Arbuscular Mycorrhizal Fungi Promote Lead Immobilization by Increasing the Polysaccharide Content Within Pectin and Inducing Cell Wall Peroxidase Activity

The mechanism by which arbuscular mycorrhizal (AM) fungi immobilize lead (Pb) within the cell wall is unclear. Therefore, the aim of this study was to investigate the mechanism by which AM fungi immobilize Pb within the cell wall by measuring the Pb content in the cell wall, the polysaccharide and the uronic acid contents of different cell wall fractions, and the activity of cell wall peroxidase. Mycorrhizal-associated Medicago truncatula had higher shoot and root biomass than nonmycorrhizal-associated M. truncatula. AM inoculation increased the content of Pb in the cell wall under Pb stress. The polysaccharide content in the pectin and hemicellulose fractions were increased by AM inoculation with or without Pb stress. In AM-associated roots, the cell wall peroxidase activity increased in response to Pb stress. However, Pb stress did not affect the cell wall peroxidase activity in non-AM-associated roots. Correlation analysis suggested that MtPrx05 and MtPrx10 may participate in polysaccharide cross-linking and cell wall stiffening. The Pb stress resistance mechanism of AM-associated roots may involve cell wall stiffening. Taken together, the results show that AM inoculation may improve host plant growth and increase Pb immobilization in the cell wall by increasing the polysaccharide content within pectin and hemicellulose and by inducing cell wall peroxidase activity. Both the polysaccharide composition and cell wall peroxidase have important contributions to the resistance of mycorrhizal-associated plants.