Structural studies showed that tumors 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 tumor.Flexible gateways were structurally similar to compressed plasmodesmata but were able to accommodate complete SRBSDV virions (about 80 nm diameter).Virions were also found in sieve pore gateways, providing evidence for the movement of a virus with large virions within phloem tissue and suggesting that the unusual neovascularization of plant virusinduced tumors facilitated virus spread.A working model for the spread of tumor-inducing reoviruses in plants is presented.