Role of arbuscular mycorrhizal fungi (Rhizophagus irregularis) on mercury tolerance of Medicago truncatula in relation to mercury and zinc concentration

Abstract

Mercury (Hg) pollution of soils is a critical environmental problem. To rehabilitate Hg contaminated soils, arbuscular mycorrhizal fungi (AMF)-based phytoremediation may be supportive, yet the functional potential of AMF in response to Hg exposure is unclear. In a greenhouse experiment, we assessed the response of Medicago truncatula (biomass, Hg tolerance index (TI), Hg partitioning) to different Hg concentrations [0 (Hg0), 25 (Hg25), 50 (Hg50) µg g-1] in treatments with (AM) and without (NM) inoculation of the AMF Rhizophagus irregularis. Additionally, zinc (Zn) uptake and the expression of two Zn transporter genes (MtZIP2, MTZIP6) were examined, because Hg and Zn share the same outer electronic configuration, inferring a potential competition for the same transporters. Although AM plants revealed lower biomass than NM plants, they showed a higher Hg TI. Plant roots were identified as dominant Hg reservoirs. At Hg25, R. irregularis decreased the Hg translocation from roots to stems, while Hg translocation was increased at Hg50. Hg in leaves originated mainly from atmospheric uptake. A lower Hg concentration in leaves of AM than NM plants was found, indicating a regulatory effect of R. irregularis on stomata functioning. The negative relationship between Hg and Zn concentrations in the roots of AM and NM plants implied a potential competition for the same transporters, although the expression of Zn transporters was upregulated by AMF inoculation at all Hg levels. In conclusion, this baseline study demonstrated that R. irregularis contributed to Hg tolerance of M. truncatula, suggesting the potential of R. irregularis for Hg-contaminated phytoremediation.