Biomineralised barbed botanics bite back!

They almost literally bite. Glochidiate trichomes in some species of Loasa contain nanocrystalline apatite-cellulose composite, similar to the material found in teeth.
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Generally, plants don’t have the ability to run away if threatened by organisms that would cause them harm, e.g. by eating them. However, that doesn’t mean that they are defenceless in the face of such feeding attempts. Indeed, they are often armed with an impressive and comprehensive cocktail of chemicals that help to deter those who would dine upon their tasty tissues. And they also have an awesome armoury of surface-mounted hairs, prickles, thorns, spines, trichomes, and other enations that deter all but the most hungry of herbivores.

Loasa pallida leaf hairs
Loasa pallida leaf hairs. Image: Hans-Jürgen Ensikat et al. (2016)

Now, adding to this seriously hardcore catalogue of defences is the announcement by Hans-Jürgen Ensikat et al. that some plants are fighting back with added ‘bite’. Studying members of the Loasaceae (the rock nettle family) with electron microscopy and elemental analysis techniques, the team found that the cell walls of the tips of the stinging hairs and hooks of glochidiate trichomes of five species of the genus Loasa contain nanocrystalline apatite-cellulose composite material.

This apatite – a form of calcium phosphate – is similar to the material found in teeth and bones of vertebrate animals, but is the first report of this form of biomineralisation in higher plants. Its presence in these defensive structures appears to reinforce the stinging hairs thereby helping them deliver the necessary ‘message’ – a painful sting from chemicals injected into the animal by the hypodermic nature of the stinging hair whose tip is broken off by contact with the animal. Ouch!

However, arguably even more ingenious, is the news that other plants are very economical in their defence capabilities in using an environmentally produced coating. Examining naturally sand-coated Abronia latifolia (the aptly named sand verbena) and plants of Navarretia mellita supplemented with a sandy layer, Graduate Student Eric ‘Rick’ LoPresti and Prof. Richard ‘Rick’ Karban (University of California at Davis (USA), Department of Entomology and Nematology) conclude that this mineral mantle has a protective role because such plants are less chewed by herbivores than those that are not so adorned. They further deduce that it is a physical effect rather than one of camouflage whereby the plants are hidden from a herbivore’s sight by blending in as part of the sandy background.

So, consumption-confounding psammophory (‘sand-carrying’) is apparently not ‘cryptic’. Importantly, this elegant study is the first experimental support for the hypothesis that a sand covering protects plants from herbivory.* And, why waste expensive and valuable resources on creating armour-coating when it’s provided by Mother Nature? Non-intelligent plants? I think not!

 

* But, all is not yet sorted. For the hypothesis-testing-hungry human ‘herbivores’, listed in Norbert Jürgens’ review of the psammophorous species of southern Africa is approximately eight potential roles for this phenomenon in the life of plants. So, there’s clearly more to examine in the intriguing lifestyle of these belligerent botanics that really show ‘true grit’ in surviving in some of the planet’s harshest environments.

References

Hans-Jürgen Ensikat, Thorsten Geisler, Maximilian Weigend, 2016, 'A first report of hydroxylated apatite as structural biomineral in Loasaceae – plants’ teeth against herbivores', Scientific Reports, vol. 6, p. 26073 http://dx.doi.org/10.1038/srep26073

Christoph Neinhuis, Ute Müller-Doblies, Dietrich Müller-Doblies, 2008, 'Psammophora and other sand-coated plants from southern Africa', Feddes Repertorium, vol. 107, no. 5-6, pp. 549-555 http://dx.doi.org/10.1002/fedr.19961070512

N. Jürgens, 2008, 'Psammophorous plants and other adaptations to desert ecosystems with high incidence of sandstorms', Feddes Repertorium, vol. 107, no. 5-6, pp. 345-359 http://dx.doi.org/10.1002/fedr.19961070510


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