Researchers have demonstrated a technique for successfully encapsulating bacteria that can then be stored and applied to plants to improve plant growth and protect against pests and pathogens. The technique opens the door to creating a wide range of crop applications that allow farmers to make use of these beneficial bacteria in conjunction with agrochemicals.
"Many of the beneficial bacteria we know of are fairly fragile, making it difficult to incorporate them into practical, shelf-stable products that can be applied to plant roots or leaves," says John Cheadle, co-lead author of a paper on the work and a Ph.D. student at North Carolina State University. "The technique we demonstrate here essentially stabilizes these bacteria, making it possible to develop customized probiotics for plants."
At issue are plant growth-promoting bacteria (PGPBs), which are microbes that benefit plant health and growth, helping plants extract nutrients from the environment and protecting them from pests or pathogens.
"A longstanding challenge for making use of these bacteria has been that if you tried to come up with a single application that combined them with agrochemicals, like pesticides or fertilizers, the bacteria would die," says Saad Khan, co-corresponding author of the paper and INVISTA Professor of Chemical and Biomolecular Engineering at NC State. "We wanted to develop a solution that would allow bacteria to be used in conjunction with chemicals already in widespread use by growers."
"By the same token, a healthy plant microbiome allows the plants to make better use of nutrients available in the soil and more resistant to pathogens," says Tahira Pirzada, co-corresponding author and a research scholar at NC State. "This may allow growers to use less fertilizer and pesticides without hurting crop production."
The new technique revolves around a custom-made emulsion, with only a handful of ingredients. One part of the emulsion consists of a saline solution that contains PGPBs. For the proof-of-concept demonstration, the researchers used the bacteria Pseudomonas simiae and Azospirillum brasilense. P. simiae acts as a biopesticide by promoting pathogen resistance; A. brasilense acts as a biofertilizer by fixing nitrogen.
The second part of the emulsion consists of a biodegradable oil and a biodegradable polymer derived from cellulose. The polymer can be loaded with agrochemical active ingredients, which means the emulsion can incorporate these ingredients without relying on environmentally harmful organic solvents, which are typically used in pesticide formulations.
When the two parts of the emulsion are mixed together, the oil is broken into droplets that are distributed throughout the saline solution. The cellulose polymer sticks to the surface of these droplets, preventing the droplets from merging back together.
Essentially, the emulsion is a salad dressing with the oil droplets held in suspension throughout the saline solution. In practical terms, this would allow the PBPGs to be applied simultaneously with agrochemicals using the same emulsion.
To see how well the emulsion worked, the researchers did two tests.
First, the researchers compared the survival of PBPGs in the emulsion to the survival of PBPGs in the saline solution alone. Samples of each were stored at room temperature. After four weeks, the population of P. simiae in the emulsion was 200% higher than the population in saline; the population of A. brasilense in the emulsion was 500% higher.
Second, the researchers wanted to see how well pesticides would work when incorporated into the emulsion. For this, the researchers incorporated the pesticide fluopyram into the emulsion. They also added fluopyram to the saline solution by itself. The researchers then introduced C. elegans nematodes - which serve as a proxy for pests - into the emulsion and the saline solution.
"Not surprisingly, the pesticide in saline solution killed the pest very quickly - all of the pests were killed within an hour," says Mariam Sohail, co-lead author of the paper and a recent Ph.D. graduate from NC State. "The emulsion worked more gradually, killing 95% of the pests within 72 hours. This is valuable to know, since it suggests our technique could be used strategically to provide sustained protection against specific pests or pathogens.
"Ultimately, we found our technique allows us to incorporate multiple active ingredients into a single delivery system and allows the PGPBs to survive and thrive," Sohail says.
"We also demonstrated that the emulsion improved the survival and reproductive success of these bacteria when applied to soil, as compared to applying the bacteria to the soil without the emulsion," Cheadle says.
"Next steps will involve greenhouse testing and, later, microplots," says Khan. "We will likely want to evaluate different PGPBs and other active ingredients to see how they perform with different targeted plant species."
The paper, "Pickering Emulsion for Enhanced Viability of Plant Growth Promoting Bacteria and Combined Delivery of Agrochemicals and Biologics," is published open access in the journal Advanced Functional Materials. A third co-corresponding author on the paper is Nathan Crook, an assistant professor of chemical and biomolecular engineering at NC State. The paper was co-authored by Rishum Khan, a former masters student at NC state; Hrishikesh Mane and Katie Ernst, Ph.D. students at NC State; Khandoker Samaher Salem, a postdoctoral researcher at NC State; Adriana San Miguel, an associate professor of chemical and biomolecular engineering at NC State; and Charles Opperman, a professor of entomology and plant pathology at NC State.
This work was funded in part by the Global One Health Academy at North Carolina State University.
