Multi-tasking phytochelatin synthases

Phytochelatins are essential for cadmium and arsenic detoxification in plants, some fungi, and animals. It is mysterious, that the responsible enzymes, phytochelatin synthases (PCS), are constitutively expressed and so widespread in nature. Phylogenetic analysis indicates multiple horizontal transfers of PCS genes, but a bacterial origin appears unlikely. Differences between bacterial and eukaryotic PCS proteins in structure and activity had indicated bi-functionality of phytochelatin synthases as peptidases and transpeptidases. Recent observations indicate that PCS indeed serve physiological functions that most likely are much more prevalent than cadmium or arsenic detoxification. First, PCS-deficient Arabidopsis thaliana mutants are hypersensitive to zinc suggesting a role of phytochelatin synthesis, i.e. the formation of metal-binding peptides from glutathione in a transpeptidase reaction, in Zn homeostasis. Second, these mutants are also impaired in defense responses conferring resistance to incompatible pathogens (= nonhost resistance). The latter is hypothesized to be attributable to an involvement of PCS as a peptidase in indole glucosinolate metabolism. Possibly, micronutrient homeostasis and nonhost resistance are closely connected as PCS are not the only proteins involved in both processes. (C) 2009 Elsevier Ireland Ltd. All rights reserved.