IDENTIFYING BICISTRONIC SEGMENTS IN THE GENOME OF THE AGENT CAUSING THE 'MAL DE RIO CUARTO' (MRCV) DISEASE
L CONCI1, F GUZMAN1, M del VAS2, A DISTÉFANO2, I LAGUNA1 and H HOPP2
1IFFIVE-INTA, Camino 60 (5719). Cordoba, Argentina; 2IBM-CICV, INTA-Castelar, CC 77 (1708) Morn, Argentina
Background and objectives
The agent causing the 'Mal de Rio Cuarto' (MRCV) disease is a reovirus which severely affects maize crops in Argentina. It produces losses ranging between 10 and 100 million dollars per year, depending on the agricultural period. Infected maize plants show symptoms such as internode shortening, dwarfing, total or partial atrophy of both male and female flowers, and concomitant reductions or even total suppression of grain yield. The unequivocal symptom of virus infection is an overgrowth of tissue on the veins in the back of the leaves, termed enation. The severity of symptoms is associated to plant genotype, age at which infection occurs, and weather conditions. Even though the virus is capable of infecting not only maize but also other crops of economic importance such as sorghum, oat, wheat, barley, millet and rye, the damage to these crops is still to be assessed. The MRCV has also been found in a number of weeds, mostly Gramineae, which not only operate as a reservoir for the virus from year to year, but also provide shelter for the planthopper Delphacodes kuscheli Fennah (Homoptera, Delphacidae) which has been identified to be the MRCV vector.
Initial research on the 'Mal de Rio Cuarto' disease led to the assumption that it was caused by a fijivirus bearing great similarity to both MRDV  observed in the Mediterranean area, and RBSDV detected in rice crops in South-East Asia. This virus genus shows a genome divided into 10 dsRNA segments within a complex capsid consisting of at least six polypeptides.
Some biological features, as well as serological and molecular hybridization tests, suggest that it is either different from or distantly related to the fijiviruses mentioned above.
Results and conclusions
Part of the virus genome has been cloned and nearly 25% sequenced (approximately 7000 nt) including parts of some of the 10 genomic segments (S). The sequencing of 1003 bp of virus S7 (50% of the segment) was performed and homology on the nucleotide level of 56.5% to MRDV S6 and 57.5% to RBSDV S7 was confirmed. This homology rose to 65% when the derived amino-acid sequence was analyzed. The presence of two encoding regions separated by a 52-bp intercistronic region could be determined. This size coincides with published information for S6 and S7 of other fijiviruses .
70% (1189 nt) of MRCV S8 has been sequenced. This clone comprises an extended encoding region, then a 158-bp broad intergenic region, and the beginning of a new region. This fact would also indicate a bicistronic structure for this segment already described in other fijiviruses . Recognition sequences typical of eukaryotic ribosomes close to the ATG where the second ORF initiates were also identified. A homology on the nucleotide level of 72.5% to MRDV S8 was determined and the amino-acid sequence deduced from it showed an homology of 75% to S8 of the same virus.
These results reveal that the MRCV belongs to the Fijiviridae family, displaying its typical structure with at least two bicistronic segments . Homology percentages with MRDV and RBSDV ranged from 56.5 to 72.5%, confirming that MRCV is an entity either different from or distantly related to the fijiviruses mentioned above, although they could suggest a common origin for all of them.
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2. Marzachi C, Antoniazzi S, d'Aquilio M, Boccardo G, 1996. European Journal of Plant Pathology 102, 601-605.