New insight into silica deposition in horsetail (Equisetum arvense)

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.

Callose-associated silica deposition in Arabidopsis

“…Our results for callose induction strongly support research identifying a role for callose in resistance to powdery mildew in Arabidopsis [4] while our findings for silica deposition suggest a mechanism for silicon-induced resistance to the same pathogen in Arabidopsis [7,8]. We reported previously that callose-associated silica deposition provided protection against powdery mildew in Equisetum arvense (horsetail) and 8 we speculated that biogenic silica presented a physical barrier to the entry of the pathogen [3]. The coincident deposition of biogenic silica and callose in Arabidopsis may provide resistance to powdery mildew by a similar mechanism.…”

“…The coincident deposition of biogenic silica and callose in Arabidopsis may provide resistance to powdery mildew by a similar mechanism. In horsetail we were able to demonstrate almost mirror-like depositions of callose and silica during the development of stomata [3]. Herein we have seen similarities with the development of trichomes with silica deposition appearing to mirror the role of callose in the development of these epidermal hair cells [9] (Figure 4).…”

“…The biochemical machinery which differentiates silica accumulators such as rice and horsetail from non-accumulators such as Arabidopsis remains to be understood and is the subject of a significant research effort. Of particular interest is the plant extracellular matrix as a factor in templating biogenic silica deposition [2] and we have identified the -1-3-glucan callose as a biomolecule involved in silica deposition in horsetail [3]. We were able to show that not only does silica deposition in horsetail mirror callose deposition but also that in vitro callose could induce the formation of silica from an under-saturated solution of Si(OH) 4 .…”

“…PDMPO labelling of Arabidopsis-derived silica We used an established method for the identification of biogenic silica in plant tissues [3]. Briefly, silica residues collected from filters were suspended in 20 mM PIPES at pH 7 and containing 0.125M 2-(4-pyridyl)-5-((4-(2-dimethylaminoethylaminocarbamoyl)-methoxy) phenyl) oxazole (PDMPO; LysoSensor Yellow/Blue DND-160, 1 mM in DMSO).…”

Roles of silica and lignin in horsetail (Equisetum hyemale), with special reference to mechanical properties

“…The silica content in horsetail amounts to 25% of the dry weight and is derived from silicic acid from the soil, 8 and a part of deposition mechanisms of silica has recently been reported. 16 Lignin functions in binding other cell wall components such as cellulose microfibrils, and hemicelluloses and gives rigidity and/or mechanical strength to plants. 17,18 Mutants with less lignin content showed decreased mechanical strength.…”