For the first time, researchers have found calcium phosphate in the structure of plants – in this case, used to harden the needle-like hairs used to defend against predators.
“The mineral composition of the stinging hairs is very similar to that of human or animal teeth,” says Weigend, who has been studying rock nettles for more than two decades. “This is essentially a composite material, structurally similar to reinforced concrete”, adds Weigend. While the structure of the trichomes are made of the fibrous typical of plant cell walls, they are densely encrusted with tiny crystals of calcium phosphate, making the stinging hairs unusually rigid.
The researchers are not clear as to why these plants have evolved such a unique type of biomineralization; most plants use silica or calcium carbonate as structural biominerals, so why not the rock nettles? “A common reason for any given solutions in evolution is that an organism possesses or lacks a particular metabolic pathway,” says Weigend. But since rock nettles are able metabolize silica, why the calcium phosphate?
“At present we can only speculate about the adaptive reasons for this. But it seems that rock nettles pay back in kind,” muses Weigend, “a tooth for a tooth.”