Downloaded from www.microbiologyresearch.org by IP: 93.91.26.97 On: Fri, 20 Nov 2015 00:42:57 Journal of General Virology (2000), 81, 2791–2795. Printed in Great Britain .......................................................................................................................................................................................................... SHORT COMMUNICATION Mechanical transmission of Potato leafroll virus Mike Mayo, Eugene Ryabov, Gillian Fraser and Michael Taliansky Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK Like typical luteoviruses, Potato leafroll virus (PLRV) cannot be transmitted mechanically by rubbing plants with solutions containing virus particles. However, PLRV was found to be mechanically transmissible from extracts of plants that had been inoculated by viruliferous aphids and then post- inoculated with Pea enation mosaic virus-2 (PEMV- 2). Unlike the asymptomatic infections induced by either virus alone, double infections in Nicotiana benthamiana induced necrotic symptoms with some line patterning and vein yellowing. Infective PLRV was recovered from a purified virus preparation by inoculating plants mechanically with purified virus particles mixed with PEMV-2. Similarly, Beet mild yellowing virus was readily transmitted mech- anically from mixtures containing PEMV-2. PLRV was also transmissible from mixtures made with extracts of plants infected with Groundnut rosette virus, although less efficiently than from mixtures containing PEMV-2. This novel means of trans- mitting PLRV, and perhaps other poleroviruses, should prove very useful in a number of fields of luteovirus research. One of the principal characteristics of viruses that are members of the family Luteoviridae is that they cannot be transmitted mechanically by rubbing healthy plants with extracts made from virus-infected plants (Waterhouse et al., 1988). The explanation usually offered for this inability is that, although luteoviruses are able to multiply in mesophyll cells (at least in protoplasts prepared from mesophyll cells), transport of luteovirus infection between mesophyll cells is not possible (Taliansky & Barker, 1999). More recently, it was proposed that this inability to spread is because of an RNA-mediated defence in non-vascular tissues, similar to post-transcriptional gene silencing, that cannot be overcome by luteoviruses (Voinnet et al., 1999). Whatever the cause, the effect is that any primarily infected cell does not yield an infective centre from which infection can develop. Although productive infection Author for correspondence : Mike Mayo. Fax 44 1382 562426. e-mail mailscri.sari.ac.uk starts when aphid-inoculated luteoviruses are delivered to cells in the phloem, cells in other tissues can become infected. For example, a few non-vascular cells in systemically infected Nicotiana clevelandii (Barker, 1987) and potato (van den Heuvel et al., 1995) have been found to contain Potato leafroll virus (PLRV, genus Polerovirus), epidermal and mesophyll cells of N. clevelandii leaves inoculated by viruliferous aphids carrying green fluorescent protein (GFP)-tagged PLRV accumulated PLRV (Nurkiyanova et al., 2000) and epidermal cells of tobacco leaves became infected with the luteovirus Tobacco necrotic dwarf virus after mechanical inoculation (Imaizumi & Kubo, 1980). It is also possible to transmit luteoviruses to plants without using aphids, by agroinoculation. This is a form of mechanical inoculation in which plants are injected with Agrobacterium cells that carry a Ti plasmid containing a full-length DNA copy of the virus genome (Leiser et al., 1992 ; Mutterer et al., 1999). However, the infection that follows, presumably because of transformation of cells in the vascular system, resembles that initiated by viruliferous aphids. That is, virus is largely limited to phloem tissue and cannot be transmitted mechanically from the infected plants to healthy ones. One apparently contradictory example of a mechanically transmissible member of the family Luteoviridae has been created by recent taxonomic revisions. These have added what was previously known as RNA-1 of pea enation mosaic virus to the family as a species [Pea enation mosaic virus-1 (PEMV-1)] in the revised genus Enamovirus (Mayo & D’Arcy, 1999). PEMV-1 is mechanically transmissible when inoculated together with Pea enation mosaic virus-2 (PEMV-2), now classified as an umbravirus, as well as being transmissible by aphids in the same circulative, non-propagative manner as other viruses in the family Luteoviridae. PEMV-1 is able to infect individual protoplasts but, unless ‘ helped ’ by PEMV-2, cannot spread in plants. This makes the pea enation mosaic complex unusual in that the luteoviruses that ‘ help ’ other umbraviruses to become aphid-transmissible are capable of independent multiplication, and they spread in infected plants in the absence of their umbravirus partners (Falk et al., 1999). Another unconventional result was obtained with the lettuce speckles complex. This is a mixed infection by a polerovirus, Beet western yellows virus (BWYV), and an umbravirus, Lettuce speckles mottle virus, and Falk et al. (1979) reported that it was sometimes possible to transmit BWYV 0001-7249  2000 SGM CHJB