Contaminated soils and waters pose a major environmental and human health problem, which may be partially solved by the emerging phytoremediation technology. This cost-effective plant-based approach to remediation takes advantage of the remarkable ability of plants to concentrate elements and compounds from the environment and to metabolize various molecules in their tissues. Toxic heavy metals and organic pollutants are the major targets for phytoremediation. In recent years, knowledge of the physiological and molecular mechanisms of phytoremediation began to emerge together with biological and engineering strategies designed to optimize and improve phytoremediation. In addition, several field trials confirmed the feasibility of using plants for environmental cleanup. This review concentrates on the most developed subsets of phytoremediation technology and on the biological mechanisms that make phytoremediation work.

Nitric oxide (NO) has been implicated as mediator in a variety of physiological functions, including neurotransmission, platelet aggregation, macrophage function, and vasodilation. The consumption of NO by extracellular hemoglobin and subsequent vasoconstriction have been suggested to be the cause of the mild hypertensive events reported during in vivo trials of hemoglobin-based O2 carriers. The depletion of NO from endothelial cells is most likely due to the oxidative reaction of NO with oxyhemoglobin in arterioles and surrounding tissue. In order to determine the mechanism of this key reaction, we have measured the kinetics of NO-induced oxidation of a variety of different recombinant sperm whale myoglobins (Mb) and human hemoglobins (Hb). The observed rates depend linearly on [NO] but show no dependence on [O2]. The bimolecular rate constants for NO-induced oxidation of MbO2 and HbO2 are large (k.ox,NO = 30-50 microM-1 s-1 for the wild-type proteins) and similar to those for simple nitric oxide binding to deoxygenated Mb and Hb. Both reversible NO binding and NO-induced oxidation occur in two steps: (1) bimolecular entry of nitric oxide into the distal portion of the heme pocket and (2) rapid reaction of noncovalently bound nitric oxide with the iron atom to produce Fe(2+)-N=O or with Fe(2+)-O-O delta- to produce Fe(3+)-OH2 and nitrate. Both the oxidation and binding rate constants for sperm whale Mb were increased when His(E7) was replaced by aliphatic residues. These mutants lack polar interactions in the distal pocket which normally hinder NO entry into the protein. Decreasing the volume of the distal pocket by replacing Leu(B10) and Val(E11) with aromatic amino acids markedly inhibits NO-induced oxidation of MbO2. The latter results provide a protein engineering strategy for reducing hypertensive events caused by extracellular hemoglobin-based O2 carriers. This approach has been explored by examining the effects of Phe(B10) and Phe(E11) substitutions on the rates of NO-induced oxidation of the alpha and beta subunits in recombinant human hemoglobin.

The bioaccumulation of arsenic by plants may provide a means of removing this element from contaminated soils and waters. However, to optimize this process it is important to understand the biological mechanisms involved. Using a combination of techniques, including x-ray absorption spectroscopy, we have established the biochemical fate of arsenic taken up by Indian mustard (Brassica juncea). After arsenate uptake by the roots, possibly via the phosphate transport mechanism, a small fraction is exported to the shoot via the xylem as the oxyanions arsenate and arsenite. Once in the shoot, the arsenic is stored as an As III -tris-thiolate complex. The majority of the arsenic remains in the roots as an As III -tris-thiolate complex, which is indistinguishable from that found in the shoots and from As III -tris-glutathione. The thiolate donors are thus probably either glutathione or phytochelatins. The addition of the dithiol arsenic chelator dimercaptosuccinate to the hydroponic culture medium caused a 5-fold-increased arsenic level in the leaves, although the total arsenic accumulation was only marginally increased. This suggests that the addition of dimercaptosuccinate to arsenic-contaminated soils may provide a way to promote arsenic bioaccumulation in plant shoots, a process that will be essential for the development of an efficient phytoremediation strategy for this element.Arsenic may play an essential role in animal nutrition (Uthus, 1992(Uthus, , 1994, perhaps in Met metabolism, but there is no doubt that the element is principally renowned for its toxicity (National Research Council, 1977). Indeed, arsenic toxicity in humans has recently become evident on a very large scale in Bangladesh (Dhar et al., 1997), and the National Research Council has recently recommended that the maximum contaminant level standard for drinking water in the U.S. be lowered from the current value of 50 g L Ϫ1 (National Research Council, 1999). Arsenic is also toxic to plants and microorganisms and has been used in pesticides, herbicides, preservatives, and pharmaceuticals (National Research Council, 1977). Many of these uses continue today, and therefore it is important to remediate past contamination (Dutre et al., 1998). In this paper we address arsenate uptake by Indian mustard (Brassica juncea) plants growing hydroponically. Our data suggest that arsenate (As V ) enters the roots as a phosphate analog and is promptly reduced to As III . Little arsenic is transported to the aboveground tissues. The addition of dimercaptosuccinate to the hydroponic growth solution caused significant amounts of arsenic to move into the shoot, perhaps offering a way of removing arsenate from contaminated soils. MATERIALS AND METHODS Plant GrowthIndian mustard (Brassica juncea [L.] Czern. variety 426308) (Kumar et al., 1995) plants were grown under microbiologically controlled conditions such that their roots were maintained axenically. Seeds were surfacesterilized in 2.6% (w/v) sodium hypochlorite for 30 min, rinsed four times in autoclaved de-i...