Abstract
Trichome biomineralization is widespread in plants but detailed chemical patterns and a possible influence of soil chemistry are poorly known. We explored this issue by investigating trichome biomineralization in 36 species of Mediterranean Brassicaceae from ultramafic and calcareous soils. Our aims were to chemically characterize biomineralization of different taxa, including metallophytes, under natural conditions and to investigate whether divergent Ca, Mg, Si and P-levels in the soil are reflected in trichome biomineralization and whether the elevated heavy metal concentrations lead to their integration into the mineralized cell walls. Forty-two samples were collected in the wild while a total of 6 taxa were brought into cultivation and grown in ultramafic, calcareous and standard potting soils in order to investigate an effect of soil composition on biomineralization. The sampling included numerous known hyperaccumulators of Ni. EDX microanalysis showed CaCO3 to be the dominant biomineral, often associated with considerable proportions of Mg—independent of soil type and wild versus cultivated samples. Across 6 of the 9 genera studied, trichome tips were mineralized with calcium phosphate, in Bornmuellera emarginata the P to Ca-ratio was close to that of pure apatite-calcium phosphate (Ca5(PO4)3OH). A few samples also showed biomineralization with Si, either only at the trichome tips or all over the trichome. Additionally, we found traces of Mn co-localized with calcium phosphate in Bornmuellera emarginata and traces of Ni were detected in trichomes of the Ni-hyperaccumulator Odontarrhena chalcidica. Our data from wild and cultivated plants could not confirm any major effect of soil chemistry on the chemistry of trichome biominerals. Hyperaccumulation of Ni in the plants is not mirrored in high levels of Ni in the trichomes, nor do we find large amounts of Mn. A comparison based on plants from cultivation (normal, calcareous and serpentine soils, Mg:Ca-ratios ca 1:2 to 1:20) shows at best a very weak reflection of different Mg:Ca-ratios in the mineralized trichomes. The plants studied seem to be able to maintain highly conserved biomineralization patterns across a wide range of soil chemistries.
Highlights
Our first hypothesis is that the divergent soil chemistry of serpentine soils is reflected, for example, in the magnesium content of plants grown on this substrate
We find no conclusive evidence for this hypothesis from our data—plants appear to exert complete control over trichome mineralization, more or less independent of soil composition
Magnesium is a widespread minor component, apparently playing a subordinate role in wild plants collected from calcareous substrates and a slightly more prominent one in the trichomes of plants collected from serpentine soils
Summary
Biomineralization plays a prominent role in the animal kingdom but it has received comparatively little attention in plants. It comes in essentially two different forms—the deposition of intracellular crystals or crystal complexes such as phytoliths and the mineral incrustations of plant surfaces, especially plant trichomes. Several hypotheses have been offered about the function of biomineralization in plants, including herbivore defense in the form of tissue rigidity and mechanical support, regulation of cytoplasmic calcium levels, detoxification of aluminum, heavy metals and/or oxalic acid, light gathering and scattering [1]. Plant trichome structure and chemical constituents often change according to function. This is especially true of trichomes exposed to light, herbivore damage, water stress, salinity or heavy metals [2]. Widely considered the most important function of mineralized trichomes, is an example of this structural alteration, with effectiveness contingent upon trichome shape, size, density, placement and physical properties [2,3,4]
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