The use of transgenic fruit trees as a resistance strategy for virus epidemics: the plum pox (sharka) model

Sharka or plum pox, caused by Plum pox virus (PPV: genus Potyvirus; Family Potyviridae), is the most serious disease of Prunus. Most cultivated Prunus species are highly susceptible and conventional breeding has not produced highly resistant and commercially acceptable varieties. Success in developi ng virus-resistant herbaceous crops through genetic engineering led us to i nvestigate this approach for resistance to PPV. Our programme aims to devel op a biotechnological approach to PPV control that is effective and shown t o be environmentally safe. The programme began with the cloning of the PPV coat protein (CP) gene and the development of a transformation system for p lum (Prunus domestica). The CP construct was first tested in Nicotiana bent hamiana in which it proved effective in producing transgenic plants with va rying levels of CP expression. Some of these plants, particularly low PPV C P expressers, were resistant to PPV, or recovered from initial infection. B ased on these results plum was transformed using the Agrobacterium tumefaci ens system and both low and high PPV CP-expressing transgenic plum lines we re obtained. These were inoculated with PPV by bud grafts in the greenhouse . Line C-5 proved to be highly resistant. It contained multiple copies of t he insert, produced low levels of PPV CP mRNA, no detectable CP and the ins ert appeared to be methylated. These characteristics all suggest that the r esistance of the C-5 clone is based on post-transcriptional gene silencing (PTGS). Field tests of C-5 and other transgenic lines in Poland, Romania an d Spain have demonstrated that such trees when inoculated by bud-grafts all ow a low level of PPV multiplication, from which they rapidly recover. C-5 plants exposed to natural infection for 3 years did not become infected, wh ereas control trees were infected in the first year. Hybrid plums having th e C-5 PPV CP insert inherited from C-5 are virus-resistant, demonstrating t he usefulness of C-5 as a parent in developing new PPV-resistant plum varie ties. Research is in progress on the biorisks of PPV CP transgenic plants. Gene constructs that either produce no CP or CP that cannot be transmitted by aphids have been developed, tested in N. benthamiana and transferred to plum. Studies have begun on the potential for synergistic interactions betw een the PPV CP gene and the other common viruses of Prunus spp. In the futu re we will be participating in investigating the toxicity or/and the allerg enicity of transgenic fruit products and, more importantly, transgenic line s will be developed that express transgenes only in vegetative parts of the plant and not in the fruit. (C) 2000 Elsevier Science B.V. All rights rese rved.