In higher plants, root growth is achieved by pronounced elongation of cells that are descendants of the root apical meristem (RAM), a specialized structure present at the root tip, which produces cells for virtually all of the root tissues formed during postembryonic development. In this localized microenvironment, meristematic cells can reside for potentially an indeterminate period of time and produce progeny cells while self-renewing, thus exhibiting key features of a stem-cell niche (Ohlstein et al. 2004; Li and Xie 2005; Dinneny and Benfey 2008; Morrison and Spradling 2008). This behavior relies on the capacity of RAM cells to undergo asymmetric cell division, which is another defining feature of stem-cells dictating that one daughter cell retains the meristematic fate and another daughter cell is programmed to differentiate into a specialized cell (McCulloch and Till 2005; Scheres 2007; Morrison and Spradling 2008). The acquisition of differential potential of the two daughter cells can result from an unequal partitioning of cell fate determinants through the asymmetric positioning of the cell division plate as well as from a different signaling from their surroundings.

The maintenance of the RAM is assured by a balance between the production of new meristematic cells and their displacement toward the differentiation process. However, in some cases, the RAM is genetically determined to become exhausted, and root growth shifts from an indeterminate to a determinate developmental pattern. Recently, an extensive review on the determinacy/indetermi-nacy of root growth has been provided by Shishkova et al. (2008), but it is outside the scope of this chapter.

Here, we review the recent advances in understanding transcriptional networks and hormone cross-talk underlying the establishment and regulation of the RAM. In particular, signals between the organizing region and the stem-cell population in the RAM as well as signals released by neighboring cells will be emphasized. Mainly, we focus on studies on the model plant, Arabidopsis thaliana.

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