Phototropism • Gravitropism • Thigmotropism Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses 26.2 Plant Responses • Phototropism – Cells on dark side of plant grow faster, so plant bends toward the light – positive phototropism – Pigment related to riboflavin acts as a receptor which somehow causes the release of the hormone auxin, which migrates to the shady side, causing increased growth. – Auxin is produced in the Phototropism short clip: shoot tips ..\..\Biology\Biology Clipart Movies Animations Sounds\Biology movies\phototrop1.2[1].mov – Roots are either insensitive to light or show negative phototropism Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses 26.2 Plant Responses • Gravitropism – Roots show positive gravitropism • If root cap removed, root doesn’t respond to gravity • Statoliths in cap – Stems show negative gravitropism – Auxin moves to lower side of stem or root in response to gravity (may be newer theories) Section of pea root, showing • Auxin inhibits growth of root statoliths settled at bottom of cells cells on lower side root grows down • Stimulates growth of stem Gravitropism(negative) in stems movie. cells on lower side stem Time lapse/actual time 1 hour: ..\..\Biology\Biology Clipart Movies Animations grows up Mills AP Bio 2003/2013 Sounds\Biology movies\gravitropism.mov Chapter 26 Control of Plant Growth Responses 26.2 Plant Responses • Thigmotropism – Unequal growth due to contact with a solid object – Cells opposite the side being touched elongate – Hormones auxin and ethylene may be involved. Thigmotropism movie – Pictures taken at 10 min intervals– morning glory vine grows: ..\..\Biology\Biology Clipart Movies Animations movies\Thigmotropism vine Mills AP BioSounds\Biology 2003/2013 winding.mov Chapter 26 Control of Plant Growth Responses 26.2 Plant Responses • Nastic Movements – Independent of direction of stimulus – Types • Seismonastic movements – Respond to touch, shaking or thermal » Ex: Mimosa – sleeping grass, Venus flytrap • Sleep movements – Occur daily in response to light and dark changes » Ex: Prayer plant – folds Mimosa plant after being touched up at night – Rhythmic sleep behavior = ..\..\Biology\Biology Clipart Movies Circadian rhythms Animations Sounds\Biology movies\nastic sensitive_plant_open[2].avi – Circadian rhythms can persist even without the stimulus d/t biological clocks Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses 26.2 Plant Responses • Greenhouse-grown plants were placed in continuous light. The diurnal opening/closing of the leaflets (nyctinasty) is clearly observed. – Nastic movement over time movie – Also (nyctnasty in silk plant) http://employees.csbsju.edu/ssaupe/images/Movies/silk_tree.avi – From http://employees.csbsju.edu/ssaupe/biol327/Lab/movie/movies.htm Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses 26.2 Plant Responses • Nastic Movements-how they work – Stimulus causes K ions to move out of a specialized area of cells (pulvinus) at base of leaf. – Water follows by osmosis decreased turgor Trigger Hairs pressure. – Stimulus on one leaf can cause other leaves to close, so must have some system of communication. Three sensitive trigger hairs on each leaflet Venus Flytrap movies: ..\..\Biology\Biology Clipart Movies Animations Sounds\Biology movies\venus flytrap movie.mov ..\..\Biology\Biology Clipart Movies Animations Sounds\Biology movies\Venus Fly Trap.wmv ..\..\Biology\Biology Clipart Movies Animations Mills AP Bio 2003/2013 Sounds\Biology movies\Venus flytrap eating a spider.wmv Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses 26.2 Plant Responses • Photoperiodism – Physiological responses brought about by changes in the length of day or night. • Three plant groups – Short-day plants • Flower when the day length is shorter than a critical length. – Long-day plants • Flower when the day length is longer than a critical length. – Day-neutral plants • Flowering not dependent on day length. – Plant flowering actually depends on a critical length of darkness. – Short days correspond with long nights. Kolonchoe – short day plantMills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses If light at least critical length or shorter flowers More accurate: If light at least critical length or longer flowers 26.2 Plant Responses More accurate: If dark at least critical length or longer (regardless of length of light) and continuous flowers If dark at least critical length or shorter (regardless of length of light) flowers flash of light during dark will inhibit flowering If critical darkness longer than critical length, but interrupted by a flash of light, flowering will occur. Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses 26.2 Plant Responses • Phytochromes – Theory: pigment molecules that detect photoperiods, and directs flowering of plants – Blue green leaf pigments that exist in two forms: • Pr – phytochrome red (inactive form) – absorbs red light 660mm wavelength – gets converted to Pfr • Pfr – phytochrome far red (active form) – absorbs red light 730nm – gets converted to Pr – Sunlight has more red than far red wavelengths, shade and sunset have more far red wavelengths • So in sunlight, Pr absorbs red light and gets converted to Pfr, which is the active form and induces flowering etc. – Phytochrome conversion may be the first step in reception transduction reaction pathway? Mills AP Bio 2003/2013 Phytochrome Animation Mills AP Bio 2003/2013 Chapter 26 Control of Plant Growth Responses 26.2 Plant Responses • Phytochromes – other functions – Pr Pfr (in sunlight) • Promotes seed germination, leaf expansion, stem branching, and inhibits stem elongation – Presence of Pfr tells some seeds that conditions are right for germination • Some seeds need dark to germinate, some do better with some light – In presense of more far red (shade), Pfr increases, leading to shade avoidance strategies – stem elongation, flowering to produce seeds Mills AP Bio 2003/2013 The End Mills AP Bio 2003/2013