Metal tolerance in plants: chelation and sequestration

虽然我们还没有完全了解一些植物在金属污染环境中茁壮成长的因素，但耐受性似乎可以分为两类：金属排斥和金属超积累。可以通过植物引起的土壤变化来实现排除。超积累被认为与金属结合分子（螯合剂）的产生和/或金属离子在植物中不能发挥其毒性作用的部分的隔离有关。植物中含有两大类螯合剂：植物螯合剂和低分子量有机酸。虽然暴露于一些金属会诱导植物螯合素的合成和有机酸的过量产生，但某些物种的金属耐受性似乎与这些螯合剂的产生无关。这项研究计划的长期目标是确定植物螯合素和有机酸对螯合和隔离有毒金属的相对重要性。当前的目标包括：1)确定植物对金属暴露的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；3)确定潜在有毒金属的亚细胞定位。有机酸的定量将采用高效液相色谱标准方法和毛细管电泳新方法。CE还将用于直接测量与植物产生的螯合剂结合的金属量，使用我的实验室正在开发的一种新方法。微同步加速器x射线荧光和二次离子质谱法将用于在微米尺度上绘制植物内部和根外土壤中的金属离子。该研究将为进一步了解植物的金属耐受性机制提供理论依据，并可应用于植物修复（利用植物净化污染土壤）领域。

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).