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A system was developed which allows the transfer of foreign genes into apricot cultivars. We report the transformation and regeneration of Prunus armeniaca plants with Agrobacterium tumefaciens strain LBA 4404 containing various binary plasmids, pBinGUSint, carrying the marker gene ß-glucuronidase (GUS) and pBinPPVm, carrying the coat protein gene of Plum Pox Virus (PPV). The marker gene GUS was used for optical evaluation of the efficiency of the transformation system. The coat protein gene of PPV was used to introduce coat protein mediated resistance against one of the most important pathogens of stone fruit trees in Europe and the whole Mediterranean area. This is the first report of the successful integration of a viral coat protein gene into a fruit tree species, opening a new perspective on the control of the disease.

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Transgenic Nicotiana benthamiana and N. clevelandii plants expressing the coat protein of Plum Pox Virus under the control of the 35S promoter from Cauliflower Mosaic Virus were engineered by Agrobacterium tumefaciens mediated transformation. The phenomenon of virus resistance was observed at different levels when transgenic plants, expressing the coat protein and control plants were compared after challenge infection with Plum Pox Virus. N. clevelandii coat protein transgenic plants circumvent virus accumulation. After an initial increase in virus titer similar to the control plants, some coat protein expressing plants showed a reduced accumulation of virus and inhibition of the systemic spread, characterized by decrease of the virus titer and formation of new symptomless leaves. In other N. clevelandii coat protein expressing plants virus accumulation was inhibited and disease symptoms never appeared. N. benthamiana coat protein expressing plants were also protected. After a temporary virus accumulation, virus titer decreased without the appearance of symptoms with the exception of a few plants, which showed a delay of thirty days in the development of symptoms post challenge infection.

Applying the new method for culture initiation 16 different cultivars of Malus domestica could be established in vitro from shoot tips of adult orchard trees. Actively growing shoot tips were cleaned and surface disinfested, dissected to 2-3 mm and placed on a modified MS-medium with 4.4/xM BA. Explants were covered for 24 h with 200/zl of a 0.1% solution of 8-hydroxy-quinolinol-sulfate (8-HQS) and transferred to a medium containing both auxin and cytokinin after 2 weeks. The application of 8-HQS induced a strong reduction of the infection rate and inhibited the browning of the explants and the media. After 7 days yields of 50-90% sterile explants could be obtained in comparison to 100% losses of untreated shoot tips. After 60 days variable rates of actively growing shoots could be observed, depending on the genotypes. The described method allows a successful establishment of fruit trees from adult orchard material on one hand by strongly reducing the browning, caused by the oxidation of polyphenolic compounds by polyphenoloxidases, and on the other hand 8-HQS can strongly increase the yield of explants without contamination, independently from the vegetation period and the phytosanitary state of the donor material.Abbreviations: BA-6-benzyladenine, IBA-indolebutyric acid, 8-HQS-8-hydroxy-quinolinol-sulfate