Family: Kitaviridae
Genus: Higrevirus
Distinguishing features
Viruses assigned to the genus Higrevirus form a monophyletic group based on a well-supported maximum likelihood tree inferred from the RNA-directed RNA polymerase (RdRP). Higrevirus genomes are split into three segments and encode a novel specialized transport module called binary movement block (BMB). Hibiscus green spot virus 2 (HGSV2, Higrevirus waimanalo) causes non-systemic infections and is transmitted by mites of the genus Brevipalpus.
Virion
Morphology
Virions of HGSV2 isolate WAI 1/1 (Hawaii, USA) are bacilliform particles with sizes of 33 to 120 nm (length) and 14 to 70 nm (width) (Melzer et al., 2012, Olmedo-Velarde et al., 2024). In micrographs of ultrathin sections of C. aurantium leaves infected by HGSV2 isolate DMs01 (Brazil), spherical virus-like particles (VLPs) of 55 nm have been observed (Pereira et al., 2025).
Nucleic acid
Genomic segments of higreviruses consist of three positive-sense, single-stranded (ss) RNA molecules. RNA1 is 8.4–8.9 kb, whereas RNA2 and RNA3 are shorter, and of more variable length, ranging from 2.4 to 3.9 kb (Melzer et al., 2012, Li et al., 2023).
Genome organization and replication
Higreviruses possess segmented, positive-sense single-stranded RNA (ssRNA) genomes composed of three RNA molecules. Despite this shared architecture, the higreviruses HGSV2, pistachio virus X (PisVX, Higrevirus pistaciae), and Phellodendron-associated higre-like virus (PaHLV, Higrevirus amurense), exhibit notable differences in genome size and ORF composition (Melzer et al., 2012, Mohammadi et al., 2021, Li et al., 2023). RNA1 is the most conserved segment. It is monocistronic or bicistronic, but always encodes the RdRP, which contains the canonical methyltransferase (Pfam01660), helicase (Pfam01443), and polymerase (Pfam00978) domains. HGSV2 RNA1 contains a small orphan overlapping ORF (p11), whereas the RNA1 of PisVX and PaHLV encodes only the RdRP. RNA2 generally encodes movement-associated proteins, including the conserved BMB1 and BMB2 orthologs, which are sufficient for cell-to-cell movement. HGSV2 RNA2 is tetracistronic, whereas PisVX and PaHLV contain fewer ORFs, reflecting a more compact and often truncated RNA2 segment. RNA3 is the most variable segment across the genus. All three viruses encode an SP24 (Pfam16504) ortholog and additional orphan ORFs with multiple transmembrane helices (TMHs). HGSV2 RNA3 contains two or three ORFs (depending on the isolate), whereas the RNA3 of PisVX and PaHLV is larger and displays a genomic organization that appears intermediate between HGSV2 RNA3, cilevirus RNA2, and blunervirus RNA3 (Ramos-González et al., 2023).
Biology
Higreviruses have been detected in the Americas and Asia (Melzer et al., 2012, Mohammadi et al., 2021, Li et al., 2023, Pereira et al., 2025). Their natural host range includes shrubs and trees, encompassing ornamentals and economically important crops such as hibiscus (Hibiscus spp.), citrus (Citrus spp.) and pistachio (Pistacia vera). Experimentally, HGSV2 infects Arabidopsis thaliana, Phaseolus vulgaris, Nicotiana tabacum, and N. benthamiana and it has been successfully transmitted by viruliferous Brevipalpus azores, B. yothersi, and B. papayensis (Olmedo-Velarde et al., 2024, Pereira et al., 2025). Proteins BMB1 and BMB2, encoded by the RNA2 ORFs 2 and 3 of higreviruses, together form a novel block of plant virus movement genes termed the binary movement block (BMB) (Lazareva et al., 2017b, Lazareva et al., 2021, Atabekova et al., 2022).
Species demarcation criteria
The demarcation criteria for species of the genus Higrevirus are based on (1) the extent of the serological relationship as determined by immunodiffusion and/or ELISA, (2) less than 85% aa sequence identity for all ORF encoded proteins, (3) natural host range and, (4) vector species and particularities in the transmission process.