Herbivores, both in nature and the human self-identification kind, think plants love to be eaten. But a new study shows that may not be so -- they may be biting back.
Bones and teeth in the animal kingdom largely consist of a very tough mineral substance called calcium phosphate. A new study also found calcium phosphate is a structural biomineral in higher plants, providing the “bite” to the stinging hairs of representatives of the rock nettle family (Loasaceae). Calcium phosphate hardens the trichomes, which serve as a defense against being eaten, because when the tongue touches the trichomes of rock nettles, the tips of the stinging hairs break off and a painful cocktail pours out into the sensitive tissue.
This is different than stinging nettles, which fortify their needle-like hairs with silica.
Calcium phosphate has never been documented as a structural biomineral in higher plants and the authors discovered it by investigating the hypodermic syringe-looking nettles with an electron microscope.
Structurally similar to reinforced concrete
It could be shown that especially the mechanically highly stressed tips of the hairs are densely infused with tiny crystals of calcium phosphate, making them a kind of natural reinforced concrete.
Why did rock nettles evolve this particular type of biomineralization instead of silica or calcium carbonate as other plants di? It is obviously a particular metabolic pathway -- rock nettles are able metabolize silica and use it as a structural biomineral, side by side with calcium phosphate -- but that doesn't explain why calcium phosphate is used in the stinging hairs tips, the same substance that their enemies have in their mouths.
It is quite accurately a tooth for a tooth.
Citation: Hans-Jürgen Ensikat, Thorsten Geisler & Maximilian Weigend: A first report of hydroxylated apatite as structural biomineral in Loasaceae – plants‘ teeth against herbivoren, Scientific Reports, DOI: 10.1038/srep26073