Il miglioramento genetico per la resistenza a sharka in pesco: risultati del progetto italiano PPVCON

Sharka is a worldwide serious disease with a severe impact on the productivity and fruit quality of Prunus species. It is caused by Plum Pox Virus (PPV), a virus against which no chemical or biological curative treatments are available. PPV is easily transmitted by aphids and by grafting; so, despite the considerable efforts made in many countries, PPV has spread in all the most important Prunus growing areas. The current short-term solution is the eradication of infected trees and the plantation of virus-free material. In Italy, the virus presence was first identified in 1973 on apricot trees in Val Venosta area (Northern Italy). Since then, and despite the precautionary measures put in place, it has been spreading in almost all Italian regions, posing a serious threat to the stone fruit industry. In Emilia Romagna region (southern Po valley) it has been found since the early ‘80s, but the situation has worsened over the past decade, when outbreaks of the M strain of the virus have been identified. The M strain is characterized by a fast spreading by aphid vectors. The Italian Ministry of Agriculture sponsored the National Project PPVCON, aimed to fight the virus threat through breeding actions in order to increase tolerance or introduce resistance into peach and improve knowledge on the virus biology and host interaction as well. No source of resistance has yet been found in peach, thus the isolation and introgression into P. persica of genes coding for the resistance to Sharka disease would be of significant economic and environmental benefit. In France, the resistance was found in P. davidiana (clone P1908), a species related to peach. In the offspring obtained from crossing ‘Summergrand’ (yellowflesh nectarine) and P1908, six QTL (Quantitative Trait Loci) were identified. One of them, highly correlated to the resistance, appears to be homologous to a QTL found in apricot. High level of resistance was also observed in several almond cultivars and the potential role of this species as a source of resistance to PPV in peach breeding programs has been demonstrated (Gradziel, 2002; Martínez-Gómez et al., 2004). As an alternative or complementary approach, genetically engineered resistance by transgenes obtained from the virus itself is being tested. Transgenic PPV resistant Prunus domestica plants containing the PPV-CP (coat protein) gene have been obtained . Field tests performed on ‘C5’ selection, one of these transgenic clones (now patented as ‘Honey Sweet’), clearly demonstrated to be resistant to PPV infection either through aphid vectors and by graft inoculation. Despite ethical-legislative limitations to the use of genetically modified organisms (GMO) in the European Union, at present, the goal of obtaining transformed peach trees is not reached yet due to its recalcitrance to in vitro transformation and regeneration. For these reasons, the actions of the PPVCON project have been oriented exclusively on traditional breeding techniques. More than 3500 seedlings from about 100 crossing combinations were obtained from the two available sources (P. davidiana and P. dulcis) and are currently under evaluation. In order to assess PPV resistance, seedlings were propagated onto infected symptomatic ‘GF305’ seedling peach rootstocks, highly susceptible to PPV. The absence of visible symptoms was further investigated with serological (DAS-ELISA) and molecular (Real-Time RTPCR) tests. After several cycles of inoculations, 34 selections were classified as putatively resistant. In order to find molecular markers linked to the resistance, a progeny obtained by crossing the ‘SD 81’ resistant hybrid (P. persica x P. davidiana) and the susceptible peach cultivar ‘Maria Aurelia’ has been analyzed. This 88 individuals population was genotyped with microsatellite markers and a linkage map was constructed. A QTL analysis showed two regions involved in PPV resistance in LG1 and LG7. Two molecular markers linked to these regions were also identified. The pomological evaluation of all the progenies has just started: phenotypically, most show intermediate characteristics compared to parents, although the traits of low fruit quality inherited from the pollen parent (resistant hybrid) are prevalent. The way to obtain resistant/tolerant cultivars with high fruit quality is still long but it seems to be the only feasible strategy to keep this species at a commercial stage, at least where Sharka disease is endemic.

Keywords: silencing, Prunus, transgenics, interspecific hybrids, ELISA, quantitative trait loci (QTL)