Shrinkage of Genome Size in a Plant RNA Virus upon Transfer of an Essential Viral Gene into the Host Genome

Tromas, Nicolas; Zwart, Mark P.; Forment, Javier; Elena, Santiago F.

TEV-ÁNIb Has Greater Within-Host Fitness than TEV in N. tabacum 35S::NIb Li and Carrington (1995) generated transgenic N. tabacum cv. Xanthi plants expressing TEV NIb under control of the Cauliflower mosaic virus (CaMV) 35 S promoter, referred to here as N. tabacum 35S::NIb. TEV variants with disruption of the polymerase GDD amino acid motif, the nuclear localization signal, or the NIb N-terminal cleavage site could replicate in these transgenic plants, whereas replication was not supported in wild-type plants. Moreover, a TEV variant missing NIb altogether could also infect these transgenic plants and induce symptoms after 7 dpi, similar to the wild-type virus. Tromas and Elena (2010) also generated a TEV variant missing the NIb cistron, referred to here as TEV-ÁNIb. TEV-ÁNIb can infect N. tabacum 35S::NIb, but not the wild-type N. tabacum, in agreement with previous results (Li and Carrington 1995) . TEV is the full-length wild-type virus, capable of infecting both wild-type N. tabacum and 35S::NIb transgenic plants. When wild-type N. tabacum plants were coinoculated with different mixtures of TEV:TEV-ÁNIb (1:100 to 100:1), we detected only infection by TEV. Coinfected wild-type plants were probably not observed because the low cellular multiplicity of infection of TEV in N. tabacum (Tromas et al. 2014 ) precludes cellular coinfection, especially early in infection.