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

The highly efficient C4 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 C4 monocot maize, Kranz patterning is regulated by redundant function of SCARECROW 1 (ZmSCR1) and a previously uncharacterized homeolog 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 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.\n\nSummary statementTwo duplicated maize SCARECROW genes control the development of the endodermis in roots and the mesophyll in leaves