On fate and flexibility in stomatal development

In plants, the development of the epidermis, and the specialized stomatal lineage within it, exemplifies an old developmental problem that is newly relevant in this current era of stem cell biology: How can a tissue maintain flexibility and change its development midcourse yet still reliably generate differentiated and patterned cells? In this perspective, we endeavor to create a conceptual framework for the widespread questions in development that are raised by observations of stomatal development pathways in "default" settings and in response to environmental challenges. These general issues are related to the molecular pathways and networks recently elucidated for Arabidopsis stomatal development. Finally, the utility of developmental approaches for solving problems of signaling specificity are explored, emphasizing the specific use of the stomatal lineage as an in vivo testing ground for hormone and mitogen-activated protein kinase (MAPK) signaling cascades.