Phyllotaxis : biological mechanisms
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Phyllotaxis: biological
mechanisms
Seth Donoughe
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Phyllotaxis
A few simple assumptions about leaf growth -->
Mathematical models -->
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/iaa28forweb.gif• http://upload.wikimedia.org/wikipedia/commons/2/2c/Auxin.jpg• http://images.the-scientist.com/content/images/articles/53126/69
-1.jpg• http://www.uic.edu/classes/bios/bios100/labs/meristem.jpg