Organ Formation at the Vegetative Shoot Meristem.

In higher plants, organ formation is critical for generating the vegetative portion of the plant. Above-ground organ formation occurs at a collection of stem cells termed the shoot meristem (SM), which is established during embryogenesis. How the SM functions as a site of continuous organ formation is a central question for plant developmental biology. This is not only because all above-ground organs are initiated by the SM but also because their position and identity are established there. The SM retains the capability to form organs through two fundamental processes (Figure 1). The first function of the SM is to maintain a pool of undifferentiated cells. Without undifferentiated cells to draw upon, new organ initiation would not be possible. The second process is to direct appropriately positioned undifferentiated cells toward organ formation and eventual differentiation. Although the structure of meristems is variable across the plant kingdom, all complex multicellular plants regulate the balance between an undifferentiated and differentiated fate at their respective meristems. Thus, unraveling the mechanisms by which model plants such as Arabidopsis regulate meristem development is expected to have implications for a wide variety of plant species. The SM has been the focus of many studies over the past several decades. These studies have investigated the diversity, morphology, histology, cell division patterns, and cell lineages of the SM. Because this work has been summarized in detail elsewhere (Steeves and Sussex, 1989; Lyndon, 1990, 1994), this review focuses on recent advances in understanding the genetic control of organ formation at the SM.