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First published online 24 January 2007
doi: 10.1242/dev.000497

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Establishing leaf polarity: the role of small RNAs and positional signals in the shoot apex

¹ Cold Spring Harbor Laboratory, One Bungtown Road, Cold Spring Harbor, NY 11724, USA.
² Watson School of Biological Sciences, One Bungtown Road, Cold Spring Harbor, NY 11724, USA.

^* Author for correspondence (e-mail: timmerma{at}cshl.edu)

SUMMARY

The flattening of leaves results from the juxtaposition of upper (adaxial) and lower (abaxial) domains in the developing leaf primordium. The adaxial-abaxial axis reflects positional differences in the leaf relative to the meristem and is established by redundant genetic pathways that interpret this asymmetry through instructive, possibly non-cell autonomous, signals. Small RNAs have been found to play a crucial role in this process, and specify mutually antagonistic fates. Here, we review both classical and recently-discovered factors that contribute to leaf polarity, as well as the candidate positional signals that their existence implies.