Herbicide harms plant-friendly bacteria

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Farmers fighting weeds with herbicide may also be unintentionally killing bacteria that benefit the soil and guard against fungus, new research suggests.

Researchers found that using the weed-killing herbicide glyphosate negatively affected Pseudomonas, a soil-friendly kind of bacteria.

“Beneficial Pseudomonas in the soil can help crops thrive. They can produce plant-stimulating hormones to promote plant growth and antifungals to defeat problematic fungi—such as Pythium and Fusarium—found in agricultural soil, but previous studies reported that the abundance of beneficial bacteria decreased when the herbicide glyphosate seeps underground,” says Ludmilla Aristilde, an assistant professor of biological and environmental engineering at Cornell University. “Our study seeks to understand why this happens.”

Soil bacteria require their proteins—composed of amino acids—and their metabolism to support cellular growth and the production of important metabolites to sustain their underground fight. But glyphosate applied to crops can drain into the soil and disrupt the molecular factories in the bacterial cells in some species, interfering with their metabolic and amino acid machinery.

The new findings show that glyphosate does not target the amino acid production and metabolic gadgetry equally among the Pseudomonas species. For example, when Pseudomonas protegens, a bacteria used as a biocontrol agent for cereal crops, and Pseudomonas fluorescens, used as a fungus biocontrol for fruit trees, were exposed to varying glyphosate concentrations, the researchers noted no ill effects.

In two species of Pseudomonas putida, used in soil fungus control for corn and other crops, however, the bacteria had notably stunted growth, says Aristilde, who is a faculty fellow at the university’s Atkinson Center for a Sustainable Future.

“Thus, if a farmer is using Pseudomonas fluorescens as a biocontrol, then it is probably okay to use glyphosate,” Aristilde says. “But if the farmer uses Pseudomonas putida to control the fungus in the soil, then glyphosate is more likely to prevent the bacteria from doing its job.”

Why farmers may want to keep, not kill, weeds

The study offers molecular details for why glyphosate adverse effects on Pseudomonas are species-specific.

“That’s actually good news because—as a society—we will likely not stop using herbicide completely,” says Aristilde. “If that is the case, farmers need to know which beneficial soil biocontrol they’re using can be susceptible. If they’re using a strain that is susceptible and conflicting with their herbicide application, then it is a problem. That’s the bottom line.”

The study appears in Frontiers of Environmental Science.

The US Department of Agriculture’s National Institute of Food and Agriculture, the National Science Foundation, and the Academic Venture Fund at Cornell University’s Atkinson Center for a Sustainable Future supported the research.

Source: Cornell University