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

植物螯合肽的合成，谷胱甘肽衍生的金属结合肽，长期以来一直被认为是各种物种对镉和砷解毒所必需的。然而，考虑到这些相当零星需要的功能，负责的酶植物螯合素合成酶的组成表达和广泛分布一直是神秘的。最近，有证据表明植物螯合素合成酶的两种重要生理功能可归因于两种不同的酶活性。转肽酶活性导致锌激活的植物螯合素合成，这有助于植物体内微量营养素锌的积累——鉴于人类普遍缺锌，这是一个重要的作用——并有助于对锌水平升高的耐受性。植物螯合素合成酶的肽酶活性似乎对植物对潜在病原体的非寄主抗性很重要。该项目旨在通过三种不同的方法来阐明这些功能的分子基础：在各种条件下生长的野生型和几种植物螯合素合成酶突变植物的微量营养素状况的全面分析，高分辨率代谢物分析以确定肽酶活性的底物和产物，以及诱变以揭示金属活化的结构决定因素和酶的不同活性。

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.