Maintenance of the SAM

During organ formation, cells are continuously lost from the SAM, and these cells have to be replaced through cell divisions in the stem cell zone in order to maintain a functional SAM. The rate of cell divisions and the size of the stem cell population has to be coordinated with the rate of organ formation, which may vary during different developmental phases. How is this coordination achieved, and how can cells in the SAM communicate their position and their fate to each other? An elegant study used periclinal chimeras of tomato differing in the genetic makeup of meristem cell layers to address this question (Szymkowiak & Sussex 1992). In one experiment, such chimeras were generated by grafting between wild-type tomato plants and the tomato mutant fasciated, which has larger floral meristems and produces more carpels. When the outer two meristem layers were composed of wild-type cells, and only the L3 layer carried the fasciated mutation, the whole floral meristem increased in size and developed extra carpels, indicating that the cells in the L1 and L2 perceived and responded to a size increasing signal from the L3. The molecular basis of this signalling process has not been identified in tomato, but mutants similar to fasciated have been analysed in other species, like Arabidopsis thaliana.

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