SOME COAT PROTEIN CONSTITUENTS FROM STRAWBERRY LATENT RINGSPOT VIRUS EXPRESSED IN TRANSGENIC TOBACCO PROTECT PLANTS AGAINST SYSTEMATIC INVASION FOLLOWING ROOT INOCULATION BY NEMATODE VECTORS

The coding sequences in RNA2 for the coat proteins (CP) of strawberry latent ringspot virus (SLRSV) were modified and amplified using polyme rase chain amplification reactions (PCR) to facilitate their expressio n in Agrobacterium tumefaciens-transformed Nicotiana tabacum Xanthi-nc . The coding sequences for the smaller capsid protein (S, 29kDa) and t hat for the theoretical precursor of L and S (P, 73kDa) had ATG 'initi ation' codon sequences added at the 5'-proximal Ser/Gly (S/G) cleavage site in the unmodified sequence. The sequence coding for the larger o f the two 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 plant a to a maximum concentration of 0.01% of total extractable proteins bu t did not assemble into virus-like particles. When challenged by mecha nical inoculation with virus particles or viral RNA, and compared with control plants, tobacco plants (primary transgenic clones or S1 and S 2, kanamycin-resistant seedlings) expressing the virus capsid subunits separately, or their precursor, decreased the accumulation of SLRSV p articles in inoculated leaves and fewer plants became invaded systemic ally. In experiments in which the roots of seedlings were exposed to S LRSV-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 t o virus inoculation by nematode vectors.