ABSTRACT
In all, 70 of 296 leaf samples (23.6%) collected from faba bean (Vicia faba) plants showing leaf yellowing and stunting in Ethiopia gave nanovirus-positive reactions when studied by triple-antibody sandwich enzyme-linked immunosorbent assay using broad-spectrum monoclonal antibodies (MAbs) specific to nanoviruses. Further analysis of these samples with seven discriminating MAbs revealed contrasting epitope profiles that were categorized into roughly three serogroups, designated A, B, and C. Serogroup A was found in 89% of the nanovirus-positive samples whereas serogroups B and C were infrequently encountered. Sequence analysis of DNA-S and DNA-U1 of serogroup A, B, and C isolates suggested that each represents a distinct nanovirus species. Serogroup A comprised isolates of Faba bean necrotic stunt virus reported earlier only from Ethiopia and Morocco. The DNA-R, -S, -U1, and -U2 sequences of a serogroup B isolate closely resembled those of Faba bean necrotic yellows virus, providing first molecular evidence for its occurrence in Ethiopia. Sequence analysis of the eight genomic DNAs of a representative serogroup C isolate (Eth-231) showed that it shared overall nucleotide and amino acid sequence identities of only ≤70 and ≤74%, respectively, with other nanoviruses. This suggests that Eth-231 represents a new nanovirus species, for which the name faba bean yellow leaf virus is proposed.
ABSTRACT
Using monoclonal antibodies raised against a Faba bean necrotic yellows virus (FBNYV) isolate from Egypt and a Faba bean necrotic stunt virus (FBNSV) isolate from Ethiopia, a striking serological variability among nanovirus isolates from faba bean in Morocco was revealed. To obtain a better understanding of this nanovirus variability in Morocco, the entire genomes of two serologically contrasting isolates referred to as Mor5 and Mor23 were sequenced. The eight circular ssDNA components, each identified from Mor5- and Mor23-infected tissues and thought to form the complete nanovirus genome, ranged in size from 952 to 1,005 nt for Mor5 and from 980 to 1,004 nt for Mor23 and were structurally similar to previously described nanovirus DNAs. However, Mor5 and Mor23 differed from each other in overall nucleotide and amino acid sequences by 25 and 26%, respectively. Mor23 was most closely related to typical FBNYV isolates described earlier from Egypt and Syria, with which it shared a mean amino acid sequence identity of about 94%. On the other hand, Mor5 most closely resembled a FBNSV isolate from Ethiopia, with which it shared a mean amino acid sequence identity of approximately 89%. The serological and genetic differences observed for Mor5 and Mor23 were comparable to those observed earlier for FBNYV, FBNSV, and Milk vetch dwarf virus. Following the guidelines on nanovirus species demarcation, this suggests that Mor23 and Mor5 represent isolates of FBNYV and FBNSV, respectively. This is the first report not only on the presence of FBNSV in a country other than Ethiopia but also on the occurrence and complete genome sequences of members of two nanovirus species in the same country, thus providing evidence for faba bean crops being infected by members of two distinct nanovirus species in a restricted geographic area.
