Chromium detoxification in arbuscular mycorrhizal symbiosis mediated by sulfur uptake and metabolism

It has been well demonstrated that arbuscular mycorrhizal (AM) symbiosis can improve plant chromium (Cr) tolerance, but the detoxification mechanisms are still unsolved. In the present study the role of AM symbiosis in plant sulfur (S) acquisition and metabolism related to Cr detoxification was investigated. Medicago truncatula plants were cultivated with/without AM fungus Rhizophagus irregularis in Cr(VI) contaminated soils at two soil S and phosphorus (P) concentrations. Sulfur uptake, transport and metabolism by plants were determined by biochemical and molecular methods. Meanwhile, Cr uptake and transformation were characterized by using spectroscopic methods, including synchrotron based X-ray absorption near edge structure (XANES) analysis. The results indicated that mycorrhizal inoculation improved plant S acquisition by up-regulating the expression of high affinity sulfate transporter genes (i.e. MtSULTR1.1, MtSULTR1.2) in plant roots. Mycorrhizal colonization also systematically regulated S transport and metabolism for Cr(VI) detoxification. XANES analysis revealed the Cr immobilization in AM roots by phosphate and histidine-like ligands rather than thiol groups. Correlation analysis further confirmed possible involvement of S metabolites in the relief of Cr induced oxidative stress. Mycorrhizal effects were more pronounced under low P condition. The study allowed a new insight into the mechanisms of enhanced plant Cr tolerance by AM symbiosis.