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Green shoots of geminivirology
Physiological and Molecular Plant Pathology (2002) 60, 215±218 doi:10.1006/pmpp.2002.0400, available online at http://www.idealibrary.com on E D I TO R I A L
Green shoots of geminivirology B . D . H A R R I S O N * and D . J . ROB I N S O N Scottish Crop Research Institute, Invergowrie, Dundee, DD2 5DA, UK
Advances in knowledge about geminiviruses (members of the family Geminiviridae), which cause several plant diseases of great economic importance, have been increasingly rapid since the somewhat belated discovery of their characteristic geminate (twinned) particles in 1974 [2] and of their circular single-stranded genomic DNA in 1977 [12, 14]. Indeed, they have become one of the best studied families of plant viruses and been shown to possess several unique features. This special issue contains a set of short reviews that describe some of these features and discuss some of the fastest-developing aspects of geminivirology. The family Geminiviridae comprises four genera, which are separated on the basis of their molecular and biological properties. The layout of the genomes of viruses in these four genera, Mastrevirus, Begomovirus, Curtovirus and Topocuvirus, is shown in Fig. 1. of the accompanying review by Gutierrez [13]. Most mastreviruses infect plants in the family Poaceae but a few have dicotyledonous hosts. Their vectors are leafhoppers, which transmit them in a persistent circulative manner. The genus includes about a dozen species and takes its name from that of the type species, Maize streak virus (MSV). MSV causes maize streak, one of the three most economically important plant virus diseases in Africa, and ®rst noticed about a century ago [10]. Other mastreviruses occur in Asia, Australia or Paci®c islands, and one, Wheat dwarf virus, is found in Europe. Mastreviruses have a single genomic DNA molecule of about 2600 nt which, like all geminivirus genomes, is transcribed bidirectionally. The mastrevirus genome encodes four proteins, two on the DNA strand that is contained in virus particles (the viral strand) and two on the complementary strand [25] (see Fig. 1 in [13]). The viral strand open reading frames (ORFs) code for the coat protein and for a protein required for cell-to-cell movement (MP; previously also known at dierent times as V1 and V2) of the virus. The proteins encoded by the ORFs on the complementary strand are both involved in viral DNA replication, and are translated from spliced * To whom all correspondence should be addressed. E-mail: [email protected]
and unspliced versions of the same mRNA. The smaller protein, RepA, is translated from the unspliced mRNA, whereas the template for translation of the larger protein, Rep, is produced by removal of an intron that includes the stop codon of RepA. Species in the genus Begomovirus (type species: Bean golden mosaic virus-Puerto Rico) infect dicotyledons and are transmitted in the persistent circulative manner by the white¯y Bemisia tabaci, which is considered to be a species complex [7, 26]. B. tabaci occurs worldwide in tropical, subtropical and warm temperate regions and has evolved into dierent biotypes in dierent areas [26]. Begomoviruses cause several of the world's most economically important plant virus diseases, including cassava mosaic, which was recorded in Africa more than a hundred years ago [35], cotton leaf curl, tomato leaf curl and yellow leaf curl, and mungbean yellow mosaic. Together, these diseases cause yield losses valued at billions of pounds sterling per annum. More than 70 begomovirus species have been described. Begomovirus genomes are of two kinds, monopartite and bipartite. The bipartitite genomes comprise two DNA species of similar size (2.5±2.8 kb), referred to as DNA A and DNA B. The nucleotide sequences of DNA A and DNA B are quite dierent, except for a short ``common region'' of 200±400 nucleotides that is very similar or often identical in the two DNAs. The common region includes a stem-loop structure, with the loop containing the nonanucleotide TAATATTAC, which is conserved in the genomes of all four geminivirus genera. The ®nal A of the nonanucleotide is the origin of rollingcircle DNA replication [19, 30]. The single genomic DNA of the begomoviruses with monopartite genomes is homologous to DNA A of the bipartite genomes, and for convenience is often referred to as DNA A. The DNA A encodes at least four proteins: the coat protein on the viral strand, and three proteins, AC1, AC2 and AC3, from overlapping ORFs on the complementary strand (see Fig. 1 in [13]). AC1, the replication-associated protein or Rep, is required for the initiation of DNA replication [19], AC2 (the transcriptional activator protein or TrAP) activates transcription of the viral sense genes in both DNA A and DNA B [32, 33], and
c 2002 Elsevier Science Ltd. All rights reserved. 0885-5765/02/$ - see front matter *
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Editorial TAATATT AC
TAGTATT AC
C1 DNA β
DNA 1
Rep
F I G . 1. Genetic organization of ancillary DNA molecules associated with some begomoviruses. Left: DNA b associated with Ageratum yellow vein virus (1347 nt); right: DNA 1 associated with Cotton leaf curl virus (1376 nt). Arrows indicate ORFs encoding Rep, a replication-associated protein, and C1, a protein of unknown function. The highly conserved TAA/GTATTqAC sequences are also indicated. The downward arrow (q) indicates the initiation site for rolling-circle DNA replication.
AC3 (REn) is an enhancer of DNA replication [34]. Two other ORFs occur in the DNA A of some but not all begomoviruses. AV2, which occurs in begomoviruses from the Old World but not in those from the New World, overlaps the coat protein gene on the viral strand and encodes the precoat protein, which may have a role in virus movement [24]. AC4, contained within the AC1 ORF on the complementary strand of many begomoviruses, seems to have a role in symptom development and/or virus movement in some instances [17, 27] but not in others [8, 9]. DNA B, when present, encodes two proteins: BV1, the nuclear shuttle protein (NSP), which controls the transport of viral DNA between the nucleus and cytoplasm, on the viral strand, and BC1, which mediates virus cell-to-cell movement, on the complementary strand [23]. The genus Curtovirus (type species: Beet curly top virus; BCTV) includes only a few species; these occur in North America and, in one instance, also in Turkey. BCTV causes economically important diseases in sugar beet and several other crops. Beet curly top disease was ®rst described in the late 19th century, and both the disease and BCTV were the subject of many seminal papers on virus/plant/vector interactions in the ®rst half of the 20th century [1]. Curtoviruses infect dicotyledons and are transmitted in the persistent circulative manner by leafhoppers, notably the beet leafhopper, Eutettix tenellus. The curtovirus genome consists of a single, circular ssDNA molecule of around 3 kb. The viral strand encodes three proteins in overlapping ORFs: the coat protein, a cell-to-cell movement protein, and V2, which seems to control the balance between ssDNA and dsDNA synthesis. The four complementary strand ORFs are arranged in a very similar way to those in begomovirus DNA A (see Fig. 1 in [13]). Rep and REn encode proteins that have similar functions to their begomovirus counterparts [16]. C2, the positional analogue of the begomovirus TrAP protein, is of unknown function [16], whereas C4, equivalent to begomovirus AC4, can initiate cell division and aects symptom development [18].
Tomato pseudo-curly top virus is the sole species in the fourth geminivirus genus, Topocuvirus. It is transmitted by the treehopper, Micrutalis malleifera, and occurs in the southern United States. Its genome design resembles that of BCTV but contains only two genes in the viral strand [4] (see Fig. 1 in [13]), which encode the coat protein and a movement protein. For a plant virus to survive in nature, it must be able to replicate its genome, to assemble genomic nucleic acid into nucleoprotein particles in infected cells, to spread within a plant and to be transmitted from plant to plant. Recent work on the requirements for geminivirus DNA replication has clari®ed the nature of the essential interplay between virus-encoded proteins, and their interactions with structures in the large (or only) viral intergenic region. This topic, and the ways in which viral gene products interact with host proteins to subvert the DNA-synthesizing machinery of the plant, are covered by Gutierrez [13]. His review brings out the emerging dierences between mastreviruses and begomoviruses in these events. Geminivirus transport within the plant is discussed in the review by Gafni and Epel [11], who examine virus intercellular and intracellular movement against the background of the transport processes that take place in uninfected cells. Modern cell biological techniques, such as visualization of ¯uorescently labelled proteins and nucleic acids in living cells by confocal laserscanning microscopy, have played key parts in several recent discoveries, which again point to dierences between mastreviruses and begomoviruses. Two other reviews focus on speci®c genera of geminiviruses. Boulton [3] discusses mastrevirus gene products, their interactions, and how the viral functions are integrated to produce a viable reproductive strategy for these viruses. As information has increased, so it has become increasingly evident how complex and subtly regulated is the system that has evolved. The second review of this kind, on begomovirus coat protein [15], takes a very dierent approach by examining the properties of a single viral protein that turns out to
Editorial have a multiplicity of functions. The resulting insights lead to an appreciation of the sophistication of the design requirements of this protein. Moreover, variation in its structure gives strong clues to the past course of begomovirus evolution and provides a serological basis for begomovirus detection and identi®cation. An important aspect of geminivirology, which has expanded recently but is referred to only in passing in the above four reviews, is the existence and properties of a variety of circular single-stranded DNA molecules about half the size of those of geminivirus genomic DNA. These small molecules are of three main kinds. One kind consists of defective genome segments that have one or more deletions, sometimes together with inversions, rearrangements, duplications and/or insertion of alien sequences [20, 31]. The defective molecules found in cultures of the bipartite-genome begomovirus African cassava mosaic virus are derived from DNA B by deletion of about half the molecule, including the whole of the BV1 gene and the downstream part of the BC1 gene. They interfere with replication of genomic ssDNA, and are comparable with the DI RNAs associated with some animal-infecting RNA viruses [31]. A variety of dierent defective molecules is associated with the monopartite-genome begomovirus Cotton leaf curl virus (CLCuV), although one form seemed to dominate in each infected plant [20]. One example contained a fragment derived from DNA b (see below) [6]. The defective molecules may be produced in larger amounts than genomic DNA. They all replicate only in association with genomic DNA, and they all contain sequences from its common (or intergenic) region. The second kind of molecule enables monopartite-genome begomoviruses to produce disease symptoms that are characteristic of ®eld-infected plants, such as yellow vein in Ageratum [29] and leaf curl and enations in cotton [6]. These molecules, known as DNA b, again depend on genomic DNA for replication, and contain the stem-loop and nonanucleotide sequence characteristic of all geminiviral DNAs. They seem to act like satellite nucleic acid. A DNA b associated with CLCuV appears to be a suppressor of post-transcriptional gene silencing that enables the virus to attain greater concentrations in tissues [21]. One ORF is conserved among the examples of DNA b that have been completely sequenced (Fig. 1), and mutation of this ORF inactivates the symptomaecting property of the DNA b of CLCuV, although the DNA can still be replicated [5]. The third kind of small molecule, which is found in cultures of Ageratum yellow vein virus [28] and CLCuV [22], is called DNA 1, and contains a stem-loop structure with the nonanucleotide TAGTATTAC in the loop. This is a characteristic of nanovirus DNAs, and indeed DNA 1 encodes a protein that resembles a replication-associated protein of nanoviruses (Fig. 1); it has no known function other than in self-replication of DNA 1 [28]. All three kinds of small
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molecule can be packaged in viral coat protein and are transmitted by vector insects in association with the virus in question. The apparently obligatory association of a nanoviruslike component with a virus in a dierent family, a novel phenomenon, has encouraged geminivirologists to take a closer interest in nanoviruses, which belong to a recently recognized genus of plant viruses (Nanovirus: this has anities to animal viruses in the family Circoviridae). Nanoviruses occur in warm temperate or tropical regions, they infect monocotyledons or dicotyledons, and they are transmitted in the persistent manner by aphids. Their genomes consist of six or more molecules of circular ssDNA, each of about 1 kb and typically encoding a single protein. Some of the proteins undoubtedly have functions equivalent to those of geminiviruses. This suggests that perhaps nanoviruses are closely allied to geminiviruses but have a separate DNA component for each open reading frame. This group of reviews deals with dierent interrelated and rapidly developing topics. A recurring feature is the multiplicity of interactions among viral proteins and nucleic acids and, increasingly, between viral and host components. Further technological advances can be expected to lead, within a few years, to a much deeper understanding of the nature and importance of these interactions and, perhaps, to the development of practicable methods of preventing them. So, impressive though they are, the achievements reviewed here probably just represent the green shoots in what should prove an increasingly productive ®eld of research. The Scottish Crop Research Institute is grant-aided by the Scottish Executive Environment and Rural Aairs Department. REFERENCES 1. Bennett CW. 1971. The Curly Top Disease of Sugarbeet and Other Plants. Monograph 7. St Paul, Minnesota: American Phytopathological Society, 81 p. 2. Bock KR, Guthrie EJ, Woods RD. 1974. Puri®cation of maize streak virus and its relationship to streak diseases of sugar cane and Panicum maximum. Annals of Applied Biology 77: 289±296. 3. Boulton M. 2002. Functions and interactions of mastrevirus gene products. Physiological and Molecular Plant Pathology 60: 243±255. 4. Briddon RW, Bedford ID, Tsai JH, Markham PG. 1996. Analysis of the nucleotide sequence of the treehoppertransmitted geminivirus, tomato pseudo-curly top virus, suggests a recombinant origin. Virology 219: 387±394. 5. Briddon RW, Bull SE, Bedford ID, Mansoor S, Zafar Y, Malik KA, Markham PG. 2001. Analysis of DNA b; an essential component of cotton leaf curl disease. Abstracts of the 3rd International Geminivirus Symposium, Norwich, UK, 24±28 July 2001, p14.
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6. Briddon RW, Mansoor S, Bedford ID, Pinner MS, Saunders K, Stanley J, Zafar Y, Malik KA, Markham PG. 2001. Identi®cation of DNA components required for induction of cotton leaf curl disease. Virology 285: 234±243. 7. Brown JK, Frohlich DR, Rosell RC. 1995. The sweetpotato or silverleaf white¯ies: biotypes of Bemisia tabaci or a species complex?. Annual Review of Entomology 40: 511±534. 8. Elmer JS, Brand L, Sunter G, Gardiner WE, Bisaro DM, Rogers SG. 1988. Genetic analysis of the tomato golden mosaic virus. II. The product of the AL1 coding sequence is required for replication. Nucleic Acids Research 16: 7043±7060. 9. Etessami P, Saunders K, Watts J, Stanley J. 1991. Mutational analysis of complementary-sense genes of African cassava mosaic virus DNA A. Journal of General Virology 72: 1005±1012. 10. Fuller C. 1901. Mealie variegation. 1st Report of the Government Entomologist, Natal, 1899±1900, pp 17±19. 11. Gafni Y, Epel BL. 2002. The role of host and viral proteins in intra- and intercellular tracking of geminiviruses. Physiological and Molecular Plant Pathology 60: 231±241. 12. Goodman RM. 1977. Single-stranded DNA genome in a white¯y-transmitted plant virus. Virology 83: 171±179. 13. Gutierrez C. 2002. Strategies of geminivirus DNA replication and cell cycle interference. Physiological and Molecular Plant Pathology 60: 219±230. 14. Harrison BD, Barker H, Bock KR, Guthrie EJ Meredith G, Atkinson M. 1977. Plant viruses with circular single-stranded DNA. Nature 270: 760±762. 15. Harrison BD, Swanson MM, Fargette D. 2002. Begomovirus coat protein: serology, variation and functions. Physiological and Molecular Plant Pathology 60: 257±271. 16. Hormuzdi SG, Bisaro DM. 1995. Genetic analysis of beet curly top virus: examination of the roles of L2 and L3 genes in viral pathogenesis. Virology 206: 1044±1054. 17. Jupin I, de Kouchkovsky F, Jouanneau F, Gronenborn B. 1994. Movement of tomato yellow leaf curl geminivirus (TYLCV): involvement of the protein encoded by ORF C4. Virology 204: 82±90. 18. Latham JR, Saunders K, Pinner MS, Stanley J. 1997. Induction of plant cell division by beet curly top virus gene C4. The Plant Journal 11: 1273±1283. 19. Laufs J, Jupin I, David C, Schumacher S, HeyraudNitschke F, Gronenborn B. 1995. Geminivirus replication: genetic and biochemical characterization of Rep protein function, a review. Biochimie 77: 765±773. 20. Liu Y, Robinson DJ, Harrison BD. 1998. Defective forms of cotton leaf curl virus DNA-A that have dierent combinations of sequence deletion, duplication, inversion and rearrangement. Journal of General Virology 79: 1501±1508. 21. Mansoor S, Briddon RW, Vionnett O, Markham PG, Baulcombe D. 2001. Understanding the role of DNA b in cotton leaf curl disease in Pakistan. Abstracts of the 3rd
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International Geminivirus Symposium, Norwich, UK, 24±28 July 2001, p58. Mansoor S, Khan SH, Bashir A, Saeed M, Zafar Y, Malik KA, Briddon R, Stanley J, Markham PG. 1999. Identi®cation of a novel circular single-stranded DNA associated with cotton leaf curl disease in Pakistan. Virology 259: 190±199. Noueiry AO, Lucas WJ, Gilbertson RL. 1994. Two proteins of a plant DNA virus coordinate nuclear and plasmodesmatal transport. Cell 76: 925±932. Padidam M, Beachy RN, Fauquet CM. 1996. The role of AV2 (``precoat'') and coat protein in viral replication and movement in tomato leaf curl geminivirus. Virology 224: 390±404. Palmer KE, Rybicki EP. 1998. The molecular biology of mastreviruses. Advances in Virus Research 50: 183±234. Perrin TR. 2001. The Bemisia tabaci species complex. Crop Protection 20: 725±737. Rigden JE, Krake LR, Rezaian MA, Dry IB. 1994. ORF C4 of tomato leaf curl geminivirus is a determinant of symptom severity. Virology 204: 847±850. Saunders K, Stanley J. 1999. A nanovirus-like DNA component associated with yellow vein disease of Ageratum conyzoides: evidence for interfamilial recombination between plant DNA viruses. Virology 264: 142±152. Saunders K, Bedford ID, Briddon RW, Markham PG, Wong SM, Stanley J. 2000. A unique virus complex causes Ageratum yellow vein disease. Proceedings of the National Academy of Sciences of the United States of America 97: 6890±6895. Stanley J. 1995. Analysis of African cassava mosaic virus recombinants suggests strand nicking occurs within the conserved nonanucleotide motif during the initiation of rolling circle DNA replication. Virology 206: 707±712. Stanley J, Townsend R. 1985. Characterisation of DNA forms associated with cassava latent virus infection. Nucleic Acids Research 13: 2189±2206. Sunter G, Bisaro DM. 1991. Transactivation in a geminivirus: AL2 gene product is needed for coat protein expression. Virology 180: 416±419. Sunter G, Bisaro DM. 1992. Transactivation of geminivirus AR1 and BR1 gene expression by the viral AL2 gene product occurs at the level of transcription. Plant Cell 4: 1321±1331. Sunter G, Hartitz MD, Hormuzdi SG, Brough CL, Bisaro DM. 1990. Genetic analysis of tomato golden mosaic virus: ORF AL2 is required for coat protein accumulation while ORF AL3 is necessary for ecient DNA replication. Virology 179: 69±77. Warburg O. 1894. Die Kulturp¯anzen Usambaras. Mitteilungen von Forschungsreisenden und Gelehrten aus den deutschen Schutzgebieten 7: 131±199.
Green shoots of geminivirology B . D . H A R R I S O N * and D . J . ROB I N S O N Scottish Crop Research Institute, Invergowrie, Dundee, DD2 5DA, UK
Advances in knowledge about geminiviruses (members of the family Geminiviridae), which cause several plant diseases of great economic importance, have been increasingly rapid since the somewhat belated discovery of their characteristic geminate (twinned) particles in 1974 [2] and of their circular single-stranded genomic DNA in 1977 [12, 14]. Indeed, they have become one of the best studied families of plant viruses and been shown to possess several unique features. This special issue contains a set of short reviews that describe some of these features and discuss some of the fastest-developing aspects of geminivirology. The family Geminiviridae comprises four genera, which are separated on the basis of their molecular and biological properties. The layout of the genomes of viruses in these four genera, Mastrevirus, Begomovirus, Curtovirus and Topocuvirus, is shown in Fig. 1. of the accompanying review by Gutierrez [13]. Most mastreviruses infect plants in the family Poaceae but a few have dicotyledonous hosts. Their vectors are leafhoppers, which transmit them in a persistent circulative manner. The genus includes about a dozen species and takes its name from that of the type species, Maize streak virus (MSV). MSV causes maize streak, one of the three most economically important plant virus diseases in Africa, and ®rst noticed about a century ago [10]. Other mastreviruses occur in Asia, Australia or Paci®c islands, and one, Wheat dwarf virus, is found in Europe. Mastreviruses have a single genomic DNA molecule of about 2600 nt which, like all geminivirus genomes, is transcribed bidirectionally. The mastrevirus genome encodes four proteins, two on the DNA strand that is contained in virus particles (the viral strand) and two on the complementary strand [25] (see Fig. 1 in [13]). The viral strand open reading frames (ORFs) code for the coat protein and for a protein required for cell-to-cell movement (MP; previously also known at dierent times as V1 and V2) of the virus. The proteins encoded by the ORFs on the complementary strand are both involved in viral DNA replication, and are translated from spliced * To whom all correspondence should be addressed. E-mail: [email protected]
and unspliced versions of the same mRNA. The smaller protein, RepA, is translated from the unspliced mRNA, whereas the template for translation of the larger protein, Rep, is produced by removal of an intron that includes the stop codon of RepA. Species in the genus Begomovirus (type species: Bean golden mosaic virus-Puerto Rico) infect dicotyledons and are transmitted in the persistent circulative manner by the white¯y Bemisia tabaci, which is considered to be a species complex [7, 26]. B. tabaci occurs worldwide in tropical, subtropical and warm temperate regions and has evolved into dierent biotypes in dierent areas [26]. Begomoviruses cause several of the world's most economically important plant virus diseases, including cassava mosaic, which was recorded in Africa more than a hundred years ago [35], cotton leaf curl, tomato leaf curl and yellow leaf curl, and mungbean yellow mosaic. Together, these diseases cause yield losses valued at billions of pounds sterling per annum. More than 70 begomovirus species have been described. Begomovirus genomes are of two kinds, monopartite and bipartite. The bipartitite genomes comprise two DNA species of similar size (2.5±2.8 kb), referred to as DNA A and DNA B. The nucleotide sequences of DNA A and DNA B are quite dierent, except for a short ``common region'' of 200±400 nucleotides that is very similar or often identical in the two DNAs. The common region includes a stem-loop structure, with the loop containing the nonanucleotide TAATATTAC, which is conserved in the genomes of all four geminivirus genera. The ®nal A of the nonanucleotide is the origin of rollingcircle DNA replication [19, 30]. The single genomic DNA of the begomoviruses with monopartite genomes is homologous to DNA A of the bipartite genomes, and for convenience is often referred to as DNA A. The DNA A encodes at least four proteins: the coat protein on the viral strand, and three proteins, AC1, AC2 and AC3, from overlapping ORFs on the complementary strand (see Fig. 1 in [13]). AC1, the replication-associated protein or Rep, is required for the initiation of DNA replication [19], AC2 (the transcriptional activator protein or TrAP) activates transcription of the viral sense genes in both DNA A and DNA B [32, 33], and
c 2002 Elsevier Science Ltd. All rights reserved. 0885-5765/02/$ - see front matter *
216
Editorial TAATATT AC
TAGTATT AC
C1 DNA β
DNA 1
Rep
F I G . 1. Genetic organization of ancillary DNA molecules associated with some begomoviruses. Left: DNA b associated with Ageratum yellow vein virus (1347 nt); right: DNA 1 associated with Cotton leaf curl virus (1376 nt). Arrows indicate ORFs encoding Rep, a replication-associated protein, and C1, a protein of unknown function. The highly conserved TAA/GTATTqAC sequences are also indicated. The downward arrow (q) indicates the initiation site for rolling-circle DNA replication.
AC3 (REn) is an enhancer of DNA replication [34]. Two other ORFs occur in the DNA A of some but not all begomoviruses. AV2, which occurs in begomoviruses from the Old World but not in those from the New World, overlaps the coat protein gene on the viral strand and encodes the precoat protein, which may have a role in virus movement [24]. AC4, contained within the AC1 ORF on the complementary strand of many begomoviruses, seems to have a role in symptom development and/or virus movement in some instances [17, 27] but not in others [8, 9]. DNA B, when present, encodes two proteins: BV1, the nuclear shuttle protein (NSP), which controls the transport of viral DNA between the nucleus and cytoplasm, on the viral strand, and BC1, which mediates virus cell-to-cell movement, on the complementary strand [23]. The genus Curtovirus (type species: Beet curly top virus; BCTV) includes only a few species; these occur in North America and, in one instance, also in Turkey. BCTV causes economically important diseases in sugar beet and several other crops. Beet curly top disease was ®rst described in the late 19th century, and both the disease and BCTV were the subject of many seminal papers on virus/plant/vector interactions in the ®rst half of the 20th century [1]. Curtoviruses infect dicotyledons and are transmitted in the persistent circulative manner by leafhoppers, notably the beet leafhopper, Eutettix tenellus. The curtovirus genome consists of a single, circular ssDNA molecule of around 3 kb. The viral strand encodes three proteins in overlapping ORFs: the coat protein, a cell-to-cell movement protein, and V2, which seems to control the balance between ssDNA and dsDNA synthesis. The four complementary strand ORFs are arranged in a very similar way to those in begomovirus DNA A (see Fig. 1 in [13]). Rep and REn encode proteins that have similar functions to their begomovirus counterparts [16]. C2, the positional analogue of the begomovirus TrAP protein, is of unknown function [16], whereas C4, equivalent to begomovirus AC4, can initiate cell division and aects symptom development [18].
Tomato pseudo-curly top virus is the sole species in the fourth geminivirus genus, Topocuvirus. It is transmitted by the treehopper, Micrutalis malleifera, and occurs in the southern United States. Its genome design resembles that of BCTV but contains only two genes in the viral strand [4] (see Fig. 1 in [13]), which encode the coat protein and a movement protein. For a plant virus to survive in nature, it must be able to replicate its genome, to assemble genomic nucleic acid into nucleoprotein particles in infected cells, to spread within a plant and to be transmitted from plant to plant. Recent work on the requirements for geminivirus DNA replication has clari®ed the nature of the essential interplay between virus-encoded proteins, and their interactions with structures in the large (or only) viral intergenic region. This topic, and the ways in which viral gene products interact with host proteins to subvert the DNA-synthesizing machinery of the plant, are covered by Gutierrez [13]. His review brings out the emerging dierences between mastreviruses and begomoviruses in these events. Geminivirus transport within the plant is discussed in the review by Gafni and Epel [11], who examine virus intercellular and intracellular movement against the background of the transport processes that take place in uninfected cells. Modern cell biological techniques, such as visualization of ¯uorescently labelled proteins and nucleic acids in living cells by confocal laserscanning microscopy, have played key parts in several recent discoveries, which again point to dierences between mastreviruses and begomoviruses. Two other reviews focus on speci®c genera of geminiviruses. Boulton [3] discusses mastrevirus gene products, their interactions, and how the viral functions are integrated to produce a viable reproductive strategy for these viruses. As information has increased, so it has become increasingly evident how complex and subtly regulated is the system that has evolved. The second review of this kind, on begomovirus coat protein [15], takes a very dierent approach by examining the properties of a single viral protein that turns out to
Editorial have a multiplicity of functions. The resulting insights lead to an appreciation of the sophistication of the design requirements of this protein. Moreover, variation in its structure gives strong clues to the past course of begomovirus evolution and provides a serological basis for begomovirus detection and identi®cation. An important aspect of geminivirology, which has expanded recently but is referred to only in passing in the above four reviews, is the existence and properties of a variety of circular single-stranded DNA molecules about half the size of those of geminivirus genomic DNA. These small molecules are of three main kinds. One kind consists of defective genome segments that have one or more deletions, sometimes together with inversions, rearrangements, duplications and/or insertion of alien sequences [20, 31]. The defective molecules found in cultures of the bipartite-genome begomovirus African cassava mosaic virus are derived from DNA B by deletion of about half the molecule, including the whole of the BV1 gene and the downstream part of the BC1 gene. They interfere with replication of genomic ssDNA, and are comparable with the DI RNAs associated with some animal-infecting RNA viruses [31]. A variety of dierent defective molecules is associated with the monopartite-genome begomovirus Cotton leaf curl virus (CLCuV), although one form seemed to dominate in each infected plant [20]. One example contained a fragment derived from DNA b (see below) [6]. The defective molecules may be produced in larger amounts than genomic DNA. They all replicate only in association with genomic DNA, and they all contain sequences from its common (or intergenic) region. The second kind of molecule enables monopartite-genome begomoviruses to produce disease symptoms that are characteristic of ®eld-infected plants, such as yellow vein in Ageratum [29] and leaf curl and enations in cotton [6]. These molecules, known as DNA b, again depend on genomic DNA for replication, and contain the stem-loop and nonanucleotide sequence characteristic of all geminiviral DNAs. They seem to act like satellite nucleic acid. A DNA b associated with CLCuV appears to be a suppressor of post-transcriptional gene silencing that enables the virus to attain greater concentrations in tissues [21]. One ORF is conserved among the examples of DNA b that have been completely sequenced (Fig. 1), and mutation of this ORF inactivates the symptomaecting property of the DNA b of CLCuV, although the DNA can still be replicated [5]. The third kind of small molecule, which is found in cultures of Ageratum yellow vein virus [28] and CLCuV [22], is called DNA 1, and contains a stem-loop structure with the nonanucleotide TAGTATTAC in the loop. This is a characteristic of nanovirus DNAs, and indeed DNA 1 encodes a protein that resembles a replication-associated protein of nanoviruses (Fig. 1); it has no known function other than in self-replication of DNA 1 [28]. All three kinds of small
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molecule can be packaged in viral coat protein and are transmitted by vector insects in association with the virus in question. The apparently obligatory association of a nanoviruslike component with a virus in a dierent family, a novel phenomenon, has encouraged geminivirologists to take a closer interest in nanoviruses, which belong to a recently recognized genus of plant viruses (Nanovirus: this has anities to animal viruses in the family Circoviridae). Nanoviruses occur in warm temperate or tropical regions, they infect monocotyledons or dicotyledons, and they are transmitted in the persistent manner by aphids. Their genomes consist of six or more molecules of circular ssDNA, each of about 1 kb and typically encoding a single protein. Some of the proteins undoubtedly have functions equivalent to those of geminiviruses. This suggests that perhaps nanoviruses are closely allied to geminiviruses but have a separate DNA component for each open reading frame. This group of reviews deals with dierent interrelated and rapidly developing topics. A recurring feature is the multiplicity of interactions among viral proteins and nucleic acids and, increasingly, between viral and host components. Further technological advances can be expected to lead, within a few years, to a much deeper understanding of the nature and importance of these interactions and, perhaps, to the development of practicable methods of preventing them. So, impressive though they are, the achievements reviewed here probably just represent the green shoots in what should prove an increasingly productive ®eld of research. The Scottish Crop Research Institute is grant-aided by the Scottish Executive Environment and Rural Aairs Department. REFERENCES 1. Bennett CW. 1971. The Curly Top Disease of Sugarbeet and Other Plants. Monograph 7. St Paul, Minnesota: American Phytopathological Society, 81 p. 2. Bock KR, Guthrie EJ, Woods RD. 1974. Puri®cation of maize streak virus and its relationship to streak diseases of sugar cane and Panicum maximum. Annals of Applied Biology 77: 289±296. 3. Boulton M. 2002. Functions and interactions of mastrevirus gene products. Physiological and Molecular Plant Pathology 60: 243±255. 4. Briddon RW, Bedford ID, Tsai JH, Markham PG. 1996. Analysis of the nucleotide sequence of the treehoppertransmitted geminivirus, tomato pseudo-curly top virus, suggests a recombinant origin. Virology 219: 387±394. 5. Briddon RW, Bull SE, Bedford ID, Mansoor S, Zafar Y, Malik KA, Markham PG. 2001. Analysis of DNA b; an essential component of cotton leaf curl disease. Abstracts of the 3rd International Geminivirus Symposium, Norwich, UK, 24±28 July 2001, p14.
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