Increased lead accumulation in a single gene mutant of pea (Pisum sativum L.).

Abstract

Lead (Pb) is ranked number two on the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) priority list of hazardous substances, and is considered a major hazardous chemical found on 47% of the United States Environmental Protection Agency (USEPA) national priorities list sites (Hettiarachchi and Pierzynski, 2004). Lead poses a significant risk to humans, especially children. In the US alone it has been estimated that Pb poisoning affects more than 800,000 children between the ages of one and five (Pirkle et al., 1998). Soil is a main pathway of human Pb exposure (Madhavan et al., 1989; Mielke and Reagan, 1998). Pb is also one of the more persistent metals, and is estimated to have a soil retention time of 150–5,000 years (Shaw, 1990). Engineering-based technologies such as leaching with acids, excavation, and electro-physical separation for abating Pb in soils are not only costly but also harmful to the soil’s physical and chemical properties. Over the last 20 years there has been increasing interest in plant-based bioremediation or phytoremediation. Phytoremediation encompasses several strategies including phytostabilization and phytoextraction (Cunningham et al., 1995; Salt et al., 1998, Huang and Chen, 2003). Phytostabilization is the use of plants and soil amendments to reduce the intrinsic hazard of Pb-contaminated soil by reducing Pb bioavailability in the soil. Phytoextraction is the use of plants to remove Pb from contaminated soils (Huang et al., 1997a, 1997b; Lasat, 2002). Plant roots absorb Pb from soil and transport it to the shoots. Through the continued cultivation of selected plant species on Pb-contaminated soil and the harvest of shoots, the soil could be decontaminated. Since plant cultivation and harvest are less expensive than the engineering-based practices, phytoextraction represents an attractive alternative for the cleanup of Pb-contaminated soils. The success of Pb phytoextraction depends on two key factors: Pb bioavailability in soils, and the plant’s ability to absorb Pb from soils to roots and translocate it from roots to shoots. A plant capable of accumulating Pb at a concentration of 1,000 mg kg or higher in shoots is considered to be a Pb hyperaccumulator (Baker and Brooks, 1989; Brooks, 1998; Lasat, 2002). Approximately 400 plant species were reported to hyperaccumulate metals (Baker and Brooks, 1989; Brooks, 1998), however, there is no known Pb hyperaccumulator (Lasat, 2002). A possible reason for the lack of naturally occurring Pb hyperaccumulators could be that Pb occurs largely in insoluble precipitates such as phosphates, carbonates, and hydroxyloxides, which are unavailable to plant roots (Hettiarachchi and Pierzynski, 2004). Thus, plants may not have developed effective mechanisms for Pb absorption. Using Arabidopsis thaliana as a model system, Chen et al. (1997) screened ethyl methanesulfonate (EMS) mutated populations for Pb hyperaccumulators. Results showed that selected mutants not only accumulated Pb but also other metals such as Ca, Fe, Mg, Mn, and Zn. It was postulated that a mutant, after losing its selectivity for a particular metal, might accumulate other metals (Chen et al., 1997). In a study of Fe accumulation in the ‘bronze’ mutant, E107 (brz brz), of pea (Pisum sativum L.), Welch and LaRue (1990) found that extremely high concentrations of Fe as well as high levels of Mn, Cu, Ca, and Mg were accumulated in shoots. The increased Fe accumulation J. Chen (&) University of Florida, IFAS, Mid-Florida Research and Education Center, 2725 Binion Road, Apopka, FL 32703, USA e-mail: jjchen@ifas.ufl.edu