The family Kitaviriridae contains members that infect plants and cause non-systemic diseases, which are economically important in some high-value horticultural crops. Though important, our understanding about kitaviruses is still limited when compared with other plant virus groups. This can be partly attributed to the lesser number of known viruses and viral isolates, which is also a constraint for defining species demarcation criteria of the kitaviral genera. Lesser sequence identity values among the known members of each kitaviral genera reveals the huge gap in kitaviral phylogenetic diversity. Till date, only 11 kitaviral species are recognized by ICTV and seven new members are yet to be recognized (Ramos-González et al., 2023).
On the other hand, DDVD studies have expanded the phylogenetic diversity and host range of many plant virus groups (Nibert et al., 2016, Bejerman et al., 2021, Bejerman and Debat, 2022, Bejerman et al., 2022, Kavi Sidharthan et al., 2022, Sidharthan et al., 2022a, b, Bejerman et al., 2023, Sidharthan et al., 2023a, b). In a DDVD study, sequences of a novel blunervirus was identified in transcriptome-assembled contigs of Paulownia tomentosa (Morozov et al., 2020). This hinted the possibility of finding sequences of novel kitaviruses in various plant transcriptome libraries in SRA database, which contains far more transcriptome data than TSA database (Edgar et al., 2022). Thus, in the present study, we explored the public domain plant SRA libraries for novel kitaviral sequences and identified fifteen putative novel blunerviruses, a 2.5-fold expansion of the genus Blunervirus, in fifteen plant species. Until now, blunerviruses were identified in only angiospermic and dicot hosts (Ramos-González et al., 2023). By identifying three new blunerviruses, each in gymnospermic and monocot hosts, the present study expanded the host range of blunerviruses. Interestingly, nearly half of the novel blunerviruses are identified in tree hosts. Though nearly-half of the bluerviruses (atleast eight) were identified in plants grown in Asian continent in the current study, the remaining viruses were identified in plants grown in American and European continents, suggesting the wider geographical distribution of blunerviruses (Ramos-González et al., 2023).
ChrBlV1 was identified in an SRA library derived from diseased Chrysanthemum morifolium plants co-infected with two carlaviruses (Chirkov et al., 2022) and a novel secovirus (Sidharthan et al., 2023b). Similarly, CtBlV1 was identified in an SRA library that also contained reads of a novel secovirus (Sidharthan et al., 2023b). On the other hand, OaBlV1 was identified in an SRA library derived from apparently heathy-looking oak leaf samples (ter Horst et al., 2023).
Largely, the genome organization of identified novel blunerviruses resembled that of known blunerviruses. The unusually longer (8.5 kb) NaBlV1 RNA1 that coded for MET–HEL–RdRp polyprotein, resembled that of Ailanthus blunervirus 1 (AiBV1) (Zhang et al., 2024), and the encoded polyprotein by both the viruses were phylogenetically related. However, individual MET–HEL and HEL–RdRp polyprotein-encoding genome segments could not be identified for NaBlV1 in the respective library. Like other blunerviruses, RNA3 ORF3 of all identified viruses (RNA4 ORF4 in AiBV1) encodes the negevirus SP24 ortholog and ORF2 encodes protein with an SP and/or TMHs (mostly C-terminal), while the ORFs 1 and 5 are ORFans (Ramos-González et al., 2023). Similar to AiBV1, apple blunervirus 1 (ApBV1) and Paulownia tomentosa blunervirus, MP-encoding RNA4 genome segment could not be recovered for four identified viruses in the current study. However, further studies are needed to ascertain the lack of MP-encoding genome segment in these four identified viruses. Except for the viruses identified in poaceous hosts, obvious host-virus coevolution pattern was not observed in MET–HEL and HEL–RdRP-based phylogenetic trees. Lesser sequence identities among the proteins encoded by identified and known blunerviruses, reiterates the existence of huge unexplored diversity in the family Kitaviridae (Ramos-González et al., 2023).
Polyprotein sequences of identified viruses shared < 75% amino acid identities among each other and with known blunerviruses, but were clustered together with known blunerviruses in phylogenetic analysis. Based on the polyprotein sequence-based species demarcation criteria of blunerviruses (Melzer et al., 2019) and the consensus statement report of Simmonds et al. (2017), the identified viruses can be regarded as novel members of the genus Blunervirus. In the current study, host assignment is solely based on the metadata provided in the public domain sequence repositories, which should be regarded cautiously until further validation. Further studies can be undertaken to understand the biological properties and geographical spread of identified novel blunerviruses and the economic importance of the diseases they incite. Genome sequences recovered in this study will be useful for developing detection assays for the identified viruses.