Assigning gene function in biosynthetic pathways: camalexin and beyond.
Camalexin (3-thiazol-2′-yl-indole) is the major phytoalexin in Arabidopsis thaliana (Glawischnig, 2007) and is involved in defense against a wide range of pathogens, such as Botrytis cinerea and Alternaria brassicicola (Kagan and Hammerschmidt, 2002; Denby et al., 2004). The pathway leading to this model phytoalexin has been almost fully elucidated, and recent focus has been on the biosynthetic origin of the thiazole ring. Several publications have shown that glutathione, and not Cys, is the direct source of the heterocycle and have demonstrated that both glutathione and Cys conjugates of indole-3-acetonitrile (IAN) are intermediates in the pathway (Böttcher et al., 2009; Geu-Flores et al., 2011; Su et al., 2011). However, two recent studies published in this journal arrived at different conclusions regarding the conversion of the glutathione conjugate (γ-Glu-Cys[IAN]-Gly) to the Cys conjugate (Cys[IAN]), particularly regarding the family of enzymes cleaving off the γ-Glu residue. Su et al. (2011) reported that known members of the γ-glutamyl transpeptidase (GGT) family conducted this reaction, whereas we (Geu-Flores et al., 2011) found that members of the newly found γ-glutamyl peptidase (GGP) family performed the same reaction. This has created some confusion in the literature, with several subsequent depictions of the pathway containing both GGTs and GGPs (Ahuja et al., 2012; Saga et al., 2012), although some have made a clear distinction (Bednarek, 2012). Based on the results from these two reports, together with other published data and additional experimental results included here, we argue that GGPs and not GGTs are the γ-glutamyl cleaving enzymes in the camalexin pathway in Arabidopsis. Our line of argumentation leads to a discussion regarding the prerequisites for identification of specific genes as responsible for particular enzymatic steps in biosynthetic pathways.