Fig. 3.

A comparison of the regulatory mechanism for stem cell control between angiosperms and mosses. In the angiosperm shoot apical meristem (SAM), the WUSCHEL-CLAVATA (WUS-CLV) signaling module plays a central role in stem cell maintenance. WUS is directly promoted by cytokinin signaling, and cytokinin biosynthesis is under the control of Class I KNOX (KNOXI). Auxin triggers organ differentiation but it also acts synergistically with cytokinin through suppression of type-A ARR (A-ARR) by Monopterous (MP). In addition, auxin controls expression of APETALA2/Ethylene Responsive Factor (AP2/ER) transcription factors such as AINTEGMENTA/PLETHORA (AIL/PLT), which are involved in pluripotency of the stem cell. Outside the stem cells, ASYMMETRIC LEAVES1 (AS1) and ASYMMETRIC LEAVES2 (AS2) recruit the Polycomb Repressive Complex (PRC) and stably repress KNOXI expression. On the other hand, in mosses, the WUS-CLV signaling module does not exist. However, cytokinin promotes stem cell fate and CLV represses it through an unknown pathway, as in angiosperms. It is likely that type-B ARR (B-ARR) works under the cytokinin pathway although remains to be determined. Auxin is also important for organogenesis, and a synergistic pathway with cytokinin and stem cell regulation through an AP2/ERF transcription factor is probably conserved. Furthermore, repression of meristematic genes by the PRC (PpLHP1/PpFIE) outside the stem cells is also a shared feature between mosses and angiosperms, although the mechanism of PRC recruitment and the target genes to be repressed are unknown