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Spinach transformed into a “bomb sniffer” capable of communicating with humans? Though initially discredited by the scientific community, we have known since 1983 that plants communicate with one another through volatile organic compounds to their neighbours warning them of dangers, such as herbivores or aphids [1]. Additionally, garden pea plants can inform their neighbours of impending drought using their root systems [2]. Some plants may even communicate through sound. A clicking sound was recorded from young corn plants grown in water, which appeared to cause neighbouring roots to reroute towards the sound [3]. Another fantastic example is the concave structure of a particular carnivorous plant native to Borneo, is thought to reflect the echolocation of bats, thus helping them find the plant (and resulting in the fertilisation of the soil with their guano) [4].  Now, with a little help from engineers at the Massachusetts Institute of Technology (MIT), spinach plants (Spinacia oleracea) have broken the human/plant communication barrier [5]. Popeye, the Sailor Man Strano, described the potential of “plant nanobionics” in a recent Nature Materials paper. The team have opened up previously unimaginable applications for plants, from terrorism prevention to drought detection.

Even though I have no background in nanotechnology, I found this paper to be both fascinating and accessible. Plant nanobionics is an emerging field, and it is important that scientists involved communicate as clearly as possible to the public to prevent confusion with/or experiencing the same backlash as genetic manipulation. So without further ado.

Plant nanobionics embeds non-native functions into plants, interfacing them with specifically designed nanoparticles. Plants are extremely sensitive to changes in their environment, and their extensive root networks are continuously sampling groundwater which is passed up to the leaves, see Figure 1 [5]. By taking advantage of their natural capabilities, and incorporating some nanotechnology, the engineered plants in this study were able to signal the change via email. The researchers point out that by having plants which can relay information like this, could allow for earlier preparation for situations like droughts and faster responses to possible security threats. During testing, the team found that when a nitroaromatic was present in groundwater, which is sampled by the plant, the embedded carbon nanotubes were able to signal the change.

Screen Shot 2017-03-17 at 14.24.15
Figure 1 – Diagrammatic depiction of detection set-up with nanobionic plant and a Raspberry Pi CCD detector. The plant acts as if it were a fluidic device, sampling water from the environment. As the leaves transpire, water and other analytes are drawn up from the roots into the stem and towards leaves. Adapted from Wong et al. 2016.

In the study researchers applied a solution of nanoparticles to the underside of the spinach leaves, inserting them into the mesophyll layer. The near-infrared fluorescent nanosensors are made up of single-walled carbon nanotubes, which are tiny hollow carbon rods. One of the nanosensors recognises nitroaromatics via infrared fluorescent emission and is attached to the peptide Bombolitin II, acts as the active sensor. When the molecule of interest binds to this “chaperone sensor” complex wrapped around a nanotube, the fluorescence of the tube is quenched. An infrared camera is triggered by the drop in fluorescence, and so long as the camera is connected to a computer, can be set up to send an email alert. A second nanosensor, bound to polyvinyl alcohol, a acts as a continuous reference signal, allowing comparison between the two signals to determine if anything was detected.

They chose picric acid, a nitroaromatic compound, as the analyte to be detected. This compound is common to explosives and was applied to the plants using either direct application (through the leaf surface) or through root uptake. The analytes accumulate inside leaf tissues, specifically the mesophyll layer, resulting in relative changes in emission intensity. To obtain the data, MIT engineers used a laser to prompt the nanotubes to emit near-infrared fluorescent signals detected by a camera connected to a Raspberry Pi computer. The researchers describe these sensors as real-time providers of information from the plant, capable of telling us about the environment they face. Taking approximately 10 minutes from the introduction of the chemical analyte in water before a signal was picked up in the leaves.

Plant nanobionics holds some seriously incredible potential. For this study, the team chose spinach as they wanted to demonstrate that the technology can be applied to “any living plant”. Before developing “bomb sniffing” spinach, Strano demonstrated the capabilities of nanoparticles in augmenting a plants’ photosynthetic abilities in Arabidopsis, a model plant. In that instance to detect a pollutant from combustion, nitric oxide.

The MIT engineers reckon that plants could be designed to communicate just about any change in their environment, as nanotubes could be engineered to glow differently when they detect different compounds. Plus these functionalities should be introducible into wild-type plants rapidly and efficiently, directly applying a solution to the spinach leaves inserted enough sensors for detection.

Botanists could use this nanotechnology to discover more about the inner workings of plants. Strano and his team have also engineered spinach plants to detect dopamine, which influences root growth, and are working towards additional sensors to track chemicals that plants use to relay information within their own tissues [6].

The real-time monitoring of plant health would allow easy and rapid access to information that could directly affect yield and margins in precision agriculture. Possibly also usable to maximise yields of rare compounds synthesised by plants, such as drugs used to treat cancer. There are foreseeable ways to use this approach to produce nanobionic plants which have a boosted photosynthetic capabilities, able to monitor multiple chemicals simultaneously or even transmit radio signals.

Great, so what are the snags? Even though the Raspberry Pi and camera are pretty cheap, currently the infrared cameras are only capable of picking up a signal from a meter away. The researchers deemed this distance required for detection a limiting factor for use under many situations, and are working on ways to increase the signal range. They also found that the Raspberry Pi CCD silicon detector is not as sensitive to higher wavelengths of near-infrared fluorescence, compared to a fancy “two-dimensional (2D) array InGaAs detector (Princeton Instruments OMA V)” though they are still extremely excited by the camera-Raspberry Pi combinations potential. 

Some other challenges facing the technology include the significant plant-to-plant variation in transport rates and root permeability, which can make quantitative calibration difficult. The spinach used in the study were three-week-old plants, a more mature/larger plant may take longer than 10 minutes to accumulate enough of the analyte for detection to occur. Furthermore, unlike genetically engineered plants designed to have increased photosynthetic capability, plant-based nanobiotechnology lacks the advantage of having the plants replicate the system through to their offspring. There is, therefore, greater difficulty in scaling when compared to genetic engineering methods.

What is worse than a triffid? Probably one that could communicate its demands.

Links Out:

The Study by MIT Engineers – get your information from the horse’s mouth.

MIT’s blog – keep an eye out for the inevitable updates on this tech.

Plant talk – some more detail on the ways plants communicate with one another.

[1] Karban, R. et al. (2000) Oecologia, 125: 66-71.

[2] Falik, O., et al. (2011) PLoS ONE 6(11): e23625.

[3] Gagliano, M., et al. (2012) Trends in Plant Science, 17(6), 323 – 325.

[4] Schöner, M. G., et al. (2015) Current Biology, 25(4), 1911 – 1916.

[5] Wong, M.H. et al. (2016) Nature Materials. 16, 264–272.

[6] Trafton, A., (2016) MIT News


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