Eye on Organics

Two-photon excitation microscopy (TPEM), a fluorescence technique commonly used to image cell and tissue samples of plants and animals, has been used for the first time to visualize how organic compounds from pesticides or air pollution migrate through the cellular structure of living plants. The researchers who developed the novel application believe that it could help improve the design and use of pesticides, track the global fate of persistent organic pollutants (POPs), and design new strategies for bioremediation of contaminated soil.

In TPEM, a laser is used to excite a biological sample with two low-energy photons, which, upon combining at a focal point, have sufficient energy to induce fluorescence without damaging the prepared sample. Fluorescent dyes, green fluorescent protein tags, or other markers typically are needed to highlight specific cell structures. The detected fluorescence in turn can be used to generate an image for further study.

Chemistry Ph.D. student Edward Wild, environmental chemistry professor Kevin C. Jones, and colleagues at Lancaster University, in England, came to realize that the natural fluorescence of some cell structures and aromatic organic compounds could be visualized without the need for fluorescence markers [Environ. Sci. Technol., 38, 4195 (2004)].

Using anthracene as a model compound, they developed images that show the chemical as it migrates over a four-day period from the waxy outer surface of corn leaves to the epidermal cell wall and then into the aqueous cytoplasm of the epidermal cells. Optical filters allowed the components of the leaves and the anthracene to be visualized as different colors. Anthracene was chosen to be representative of POPs, which before now were believed to partition nearly exclusively into the lipophilic layer on leaf surfaces. The Lancaster team's results have challenged that assumption by showing that organic compounds can migrate into essentially all parts of the leaf structure.

Plants can be invaluable biomonitors, notes chemistry professor Donald Mackay of Trent University, Peterborough, Ontario. But research in this area has been plagued by a lack of fundamental understanding of the mechanisms of uptake--where POPs go, how they get there, and how fast they migrate into the plant tissues. "This new technique promises to elucidate those mechanisms," Mackay says. "We can see the journey of the POPs for the first time. This should enable more reliable models and predictive methods to be constructed."