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Pathogen Biology

Virions of TSWV are complex compared to many plant viruses. There are three RNAs in the virus genome that are individually encapsidated, and are collectively bound by a membrane envelope that is of host origin (Figure 7). This complex virion structure is a characteristic that distinguishes TSWV from most other plant viruses. TSWV virions are roughly spherical and are 80-110 nm in diameter (Figure 8). Two virus proteins processed during replication to contain sugars, i.e. glycoproteins (GPs), are dispersed throughout the surface of the viral envelope. These proteins are called glycoprotein C (G_C) and glycoprotein N (G_N) and differ slightly in size (Figure 7). Inside the viral envelope is each of the three viral RNAs, individually bound by multiple copies of a nucleocapsid protein. The three RNAs differ in size and are called Large (L), Middle (M) and Small (S) (Figure 9). Also inside the envelope are several copies of a virus-encoded "replicase" protein that is required to initiate virus replication in a new host.

Figure 7	Figure 8
Figure 9

The GPs in the envelope function in the maturation and assembly of virions, and appear to play a role in the acquisition of TSWV by thrips. Envelope-deficient isolates of TSWV, generated by serial mechanical passage in plants, are infectious in plants but are not transmitted by thrips. This is evidence that the GPs are not required for replication in plants, but are required for virus infection of thrips leading to subsequent virus replication in and transmission by thrips.

In addition to the GPs, the M RNA segment encodes a non-structural protein (NSm). The NSm is unique to tospoviruses in the family Bunyaviridae, and is thought to be an adaptation of tospoviruses to plants to facilitate tospovirus movement from cell to cell through plant cell walls via the plasmodesmata. Because enveloped particles are too large to be transported through plasmodesmata, the role of NSm is to form tubules that facilitate movement of nucleocapsids (RNA plus protein) from cell to cell (Figure 10). In addition to the nucleocapsid protein, the S RNA segment encodes a non-structural protein (NSs). Crystalline-like structures of NSs are produced in infected insect cells (Figure 11) and plant cells. The NSs protein has RNA silencing suppressor activity, and may play a role in post-transcriptional gene silencing or RNA metabolism.

Figure 10

Figure 11

TSWV is one member of the dozen or so different viruses in the genus Tospovirus. One striking difference in these viruses is the variation in their host ranges. TSWV is renowned for having an extensive host range, whereas other members of the genus Tospovirus such as Peanut yellow spot virus or Iris yellow spot virus have narrow host ranges. The genetic diversity in this virus group may be fostered by their replication in both different species of plants and different species of thrips. Although genes to confer resistance to TSWV have been found in some germplasm lines and used to develop new cultivars, there has been rapid adaptation of new forms of the virus to cultivars that have been released. Thus, there are virtually no cultivars of major crops with significant levels of resistance to TSWV that have remained resistant in the field for more than a few years.