The sensors detect two signaling molecules that plants use to coordinate their response to stress: hydrogen peroxide and salicylic acid.

Researchers found that plants produce these molecules at different times for each type of stress, creating distinctive patterns that could serve as an early warning system. Farmers could use these sensors to monitor potential threats to their crops, allowing them to intervene before the crops are lost, researchers say.

The researchers found that the 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 professor Michael Strano, one of the senior authors of the study.

Plants respond to different kinds of stress in different ways. The sensors consist of tiny carbon nanotubes wrapped in polymers. By changing the three-dimensional structure of the polymers, the sensors can be tailored to detect different molecules, giving off a fluorescent signal when the target is present.

To embed the nanosensors into plants, the researchers dissolve them in a solution, which is then applied to the underside of a plant leaf. The sensors can enter leaves through pores called stomata and take up residence in the mesophyll, the layer where most photosynthesis takes place. When a sensor is activated, the signal can be easily detected using an infrared camera.

In this study, the sensors for hydrogen peroxide and salicylic acid were applied to pak choi, a leafy green vegetable also known as bok choy. Then, researchers exposed the plants to four different types of stress — heat, intense light, insect bites, and bacterial infection — and found that the plants generated distinctive responses to each.

Each type of stress led the plants to produce hydrogen peroxide within minutes, reaching maximum levels within an hour and then returning to normal. Heat, light, and bacterial infection all provoked salicylic acid production within two hours of the stimulus, but at distinct time points. Insect bites did not stimulate salicylic acid production at all.

For stress such as an insect bite, a plants’ response includes the production of chemical compounds that insects don’t like, driving them away from the plant. Salicylic acid and hydrogen peroxide can also activate signaling pathways that turn on the production of proteins that help plants respond to heat and other stresses.

This technique is the first that can obtain real-time information from a plant, and the only one that can be applied to nearly any plant. Most existing sensors consist of fluorescent proteins that must be genetically engineered into a specific type of plant and can’t be universally applied.

The researchers are now adapting these sensors to create sentinel plants that could be monitored to give farmers a much earlier warning when their crops are under stress.

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Bacteria that can convert nitrogen gas to ammonia could not only provide nutrients that plants need, but also help regenerate soil and protect plants from pests. However, these bacteria are sensitive to heat and humidity, so it’s difficult to scale up their manufacture and ship them to farms.

To overcome that obstacle, MIT chemical engineers have devised a metal-organic coating that protects bacterial cells from damage without impeding their growth or function. In a new study, they found that these coated bacteria improved the germination rate of a variety of seeds, including vegetables such as corn and bok choy.

This coating could make it much easier for farmers to deploy microbes as fertilizers, says Ariel Furst, chemical engineering assistant professor at MIT and the senior author of the study.

“We can protect them from the drying process, which would allow us to distribute them much more easily and with less cost because they’re a dried powder instead of in liquid,” she says. “They can also withstand heat up to 132 degrees Fahrenheit, which means that you wouldn’t have to use cold storage for these microbes.”

Nitrogen-fixing bacteria, which convert nitrogen gas to ammonia, can aid farmers without the use of synthetic chemicals.

Some farmers have already begun deploying these “microbial fertilizers,” growing them in large onsite fermenters before applying them to the soil. However, this is cost-prohibitive for many.

Shipping these bacteria to rural areas is not currently a viable option, because they are susceptible to heat damage. The microbes are also too delicate to survive the freeze-drying process that would make them easier to transport.

To protect the microbes from both heat and freeze-drying, Furst decided to apply a coating called a metal-phenol network (MPN), which she has previously developed to encapsulate microbes for other uses, such as protecting therapeutic bacteria delivered to the digestive tract.

The coatings contain two components — a metal and an organic compound called a polyphenol — that can self-assemble into a protective shell.

The metals used for the coatings, including iron, manganese, aluminum and zinc, are considered safe as food additives. Polyphenols, which are often found in plants, include molecules such as tannins and other antioxidants.

For this study, the researchers created 12 different MPNs and used them to encapsulate Pseudomonas chlororaphis, a nitrogen-fixing bacterium that also protects plants against harmful fungi and other pests. They found that all of the coatings protected the bacteria from temperatures up to 50 C, and also from relative humidity up to 48 per cent. The coatings also kept the microbes alive during the freeze-drying process.

Using microbes coated with the most effective MPN — a combination of manganese and a polyphenol called epigallocatechin gallate — the researchers tested their ability to help seeds germinate in a lab dish.

They heated the coated microbes to 50 C before placing them in the dish, and compared them to fresh uncoated microbes and freeze-dried uncoated microbes.

The researchers found that the coated microbes improved the seeds’ germination rate by 150 per cent, compared to seeds treated with fresh, uncoated microbes. This result was consistent across several different types of seeds, including dill, corn, radishes and bok choy.

Furst has started a company called Seia Bio to commercialize the coated bacteria for large-scale use in regenerative agriculture. She hopes the low cost of the manufacturing process will help make microbial fertilizers accessible to small-scale farmers who don’t have the fermenters needed to grow such microbes.

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A team of engineers has coated seeds with silk that has been treated with a kind of bacteria that naturally produce a nitrogen fertilizer, to help the germinating plants develop. Tests have shown that these seeds can grow successfully in soils that are too salty to allow untreated seeds to develop normally. The researchers hope this process, which can be applied inexpensively and without the need for specialized equipment, could open up areas of land to farming that are now considered unsuitable for agriculture.

The findings were published in the journalPNAS, in a paper by graduate students Augustine Zvinavashe and Hui Sun, postdoc Eugen Lim, and professor of civil and environmental engineering Benedetto Marelli.

The work grew out of Marelli’s previous research on using silk coatings as a way to extend the shelf life of seeds used as food crops. “When I was doing some research on that, I stumbled on biofertilizers that can be used to increase the amount of nutrients in the soil,” he says. These fertilizers use microbes that live symbiotically with certain plants and convert nitrogen from the air into a form that can be readily taken up by the plants.

Not only does this provide a natural fertilizer to the plant crops, but it avoids problems associated with other fertilizing approaches, he says: “One of the big problems with nitrogen fertilizers is they have a big environmental impact, because they are very energetically demanding to produce.” These artificial fertilizers may also have a negative impact on soil quality, according to Marelli.

Although these nitrogen-fixing bacteria occur naturally in soils around the world, with different local varieties found in different regions, they are very hard to preserve outside of their natural soil environment. But silk can preserve biological material, so Marelli and his team decided to try it out on these nitrogen-fixing bacteria, known as rhizobacteria.

“We came up with the idea to use them in our seed coating, and once the seed was in the soil, they would resuscitate,” he says. Preliminary tests did not turn out well, however; the bacteria weren’t preserved as well as expected.

That’s when Zvinavashe came up with the idea of adding a particular nutrient to the mix, a kind of sugar known as trehalose, which some organisms use to survive under low-water conditions. The silk, bacteria, and trehalose were all suspended in water, and the researchers simply soaked the seeds in the solution for a few seconds to produce an even coating. Then the seeds were tested at both MIT and a research facility operated by the Mohammed VI Polytechnic University in Ben Guerir, Morocco. “It showed the technique works very well,” Zvinavashe says.

The resulting plants, helped by ongoing fertilizer production by the bacteria, developed in better health than those from untreated seeds and grew successfully in soil from fields that are presently not productive for agriculture, Marelli says.

In practice, such coatings could be applied to the seeds by either dipping or spray coating, the researchers say. Either process can be done at ordinary ambient temperature and pressure. “The process is fast, easy, and it might be scalable” to allow for larger farms and unskilled growers to make use of it, Zvinavashe says. “The seeds can be simply dip-coated for a few seconds,” producing a coating that is just a few micrometers thick.

The ordinary silk they use “is water soluble, so as soon as it’s exposed to the soil, the bacteria are released,” Marelli says.

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