New insights into the formation of silica aggregates in sorghum roots

Even though silica and lignin can provide comparable stiffness, they cannot be considered as structural equivalents due to contrasting hydrophobicity.
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Silica deposition in the cell walls of grasses improves their mechanical properties and helps plants to withstand various stress conditions. Silicification is often viewed as a structural equivalent to lignification, providing similar strength while requiring lower metabolic costs. Its mechanism is still not understood and silica–cell wall interactions are elusive.

Soukup et al. investigate the formation of silica aggregates in Sorghum bicolor roots, the role of cell wall components in their formation, and the relationship between silica and lignin. The study reports that active uptake of silicic acid is provided at the root apex, where silicon transporters Lsi1 and Lsi2 are expressed. The locations of silica aggregation are established during the development of tertiary endodermal cell walls, even in the absence of silicon. Silica aggregation takes place in non-lignified spots in the endodermal cell walls, which progressively accumulate silicic acid, and its condensation initiates at arabinoxylan–ferulic acid complexes.

Remains of sorghum primary roots after mineralization with sulphuric acid.
Remains of sorghum primary roots after mineralization with sulphuric acid.

The authors propose a model in which the formation of silica aggregates in sorghum roots is predetermined by a modified cell wall architecture and takes place as governed by endodermal development. The interaction with silica is provided by arabinoxylan–ferulic acid complexes and interferes with further deposition of lignin. However, as silica and lignin exhibit contrasting hydrophobicity, they cannot be considered as equivalent cell wall modifications.

Reference

Soukup, M., Martinka, M., Bosnić, D., Čaplovičová, M., Elbaum, R., & Lux, A. (2017). Formation of silica aggregates in sorghum root endodermis is predetermined by cell wall architecture and development. Annals of Botany, 120(5), 739–753. https://doi.org/10.1093/aob/mcx060


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