Specially tagged “sentinel plants” could soon provide an early warning of crop problems such as insect damage or bacterial infection. These plants would utilize two “glowing” sensors that react to stress-related compounds in the leaves.

When plants are subjected to stress, their leaves produce chemicals known as signaling molecules which trigger their adaptive responses. If a plant is being eaten by insects, for example, its leaves may produce a signaling molecule that triggers the rest of the plant to produce a chemical which repels insects.

Two of the most commonly used signaling molecules are hydrogen peroxide and salicylic acid.

Four years ago, MIT’s Prof. Michael Strano and colleagues created a leaf-integrated sensor that fluoresces in the presence of hydrogen peroxide. The “sensor” actually consists of a multitude of single-walled carbon nanotubes, each one wrapped in a strand of synthetic DNA known as an oligomer.

When a carrier solution containing these “corona phase molecular recognition” (CoPhMoRe) nanosensors is applied to the underside of a leaf, the tiny objects make their way through minuscule openings in the leaf’s surface called stomata. The nanosensors end up in the mesophyll, which is the internal layer of the leaf in which most photosynthesis occurs.

When hydrogen peroxide is subsequently produced in that layer, it binds with the nanosensors, causing them to fluoresce. That fluorescence can easily be detected using an infrared camera.

While hydrogen peroxide production alone can indicate the presence of certain plant stressors, being able to also detect salicylic acid would be even more useful. With that fact in mind, Strano’s team has now altered the structure of the oligomer used in the technology, creating a second type of CoPhMoRe nanosensor that fluoresces upon binding with salicylic acid instead of hydrogen peroxide.

MIT

In lab tests performed on pak choi (aka bok choi) plants, solutions containing the two different types of nanosensors were applied to different parts of the same leaf. Those plants were then subjected to stressors such as harsh light, heat, bacterial infection and insect bites.

It was found that the first three of these stressors resulted in the production of hydrogen peroxide within a matter of minutes, followed by the production of salicylic acid sometime within a two-hour window thereafter. The exact amount of time that it took for the salicylic acid to appear, however, varied in a consistent fashion according to the type of stressor.

This means that if a few CoPhMoRe-enhanced plants (within a crop of regular plants) were to be constantly monitored by an infrared camera, farmers could tell if they were experiencing the early stages of light, heat or bacterial stress based on how much time elapsed between their leaves initially producing hydrogen peroxide and then producing salicylic acid.

If only hydrogen peroxide were to be produced, that would mean insect bites were the culprit. And of course, if neither of the signaling molecules were being produced, that would mean the plants were doing fine.

“These two sensors together can tell the user exactly what kind of stress the plant is undergoing. Inside the plant, in real time, you get chemical changes that rise and fall, and each one serves as a fingerprint of a different stress,” says Prof. Strano. “We’re incorporating this technology into diagnostics that can give farmers real-time information much faster than any other sensor can, and fast enough for them to intervene.”

A paper on the study was recently published in the journal Nature Communications.

Source: MIT