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Plums are one of the most widely cultivated stone-fruits in Europe and other temperate areas of the world, but farmers have been fighting a seemingly never-ending battle against the plum pox virus. This is a fruit-destroying disease commonly known as ‘sharka’ and was first reported in Bulgaria in 1917. The virus travels from orchard to orchard via aphids, and from region to region through the transport of infected budwood.
A century later, European researchers in the INTEREST project extensively studied a novel plum cultivar — the HoneySweet — that could protect the stone-fruit industry from future losses.
While the HoneySweet plum is deregulated in the US it remains subject to strict controls in Europe. The INTEREST project aimed to provide evidence of its effectiveness and safety that could clear the way for more widespread cultivation.
Genetically modified fruit trees offer a viable tool in the battle against diseases that have an enormous impact on farmers, the agricultural industry and, ultimately, on consumers, the project’s researchers concluded.
The HoneySweet plum has been genetically modified to be resistant to the plum pox virus (PPV). It was developed by Ralph Scorza at the United States Department of Agriculture, Michel Ravelonandro at the Institut National de la Recherche Agronomique in France and Dennis Gonsalves at Cornell University in the US.
In the INTEREST project, Ravelonandro and Scorza, as well as Jaroslav Polak of the Crop Research Institute in the Czechia, built on more than a decade of field testing to evaluate how the pest-resistant tree could be a model for other transgenic cultivars. They also focused on concerns about introducing genetically modified crops into European agriculture.
“Research progress in molecular virology and plant transformation led us to challenge PPV with a pathogen-derived resistance approach. By introducing a segment of the PPV genome into the Prunus domestica plum, the tree is effectively protected against the virus, similar to how a vaccine works in humans,” explains Ravelonandro.
Protected plums
The researchers studied plants grown in different agro-climatic conditions in the Czechia and the US and in greenhouse-controlled conditions in France, while monitoring levels of PPV virus exposure.
Among other methods, they tested virus resistance using artificial graft inoculation, whereby an infected part of a susceptible plant is grafted onto virus-free rootstocks. This determined that PPV only appears in leaves situated close to the inoculation point of the HoneySweet plum, suggesting that the virus is unable to spread to other tissues of resistant trees. And they found that HoneySweet's fruit quality and quantity was not affected by PPV infection, even when other severe plum viruses were introduced to increase the viral load.
By growing trees protected genetically from PPV, pesticide use is reduced, mitigating the negative impacts of agriculture on the environment, while yields and fruit quality are maintained, and the risk of a PPV epidemic wiping out an entire orchard – and potentially a grower’s livelihood – is eliminated.
Importantly, Ravelonandro and his colleagues confirmed that viral resistance can be inherited via conventional breeding methods and will remain a dominant trait when cross-breeding with other cultivars, simplifying reproduction of disease-resistant trees.
The same technique could also be applied to other varieties of stone-fruit tree affected by PPV and similar viruses, including apricots, peaches, cherries and nectarines.
“Virus resistance studies in an open field were legally completed here in Europe in accordance with EU regulatory rules, and were successful,” says Ravelonandro. . “We believe that plum-tree growers can significantly benefit from such scientific progress; however, the use of these trees should comply with the Cartagena protocol that aims to protect natural biological diversity from potential risks posed by genetically modified organisms.”
Working closely with other researchers worldwide to exchange information and share expertise, the INTEREST team is preparing a dossier to be submitted to the EU authorities charged with assessing the safe use of HoneySweet plum trees and the consumption of their fruits against EU guidelines.
The spread of PPV across Europe and around the world is estimated to have cost the agricultural sector €10 billion globally over the last 30 years. With climate change likely to further increase the spread of PPV and other diseases, genetic modification to help plants adapt may play a significant role in protecting agricultural yields, crop quality, and food supply and security.