Rhizophagus irregularis regulates antioxidant activity and gene expression under cadmium toxicity in Medicago sativa

Cadmium (Cd) is a phytotoxic heavy metal (HM) that can induce generation of reactive oxygen species (ROS). Arbuscular mycorrhizal fungi (AMF) are considered as bio-ameliorators that help to mitigate HM-derived oxidative stress. The objective of this study was to assess AM fungus Rhizophagus irregularis on changes in enzymatic activity and transcription of antioxidants of Medicago sativa under Cd stress. Increased superoxide dismutase (SOD) and ascorbate peroxidase (APX) transcripts accumulation under Cd stress did not lead to improvement in their activities which could be related to inactivation of enzyme resulting from excess ROS or Cd binding to the active site. However, more pronouncedly enhanced the activity of CAT instead of SOD, APX, and GR in the roots of M. sativa under Cd stress conditions suggested that antioxidant enzymes differed in sensitivities to Cd exposure. AMF symbiosis significantly reduced transcripts abundance of MsCu/Zn SOD and MsCAT genes compared with NM plants indicating that metal sequestration within hyphal fungi probably made Cd concentration insufficient in root cells for induction of these genes. However, GR activity was not affected by its gene expression under Cd stress. This phenomenon might be attributed to AMF-mediated post-transcription regulation. The results presented here could provide an enlightenment to the capability of Cd-induced ROS-scavenging system in M. sativa colonized with R. irregularis.