Crinivirus replication and host interactions

Abstract

Criniviruses comprise one of the genera within the family Closteroviridae. Members in this family are restricted to the phloem and rely on whitefly vectors of the genera Bemisia and/or Trialeurodes for plant-to-plant transmission. All criniviruses have bipartite, positive-sense single-stranded RNA genomes, although there is an unconfirmed report of one having a tripartite genome. Lettuce infectious yellows virus (LIYV) is the type species of the genus, the best studied so far of the criniviruses and the first for which a reverse genetics system was developed. LIYV RNA 1 encodes for proteins predicted to be involved in replication, and alone is competent for replication in protoplasts. Replication results in accumulation of cytoplasmic vesiculated membranous structures which are characteristic of most studied members of the Closteroviridae. These membranous structures, often referred to as Beet yellows virus (BYV)-type vesicles, are likely sites of RNA replication. LIYV RNA 2 is replicated in trans when co-infecting cells with RNA 1, but is temporally delayed relative to RNA 1. Efficient RNA 2 replication also is dependent on the RNA 1-encoded RNA-binding protein, P34. No LIYV RNA 2-encoded proteins have been shown to affect RNA replication, but at least four, CP (major coat protein), CPm (minor coat protein), Hsp70h, and P59 are virion structural components and CPm is a determinant of whitefly transmissibility. Roles of other LIYV RNA 2-encoded proteins are largely as yet unknown, but P26 is a non-virion protein that accumulates in cells as characteristic plasmalemma deposits which in plants are localized within phloem parenchyma and companion cells over plasmodesmata connections to sieve elements. The two remaining crinivirus-conserved RNA 2-encoded proteins are P5 and P9. P5 is 39 amino acid protein and is encoded at the 5′ end of RNA 2 as ORF 1 and is part of the hallmark closterovirus gene array. The orthologous gene in BYV has been shown to play a role in cell-to-cell movement and indicated to be localized to the endoplasmic reticulum as a Type III integral membrane protein. The other small protein, P9, is encoded by ORF 4 overlaps with ORF 3 that encodes the structural protein, P59. P9 seems to be unique to viruses in the genus Crinivirus, as no similar protein has been detected in viruses of the other two genera of the Closteroviridae.

Closteroviridae

“…The close association of viruses with phloem tissues accounts for some of the transmission traits, such as the length of the acquisition and transmission feeding periods and the lack of transmission during brief probings. CPm is a determinant of whitefly transmissibility (Kiss et al, 2014). Viral species infecting woody hosts are spread over long distances through infected propagating material (primarily nursery productions), rather than vectors.…”

A novel block of plant virus movement genes

“…It consists of five proteins encoded by a quintuple gene block (QGB), namely a small transmembrane protein, a heat shock protein 70 (Hsp70) chaperone-like protein (Hsp70h) with enzymatic ATPase activity, coat protein (CP) and two distant coat protein homologues, p64 and CPm (Dolja et al, 2006). Coordinated incorporation of the last four proteins (CP, CPm, Hsp70h and p64) into filamentous virions, which are targeted to PD with the aid of the small transmembrane protein, is required for virus cell-to-cell movement (Dolja et al, 2006;Kiss et al, 2013). The Hsp70h ATPase activity is essential for Beet yellows virus (BYV) transport through PD.…”

Accumulation of 24 nucleotide transgene‐derived siRNAs is associated with crinivirus immunity in transgenic plants

“…Lettuce infectious yellows virus (LIYV) is a type member in the genus Crinivirus that includes the whitefly‐transmitted members of the family Closteroviridae (Tzanetakis etal ., ). Criniviruses are characterized by their large bipartite genomes of positive‐sense, single‐stranded RNA of approximately 15–18 kb in total (Kiss etal ., ; Tzanetakis etal ., ). RNA1 encodes proteins that are predominantly associated with replication: the 5′‐terminal replicative module consisting of open reading frames (ORFs) 1a and 1b that code for the conserved domains of papain‐like cysteine proteinase (PRO), methyltransferase (MTR), helicase (HEL) and RdRp (Yeh etal ., ).…”