CHAMPAIGN, Ill. — Phytoplasmas are bacteria that can invade the vascular tissues of plants, causing many different crop diseases. While most studies of phytoplasmas begin by examining plants showing disease symptoms, a new analysis focuses on the tiny insects that carry the infectious bacteria from plant to plant. By extracting and testing DNA from archival leafhopper specimens collected from natural areas, the study identified new strains of phytoplasmas and found new associations between leafhoppers and phytoplasmas known to harm crop plants
Reported in the journal Biology, the study is the first to look for phytoplasmas in insects from natural areas, said Illinois Natural History Survey postdoctoral researcher Valeria Trivellone, who led the research with entomologist state INHS Christopher Dietrich. It is also the first to use a variety of molecular approaches to detect and identify phytoplasmas in leafhoppers.
The research team included Illinois Natural History postdoctoral researchers Yanghui Cao and Valeria Trivellone, front left and right, and INSS State Entomologist Christopher Dietrich.
Photo by Fred Zwicky
delete
Edit the embedded media in the Files tab and re-embed as needed.
“We compared traditional molecular techniques with next-generation sequencing approaches and found that the newer techniques outperformed the traditional ones,” Trivellone said. These methods will allow researchers to target more regions of phytoplasma genomes to get a clearer picture of different bacterial strains and how they damage plants, he said.
“One thing that’s really new about this study is that we’ve focused on disease vectors, leafhoppers, and not plants,” Dietrich said. The standard approach to looking for phytoplasmas in plants is much more labor intensive, requiring scientists to extract DNA from a plant that appears to be diseased and check for phytoplasmas, he said.
The researchers used both traditional and newer genetic sequencing techniques to search for phytoplasmas in leafhoppers.
Photo by Fred Zwicky
delete
Edit the embedded media in the Files tab and re-embed as needed.
“But even when you identify the phytoplasma, you don’t know which leafhopper or other vector transmitted it to the plant,” Dietrich said. “So the researchers have to go back into the field to collect all the possible insect vectors. Then they do transmission experiments, where they let the cicadas feed on an infected plant and then put them on an uninfected plant to see if spread the disease.”
Because such research is laborious and slow, “we still don’t have a good idea of which insects are spreading the most phytoplasmas among plants,” Dietrich said. “That really limits your ability to establish an effective management strategy.”
For the new study, the researchers turned to grasshopper specimens in the INHS insect collection. Dietrich had collected many of these insects over a 25-year period as part of his work to classify their genetic relationship and evolution. The researchers examined 407 grasshopper species collected around the world in areas less disturbed by human development. The 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 novel sequencing approaches. The latter are less expensive and more informative than traditional methods, the researchers report. Of the insects sampled, 41 tested positive for phytoplasmas, and the researchers obtained usable phytoplasma sequence data from 23 leafhoppers. Phytoplasmas include those that cause a disease known as aster yellows, which inhibits photosynthesis and reduces the productivity of several different crop plants. These phytoplasmas were found in several new leafhopper species never before identified as disease vectors.
“These leafhoppers can transmit phytoplasmas to wild plants in natural areas,” Trivellone said.
The study found phytoplasmas in regions of the world where such diseases had not been reported and identified several new strains of the bacteria. He also found previously unreported associations between some phytoplasmas and grasshopper species.
Scientists lack tools to target bacteria on asymptomatic plants to prevent disease outbreaks, so controlling phytoplasmas involves using pesticides to kill insect vectors.
“Because insecticides are only partially specific to the target insects, they also kill a variety of beneficial insects, which is not sustainable,” Trivellone said.
Dietrich examines leafhopper species collected over a 25-year period and now in the INHS collection.
Photo by Fred Zwicky
delete
Edit the embedded media in the Files tab and re-embed as needed.
“We’re discovering that there are a lot of new phytoplasmas in nature that no one has ever seen before,” Dietrich said. “They don’t cause disease symptoms in the native plants they’ve been associated with for perhaps millions of years. They only start causing disease when they jump to a new host that hasn’t been exposed to the phytoplasma before.”
Dietrich said the new findings parallel those seen in emerging infectious diseases in humans originating in wildlife. “That’s why we need to look more widely across nature and see what’s out there.”
The National Science Foundation supports this research.
INHS is a division of the Prairie Research Institute at the University of Illinois Urbana-Champaign.