Plant Responses to Signals

Chapter 39

• Plants have to respond to gravity and other stimuli in environment.

• Growth pattern in plants - reaction to light.

• Shoot reaches sunlight - starts process called greening.

• Shoots start to grow - entire plant begins to make chlorophyll.

• Begins signal transduction pathway like one seen in animal cells.

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• Signal transduction pathway promotes cell activity in plant.

http://www.ercim.org/publication/Ercim_News/enw44/sillion.jpg

Hormones

• Plants produce hormones that regulate growth and development.

• Hormones - chemical signals produced in one part of body, transported to other parts.

• Growth towards or away from stimuli (regulated by hormones) - tropism.

http://www.biologie.uni-hamburg.de/b-online/library/cat-removed/tropism.gif

• Growth of shoot towards light -phototropism (positive).

• Hormone responsible for growth -auxin.

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• Auxin produced in large quantities in apical meristem - growth occurs.

• Auxin used on cut stems to promote root growth.

• Auxins used as growth inhibitor for some plants - used as pesticides.

http://botit.botany.wisc.edu/images/130/Growth_Substances/Auxins/root_formation/

• Cytokinins stimulate cytokinesis (cell division)

• Cytokinins produced in actively growing tissues, particularly roots, embryos, and fruits.

• Both cytokinins and auxins present, cells divide.

http://trilliumresearch.org/images/htr_web_images_research/05_rp_03_30_md.jpg

Shoots forming with addition of cytokinins

• Cytokinin levels raised, shoot buds form.

• Auxin levels raised, roots form. • Cytokinins also slow down

aging process of some plant organs - florists use sprays to keep flowers fresh.

http://www.gbpetalpusher.com/flowers/flower5-big.jpg

• Gibberellins stimulate growth in leaves and stems - little effect on root growth.

• Stems, gibberellins stimulate cell elongation and cell division.

• Gibberellins applied to dwarf plants - grow to normal height.

• Applied to normal plants - nothing happens.

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• Many plants - both auxin and gibberellins must be present for fruit to set.

• Seeds have large amount of gibberellins - signals seed to break dormancy.

http://www.science.org.au/sats2004/images/helliwell2.jpg

• Abscisic acid promotes plant to become dormant; thought to help leaves drop in fall.

• Sometimes seed will need to have all abscisic acid removed (through washing) to break dormancy.

• Also helps to withstand drought - sends plant into dormancy until the conditions are favorable again.

http://www.eco-systems.org/images/Premature_sugar_maple_leaf_drop_along_town_road_in_August_2000_.jpg

• Ethylene promotes leaf dropping as well as fruit ripening.

• If fruit producing ethylene placed with fruits that are not, those fruits will also ripen in response to hormone.

• By losing leaves during fall, plants prevent drying out during winter.

http://www.pakupaku.info/knowledge/images/ethylene.gif

Responses to light

• Plants require light to grow; can absorb various aspects of spectrum of light.

• Respond differently to different wavelengths of light.

• 2 different types of plants, short day and long day.

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http://oceancolor.gsfc.nasa.gov/SeaWiFS/ICONS/spectrum.gif

• Short-day plants - long-night plants -require minimum length of uninterrupted darkness.

• Long-day plans - short-night plants - require period of continuous darkness interrupted by few minutes of light.

• Response to light - photoperiodism.

http://www.berrypropco.co.nz/variety_pics/par.gif

http://plantfacts.ohio-state.edu/hcs300/devel2.htm

• Typically, red light used to interrupt nighttime cycle.

Tropisms

• Roots - positive gravitropism (grow in direction of gravity); shoots - negative gravitropism (grow against direction of gravity).

• Thigmotropism - response to touch; in some plants, causes plant to coil around an object (like tendril). QuickTime™ and a

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• Some plants cannot grow in extreme temperatures or salinities; others thrive in them.

• Freezing of cytoplasm can kill plant because excess ions can accumulate. QuickTime™ and a decompressorare needed to see this picture.

http://www.learnnc.org/lp/media/collections/cede/resized/cedebwr07.jpg

Marsh grasses are often tolerate ofextreme salinities

Plant defenses• Plants susceptible to many

different bacteria and viral infections because of place in food chain.

• Eaten by herbivores - need protection against excess herbivory – use physical defenses, such as thorns, and chemical defenses, such as production of toxic compounds.

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http://www.learner.org/jnorth/images/graphics/monarch/PlantDefense01.jpg

• Some plants able to secrete compounds that kill insect eating it.

• Most plants resistant to pathogens automatically because they are able to detect infection and kill it off right away.

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http://138.23.152.128/images/leaf.jpg