Hydrogen peroxide modulates silica deposits in sorghum roots.

Hydrated silica (SiO2·nH2O) aggregates in the root endodermis of grasses. Application of soluble silicates (Si) to roots is associated with variations in the balance of reactive oxygen species (ROS), increased tolerance to a broad range of stresses affecting ROS levels, and early lignin deposition. In sorghum (Sorghum bicolor L.), silica aggregation is patterned in an active silicification zone (ASZ) by a special type of aromatic material with a signature typical to lignin. Since lignin polymerization is mediated by ROS, we studied the formation of root lignin and silica under varied conditions of ROS by modulating hydrogen peroxide (H2O2) concentration in the growth medium. Sorghum seedlings were grown hydroponically and supplemented with Si, H2O2, and KI, a salt that catalyzes H2O2 decomposition. Lignin and silica deposits in the endodermis were studied by histology, scanning electron and Raman microscopies. Cell wall composition was quantified by thermal gravimetric analysis. The endodermal H2O2 concentration correlated to the extent of the aromatic polymer deposition along the root, but did not affect its patterning in spots. Our results show that the ASZ spots were necessary for root silica aggregation, and suggest that silicification is intensified under oxidative stress as a result of increased ASZ lignin-like deposition.