Bacteria perform a number of functions in nature: sometimes good and sometimes bad. It’s a kind of Dr.-Jekyll-and-Mr.-Hyde form in nature, and this makes it hard on nurseries, too. There are benign soilborne bacteria that can turn pathogenic in the same setting, and researchers are learning why.

One such bacterium is Rhodococcus, and while it can be a plant-associated bacteria working as a beneficial, some forms can be pathogenic. A team of Oregon State University researchers, including Elizabeth Savory, Skylar Fuller and Alexandra Weisberg, turned to genome sequencing to identify species of the bacteria that move from beneficial and stimulating plant growth, to being a pathogen that deforms plant tissues.

Researchers published new findings in the journal eLife looking at how this bacteria goes from “good” to “bad.” What they learned is that the change is made possible by DNA molecules known as plasmids, according to Jeff Chang, a microbial genomicist in OSU’s College of Agricultural Sciences and the leader of the study. A plasmid is a DNA molecule that is maintained separately from the chromosome of the bacteria.

Transition a challenge
Chang says the ease of this transition is unusual and presents a difficult challenge for nurseries. “Beneficial strains of Rhodococcus cause growth of the plant that could be misinterpreted as disease symptoms. We traced how the beneficial and pathogenic members of Rhodococcus are moving from plant to plant and nursery to nursery.” With that information, nurseries can work to prevent the spread of the bacteria.

Essentially, the plasmids are traveling from bacterium to another, which increases the chance of changing from a beneficial to a pathogen. That mechanism is what the researchers observed, and the team used genomic tools to study plasmid patterns. It was Weisberg, a postdoctoral fellow in Chang’s lab, who made the observation.

The bacteria infect mainly herbaceous perennials, including Shasta daisy, speedwell and chrysanthemum. While woody hosts are rarer, there are a few, including butterfly bush and false spirea. The evolutionary transition by the movement of plasmids has potential to make new lines of pathogenic Rhodococcus in nurseries and other environments, Chang says.

Study results will affect Oregon’s $900 million greenhouse and nursery industry. Those deformed plants aren’t aesthetically pleasing, and diseased plants can’t be shipped out of state. The only alternative is to destroy infected plants.

New kit developed
To help nurseries, the university developed molecular tests that work in commercially available kits that allow nurseries to quickly discriminate between the beneficial and pathogenic strains of the bacteria. OSU has filed for a patent for the molecular tools, which were developed by Fuller.

Adds Weisberg: “These plasmids can be transferred from bacterium to another, which makes tracking the disease difficult. Tracing the plasmids separately from the chromosome was the key to understanding how these transitions relate to each other.”

The research was supported by the USDA National Institute of Food and Agriculture, and Savory and Weisberg’s fellowships were also funded by USDA NIFA. Fuller’s research was supported by OSU’s Department of Botany and Plant Pathology.

Source: Oregon State University