In the Arabidopsis root, xylem is organised into a central file of metaxylem that is flanked by protoxylem. Xylem fate is determined by HD-ZIP III transcription factors; high levels promote metaxylem formation whereas low levels specify protoxylem. The factors that downregulate HD-ZIP III levels are known but those that promote HD-ZIP III expression have remained elusive. Yunde Zhao, Ykä Helariutta, Jan Dettmer and colleagues now show that auxin biosynthesis promotes HD-ZIP III expression and metaxylem specification in Arabidopsis (p. 1250). The authors first isolate mutants that display defective xylem patterning and HD-ZIP III gene downregulation. These mutants harbour mutations in the gene encoding TRYPTOPHAN SYNTHASE BETA-SUBUNIT 1. Tryptophan is a precursor in the auxin biosynthesis pathway and, accordingly, the mutants exhibit aberrant auxin levels. They also show that metaxylem development and HD-ZIP III levels are defective in other auxin biosynthesis mutants. Based on their findings, the authors propose that tryptophan-dependent auxin biosynthesis is required for metaxylem formation.