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Pathogen Biology

SBWMV is the type member of the Furovirus genus. Members of the genus are characterized by rigid rod-shaped particles (Figure 8) and positive sense (directly serving as mRNA) RNA genomes consisting of two molecules that are packaged into separate particles (Figure 8). The two particles differ in length, and the shorter particle (138-160 nm) is 10 to 20 times more prevalent in infected tissue than the longer particle (281-300 nm), but both particles are required for infection.

Figure 8

The longer particle contains RNA 1, which is approximately 7100 nucleotides long and encodes three proteins. Two of these, measuring 150 kDa and 209 kDa, are associated with virus replication (Figure 9). The 37 kDa protein is a cell-to-cell movement protein. The 150 kDa and 209 kDa proteins are translated directly from the message sense viral RNA, whereas the 37 kDa protein is expressed via a subgenomic mRNA.

Figure 9

The shorter particle contains RNA 2 (approximately 3600 nucleotides), which also encodes three different proteins (Figure 9). The first is the 19 kDa coat or capsid protein (CP). Sometimes, the coat protein UGA termination codon is suppressed allowing translation of an 84 kDa CP-readthrough protein, which is thought to be required for virus transmission by its vector, Polymyxa graminis (Figure 10). The third protein is a 19 kDa cysteine-rich protein that is expressed via a subgenomic mRNA and may function as a suppressor of post transcriptional gene silencing (Figure 9). Post transcriptional gene silencing is a mechanism that can provide host resistance to a virus.

Figure 10

Systemic infection of plants is temperature dependent. Virus movement from the roots (site of infection) to the leaves occurs at temperatures less than 20 °C (68 °F). Higher temperatures limit virus movement into the leaves and if the temperatures rise after some leaves are infected, no further infection of developing leaves will occur. If SBWMV-infected plants are grown at high temperatures (25-30 °C/ 77-86 °F), or for prolonged periods of time, or if the virus is maintained by repeated mechanical inoculation, the virus tends to delete specific sequences in the gene encoding the CP-readthrough protein (Figure 11). There is a tendency for virus strains that contain large deletions to be more virulent when mechanically inoculated onto wheat plants. Stable deletion mutants of one particular size can predominate, but this tends to be cultivar specific. A 759 nucleotide deletion predominated in the virus maintained in the cultivar ‘Galahad’ after repeated mechanical inoculations, while a stable deletion of 1058 nucleotides predominated in the virus maintained in the cultivar ‘Michigan Amber.’ Since these deletions are generated and maintained in the absence of transmission by the vector, the CP readthrough domain has been implicated in vector transmission.

Figure 11

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