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.

SummaryIntracellular malaria parasites require lipids for growth and replication. They possess a prokaryotic type II fatty acid synthesis (FAS II) pathway that localizes to the apicoplast plastid organelle and is assumed to be necessary for pathogenic blood stage replication. However, the importance of FAS II throughout the complex parasite life cycle remains unknown. We show in a rodent malaria model that FAS II enzymes localize to the sporozoite and liver stage apicoplast. Targeted deletion of FabB/F, a critical enzyme in fatty acid synthesis, did not affect parasite blood stage replication, mosquito stage development and initial infection in the liver. This was confirmed by knockout of FabZ, another critical FAS II enzyme. However, FAS II-deficient Plasmodium yoelii liver stages failed to form exo-erythrocytic merozoites, the invasive stage that first initiates blood stage infection. Furthermore, deletion of FabI in the human malaria parasite Plasmodium falciparum did not show a reduction in asexual blood stage replication in vitro. Malaria parasites therefore depend on the intrinsic FAS II pathway only at one specific life cycle transition point, from liver to blood.

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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

Irradiation-attenuated sporozoite vaccinations confer sterile protection against malaria infection in animal models and humans. Persistent, nonreplicating parasite forms in the liver are presumably necessary for the maintenance of sterile immunity. A novel vaccine approach uses genetically attenuated parasites (GAPs) that undergo arrested development during liver infection. The fate of GAPs after immunization, their persistence in vaccinated animals, and the immune mechanisms that mediate protection are unknown. To examine the developmental defects of genetically attenuated liver stages in vivo, we created deletions of the UIS3 and UIS4 loci in the Plasmodium yoelii rodent malaria model (Pyuis3[-] and Pyuis4[-]). The low 50% infectious dose of P. yoelii in BALB/c mice provides the most sensitive infectivity model. We show that P. yoelii GAPs reach the liver, invade hepatocytes, and develop a parasitophorous vacuole but do not significantly persist 40 h after infection. A single dose of Pyuis4(-) sporozoites conferred complete protection, but full protection by Pyuis3(-) sporozoites required at least 2 immunizations. CD8(+) T cells were essential for protection, but CD4(+) T cells were not. Our results show that genetically distinct GAPs confer different degrees of protective efficacy and that live vaccine persistence in the liver is not necessary to sustain long-lasting protection. These findings have important implications for the development of a P. falciparum GAP malaria vaccine.

A major goal of phytoremediation is to transform fast-growing plants with genes from plant species that hyperaccumulate toxic trace elements. We overexpressed the gene encoding selenocysteine methyltransferase (SMT) from the selenium (Se) hyperaccumulator Astragalus bisulcatus in Arabidopsis and Indian mustard (Brassica juncea). SMT detoxifies selenocysteine by methylating it to methylselenocysteine, a nonprotein amino acid, thereby diminishing the toxic misincorporation of Se into protein. Our Indian mustard transgenic plants accumulated more Se in the form of methylselenocysteine than the wild type. SMT transgenic seedlings tolerated Se, particularly selenite, significantly better than the wild type, producing 3-to 7-fold greater biomass and 3-fold longer root lengths. Moreover, SMT plants had significantly increased Se accumulation and volatilization. This is the first study, to our knowledge, in which a fast-growing plant was genetically engineered to overexpress a gene from a hyperaccumulator in order to increase phytoremediation potential.Although selenium (Se) is a necessary micronutrient for humans and animals at very low doses, it is extremely toxic at higher doses (Wilber, 1983). Excess Se has been implicated in birth defects, sterility, and disease in animals, fish, and wildlife and loss of hair, teeth, and nails, fatigue, and even death in humans (Moxon, 1937;Eisler, 1985;Lemly and Smith, 1987;Sorenson, 1991). Environmental Se pollution is a worldwide problem. Anthropogenic Se pollution arises from many sources, such as aqueous discharges from electric power plants, coal ash leachates, refinery effluents, and industrial wastewater (American Medical Association, 1989). Selenium also occurs naturally in soils formed from Se-bearing shales. This leads to Se-contaminated irrigation drainage water, one of the most serious agricultural problems in the western United States and other areas with similar environments and geological conditions (Presser and Ohlendorf, 1987).Cleaning up Se-contaminated soil and water is a major concern. Phytoremediation, using plants to remove, stabilize, or detoxify pollutants, is a promising technology for remediating Se-contaminated soil and water (Terry et al., 2000). In a process called phytoextraction, plants extract Se from soils and water into their tissues, which can be harvested and removed. Selenium is unusual among trace elements because it can also be removed from the ground ecosystem by phytovolatilization, i.e. plants metabolize inorganic Se to relatively nontoxic, volatile forms (dimethyl selenide [DMSe] and dimethyl diselenide [DMDSe]), which escape to the atmosphere (Lewis et al., 1966;Terry et al., 2000). Selenium phytoremediation has been achieved under field conditions using fast-growing plant species, such as Indian mustard (Brassica juncea; Bañ uelos et al., 1997), which accumulates Se to hundreds of parts per million (Bañ uelos and Schrale, 1989). Hyperaccumulating plant species, such as Astragalus bisulcatus, have adapted to seleniferous soils and accum...

Background: Plasmodium sporozoites migrate to the liver where they traverse several hepatocytes before invading the one inside which they will develop and multiply into thousands of merozoites. Although this constitutes an essential step of malaria infection, the requirements of Plasmodium parasites in liver cells and how they use the host cell for their own survival and development are poorly understood.

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