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Apple trees’ natural response to virus mirrors genetic modification mechanism, study shows

apple tree fruit

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Apple trees respond to a common viral infection by targeting a gene in the same pathway that genetic scientists are aiming for, scientists at the University of Manchester find. The discovery published in Current Biology shows that the fruit trees, which develop rubbery branches when infected, are a reflection of how scientists try to genetically modify trees.

Apple rubbery wood virus (ARWV), which causes rubbery wood disease, has now been largely — if not completely — eliminated from commercial apple trees. However, an extensive study in the UK in the 1950s, when ARWV was widespread, found that in some cases over 50% of apple trees sampled were infected with the virus. The widespread presence of the virus around the world is well documented.

Although their branches become more flexible, no ill effects have been identified for people who have eaten fruit from the infected trees and there are no ill effects on the environment.

The study also provides important insight into how scientists might one day process woody plant tissue so that it efficiently produces sugars needed for biofuels. Woody plant material represents a huge renewable resource that has the potential to produce biofuels and other chemicals with more favorable net carbon dioxide emissions. However, scientists have not yet found an efficient way to release the significant amount of sugars, which are estimated to be about 70%.

The scientific team showed that the symptoms of ARWV infections stem from a reduction in lignin – a complex organic polymer that forms the main structural material that supports the tissues of most plants.

Using next generation sequencing (NGS) to analyze the expression of all genes in the rubbery apple tree branches, they found that phenylalanine ammonia lyase (PAL), an enzyme responsible for lignin biosynthesis, was suppressed by the plant in response on the infection.

The response to ARWV infection results in the plant generating multiple small interfering RNAs known as (vasiRNAs). The vasiRNAs then target several of the plant’s own genes to be downregulated — or broken down — in what’s believed to be part of an antiviral defense response.

One of the genes downregulated by the plant is PAL, and this leads to a decrease in lignin biosynthesis which makes the branches more flexible and facilitates the release of sugars.

The mechanism used by the apple rubbery wood virus to alter lignin is very similar to how scientists have altered lignin in genetically modified trees to make it easier to process. Despite the changed lignin, the trees manage to grow normally.

Lead author Professor Simon Turner said: “Widespread genetic engineering of many plants is limited by regulatory hurdles and public opposition, and this appears to be the case for trees in particular. These research findings are an important contribution to that debate.

“Our work shows that technologies that are considered new and under regulatory scrutiny have similarities to events that are considered natural.

“It appears that the ARWV infections, unbeknownst to us, have produced something akin to a massive field trial.

“Because the disease has been around the world for decades, even conservative estimates suggest that many thousands of infected apple trees were propagated.

“Millions of apples from ARWV-infected trees were eaten with no known adverse health or environmental effects, despite the siRNA-induced changes in lignin caused by the plant’s response to the virus.”

He added: “Currently, the biofuel industry uses huge tracts of farmland to produce cornstarch which is used to produce 60 billion liters of bioethanol.

“That’s relatively inefficient in terms of CO2 savings, but can also affect global food production systems.

“But our better understanding of this mechanism may one day unlock the potential to isolate the sugars in the woody tissue, making biofuel production much more efficient.”

CRISPRing trees for a climate-friendly economy

More information:
Holly Allen et al, Flexible and digestible wood caused by virus-induced alteration of cell wall composition, Current Biology (2022). DOI: 10.1016/j.cub.2022.06.005

Provided by the University of Manchester

Quote: Apple Trees Natural Response to Virus Reflects Genetic Modification Mechanism, Study Shows (2022, Aug 8), retrieved Aug 8, 2022 from https://phys.org/news/2022-08-apple-trees-natural-response-virus . html

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