Structural basis for a root silicon-based barrier against cadmium, chromium, and salt stress in plants

Silicon (Si) forms a defensive barrier in roots of Si-accumulators such as rice, blocking the absorption of toxic metals and salt, but many dicotyledons do not accumulate Si, limiting the Si application in agriculture. This may be related to differences in their cell walls: mixed-linkage glucan (MLG) exists in cell walls of monocotyledons but is absent in dicotyledons. In vivo and in vitro experiments, material characterization, and genetic modification approaches were employed to investigate the structural roles of MLG in root Si barriers and its effects on plant stress resistance. Both the roots of Si transporter Lsi2 (low silicon rice 2) defective rice mutants and dead rice continued to accumulate Si, showing root Si deposition is controlled by cell wall components, but not transporters. MLG-induced Si polymerization in vitro and CslF6 (encoding an MLG synthase) controlled root MLG and Si concentrations. Overexpression of rice CslF6 in Arabidopsis, an Si non-accumulator, enabled them to form Si barriers, enhancing resistance to cadmium, chromium, and salt stress. These findings suggest that MLG serves as a structural basis for root Si barriers and that genetic modification combined with Si supply is a promising strategy to endow Si non-accumulators with Si-mediated stress resistance.