1Station de Pathologie Vegetale, INRA-Bordeaux, 33883 Villenave d'Ornon, France; 2Appalachian Fruit Research Station, USDA, Kearneysville, West Virginia, USA

Background and objectives
Plum pox is a most important disease causing severe damage in stone fruit crops. Resistant plants have not been found by classical breeding: only Prunus tolerant to PPV have been produced. To compare the resistance traits shown by these plants, similar studies have been made with the transgenic C-5 plum recently demonstrated as resistant to PPV. Our objective was to investigate whether the engineered CP gene is effective against diverse PPV isolates exhibiting significant changes in nucleotide sequences or spreading with a high velocity in natural conditions.

Results and conclusions
One transgenic line, plum C-5, has been recently demonstrated as resistant to PPV infection [1]. To characterize the resistance phenotype developed by these plants, transgenic scions C-5 have been propagated and assayed to PPV infection with transgenic and non-transformed control plants. Plants were respectively inoculated with 10 PPV isolates belonging to the common serotypes D or M and notably the two atypical strains El amar (Egyptian) and Sour Cherry. Results of PPV ELISA, immunoblot and ICIPCR showed that no C-5 plums were infected. To understand these results we carried out comparative studies of the nucleotide sequences of the engineered PPV CP gene and those of the PPV challenger isolates. These molecular studies led us to identify a core sequence highly homologous in the PPV CP gene that might be the key sequence, possibly inducing the co-suppression phenomenon of the virus transgene and the PPV genome challenger. Can such results mean that the resistance phenotype shown by the C-5 plum is efficient and stable?

1. Ravelonandro M, Scorza R, Bachelier JC, et al., 1997. Plant Disease 81, 1231-1235.