Elevated auxin biosynthesis and transport underlie high vein density in C leaves.

High vein density, a distinctive trait of C leaves, is central to both C-to-C evolution and conversion of C to C-like crops. We tested the hypothesis that high vein density in C leaves is due to elevated auxin biosynthesis and transport in developing leaves. Up-regulation of genes in auxin biosynthesis pathways and higher auxin content were found in developing C leaves compared with developing C leaves. The same observation held for maize foliar (C) and husk (C) leaf primordia. Moreover, auxin content and vein density were increased in loss-of-function mutants of , a suppressor of auxin biosynthesis. Treatment with an auxin biosynthesis inhibitor or an auxin transport inhibitor led to much fewer veins in new leaves. Finally, both auxin efflux transporter and influx transporter mutants showed reduced vein numbers. Thus, development of high leaf vein density requires elevated auxin biosynthesis and transport.