Plants are excellent in situ bioremediators for complex waste sites such as mine tailings, landfills, and dredged aquatic sediments (Bert et al. 2009). Roots penetrate the soil15 and extract toxic heavy metals and organic compounds. Contaminants are either sequestered in the plant tissues, or degraded to less toxic organic compounds. Plant “sponges” are removed from the site and destroyed. Activities that indirectly promote remediation include the stabilization of soils (i.e., minimizing pollutant dispersal) and the enrichment of diverse and metabolically active bacterial communities in the rhizosphere.

Some 350 plants naturally absorb toxic chemicals (Thieman and Palladino 2012). Fast-growing woody plants (e.g., poplar and juniper) and high biomass crop plants (e.g., alfalfa) are prime candidates (Thieman and Palladino 2012). In one study, wild rice, alfalfa, ryegrass, and tall fescue removed PCBs from an electronics dump site in China (Shen et al. 2009). Sunflowers, shrub tobacco, poplars, and Indian mustard are all effective phytoremediators of heavy metals (Hur et al. 2011).

Natural phytoremediation is limited by slow and sometimes incomplete pollutant detoxification (Hur et al. 2011). Thus, transgenic plants have been created to enhance or confer the phytoremediation properties of plants. Transgenic plants overexpress transporters or catabolic enzymes native to the plant or carry transgenes from bacteria (Hur et al. 2011). For example, yellow poplars that expresses merA (encoding a bacterial mercury ion reductase) detoxify mercury ten times faster than non-GM controls (Rugh et al. 1998). This translates into the same amount of remediation in half the time16 (Hur et al. 2011). Thanks to rapid advances in CRISPR/Cas9, we are likely to see many more transgenic phytoremediators developed (Mosa et al. 2016). There is already one report of a phytoremediating CRISPR/Cas9 transgenic poplar.

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