1.11.36
A DS RNA PHYTOREOVIRUS ASSOCIATED WITH TOBACCO EXHIBITING A FORM OF LEAF CURL IN SOUTH AFRICA

CHRISTINE REY1, G BOCCARDO2 and ELVERA D'ANDREA1

1Department of Microbiology, University of the Witwatersrand, PO Wits2050, Johannesburg, South Africa; 2Instituto di Fitovirologia Applicata, Str. delle Cacce 73,10135 Torino, Italy

Background and objectives
Three different forms of leaf curl, named class I, II and III, based on different symptomatologies, have been described in Zimbabwe and South Africa [1]. Class 11 aetiology is suspected to be a genetic disorder, while class III tobacco leaf curl is typical of a geminivirus infection [1]. Numerous attempts over the past 20 years to isolate a virus from class I tobacco in South Africa had failed until recently, when reovirus-like particles were isolated. While the leaf curl symptoms are identical to those reported in the literature for wound tumour virus (WTV) [2], the aim of this research was not to establish pathogenicity but rather to isolate and characterize dsRNAs from leaf curl-infected tobacco and to attempt to demonstrate the association of these dsRNAs with phytoreoviruses.

Results and conclusions
Twelve dsRNAs, with a total apparent molecular weight of 16x106, were consistently isolated from leaf-curl (class I) tobacco over several years. The dsRNAs were found to be nonhomologous distinct species lacking poly(A) tails, and demonstrated a unique banding pattern on non-denaturing PAGE gels, when compared to other reoviruses. The closest similarity was with the dsRNAs of WTV. Positive cross-reactvity of a purified virus preparation from class I tobacco with an antiserum was observed in indirect ELISAs and Western blots. SDS-PAGE and Western blots revealed three proteins (120, 95 and 39 kDa apparent molecular weights) of similar MW to those of W'TV, the only dicot-infecting phytoreovirus reported to date. Isolation of icosahedral particles (outer core of 60-65 nm in diameter and an inner core of 40-45 nm) was achieved.

Results from this study establish the presence of 12 dsRNAs, typical of the genus Phytoreovirus, but showing a unique pattern. Isolation of viral particles with an inner and outer core (characteristic of phytoreoviruses), serological cross-reactivity with antiserum to VTN in ELISA and WB, and similarity of symptoms in tobacco with WTV in several indicator hosts, indicate the presence of a phytoreovirus in leaf curl-infected tobacco. This is the first report of a phytoreovirus in tobacco, the first report in Africa, and only the second field isolation of a dicot phytoreovirus.

References
1. Paldmadis M, Dusterwald G, Duyver L, Rey MEC, 1997. Plant Pathology 46, 282-290.
2. Black LM, 1945. American Journal of Botany 32, 408-415.