Family: Alphaflexiviridae

Genus: Mandarivirus

Distinguishing features

This genus includes two closely related species, Citrus yellow vein clearing virus and Indian citrus ringspot virus, members of which infect citrus in west Asia and are transmitted by aphids (Loconsole et al., 2012, Rustici et al., 2002). There are six ORFs in the genome and the capsid protein (CP) is the largest of any plant-infecting member of the family. 

Virion

Morphology

Virions are flexuous filaments of 650 nm modal length, 13 nm in diameter, with clearly visible cross-banding (Figure 1. Mandarivirus). 

Figure 1. Mandarivirus.  Electron micrograph of particles of an isolate of Indian citrus ringspot virus, stained in 1% uranyl acetate. The bar represents 100 nm.

Physicochemical and physical properties

Mandariviruses form a single band in caesium sulphate density gradients. Purified preparations show maximum absorption at 260 nm with a A_260/280 ratio of 1.1 (corrected for light scattering). 

Nucleic acid

Virions of Indian citrus ringspot virus (ICRSV) contain a single molecule of linear single-stranded RNA of 7560 nt, excluding the 3′-poly(A) tail (Figure 2. Mandarivirus). 

Proteins

The only structural protein of ICRSV is the CP, composed of 325 aa (34 kDa). 

Carbohydrates

None reported. 

Genome organization and replication 

The genomic RNA of ICRSV comprises six ORFs on the positive-strand, a 5′-UTR of 78 nt and a 3′-UTR of 40 nt, followed by a poly(A) tail (Figure 2. Mandarivirus). ORF1 encodes the replication-associated protein (Rep). ORF2, ORF3 and ORF4 form the triple gene block (TGB). ORF5 encodes the CP. ORF6 encodes a putative protein of unknown function that shows limited similarity with nucleic acid-binding proteins encoded by ORF6 of allexiviruses and carlaviruses. 

Figure 2. Mandarivirus.  Genome organization of Indian citrus ringspot virus (ICRSV) showing the relative positions of the ORFs and their expression products. Mtr, methyltransferase; Hel, helicase; RdRP, RNA-directed RNA polymerase; TGB, triple gene block; CP, capsid protein; NB, nucleic acid binding protein.

Biology

Mandariviruses infect citrus but cause distinct symptoms on citrus and experimental hosts depending on the virus. Citrus yellow vein clearing virus (CYVCV) causes vein clearing disease and can be mechanically transmitted and by aphids (Loconsole et al., 2012). ICRSV causes a serious disease of citrus, especially Kinnow mandarin, in India, with bright yellow ringspots on mature leaves, followed by rapid decline of the tree (Rustici et al., 2002). Experimentally ICRSV can be mechanically inoculated to leaves of Chenopodium quinoa, C. amaranticolor, Glycine max, Vigna unguiculata and Phaseolus vulgaris, giving local lesions, but systemic infection only in P. vulgaris. No natural vector is known, but ICRSV is transmitted by grafting and persists in the host. 

Antigenicity

Particles are immunogenic. Rabbit antisera can have titers of 1/128 and 1/2048 in gel diffusion and EM decoration, respectively. 

Derivation of names

Mandari: from mandarin (Citrus reticulata), the host of members of the type species, Indian citrus ringspot virus. 

Species demarcation criteria 

Members of different species should have less than 72% nt identity (or 80% aa identity) between their respective CP or Rep genes, although reaction with antisera and biological properties should also be taken into account. 

Related, unclassified viruses

Virus names and virus abbreviations are not official ICTV designations.