Silica in plants: biological, biochemical and chemical studies

Abstract

Background: The incorporation of silica within the plant cell wall has been well documented by botanists and materials scientists; however, the means by which plants are able to transport silicon and control its polymerization, together with the roles of silica in situ, are not fully understood.

Recent progress: Recent studies into the mechanisms by which silicification proceeds have identified the following: an energy-dependent Si transporter; Si as a biologically active element triggering natural defence mechanisms; and the means by which abiotic toxicities are alleviated by silica. A full understanding of silica formation in vivo still requires an elucidation of the role played by the environment in which silica formation occurs. Results from in-vitro studies of the effects of cell-wall components associated with polymerized silica on mineral formation illustrate the interactions occurring between the biomolecules and silica, and the effects their presence has on the mineralized structures so formed.

Scope: This Botanical Briefing describes the uptake, storage and function of Si, and discusses the role biomolecules play when incorporated into model systems of silica polymerization as well as future directions for research in this field.

Fig. 1.

The polymerization of monomeric silicic acid to form larger silica particles proceeds though various condensation reactions with dimers, oligomers and aggregates as intermediates.

Fig. 2.

Electron micrographs of silica structures from plants. (A) Gel-like (left) and globular silica (right) from the early-diverging plant Equisetum arvense at very early stages of development of the silica structures. (B) Stomata of mature E. arvense surrounded by pilulae encrusted with rosettes; the observed specimens contain 0·1 % w/w C, with the remainder being silica; left, upper surface; right, lower surface. (C) Inorganic material from acid-digested Cucurbita (marrow) leaves (right-hand image shows the Si elemental map).