High resistance and control of biological risks in transgenic plants expressing modified plum pox virus coat protein

Transgenic plums transformed with the plum pox virus coat protein (PPV CP) gene displayed a resistance to the sharka disease (Ravelonandro et al., 199 7). However, the expression of PPV CP in transgenic plants may lead to comp lementation of deficient characteristic of an incoming potyvirus. Indeed, a n aphid-intransmissible strain of zucchini yellow mosaic virus (ZYMV-NAT) c ould be transmitted when encapsidated by the engineered PPV CP (Lecoq et al ., 1993). To control such a risk, new PPV CP constructs were designed and i ntroduced into Nicotiana benthamiana genome. In the first construct, the DA G amino acid triplet involved in the potyvirus aphid-transmission was delet ed. The second construct encoded a truncated PPV CP lacking its first 140 a mino acids. Tn the last construct, the nucleotides encoding the charged ami no-acids R-220, Q(221) and D-264 localized in the core of the PPV CP were r emoved. A bacterial expression system was developed to show that these dele tions prevent the assembly of the PPV CP subunits. For each construct, seve ral transgenic lines were produced and first challenged with several strain s of PPV Two phenotypes of resistance were observed: recovery and immunity. Their biochemical characterization showed that the resistance was RNA-medi ated and therefore can be classified as homology-dependent (Jacquet et al., 1998a). Resistant lines producing high level of wild type or modified PPV CP were then inoculated with ZYMV-NAT to perform an aphid-transmission assa y. Results of these experiments demonstrated that the use of modified forms of PPV CP genes in transgenic plants provide a good way to control the bio logical risks associated with heteroencapsidation (Jacquet et nl., 1998b).