Control Systems in Plants Ch. 39 A grass seedling growing towards light

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Control Systems in PlantsCh. 39A grass
seedling growing towards light
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Control Systems

• Keep track of the time of day
• Notice seasons
• Sense gravity
• Sense direction of sunlight

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Plant Hormones

• Hormone
• compound produced by one part of an organism that
  is transported and triggers a response in another
  part

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Plant Hormones

• Auxins
• main site of production is the apical meristem
• increases plasticity of cell wall
• promotes elongation of shoots
• promotes formation of adventitious roots fruit
  growth
• used as herbicides
• 2,4-D
• IAA

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Figure 39.8 Apical dominance with apical bud
(left), apical bud removed (right)
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Plant Hormones

• Cytokinins
• anti-aging
• stimulates cell division
• activates proteins for mitosis
• stimulates axillary bud growth
• Cytokinins and Auxins
• together control cell division and
  differentiation

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Plant Hormones

• Gibberellins
• Primarily produced in roots and young leaves
• imbibition triggers its release
• promotes internode elongation (stem elongation)
• promotes seed germination (breaks dormancy)
• places a role in fruit growth

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Figure 39.11 The effect of gibberellin treatment
on seedless grapes
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Plant Hormones

• Abscisic acid
• produced in terminal bud
• prepares plant for winter
• closes stomata
• inhibits growth

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Plant Hormones

• Ethylene
• gas
• ripens fruit
• leaf abscission
• Stimulus is shortening days/longer nights and
  cooler temps. (to a degree)

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Plant Hormones

• Oligosaccharins
• plant defense against pathogens
• cell growth and differentiation
• flower development

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Tropisms

• Growth responses that result in curvatures of the
  whole plant toward/away from a stimulus
• Phototropism
• stimulus is light
• Gravitropism
• stimulus is gravity
• Positive downward (roots)
• Negative upward (shoots)

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Tropisms
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Tropisms

• Thigmotropism
• stimulus is touch

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Turgor Movements
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Turgor Movements

• Reversible movements caused by changes in turgor
  pressure
• Rapid Leaf movements
• reduce water loss or protect from herbivores
• touch causes leaf to collapse (causes a rapid
  loss of turgor pressure by cells causing them to
  become flaccid)
• motor cells lose K
• 10 minute restoration
• Mimosa clip

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Figure 39.27 Rapid turgor movements by the
sensitive plant (Mimosa pudica)
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Turgor Movements

• Sleep movements
• lowering of leaves to vertical position in
  evening
• raising leaves to a horizontal position in the
  morning
• one side of plant is turgid while the other is
  flaccid
• Daily changes in turgor pressure

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Figure 39.21 Sleep movements of a bean plant
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Control of Daily and Seasonal Responses

• Circadian rhythms
• does not require environmental cues
• light-dark cycle
• Entrained from environment internally set

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Figure 39.x1 Biological clocks
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Control of Daily and Seasonal Responses

• Photoperiodism
• Detect time of year by the photoperiod
• physiological response to night length
• Effects seasonal events such as flowering and
  seed germination

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Photoperiodism

• Critical night length
• day length does not trigger flowering
• If daytime is interrupted, no effect on flowering
• If night period is interrupted by a short period
  of light, plants do not flower
• Could be effected by a single exposure or may
  require several exposures

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Photoperiodism

• Short day plants
• night is longer than a critical length
• Flower in late summer, fall
• Long day plants
• night is shorter than a critical length
• Flower in late spring, summer
• Neutral day plants
• unaffected by photoperiod
• triggered by maturity

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Phytochromes

• Protein containing a chromophore responsible for
  a plants response to the photoperiod
• Alternate between 2 forms (Pr and Pfr)
• Pr only converts to Pfr in presence of light
• Pfr triggers many plant responses
• Pfr degrades back to Pr at night

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Figure 39.20 Phytochrome a molecular switching
mechanism
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Phytochromes

• Photoreceptors
• uses phytochrome to tell if light is present
• Red light most effective in interrupting night
  length
• Entrain the biological clock
• night length measured
• extremely accurate

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Figure 39.22 Photoperiodic control of flowering
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Figure 39.22 Photoperiodic control of flowering
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Figure 39.23 Reversible effects of red and
far-red light on photoperiodic response
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Figure 39.24 Experimental evidence for a
flowering hormone(s)
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Environmental Stress

• An environmental condition that can have an
  adverse effect on a plants growth, reproduction,
  survival
• Water deficit, oxygen deprivation, salt stress,
  heat, cold, herbivores

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Water Deficit

• Control systems in both leaves and roots
• Guard cells lose turgor and close
• Mesophyll releases abscisic acid
• Young leaf growth is inhibited
• Wilting reduces surface area
• Shallow root growth inhibited deeper roots
  continue to grow

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Oxygen Deprivation

• Waterlogged soil lacks air spaces to hold oxygen
• May form air tubes from roots to the surface
• Submerged roots may be continuous with aerial
  roots

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Salt

• Lowers water potential of soil causing a water
  deficit even if enough water is present
• Produce compatible solutes in response to
  moderately saline soils
• Keeps water potential of cells more negative than
  the soil solution w/o admitting toxic quantities
  of salt

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Heat

• Transpiration reduces temperature and keeps
  enzymes from denaturing
• Produce shock proteins
• a back up plan to transpiration

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Cold

• Subfreezing temps cause ice crystals to form in
  protoplast
• death
• Lipids become locked and causes a loss of
  fluidity in membranes
• Alter lipid composition by increasing saturated
  fatty acids

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Herbivores

• Chemical
• Distasteful
• Toxic
• Recruit predatory animals
• Plants attract wasps that lay eggs in caterpillars

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Figure 39.29 A corn leaf recruits a parasitoid
wasp as a defensive response to an herbivore, an
army-worm caterpillar
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Defense Against Pathogens

• Gene-for-gene recognition
• Resistance to a disease depends on a precise
  match up between an allele in a plant and an
  allele in the pathogen

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Defense Against Pathogens

• Hypersensitive response
• chemical signaling system to resist infection
• phytoalexins
• Systemic acquired resistance
• protects unaffected tissues from a pathogen
  spreading

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Figure 39.31 Defense responses against an
avirulent pathogen
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• A good candidate for one of the hormones
  responsible for activating SAR is salicylic acid.
• A modified form of this compound, acetylsalicylic
  acid, is the active ingredient in aspirin.
• Centuries before aspirin was sold as a pain
  reliever, some cultures had learned that chewing
  the bark of a willow tree (Salix) would lessen
  the pain of a toothache or headache.
• In plants, salicylic acid appears to also have
  medicinal value, but only through the stimulation
  of the systemic acquired resistance system.

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Extreme Plant AdaptationsA conclusion to the unit