Rapid vascular movement of tobraviruses does not require coat protein: evidence from mutated and wild-type viruses

Early studies of the tobravirus Tobacco rattle virus (TRV) described two types of virus isolate with apparently different disease characteristics. M-type isolates, which contain both viral genomic RNAs and form virus particles, could be passaged by mechanical inoculation and produced rapid but short-lived systemic symptoms. In contrast, NM-type isolates, which contain only RNAI and do not form virus particles, were difficult to passage by mechanical inoculation and were very slow to produce systemic symptoms. From the early observations on such isolates made in the 1960s, it has become accepted that M isolates with encapsidated TRV particles move rapidly through the vascular system whereas NM isolates containing only unencapsidated TRV RNAI move only slowly via plasmodesmata from cell to cell and take many weeks to reach the upper parts of plants. However, we show that NM isolates of TRV and another tobravirus Pea early-browning virus (PEBV) move into systemic tissue of N. benthamiana and N. clevelandii by 6 days post inoculation, suggesting that this rapid movement occurs via the vasculature. The systemic movement of TRV and PEBV mutants lacking functional coat protein that have been modified to express the green fluorescent protein were examined by confocal microscopy. This confirmed that the tobraviruses do not require the CP for long distance movement via the phloem, a property that is shared with only a small group of plant viruses.