Signalling between the shoot apical meristem and developing lateral organs.

A characteristic feature of plant development is the extensive role played by cell-cell signalling in regulating patterns of growth and cell fate. This is particularly apparent in the shoot apical meristem (SAM) where signalling is involved in the maintenance of a central undifferentiated stem cell population and the formation of a regular and predictable pattern of leaves, from the meristem periphery. Although these two functions occur in different regions of the meristem, their activity must be coordinated to maintain meristem integrity. The role of signalling in the SAM was first characterised over 60 years ago by elegant surgical experiments. These studies showed that existing leaf primordia determine future sites of organ formation in adjacent regions of the SAM, a finding that laid the foundation for subsequent studies into the mechanisms controlling phyllotaxy. Recent studies have identified auxin as a likely signal promoting organ formation and shown that young primordia play an important role in determining its distribution in the SAM. These pioneering surgical experiments also revealed that signals from the meristem regulate the development of organ primordia. In this case a meristem signal promotes the formation of cell types found in the top/adaxial half of the emerging leaf. While the identity of this signal remains elusive, the recent characterisation of a small family of PHABULOSA-like (PHB-like) transcription factor genes has provided important clues to its nature. These genes, which promote adaxial cell identity, are regulated by microRNAs (miRNAs) raising the exciting possibility that the meristem signal is either a miRNA or part of a pathway regulating the distribution of miRNAs.