Tissue Specificity of Zea mays Infection by Maize Streak Virus. Andrew P. Lucy. Department of Virus Research, John Innes Centre, Norwich Research Park, Colney, Norwich, NR4 7UH, U.K. Margaret I. Boulton, Jeffrey W. Davies, and Andrew J. Maule. Department of Virus Research, John Innes Centre, Norwich Research Park, Colney, Norwich, NR4 7UH, U.K. MPMI 9:22-31. Accepted 27 September. Copyright 1996 The American Phytopathological Society.

Maize streak virus (MSV) is a single-stranded DNA virus and type member of subgroup I of the Geminiviridae, members of which have been considered generally as viruses limited to phloem tissues. We have studied this characteristic and the extent of MSV penetration into the shoot apical merislem of maize using immuno-histochemical and in situ hybridization techniques. This approach has been used to investigate the suggestion that host cell division might be required for the replication of MSV. The results show that, within the shoot apex, MSV is present only in the vascular tissues and does not invade the apical meristem. In mature leaves, virus is located only in areas of the leaf displaying the characteristic chlorotic streak symptoms of infection and, in contrast to the situation in apical and stem tissues, it is no longer restricted to the vasculature. Viral coat protein and both positive and negative strands of the DNA genome were found in mesophyll, vascular-associated parenchyma, and bundle sheath cells of the leaf. MSV was not usually found in nonphotosynthetic tissues outside of the vasculature. Localization of both double-stranded viral DNA and transcripts encoding proteins involved in virus replication, identified cells in which virus replication was active. Further spatial comparison with the distribution of transcripts of histone H2b, an S-phase specific gene, implied that host DNA replication was not essential for viral replication.