Genomic diversity, host associations, and tissue tropism of Hydrangea ringspot virus: A global and regional perspective.

Hydrangea ringspot virus (HdRSV) is an emerging plant virus infecting ornamental hydrangea species worldwide, yet its genomic diversity and host associations remain poorly understood. To expand the available genomic resources and assess HdRSV variability, we screened 210 publicly available Hydrangea spp. transcriptomes from diverse tissues, complemented with four newly generated H. macrophylla transcriptomes from Colombia. Viral genomes were assembled from infected samples and analyzed to infer phylogenetic relationships, lineage distribution, and relative viral RNA abundance. Two well-supported phylogenetic lineages (HdRSV-L1 and HdRSV-L2) were recovered from both full-genome and replicase coding sequence (CDS) analyses. HdRSV was detected across all host tissues examined, with the highest median viral loads in roots, followed by stems and leaves. H. macrophylla harbored both viral lineages, while H. serrata was exclusively infected by HdRSV-L1. Cultivar-level analysis revealed marked differences in viral abundance, with lineages showing distinct tissue preferences but no co-infection patterns, except in the 'Bailer' cultivar. Comparative analysis of the replicase CDS identified a single lineage-defining nonsynonymous mutation (C1578T; Thr → Ile), fixed in 90% of HdRSV-L2 genomes, corresponding to a polar-to-nonpolar amino acid change potentially associated with structural adaptation. Together, these findings provide the most comprehensive overview to date of HdRSV genomic diversity, host and tissue distribution, and molecular variation, offering new insights into the evolution and epidemiology of this understudied plant virus.