Virus Resistant Plums
The plum population in Europe was infected by Sharka (otherwise called Plum Pox) which was caused by the plum pox virus (PPV). Some species were also infected with the papaya ringspot virus (PRV). Due to these infections, there were severe economic losses in Europe. Because of this, the University of Hohenheim, Germany started research to find out how to create a plum plant that was completely immune to the viruses.
Finding the Procedure
The procedure they first used to make the plants virus resistant was breeding the plants with sharka tolerant varieties that were bred in Čačak, Serbia. After trying this procedure, though, they noticed that the plants were still being infected by PPV. Even though the symptoms were exceedingly less severe than before, the plants were still being weakened during dry, warm years, which made them more susceptible to PPV infection. They then looked for another approach that was effective and environmentally safe. Finally, they found a highly effective procedure that created transgenic plants resistant to the PPV virus.
First, they cloned the PPV coat protein (CP) gene and developed a transformation system for plums. It was first tested in Nicotiana benthamiana and created transgenic plants which expressed varying levels of CP. When this proved to work, they began transforming the plum plants to obtain both low and high PPV CP-expressing transgenic plum lines. These plants were cultured in a greenhouse maintained at 22-24 degrees Celsius with an 18 hour day length maintained by high-intensity lighting. Afterwards they kept the plants at 5 degrees Celsius to put them into a Cold Induced Dormancy for two months. Then, these lines were innoculated with PPV bud grafts in the greenhouse for 5-8 months. One line in particular, line C-5 (nicknamed "Jojo"), was highly resistant to the PPV virus. This line contained multiple copies of the insert and low levels of the PPV CP mRNA. These things are evidence that the C-5 clone went through post-transcriptional gene silencing (PTGS). In this process, mRNA is degraded in the cytoplasm almost immediately after synthesis. When this procedure was performed in Poland, Romania, and Spain, the same results ensued. Also, when C-5 plants were exposed to natural infection for 3 years, they did not become infected, whereas control trees were infected within a year.
This procedure was very successful in fixing the sharka problem in Europe. They were able to find a way to breed plum plants that were completely resistant to the plum pox virus. The advantage to this finding is that it fixed the economic losses that happened in Europe due to infected plums. However, this procedure is extremely time consuming and expensive. In order to create a resistant plant, months have to go into the breeding and cloning processes.