Phyllotaxis: biological mechanisms

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Phyllotaxis biological mechanisms
Seth Donoughe
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Phyllotaxis

• A few simple assumptions about leaf growth --gt
• Mathematical models --gt
• Patterns observed in plants

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First available space

• Mitchison (1977)
• Based on contact-circles and an expanding apex.
• Diffusion of inhibitor
• Transport of inhibitor
• Depletion or competition for a compound

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Largest available place

• Douady and Couder (1992)
• Maximize light gathering
• Never overlap

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• First available place or largest available
  space?
• Diffusion of an inhibitor? Active transport? A
  more complicated mechanism?

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Plant hormones

• There are several major classes
• Influence virtually all aspects of plant life
  cycle
• One important class
• Auxins

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Auxin

• Creates top - bottom (apical - basal) polarity
• Mediates phototropism
• Induces vascular tissue growth
• Produced in the shoot apical meristem and is
  transported downward
• Influx and efflux carriers accomplish this.

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Auxin controls the production of leaf primordia
at the shoot apical meristem, and is therefore
likely involved in phyllotaxis
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PIN1 is an auxin efflux carrier
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PIN1 is an auxin efflux carrier
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A series of proteins is needed to direct
successful leaf growth
Involving at least an auxin-dependent
transcription factor, a negative regulator of
auxin response, and the proteins for auxin
transport.
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PIN1 and a uniform distribution of auxin combine
to create the phyllotactic pattern

• Auxin begins primordium formation
• PIN1 preferentially directs auxin towards the
  growing leaf.
• The leaf becomes an auxin-sink
• As a result there is a minimum distance to the
  next primordium
• Positive feedback and lateral inhibition

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Contact-circles supported
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Photo credit and references

• Reinhart, et al. (2003). Regulation of
  phyllotaxis by polar auxin transport.
• Douady and Couder. (1991)
• Mitchison. (1977)
• http//www-bioc.rice.edu/bartel/projects/iaa28for
  web.gif
• http//upload.wikimedia.org/wikipedia/commons/2/2c
  /Auxin.jpg
• http//images.the-scientist.com/content/images/art
  icles/53126/69-1.jpg
• http//www.uic.edu/classes/bios/bios100/labs/meris
  tem.jpg