Transgenic Indian Mustard Overexpressing Selenocysteine
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Environ. Sci. Technol. 2007, 41, 599-605 With irrigation of soils in these regions, Se and other salts Transgenic Indian Mustard are leached into the subsurface drainage water, which may Overexpressing Selenocysteine be channeled and stored in evaporation ponds. The bioac- cumulation of Se in drainage water and its subsequent Lyase or Selenocysteine accumulation in surface waters represents a serious envi- ronmental hazard (as typified by the environmental disaster Methyltransferase Exhibit Enhanced which occurred at the Kesterson Reservoir in the San Joaquin Valley in California in the 1980s) (2, 3). An attempt was made Potential for Selenium to deal with the drainage water problem by channeling the Se- and salt-laden drainage water through the San Luis Drain Phytoremediation under Field (SLD), small sloughs, and the San Joaquin River toward the Conditions San Francisco Bay Delta (4). However, the final construction and use of the SLD was abandoned because of objections raised by environmentalists and by state and federal agencies. | GARY BAN˜ UELOS,² DANIKA L. LEDUC,³, This, in turn, created a new problem of how to deal with the § ELIZABETH A. H. PILON-SMITS, AND large quantities of Se-, salt-, and boron-contaminated ,³ NORMAN TERRY* sediment left in the drain after it was abandoned (5). ARS-USDA, 9611 South Riverbend Avenue, When compared with other methods of dealing with Se- Parlier, California 93648, Department of Plant and Microbial contaminated sediments such as soil washing, excavation, Biology, 111 Koshland Hall, University of California, and reburial of the contaminated sediment (6), phytoreme- Berkeley, California 94720-3102, and Biology Department, diation offers a much less expensive alternative to managing Colorado State University, Anatomy/Zoology Building, Se in the drainage sediments. Plants can be used to remove Fort Collins, Colorado 80523 Se from soil by absorbing and transporting it to above-ground tissues that are then harvested and disposed (7-9), as well as volatilization of Se by plants (10) and/or microbes (11), Two new transgenic Indian mustard [Brassica juncea (L.) to the atmosphere. However, in drainage sediments, the high Czern.] lines were tested under field conditions for salt and boron (B) levels that are present along with the excessive Se create such harsh growing conditions that the their ability to accumulate selenium (Se) from Se- and boron- uptake and volatilization of Se for many plant species is contaminated saline sediment. The transgenic lines impaired (7, 9, 12, 13). Even under less adverse growing overexpress genes encoding the enzymes selenocysteine conditions, the effective use of phytoremediation as a lyase (cpSL) and selenocysteine methyltransferase technology for the cleanup of Se is limited by the slow rate (SMT), respectively. In the first Spring planting, cpSL, of the biological processes involved. SMT, and wildtype plants (WT) were compared, while SMT Because there is a need to develop plants that can remove and WT were compared in a second, Fall planting. In Se from soil at much faster rates than are presently observed, the Spring planting, shoots of the cpSL transgenic plants we began a program of genetic engineering to increase the accumulated 2-fold more Se (p < 0.01), had 1.8 times higher efficiency of Se phytoremediation. Our goal was to generate leaf Se concentrations (p < 0.01), and grew better on plants with enhanced abilities for Se uptake and volatilization contaminated soil than WT. The SMT plants had a 1.7- (14). This research led to the first successful field trial using transgenic plants for phytoremediation (12). In this study, fold higher leaf Se concentration than WT (p < 0.05). In transgenic Indian mustard (Brassica juncea L. Czern) lines the Fall planting, the SMT transgenic plants accumulated 1.6- that were developed for accumulating Se and heavy metals fold more Se in their shoots than WT (p < 0.01) with Se (described in full by Pilon-Smits et al. (15) and Zhu et al. (16, concentrations being higher in both leaves and stems. These 17)) were tested in the field by growing them in poor-quality results conclusively demonstrate that cpSL and SMT drainage sediment. The transgenic B. juncea lines, which transgenic lines have significantly greater Se phytoremediation overexpressed genes encoding the enzymes adenosine potential than wildtype Indian mustard. Further, this triphosphate sulfurylase (APS), γ-glutamyl-cysteine syn- study confirms the importance of field testing for evaluating thetase (ECS), and glutathione synthetase (GS), accumulated future transgenic lines. 4.3-, 2.8-, and 2.3-fold more Se in their leaves than wildtype plants, respectively (p < 0.05). The research program on the genetic engineering of Indian mustard has been continued with the development of two Introduction new lines, selenocysteine lyase (cpSL, 18) and selenocysteine Selenium (Se) contamination of irrigation drainage water methyltransferase (SMT, 19), respectively (Figure 1). The cpSL represents one of the most serious problems confronting transgenic plants were developed in order to diminish the agriculture in Se-rich regions of the Western United States toxic, nonspecific incorporation of selenocysteine into (e.g., California and Colorado) and other parts of the world proteins. This is achieved by virtue of the fact that seleno- with similar environmental and geological formations (1). cysteine lyase breaks down selenocysteine into elemental Se and alanine (Figure 1). The mouse SL enzyme was chosen * Corresponding author phone: (510) 642-3510; fax: (510) 642- because of its reported high activity toward SeCys but 4995; e-mail: [email protected]. negligible activity toward cysteine (20). Therefore, by intro- ² ARS-USDA. ducing the mouse SL gene in the plant it was hoped to ³ University of California. manipulate Se fluxes without interfering with S metabolism. § Colorado State University. | Present address: Department of Chemistry and Biochemistry, The SMT transgenic plants (which overexpress a gene cloned California State University, East Bay, 25800 Carlos Bee Blvd., Hayward, from a Se hyperaccumulator, Astragalus bisulcatus) were CA, 94542-3087. developed to generate fast-growing Indian mustards with 10.1021/es061152i CCC: $37.00 2007 American Chemical Society VOL. 41, NO. 2, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 599 Published on Web 12/06/2006 Central Regional Plant Introduction Station, Ames, IA, and propagated. The transgenic lines were obtained by trans- formation of this genotype with different DNA constructs. The (homozygous) transgenic lines were tested only under laboratory conditions and selected previously for high expression of the introduced gene. Transgenic B. juncea cpSL plants were obtained by overexpression of the mouse (Mus musculus) selenocysteine lyase (SL) cDNA fused to the transit sequence of ferredoxin from Silene pratensis, under the control of the constitutive CaMV 35S promoter, as described by Pilon et al. (20) and Garifullina et al. (18). Transgenic SMT plants were obtained as described by LeDuc et al. (19)by expression of the Astragalus bisulcatus SMT gene (encoding the enzyme selenocysteine methyltransferase), driven by the 35S CaMV promoter. For this field study, one representative line was chosen for each type of transgenic (SMT 3-11 and cpSL C3), as well as untransformed B. juncea. Plant Growth Conditions. Transgenic and wildtype plants were planted during two different growing seasons (Spring vs Fall) at two different locations on the sediment field plot during 2004 on the USDA-ARS Research Facility in Parlier, CA. For the Spring planting, the following lines were FIGURE 1. Simplified schematic overview of selenium assimilation evaluated: untransformed B. juncea (WT) and B. juncea in plants. The inorganic forms of selenium, selenate and selenite, are reduced to selenocysteine. Overexpression of SMT results in overexpressing selenocysteine methyltransferase (SMT) and the methylation of selenocysteine. Overexpression of cpSL converts chloroplastic selenocysteine lyase (cpSL). For the Fall plant- selenocysteine into elemental selenium and alanine. Other metabolic ing, the SMT and WT lines were evaluated. The plants were fates of selenocysteine are the production of selenomethionine initially grown in germination trays under controlled green- and the eventual conversion to the substantially less toxic, volatile house conditions at 24 ( 2 °C with an average photosynthetic - - dimethylselenide or the highly toxic (nonspecific) incorporation of flux of approximately 450 µmol m 2 s 1. The plantlets were selenocysteine into proteins (not shown). Overexpression of cpSL lightly fertilized with a non-sulfur-containing ammonium and SMT were two strategies used to outcompete the latter, toxic nitrate fertilizer (15-15-15) and hardened by increasing the process. temperature to 32 ( 2 °C during both growing seasons and by reducing the amount of irrigation water. For the Spring the ability to hyperaccumulate Se by methylating seleno- and Fall plantings, 25 d old plants were ready to transplant cysteine into nontoxic methylselenocysteine (Figure 1, 19). from the greenhouse to the sediment and control field plots In the present study, the cpSL and SMT transgenics were in May and in September 2004, respectively. Two beds were tested for the first time in the field to assess the potential randomly selected from the sediment and control plots, ability of these newly obtained transgenic Indian mustard respectively, for each growing season.