The University of Manchester researchers discovered that apple trees respond to a common viral infection by targeting a gene in the same pathway that genetic scientists are interested in. The discovery, published in Current Biology, demonstrates that infected fruit trees develop rubbery branches, mirroring how scientists are attempting to genetically modify trees.
Apple rubbery wood virus (ARWV), the causative agent of apply rubbery wood disease, has been largely, if not completely, eradicated from commercial apple trees. However, during the 1950s, when ARWV was widespread in the UK, an extensive survey revealed that in some cases, more than 50% of apple trees sampled were infected with the virus. The virus's presence is well documented all over the world.
Despite the fact that their branches become more flexible, no adverse effects on humans who ate fruit from infected trees have been recorded, nor have there been any negative environmental consequences.
The research also sheds light on how scientists might one-day process woody plant tissue to efficiently produce sugars needed for biofuels. Woody plant material is a vast renewable resource with the potential to produce biofuels and other chemicals with lower net CO2 emissions. However, scientists have yet to devise an efficient method for releasing its substantial store of sugars, which is estimated to be around 70%. The researchers discovered that the symptoms of ARWV infections are caused by a decrease in lignin, a complex organic polymer that forms a key structural material that supports the tissues of most plants.
They discovered that the plant suppressed phenylalanine ammonia lyase (PAL), an enzyme responsible for lignin biosynthesis, in response to the infection by using next-generation sequencing (NGS) to analyze the expression of all the genes in the rubbery apple tree branches.
In response to ARWV infection, the plant produces a large number of small interfering RNAs, known as (vasiRNAs). The vasiRNAs then target several of the plant's own genes for downregulation (or degradation) as part of antiviral defense response. PAL is one of the genes that the plant downregulates, which results in a decrease in lignin biosynthesis, which increases branch flexibility and facilitates sugar release.
The mechanism by which the apple rubbery wood virus alters lignin is similar to how scientists alter lignin in genetically modified trees to make it easier to process. The trees continue to grow normally despite the altered lignin. Professor Simon Turner, the study's lead author, stated, "Many plants' widespread genetic engineering is hampered by regulatory barriers and public opposition, and this appears to be especially true for trees. These research findings contribute significantly to that debate.
"Our research shows that technologies that are new and subject to regulatory oversight have similarities to events that occur naturally. "Unbeknownst to us, the ARWV infections appear to have been conducting a large field trial. "Since the disease has been present for many decades, even conservative estimates suggest that thousands of infected apple trees have been propagated.
"Despite the siRNA-induced lignin alterations caused by the plant's response to the virus, millions of apples from ARWV-infected trees were eaten with no known adverse health or environmental consequences." He continued, "Currently, the biofuel industry consumes vast amounts of agricultural land in order to produce corn starch, which is used to produce 60 billion litres of bioethanol.
"This is inefficient in terms of CO2 savings, but it may have an effect on global food production systems. "However, as we gain a better understanding of this mechanism, we may one day be able to isolate the sugars within the woody tissue, making biofuel production much more efficient."
