Fig. 4. Model for the regulation of stem cells in the Arabidopsis shoot meristem. The CZ comprises both the stem cells (blue) and the OC (red), expressing CLV3 and WUS, respectively (insets: in situ hybridizations). WUS activity in the OC results in specification of stem cell identity in the three outermost cell layers, which in turn signal back by CLV3, restricting the WUS expression domain. Note that the expression domains in this figure are estimated based on comparison of separate experiments and not on simultaneous detection of the genes.