Lessons from the vegetative shoot apex - Intercellular Communication in Plants - page 170

are a staggering number of RLKs with no known function, suggesting that receptor-
ligand interactions are likely to be the most common form of signalling in plants. It
would therefore not be surprising to find many more CLV-like pathways operating in
the meristem. However as can be seen from this and other chapters, not all signalling
in plants involves the perception of ligands. A number of transcriptions factors, such
as the KNOX genes, move through the meristem and may form concentration gradi-
ents that activate different sets of target genes. This type of signalling may rely on
the developmental regulation of symplastic fields within the meristem, an area that
is still poorly understood. The formation of organs also requires the movement and
perception of a signalling molecule, which in this case is auxin. How cells perceive
and respond to increased levels of auxin remains to be determined. Similarly, under-
standing how auxin-regulated pathways interact with other signalling pathways in
the meristem will be particularly interesting. The addition of microRNAs to the list
of potential signalling molecules is intriguing, but not necessarily unexpected. RNA
movement in plants is well known and has been extensively characterised. However
it remains to be seen whether localised movement of RNA or microRNAs within the
meristem, or between the meristem and emerging organs, has a role in regulating
meristem functions. Thus studies of the shoot apical meristem have provided unique
insights into various types of signalling systems that operate within plants, and are
likely to do so well into the future.
Acknowledgements
The author thanks Gwyneth Ingram, David Smyth and members of the Smyth lab-
oratory for thoughtful discussions and comments on this manuscript.
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