The gene that encodes the PPV protein was separated from the virus and inserted into the plum's DNA. The transgenic shoots (coded as C5) were placed in vitro to grow roots and were then propagated via grafts. The type of plum chosen was the Prunus domestica and the new PPV-resistant variety was called HoneySweet.
To verify its resistance to Sharka, the trees were placed in greenhouses and studied for five years. Portions of infected plant tissue were grafted onto them, but they never developed the disease. Some trees were also cultivated in Czech Republic, Poland, Romania and Spain (where the PPV is now indigenous) and, after 20 years of tests, they were never infected through aphids.
A detailed description of 'HoneySweet' can be found in the patent (US PP15154 P2).
'HoneySweet' was liberalised in the US by the FDA (Food and Drug Administration) and the EPA (Environmental Protection Agency) for the production of fruit and the development of new PPV-resistant varieties, as trees were considered safe for both the environment and the health of consumers. At the moment, 'HoneySweet' has not been cleared for cultivation in the EU or other areas outside the US.
Source: Scorza R., Ravelonandro M. , Callahan A., Zagrai I., Polak J., Malinowski T., Cambra M., Levy L., Damsteegt V., Krıska B., Cordts J., Gonsalves D., Dardick C., 'HoneySweet' (C5), the First Genetically Engineered Plum pox virus–resistant Plum (Prunus domestica L.) Cultivar', May 2016, HORTSCIENCE 51(5), pagg. 601–603. hortsci.ashspublications.org/content/51/5/601.extract