Abstract
Virion distribution and ultrastructural changes induced by the infection of maize or rice with four different reoviruses were examined. Rice black streaked dwarf virus (RBSDV, genus Fijivirus), Rice ragged stunt virus (RRSV, genus Oryzavirus), and Rice gall dwarf virus (RGDV, genus Phytoreovirus) were all phloem-limited and caused cellular hyperplasia in the phloem resulting in tumors or vein swelling and modifying the cellular arrangement of sieve elements (SEs). In contrast, virions of Rice dwarf virus (RDV, genus Phytoreovirus) were observed in both phloem and mesophyll and the virus did not cause hyperplasia of SEs. The three phloem-limited reoviruses (but not RDV) all induced more flexible gateways at the SE-SE interfaces, especially the non-sieve plate interfaces. These flexible gateways were also observed for the first time at the cellular interfaces between SE and phloem parenchyma (PP). In plants infected with any of the reoviruses, virus-like particles could be seen within the flexible gateways, suggesting that these gateways may serve as channels for the movement of plant reoviruses with their large virions between SEs or between SEs and PP. SE hyperplasia and the increase in flexible gateways may be a universal strategy for the movement of phloem-limited reoviruses.
Highlights
In plants, symplastic infection allows viruses to invade multiple types of cell, including mesophyll cells (MC), phloem parenchyma (PP), companion cells (CC) and sieve elements (SE)[1]
Virions of RBSDV, RGDV and RRSV were abundant in sap from vascular tissues but rare or absent in mesophyll tissue extracts
RDV virus-like particles (VLPs) were distributed in mesophyll (Fig. 1N), SE (Fig. 1O) and PP (Fig. 1P) cells, where they were located in the cytoplasm rather than in the nucleus
Summary
Symplastic infection allows viruses to invade multiple types of cell, including mesophyll cells (MC), phloem parenchyma (PP), companion cells (CC) and sieve elements (SE)[1]. We recently reported a uniquely-structured channel, termed the flexible gateway[23], on the SE-SE interface within tumors induced by Southern rice black-streaked dwarf virus (SRBSDV)[23], a novel member of genus Fijivirus, family Reoviridae This intercellular channel resembles PD in morphology but has a similar capacity to the sieve pore[23]. TEM shows a central pith structure and osmiophobic CW modification at the periphery resulting in very little open space within the channel but it has the capacity to accommodate the large virions of SRBSDV, about 80 nm in diameter[23] These flexible gateways are concentrated on the sieve plate but are frequent on the non-SP SE-SE interface of the SE hypertrophied region in SRBSDV-infected phloem but they are much less frequently observed in normal sieve tubes without hyperplasia. Our findings provide more insight into the cellular distribution and movement patterns of plant reoviruses
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