Transgenic Citrate-Producing Plants for Lead Phytoremediation

In 1991, the U.S. Secretary of Health and Human Services called lead ¿the number one environmental threat to the health of children in the United States.¿ Lead exposure can cause premature birth and impair a child¿s mental and physical development. In adults, lead exposure can cause kidney damage, high blood pressure, and other problems. The U.S. Environmental Protection Agency estimates that 12,000,000 homes exceed the new 400 ppm standard for soil lead in play areas. Soil lead at small arms firing ranges, manufacturing plants, and other government and industrial sites poses similar challenges.

A promising alternative to excavation and replacement of lead-contaminated soil is phytoremediation, whereby living plants remove lead from firing ranges, industrial sites, and residences. Phytoremediation relies on crop species and chelators that facilitate higher plant uptake rates. The annual cost of chelators can range up to $20,000 per acre. Furthermore, sites with sandy, well-drained soil may need a water-impermeable liner to prevent slowly degrading chelators such as ethylenediamine tetraacetic acid from leaching metals into groundwater. Costs of a liner can double the total cost of phytoextraction, rendering phytoremediation impractical for certain sites.

In this Phase II project, Edenspace Systems Corporation seeks to demonstrate the use of transgenic plants that exude significant amounts of a rapidly biodegradable chelator¿citric acid¿from their roots, enabling a cost savings of more than 70 percent in lead phytoextraction. A citrate synthase (CS) transgene was placed under the control of different plant promoters to overexpress the gene in roots. In Phase I, 100 lines of CS-transgenic tobacco, Nicotiana tabacum, were created and tested. Preliminary results with five hemizygous strains are promising, suggesting that higher-performing homozygous strains created at the end of Phase I may hyperaccumulate lead from soil (bioconcentration factor >1). Significantly, little soluble lead remained in the soil, indicating that the plants may successfully address leaching concerns. In Phase II, the highest performing transgenic lines will be demonstrated at a small arms firing range in Maryland and a residential site in Massachusetts. Phase II objectives also include concentrating lead in harvested plants to facilitate disposal or recycling, and mating transgenic lines of a proven lead-accumulating plant species, Brassica juncea.

At many sites, applying rapidly biodegradable citric acid precisely at the root/soil interface where metal uptake occurs may eliminate the need for expensive chelating agents and liners. Attainment of the project¿s goals therefore may reduce the substantial public health hazard of soil lead by realizing phytoremediation¿s low-cost potential.