Metal tolerance in plants: chelation and sequestration

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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).