Phytoplasmas are bacteria that can invade the vascular tissues of plants, causing many different crop diseases. While most phytoplasma studies begin by looking at plants showing disease symptoms, a new analysis focuses on the tiny insects that carry 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 in natural areas, said Illinois Natural History Survey postdoctoral researcher Valeria Trivellone, who led the research with state entomologist from INHS Christopher Dietrich. He is also the first to use a variety of molecular approaches to detect and identify phytoplasmas in leafhoppers.
“We compared traditional molecular techniques with next-generation sequencing approaches, and found that the new techniques outperformed traditional techniques,” 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, she said.
“One thing that’s really new in this study is that we focused on the disease vectors, the leafhoppers, not the plants,” Dietrich said. The standard approach to finding 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.
“But even when you identify the phytoplasma, you don’t know which leafhopper or other vector transmitted it to the plant,” Dietrich said. “So researchers have to go back into the field to collect all the potential insect vectors. Then they do transmission experiments, where they let the leafhoppers feed on an infected plant, then place them on an uninfected plant to see if she catches the disease.”
Because this research is laborious and slow, “we still don’t have a good idea of which insects spread the most phytoplasmas between plants,” Dietrich said. “It really limits your ability to put together an effective management strategy.”
For the new study, the researchers turned to leafhopper specimens from the INHS insect collection. Dietrich had collected many of these insects over a 25-year period as part of his work classifying their genetic relatedness and evolution. The researchers looked at 407 leafhopper species collected around the world from 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 samples and processed each one, using traditional and newer 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 included those that cause a disease known as aster yellows, which inhibits photosynthesis and reduces the productivity of several different crop plants. These phytoplasmas have been found in several new leafhopper species never previously identified as disease vectors.
“These leafhoppers can transmit phytoplasmas to wild plants in natural areas,” Trivellone said.
The study found phytoplasmas in areas of the world where such diseases had not been reported and identified several new strains of bacteria. He also found previously unreported associations between certain phytoplasmas and leafhopper species.
Scientists have no tools to target bacteria in symptomless plants to prevent outbreaks. Phytoplasma control therefore involves the use of pesticides to kill insect vectors.
“Because insecticides are only partially specific to target insects, they also kill a variety of beneficial insects, which is not sustainable,” Trivellone said.
“We are finding that there are many 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.”
The new findings parallel those seen in emerging infectious diseases of humans from wildlife, Dietrich said. “That’s why we need to look wider 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 at Urbana-Champaign.