Redundant SCARECROW genes pattern distinct cell layers in roots and leaves of maize

The highly efficient C 4 photosynthetic pathway is facilitated by 'Kranz' leaf anatomy. In Kranz leaves, closely spaced veins are encircled by concentric layers of photosynthetic bundle sheath (inner) and mesophyll (outer) cells. Here, we demonstrate that, in the C 4 monocot maize, Kranz patterning is regulated by redundant function of SCARECROW 1 (ZmSCR1) and a previously uncharacterized homeologue: ZmSCR1h. ZmSCR1 and ZmSCR1h transcripts accumulate in ground meristem cells of developing leaf primordia and in Zmscr1;Zmscr1h mutant leaves, most veins are separated by one rather than two mesophyll cells; many veins have sclerenchyma above and/or below instead of mesophyll cells; and supernumerary bundle sheath cells develop. The mutant defects are unified by compromised mesophyll cell development. In addition to Kranz defects, Zmscr1;Zmscr1h mutants fail to form an organized endodermal layer in the root. Collectively, these data indicate that ZmSCR1 and ZmSCR1h redundantly regulate cell-type patterning in both the leaves and roots of maize. Leaf and root pathways are distinguished, however, by the cell layer in which they operate - mesophyll at a two-cell distance from leaf veins versus endodermis immediately adjacent to root vasculature.