Phase change and the regulation of shoot morphogenesis in plants.

The shoot system of higher plants passes through several different phases during its development. Each of these phases is characterized by a unique set of morphological and physiological attributes. The intermediate character of the structures produced during phase changes and the phenotypes of mutations that affect this process demonstrate that these phases are specified by independently regulated, overlapping developmental programs. Transitions between phases appear to be initiated by factors extrinsic to the shoot apical meristem; the ability of the shoot to respond to such factors and to remain in a particular phase of development is regulated by factors intrinsic to the meristem. The possibility that developmental phases are maintained by epigenetic cell states and the role of DNA methylation in this process are discussed.