Indolic glucosinolates at the crossroads of tryptophan metabolism

In plants the amino acid tryptophan (Trp) is used to synthesize proteins and a wide variety of compounds that control development and defense. Recent studies in the reference plant Arabidopsis thaliana have elucidated a number of tryptophan secondary metabolism pathways derived from the Trp metabolite, indole-3-acetaldoxime (IAOx), particularly the pathway for synthesis of indolic glucosinolate (IG) herbivory defense compounds. Analyses of mutants, natural variants, and transgenic strains with perturbations in the IG pathway have revealed that regulation of this pathway is networked at many levels, including interfaces with Trp synthesis, other Trp secondary metabolism pathways, other glucosinolate synthesis pathways, and sulfur metabolism. Transcriptional regulatory mechanisms have been particularly well characterized, but additional mechanisms such as metabolic channeling may also contribute to the homeostasis of Trp secondary metabolism. The IG pathway thus serves as a paradigm for regulatory cross-talk between primary and secondary metabolism and among inter-related secondary metabolic processes.