Chromium immobilization by extraradical mycelium of arbuscular mycorrhiza contributes to plant chromium tolerance

• P supply cannot improve plant Cr tolerance as AM symbiosis does.
• AM symbiosis can restrain Cr translocation from roots to shoots.
• Cr can be taken up by extraradical mycelium but immobilized in mycorrhizal roots.

Arbuscular mycorrhizal (AM) fungi, as important plant mutualists, can protect host plants against environmental stresses, including heavy metal contaminations. It is generally accepted that improvement of plant P nutrition by AM symbiosis plays an important role in plant tolerance to heavy metals. In the present study, we tested if exogenous P amendment to the chromium (Cr) contaminated soil could match the positive effects of AM symbiosis on plant Cr tolerance for the highly mycorrhizal dependent plant—dandelion (Taraxacum platypecidum Diels.). Experimental results showed that P addition could not enhance plant growth as well as AM symbiosis did. AM fungi could immobilize Cr in mycorrhizal roots besides enhancing plant P acquisition. Cr distribution pattern in principal roots as revealed by synchrotron radiation micro-focused X-ray fluorescence (SR μ-XRF) analysis supported the stabilization of Cr in mycorrrihzal roots. Furthermore, by using a three-compartment cultivation system, we demonstrated that extraradical mycelium (ERM) could take up and transport Cr to mycorrhizal roots, but restrained Cr translocation from roots to shoots, and thus contributed to Cr immobilization in roots and relieved Cr phytotoxicity.