Structural studies showed that tumours induced by Southern rice black-streaked dwarf virus (SRBSDV; genus Fijivirus, family Reoviridae) were highly organized, modified phloem, composed of sclerenchyma, vessels, hyperplastic phloem parenchyma and sieve elements (SEs). Only parenchyma and SEs were invaded by the virus. There was a special region that consisted exclusively of SEs without the usual companion cells and a new flexible type of intercellular gateway was observed on all SE-SE interfaces in this region. These flexible gateways significantly increased the intercellular contacts and thus enhanced potential symplastic transport in the tumour. Flexible gateways were structurally similar to compressed plasmodesmata but were able to accommodate complete SRBSDV virions (~80 nm diameter). Virions were also found in sieve-pore gateways, providing strong evidence for the movement of a virus with large virions within phloem tissue and suggesting that the unusual neovascularization of plant virus-induced tumours facilitated virus spread. A working model for the spread of tumour-inducing reoviruses in plants is presented.

Southern rice black-streaked dwarf virus (SRBSDV) is a recently described member of the genus Fijivirus, family Reoviridae. The roles of the proteins encoded by the SRBSDV genome have rarely been studied. In a yeast two-hybrid (YTH) assay in which SRBSDV P6, a putatively multifunctional protein, was used as bait and an SRBSDV cDNA library was used as prey, there was a strong interaction between the P6 and P5-1 proteins. The interaction was confirmed by bimolecular fluorescence complement (BiFC) assay in plant cells. YTH analysis using truncated mutants showed that the N-terminal region (amino acids 9-231) of P5-1 is necessary for binding P5-1 to P6 and that the N-terminal fragment (amino acids 1-93) of P6 is necessary for its interaction with P5-1. SRBSDV P5-1 formed granules positioned at the cell periphery in Nicotiana benthamiana leaves; P6 was present in both the cytoplasm and the nucleus and formed punctate bodies associated with the cell periphery. Immunogold labeling showed that both P6 and P5-1 localized within viroplasms in infected cells of rice plants. These results suggest that the interaction between P5-1 and P6 of SRBSDV may be involved in the formation of viroplasms.

Rice black-streaked dwarf virus (RBSDV) is a recognized member of the genus Fijivirus, family Reoviridae. Genome segment S5 has a putative second ORF partially overlapping the major ORF but in a different reading frame. This putative ORF is present in a published sequence and in two Chinese isolates now sequenced. Antibodies were raised against purified P5-1 and P5-2 fusion proteins expressed in a prokaryotic system. In western blots, these antibodies reacted with proteins of about 106 and 27 kDa, respectively, as predicted by sequence analysis. In immunoelectron microscopy, antibodies to P5-1 reacted with viroplasms, indicating that P5-1 is a component of viroplasms, but no labeling was observed with P5-2 antisera. Northern blot assays showed that the genome segment S5 was transcribed as a single mRNA with no subgenomic RNA. These results show that S5 is functionally bicistronic in infected plants. Possible translational mechanisms for P5-2 are discussed.

Southern rice black-streaked dwarf virus (SRBSDV) is a novel putative member of the genus Fijivirus, family Reoviridae. We report here the genomic sequences of a Vietnamese isolate (SRBSDV-V). The total genome of SRBSDV-V has 29 115 nucleotides (nt), nine nt shorter than SRBSDV-GD or -HN, but similar in organization to these two Chinese isolates. Nucleotide diversities among SRBSDV isolates were much lower than those among the corresponding ORFs of the available RBSDV isolates and there was a lower purifying selection pressure on SRBSDV than RBSDV, providing first molecular evidence for the view that SRBSDV is of recent origin.