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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.
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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.
Signal transduction pathway promotes cell activity in plant.
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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.
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Growth of shoot towards light -phototropism (positive). • Hormone responsible for growth -auxin.
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/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.
Shoots forming with addition of cytokinins http://trilliumresearch.org/images/htr_web_images_research/05_rp_03_30_md.jpg
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.
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.
Many plants - both auxin and gibberellins must be present for fruit to set. • Seeds have large amount of gibberellins - signals seed to break dormancy.
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.
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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.
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.
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.
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).
Some plants cannot grow in extreme temperatures or salinities; others thrive in them. • Freezing of cytoplasm can kill plant because excess ions can accumulate.
http://www.learnnc.org/lp/media/collections/cede/resized/cedebwr07.jpghttp://www.learnnc.org/lp/media/collections/cede/resized/cedebwr07.jpg Marsh grasses are often tolerate of extreme 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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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.
