Phyllotaxis: biological mechanisms Seth Donoughe

Phyllotaxis A few simple assumptions about leaf growth --> Mathematical models --> Patterns observed in plants

“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

“Largest available place” Douady and Couder (1992) –Maximize light gathering –Never overlap

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

Plant hormones There are several major classes –Influence virtually all aspects of plant life cycle One important class: –Auxins

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.

Auxin controls the production of leaf primordia at the shoot apical meristem, and is therefore likely involved in phyllotaxis

PIN1 is an auxin efflux carrier

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.

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

Contact-circles supported

Photo credit and references Reinhart, et al. (2003). Regulation of phyllotaxis by polar auxin transport.Reinhart, Douady and Couder. (1991) Mitchison. (1977) scientist.com/content/images/articles/53126/69-1.jpghttp://images.the- scientist.com/content/images/articles/53126/69-1.jpg