BEHAVIOUR OF TRANSGENIC PLUM POX VIRUS-RESISTANT PRUNUS DOMESTICA L. CLONE C5 GROWN IN THE OPEN FIELD UNDER A HIGH AND PERMANENT INFECTION PRESSURE OF THE PPV-REC STRAIN

Abstract

Transgenic plum (Prunus domestica L.) clone C5 was grafted on virus-free St. Julien rootstocks and inoculated by bud grafting with the recombinant strain (PPVRec) of the Plum pox virus (PPV). Non inoculated C5 plums were used as controls. Transgenic plums infected with PPV-Rec and control plants were evaluated for six years in the open field. PPV symptoms, mild diffuse spots and rings, appeared on the basal leaves the second year after inoculation. The presence of PPV was confirmed by DAS-ELISA, ISEM, and RT-PCR. Severe PPV symptoms appeared in the leaves of shoots grown from non-transgenic inoculum buds. A reduction of symptoms and a decrease of the relative PPV concentration, as determined by DAS-ELISA in transgenic C5 plants, were observed from the third to the fifth year after virus inoculation. Very mild symptoms in older leaves of the basal branches of trees subsided by the end of the vegetative period, and were difficult to observe. The level of PPV detection by DAS-ELISA dropped from June to August together with a reduction of symptoms. PPV was undetectable by DAS-ELISA in more than half of the trees by the end of August. PPV-Rec was detected by RT-PCR in the basal and middle parts of the C5 scions. The upper leaves did not contain a detectable level of the virus. These results show the high level and stability of PPV resistance in graft-inoculated C5 trees.