Abstract
Biomineralization is a common phenomenon in plants and has been shown to be chemically, functionally and topologically diverse. Silica and calcium carbonate have long been known as structural plant biominerals and calcium phosphate (apatite)–long known from animals–has recently been reported. Strikingly, up to three different biominerals may occur in a single trichome in, e.g., Urticaceae and Loasaceae, and in combination with organic compounds, can form organic/inorganic composite materials. This article presents an extension of previous studies on the distribution of these biominerals in Loasaceae trichomes with a focus on their spatial (three-dimensional) distribution and co-localization with organic substances. Light microscopy and scanning electron microscopy with high-resolution EDX element analyses of sample surfaces and sections illustrate the differential distribution and composition of the different biomineral phases across cell surfaces and cell walls. Raman spectroscopy additionally permits the identification of organic and inorganic compounds side by side. All three biominerals may be found in a nearly pure inorganic phase, e.g., on the plant surfaces and in the barbs of the glochidiate trichomes, or in combination with a larger proportion of organic compounds (cellulose, pectin). The cell lumen may be additionally filled with amorphous mineral deposits. Water-solubility of the mineral fractions differs considerably. Plant trichomes provide an exciting model system for biomineralization and enable the in-vivo study of the formation of complex composite materials with different biomineral and organic compounds involved.
Highlights
Biomineralization is a well-known phenomenon in plants and animals (Skinner and Jahren, 2003)
Scanning electron microscopy (SEM) with integrated energydispersive X-ray spectroscopy (EDX) for element analysis is an excellent tool for identification of biominerals
For the present study we investigated three taxa of Loasaceae (Blumenbachia, Caiophora, Loasa) with trichomes, each having quite similar morphologies, but remarkable differences in biomineralization
Summary
Biomineralization is a well-known phenomenon in plants and animals (Skinner and Jahren, 2003). Biomineralization is usually found at the level of individual cells, either in epidermal cell walls or as intracellular structures such as cystoliths. Calcium oxalate is generally found as intracellular biomineral whereas calcium carbonate is both found as intracellular cystoliths and in the cell walls of trichomes and stinging hairs (Franceschi and Horner, 1980; Lanning and Eleuterius, 1989; He et al, 2014). Silica is the hardest biomineral and widely reported in the form of deposits in the outer cell walls of the epidermis and trichomes, and as intracellular phytoliths (Nawaz et al, 2019; Gallaher et al, 2020). Abrasive grainy cell wall inclusions and mineralized sharp tips of trichomes, as well as a dense cover of stiff mineralized hairs forming a physical barrier, are assumed to provide protection against grazing animals (Figure 1)
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