Bacterial speck is a common disease affecting tomatoes that can result in lower yields for growers. A new study led by researchers at Penn State gives new clues on how a plant's microbiome can be used to combat the pathogen.
The research - published in the journal Environmental Microbiome - examined how disease suppressive microbiomes of a tomato plant's phyllosphere, the portion of the plant above ground, differed from the microbiomes of plants that were conducive to bacterial speck.
The team found that a number of populations of Xanthomonas and Pseudomonas bacteria were present on the plants that had developed a resistance against bacterial speck, suggesting they play a role in suppressing the disease.
Kevin Hockett, associate professor of microbial ecology in the College of Agricultural Sciences and lead author on the paper, said the findings could eventually help lead to new treatments for plants, as well as open up opportunities for further research.
"If we can learn more about which microbes are driving down the disease, it's possible that we could isolate and combine them in the future for growers to use as a treatment," he said. "Additionally, some of the most important crop diseases are fungal, so if we can show that this process works for fungi, that could open up even more research and possible applications."
The study was inspired by the way some soil microbiomes can develop season over season to eventually suppress plant disease. Hockett explained that if a crop is sensitive to a particular disease and a grower plants the crop in the same spot year after year, in some cases microbes in the soil will eventually shift to suppress that pathogen and the disease will go down.
He said that while this process has been observed, its exact mechanisms aren't well known, and it's not clear if this could happen above ground, too.
"We've seen this with soils, which makes sense because the same soil is there year after year," Hockett said. "In the case of the above ground portions of plants - leaves, flowers, fruits - all of that gets harvested and removed from the field or tilled under. So, we were curious about whether we could replicate this process on the plant's leaves."
In a previous paper, the researchers found that yes, a plant's microbiome can changel to suppress the bacteria that causes bacterial speck. But because microbiomes are made up of many types of microorganisms - including bacteria, fungi and viruses - the researchers were interested in learning precisely which microbes were driving this disease suppression.
For the current study, the researchers started by spraying tomato plants with the bacteria that causes bacterial speck. After a few days, they "passaged" the microbiome by choosing the plants that had the least amount of disease, washing the leaves into a solution to collect the microbiome, and then spraying the solution onto a new plant.
The team also included a control group, in which they performed the passaging protocol on different plants that did not have a pathogen applied. Because there was no disease, they chose the leaves to passage at random. They did this process nine times before collecting and analyzing the microbiome of the final passage.
Now that they know more about the composition of the disease-suppressive microbiome, Hockett said they have better clues about which microbes to target for possible future treatments.
"It may be that the whole microbial community is necessary to be effective, but the first thing we want to do is go in and start pulling this community apart to identify who are really the important players for disease suppression," he said. "Because it may be that there are some members that got selected during the passaging but they don't really contribute anything to disease suppression."
Hanareia Ehau-Taumaunu, postdoctoral scientist at the Bioprotection Aotearoa and Bioeconomy Science Institute; Terrence Bell, assistant professor at the University of Toronto; and Javad Sadeghi, postdoctoral fellow at the University of Toronto, also co-authored this study.
The U.S. Department of Agriculture's National Institute of Food and Agriculture, Fulbright New Zealand Science and Innovation Graduate Award, Indigo Agriculture Phytobiomes Fellowship, Penn State One Health Microbiome Center, Northeast SARE Graduate Award, and Ngārimu VC and 28th (Māori) Battalion Memorial Doctoral Scholarship, and PA Vegetable Growers Association helped support this research.
