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Nanoscale nutrients can protect plants from fungal diseases

Most, if not all, kitchen products are likely to be threatened by fungal diseases. The threat is met with the world’s staple foods such as rice, wheat, potatoes and corn (SN: 22/09/05). Pathogenic fungi are also provided for our coffee, sugar cane, bananas and other economically important crops. Annually, fungal diseases destroy a third of all crops and pose a serious threat to global food security.

To stop the spread of fungal diseases, farmers spray the soil with toxic chemicals that ravage the soil, without sparing even the beneficial microbes that abound in the soil. Or cover plants with fungicides. But the use of fungicides is only effective in the short term – until pathogenic fungi evolve by resisting these synthetic chemicals.

Now, a new idea is taking root: it helps plants resist by giving them the tools to fight their own battles. A team led by Jason White, an environmental toxicologist at the Connecticut Agricultural Experiment Station in New Haven, is fortifying crops with nutrients formed in nanosimensioned packages, which increase the innate immunity of plants to pathogenic fungi more efficiently than traditional feeding of plants. Over the past few years, researchers have devised several nanonutrient blends that increase resistance to soybean, tomato, watermelon, and recently eggplant fungi, as reported in the April plant disease.

The concept “addresses the challenge at the source rather than trying to put help on the (problem),” says Leanne Gilbertson, an environmental engineer at the University of Pittsburgh who did not participate in the research. White’s strategy provides plants with the nutrients they need to trigger enzyme production to protect themselves from a pathogenic attack. Without the introduction of synthetic chemicals, the strategy avoids any chance of malignancies developing resistance, she says.

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The researchers ’approach to nanomaterials is inspired by their earlier discovery that nanoparticles transported from maize roots can recede from leaves. The researchers immersed half of the root fibers of a single corn plant in a formulation of copper nanoparticles and the other half in pure water. White and colleagues reported in 2012 on Environmental Science & Technology. Copper appeared in the roots submerged in water, pointing back and forth between roots and shoots. That finding suggested that nanoparticles can be applied directly to leaves in the first place, even when the target target was the roots.

Using the leaves as an entry point solves a perennial problem: the delivery of dissolved nutrients by the soil is inefficient. Chemicals can decompose in the soil, vaporize into the atmosphere, or leach. Only about 20 percent of the watered nutrients end up reaching the target areas of a plant. “By using nanoscale shape, we can deliver (nutrients) more efficiently where we want it and where the plant needs it,” says White.

To see if this approach could provide nutrients needed specifically for defense against hostile fungi, White and colleagues conducted tests on eggplants and tomatoes. The team sprayed metal nanoparticles on the leaves and shoots of young plants and then infected the plants with pathogenic fungi. Plants treated with nanoparticles had high levels of nutritional metals in the roots and higher yields compared to plants fed easily dissolved nutrients, the team reported in 2016 in Environmental Science: Nano.

The researchers found that the nanoparticles did not harm the fungi: they still thrived between nanoparticles in the environment without the host plant being present. In contrast, the antifungal properties of nanoparticles derive from providing food to plants – equivalent to humans taking nutritional supplements – that allow plants to mount an adequate defense on demand.

What makes nanonutrients more potent than common fertilizers is the sweet spot of their sizes, which control the rate at which they dissolve, says Fabienne Schwab, an environmental chemist who is not involved in research. Nanonutrients are thousands of times smaller than the diameter of human hair and thousands of times larger than easily dissolved nutrient salts. They have a large, exposed surface, so they dissolve faster than a stronger piece of the same nutrient. However, nanonutrients are large enough that they do not dissolve at the same time: they can gradually release nutrients over weeks. In contrast, easily dissolved nutrients give plants a temporary increase in nutrients, similar to the sugar rush.

“When you use (nutrients) at the nanometer scale, you can tune the solubility pretty much as you like,” says Schwab of the Adolphe Merkle Institute in Freiburg, Switzerland.

Only size cannot be adjusted: shape, composition, and surface chemicals can be modified to stimulate a plant’s different levels of responses. For example, White and colleagues found that thin copper oxide sheets in nanometers were better than spherical copper nanoparticles to prevent Fusarium virguliform infection in soybeans. The key to its effectiveness lay in the faster release of charged copper atoms into the dwarf leaves and a strong adhesion to the leaf surfaces. Copper nanomaterials have restored soybean masses and photosynthesis rates to disease-free plant levels, the team reported in Nature Nanotechnology in 2020.

“It’s a very promising technology,” Schwab says, but adds that there are other aspects to consider before implementing it. If agricultural nanotechnology is to achieve widespread use, it must abide by environmental and safety regulations, as well as, perhaps even more challengingly, overcome consumer caution. So far, White and colleagues have found no residual nanonutrients in their products that would end up on the consumer dining table. But other implications, such as the persistence of nanomaterials in the environment and the dangers it poses to human handlers, have not yet been fully understood.

“People in general get nervous when you talk about nanotechnology and food,” White says. But he says his group is not using any exotic material, whose health impacts remain complete enigmas. Instead, "we're using nutrients that plants need (that) they just can't get enough of."

White says he ate the eggplants, tomatoes and watermelons he grew for his research. And perhaps that is the best reassurance consumers can get: a toxicologist who tests the literal fruit of his work.

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