The alleviation mechanisms of cadmium toxicity in Broussonetia papyrifera by arbuscular mycorrhizal symbiosis varied with different levels of cadmium stress.

The alleviation of cadmium (Cd) toxicity in Broussonetia papyrifera by arbuscular mycorrhizal (AM) fungi are still not completely elucidated. This study investigated the effects of Rhizophagus irregularis on physiological and biochemical characteristics, and molecular regulation in B. papyrifera under different levels of Cd (0, 30, 90 and 270 mg kg Cd) stress. Results showed that (1) AM symbiosis improved the growth and photosynthesis, enhanced ROS levels as stress signaling and maintained ROS balance under low and medium Cd stress. (2) AM symbiosis regulated AsA-GSH cycle to mitigate ROS overproduction under high Cd stress. (3) AM fungus can chelate more Cd under high Cd stress, increasing soil pH and GRSP content. (4) AM plants can fix or chelate more Cd by P in leaves and reserve more P in stems under high Cd stress. (5) AM symbioses increased root net Cd influx and uptake under medium Cd stress but inhibited under high Cd stress, with upregulation of genes related heavy metals (HMs) transport under medium Cd stress and inhibited the transcription of genes related HMs transport under high Cd stress. Therefore, the alleviation mechanisms of Cd toxicity in B. papyrifera by R. irregularis symbiosis depends on the levels of Cd stress.