Epidermal cell fate and patterning in leaves.

that ma- ture differentiated cells are distributed in a characteristic pattern. One of the simplest possible patterns in tissues is that in which a minimum distance is maintained between dif- ferentiated cells in a two-dimensional sheet of cells (Wolpert, 1971). Such a pattern could be created by several different mechanisms. For example, the initial positioning of precur- sor cells could be random within a field of equally compe- tent cells, with adjacent cells subsequently prevented from assuming the precursor cell fate by lateral inhibition. Alter- natively, a prepatterning could exist so that the selection or placement of the precursor cells is nonrandom. Regardless of how precursor cells are placed, the production of new cells from a precursor cell can also contribute to the final spacing pattern (Sachs, 1978). Although the molecular inter- actions guiding patterning are known for such model sys- tems as epidermal bristle formation in Drosophila (Ghysen et al., 1993), little is known about the nature of the intercellular signaling that establishes cell patterning in plants (see Clark, 1997; Kerstetter and Hake, 1997; Laux and Jurgens, 1997; McLean et al., 1997; Poethig, 1997; and Schiefelbein et al., 1997, in this issue, for further discussion). The epidermis of plant leaves provides an excellent