Silicon mediated redox homeostasis in the root-apex transition zone of rice plays a key role in aluminum tolerance

The supply of silicon (Si) mitigates the aluminum (Al) toxicity on plant root growth, while the underlying mechanism remains unknown. Transition zone (TZ) emerges as the Al-toxicity target of plant root apex. The objective of the study was to evaluate the effect of Si on redox homeostasis in root-apex TZ of rice seedlings under Al stress. Si alleviated Al toxicity as revealed by promotion of root elongation and less Al accumulation. In Si-deprived plants, treatment with Al altered the normal distribution of superoxide anion (O2·-) and hydrogen peroxide (H2O2) in root tip. Al induced a significant increase of reactive oxygen species (ROS) in root-apex TZ, resulting in the peroxidation of membrane lipid and loss of plasma membrane integrity in root-apex TZ. However, Si greatly increased the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and enzymes involved in ascorbate-glutathione (AsA-GSH) cycle in root-apex TZ under Al stress, and enhanced AsA and GSH contents, which reduced ROS and callose contents, thereby reducing malondialdehyde (MDA) content and Evans blue uptake. These results allow to precise the changes of ROS in root-apex TZ after exposure to Al, and the positive role of Si in maintaining redox balance in root-apex TZ.