Metal tolerance in plants: chelation and sequestration

虽然我们还不完全了解允许某些植物在金属污染环境中茁壮成长的因素，但耐受性似乎分为两类：金属排斥和金属过度积累。可以通过植物引起的土壤变化来实现排异。超积累被认为涉及金属结合分子(螯合剂)的产生和/或金属离子在植物部分不能发挥其毒性作用的地方的隔离(S)。植物含有两类主要的螯合剂：植物螯合素和低分子有机酸。虽然暴露在多种金属中会导致植物络合素的合成和有机酸的过量生产，但某些物种对金属的耐受性似乎不依赖于这些螯合剂的产生。这项研究计划的长期目标是确定植物螯合素和有机酸在螯合和隔离有毒金属方面的相对重要性。眼下的目标包括1)确定植物对金属暴露直接反应产生的有机酸的类型和数量，2)确定这些分子的数量是否足以结合金属从而防止毒性，以及3)确定潜在有毒金属的亚细胞定位。这些有机酸将使用标准的高效液相色谱方法和毛细管电泳法(CE)进行定量。使用我的实验室正在开发的一种新方法，CE还将用于直接测量与植物生产的螯合剂结合的金属量。微型同步加速器x射线荧光和二次离子质谱仪将被用来绘制微米尺度的植物内部和紧邻根外土壤中的金属离子图谱。这项研究将提供更好的了解植物耐受金属的机制，并可应用于植物修复领域(利用植物净化受污染的土壤)。

Although we do not yet fully understand the factors that permit some plants to thrive in metal-polluted environments, tolerance appears to fall into two categories: metal exclusion and metal hyperaccumulation. Exclusion may be achieved by plant-induced changes in the soil. Hyperaccumulation is thought to involve the production of metal-binding molecules (chelators) and/or the sequestration of metal ions in parts of the plant where the metal can't exert its toxic effect(s). Plants contain two major classes of chelators: phytochelatins and low molecular weight organic acids. While exposure to a number of metals induces the synthesis of phytochelatins and the overproduction of organic acids, metal-tolerance in some species seems to be independent of the production of these chelators. The long term goal of this research program is to determine the relative importance of phytochelatins and organic acids to the chelation and sequestration of toxic metals. Immediate objectives include 1) identify the types and amounts of organic acids produced by plants in direct response to metal exposure, 2) determine if the amounts of these molecules are sufficient to bind metals and thereby prevent toxicity and 3) determine the subcellular localization of potentially toxic metals.  Organic acids will be quantified using standard methods for high performance liquid chromatography and a new method for capillary electrophoresis (CE). CE will also be used to measure directly the amount of metals bound to plant-produced chelators using a novel method that is under development in my lab. Micro-synchrotron x-ray fluorescence and secondary ion mass spectrometry will be used to map metal ions at the micron scale inside the plant and in the soil immediately outside the root.  This research will provide a better understanding of the mechanisms of metal tolerance in plants and can be applied to the field of phytoremediation (using plants to cleanse contaminated soils).