AbstractThe coding sequences in RNA2 for the coat proteins (CP) of strawberry latent ringspot nepovirus (SLRSV) were modified and amplified using polymerase chain amplification reactions (PCR) to facilitate their expression in Agrobacterium tumefaciens-transformed tobacco cv. Xanthi-nc. The coding sequences for the smaller coat protein (S, 29kDa) and that for the theoretical precursors of L and S (P, 73 kDa) had ATG initiation codon sequences added at the 5'-proximal Ser/Gly (S/G) cleavage site in the unmodified sequence. The sequence coding for the larger of the 2 proteins of mature SLRSV capsids (L, 44kDa) had an ATG codon added at its 5' S/G site and a TAG stop codon sequence added at the 3'-proximal S/G site. The P, L and S proteins were expressed in planta to a max. concn of 0.01% of total extractable proteins but did not assemble into virus-like particles. When challenged by mechanical inoculation with virus particles or viral RNA, and compared with control plants, tobacco plants (primary transgenic clones or S1 and S2, kanamycin-resistant seedlings) expressing the virus capsid subunits separately, or their precursor, decreased the accumulation of SLRSV particles in inoculated leaves and fewer plants became invaded systematically. In experiments in which the roots of seedlings were exposed to SLRSV-carrying vector nematodes (Xiphinema diversicaudatum), SLRSV was detected in the roots of non-transformed control tobacco plants (6/20) and in transgenic tobacco expressing the L protein (7/40), but not in any of 25 tobacco plants expressing the S protein or in 35 expressing the P protein. This is the second example of CP-mediated resistance to virus inoculation by nematode vectors.