Potential Use of Transgenic Plums Resistant to Plum Pox Virus Field Infection

Research to date indicates that post-transcriptional gene silencing (PTGS) is an effective strategy for the development of stable high-level resistance to Plum pox virus in plum. Field tests in Poland, Romania, and Spain show that after 5–6 years of natural aphid vectored inoculation, trees of the PTGS clone C5 remain virus-free. The effectiveness of transgene-based PTGS for imparting potyvirus resistance and the relatively rapid incorporation of this trait into Prunus germplasm suggests continued exploration and employment of this strategy for producing virus resistant stone fruits and other woody perennial tree fruits. INTRODUCTION Plum pox virus (PPV), the causal agent of sharka disease, is one of the most serious viruses affecting stone fruits, a group of commercially-produced Prunus species that includes peach, plum, apricot, and cherry (Nemeth, 1994). PPV is spread by aphids and through the use of infected budwood. Symptoms of plum pox infection include leaf chlorosis, fruit chlorosis and deformation, premature fruit drop, and tree decline, particularly in the presence of other common Prunus viruses (Roy and Smith, 1994). Originally reported from Bulgaria in 1932, PPV has spread throughout Europe where it has caused the destruction of over 100 million stone fruit trees. The scarcity of natural resistance and the lack of efficient control measures have made it difficult to stop the spread of PPV. Quarantine, plant sanitation, and eradication are the only available control strategies. Serious consequences result when these measures fail. Recent outbreaks of PPV in Chile, U.S., and Canada illustrate the potential for spread of this disease and the devastating consequences to the fruit industry. For example, during the first 2.5 years following the appearance of PPV in a limited area of Pennsylvania over 550 ha of stone fruit orchards were destroyed in order to contain and ostensibly eradicate the virus. Important and useful work is underway on the development of resistant cultivars of stone fruits utilizing naturally occurring resistance genes (Hartmann, 2002; Kegler et al., 1998; Polak et al., 2002). Yet there are few highly resistant genotypes, and the number of genes affecting resistance, their inheritance, and their effectiveness against various strains of PPV are areas of inquiry that require further study. It is clear that a multifaceted approach to developing resistant plants is necessary. Using plum (Prunus domestica L.) as a model, we have shown that through genetic transformation, stone fruits highly resistant to PPV can be developed (Scorza et al., 1994; Ravelonandro et al., 1997). This resistance is based on post-transcriptional gene silencing (PTGS) (Turner and Schuch, 2000) induced by the PPV coat protein transgene that was inserted into plum Proc. XXVI IHC – Genetics and Breeding of Tree Fruits and Nuts