EDITORIAL published: 27 December 2013 doi: 10.3389/fmicb.2013.00411 e-Book on Closteroviridae

Ricardo Flores 1*,PedroMoreno2,BryceFalk3, Giovanni P.Martelli 4 and William O. Dawson 5

1 Plant Stress Biology, Instituto de Biología Molecular y Celular de Plantas, Consejo Superior de Investigaciones Científicas-Universidad Politécnica de Valencia, Valencia, Spain 2 Centro de Protección Vegetal y Biotecnología, Instituto Valenciano de Investigaciones Agrarias, Moncada, Valencia, Spain 3 Department of Plant Pathology, University of California, Davis, CA, USA 4 Department of Soil, Plant and Food Sciences, University Aldo Moro of Bari, Bari, Italy 5 Department of Plant Pathology, Citrus Research and Education Center, University of Florida, Lake Alfred, FL, USA *Correspondence: rfl[email protected]

Edited by: Akio Adachi, The University of Tokushima Graduate School, Japan

Keywords: closterovirus, ampelovirus, crinivirus, Citrus tristeza virus, citrus tristeza

This e-Book on the virus family Closteroviridae provides an In this same context, Lee and Keremane (2013) elaborate on overview on some representative members of this family. Most the history of CTV in Florida and on the methods developed articles are reviews on different fundamental and applied aspects, to select mild isolates that could protect against strains inducing but a few are original contributions focused on more specific decline of trees grafted on sour orange, a rootstock much valued issues. Even if biased toward closteroviruses, and more explicitly for the quality of the fruit produced and for its tolerance to citrus toward Citrus tristeza virus (CTV) due to its economic rele- blight, a disease of unknown etiology, and to phytophthora root vance, this compilation altogether results an attractive blend that rot. The final aim was to identify mild isolates that when inocu- we hope will attract the attention of the aficionados of this lated in the existing field trees could extend their productive life fascinating virus family. and facilitate a more graduate replanting with trees propagated First, Dawson et al. (2013) discuss CTV-host interactions, on tolerant rootstocks. highlighting that in contrast to movement of viruses in herba- Also on a historical framework, Wang et al. (2013) have exam- ceous plants that largely occurs through adjacent cells, CTV ined the collection of Californian CTV isolates maintained in infection relies mainly on long-distance movement. Moreover, the Citrus Clonal Protection Program (CCPP) in Riverside since infection of certain citrus species requires different combinations 1914. Analyses of isolates from this collection using multiple of three CTV genes, possibly acquired by the virus to expand its molecular markers have found genotypes T36, VT, and T30 at host range. Regarding pathogenesis, it is unknown why the virus high frequencies, with T30 and T30 + VT being the most abun- incites severe disease in some citrus species and not in others, but dant. Phylogenetic reconstructions using the CTV coat protein p23, a CTV-specific protein that is a suppressor of RNA silencing gene have resulted in seven clades: five associated with standard and a regulator of viral RNA synthesis appears to be the cause of genotypes (T36, VT, T30, T3, and B165/T68) and two unrelated. some tristeza syndromes, particularly seedling yellows (SY). Reduced phylogenetic diversity and virulence was observed in Flores et al. (2013) describe the properties of this protein isolates collected in central California between 1957 and 2009 (p23), with a putative zinc-finger domain and some basic motifs, in comparison to those of southern California collected in early which is unique to CTV. Besides the functions mentioned above, times (1957–2009). Biological characterization also indicated a p23: (i) elicits CTV-like symptoms when expressed ectopically as reduced number and less virulent SP isolates compared to SY a transgene in several citrus species, (ii) enhances systemic infec- isolates introduced to California. tion and virus accumulation in p23-transgenic sour orange, and Ambrós et al. (2013) have tried to extend their CTV genetic (iii) releases the virus from the phloem in some p23-transgenic system, which is based on agroinfiltration of Nicotiana ben- citrus species. Furthermore, p23 accumulates preferentially in thamiana with T36-based plasmids and results in systemic infec- the nucleolus—being the first closterovirus protein with such tion and production of enough CTV virions to infect citrus by subcellular localization—as well as in plasmodesmata. slash inoculation. Using oncogenic Agrobacterium strains they While one of the economically relevant CTV syndromes have observed induction of tumors expressing GUS in differ- (decline) can be managed by using resistant/tolerant rootstocks, ent plant species, including citrus, but systemic infection only the other (stem pitting, SP) cannot. Folimonova (2013) reports in N. benthamiana. Moreover, mechanical inoculation of CTV on the recent progress achieved on elucidating how cross- virions to N. benthamiana agroinfiltrated previously with a protection may work in the citrus/CTV pathosystem. Only iso- silencing suppressor resulted in systemic infection with T36, lates that belong to the same strain or genotype group of the but not T318A, which replicates in protoplast of this plant virus (there are six) cross-protect against each other, while iso- to the same extent as T36. Finally, T36 was graft-transmitted lates from different strains do not. Intriguingly, the mechanism of from infected to healthy N. benthamiana plants agroinfiltrated cross-protection (or superinfection exclusion) by CTV requires a previously with a silencing suppressor. These data indicate specific viral protein, p33. These findings open the door for the that extension of this genetic system still needs considerable selection of protecting isolates. improvement.

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Harper (2013), by examining the complete genome phylo- virus 1 and Grapevine leafroll-associated virus 7, form a distinct genies of 36 CTV sequences, tackles their classification into six clade within the family Closteroviridae. strains or genotypes (T36, VT, T3, RB, T68, and T30 exhibit- The chapter by Dolja and Koonin (2013) summarizes advances ing a wide range of phenotypic characteristics) and dissects the in closterovirus research during the last several years, explores major evolutionary processes that led to their formation: (i) the relationships between virus biology and vector design, and ancestral diversification of the major CTV lineages, (ii) conser- outlines the most promising directions for future application vation and co-evolution of the major functional domains within, of closterovirus-based vectors. These vectors offer high genetic though not between CTV genotypes, and (iii) extensive recom- capacity and stability, together with applicability to important bination between lineages that have given rise to new genotypes. woody plants such as citrus and grapevine. The description of Knowledge of the selective pressures acting upon CTV strains is the problems found (and their solutions) when designing vec- crucial to the development of cross-protection programs for syn- tors derived from the Grapevine leafroll associated virus 2 is thesis of CTV-based viral vectors for field release, and for breeding particularly illustrative. of new resistant citrus cultivars. Maree et al. (2013) provide an overview on Grapevine leafroll- Rubio et al. (2013) deal with a similar topic, but expanded to associated virus 3 (GLRaV-3), the type species for the genus the family Closteroviridae. They conclude that the major factors Ampelovirus, which is regarded as the most important causative that have shaped the genetic structure and diversity of this fam- agent of grapevine leafroll disease (GLD). Complete genome ily of viruses comprise: (i) a strong negative pressure that seems sequencing of several isolates has revealed the existence of genetic responsible for the high genetic stability of some viruses, (ii) variants, and characterization of the subgenomic RNAs has sup- human transport of infected propagative material that has caused plied insights into the replication strategy and the putative func- dispersion of genetically similar virus genotypes, (iii) recombina- tion of some viral proteins. Deep sequencing, apart from being tion between divergent sequence variants resulting in generation a fine diagnostic tool, has furnished a more penetrating view of of new genotypes, (iv) interactions between virus strains or the complexity of viral infections and of the underlying plant between different viruses in mixed infections that may affect the pathogen interactions. final outcome, and (v) genetic drift caused by host change or Almeida et al. (2013) discuss the ecology and management of insect transmission leading to changes in the viral population. GLD, focusing primarily on GLRaV-3, the most important virus Bar-Joseph and Mawassi (2013) focus on the finding that the species within the complex causing this disease. After introducing molecular characterization of CTV and other members of the various aspects of GLD biology and ecology, the authors report Closteroviridae has revealed that, in addition to genomic and on disease management case studies from four different coun- subgenomic RNAs, infected plants often contain one or more tries and continents (South Africa, New Zealand, California-USA, double-stranded defective RNAs (dRNAs) of various sizes, most and France), and end highlighting scientific gaps that must be of which contain diverse internal deletions flanked by the two filled for the development of knowledge-based sustainable GLD genomic termini. The roles and biological functions of dRNAs management practices. remain terra incognita, but one possibility is that these abundant Moving to the genus Crinivirus with a bipartite genome, Kiss double-stranded RNAs are used as a buffering system to protect et al. (2013) review replication and interactions with the host the large and fragile viral single-stranded RNA genomes from of the type species, Lettuce infectious yellows virus (LIYV). LIYV being targeted by the RNA silencing defense of the host. RNA1 encodes proteins involved in replication, which results Gushchin et al. (2013) report on an intriguing observation in formation of vesiculated membranous structures where most related to replication of Beet yellows virus (BYV), the type species likely this process occurs (see above). Four of the LIYV RNA2- of genus Closterovirus. Infection by eukaryotic viruses induces encoded proteins, CP (major coat protein), CPm (minor coat formation of membranous compartments, wherein replication protein), Hsp70h, and p59 are virion structural components, and occurs. Specifically, complexes from cell membranes of endo- CPm is a determinant of whitefly transmissibility. P5 is a small plasmic reticulum (ER) or mitochondrial origin appear in clos- protein encoded at the 5 end of RNA2 and its ortolog in BYV terovirus infections. Computer-assisted analysis predicts several is localized to the ER and plays a role in cell-to-cell movement. putative membrane-binding domains in the central region (CR) The other small protein, p9, is unique to members of the genus of the BYV polyprotein 1a. Transient expression in N. benthami- Crinivirus. ana of a hydrophobic segment of the CR results in reorganization A previous study using an AlexaFluor-based immunofluores- of ER into ∼1-µm mobile globules, suggesting that this segment cent localization assay has shown that retention of LIYV virions may be involved in the formation of multivesicular complexes in in the anterior foregut of its whitefly vector is required for virus BYV-infected cells. transmission. Ng (2013), by incorporating photostable fluores- Melzer et al. (2013) have used pyrosequencing to character- cent nanocrystals, such as quantum dots (QDs), has improved ize the genomes of closteroviruses infecting a single common theassayforthein vitro and in situ localization of LIYV virions. green ti plant (Cordyline fruticosa L.) in Hawaii. Besides confirm- Immunoblot analyses resulted in a virus detection limit compara- ing the presence of Cordyline virus 1 (CoV-1), sequence analysis ble to that of DAS-ELISA, and in membrane feeding experiments has unveiled three additional closteroviruses (CoV-2 to -4), which they revealed that specific virion retention in whitefly vectors based on the divergence of several viral proteins, represent four corresponded with successful transmission. distinct closterovirus species. Phylogenetic reconstructions indi- Finally, the article by Tzanetakis et al. (2013) provides a cate that CoV-2, CoV-3, and CoV-4, together with Little cherry detailed review on the epidemiology of the genus Crinivirus,

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most of whose members have been characterized in the last 20 Lee, R. F., and Keremane, M. (2013). Mild strain cross protection of tristeza: a years. Criniviruses have emerged as a major agricultural threat review of research to protect against decline on sour orange in Florida. Front. to important horticultural crops—including tomato, potato, let- Microbiol. 4:259. doi: 10.3389/fmicb.2013.00259 Maree, H. J., Almeida, R. P. P., Bester, R., Chooi, K. M., Cohen, D., Dolja, V. V., tuce, and cucumber—at the end of the twentieth century with et al. (2013). Grapevine leafroll-associated virus 3. Front. Microbiol. 4:82. doi: the establishment and naturalization of their whitefly vectors, 10.3389/fmicb.2013.00082 belonging to genera TrialeurodesandBemisia, in temperate cli- Melzer, M. J., Sugano, J. S., Uchida, J. Y., Borth, W. B., Kawate, M. K., mates around the globe. and Hu, J. S. (2013). Molecular characterization of closteroviruses infecting Cordyline fruticosa L. in Hawaii. Front. Microbiol. 4:39. doi: 10.3389/fmicb.2013. 00039 REFERENCES Ng, J. C. K. (2013). A quantum dot-immunofluorescent labeling method to inves- Almeida, R. P.P., Daane, K. M., Bell, V. A., Blaisdell, G. K., Cooper, M. L., Herrbach, tigate the interactions between a crinivirus and its whitefly vector. Front. E., et al. (2013). Ecology and management of grapevine leafroll disease. Front. Microbiol. 4:77. doi: 10.3389/fmicb.2013.00077 Microbiol. 4:94. doi: 10.3389/fmicb.2013.00094 Rubio, L., Guerri, J., and Moreno, P. (2013). Genetic variability and evolutionary Ambrós, S., Ruiz-Ruiz, S., Peña, L., and Moreno, P. (2013). A genetic system for cit- dynamics of viruses of the family Closteroviridae. Front. Microbiol. 4:151. doi: rus tristeza virus using the non-natural host Nicotiana benthamiana: an update. 10.3389/fmicb.2013.00151 Front. Microbiol. 4:165. doi: 10.3389/fmicb.2013.00165 Tzanetakis, I. E., Martin, R. R., and Wintermantel, W. M. (2013). Epidemiology of Bar-Joseph, M., and Mawassi, M. (2013). The defective RNAs of Closteroviridae. criniviruses: an emerging problem in world agriculture. Front. Microbiol. 4:119. Front. Microbiol. 4:132. doi: 10.3389/fmicb.2013.00132 doi: 10.3389/fmicb.2013.00119 Dawson,W.O.,Garnsey,S.M.,Tatineni,S.,Folimonova,S.Y.,Harper,S.J.,and Wang, J., Bozan, O., Kwon, S.-J., Dang, T., Rucker, T., Yokomi, R. K., et al. (2013). Gowda, S. (2013). Citrus tristeza virus-host interactions. Front. Microbiol. 4:88. Past and future of a century old citrus tristeza virus collection: a California doi: 10.3389/fmicb.2013.00088 citrus germplasm tale. Front. Microbiol. 4:366. doi: 10.3389/fmicb.2013. Dolja, V. V., and Koonin, E. V. (2013). The closterovirus-derived gene expression 00366 and RNA interference vectors as tools for research and plant biotechnology. Front. Microbiol. 4:83. doi: 10.3389/fmicb.2013.00083 Flores, R., Ruiz-Ruiz, S., Soler, N., Sánchez-Navarro, J., Fagoaga, C., López, C., et al. Received: 13 December 2013; accepted: 14 December 2013; published online: 27 (2013). Citrus tristeza virus p23: a unique protein mediating key virus–host December 2013. interactions. Front. Microbiol. 4:98. doi: 10.3389/fmicb.2013.00098 Citation: Flores R, Moreno P, Falk B, Martelli GP and Dawson WO (2013) e-Book on Folimonova, S. Y. (2013). Developing an understanding of cross-protection by Closteroviridae. Front. Microbiol. 4:411. doi: 10.3389/fmicb.2013.00411 citrus tristeza virus. Front. Microbiol. 4:76. doi: 10.3389/fmicb.2013.00076 This article was submitted to Virology, a section of the journal Frontiers in Gushchin, V. A., Solovyev, A. G., Erokhina, T. N., Morozov, S. Y., and Agranovsky, Microbiology. A. A. (2013). Beet yellows virus replicase and replicative compartments: parallels Copyright © 2013 Flores, Moreno, Falk, Martelli and Dawson. This is an open- with other RNA viruses. Front. Microbiol. 4:38. doi: 10.3389/fmicb.2013.00038 access article distributed under the terms of the Creative Commons Attribution License Harper, S. J. (2013). Citrus tristeza virus: evolution of complex and varied geno- (CC BY). The use, distribution or reproduction in other forums is permitted, provided typic groups. Front. Microbiol. 4:93. doi: 10.3389/fmicb.2013.00093 the original author(s) or licensor are credited and that the original publication in this Kiss, Z. A., Medina, V., and Falk, B. W. (2013). Crinivirus replication and host journal is cited, in accordance with accepted academic practice. No use, distribution or interactions. Front. Microbiol. 4:99. doi: 10.3389/fmicb.2013.00099 reproduction is permitted which does not comply with these terms.

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