Stinging hair morphology and wall biomineralization across five plant families: Conserved morphology versus divergent cell wall composition.

Abstract

Stinging hairs are striking examples of plant microengineering-the plant equivalent of the hypodermic syringe. The requisite mechanical properties are mostly achieved by cell wall mineralization. Stinging hairs of Urtica dioica (Urticaceae) are known to be mineralized with silica and calcium carbonate and those of Loasaceae also with calcium phosphate, but no comparative study has been provided across different taxa with stinging hairs. Light microscopy and scanning electron microscopy (SEM) with cryo-SEM and energy-dispersive x-ray spectroscopy were used to analyze morphology and biomineralization of stinging hairs of 43 species from the families Caricaceae, Euphorbiaceae, Loasaceae, Namaceae, and Urticaceae. Stinging hair morphology is similar across the taxa studied, in striking contrast to the divergent patterns of biomineralization. Trichome bases are mostly calcified, sometimes silicified, the shafts are mostly calcified, and the apices silicified (Urticaceae), and contain calcium phosphate (Caricaceae, Namaceae), both silica and calcium phosphate (some Loasaceae), or no minerals (Cnidoscolus, Euphorbiaceae). Some stinging hairs are superficially thinly coated with silica over a cell wall otherwise mineralized with calcium carbonate or calcium phosphate. Mineralization patterns are surprisingly diverse and involve three different biominerals deposited in different parts of individual trichomes with calcium phosphate a common component. The physical properties of different wall regions of the stinging trichomes are thus fine-tuned to optimize their function via modulation of wall thickness and differential element deposition. Similar function is apparently achieved through divergent wall compositions.