The Effects of Plant Hormones

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The Effects of Plant Hormones

• Chapter 33

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Chapter Overview

• Overview of Plant Hormones
• Discovery of Auxin
• Details of the Five Major Classes of Plant
  Hormones
• Gravitropism
• Phototropism

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What is a Hormone?
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Table 33-1
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Charles and Francis Darwin
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Fritts Went
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Auxin

• Synthesis- highest concentration in shoot tips
• Movement- actively transported by carrier protein
  from cell to cell
• Causes Changes in Gene Expression

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Key Effects of Auxin

• Promotes apical dominance
• Involved in Phototropism and Gravitropism
• Synthetic Auxins (2,4 D) act as potent herbicides
• Seedless Fruits
• tomatoes
• cucumbers
• Cell Culture- promotes roots tissue
  differentiation

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Gibberellins

• Elongation of stems is one major effect
• High levels produced in actively growing areas,
  including tips of roots and shoots, and
  developing leaves and flowers
• Movement?
• Action more confined to stems
• Involved in biochemical changes triggered when
  seed dormancy is broken

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Cytokinins

• extremely low in abundance (1mg in 60 kg corn)
• produced at highest levels in roots
• transported in xylem
• promotes cell division
• cytokinin/auxin in cell culture
• promotes lateral buds
• activity modulated by Ca2 ions

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Agrobacterium tumafaciens

• Contains a Ti (Tumor-Inducing) Plasmid
• Genes on Ti plasmid direct synthesis of auxin and
  cytokinin
• Disarmed plasmids used in biotechnology

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History of Ethylene

• China
• Puerto Rico
• Leakage in Gas Lights

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Ethylene

• Synthesized throughout the plant
• Fruit Ripening
• Self-Regulatory Molecule
• Promotes Abscission of leaves and fruits
• Leads to triple response if applied to seedlings
  (thickening, shortening, and hooking of
  hypocotyl)

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Abscisic Acid

• inhibitory hormone
• antagonistic to gibberellins and auxin
• role in stomatal opening and closing

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Other Plant Hormones

• Salicylic Acid (Role in SAR)
• Jasmonic Acid
• Systemin
• Brassinosteroids (Acts similar to IAA)

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Gravitropism

• Shoots tend to grow up and roots tend to grow
  down
• What portion of a root is required for
  gravitropism?
• How do plants sense which way is up?
• We really dont know

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Gravitropism

• What happens when gravity is sensed?
• Calcium ions re-distributed
• IAA redistributed with highest levels on cells on
  bottom of horizontal root and shoot
• IAA leads to cell elongation in shoots
• IAA inhibits cell elongation in roots
• How are Ca2 ions redistributed? How is Auxin
  re-distributed?

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Phototropism

• Shoots tend to bend toward light
• What type of light activates phototropism?
• 4 Genes involved in blue light perception/signal
  transduction are found in Arabidopsis thaliana
• NPH1 (non-phototropic hypocotyl)
• plasmsa membrane protein with kinase activity
• has the carotenoid zeaxanthin as chromophore

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Photoperiod and Flowering

• Some species flower depending only on the
  maturity or size of plant (cucumbers, peas, and
  tomatoes)
• Some species always flower around the same time,
  regardless of size (soybean)
• Importance?

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Photoperiod and Flowering

• Short-day plants
• Long-day plants
• Is it the length of the light period or the dark
  that is important?

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Which Wavelength of Light is Effective at
Interrupting the Night?

• Red light (660nm)
• Far Red (730nm) reversed the effects of red
  light
• Mechanism?

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Phytochrome

• Photoreceptor absorbs both red and far-red light
• Pr absorbs red light and converts to Pfr, which
  converts back to Pr if absorbing far-red light
  (40 Pr and 60 Pfr)
• Only the Pfr is biologically active
• At night, Pfr slowly converts back to Pr
• Pfr inhibits flowering in short-day plants, but
  promotes it in long-day plants