Figure 7.

Model for development of maize sheath margins. A, Cartoon of a maize shoot apex, showing the SAM (yellow) and blade (dark green) and sheath (light green) domains of a young primordium. The disc of insertion contains, in addition to the unelaborated node and stem, the recruited but as yet unexpanded domains of the lower leaf sheath. B, Fate mapping and sector analyses of maize leaf development (Scanlon and Freeling, 1997) reveal that the founder cells that give rise to the margins of the maize leaf blade (dark green) do not overlap the meristem flank; however, the founder cells that give rise to both the right and left margins of the leaf sheath (light green) are derived from a common domain that overlaps the SAM (yellow). Clonal analyses also indicate that the outermost margins of the maize leaf are derived from the L1 (protodermal) cell layer of the SAM (Poethig and Szymkowiak, 1995). C, Model for the development of left and right sheath margins from a single cell layer of the SAM. Sheath margins are initialized as a single cell layer; the right and left domains are not yet resolved (left). Periclinal divisions generate multiple cell layers from the L1 (middle). Later, the right and left margin domains are specified and emerge as separate edges of the leaf during the P4 stage of development (right).