‘Lamon bean’ is a protected geographical indication (PGI) for a product of four varieties of bean (Phaseolus vulgaris L.) grown in a specific area of production, which is located in the Belluno district, Veneto region (N.E. of Italy). The ‘Lamon bean’ has been threatened by severe virus epidemics that have compromised its profitability. The full virome of seven bean samples showing different foliar symptoms was obtained by MinION sequencing. Evidence that emerged from sequencing was validated through RT-PCR and ELISA in a large number of plants, including different ecotypes of Lamon bean and wild herbaceous hosts that may represent a virus reservoir in the field. Results revealed the presence of bean common mosaic virus (BCMV), cucumber mosaic virus (CMV), peanut stunt virus (PSV), and bean yellow mosaic virus (BYMV), which often occurred as mixed infections. Moreover, both CMV and PSV were reported in association with strain-specific satellite RNAs (satRNAs).
Considering the wide diversity of species and viral strains that could be detected in Lamon bean, a fully comprehensive and sensitive diagnostic tool, able to explore such a great variability, was highly needed.
For this reason, researchers took advantage of the advent of High Throughput Sequencing (HTS) approaches, which in the last two decades have allowed great advances and have promoted discovery, diagnostics, and evolutionary studies [3]. Plant virus research has been heavily impacted by the development of HTS techniques for the identification of emerging viruses, genome reconstruction, analysis of population structures, evolution of novel viral strain(s), and much more [4][5][6][7][8][9]. Single-molecule sequencing technologies, often called “third generation sequencing”, provide greater advantages over second-generation approaches, including the production of long read lengths that are easier to map to a reference sequence and that facilitate de novo assembly, short run times, small amounts of input nucleic acids (DNA or RNA), and low cost for a single run [3]. The consistent improvement in HTS technologies has occurred in parallel with the continuous development of suitable bioinformatic tools for data analysis and with the online availability of a wide range of biological data sets, which have allowed a deeper comprehension of HTS results [6][7].
In this entry, a third-generation sequencing platform, namely MinION, provided by Oxford Nanopore Technologies, was exploited to obtain the virome of seven bean samples that exhibited different foliar symptoms resembling those of virus disease. The picture that emerged from the sequencing results was consolidated through RT-PCR and ELISA assays in many samples, inclusive of different varieties of bean grown in the Lamon area and various families of herbaceous plants, which may represent a virus reservoir in the field.


A total of 59 samples were tested by RT-PCR assays to confirm the presence of the viruses detected by MinION sequencing, BCMV, CMV, PSV and BYMV (Table 1). Three Lamon beans were negative to all tested viruses (9%). Single infections caused by BCMV, and PSV were detected in 39% and 13% of Lamon beans, respectively, while CMV was reported in 21% of bean samples, exclusively in co-presence with either BCMV (9%), PSV (3%), or both (9%). Mixed infection by BCMV and PSV was also reported in 12% of Lamon beans. BYMV was identified, exclusively by MinION sequencing, in two samples (6%) either as a single infection (LB-21, NB06) or in co-presence with BCMV (LB-4, NB11). All herbaceous hosts were negative for the presence of BCMV. CMV was exclusively reported in Trifolium pratense L. (Leguminosae), both as a single infection (4%) and in co-presence with PSV (8%). Single infection caused by PSV was detected in Trifolium pratense L. (8%), Chrysanthemum sp. (4%) and, for the first time, in Solanum tuberosum L. (8%). Members of the Amaranthaceae (Chenopodium album L., Amaranthus retroflexus L., and Achillea millefolium L.), Apiaceae (Daucus carota L.), and Asteraceae (Taraxacum officinale L.) were negative to all tested viruses (68%). The results of ELISA tests are reported in Table 2 and revealed BCMV presence in samples collected in all the surveyed municipalities, with an incidence ranging from 40% to 100%. CMV was serologically identified in 50% and 62% of the samples belonging to the two municipalities (Sovramonte and Belluno) with incidence of 25 and 30%, respectively.
Sequences obtained by Sanger were used for assignment to pathotypes/subgroups according to recent comprehensive studies of BCMV [10], CMV [11], and PSV [12]. The results revealed that 79% of BCMV sequences fall into pathotype VII, sharing 97.7–99.2% identity with NL-4 strain (DQ666332) [13]. Three BCMV sequences (13%) were included in the pathotype I, exhibiting from 99.0% to 99.6% of sequence similarity to the NL-1 strain (KM023744) [14]. The remaining BCMV sequences (8%) fall into pathotype VI, sharing 95.6–99.2% identity with recombinant RU-1 (GQ219793) [15] and RU-1-CA (MH024843) [16] strains. All PSV sequences belonged to subgroup IA, showing a sequence similarity that ranged from 92.7% to 99.3% with the PSV-ER strain (U15730) [17]. CMV sequences can be assigned to the subgroups IA and IB, exhibiting 99.3% identity with Fny-CMV strain (D10538) and 95.3–96.4% sequence similarity with As-CMV strain (AF013291), respectively [18][19][20]. Results of BLAST analyses are summarized in Table 3. Phylogenetic analyses (Figure 3) assigned reference sequences of the isolates of BCMV, PSV and CMV detected in Lamon bean plants to the respective clusters congruently with the BLAST results outlined above. The phylogenetic analysis carried out with the introduction of other sequence variants found in the survey produced similar results (not shown).
This entry is adapted from the peer-reviewed paper 10.3390/plants11060779