Nothing in Biology Makes Sense: Making sense of ubiquitous plant defenses

A giraffe, dodging thorns like a pro. Photo by Colin Beale, via Nothing in Biology Makes Sense.

We have a second post at the collaborative blog Nothing in Biology Makes Sense! this week, in which ecologist Colin Beale (guest posting from Safari Ecology) tackles the question of why so many African savannah plants grow thorns:

At one level the answer is obvious—there are an awful lot of animals that like to eat bushes and trees in the savanna. Any tree that wants to avoid this would probably be well advised to grow thorns or have some other type of defence mechanism to protect itself. But then again, perhaps the answer isn’t so obvious: all those animals that like to eat bushes seem to be eating the bushes perfectly happily despite the thorns. So why bother having thorns in the first place?

To find out why, and see more of Colin’s great photos, go read the whole thing. ◼

Making themselves at home: Spider mites disable plant defenses, then spin their own

Tomatoes, one of many plants that play unwilling host to red spider mites. Photo by sylvar.

ResearchBlogging.orgPlant-eating insects must overcome some of the cleverest weaponry in the living world—from poisonous latex to sticky hairs—just to find a meal or a place to lay eggs. Many deal with their host plants’ toxic defenses by digesting them or sequestering them safely for personal use, but the red spider mite Tetranychus evansi simply turns them off.

Tetranychus evansi eats a wide range of plants, from tomatoes to potatoes. One female mite can eat enough to lay 50%-70% of her weight in eggs every day, and while that isn’t much on the scale of a single, miniscule red mite, it adds up quickly when colonies build into dense clusters on host plants, sucking them dry and covering them in webs of spun silk.

Most host plants respond to such an onslaught by ramping up production of chemicals that make them unpalatable to herbivores, or that interfere with the mites’ ability to digest plant tissue. However, a team of Dutch and Brazilian biologists recently found that T. evansi somehow short-circuits this response [$a].

The team, whose senior author is the Dutch biologist Arne Jannsen, discovered that mites raised on leaf tissue from tomato plants previously attacked by T. evansi survived longer and laid more eggs than mites raised on tissue from plants that had never been attacked. Analsyis of RNA from tomato leaves attacked by the mites revealed that they were producing fewer of the signalling proteins associated with responding to insect damage than leaves damaged by another, related mite species—and one protein was produced at lower rates than in undamaged leaves!

Mites, up close. Photo via AgroLink.

In other words, the mites were not just preventing the host plant from boosting its defences in response to a mite attack—they were suppressing the defenses below what they would be without an immediate threat. Like a burglar cutting the power to a home security system, T. evansi can somehow prompt a hostile host to become more hospitable.

This raises another problem, however. With its defenses down, the host plant is also more hospitable to other insect herbivores, which could reduce the plant’s value to T. evansi, or even activate the alarms the mites have managed to suppress. A second study by the same team suggests that this may be part of the function of the webs T. evansi spins as it consumes its host.

In this second round of experiments, the group returned to the closely related mite Tetranychus urticae, which was used to stimulate plant defenses in the first study. Earlier work had found that some strains of T. urticae can tolerate or suppress host plant defenses [$a], though not nearly as effectively as T. evansi. That earlier work found that non-suppressing mite strains could benefit from living on the same plant as a suppressing strain, and the new study first demonstrated that this effect is even stronger when T. urticae shares a plant with T. evansi.

A whole lot of (presumably happy) mites. Photo via AgroLink.

In contrast, T. evansi colonies fared worse in the presence of the non-suppressing mites, whether fed leaves that had already been attacked by T. urticae, or placed on a mite-free leaf of a plant with another leaf infested by the non-suppressing species. All else being equal, T. urticae benefits from the defense-suppressing activity of T. evansi, but reduces the value of the host plant to T. evansi.

Faced with this freeloading competitor, T. evansi apparently replaces the disabled plant defenses with webbing. The team found that even though T. urticae thrived when given evansi-chewed tomato leaves, the non-suppressing mites had difficulty colonizing leaves covered in T. evansi webs. Moreover, T. evansi introduced onto a plant with the non-suppressing mites spun more webbing than when introduced onto a mite-free plant; but they didn’t ramp up web-spinning when sharing a plant with another colony of their own species, suggesting that the mites can respond to competition by building up their defenses.

So not only does T. evansi possess the means to turn off its hosts’ biological security system, it erects its own defenses to protect the plant from one competitor that might try to take advantage of the situation. How, exactly, the mites interfere with plants’ defensive responses will be an interesting future line of study. I’d also be very interested to see whether other herbivorous insects—things larger than other mites, and not so easily put off by some silk security fencing—also preferentially attack plants disabled by T. evansi. ◼

References

Kant, M., Sabelis, M., Haring, M., & Schuurink, R. (2008). Intraspecific variation in a generalist herbivore accounts for differential induction and impact of host plant defences Proc. Royal Soc. B, 275 (1633), 443-52 DOI: 10.1098/rspb.2007.1277

Sarmento, R., Lemos, F., Dias, C., Kikuchi, W., Rodrigues, J., Pallini, A., Sabelis, M., & Janssen, A. (2011). A herbivorous mite down-regulates plant defence and produces web to exclude competitors. PLoS ONE, 6 (8) DOI: 10.1371/journal.pone.0023757

Sarmento, R., Lemos, F., Bleeker, P., Schuurink, R., Pallini, A., Oliveira, M., Lima, E., Kant, M., Sabelis, M., & Janssen, A. (2011). A herbivore that manipulates plant defence. Ecology Letters, 14 (3), 229-36 DOI: 10.1111/j.1461-0248.2010.01575.x

Big herbivores shape plant community response to global warming

ResearchBlogging.orgThe cover article from this week’s PNAS has important implications for how we plan for, and deal with, climate change. Post and Pedersen report that the way an arctic plant community changes in response to warming depends heavily on the presence of large herbivores [$-a], like muskoxen and caribou.


Photo by Giant Ginkgo.

Previously, it was thought that one effect of global climate change would be for woody shrubs and dwarf trees to become more common in arctic and subarctic plant communities. This increase in woody plants could trap more atmospheric carbon and increase the albedo of the land – meaning more heat could be reflected back into space. Both of which effects might help slow a warming global climate.

However, Post and Pedersen show that large herbivores can reduce this shift in community composition. In huge five-year study, they set up experimental plots from which caribou and muskoxen were either excluded by fencing, or not excluded. Within each class of plot, they also placed 1.5-m “open-topped chambers” (OTCs) made of fiberglass – basically, cylinders that help trap solar heat, warming the ground inside. In the fenced plots, the plant communities inside the OTCs shifted toward more woody species; but in the unfenced plots, where large herbivores could reach in and graze inside the warmed cylinders, plant communities didn’t develop greater cover by woody species.

Now, it’s not surprising that large herbivores can have a profound effect on the plant species that grow in their grazing land. Where I come from, in the northeast U.S., large swaths of forest have been dramatically altered [$-a] by a population explosion of white-tailed deer freed from their natural predators. But Post and Pedersen have drawn a connection between this effect and the ways in which natural communities may respond to the most dramatic environmental change in human history. It just goes to show what a massively complex system we humans are tinkering with, and how little we know about what that tinkering may ultimately do.

References

E. Post, C. Pedersen (2008). Opposing plant community responses to warming with and without herbivores PNAS, 105 (34), 12353-8 DOI: 10.1073/pnas.0802421105

F.L. Russell, D.B. Zippin, N.L. Fowler (2001). Effects of white-tailed deer (Odocoileus virginianus) on plants, plant populations and communities: A review The American Midland Naturalist, 146 (1), 1-26 DOI: 10.1674/0003-0031(2001)146[0001:EOWTDO]2.0.CO;2