Activities and physiological roles of phytochelatin synthases, bi-functional enzymes involved in metal homeostasis and nonhost resistance

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The synthesis of phytochelatins, glutathione-derived metal-binding peptides, has long been known to be essential for the detoxification of cadmium and arsenic in various species. Given these rather sporadically required functions, however, constitutive expression and widespread distribution of the responsible enzymes, phytochelatin synthases, have been mysterious. Recently, evidence was reported for two important physiological functions of phytochelatin synthases that are attributable to two different enzymatic activities. Transpeptidase activity results in Zn-activated phytochelatin synthesis which contributes to the accumulation of the micronutrient Zn in plants – an important effect in light of widespread Zn deficiency in humans – and to tolerance of elevated Zn levels. The peptidase activity of phytochelatin synthases appears to be important for nonhost resistance of plants against potential pathogens. This project aims to elucidate the molecular basis for these functions using essentially three different approaches: thorough analysis of micronutrient status in wildtype and several phytochelatin synthase mutant plants grown under a variety of conditions, high-resolution metabolite profiling to identify substrates and products of the peptidase activity, and mutagenesis to unravel the structural determinants of metal activation and the different activities of the enzyme.