Plant Responses to Signals

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Plant Responses to Signals

• Chapter 39

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• Plants have to respond to gravity and other
  stimuli in environment.
• Growth pattern in plants - reaction to light.

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• 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.

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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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http//www.biologie.uni-hamburg.de/b-online/librar
y/cat-removed/tropism.gif
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• 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.

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http//botit.botany.wisc.edu/images/130/Growth_Sub
stances/Auxins/root_formation/
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• Cytokinins stimulate cytokinesis (cell division)
• Cytokinins produced in actively growing tissues,
  particularly roots, embryos, and fruits.
• Both cytokinins and auxins present, cells divide.

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Shoots forming with addition of cytokinins
http//trilliumresearch.org/images/htr_web_images_
research/05_rp_03_30_md.jpg
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• 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.

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• 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.

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http//www.science.org.au/sats2004/images/helliwel
l2.jpg
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• 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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http//www.eco-systems.org/images/Premature_sugar_
maple_leaf_drop_along_town_road_in_August_2000_.jp
g
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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.

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http//www.pakupaku.info/knowledge/images/ethylene
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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/spec
trum.gif
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• 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.

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http//www.berrypropco.co.nz/variety_pics/par.gif
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http//plantfacts.ohio-state.edu/hcs300/devel2.htm
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• Typically, red light used to interrupt nighttime
  cycle.

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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).

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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.

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http//www.learnnc.org/lp/media/collections/cede/r
esized/cedebwr07.jpg
Marsh grasses are often tolerate of extreme
salinities
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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/mona
rch/PlantDefense01.jpg
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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.

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http//138.23.152.128/images/leaf.jpg