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.
Revenge of the plants? It’s hard for the mind not to wander into B-movie territory when considering what researchers from Bonn University recently discovered: The first plants found to have calcium phosphate as a structural biomineral.
Calcium phosphate is widely found in the animal kingdom; it’s a hard mineral substance of which bones and teeth are largely comprised. Now the researchers have confirmed its presence in the stinging hairs of rock nettles (Loasaceae), a “well-defended” plant native to the South American Andes.
The mineral acts to reinforce the trichomes, the tiny ouchie stinging hairs that serve as a potent reminder for herbivores to back off. When an animal’s tongue comes into contact with the trichomes, the hardened tips break off and a “painful cocktail” floods the tissue. “The mechanism is very similar to that of our well-known stinging nettles," says Dr. Maximilian Weigend of the Nees-Institut for Biodiversity of Plants at Bonn University.
But while the hairs of stinging nettles are hardened with silica, the calcium phosphate makes the rock nettles different.
"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."
Up next, "Attack of the Man-Eating Plants" coming soon to a theater near you?