Plants vary considerably in their physiological response to selenium (Se). Some plant species growing on seleniferous soils are Se tolerant and accumulate very high concentrations of Se (Se accumulators), but most plants are Se nonaccumulators and are Se-sensitive. This review summarizes knowledge of the physiology and biochemistry of both types of plants, particularly with regard to Se uptake and transport, biochemical pathways of assimilation, volatilization and incorporation into proteins, and mechanisms of toxicity and tolerance. Molecular approaches are providing new insights into the role of sulfate transporters and sulfur assimilation enzymes in selenate uptake and metabolism, as well as the question of Se essentiality in plants. Recent advances in our understanding of the plant's ability to metabolize Se into volatile Se forms (phytovolatilization) are discussed, along with the application of phytoremediation for the cleanup of Se contaminated environments.

inhibited SeO 3 and SeMeth uptake by 33% and 15± 25%, respectively, as compared to an inhibition of 90% of SeO 4 uptake. Similar results were observed with regard to sulfate eects on volatilization. We conclude that reduction from SeO 4 to SeO 3 appears to be a ratelimiting step in the production of volatile Se compounds by plants. Inhibitory eects of sulfate on the uptake and volatilization of Se may be reduced substantially if Se is supplied as, or converted to, SeO 3 and/or SeMeth rather than SeO 4 .

Soybean plants (Glycine max [L.] Merr var Amsoy 71) were grown in growth chambers with high-phosphorus (high-P) and low-phosphorus (low-P) culture solutions. Low-P treatment reduced shoot growth significantly 7 days after treatment began. Root growth was much less affected by low-P, there being no significant reduction in root growth rate until 17 days had elapsed. The results suggest that low-P treatment decreased soybean growth primarily through an effect on the expansion of the leaf surface which was diminished by 85%, the main effect of low-P being on the rate of expansion of individual leaves. Low-P had a lesser effect on photosynthesis; light saturated photosynthetic rates at ambient and saturating CO2 levels were lowered by 55 and 45%, respectively, after 19 days of low-P treatment. Low-P treatment increased starch concentrations in mature leaves, expanding leaves and fibrous roots; sucrose concentrations, however, were reduced by low-P in leaves and increased in roots. Foliar F-2,6-BP levels were not affected by P treatment in the light but in darkness they increased with high-P and decreased with low-P. The increase in the starch/sucrose ratio in low-P leaves was correlated primarily with changes in the total activities of enzymes of starch and sucrose metabolism.Suboptimal phosphorus supply diminishes photosynthetic C02-fixation rates (2,24) and the expansion of the photosynthetic leaf surface (5,22). It may also lead to decreased cytosolic orthophosphate levels (26). Orthophosphate (Pi) is thought to regulate the activities of several enzymes involved in starch and sucrose metabolism in vivo and the export of C out of the chloroplast via the Pi-translocator (3). However, much of the evidence in support of Pi as a key regulator of carbon partitioning has been obtained with in vitro systems. In this paper, we study the nutritional effects of Pi on growth, photosynthesis, and starch/sucrose metabolism in an intact plant system; soybean plants were treated with sufficient P for optimal plant growth (high-P) and with suboptimal P levels (low-P). The high-P and low-P solutions were similar except that the high-P solution contained 200 Mm KH2PO4 and the low-P solution contained 10 Mm KH2PO4. Both high-P and low-P culture solutions contained 9.125 mM N03-N, 0.625 mM NH4+-N, 4 mm K+, 1.0 mm S, 2.5 mm Ca2+, 1.0 MM Mg2+, 250 Mm Na+, 250 gM Si, 500 Mm C1-, 50 Mm FeHEDTA, 25 Mm B, 1.0 gM Mn, 1.0 gM Zn, 0.4 gM Mo, and 0.4 uM Cu. METHODS AND MATERIALSThe presence of Si in combination with a 10-fold reduction in the Mn concentration in solution ameliorated a putative Mn toxicity symptom in both high-P and low-P plants. Gas ExchangeGas exchange analyses were performed on randomly selected high-P and low-P plants at 18 to 20 d after transplant. Fully expanded trifoliates at the second or third node numbered basipetally from the first primary trifoliate were used for both irradiance and CO2 saturation curve determinations using steady state gas exchange equipment described previously (25). Co2 concentrations of...

In earlier studies, the assimilation of selenate by plants appeared to be limited by its reduction, a step that is thought to be mediated by ATP sulfurylase. Here, the Arabidopsis APS1 gene, encoding a plastidic ATP sulfurylase, was constitutively overexpressed in Indian mustard (Brassica juncea). Compared with that in untransformed plants, the ATP sulfurylase activity was 2-to 2.5-fold higher in shoots and roots of transgenic seedlings, and 1.5-to 2-fold higher in shoots but not roots of selenate-supplied mature ATP-sulfurylaseoverexpressing (APS) plants. The APS plants showed increased selenate reduction: x-ray absorption spectroscopy showed that root and shoot tissues of mature APS plants contained mostly organic Se (possibly selenomethionine), whereas wild-type plants accumulated selenate. The APS plants were not able to reduce selenate when shoots were removed immediately before selenate was supplied. In addition, Se accumulation in APS plants was 2-to 3-fold higher in shoots and 1.5-fold higher in roots compared with wild-type plants, and Se tolerance was higher in both seedlings and mature APS plants. These studies show that ATP sulfurylase not only mediates selenate reduction in plants, but is also rate limiting for selenate uptake and assimilation.

To investigate rate-limiting factors for glutathione and phytochelatin (PC) production and the importance of these compounds for heavy metal tolerance, Indian mustard (Brassica juncea) was genetically engineered to overexpress the Escherichia coli gshI gene encoding ␥-glutamylcysteine synthetase (␥-ECS), targeted to the plastids. The ␥-ECS transgenic seedlings showed increased tolerance to Cd and had higher concentrations of PCs, ␥-GluCys, glutathione, and total non-protein thiols compared with wild-type (WT) seedlings. When tested in a hydroponic system, ␥-ECS mature plants accumulated more Cd than WT plants: shoot Cd concentrations were 40% to 90% higher. In spite of their higher tissue Cd concentration, the ␥-ECS plants grew better in the presence of Cd than WT. We conclude that overexpression of ␥-ECS increases biosynthesis of glutathione and PCs, which in turn enhances Cd tolerance and accumulation. Thus, overexpression of ␥-ECS appears to be a promising strategy for the production of plants with superior heavy metal phytoremediation capacity.Heavy metals and metalloids such as Cd, Pb, Hg, As, and Se are an increasing environmental problem worldwide. Plants can be used to remove heavy metals by accumulating, stabilizing, or biochemically transforming them. This cost-effective and environment-friendly technology has been called "phytoremediation" (Salt et al., 1995). Hyperaccumulators-plant species that accumulate extremely high concentrations of heavy metals in shoots-offer one option for the phytoremediation of metal-contaminated sites. However, hyperaccumulators tend to grow slower and produce little biomass (Brooks, 1994). An alternative approach is to genetically engineer fast-growing species to improve their metal tolerance and metal-accumulating capacity. A suitable target species for this strategy is Indian mustard (Brassica juncea), which has a large biomass production, a relatively high trace element accumulation capacity (Dushenkov et al., 1995), and can be genetically engineered .Non-protein thiols (NPTs), which contain a high percentage of Cys sulfhydryl residues in plants, play a pivotal role in heavy metal detoxification. The reduced form of glutathione (␥-Glu-Cys-Gly, GSH) is one of the most important components of NPT metabolism. GSH may play several roles in heavy metal tolerance and sequestration. It protects cells from oxidative stress damage, such as that caused by heavy metals in plants (Gallego et al