Plants can’t flee from predators or seek shelter in inclement weather. Instead, they have to be really good at sensing and responding to subtle changes in their environment. That makes them great candidates for sensors. There is a whole field of research that takes advantage of plants’ sensitivity to their environment, and it could make waves in everything from medicine to farming to national security.
Plants are smart. They know when they’re under attack, and by what. That fresh-cut grass smell is just the plant’s version of a tornado siren. And the signals released from a mechanical wound like a lawnmower are different from the signals released when a plant is being eaten by an insect. Their responses are so specific, that plants exposed to different pathogens, or even the same pathogen with a mutation, will emit a different cocktail of airborne chemicals.
The chemicals released when a plant is threatened serve as communication signals that are picked up by neighboring plants and even friendly insects. Plants can communicate with humans too, by presenting a visible signal in response to a stimulus. For example, plants have been engineered to turn from green to white when triggered. Plants with the gene for the jelly fish-derived green fluorescent protein can also glow in response to stimuli. Theoretically, genetic engineers can wire plants to link a response like de-greening or fluorescence to any number of external signals. The plant response can even be connected to an electronic monitor like a smartphone via infrared-sensing nanoparticles.
That’s the basis for research programs devoted to engineering plants as sentinels. The US military’s Defense Advanced Research Projects Agency (DARPA) invests heavily in developing plants as sentinels to trigger the presence of explosives and pathogens via the Advanced Plant Technologies program.
The trick is getting plants to identify the right triggers. They evolved to respond to things like insects, sunlight, and moisture, not bombs or anthrax. But sometimes, a molecule we’d like plants to respond to might be similar to one they already recognize. For instance, there are several genes in the lab plant Arabidopsis that are activated by the presence of TNT.
These genes probably didn’t evolve to respond to TNT. Instead, they might be triggered by molecules that are chemically similar to the explosive material. If we identify sensors that evolved for responding to one signal, we can adjust them to recognize one that’s slightly different—like putting an extra insole in a shoe that’s too big. For instance, studies have shown it’s possible to tweak bacterial signaling proteins to detect explosives and potential biowarfare agents.
We develop immunity against viruses by making antibodies, which are proteins that bind very specifically to a certain pathogen. We could engineer plants to produce the antibody for smallpox, for example, and then produce a signal when the virus is present. Scientists at the University of Tennessee have already done preliminary work that shows it’s possible to engineer plants to detect pathogens and respond with a signal.
In some cases, a more dramatic approach may be necessary. Scientists have shown that it’s possible to design proteins that have all kinds of novel functions. So if we can’t find a sensor for a target molecule in nature, we could design one and build it like a genetic 3D printer.
And sometimes, a sensor isn’t even necessary. There are all kinds of chemicals that plants naturally absorb from their environment. Some of those, like formaldehyde and chloroform, we’d prefer to do without. Scientists at the University of Washington have shown it’s possible to engineer plants to detoxify the air by breaking down chemicals they already take in. For their work, they used a gene from a rabbit that’s analogous to a gene expressed in human livers.
Other researchers have shown that plants can be engineered to turn up their natural ability to take up heavy metals in order to detoxify the soil. That means we can use plants to not only signal threats in our environment, but also to combat them.
In addition to helping us, this sense and response ability could be co-opted to help plants help themselves. For example, it is sometimes beneficial for a crop to express traits only under certain conditions. Crops that have been engineered for drought resistance, for instance, typically perform poorly in non-drought conditions. If these crops could be further engineered to only activate the drought-tolerant traits when they sense a drought, then farmers wouldn’t have to forecast the weather to maximize their potential.
From slug-inspired wound dressings to honeycomb-shaped solar panels, scientists are constantly inspired by nature. Plants learned to sense and respond to subtleties in their environment long before we did. But the process is orchestrated in the same way as all of life’s inner workings: by a network of genes. As we learn more about gene networks, we can begin to build artificial ones that mimic natural systems—borrowing from evolution to turn plants into sentries.
Lawnmower photo: Kevin Doncaster/Flickr