Abstract
BackgroundThe horsetails (Equisetum sp) are known biosilicifiers though the mechanism underlying silica deposition in these plants remains largely unknown. Tissue extracts from horsetails grown hydroponically and also collected from the wild were acid-digested in a microwave oven and their silica 'skeletons' visualised using the fluor, PDMPO, and fluorescence microscopy.ResultsSilica deposits were observed in all plant regions from the rhizome through to the stem, leaf and spores. Numerous structures were silicified including cell walls, cell plates, plasmodesmata, and guard cells and stomata at varying stages of differentiation. All of the major sites of silica deposition in horsetail mimicked sites and structures where the hemicellulose, callose is known to be found and these serendipitous observations of the coincidence of silica and callose raised the possibility that callose might be templating silica deposition in horsetail. Hydroponic culture of horsetail in the absence of silicic acid resulted in normal healthy plants which, following acid digestion, showed no deposition of silica anywhere in their tissues. To test the hypothesis that callose might be templating silica deposition in horsetail commercially available callose was mixed with undersaturated and saturated solutions of silicic acid and the formation of silica was demonstrated by fluorimetry and fluorescence microscopy.ConclusionsThe initiation of silica formation by callose is the first example whereby any biomolecule has been shown to induce, as compared to catalyse, the formation of silica in an undersaturated solution of silicic acid. This novel discovery allowed us to speculate that callose and its associated biochemical machinery could be a missing link in our understanding of biosilicification.
Highlights
The horsetails (Equisetum sp) are known biosilicifiers though the mechanism underlying silica deposition in these plants remains largely unknown
Perry and Lu (1992) suggested that the organic matrix in question might be made from polymers of carbohydrates, for example, cellulose [14], and this suggestion was reinforced recently by Fry and colleagues who speculated that the hemicellulose, callose, in horsetail cell walls might be a potential site of silica deposition [15]
PDMPO as a fluorescent marker of biosilicification Microwave-assisted acid digestion of horsetail, either grown hydroponically in the presence of silicic acid or in plants collected from the wild, resulted in silica deposits and ‘skeletons’ which were successfully labelled with the fluor PDMPO
Summary
The horsetails (Equisetum sp) are known biosilicifiers though the mechanism underlying silica deposition in these plants remains largely unknown. Often having originally been extracted from biogenic silica, have been shown to accelerate or catalyse silica deposition in saturated solutions of silicic acid [16]. Biosilicifiers, such as horsetails, harvest silicic acid from solutions which are far from saturation and deposit it as amorphous hydrated silica and it is the elucidation of this mechanism which remains the ‘Holy Grail’ of biological silicification research [5]
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