Environmental ▼News Visualizing contaminants in plants
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plant.” He acknowledges that the solubility of a compound could have a big impact on where it goes, although anthracene is not particularly watersoluble by nature. Many assumptions about how plants take up and metabolize contaminants have not yet been exEDWARD WILD
esearchers in the United Kingdom have devised the first way to “see” where contaminants reside in living plants—with a little help from a nondestructive analytical technique and imaging software. In this issue of ES&T (pp 4195– 4199), the researchers show that their approach can track the movements of a persistent organic pollutant (POP)-type chemical from the waxy cuticle into the aqueous cytoplasm of leaf cells. The investigation suggests that plants may play a greater role in storage and degradation of contaminants than previously supposed. Experts believe the results from this new method could advance crop protection, food processing, and phytoremediation. “You can put the [leaf] under the microscope and see exactly where the compound is,” explains Edward Wild, the paper’s lead author and a doctoral student advised by Kevin Jones and Gareth Thomas at Lancaster University. “There is no ‘mashing up’ the sample and then analyzing it in a traditional way.” Instead, they acquired images with two-photon excitation microscopy (TPEM), a technique gaining popularity with some in biology because two photons of longer wavelength, such as near-infrared, induce fluorescence but cause less damage to the plant. To create the 3-D pictures, Wild used reconstruction software. By combining images collected at a single wavelength, the researchers watched how anthracene spread through much of a maize leaf over a four-day period. “We chose anthracene to be representative of other POPs, like PCBs and other PAH compounds, which traditionally have been thought of as substances that partition into the lipid portions of the vegetation,” says Jones. So “we were quite surprised to see the compound going into the watery parts of the
By using two-photon excitation microscopy, researchers can visualize how anthracene moves through a maize leaf. In this composite image, which is a 3-D reconstruction of 200 images, the leaf’s cuticle, cell walls, hairs, and stomata fluoresce green. The chloroplasts are orange, and the anthracene is blue dots.
plored, says Steve McCutcheon, a senior environmental engineer at the U.S. EPA’s National Exposure Research Laboratory, who saw use for the method. “Monitoring of contaminants in food stuffs and crops and even processing of plant material for food can be very much influenced by where and how the chemicals migrate and accumulate in different tissues.” He adds that the same kind of information could dictate which plants are selected for phytoremediation applications. However, because TPEM requires that the contaminant naturally fluoresces and at a different wavelength than the plant, not all contaminants can be tracked in all plants. “While the technique is limited to certain chemicals, the information gained from such model compounds will
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provide insight to atmospheric deposition onto vegetation, and likely greater understanding of how organic contaminants migrate within plants following uptake from soil in phytoremediation or crop contamination studies,” says Joel Burken, an environmental engineer specializing in the fate of organic contaminants in phytoremediation systems at the University of Missouri, Rolla. Allan Felsot, a professor of environmental toxicology at Washington State University, praises the visual nature of this innovative work but says it won’t replace other analytical techniques. He cautions that its “widespread applicability remains to be seen.” Plant physiologist Eugene Nothnagel from the University of California at Riverside agrees that the method may have limited application. Plants are designed to use light efficiently, so only very few extra wavelengths could be used without damaging a plant, and only a small number of environmental contaminants can fluoresce in these windows, he says. In addition, most plants can take up a little of everything, and a sensitive, semiquantitative technique like TPEM will find them, which complicates the data. However, he says that when applicable, “The technique gives very good information on localization of the target chemical at a subcellular level,” which would be extremely difficult to get with traditional techniques. Back in Jones’ lab, Wild is currently using the method to track pesticides and compound degradation on and within living leaves. He says very preliminary results indicate that the pesticide movement is even more rapid and diffuse than that of the anthracene. In addition, he and his colleagues are examining how roots and root hairs handle these chemicals. —RACHEL PETKEWICH © 2004 American Chemical Society
