Mutations inNAKED-ENDOSPERMIDD genes reveal functional interactions withSCARECROWand a maternal influence on leaf patterning in C₄grasses

Leaves comprise a number of different cell-types that are patterned in the context of either the epidermal or inner cell layers. In grass leaves, two distinct anatomies develop in the inner leaf tissues depending on whether the leaf carries out C₃or C₄photosynthesis. In both cases a series of parallel veins develops that extends from the leaf base to the tip but in ancestral C₃species veins are separated by a greater number of intervening mesophyll cells than in derived C₄species. We have previously demonstrated that the GRAS transcription factor SCARECROW (SCR) regulates the number of photosynthetic mesophyll cells that form between veins in the leaves of the C₄species maize, whereas it regulates the formation of stomata in the epidermal leaf layer in the C₃species rice. Here we show that SCR is required for inner leaf patterning in the C₄speciesSetaria viridisbut in this species the presumed ancestral stomatal patterning role is also retained. Through a comparative mutant analysis between maize, setaria and rice we further demonstrate that loss of NAKED-ENDOSPERM (NKD) INDETERMINATE DOMAIN (IDD) protein function exacerbates loss of functionscrphenotypes in the inner leaf tissues of maize and setaria but not rice. Specifically, in both setaria and maize,scr;nkdmutants exhibit an increased proportion of fused veins with no intervening mesophyll cells, whereas inner leaf tissues are patterned normally inscr;nkdmutants of rice. Thus, combined action of SCR and NKD may control how many mesophyll cells are specified between veins in the leaves of C₄but not C₃grasses. Finally, we identified a maternal effect in maize in which maternally derived NKD can affect patterning of cells in leaf primordia that are initiated during embryogenesis. Together our results provide insight into the evolution of cell patterning in grass leaves, demonstrate a novel patterning role for IDD genes in C₄leaves and suggest that NKD can influence embryonic leaf development non-cell autonomously from the surrounding maternal tissue.