Shoot Meristem Formation in Vegetative Development.

The majority of the plant body is derived from the activities of groups of specialized cells, known as apical meristems, at the growing tips. In a typical flowering plant, the shoot apical meristem (SAM) gives rise to the bulk of the above-ground portion of the plant, whereas root meristems give rise to the bulk of the subterranean plant body. Of course, not all plants are typical, and numerous exceptions to these broad generalizations are found in nature. For example, aerial roots are a common feature, and rhizomes or other subterranean shoot systems have SAMs that remain beneath the soil. Whether or not a plant is typical, the apical meristems of the plant represent the site at which organs are initiated and the pattern of the shoot and root system is established. In this review, we focus on meristem formation in the vegetative shoots of flowering plants. Although the term “meristem” can be used in a broad sense to refer to any actively growing portion of the plant, we limit our discussion to those portions of the plant that generate new lateral organs. Shoot and root meristems behave in different ways. The root apical meristem gives rise to the root and root cap but otherwise forms no lateral structures (lateral roots arise endogenously at a distance from the root apex; see Schiefelbein et al., 1997, in this issue). The SAM, by contrast, may adopt one of a variety of different developmental fates. A SAM may become determinate, forming a terminal flower, tendril, or thorn, or it may display an indeterminate pattern of growth, continuously producing vegetative leaves and branches. The types of lateral organs initiated by the SAM vary tremendously as well; modified leaves such as bud scales, leaf tendrils, floral bracts, and petals may all be produced. Moreover, the fate of a SAM may change during development, as, for example, when an indeterminate vegetative SAM becomes an inflorescence meristem or a determinate floral meristem. Different types of vegetative shoot meristems may be distinguished based on their position in the plant and on the developmental context in which they form (Figure 1). The SAM forms during embryogenesis and can be found dista1 to the