Hydrogen peroxide modulates lignin and silica deposits in sorghum roots

Hydrated silica (SiO2{middle dot}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 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 solution. 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. We found that the endodermal H2O2 concentration regulated the extent of ASZ lignin deposition along the root, but not its patterning in spots. Our results show that ASZ lignin is necessary for root silica aggregation in sorghum, and that silicification is enhanced under oxidative stress as a result of increased deposition of the ASZ lignin. One sentence summaryLignin with carbonyl modifications is patterned by the activity of H2O2 to nucleate silica aggregations in sorghum roots.