Australia's Stinging Trees Use Animal-Like Venom to Protect Themselves

Photo by o2elot licensed under CC BY-SA 2.0

Australia’s stinging trees (genus Dendrocnide) are no ordinary members of the nettle family (Urticaceae). Whereas a physical encounter with most of their cousins will leave you with a mild burning sensation that usually subsides within a few hours, coming into contact with a stinging tree can leave you with excruciating pain that can last for days. Such a severe reaction to stinging trees has left scientists wondering what is going on chemically that makes these trees so darn painful.

It turns out that the stinging trees have evolved chemical defenses that are surprisingly similar to the venom produced by some spiders. The discovery of these chemicals within the stinging hairs of stinging trees is a first for the plant kingdom and likely represent a remarkable case of convergent evolution.

The structure model of stinging tree venom (left) and the stinging trichomes of D. excelsa (right). [SOURCE]

Stinging tree venom belongs to a class of compounds known as neurotoxins. Their molecular structure looks a lot like a 3D version of a frustrated scribble on a piece of paper. This convoluted structure just so happens to target mammalian pain receptors with high affinity. Once attached, they activate the sensory neurons, forcing them into overdrive. This is why the pain is so severe.

The petioles of D. excelsa are covered in stinging hairs (top). Scanning electron micrograph of trichome structure on the leaf of D. moroides (bottom). [SOURCE]

This neurotoxic venom is delivered into the body thanks to the amazing anatomy of nettle trichomes. These tiny hairs are hollow and attached to the top of a sac-like structure filled with the venom. When something brushes against the hairs, the tips break off, turning them into tiny hypodermic needles. As the victim brushes across a stem or leaf, thousands of these hairs inject minutes amount of venom into the skin. Pain is soon to follow.

Amazingly, not all animals seem to be affected by the stinging trees potent venom. Plenty of creatures from insects to birds and even some mammals will feed on the leaves and fruits of these trees, all of which are covered in venom-filled trichomes. As is always the case in biology, there is no surefire way to deter all potential predators. Inevitably some organism(s) will circumvent the deterrent through evolutionary means. Nonetheless, the discovery of animal-like venom being produced by plants is remarkable and opens up new doors into the world of chemical ecology and evolution.

Photo Credits: [1] [2] [3]

Further Reading: [1]

Gymnosperms and Fleshy "Fruits"

Fleshy red aril surrounding the seeds of Taxus baccata. Photo by Frank Vincentz licensed under the GNU Free Documentation License.

Many of us were taught in school that one of the key distinguishing features between gymnosperms and angiosperms is the production of fruit. Fruit, by definition, is a structure formed from the ovary of a flowering plant. Gymnosperms, on the other hand, do not enclose their ovules in ovaries. Instead, their unfertilized ovules are exposed (to one degree or another) to the environment. The word “gymnosperm” reflects this as it is Greek for “naked seed.” However, as is the case with all things biological, there are exceptions to nearly every rule. There are gymnosperms on this planet that produce structures that function quite similar to fruits.

Internal anatomy of a Ginkgo ovule with red arrow showing the integument.

Photo copyright Bruce Kirchoff, Licensed under CC BY 2.0

The key to understanding this evolutionary convergence lies in understanding the benefits of fruits in the first place. Fruits are all about packing seeds into structures that appeal to the palates of various types of animals who then eat said fruits. Once consumed, the animals digest the fruity bits and will often deposit the seeds elsewhere in their feces. Propagule dispersal is key to the success of plants as it allows them to not only to complete their reproductive cycle but also conquer new territory in the process. With a basic introduction out of the way, let’s get back to gymnosperms.

“Fruits” of Cephalotaxus fortunei (Cephalotaxaceae)

There are 4 major gymnosperm lineages on this planet - the Ginkgo, cycads, gnetophytes, and conifers. Each one of these groups contains members that produce fleshy structures around their seeds. However, their “fruits” do not all develop in the same way. The most remarkable thing to me is that, from a developmental standpoint, each lineage has evolved its own pathway for “fruit” production.

Ginkgo “fruits” are full of butyric acid and smell like rotting butter or vomit. Photo by H. Zell licensed under CC BY-SA 3.0

For instance, consider ginkgos and cycads. Both of these groups can trace their evolutionary history back to the early Permian, some 270 - 280 million years ago, long before flowering plants came onto the scene. Both surround their developing seed with a layer of protective tissue called the integument. As the seed develops, the integument swells and becomes quite fleshy. In the case of Ginkgo, the integument is rich in a compound called butyric acid, which give them their characteristic rotten butter smell. No one can say for sure who this nasty odor originally evolved to attract but it likely has something to do with seed dispersal. Modern day carnivores seem to be especially fond of Ginkgo “fruits,” which would suggest that some bygone carnivore may have been the main seed disperser for these trees.

“Fruits” contained within the female cone of a cycad (Lepidozamia peroffskyana). Photo by Tony Rodd licensed under CC BY-NC-SA 2.0

The Gnetophytes are represented by three extant lineages (Gnetaceae, Welwitschiaceae, and Ephedraceae), but only two of them - Gnetaceae and Ephedraceae - produce fruit-like structures. As if the overall appearance of the various Gnetum species didn’t make you question your assumptions of what a gymnosperm should look like, its seeds certainly will. They are downright berry-like!

Berry-like seeds of Gnetum gnemon. Photo by gbohne licensed under CC BY-SA 2.0

The formation of the fruit-like structure surrounding each seed can be traced back to tiny bracts at the base of the ovule. After fertilization, these bracts grow up and around the seed and swell to become red and fleshy. As you can imagine, Gnetum “fruits” are a real hit with animals. In the case of some Ephedra, the “fruit” is also derived from much larger bracts that surround the ovule. These bracts are more leaf-like at the start than those of their Gnetum cousins but their development and function is much the same.

Red, fleshy bracts of Ephedra distachya. Photo by Le.Loup.Gris licensed under CC BY-SA 3.0

Whereas we usually think of woody cones when we think of conifers, there are many species within this lineage that also have converged on fleshy structures surrounding their seeds. Probably the most famous and widely recognized example of this can be seen in the yews (Taxus spp.). Ovules are presented singly and each is subtended by a small stalk called a peduncle. Once fertilized, a group of cells on the peduncle begin to grow and differentiate. They gradually swell and engulf the seed, forming a bright red, fleshy structure called an “aril.” Arils are magnificent seed dispersal devices as birds absolutely relish them. The seed within is quite toxic so it usually escapes the process unharmed and with any luck is deposited far away from the parent plant.

The berry-like cones of Juniperus communis. Photo by Piero Amorati, ICCroce - Casalecchio di Reno, Bugwood.org licensed under Creative Commons Attribution 3.0 License.

Another great example of fleshy conifer “fruits” can be seen in the junipers (Juniperus spp.). Unlike the other gymnosperms mentioned here, the junipers do produce cones. However, unlike pine cones, the scales of juniper cones do not open to release the seeds inside. Instead, they swell shut and each scale becomes quite fleshy. Juniper cones aren’t red like we have seen in other lineages but they certainly garnish the attention of many a small animal looking for food.

I have only begun to scratch the surface of the fruit-like structures in gymnosperms. There is plenty of literary fodder out there for those of you who love to read about developmental biology and evolution. It is a fascinating world to uncover. More importantly, I think the fleshy “fruits” of the various gymnosperm lineages stand as a testament to the power of natural selection as a driving force for evolution on our planet. It is amazing that such distantly related plants have converged on similar seed dispersal mechanisms by so many different means.

Photo Credits: [1] [2] [3] [4] [5] [6] [7] [8]

Further Reading: [1] [2] [3] [4] [5] [6] [7]

Convergent Carnivores

Photo by Natalie McNear licensed under CC BY-NC 2.0

A carnivorous lifestyle has evolved independently in numerous plant lineages. Despite the similarities between genera like Nepenthes, Sarracenia, and Cepholotus they are not closely related. Researchers have wondered how the highly modified leaves of various carnivorous plant species evolved into the insect trapping and digesting organs that we see today. Thanks to a recent article published in Nature, it has been revealed that the mechanisms responsible for carnivory in plants are a case of convergent evolution.

This research all started with the Australian pitcher plant Cepholotus follicularis. More closely related to wood sorrels (Oxalis spp.) than either of the other two pitcher plant families, this species offers a unique window into the genetic controls on pitcher development. Cepholotus produces two different kinds of leaves - normal, photosynthetic leaves and the deadly pitcher leaves that have made it famous the world over.

By observing which genes are activated during the development of these different types of leaves, the research team was able to identify which alleles have been modified. In doing so, they were able to identify genes involved in producing the nectar that attracts their insect prey as well as the genes involved in producing the slippery waxy coating that keeps trapped insects from escaping. But they also found something even more interesting.

[SOURCE]

By examining the digestive fluids produced by Cepholotus as well as many other unrelated carnivorous plant species from around the world, researchers made a startling discovery. They found that the genes involved in synthesizing the deadly digestive cocktails among these disparate lineages have a similar evolutionary origin.

Although they are unrelated, the ability to digest insects seems to have its origins in defending plants against fungi. You have probably heard someone say that fungi are more similar to animals than they are plants. Well, the polymer that makes up the cell walls of fungi is the same polymer that makes up the exoskeleton of insects - chitin. By comparing the carnivorous plant genes to those of the model plant Arabidopsis, the team found that similar genes became active when plants were exposed to fungal pathogens.

It appears that carnivorous plants around the world have all converged on a system in which genes used to defend themselves against fungal infection have been co-opted to digest insect bodies. Taken together, these results show that the path to carnivory in plants is surprisingly narrow. Evolution doesn't always require the appearance of new alleles but rather a retooling of genes that are already in place. 

Photo Credits: [1] [2]

Further Reading: [1]