Epidermal cell fate and pattern in leaves.
by Larkin J. C., Marks M. D., Nadeau J., Sack F. (1996)
John C. Larkin, Louisiana State University, Baton Rouge,
Jeanette A Nadeau, University of Wisconsin, Madison
in Plant Cell. 1996;(9):1109-1120. – DOI: 10.1105/tpc.9.7.1109 –
Cell differentiation requires that undifferentiated cells first be selected before becoming committed to a specific fate. The selection of precursor cells often is coordinated so that mature 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 differentiated 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 precursor cells could be random within a field of equally competent cells, with adjacent cells subsequently prevented from assuming the precursor cell fate by lateral inhibition. Alternatively, 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 interactions guiding patterning are known for such model systems 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 Jürgens, 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 system for analyzing pattern formation because the epidermal surface is readily accessible and cell patterns can be analyzed within a plane rather than in three dimensions. The leaves of most plants contain two highly differentiated cell types in the epidermis: guard cells, which constitute stomata, and trichomes. These cells, the spacing of which is the primary focus of this review, are usually separated from each other by pavement cells. Figure 1 illustrates these three classes of cells in an Arabidopsis leaf. The same two questions can be asked regarding the developmental patterns of trichomes and stomata: How are the precursor cells selected, and how is the spacing pattern generated? However, the answers appear to be quite differ-
ent for each cell type. Although patterning in Arabidopsis is considered in the greatest detail in this article, data from other species also are discussed.