Stomatal development: new signals and fate determinants.

Stomata and pavement cells are produced by a series of asymmetric divisions and progressive fate transitions within a stem cell lineage. In Arabidopsis, this process is regulated so that new lineages can be inserted between previously differentiated cells while maintaining stomatal spacing. The small peptide EPIDERMAL PATTERNING FACTOR 1 may be a positional signal secreted by stomatal precursors to modulate behavior of nearby cells. Signal-receiving cells may use TOO MANY MOUTHS and ERECTA family receptors and a MAPK pathway to regulate initiation of new lineages, promote asymmetric division, and control the plane of spacing divisions. Cell fate transitions are controlled by basic helix-loop-helix transcription factor (bHLH), MYB, and MADS-box transcription factors, and there is evidence of miRNA regulation. These results provide insight into positive and negative influences on stomatal cell transitions and suggest points of potential environmental regulation.