Genetic and Epigenetic Regulation of Stem Cell

One of the fundamental differences between plant and animal development is that plants produce new organs throughout their lifetime, which can span hundreds of years. This reiterative process of organogenesis depends on the activity of populations of pluripotent stem cells present in specialized niches, termed meristems, at the growing tip of the root and shoot (Fig. 1). Despite their common functions in stem cell maintenance and organ formation, the root and shoot meristems are organized dif-ferently. In the root, a single layer of stem cells surrounds an organizing center comprised of a small group of infre-quently dividing cells that signal to the adjoining stem cells to inhibit their differentiation (van den Berg et al. 1997; Jiang and Feldman 2005). Each stem cell divides asymmetrically, giving rise to one daughter cell that main-tains stem cell identity through contact with the organiz-ing center and one daughter that is displaced from the niche and differentiates. The angiosperm SAM consists of a small dome of cells that is organized into central and peripheral zones, distin-guished by cell division rates and gene expression patterns. In the SAM of the model plant Arabidopsis, approximately three stem cells are maintained in each of the three most apical cell layers in the central zone (CZ) of the meristem (Fig. 1C) (see Bäurle and Laux 2003; Williams and Fletcher 2005).Their pluripotency is maintained by signals from an organizing center located immediately below the stem cell cluster. Thus, in contrast to the root, direct con-tact with the organizing center is not required to acquire or maintain shoot stem cell identity. Following stem cell divi