14 Development of the Vegetative Shoot Apical Meristem

The Arabidopsis shoot apical meristem starts as a tiny group of fewer than one hundred cells, yet this small group of cells is the source of the aboveground portion of the plant. This simple fact means that a remarkable amount of control (or perception of control signals) in the development of the plant is found in the apical meristem. One reason so little is known about the vegetative apical meristem in Arabidopsis is that it is one of the smallest shoot meristems in the angiosperms (Vaughan 1952). In Arabidopsis , the extremely small size of the apical meristem may mean that some of the processes are regulated differently. For example, a hypothetical morphological gradient that extends over several cells in a large angiosperm meristem could be limited to a gradient in a smaller group of cells or perhaps one cell (i.e., intracellular gradient) in Arabidopsis . However, the size restriction of the Arabidopsis meristem is no longer limiting due to two advances. First, genetic studies using meristem mutants allow the genes and signals involved in various processes to be defined. Second, biochemical studies can be done using an interchangeable system, Brassica oleracea . Cauliflower or B. oleracea var. botrytis is a mutant that contains an immense amplification of shoot meristems (Sadik 1962; Medford et al. 1991). Cauliflower genes are highly homologous to Arabidopsis genes (typically 95% identity), allowing the systems to be interchanged (Medford et al. 1991). Hence, cauliflower (and other related Brassicas ) can be used to collect biochemical quantities of materials, and...