Study tracking plant pathogens on planthoppers from natural areas

CHAMPAIGN, Ill. — Phytoplasmas are bacteria that can attack plant vascular tissues, causing various plant diseases. While most phytoplasma research begins by examining plants that show symptoms of disease, the new analysis focuses on tiny insects that carry infectious bacteria from plant to plant. By extracting and testing DNA from archives of planthopper specimens collected in natural areas, this study identified new phytoplasma strains and discovered new associations between planthoppers and phytoplasmas that are known to harm plants.

Reported in the journal Biology, the study is the first to look for phytoplasma in insects from a natural area, said Illinois Natural History Survey postdoctoral researcher Valeria Trivellone, who led the study with INHS State Entomologist Christopher Dietrich. It is also the first to use multiple molecular approaches to detect and identify phytoplasma in planthoppers.

The research team included Illinois Natural History postdoctoral researchers Yanghui Cao and Valeria Trivellone, front left and right, and INHS State Entomologist Christopher Dietrich.

Photo by Fred Zwicky

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“We compared the traditional molecular technique with the next-generation sequencing approach, and we found that the newer technique outperformed the traditional one,” Trivellone said. This method will allow researchers to target more regions of the phytoplasma genome to get a clearer picture of the different bacterial strains and how they damage plants, he said.

“One thing that’s really new about this research is that we’ve focused on disease vectors, on leafhoppers, and not on plants,” says Dietrich. The standard approach to searching for phytoplasma in plants is much more labor intensive, requiring scientists to extract DNA from apparently diseased plants and examine the phytoplasma, he said.

The researchers used traditional and newer genetic sequencing techniques to look for phytoplasma in planthoppers.

Photo by Fred Zwicky

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“But even when you identify a phytoplasma, you don’t know what leafhoppers or other vectors are transmitting to plants,” says Dietrich. “So researchers must return to the field to collect all potential insect vectors. Then they did a contagion experiment, where they let the planthopper eat the infected plant and then put it on the uninfected plant to see if it caught the disease.”

Because the research was painstaking and slow, “we still don’t have a good idea of ​​which insects spread the most phytoplasma among plants,” says Dietrich. “It really limits your ability to set up an effective management strategy.”

For the new study, the researchers turned to leafhopper specimens in the INHS insect collection. Dietrich has collected many of these insects over a 25 year period as part of his work classifying their genetic and evolutionary interrelationships. The researchers examined 407 planthopper species collected worldwide in areas less disturbed by human development. Specimens came from North and South America, Africa, Europe, Asia and Australia.

The team extracted total DNA from the specimens and processed each one, using both traditional and newer sequencing approaches. The latter is cheaper and more informative than traditional methods, the researchers report. Of the insect samples, 41 tested positive for phytoplasma, and the researchers obtained usable phytoplasma sequence data from 23 planthoppers. Phytoplasmas include the one causing the disease known as aster yellow, which inhibits photosynthesis and reduces the productivity of several different crop plants. This phytoplasma is found in several new leafhopper species that have never been identified as disease vectors.

“These leaf locusts can transmit phytoplasma to wild plants in natural areas,” Trivellone said.

The study found phytoplasma in regions of the world where such disease had never been reported and identified several new types of bacteria. He also found previously unreported relationships between several phytoplasmas and planthopper species.

Scientists don’t have the tools to target bacteria on asymptomatic plants to prevent disease outbreaks, so controlling phytoplasma involves using pesticides to kill insect vectors.

“Because insecticides are only partially specific to the target insect, they kill a variety of beneficial insects as well, which is not sustainable,” says Trivellone.

Dietrich studied planthopper species collected for 25 years and now in the INHS collection.

Photo by Fred Zwicky

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“We discovered that there are a lot of new phytoplasmas in nature that no one has seen before,” says Dietrich. “They don’t cause disease symptoms in the native plants with which they are associated for perhaps millions of years. They only start causing disease when they jump into a new host that has never been exposed to phytoplasma before.”

The new findings parallel those seen in human emerging infectious diseases of wildlife origin, Dietrich said. “This is why we need to look wider in nature and see what’s out there.”

The National Science Foundation supported this research.

INHS is a division of the Prairie Research Institute at the University of Illinois Urbana-Champaign.

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