Use of plum pox potyvirus as an expression vector in plants

Plum pox potyvirus (PPV) is the causal agent of a devastating disease that affects stone fruit trees of the Prunus genus. However, it is also able to infect herbaceous hosts from several different families. This broad host range confers PPV an added value regarding its interest as a putative expression vector in plants. Two strategies have been used to express foreign sequences of interest using vectors based on PPV. The N-terminal region of the capsid protein (CP) has been used to express epitopes on the surface of virion particles. Four different insertion sites were evaluated, but only three of them have rendered viable chimeras. These vectors have shown notable differences in tolerance to sequence insertions. Foreign antigenic peptides expressed in them were easily recognized by specific antibodies. Moreover, sequences cloned at one of these vectors (position 12 of CP) were able to elicit an efficient B-cell response in experimental hosts. Epitope mapping by pepscan analysis and fine tuning of cloning positions are being conducted in an attempt to select optimal sites for epitope insertion. The second approach that we have followed is the use of PPV as an expression vector of whole independent proteins. Two insertion sites have been selected, one between P1 and HC proteins (PPV-XS) and the other between NIb and CP proteins (PPV-NK). A double expression vector has been constructed, which allows production of two foreign proteins with a single vector. ifferent reporter genes have been cloned in both insertion sites. The VP60 structural protein of rabbit hemorrhagic disease virus (RHDV) was also successfully expressed making use of PPV-NK vector. Extracts from the VP60-expressing plants were able to induce a remarkable immune response against RHDV in its natural hosts, rabbits. Moreover, these animals were protected against a lethal challenge with RHDV.