Maintenance of shoot and floral meristem cell proliferation and fate.

To cope with environmental changes, animals respond by altering their behavior, but sessile plants respond by altering their growth and development pattern. One of the major differences between plant and animal development is that plants have the capacity to develop new organs postembryonically. This potential to develop new organs is attributed to sets of cells, called meristems, which are found at the growing tips of the plants. Two meristematic cell populations are generated during embryogenesis. The shoot apical meristem (SAM) generates all of the aerial parts of the plant, whereas the root apical meristem generates the underground parts. The SAM produces lateral organs from the cells on its flanks while simultaneously maintaining a central pool of pluripotent stem cells for future organogenesis. Thus, maintenance of a functional SAM requires coordination between loss of cells from the meristem by differentiation and their replenishment by stem cell division (Steeves and Sussex, 1989). Different types of lateral organs are generated by the SAM during successive phases of development. The SAM produces leaves and axillary meristems during the vegetative phase and floral meristems during the reproductive phase. Floral meristems produce flowers that usually consist of four whorls of organs. After producing these whorls, the activity of the floral meristem ceases, unlike the SAM, which continuously proliferates and produces organ primordia from its flanks. Several unanswered questions about meristem function are generating considerable interest: How do stem cells originate? How is the coordination between accumulation and loss of stem cells maintained? What are the signaling mechanisms involved in the communication between stem cells and with cells in the meristem flanks? How are shoot and floral meristems distinguished? Here, we provide an update of recent developments that address some of these key questions of SAM and floral meristem maintenance and development in Arabidopsis. ORGANIZATION OF THE SAM