Abstract
Biomineralization provides living organisms with various materials for the formation of resilient structures. Calcium phosphate is the main component of teeth and bones in vertebrates, whereas especially silica serves for the protection against herbivores on many plant surfaces. Functional calcium phosphate structures are well-known from the animal kingdom, but had not so far been reported from higher plants. Here, we document the occurrence of calcium phosphate biomineralization in the South-American plant group Loasaceae (rock nettle family), which have stinging trichomes similar to those of the well-known stinging nettles (Urtica). Stinging hairs and the smaller, glochidiate trichomes contained nanocrystalline hydroxylated apatite, especially in their distal portions, replacing the silica found in analogous structures of other flowering plants. This could be demonstrated by chemical, spectroscopic, and diffraction analyses. Some species of Loasaceae contained both calcium phosphate and silica in addition to calcium carbonate. The intriguing discovery of structural hydroxylated apatite in plants invites further studies, e.g., on its systematic distribution across the family, the genetic and cellular control of plant biomineralization, the properties and ultrastructure of calcium phosphate. It may prove the starting point for the development of biomimetic calcium phosphate composites based on a cellulose matrix.
Highlights
Loasaceae, a nearly exclusively New World plant family, are often equipped with extremely effective stinging hairs and bear a dense cover of hooked or barbed (“glochidiate”), mineralized trichomes with fascinating shapes
We report the intriguing and unambiguous discovery of nano-crystalline hydroxylated apatite in the complex trichome cover of Loasaceae, this being the first report of calcium phosphate playing a role as a structural biomineral in plants
Scanning electron microscope (SEM) images of the surfaces of several Loasa species show a dense coverage with highly diversified trichomes such as small glochids and much larger stinging hairs (Fig. 1)
Summary
A nearly exclusively New World plant family, are often equipped with extremely effective stinging hairs and bear a dense cover of hooked or barbed (“glochidiate”), mineralized trichomes with fascinating shapes. There is only a single reference to the possible occurrence of carbonated and hydroxylated apatite, sometimes referred to as dahllite, in tracheophytes[6] and a passing note on the possible presence of intracellular calcium phosphate crystals in Capsicum[13], but no evidence for structural phosphate biominerals in plants Both silica (amorphous SiO2) and calcium carbonate (CaCO3) have been shown to play important roles on the plant surface, especially in mineralized plant trichomes such as the stinging hairs of Urtica[14,15] and the glochidia of Boraginaceae[16]. We report the intriguing and unambiguous discovery of nano-crystalline hydroxylated apatite in the complex trichome cover of Loasaceae, this being the first report of calcium phosphate playing a role as a structural biomineral in plants
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