State of the art
Plum pox virus (PPV) is one of the most aggressive pathogens of stone fruit crops. It brings plants to vegetative decline and causes fruit drop and deformation of persistent fruits in peach, plum, cherry, apricot as well as wild Prunus. The disease has been recorded for the first time 80 years ago, and in the last three decades, in spite of severe quarantine rules applied in many Countries, it spread rapidly in Eastern Europe, the Mediterranean basin, Asia, South- and North-America (1, 2). PPV moves by means of grafting, aphid transportation, and possibly fruit manipulation (2, see also www.sharka.cas.psu.edu). The plum pox (poty)virus or PPV is a single-stranded RNA virus, approx. 9.7 kb. It encodes for a single polyprotein, that is processed by three proteases encoded by the virus itself to make up seven mature proteins (3, 4). These proteins are used by the virus for its assembly and replication. PPV isolates are classified into 4 main serological groups and several subgroups. The main groups are: D (Dideron), M (Marcus), EA (El Amar) and C (cherry), that differ from one another for the preferential host species and the geographic distribution (5, 6). All known strains but C are pathogenic for apricot, but the strain M is the most aggressive one, likely because of its more efficient aphid transportation (6). Recently, recombinant M+D strains have also been isolated (7).
Several sources of resistance have been identified in apricot germplasm, but most of those identified first among Eastern European accessions by field observations did not pass subsequent more severe tests (see 1 for a review). At the present knowledge, 'Harcot', 'Harlayne', 'Henderson', 'Sunglo', 'Lito', and possibly 'Stark Earli-Orange' (SEO) are considered reliable sources of resistance (8, 9, 10), harbouring genes introgressed from European and Asian wild apricot accessions (10, 11). Screening for resistance/tolerance to PPV makes use of peach GF 305 for phenotypic observations of symptoms (12, 13), serological tests based on strain-specific monoclonal antibodies and RT-PCR amplification (5, 14a, 14b, 14c, 15). Nevertheless, the screening is made difficult by the different response to the different pathogen strains and to the variability of the field and, sometimes, greenhouse tests (13). Such uncertainty in attributing an individual to a given class of resistance made difficult also the analysis of segregation in controlled crosses, so that the number of genes of resistance involved is still under debate, with one, two or more genes claimed to fit the segregation data (16, 17, 18, 19, 20). The resistance to PPV is fully expressed according to several reports when different genes are active. This has been depicted for instance in P. davidiana, where six QTLs seem involved in controlling the disease following infection by PPV and two QTLs in the inhibition of the virus movement in the host plant (21).
The comparative analysis of different Prunus linkage maps showed synteny of several QTLs along different species: for instance, a major QTL of resistance to Sharka has been identified in the linkage group 1 in apricot cvs. 'Goldrich' and 'Lito' and in P. davidiana clone 1908 as well (18, 19, 21, 22). Three main mechanisms of resistance to viruses have been described up to now, all of which could apply to the PPV-apricot interaction and give reason in some way of the different segregation patterns observed in literature: the first one is based on the pathogen recognition, where R-genes belonging to the family of nucleotide binding site/ leucine-rich repeat (NBS-LRR) trigger a hypersensitive response (HR) or other defence machinery (23); the second one, less known, is involved in the restriction of long-distance movement of the virus (24); the third one deals with the inability of virus to use one or several plant genes required for pathogen infection, such as the eukaryotic initiation factor 4E (25, 26). The first mechanism seems controlled by a single dominant gene; the second one seems under oligogenic control, and the third one controlled by a single recessive gene. The research group has at his disposal several segregating populations obtained by crossing resistant (R) by susceptible (S) apricot genotypes (14c) and, for two of these populations, the group has produced framework maps where SSRs (Simple Sequence Repeats) and RGAs (Resistance gene analogs) mainly of the NBS-LRR type have already been placed (27). In the present project, we want to finely map the chromosome regions where the phenotypic resistance maps by the means of molecular markers, thus opening the way to the identification of candidate genes and to the marker-assisted selection in apricot.
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