Beyond Bioremediation: Plants that Check for Pollution

Plants are a versatile – and attractive – subject for biological engineering. Early in human history, we domesticated wild, inedible wheat into the delicious grain of today; we matured miniscule wild strawberries into plump, juicy strains; we bred the seeds out of bananas to make them more palatable. That was engineering on a phenotypic level; on a genotypic one, we have made disease-resistant crops, drought-resistant crops, plants that degrade soil pollutants like TNT into harmless carbon compounds, and even spinach that can send emails.

Why, you ask, do we want communicative spinach leaves? To alert us to pollution, of course.

In 2017, a paper was published with research funded by the US Army Research Office. The US Army had a problem – they had used extensive plots of land for training soldiers and technology, and now, they wanted to repurpose the land. You can’t repurpose land that’s been polluted by explosives, however. That kind of pollution is dangerous for people and the environment.

Nitroaromatics are a class of chemicals with explosive properties. For example, TNT stands for 2,4,6-trinitrotoluene. It’s an aromatic molecule (it has a benzene ring, with a methyl group) with three nitrate functional groups. Another nitroaromatic, picric acid, is a common pollutant from explosives. It looks just like TNT, except instead of a methyl group, it has a hydroxide there.

At the same time, it’s really expensive to test all the soil in a region. Apparently, exact locations where explosives and their pollutants had been used weren’t recorded. How can we identify regions in need of ecological clean-up?

Enter the Wong et al. team. These researchers designed a nanobionic formula that can be injected into spinach leaves. The nanobionic particles give off infrared light. If spinach starts picking up a nitroaromatic pollutant called picric acid (found in regions where explosives have been used), the acid slowly makes its way up the roots, through the stems, and into the leaves. Once the nanobionics detect it, their fluorescence starts to dim.

Infrared light is invisible to the human eye. However, the team also set up a camera that can detect it. The camera was hooked up to a computer that reads infrared light intensity. If light intensity drops, it sends you an email telling you where the polluted soil lies.

What Exactly is a Nanobionic?

Single-walled carbon nanotubes (SWCNTs) are a fluorescent nanosensor tool that already existed prior to this study. Wong et al. (2017) just applied it to spinach plants.

Of course, they didn’t want the SWCNTs to always fluoresce. So they paired it up with a peptide called Bombolitin. This peptide bonds with nitroaromatic molecules, which allows for pollutant detection. It binds to picric acid, and the SWCNT stops glowing (infra)red.

This nanosensor was swirled up into a liquid solution for injection into spinach leaf veins. The particles were then spread throughout the leaf like any other nutrient carried in the xylem. It accumulates in parenchyma (unspecialized plant cells). It is small enough to easily pass through cell walls and membranes, and built up in the cell organelles.

Why Use Plants for This?

If you wanted to do this with abiotic technology, what would you need?

This sort of system depends on contaminated water moving up into some structure, where it is tested by a sensor. It turns out we aren’t great at this.

One option here might be nanoporous ceramics. But guess what? Water is heavy. You’d risk the ceramics pulling up too much water and breaking under the weight. You’d also have to pump the water.

Plants can do this effortlessly.

A leaf is equipped with moveable pores called stomata. These are cells that open and close, allowing the leaf to breathe. And just like you lose water when you exhale (think: the vaporous puff you can see on a cold day), plants lose water when they “breathe,” too. We call this water loss transpiration.

Water is also a remarkable molecule that binds to itself. You’ve probably noticed this before in terms of surface tension (water striders use it to walk on water!) or capillary action (why a small drop on a paper towel eventually spreads to make the whole thing wet).

Combine these two features – plant transpiration, and water’s self-adhesion – and suddenly, you’re pumping water in an effortless hydraulic system. Water transpires out of the leaves, pulling water through the xylem, encouraging roots to soak up more of it.

Effortless. Elegant. It’s the perfect hydraulic technology.

So instead of setting up a “user-supplied power source,” you can upgrade your spinach to a nanobionic variety, and let them do all the water-pumping, pollutant-testing, email-sending work for you.