Identification of an aromatic aldehyde synthase involved in indole-3-acetic acid biosynthesis in the galling sawfly (Pontania sp.) and screening of an inhibitor.

Indole-3-acetic acid (IAA), a phytohormone auxin, may be involved in insect gall induction. We previously proposed that the IAA biosynthetic pathway is Trp → indole-3-acetaldoxime → indole-3-acetaldehyde (IAAld) → IAA or Trp → IAAld → IAA. In this study, we surveyed galling sawfly enzymes responsible for the rate-limiting steps using a heterologous protein expression system and identified PonAAS2, an aromatic aldehyde synthase, that catalyzed the conversion of Trp to IAAld. The PonAAS2 gene was highly expressed in early- and mid-stage larvae that contained high concentrations of IAA, but the expression level was almost negligible in larvae that had escaped from their gall in autumn and contained very low concentrations of IAA. An inhibitor of PonAAS2, obtained by screening a chemical library, inhibited IAA production in sawfly enzyme solution by 80%, suggesting the important role of this enzyme in IAA biosynthesis in sawfly.