Plant talk

Botany students talking up their experiments. — UGA Extension/Creative Commons

There was some thinking, a while back, that talking to your plants might help them grow bigger and better.

There is no hard data supporting this theory, but there is new scientific evidence that the real problem might be getting a word in edgewise. It seems that plants “talk” among themselves all the time.

Through the release of chemical cues, technically known as volatile organic compounds, they are crying for help against leaf-chewing insects, inviting predators of bad bugs to hurry on over, and warning neighboring plants to beef up their defenses. There’s a kind of scent-based gossip circulating in any given plant community, just beyond the reach of our own human senses.

In recent experiments, lima beans were overheard “chatting” about an invasion of red spider mites. In short, measurable order, genes released into the sap stream changed its flavor to something less palatable, warned other beans to do the same, and attracted predators that feed on the nasty mites.

“Hey! I’ve got one,” the bean said to the mite-eater. “Over here, over here. Get him before he gets me.”

Tobacco plants growing in the desert of southwestern Utah responded to the saliva of hawk moth caterpillars to produce a toxin that impaired the caterpillar’s ability to digest its stolen meal. As a second line of defense, the plants also released chemicals that repelled hawk moths looking for a place to lay eggs. A paste of these specific chemicals smeared on the stems of intact plants also had the repellent effect, scientists reported.

Yet another compound manufactured by the tobacco summoned the rapacious predator Geocoris pallens, which thinks of hawkmoth eggs as a tasty treat. Not a bad day’s work for a seemingly innocuous little green plant.

Caterpillars fight back with compounds of their own. — eurleif/CC

The notion that plants can communicate among themselves was controversial as recently as 20 years ago. Biologists Ian Baldwin and Jack Schultz, two of the early pioneers, published a study suggesting that chemical signals from bug-infested maple and poplar trees could stimulate undamaged trees nearby to up their chemical defenses. The pair was laughed out of the lecture hall and found it difficult to attract funding for further research.

But the tide has changed, and now researchers are trying to understand plant communications so that they might exploit this formerly secret weapon to produce crops that can fend for themselves. We think of plants as passive creatures, but they’re really players in an ever-changing form of chemical warfare.

The study of plant compounds reveals that plants can make subtle distinctions between species of predator insects, even closely related ones. Corn, cotton and tobacco have a slightly different response to Heliothis virescnes and Heliothis zea, two leaf-chewers in the same genus. What’s more, parasites that feed on these herbivores can read the cues plants provide in their search for a victim.

Another natural plant toxin can halt invasion by fungi, which afflicts many plants with debilitating disease. The substance, osmotin, penetrates the cell walls of fungi, overtakes its protein-generating machinery, and encourages it to commit suicide. It’s another neat trick that agribusiness would like to co-opt, saving some of the $9 billion in annual crop losses attributed to fungal diseases.

Of course, in the evolutionary race for survival, the insects are fighting back. Scientists from Penn State and the University of Arkansas have isolated a compound in caterpillar saliva that suppressed a plant’s chemical defenses. The compound counteracted the plant’s attempt to fend off attack with toxins of its own, and “confused it,” in one researchers words.

But caterpillars probably won’t have the last word. Plant defense is dynamic, and plants can rapidly manufacture defensive chemicals of many kinds. In this escalating conflict, the plants and bugs are locked in a subtle battle with the war never won for long.