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Plants Punch Back: Someday Your Crops Will Defend Themselves
If plants were people, you might think they’d be peace-sign-clothed, turn-the-other-cheek pacifists.
By sensing their environment, plants are able to manipulate certain factors into their favor. Plants have a sense of taste or smell, the ability to measure and detect different colors of light, and a sense of touch, says Jack Schultz, director of the University of Missouri Bond Life Sciences Center. They then use these abilities to respond to different types of attacks.
“Plants have a set of proteins related to the ones we have in our taste buds,” explains Schultz. “They are called receptor-like kinases. While humans only have a handful of them, a little weed can have over 640 receptor-like kinases.”
Plants chemically sense their environment through the receptor-like kinases along with a variety of lesser-known sensory molecules. When disease or insects attack plants, they release a specific odor. The odors, or volatiles, serve as warnings to neighboring plants.
The goal of Schultz’s research is to exploit these unique abilities. Since only affected plants send out signals, they can be selected and treated early – before the entire field has an infestation.
Each type of attack has a different correlating odor.
“When one plant responds to an attack, it emits volatiles into the air, which helps plants nearby turn on their response, even in advance of an attack,” says Schultz.
How do plants communicate?
“All the evidence indicates plants are aware of each other and can respond to each other mainly by sensing chemicals,” says Schultz. “Those chemicals can travel through the air and some can travel through the soil. That’s how the plant can tell there’s somebody nearby.”
Plants respond to attackers, both diseases and insects, by releasing chemicals that are repellent or toxic to those predators.
“All plants respond to attacks by insects and diseases by turning on or changing the production of chemicals,” says Schultz. “A lot of those chemicals are familiar to us, because we’ve adopted them for uses.”
Did you know the caffeine in your midafternoon pick-me-up drink is actually a protectant in coffee plants against insects?
“Nicotine is a protectant in tobacco plants,” says Schultz. “Many of our drugs, both recreational and medicinal, have been derived from plants, because they have the ability to be active in animals, toxic to bacteria and other microbes, or both.”
What are some of the stresses that plants are able to adapt to?
Plants respond to many types of stresses. Here are four common types.
“There are enough insects in the world that if they reproduce well enough, they theoretically ought to be able to eat all of the plants all of the time,” says Schultz.
While outbreaks can occur, they’re an exception, not the rule. Insects only eat about 10% of worldwide plant productivity, explains Schultz. That’s because plants fight back and stop insects from eating them.
The other approach plants take is to signal natural predators of the insects to come eat the attackers.
“The plants are contributing to the control process,” says Schultz. “They help their bodyguards find the bad insects that are causing damage. They do that by emitting odors that tell the friendly insects where to find their prey or their host.”
“If the plant detects that the color of light it’s receiving has either passed through or has bounced off of something green, the genetic program for growing tall quickly will be turned on,” says Schultz.
As a result, it’s able to escape being shaded out by the other plant.
Plants close their stomates when they are drought-stressed. Research suggests they communicate this to other plants, says Schultz. That’s probably due to chemical movement in the soil. This clues in other plants that still think they are doing fine into closing their own stomates.
Most of us look at a wilting plant and assume it’s teetering on the brink of death. However, wilting is a behavior plants use to conserve water.
“Instead of thinking of wilting as a symptom of a terrible situation, another way to think of it is as an adaptive behavior to conserve water,” says Schultz.
What’s that smell?
Plants produce low concentrations of volatile organic molecules when they are stressed. Plants emit these odors in response to different attacks.
The mixture of odors is different when the plant is wounded from being stepped on, as opposed to an attack by a disease or an insect or insects, says Schultz.
If you can figure out the composition of odors the plant is emitting, you can diagnose the problem.
“If you can detect the difference, you can know which plants are attacked and which ones aren’t,” Schultz points out.
“A lot of work in this was originally funded by the military, because they were interested in what the plants were able to report from the battlefield,” says Schultz.
Schultz’s research team has partnered with engineers to develop a sensor with the ability to report the specific odors plants are emitting. The sensor would be mounted on a robotic vehicle. As it moves through the field, it would be able to detect stressed plants, identify the pest, and treat the plant. Then, it would continue through the field until finding the next infested plant. Once you know which plants are being attacked, you can treat only the attacked plants and avoid spraying the entire field.
The advantage would be to treat infestations before they get to the point of visible damage on the plant, says Schultz. By the time you see the damage, it’s already economically significant.
For example, say you have an aphid-infested soybean field. It takes a decent level of damage to reduce production in soybeans. By the time aphids cause enough visible damage, there’s already been a severe impact on yields. The goal of the sensor is to catch problems before reaching economic thresholds.
“It’s a form of precision agriculture,” says Schultz. It would be spot-treating infested plants and not treating the noninfested plants. It should save a lot of money and time, and, of course, reduce environmental impacts.”
The challenge will be getting the sensor to work in an open-air setting. In order to correctly identify the odors, it will have to be able to detect in open air, where odors get diluted. The other factor is speed, because response time will be critical, says Schultz.
“The gadget we are working on to do this is taking a while because it’s difficult to get it to work well,” says Schultz. “If we do, we will be the first people to use it in open air. Most of this research has been done in labs.”
The odors are complicated mixtures with many different chemicals present. It makes the job of detection and characterization much more difficult. The researchers are taking the approach of narrowing down the number of kinds of molecules necessary to determine if the plant has been attacked.
“We are trying to get it down to three or four chemicals of the many chemicals for a fingerprint of what has happened to the plant,” says Schultz. “That fingerprint will be different for everything we want to know. Step one will be getting that to work for one plant and one attacker, but we’re going to have to do it over again for every situation.”
Schultz expects entry into the marketplace to be years away. He says best-case scenario is five years away, but that’s only if everything goes perfectly.