Mycorrhizal and non-mycorrhizal Medicago truncatula roots exhibit differentially regulated NADPH oxidase and antioxidant response under Pb stress

The regulation of reactive oxygen species (ROS) generation and ROS scavenging is essential to understand the redox balance state under heavy metal stress. In order to figure out the effect of arbuscular mycorrhizal (AM) fungus on ROS generation and ROS scavenging ability under Pb stress, the ROS content, NADPH oxidase activity, the expression levels of MtRbohs, and the antioxidant response were evaluated in AM roots under Pb stress. AM inoculation decreased the malondialdehyde content by 18%-38% and decreased the O-2(center dot-) production rate by 20%-38% compared with non-mycorrhizal (NM) roots. The water-soluble Pb complex content was decreased by AM inoculation. Without Pb stress, the H2O2 content and NADPH oxidase activity in roots were increased by AM inoculation by 4-fold and 1-fold, respectively. Pb stress decreased H2O2 content by 12%-39% and decreased NADPH oxidase activity by 14%-43% in AM roots. When exposed to Pb stress, MtRbohC-G were down-regulated in AM roots, but MtRbohA-G were up-regulated in NM roots. AM inoculation increased the glutathione content in roots by 2-fold, 2-fold, and 3-fold under 0, 500, and 1000 mg kg(-1) Pb treatment, respectively. The antioxidant enzyme activity was increased 43%-322% by AM inoculation under 1000 mg kg(-1) Pb treatment. These results reveal that AM inoculation might maintain ROS homeostasis by decreasing Pb induced ROS, by decreasing the NADPH oxidase-mediated H2O2 generation, and by increasing the antioxidant response under Pb stress.