Plant sociality and solidarity

Plants aren’t nearly as passive as their stationary state might lead one to think.  They have similar immune system needs to animals, because they are routinely attacked by hungry microbes and larger creatures intending to munch their way around a leaf.  And, like animals, they have ways of battling the ever-evolving attackers, that allow them to adapt to changing assailants.

We've blogged recently about how plants communicate through their roots and in other ways (here) and about how they may even be altruistic (here).  Here’s another interesting paper, in PNAS, about sophisticated responses and sociality among plants ("Intake and transformation to a glycoside of (Z)-3-hexenol from infested neighbors reveals a mode of plant odor reception and defense," Sugimoto et al.).  The authors report that tomato plants affected by cutworms produce a volatile chemical called Hex Vic ((Z)-3-Hexenylvicianoside) that diffuses through the air to neighboring plants.  When the neighbors detect this, they then express genes that produce a substance that makes them resistant to cutworm attack, as well as weakens the worm.

In response to herbivory, plants emit specific blends of volatiles. When undamaged plants are exposed to volatiles from neighboring herbivore-infested plants, they begin to defend against the impending infestation of herbivores. This so-called “plant–plant signaling” has been reported in several plant species. For example, a study on the expression profiles of defense-related genes when Arabidopsis was exposed toseveral volatiles, including green leaf volatiles and a monoterpene, showed that the manner of induction varied with the gene monitored or the volatile used, suggesting that the plant responses were specific to the individual volatile compound.

Unlike defenses against microbial infection, this is a mechanism related to a mega-scale predator, an animal that is too big for the plant easily to defend itself, unless it evolves some form of toxin to kill the worms.  

Here, one might speculate about why plants under attack would help other plants, rather than just do their best to beset the cutworms on their own. Normally, one would say they ‘should’ (from a Darwinian perspective) raise their defenses as best they can, but not tip-off their neighbors, and hence competitors of the danger.  Let them fend for themselves, and if they fail, the plant with the best defenses wins an evolutionary skirmish.

Micro Toms;  Wiki source

Of course if the neighbors are clones from the same artificial domestic strain, as these appear to be (the authors say they are Micro-Toms, a hybrid strain), the evolutionary issues are moot, since whatever led to the plants' behavior didn't occur in a field of clones.  So one wouldn't be able to say this evolved so they could help their close-kin neighbors in the usual sense.  But if natural tomato seed dispersal isn't distant, then nearby plants would be close relatives, and the kinship argument might work, that the attacked plant may be harmed or even doomed, but by warning its neighbors it’s warning its kin.

But, if natural tomato seed dispersal is distant, spread in the droppings of birds that ate the fruits, e.g., than the explanation must be something different.  And indeed, if the volatile emissions are detected by non-kin, plants of other species, as Sagimoto et al. seem to suggest, below, then the kinship explanation doesn't work at all.

Because (Z)-3-hexenol is a common volatile compound emitted by most herbivore-damaged plants, andbecause we found that a wide range of plant species could accumulate (Z)-3-hexenyl glycosides after exposure to volatiles, absorption and glycosylation of exogenous airborne (Z)-3-hexenol might be a general response in plants.

In any case, here we have yet another example of a more complex living world than traditional understanding of botany has generally allowed.  It's a reminder that we should curtail our hubris when it comes to assuming we higher animals do everything best.  

Talking trees

A number of years ago, some wacky botanists suggested that trees might communicate with each other by releasing chemicals into the air, presumably for the purpose of signaling danger. Most other botanists dismissed this out of hand, saying that these substances would be far too diluted by the time they wafted to other trees that there would be no signal left, not to mention that other trees couldn't possibly have receptors for these signals.

Well, evidence for just this has been growing over the past decade, with work showing that plants do indeed release many substances that do things like facilitate communication between the organs of a single plant and between plants, as well as repel destructive herbivores. A recent paper in Current Opinion in Plant Biology (Protective perfumes: the role of vegetative volatiles in plant defense against herbivores, Sybille B Unsicker, Grit Kunert and Jonathan Gershenzon, 2009, 12:1-7, discussed this week on the BBC World Service radio program, Science in Action with Jon Stewart) adds to the understanding of the role of such chemical release in plant defenses.

Details of the plant immune system have been known for a long time, but these have generally concerned how the plant launches a cascade of internal defensive responses to attack. When a bug bites, the plant responds by killing the area around the attacker, and launching a systemic chemical defense. Now it seems that volatiles released by plants can actually repel bugs that come to feed, attracted to this huge meal that can't get away. At least one volatile mimics a compound produced by aphids when they've been attacked, warning other aphids to stay away. Other volatiles inhibit pests from depositing their eggs on or in leaves. Indeed, more mechanisms are being documented all the time.

The system is more complicated than just repelling herbivores, however. Some of these substances actually attract herbivore predators and parasitoids. For example, researchers attached artificial caterpillars to the leaves of trees infested with autumnal moth, and birds attacked these caterpillars much more frequently than those on uninfested trees, presumably because of the volatiles being released that attracted them. Other attackers are more susceptible to parasitoids when they are on an infested tree, again presumably attracted by the substances being released by the tree.

Among other things this teaches us, it is yet another beautiful example of the importance of signaling in evolution. We find this a fascinating area of research, and yet more support for the idea that if you are open to looking for cooperation, you'll find it everywhere.