Hydrogen peroxide modulates lignin and silica deposits in sorghum roots

Abstract

Silica aggregates in the root endodermis of grasses. Availability of Si to roots is associated with variations in the balance of reactive oxygen species (ROS) and increased resistance to drought, Striga, salt and heavy metals. 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 and specifically hydrogen peroxide (H2O2). Sorghum seedlings were grown hydroponically under varied Si and oxidative levels. Lignin and silica deposits in the endodermis were studied by optical, scanning electron, and Raman microscopies. Cell wall composition was quantified by thermal gravimetric analysis. Silica aggregation was catalyzed by lignin modified by carbonyls. These residues were available for silica nucleation only within 2 hours of their deposition. The endodermal H2O2 concentration regulated the intensity but not the pattern of ASZ lignin deposits. Our results imply that localized secretion of modified monolignols is necessary for patterning ASZ lignin. Fast silica aggregation may locally inhibit lignin polymerization and transiently increase H2O2 levels. Such momentary local high ROS concentrations may convey Si-induced stress tolerance in the root.