Figure 2.

A, The biosynthesis of IAA precursors, such as indole-3-glycerol phosphate and l-Trp, takes place in plastids. l-Trp, the major IAA precursor, is generated via the shikimate pathway. The subsequent l-Trp-dependent IAA biosynthesis pathways are believed to occur in the cytosol. Four putative pathways for l-Trp-dependent IAA biosynthesis in higher plants are shown: the IAOx, IAM, IPyA, and TAM pathways. The enzymes known to operate in each pathway are shown in italics. Solid pathway arrows reflect presumably active processes. Dashed pathway arrows are suggested to exist, but have yet to be proven. Modifications to past published pathways (McSteen, 2010; Normanly, 2010; Ljung, 2013) are shown in A. Abbreviations not defined in the text: IAAld, indole-3-acetaldehyde; TDCs, Trp decarboxylases. B and C, The IAA biosynthesis flux network across the four l-Trp-dependent pathways was quantified using [¹¹C]indole. Measurements were taken 1 h after incubation with tracer. In unstressed control plants (B), the IPyA pathway was dominant, the IAOx pathway through IAN was second in importance, the TAM pathway was minor, and the IAM pathway was not active. In herbivore-stressed plants (C), the IAOx pathway through IAN became the dominant pathway, and the IAM pathway was now active.