Plant Detectives Develop Tool to Track Spread of Major Crop Disease

Crown-gall disease, caused by Agrobacterium, seen on sunflowers. (By LkazenOwn work, CC BY-SA 4.0, Link)

(CN) — Researchers from Oregon State University have developed a method that targets the spread of a disease-causing bacterium — a promising development for preventing ailments of more than 100 plant species, according to a new study.

This method is a significant discovery, especially for commercial plant growers, because the spread of Agrobacterium costs the nursery industry billions of dollars a year. The bacterium causes what is known as crown-gall disease, in which plants develop overgrowths on their roots and bases, particularly in fruit trees, roses, grape vine, nursery plants, shade trees, and more.

According to the U.S. Department of Agriculture, the disease targets plant species worth a combined $16 billion annually in the United States. In this study, published Thursday in the journal Science, Oregon State scientists collaborated with researchers at the USDA Agricultural Research Service to develop a way to prevent the unhindered spread of crown-gall disease.

Their findings go further than just plant disease and can potentially be useful in tracking diseases in other fields, including the targeting of plasmids that cause antibiotic-resistant genes. With more research, it can be applied to track human, animal, and foodborne illnesses.

“Understanding the genetic basis for how pathogens emerge and diversify in agricultural ecosystems is foundational for determining their spread and assessing risks,” said Jeff Chang, a professor in Oregon State’s College of Agricultural Sciences and one of the authors of the study. “These are critical to informing policies for improving plant health and preparing against disease outbreaks to increase global food security.”

The researchers looked closely at Agrobacterium, a bacterium that causes tumors in plants, or more specifically, the DNA molecules within it known as plasmids. These molecules are responsible for the spread of diseases among plants, but they are especially potent in Agrobacterium as they have genes that allow it to essentially “genetically reprogram” the host plant to develop hairy root disease.

Furthermore, the plasmids possess genes that allow the disease to be spread horizontally rather than just to an offspring, which means one plant can pass on the disease to other nearby plants. This makes the disease especially potent and difficult to manage because with each new host, the strain of Agrobacterium can become a more harmful and novel pathogen.

Because of this, the researchers needed to first analyze and understand these plasmids in order to develop a system to track them. Before this, it was accepted as virtually impossible to identify evolutionary relationships between the transfer of genetic information among plasmids to the many genetic variations of Agrobacterium. 

This information is vital to tracking disease outbreaks, so the researchers began by investigating two classes of plasmids in particular: tumor inducing and root inducing. Both allow Agrobacterium to transfer plasmids into plants causing disease. The team was provided with hundreds of strains with plasmids from Melodie Putnam, director of the Oregon State Plant Clinic, as well as others at OSU and USDA-ARS, who helped analyze the data. 

Lead author and post-doctoral researcher Alexandra Weisberg, co-mentored by Chang and Niklaus Grünwald of the USDA-ARS Horticultural Crops Research Unit in Corvallis, made a significant discovery while analyzing the datasets. She found that after sequencing 140 strains with plasmids, they all seemed to descend from nine distinct lineages.

“Armed with this extensive genetic sequencing information about how to classify plasmids and Agrobacterium, we could infer both how bacteria move among nurseries and how the plasmids move among bacteria,” Weisberg said.

“Having whole genome sequences of Agrobacterium allowed the researchers to link nurseries on the basis of having strains with the same genome and plasmid sequences, the same genome sequence but different plasmid sequences, or different genome sequences but the same plasmid sequences,” Weisberg continued.

With this new method, the team successfully tracked at least seven cases where global distribution of plants carrying a single Agrobacterium strain-plasmid combination caused widespread transmission. The authors note one of these cases involved a nursery for wholesalers that could have been where the strain first started. They later identified two more nurseries in another part of the world affected by the same genotype-plasmid combination.

After discovering how to analyze the bacteria from the plasmid, they discovered a multitude of cases where plasmid transmission was the cause for the spread of disease. One case involved a strain-plasmid combination collected in 1964 where identical plasmid sequences were found in different parts of the world in strains 30-40 years later.

Moving forward, the authors hope their findings in plasmid study will have applications in biotechnology tools as it pertains to Agrobacterium. Some strains and plasmids have been utilized in the past to study plant functions and trait selection, and with this new understanding of relationships between plasmids, this research holds the promise of advance.

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