Strategies to confer resistance to sharka (PPV) in apricot

Sharka disease, caused by the Plum Pox Virus (PPV), is endangering apricot industry in many countries worldwide. Both PPV-D (Dideron) and PPV-M (Marcus) strains are able to determine severe crop losses, with the latter strain being the most dangerous. We selected 'Lito', among the cultivars described in literature as resistant, and crossed to the susceptible accession 'BO81604311' to produce a double pseudo-test cross progeny. A population of 118 individuals was phenotyped and genotyped to identify molecular markers linked to the genetic determinant/s of Sharka resistance. Both PPV D and M strains were inoculated in seedling replicates and symptoms visually scored for three years. ELISA and PCR assays were also done. A detailed SSR-based molecular map was developed and used for QTL analysis. A major QTL for both PPV-M and PPV-D strains was found at the top of the linkage group 1 of 'Lito', in the same region where a QTL of resistance to Sharka was found in 'Stark Early Orange', the resistant parent of 'Lito', by Lambert and coworkers (2007). To pave the way for the positional cloning of the DNA sequence, the 'Lito' × 'BO81604311' progeny has been enlarged and further 241 seedlings were genotyped with old and new SSR primers that allowed saturating the QTL region in the LG1. Phenotyping with both PPV strains is under way. A random shearing 10X BAC library prepared with inserts >100 kb is being screened with markers surrounding the QTL. Positive BACs will be physically mapped to produce a contig of the genomic region including the main QTL. Finally, as a first step towards genetic transformation of apricot for resistance to PPV, constructs for either CP overexpression or posttranscriptional silencing were used, and transgenic callus lines were recovered from Agrobacterium-inoculated mature tissues, highly recalcitrant to shoot regeneration.