Protoplasma (2002) 219: 184–196 Summary. Viral invasion of the root system of Nicotiana benthami- ana was studied noninvasively with a tobacco mosaic virus (TMV) vector expressing the green-fluorescent protein (GFP). Lateral root primordia, which developed from the pericycle of primary roots, became heavily infected as they emerged from the root cortex. However, following emergence, a progressive wave of viral inhibi- tion occurred that originated in the lateral-root meristem and progressed towards its base. Excision of source and sink tissues sug- gested that the inhibition of virus replication was brought about by the basipetal movement of a root meristem signal. When infected plants were inoculated with tobacco rattle virus (TRV) expressing the red-fluorescent protein, DsRed, TRV entered the lateral roots and suppressed the host response, leading to a reestablishment of TMV infection in lateral roots. By infecting GFP-expressing trans- genic plants with TMV carrying the complementary GFP sequence it was possible to silence the host GFP, leading to the complete loss of fluorescence in lateral roots.The data suggest that viral inhibition in lateral roots occurs by a gene-silencing-like mechanism that is dependent on the activation of a lateral-root meristem. Keywords: Tobacco mosaic virus; Nicotiana benthamiana; Root; Meristem; Viral suppression; Green-fluorescent protein. Abbreviations: GFP green-fluorescent protein; PTGS posttran- scriptional gene silencing; TMV tobacco mosaic virus; TRV tobacco rattle virus. Introduction The cell-to-cell and long-distance movement of plant viruses has been the subject of intense recent investi- gation (for reviews, see Carrington et al. 1996, Lucas and Wolf 1999, Oparka et al. 1996). In the case of phloem-mobile viruses, the distribution patterns of virus in developing sink tissues are strongly influenced by source–sink relationships (A. Roberts et al. 1997). It appears that once an infectious viral complex enters the translocation stream, its final destination is largely dependent on the flow of assimilates within the plant (reviewed by Santa Cruz 2000). If systemic viruses enter the phloem at a stage of development during which the root system is a dominant sink, it might be expected that such viruses would also be delivered to developing roots. However, almost nothing is known of the patterns of virus invasion in roots, despite the fact that several viruses invade the root systems of many commercially important crop species (Matthews 1991). It appears that in both roots and shoots the apical meristem is largely devoid of virus infection (Faccioli and Colombarini 1996, Hsu et al. 2000, Matthews 1991, Walkey et al. 1969). The inability of viruses to infect apical meristems has been exploited commercially and forms the basis of meristem tip culture, a method by which virus-free clones can be obtained by growing excised shoot tips in tissue culture (Matthews 1991, Walkey et al. 1987). In general, a zone of variable length (but usually within 100 mm of the root or shoot tip) is free of replicating virus. At present, the reasons why meristems fail to support virus replication are unknown. It is possible that a barrier to meristem inva- sion is enforced by cells some distance from the meri- stem (see Matthews 1991), although such a putative barrier has not been experimentally demonstrated. While many viruses are thought to be unable to invade meristems, virus particles from several unrelated PROTOPLASMA © Springer-Verlag 2002 Printed in Austria Inhibition of tobacco mosaic virus replication in lateral roots is dependent on an activated meristem-derived signal T. A. Valentine, I. M. Roberts, and K. J. Oparka * Unit of Cell Biology, Scottish Crop Research Institute, Dundee Received July 23, 2001 Accepted October 11, 2001 * Correspondence and reprints: Unit of Cell Biology, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, United Kingdom.