<p style="text-align: justify;">The genus <i>Prunus</i> comprises a diverse group of plants with many botanical species that have been cultivated for the last 3000 years and contains approximately 400 species of trees and shrubs. Many species and cultivars are grown for their edible fruits including almonds, apricots, cherries and peaches, in addition to plums and prunes. Other species are planted for their ornamental value and a few are commercially important for hardwood timber production.</p>

<p style="text-align: justify;">Plum species are found native throughout the Northern hemisphere and the most important commercial species of plums are generally classified in two groups, the European plums (<i>Prunus domestica</i> L.) and related forms with a hexaploid chromosome number (2n=6x=48) and the Japanese plums (<i>Prunus salicina</i>) and their hybrids with diploid chromosome numbers (2n=2x=16). <i>P. domestica</i> is believed to have originated as a natural alloploid between <i>P. cerasifera</i> and <i>P. spinosa</i> and seems to have originated in Southern Europe or Western Asia around the Caucasus Mountains and the Caspian Sea.</p>

<p style="text-align: justify;">World production of all plums was over nine million tons in 2005, with China accounting for nearly half of this total. Of the other countries in the top 10 producers, only the United States and Chile are from outside Europe. Within the United States, California is the leading producer of dried plums, accounting for 70% of the world exports for this commodity.</p>

<p style="text-align: justify;">Plum pox virus (PPV) is the causal agent of Sharka disease, the most serious virus disease of stone fruits. PPV is spread from tree to tree by aphids and through infected budwood used for grafting. Symptoms of PPV infection include leaf and fruit yellowing, fruit deformation, premature fruit drop and tree decline. PPV has spread throughout Europe where it has destroyed over 100 million stone fruit trees and has also spread to India, Egypt, Lebanon, the Azores, Chile and has also been found in the US, where there is a strict quarantine and eradication program in Pennsylvania. Few highly resistant <i>Prunus</i> varieties have been developed through conventional breeding and thus genetically engineered resistance is considered to be an important option.</p>

<p style="text-align: justify;">C5 is a genetically engineered plum clone containing the PPV coat protein gene that has been found to be highly resistant to PPV. The mechanism of resistance has been determined to be through post-transcriptional gene silencing as there is no protein expressed from the inserted coat protein gene. The resistance has proven to be highly stable, with field tests carried out over eight years in three European countries where the virus is present. No C5 trees in any of these field tests have become infected through natural aphid inoculation and graft inoculated trees show few and mild symptoms of the disease.</p>

<p style="text-align: justify;">In addition to the PVP coat protein gene, C5 contains the <i>nptII</i> gene for resistance to the antibiotic Kanamycin as a selectable marker and the <uida> gene coding for the GUS enzyme, which was used as an assayable marker for the presence of the transgene. C5 was produced by <i>Agrobacterium</i>-mediated transformation of hypocotyl slices. Molecular analysis of C5 identified five separate transgene insertions, of which one is a complete insert and the others are either partial or re-arranged fragments of the T-DNA.</uida></p>

<p style="text-align: justify;"><uida>C5 has been developed as a pre-emptive approach to the control of PPV should the disease become established in the United States. The are currently (2007) no plans to release this event commercially, but the event is available should the need arise and the resistance trait expressed in the event is available to breeders to improve other varieties of plum.</uida></p>
