Transport of Gold Nanoparticles through Plasmodesmata and Precipitation of Gold Ions in Woody Poplar

Poplar plants (Populus deltoides x nigra, DN-34) were used as a model to explore vegetative uptake of commercially available gold nanoparticles (AuNPs) and their subsequent translocation and transport into plant cells. AuNPs were directly taken up and translocated from hydroponic solution to poplar roots, stems, and leaves. Total gold concentrations in leaves of plants treated with 15, 25, and 50 nm AuNPs at exposure concentrations of 498 +/- 50.5, 247 +/- 94.5, and 263 +/- 157 ng/mL in solutions were 0.023 +/- 0.006, 0.0218 +/- 0.004, and 0.005 +/- 0.0003 mu g/g of dry weight, respectively, which accounted for 0.05, 0.10, and 0.03%, respectively, of the total gold mass added. The presence of total gold in plant tissues was measured by inductively coupled plasma mass spectrometry, while AuNPs were observed by transmission electron microscopy in plant tissues. In solution, AuNPs were distinguished from Au(III) ions by membrane separation and centrifugation. AuNPs behaved conservatively inside the plants and were not dissolved into gold ions. On the other hand, Au(III) ions were taken up and reduced into AuNPs inside whole plants. AuNPs were observed in the cytoplasm and various organelles of root and leaf cells. A distinct change in color from yellow to pink was observed as Au(III) ions were reduced and precipitated in a hydroponic solution. The accumulation of AuNPs in the plasmodesma of the phloem complex in root cells clearly suggests ease of transport between cells and translocation throughout the whole plant, inferring the potential for entry and transfer in food webs.