Camalexin originates from tryptophan and its biosynthesis involves the cytochrome P450 enzymes CYP79B2 and CYP71B15 (PAD3). Camalexin induction is a complex process, for which triggering by reactive oxygen species (ROS), salicylic acid signalling, and the glutathione status are important. Targets of the signalling cascade are the tryptophan and camalexin biosynthetic genes, which are strongly transcriptionally upregulated at the sites of pathogen infection (Glawischnig et al. 2004) .