Twenty years on: the inner workings of the shoot apical meristem, a developmental dynamo

The shoot apical meristem of angiosperm plants generates leaf, stem and floral structures throughout the plant's lifetime. To do this, the plant must maintain a population of stem cells within the meristem while at the same time carefully controlling the placement and establishment of new leaf primordia. As there is little cell rearrangement in plants, underlying patterning mechanisms must exert careful control of cell division rates and orientations to achieve the correct final form. It has been twenty years since the first genes controlling meristem development were molecularly cloned. In the intervening decades, our understanding of the inner workings directing meristem development has increased enormously. This review summarizes our current knowledge of how the meristem functions as a persistent organ generating center. The story that emerges is one in which transcription factor activity combines with the action of the classic plant growth regulators auxin and cytokinin and with the action of more recently discovered small peptides to control proliferation and cell fate in the shoot apical meristem.