Quantum dots: A novel technique to assess plant and fungal uptake of organic N

Most fungi possess the ability to acquire organic nutrients from the soil, which is an important process given that most soil nutrients are abundant in organic rather than in inorganic form. However, arbuscular mycorrhizal (AM) fungi (root symbionts) are not known to directly contribute to the cycling of soil organic N. The prevailing paradigm is that AM fungi only take up inorganic nutrients. To test this paradigm we used a novel labeling technique—quantum dots (fluorescent nanoscale semiconductors)—to spatially track the decomposition and movement of labile amino acids (soluble glycine) and recalcitrant polysaccharides (insoluble chitosan) by non-mycorrhizal and AM fungi. We hypothesized that both non-mycorrhizal and AM fungi would take up labeled amino acids based on compatible mechanical size, but only non-mycorrhizal fungi would take up recalcitrant organic N. We conjugated quantum dots to the amino groups of glycine and chitosan and incubated them with non-mycorrhizal Penicillium fungi and annual bluegrass (Poa annua) inoculated with AM fungi. In addition, substrate-free quantum dots and quantum dot/substrate mixtures lacking the binding reagent were incubated with the fungi and bluegrass samples as experimental controls. Both AM hyphae and non-mycorrhizal hyphae absorbed the glycine- and chitosan-quantum dots after 4 hours of incubation. In contrast, bluegrass roots and shoots contained glycine-quantum dots after only 2 hrs of incubation, but did not absorb any chitosan-quantum dots until 24 hrs after incubation. The two experimental controls displayed no signs of uptake during the 24 hr duration of the experiment. This study suggests that AM fungi are capable of acquiring both labile and recalcitrant forms of organic N and possibly transferring them to the plant. This means that the role of AM fungi in the global N cycle may be much more significant than previously believed.