In developing Arabidopsis leaves, the vein-forming procambial cells arise only from progenitor cells that express the homeodomain-leucine zipper III gene ATHB8. By investigating ATHB8 function,Enrico Scarpella and colleagues now identify a molecular basis for how auxin signalling affects leaf vein development (see ). By studying mutant leaves that lack ATHB8, the authors determined that ATHB8 restricts preprocambial fate to narrow domains of progenitor cells and coordinates procambium formation within and between veins. In athb8 mutants, the effects of experimentally perturbing the transport of the plant hormone auxin on both vein development and the distribution of the auxin exporter PIN1 are stronger than in wild-type plants. From their studies, the authors identified an auxin-response element in the ATHB8 promoter that is required for ATHB8 preprocambial expression and found that it is targeted by the auxin-response transcription factor MONOPTEROS. Together, these findings provide an important advance towards understanding the mechanisms by which auxin signalling events are translated into developmental patterning processes.
