1Department of Biology, Laboratory of Plant Physiology and Molecular Biology, 20014 University of Turku; 2Department of Plant Production, 00014 University of Helsinki, Finland; 3Agricultural Research Centre, Institute of Plant Protection, 31600 Jokioinen, Finland

Background and objectives
Reversion is the most important disease of blackcurrant crops worldwide. Its causal agent has remained unknown despite research over more than 50 years. Reversion disease is transmitted in nature by gall mites (Cecidophyopsis ribis), and experimentally by grafting to sensitive indicator hosts, which has been the only available method for the diagnostics of the disease.

At least two forms of the disease are known. The severe form (R), occurring mostly in Eastern Europe and Scandinavia, causes distinct leaf malformations. The leaves of infected plants are narrower, and have a decreased number of main veins, larger but fewer marginal serrations and a basal sinus that is less lobed. In addition, the plants affected with the severe form occasionally develop strong malformations of the flowers, including the absence of stamens, elongation of the style and an increase in the number of petals, and some blackcurrant cultivars also show strong proliferation of shoots. Symptoms caused by the less-severe, or common form (E) of the disease are much less pronounced than the symptoms of the R-form. The type and intensity of symptoms vary between blackcurrant cultivars, and the symptoms of the E-form of reversion may be easily overlooked due to the normal variation in blackcurrant leaf shapes. Recognition of the disease based on field symptoms is therefore unreliable.

Results and conclusions
We have recently isolated a new nepovirus from reverted blackcurrant [1]. The virus particles are spherical and 27 nm in diameter. The coat protein has a molecular mass of 56.5 kDa, and it is readily degraded to a slightly smaller form of 55.5 kDa [1, 2]. The viral genome is composed of two poly(A)-tailed RNAs of ca 7700 and 6700 nt, and a satRNA of ca 1400 nt is associated with the virus. The smaller genomic RNA (RNA2) has been sequenced and found to contain one long ORF which is followed by a 3' non-coding region of 1360 nucleotides. Specific sequence motifs, which are conserved between different nepoviruses, are found in RNA2. Several conserved amino-acid streches in the coat protein (CP) gene are found to be similar to the CP of blueberry leaf mottle nepovirus, the overall sequence similarity being 61%, and identity being 41% between the CPs of these two viruses. Based on these data, the new virus belongs to the third subgroup of nepoviruses [1, 2].

Using an IC-RT-PCR detection method, the virus has been detected in numerous samples of reverted blackcurrant obtained from many geographic locations, and from samples of gall mite vectors [3]. The isolated virus has been mechanically inoculated back to healthy blackcurrant plants, in which the acute symptoms of reversion were subsequently produced, and the virus was detected by IC-RT-PCR. These data prove that the isolated virus is the causal agent of the reversion disease, and therefore the virus is called the blackcurrant reversion virus (BRV). Pathogen-derived resistance to reversion disease is currently being tested in transgenic tobacco plants expressing the coat-protein gene of BRV in different orientations. The tested viral sequences will eventually be transformed to blackcurrant for production of reversion-resistant blackcurrant cultivars.

1. Lemmetty A, Latvala S, Jones AT et al., 1997. Phytopathology 87, 404-413.
2. Latvala S, Susi P, Kaikkinen N, Lehto K, 1998. Virus Research (in press).
3. Latvala S, Susi P, Lemmetty A et al. 1997. Annals of Applied Biology 131, 283-295.