Researchers solve brome mosaic virus puzzle

Shedding light on the different components of this crop disease was key to scientists figuring out how to prevent infection

The brome mosaic virus has baffled scientists for decades. It is a genetic puzzle that, unlike many other viruses, is divided into three particles that have been impossible to tell apart.

Until now.

BMV affects grasses such as wheat and barley and, in 2016, the infection was recorded in soybeans in Manitoba. Mosaic symptoms include irregular leaf mottling such as light and dark green or yellow streaks and patches, and the leaves can be curled or puckered leading to stunted growth and reduced yields.

A study in Ohio showed that the virus can reduce yield by as much as 61 percent in soft red winter wheat.

Brome mosaic virus is almost identical to cucumber mosaic virus, which can infect not only cucumbers but squash, melons, peppers, beans, tomatoes, carrots, celery, lettuce and other vegetables.

The disease spreads easily in warm, damp environments and many animals, especially insects, aid in the spread of the virus. In addition, human contact is a common form of spread.

Understanding the genetics of BMV has been the research focus of scientists since 1971. At the University of California, Riverside, Ayala Rao, professor of plant pathology and microbiology, has been studying BMV for decades. Previously, scientists used the average of all three particles in the virus to create a basic description of them. But to really define them individually, Rao needed to separate them and get them into their purest form.

“Without a more definitive picture of the differences between these particles, we couldn’t fully understand how they work together to initiate an infection that destroys food crops,” said Rao in a news release.

The team used genetic engineering to disable the pathogenic parts of the virus and inserted them in a host plant.

“This bacterium inserts its genome into the plant’s cells, similar to the way HIV inserts itself into human cells,” Rao said.

Inside each of the three particles is a strand of RNA — RNA1, RNA2, and RNA3. These are the genetic materials controlling the production of proteins. The proteins do different tasks, some of which are destructive causing stunted growth, lesions, or death of the host plant.

The particle they studied was found to be nearly empty with most of the RNA genome located at the capsid shell.

“However, this density is disordered in the sense that the RNA is not associated with any particular structure but, rather, with an ensemble of secondary/tertiary structures that interact with the capsid (shell) protein,” Rao and his colleagues wrote in the report.

The results showed a fundamental difference between BMV with its multiple genome particles and viruses with a single genome as a result of different demands of their lifecycles. BMV appeared to “package” its RNA molecules separately in membrane-bound complexes, whereas single genome viruses use sequence-specific packaging signals throughout their RNA.

“RNA1 and RNA2 contribute to replication, while RNA3 provides the movement protein and capsid (shell of a virus) protein to regulate the efficient cell-to-cell and systemic movement,” said Rao.

Rao added that the virus is able to make millions of copies of itself, so many that it is capable of becoming epidemic.

With one of the particles mapped, the researchers could see that the first two particles are more stable than the third. Now they expect to be able to genetically alter that stability and manipulate the level to which RNA is released into plants.

“We can make the third particle more stable, so it doesn’t release RNA and the infection gets delayed,” said Rao. “(If) we can completely control the release of the genome from the third particle, infection would not occur.”

Going forward, much more research is being done on all three particles to fully understand their structures.

The research study was published recently in the Proceedings of the National Academy of Sciences.

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