ABSTRACT
In nature, arbuscular mycorrhizal fungi (AMF) play a crucial role in the root systems of plants. They can help enhance the resistance of host plants by improving the compartmentalization of toxic metal contaminants in the cell walls (CWs). However, the functions and responses of various CW subfractions to mycorrhizal colonization under Cd exposure remain unknown. Here we conducted a study to investigate how Cd is stored in the cell walls of maize roots colonized by Funneliformis mosseae. Our findings indicate that inoculating the roots with AMF significantly lowers the amount of Cd in the maize shoots (63.6 ± 6.54 mg kg-1 vs. 45.3 ± 2.19 mg kg-1, p < 0.05) by retaining more Cd in the mycorrhized roots (224.0 ± 17.13 mg kg-1 vs. 289.5 ± 8.75 mg kg-1, p < 0.01). This reduces the adverse effects of excessive Cd on the maize plant. Additional research on the subcellular distribution of Cd showed that AMF colonization significantly improves the compartmentalization of 88.2% of Cd in the cell walls of maize roots, compared to the 80.8% of Cd associated with cell walls in the non-mycorrhizal controls. We observed that the presence of AMF did not increase the amount of Cd in pectin, a primary binding site for cell walls; however, it significantly enhanced the content of lignin and the proportion of Cd in the total root cell walls. This finding is consistent with the increased activity of lignin-related enzymes, such as PAL, 4CL, and laccase, which were also positively impacted by mycorrhizal colonization. Fourier transform infrared spectroscopy (FTIR) results revealed that AMF increased the number and types of functional groups, including -OH/-NH and carboxylate, which chelate Cd in the lignin. Our research shows that AMF can improve the ability of maize plants to tolerate Cd by reducing the amount of Cd transferred from the roots to the shoots. This is achieved by increasing the amount of lignin in the cell walls, which binds with Cd and prevents it from moving through the plant. This is accomplished by activating enzymes related to lignin synthesis and increasing the exposure of Cd-binding functional groups of lignin. However, more direct evidence on the immobilization of Cd in the mycorrhiza-altered cell wall subfractions is needed.
ABSTRACT
Populus yunnanensis Dode, a facultative metallophytic poplar, exhibits afforestation potential in barren mine tailing areas. However, the interactions and functional roles of arbuscular mycorrhizal fungus (AMF) in P. yunnanensis adaptability to heavy metal stress remain unclear. Physiological and molecular responses of P. yunnanensis plantlets to AMF (Funneliformis mosseae) under cadmium (Cd) stress (50 mg kg-1) were investigated. Results showed attenuation of Cd phytotoxicity effects on cell organelles upon AMF inoculation, which also reduced the Cd concentration in the poplar leaves, stems, and roots. Under Cd stress, AMF-blocking of metal transporter (e.g., Ca2+ channel) activity occurred, decreasing root cell Cd influx by reducing H+ efflux. Bioaugmentation of rhizosphere sediments by AMF to stabilize metals with a decreasing DTPA-extractable Cd also occurred. The AMF inoculation promoted Cd conversion into inactive, less phytotoxic forms, and helped to maintain ion homeostasis and relieve nutritional ion (e.g., Ca, Mg) disorders caused by excessive Cd. Leaf enzyme and non-enzyme antioxidant systems were triggered. Root and leaf physiological response patterns differed. The AMF regulated the poplar functional genes, and nine metal-responsive gene clusters were identified. We suggest that AMF is a functional component of P. yunnanensis phenotype extension, contributing to strong adaptability to unfavorable mine tailings conditions.