"The trees, the flowers, the plants grow in silence… silence gives us a new perspective." –Mother Teresa
When I think about plants, chatterbox isn’t the first thing that comes to mind.
But below the surface of human perception, plants constantly engage in lively and informative conversations with one another. Damaged maple tree saplings, for example, release odorous chemicals into the air to alert their neighbors to amp up defenses against herbivores. Using root systems as organic phone lines, garden pea plants warn their surrounding greenery of impending drought. Plants may even be able to communicate through sound — young corn plants grown in water make a clicking noise that reroutes neighboring roots towards themselves.
Plants constantly chat to share information about their environment. It’s a wealth of knowledge, and we may finally have a way to listen in.
By embedding nanoscale carbon microtubes into the leaves of ordinary spinach plants, an MIT team of bioengineers transformed the Popeye staple into 3-in-1 bionic sensors. The plants automatically soak up explosives in the ground water, concentrate and analyze the sample and relay the data wirelessly to camera-equipped smartphones, while alerting the user with an email.
By changing the nature of the microtubes, the system can be rewired to detect dopamine or other similar molecules in their water, soil and air.
“Our goal in this work is to show that plants can signal [environmental] information to us,” says study author Min Hao Wong to Singularity Hub. Although the team worked with spinach, the technique can be applied to any living plant to customize it into an information-emitting machine, he says.
Evolution dealt plants a pretty bad hand — they can’t move around to forage for food or evade potential threats. To deal with it, plants are exquisitely attuned to minute changes in their environment. Their extensive root system constantly sucks up groundwater, capturing and taking up any chemicals that happen to be in the water source or surrounding soil, and transports it all up to their leaves.
“Plants are very good analytical chemists,” says lead author Dr. Michael Stranos. And they’re self-powered, using only the sun as their energy source. These properties make plants compelling platforms to automatically extract and detect low concentrations of chemicals in the environment, he says.
The problem is how to do so.
Several previous studies used genetic engineering to give plants an extra gene or two to boost their analytical skills. In 2011, a team modified tobacco plants so that they lost their vibrant green color (“de-greening”) upon detecting TNT in the soil. Similar plants have also been engineered to suck up and concentrate mercury from their surroundings as a cheap way to deal with the toxic waste.
These GMO plants are easy to grow and adopt for widespread use, but they often rely on a biological response — de-greening or wilting, for example — which could take hours or days. To really achieve real-time monitoring, the scientists needed another strategy.
Here’s where carbon nanotubes come in.
Back in 2011, Stranos and his team developed a series of tiny carbon sensors that can be customized to detect a wide range of chemicals, including hydrogen peroxide, the explosive TNT and sarin gas, a chemical weapon that wreaks havoc on the nervous system.
The sensors contained two components: a single-walled carbon nanotube that emits near-infrared signals, linked to a chain of molecules that captures the target chemical. Binding of the chemical quenches the tube’s florescence, which can be detected by a camera and analyzed.
Lean green machines
In the new study, the team started with carbon nanotubes that responded to picric acid, a common component of explosives.
Using a needleless syringe, they gently painted a solution containing the nanotubes onto the spinach leaves, which were absorbed into the meaty part of the leaf where photosynthesis takes place.
The team then doused the roots with water spiked with the explosive chemical. After about 10 minutes — the amount of time needed to transport water from root to leaf — they used a camera (with its infrared filters removed) to pick up changes in near-infrared emission. The camera was connected to a Raspberry Pi, which was programmed to periodically send out email alerts.
The Raspberry Pi system is low cost, portable and requires very little energy use, making it perfect for the field, explain the authors in their paper.
Within 10 minutes, the detector reported a drop in near-infrared light from the nanotube sensor, suggesting that it had successfully captured picric acid.
The self-contained, automatic system really pushes “the interaction of nanoparticles with biological systems,” says Dr. Matthew Baker, an analytical chemist from the University of Strathclyde who was not involved in the study.
Some tweaks still need to be made before the bomb-detecting bionic spinach can be tested in the real world. For example, at the moment the camera needs to be placed within three feet to reliably capture a signal, although the team is working on expanding that distance.
Nevertheless, plant nanobionics is likely here to stay.
“We deliberately designed the platform using low-cost electronics, which allows cost-effective scalability,” says Wong. He envisions eventually planting bionic plants in urban areas, crop fields or even common households to help detect and alert us of contaminants in our environments.
The sensors could also be “turned inward” to monitor the plant and prove to be invaluable to precision farming, says Wong.
And he’s willing to bet on it. Last year, Wong started a company called Plantea that offers nanosensors as a monthly subscription to farmers to help them optimize crop growth conditions and monitor plant health.
And that’s just the start. The approach may eventually lead to nanobionic plants with boosted photosynthesis abilities that monitor multiple chemicals simultaneously or transmit radio signals.
“When you have man-made materials infiltrated into a living organism, you can have plants do things that plants don’t ordinarily do,” says mechanical engineer Dr. Michael McAlpine at the University of Minnesota (he was not involved in the research).
“Once you start to think of living organisms like plants as biomaterials that can be combined with electronic materials, this is all possible,” he says.
Image credit: Min Hao Wong