US scientists find nitrogen-fixing bacteria protect soy from aphids

An invasion of soy aphids poses a problem for soy farmers requiring application of pesticides, but a team of Penn State entomologists thinks a careful choice of nitrogen-fixing bacteria may provide protection against the insects.

 

Soy is legumes, plants that can have a symbiotic relationship with nitrogen-fixing bacteria -- rhizobia -- and therefore do not need additional nitrogen fertiliser. Each type of legume has its own rhizobia.

 

"Soy is from Asia and so there were originally no nitrogen-fixing bacteria that would colonise soy in US soils," said Consuelo De Moraes, associate professor of entomology. "The rhizobia had to be transferred here."

 

The soy aphid is also not native to North America. This pest only began to infest soy fields about 10 years ago but are now fully established pests requiring pesticide applications to avoid the loss of as much as 40 percent of the crop. The researchers investigated the relationship between the type of rhizobia colonising soy plants and the plants' infestation with the aphids.

 

Results demonstrate that plant–rhizobia interactions influence plant resistance to insect herbivores and that some rhizobia strains confer greater resistance to their mutualist partners than do others, the researchers reported in the journal Plant and Soil online.

 

The researchers looked at soy plants inoculated with the rhizobia provided by the inoculant company; without rhizobia, but with added nitrogen fertiliser, and by existing rhizobia in the soil.

 

The bacteria that were used initially to inoculate the first crops of soy are growing wild in the soil now," said the researchers, adding that they are now considered "naturally-occurring" and are different from the inoculants purchased with the soy seeds.

 

They become natural because they change through generations of contact with other rhizobia. While they may not provide as much nitrogen to the plant as commercial types, the trade off between optimal growth and heavy insect damage may still be worthwhile.

In most cases, the inoculant companies provide rhizobia for inoculation that gives plants the maximum yield, said Jennifer M. Dean, postdoctoral fellow in entomology.

 

Dean said their rhizobia are highly competitive against naturally occurring nitrogen-fixing bacteria, and that the inoculant companies treat the natural rhizobia almost as a pest.

 

Because of this, soy almost uniformly incorporates the specially developed rhizobia rather than the natural ones. However, the researchers found that the plants associated with the naturally occurring rhizobia had lower aphid densities than either the artificially fertilised plants or the plants inoculated with commercial rhizobia. They also found the same level of nitrogen in both soy plants inoculated with natural rhizobia and those inoculated with commercial varieties.

 

This is the first time anyone has shown how different strains of rhizobia can affect herbivory, said De Moraes.

 

He said this may be another tool to use to protect plants from insect herbivory and it may also be applicable to other legumes.

 

It is still unknown what the natural nitrogen-fixing bacteria do to repel aphids, but there is a plan to isolate rhizobia strains from the fields and look for the specific mechanism of how they repel the aphids, according to Dean.