We used a nanotechnological technique, quantum dots (QD), to quantify the contributions of arbuscular mycorrhizal fungi (internal root symbionts) to organic N uptake by plants, and to determine the mechanisms controlling this process. We hypothesized that arbuscular mycorrhizal fungi would prefer amino acids based on their size (molecular weight), charge (isoelectric point), environmental abundance, and hydrophobicity. To test our hypotheses, we conjugated florescent QDs (~3nm diameter) to each of the 20 proteinaceous amino acids. Each treatment was incubated with aseptically grown mycorrhizal and non-mycorrhizal seedlings of Sudan grass (Sorghum bicolor). After 24 hr of incubation, QD uptake was quantified using a standard 96 well microplate reader. Mycorrhizal contributions of individual amino acids were determined using Cohen’s d effect size, where mycorrhizal uptake (QD uptake shoot^-1) was the treatment group and non-mycorrhizal uptake (QD uptake shoot^-1) was the control group.

Results/Conclusions

Overall, we found that mycorrhizal fungi significantly contributed to plant uptake of amino acids (P<0.001, based on studen'ts t), and that 71% of the observed variance can be explained by the size, charge, abundance, and hydrophobicity of individual amino acids (r² = 0.71, P<0.001 overall, based on multivariate logistics regression). Specifically, amino acid contribution by mycorrhizal fungi was significantly and negatively related to abundance (r² = 0.40, P=0.003), and significantly and positively related to charge (r² = 0.20, P=0.049). These relationships were driven by amino acid uptake by mycorrhizal plants, which seemed to favor rare (r² =0.37, P<0.001) somewhat large amino acids (r² = 0.01, P=0.041); and by non-mycorrhizal plants, which seemed to slightly prefer acidic amino acids (r² = 0.08, P=0.014). Our results suggest that arbuscular mycorrhizal fungi improve a plant’s ability to compete for soil organic N, and that organic N availability, size, and charge may be three mechanisms that control this process.