Elevated auxin biosynthesis and transport underlie high vein density in C₄ leaves.

Abstract

High vein density, a distinctive trait of C₄ leaves, is central to both C₃-to-C₄ evolution and conversion of C₃ to C₄-like crops. We tested the hypothesis that high vein density in C₄ 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 C₄ leaves compared with developing C₃ leaves. The same observation held for maize foliar (C₄) and husk (C₃) leaf primordia. Moreover, auxin content and vein density were increased in loss-of-function mutants of <i>Arabidopsis MYC2</i>, 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 <i>Arabidopsis thaliana</i> auxin efflux transporter <i>pin1</i> and influx transporter <i>lax2</i> mutants showed reduced vein numbers. Thus, development of high leaf vein density requires elevated auxin biosynthesis and transport.