Concepts of target cells in plant differentiation.

The regeneration of whole plants from fragments of mature organs is compelling evidence that plant cells retain genetic totipotency throughout differentiation. Within the intact plant, however, strict regulatory controls operate to maintain a co-ordinated pattern of growth. Not every cell differentiates along the same developmental pathway. Cell performance is determined by mechanisms that permit subtle discriminations in recognition of and response to an array of environmental and hormonal cues. Much effort is currently directed to understanding these control systems in plants. Certain positionally differentiated cells and tissues have been characterized by their specific signal recognition and their precise responses in gene expression. In a few, the competence to respond to particular signals has been distinguished by the presence of cell-specific protein markers. This article discusses some recent studies that help towards an understanding of the target nature of cells in plant development.