Phytoremediation With Plants:

    The use of plant species for phytoremediation has been well documented.  Plants are less disruptive to the environment, more cost effective, and more aesthetically pleasing than other methods of environmental clean up.  In addition to naturally occurring plants, numerous transgenic plants have been shown in controlled studies to be able to hyperaccumulate concentrations of metals that would normally be toxic.  Some can accumulate up to 100-fold higher metal concentration than their naturally occurring hyperaccumulator counterparts⁹.  Factors that influence the efficiency of phytoremediation include bioavailability of the metal, rate of metal uptake by the plant, extent of plant tolerance to the metal or the rate of chemical transformation of the metal to a less toxic form by the plant, and the rate of translocation of the metal to different plant tissues⁹.  Each of these factors can potentially be altered using transgenic plants.

    Several strategies exist for manipulating plants to become hyperaccumulators of metals.  One of these strategies is to alter the expression of proteins that serve as membrane transporters.  Some of these transporters are highly specific, while others may allow other metals with similar chemical and physical properties to nutrient ions to cross the cell or vacuolar membranes.  The ZIP family of transporters, for instance, has been shown to allow Cd, Fe, Mn, and Zn to cross membranes.  Zn-related downregulation of transporter gene expression occurs at a 50-fold higher concentration in some transgenic plants than in the wild type, allowing a much higher accumulation of Zn to enter the cell.  Several research groups have managed to express bacterial or yeast transporters in plant cells, avoiding the effect of co-suppression due to homology of inserted plant genes to genes already present in the plant.  In addition to the ZIP family, other transporters have been characterized.  The ZAT gene of Arabidopsis and the ZTP genes of Thlaspi caerulescens have been shown to mediate transport of Zn and Ni¹⁰.  For non-specific or low-specificity transporters, however, the presence of other cations may interfere with the uptake of the target metal.  Lombi et al. showed that the presence of Fe might interfere with the uptake of Cd and Zn in some plant strains, while not affecting uptake in those with a different transporter gene¹¹.  Additionally, plant-breeding experiments have indicated that in some species, Zn tolerance and uptake are independent traits.  This was supported by the findings of Assuncao, et al.¹⁰.

                
Cartoon of the structure of a metallothionine¹³         Structure of a phytochelatin¹³
 

    An alternative strategy to altering expression of membrane transport proteins is to change the levels of expression of genes encoding for chelator molecules.  Plants and algae contain Class II and III metallothionines (MT), which are cysteine-rich proteins that bind cations at the reduced sulfur residues of the cysteines¹².  Each plant has multiple Class II MT gene families that give resistance to different metal cations.  Cu has been found to give the strongest MT response in plants, followed by Cd and Zn.  Detection of MT levels cannot be seen as a phenotype and must be measured by levels of MT RNA present¹⁰.  The exact function of MT in plants remains unclear at this point, in part because of the difficulty in isolating the proteins from plant tissue.  However, it can be hypothesized that MT may have a role in providing a pool of available micronutrient metals within the cell, or may serve to protect the cell from oxidative degradation by free radicals.  Phytochelatins (PC) are similar to MT, and were formerly classified as Class III MT.  These smaller metal-binding polypeptides are enzymatically synthesized from glutathione.  They have been shown to bind to Cd strongly, and also to Cu, Pb, Au, and Hg.  PCs and Cd form clusters with acid-labile sulfide and are stored in the vacuole¹².  Again, the exact mechanisms of binding and the purpose of storage are not fully understood, however, metal binding PCs and Class III MTs have also been implicated in metal tolerance, although they are not likely the primary chelators.  In addition to MTs and PCs, plants have been shown to synthesize organic acids in response to cations.  Histidine is a primary chelator of Ni, and Co has been shown to elicit the production of free cysteine and citric acid in Crotalaria cobalticola, a Co hyperaccumulator¹⁴.