Unlike the shoot meristem, the root meristem consists of lineage-restricted stem cells. These cells are organized around the quiescent center, four very slowly dividing cells that serve as the stem-cell niche (Figure 21-20b). Stem-cell division is asymmetric (also unlike that in the shoot), and the daughter cell that loses contact with the quiescent center divides several more times and then differentiates. A WUS homolog, WOX5, is expressed in the quiescent center and is required for stem-cell maintenance, although other transcription factors are also important. The plant hormone auxin (indole-3-acetic acid) coordinates many processes involved in plant growth and differentiation; in particular, it is essential for formation of the root meristem niche. If the quiescent center is ablated, a new niche is formed in an area of high auxin concentration. However, the effect of auxin on stem cells depends on the specific cell type. For example, in the stem cells that give rise to the root cap, auxin promotes cell differentiation by repressing WOX5 via auxin-responsive transcription factors.

Plants have an amazing capacity for regeneration. The home gardener will be familiar with the ability of leaf or stem cuttings to form roots with little inducement beyond a glass of water and a sunny windowsill. Experiments performed in the mid-twentieth century demonstrated that single cells isolated from carrot roots could regenerate entire plants when placed on media containing the appropriate mix of nutrients and hormones. After that time, an often-cited major difference between plant and animal cells was that all plant cells are totipotent. Today, however, with our ability to generate iPS cells from differentiated animals cells as well as more recent careful analyses of the cells contributing to plant regeneration, which suggest that regenerated tissue arises from preexisting populations of stem cells rather than through a process of dedifferentiation, this distinction is becoming blurred.