Evolution of cassava brown streak disease-associated viruses

Cassava brown streak disease (CBSD) has occurred in the Indian Ocean coastal lowlands and some areas of Malawi in East Africa for decades, and makes the storage roots of cassava unsuitable for consumption. CBSD is associated with Cassava brown streak virus (CBSV) and the recently described Ugandan cassava brown streak virus (UCBSV) [picorna-like (+)ssRNA viruses; genus Ipomovirus; family Potyviridae]. This study reports the first comprehensive analysis on how evolution is shaping the populations of CBSV and UCBSV. The complete genomes of CBSV and UCBSV (four and eight isolates, respectively) were 69.0-70.3 and 73.6-74.4% identical at the nucleotide and polyprotein amino acid sequence levels, respectively. They contained predictable sites of homologous recombination, mostly in the 3'-proximal part (NIb-HAM1h-CP-3'-UTR) of the genome, but no evidence of recombination between the two viruses was found. The CP-encoding sequences of 22 and 45 isolates of CBSV and UCBSV analysed, respectively, were mainly under purifying selection; however, several sites in the central part of CBSV CP were subjected to positive selection. HAM1h (putative nucleoside triphosphate pyrophosphatase) was the least similar protein between CBSV and UCBSV (aa identity approx. 55%). Both termini of HAM1h contained sites under positive selection in UCBSV. The data imply an on-going but somewhat different evolution of CBSV and UCBSV, which is congruent with the recent widespread outbreak of UCBSV in cassava crops in the highland areas (>1000 m above sea level) of East Africa where CBSD has not caused significant problems in the past.

Supervirulent pseudorecombination and asymmetric synergism between genomic components of two distinct species of begomovirus associated with severe tomato leaf curl disease in India

Isolates of two distinct begomovirus species, the severe strain of the species Tomato leaf curl New Delhi virus (tomato leaf curl New Delhi virus-[India:New Delhi:Severe:1992]; ToLCNDV-[IN:ND:Svr:92], bipartite) and the Varanasi strain of the species Tomato leaf curl Gujarat virus (tomato leaf curl Gujarat virus-[India:Varanasi:2001]; ToLCGV-[IN:Var:01], mono/bipartite) infect tomato (Lycopersicon esculentum) and cause severe yield losses in northern India. This study investigated the infectivity properties of genomic components of these two species. Both pseudorecombinants were infectious in Nicotiana benthamiana, Nicotiana tabacum and L. esculentum. Enhanced pathogenicity was observed when DNA-A of ToLCNDV-[IN:ND:Svr:92] was trans-complemented with ToLCGV-[IN:Var:01] DNA-B, and was consistently associated with an increase in accumulation of ToLCGV-[IN:Var:01] DNA-B. Mixed infection of ToLCNDV-[IN:ND:Svr:92] and ToLCGV-[IN:Var:01] always showed extremely severe symptoms, suggesting a synergistic interaction between these two viruses. Southern blot analysis of viral DNAs from infected plants showed a significantly higher level of accumulation of both ToLCNDV components and DNA-B of ToLCGV-[IN:Var:01] with no alteration to levels of DNA-A of ToLCGV-[IN:Var:01]. Symptom development and/or higher infectivity of the supervirulent pseudorecombinants correlated with the increased levels of DNA-B accumulation. Protoplast replication assays revealed that enhanced infectivity by the pseudorecombinant occurred at the level of replication, as DNA-A of ToLCNDV-[IN:ND:Svr:92] enhanced ToLCGV-[IN:Var:01] DNA-B replication, whose accumulation was in turn increased by ToLCGV-[IN:Var:01] DNA-A. This is the first report demonstrating a virulent pseudorecombinant between two distinct species of begomoviruses that infect tomato, and is the second report on synergism between begomoviruses. The results revealed that ToLCGV-[IN:Var:01] DNA-B is capable of associating with different DNA-A components, despite having different iteron sequences.

Expression of rice yellow mottle virus coat protein enhances virus infection in transgenic plants

The disease caused by rice yellow mottle virus (RYMV) is a major, economically important constraint to rice production in Africa. RYMV is mechanically transmitted by a variety of agents, including insect vectors. The production of resistant rice varieties would be an important advance in the control of the disease and increase rice production in Africa. We produced transgenic plants of the Oryza sativa japonica variety, TP309, to express a RYMV coat protein gene (CP) and mutants of the CP under the control of a ubiquitin promoter. Transgenic plants expressing genes that encode wild-type CP (wt.CP), deleted CP (DeltaNLS.CP), mRNA of the CP, or antisense CP sequences of the CP gene were characterised. Eighty per cent (80%) of independent transgenic lines analysed contained CP gene sequences. Transgenic plants were challenged with RYMV and produced two types of reactions. Most of the plants expressing antisense sequences of the CP and untranslatable CP mRNA exhibited a delay in virus accumulation of up to a week, and the level of virus accumulation was reduced compared with non-transgenic TP309 plants. Transgenic plants expressing RYMV wild-type CP (wt.CP) and deleted CP (DeltaNLS.CP) accumulated the highest levels of virus particles. These results suggest that antisense CP and untranslatable CP mRNA induced moderate resistance, whereas transgenic CP enhanced virus infection.

Revising the way we conceive and name viruses below the species level: A review of geminivirus taxonomy calls for new standardized isolate descriptors

Geminivirus taxonomy and nomenclature is increasing in complexity with time, and the growing number of geminivirus sequences deposited in gene banks requires regular taxonomic updates and calls for new descriptors to identify virus isolates unambiguously. Fauquet et al. [1] proposed a system to standardize the names of the viruses, and corresponding guidelines have been followed since, rendering nomenclature much easier. Recently, due to difficulties inherent in species identification, the ICTV Geminiviridae Study Group proposed new species demarcation criteria, the most important of which being an 89% identity threshold between complete DNA-A component nucleotide sequences of begomoviruses. This threshold has been utilised since with general satisfaction. In this paper, we review the status of geminivirus species demarcation and nomenclature for a total of 389 isolates. A small number of corrections have been made to comply with the adopted demarcation criteria but otherwise the classification system has remained robust and therefore we propose to continue using it. However, the large numbers of geminivirus sequences that have become available have led us to recognize the need for a better description of virus isolates. The pairwise comparison distribution below the taxonomic level of species identified two peaks, one at 90-91% identity that may correspond to "strains" and one at 96-98% identity that may correspond to "variants". Guidelines for descriptors for each of these levels are proposed to standardize nomenclature. As a consequence, we have revisited the status of some virus isolates to elevate them to "strains". An updated list of all geminivirus isolates currently available is provided.

International Committee on Taxonomy of Viruses and the 3,142 unassigned species

In 2005, ICTV (International Committee on Taxonomy of Viruses), the official body of the Virology Division of the International Union of Microbiological Societies responsible for naming and classifying viruses, will publish its latest report, the state of the art in virus nomenclature and taxonomy. The book lists more than 6,000 viruses classified in 1,950 species and in more than 391 different higher taxa. However, GenBank contains a staggering additional 3,142 "species" unaccounted for by the ICTV report. This paper reviews the reasons for such a situation and suggests what might be done in the near future to remedy this problem, particularly in light of the potential for a ten-fold increase in virus sequencing in the coming years that would generate many unclassified viruses. A number of changes could be made both at ICTV and GenBank to better handle virus taxonomy and classification in the future.

Sequence analysis and classification of apparent recombinant begomoviruses infecting tomato in the nile and mediterranean basins

ABSTRACT Numerous whitefly-transmitted viral diseases of tomato have emerged in countries around the Nile and Mediterranean Basins the last 20 years. These diseases are caused by monopartite geminiviruses (family Gemini viridae) belonging to the genus Begomovirus that probably resulted from numerous recombination events. The molecular biodiversity of these viruses was investigated to better appreciate the role and importance of recombination and to better clarify the phylogenetic relationships and classification of these viruses. The analysis partitioned the tomato-infecting begomoviruses from this region into two major clades, Tomato yellow leaf curl virus and Tomato yellow leaf curl Sardinia virus. Phylogenetic and pairwise analyses together with an evaluation for gene conversion were performed from which taxonomic classification and virus biodiversity conclusions were drawn. Six recombination hotspots and three homogeneous zones within the genome were identified among the tomatoinfecting isolates and species examined here, suggesting that the recombination events identified were not random occurrences.

Molecular biodiversity of cassava begomoviruses in Tanzania: evolution of cassava geminiviruses in Africa and evidence for East Africa being a center of diversity of cassava geminiviruses

Cassava is infected by numerous geminiviruses in Africa and India that cause devastating losses to poor farmers. We here describe the molecular diversity of seven representative cassava mosaic geminiviruses (CMGs) infecting cassava from multiple locations in Tanzania. We report for the first time the presence of two isolates in East Africa: (EACMCV-[TZ1] and EACMCV-[TZ7]) of the species East African cassava mosaic Cameroon virus, originally described in West Africa. The complete nucleotide sequence of EACMCV-[TZ1] DNA-A and DNA-B components shared a high overall sequence identity to EACMCV-[CM] components (92% and 84%). The EACMCV-[TZ1] and -[TZ7] genomic components have recombinations in the same genome regions reported in EACMCV-[CM], but they also have additional recombinations in both components. Evidence from sequence analysis suggests that the two strains have the same ancient origin and are not recent introductions. EACMCV-[TZ1] occurred widely in the southern part of the country. Four other CMG isolates were identified: two were close to the EACMV-Kenya strain (named EACMV-[KE/TZT] and EACMV-[KE/TZM] with 96% sequence identity); one isolate, TZ10, had 98% homology to EACMV-UG2Svr and was named EACMV-UG2 [TZ10]; and finally one isolate was 95% identical to EACMV-[TZ] and named EACMV-[TZ/YV]. One isolate of African cassava mosaic virus with 97% sequence identity with other isolates of ACMV was named ACMV-[TZ]. It represents the first ACMV isolate from Tanzania to be sequenced. The molecular variability of CMGs was also evaluated using partial B component nucleotide sequences of 13 EACMV isolates from Tanzania. Using the sequences of all CMGs currently available, we have shown the presence of a number of putative recombination fragments that are more prominent in all components of EACMV than in ACMV. This new knowledge about the molecular CMG diversity in East Africa, and in Tanzania in particular, has led us to hypothesize about the probable importance of this part of Africa as a source of diversity and evolutionary change both during the early stages of the relationship between CMGs and cassava and in more recent times. The existence of multiple CMG isolates with high DNA genome diversity in Tanzania and the molecular forces behind this diversity pose a threat to cassava production throughout the African continent.

Molecular criteria for genus and species discrimination within the family Potyviridae

A phylogenetic analysis of the optimised nucleotide (nt) alignment of the entire ORFs of a representative of each fully-sequenced species in the family Potyviridae provided strong support for several subgroups within the genus Potyvirus. A complete set of two-way comparisons was done between the sequences for the entire ORF and for each gene amongst all the 187 complete sequences from the family. Most species had 50-55% nt identity to other members of their genus in their ORFs but there were significant groups of more closely related species and species demarcation criteria were <76% nt identity and <82% amino acid identity. The corresponding thresholds for species demaracation using nt identity values for the individual genes ranged from 58% (P1 gene) to 74-78% (other genes) although a few comparisons between different species exceeded these values. For the entire ORF, genus demarcation criteria were <46% nt identity but this did not separate rymoviruses from potyviruses. Comparisons in the CI gene most accurately reflected those for the complete ORF and this region would therefore be the best for diagnostic and taxonomic studies if only a sub-portion of the genome is to be sequenced. Further comparisons were then made using all the 1220 complete capsid protein (CP) genes. These studies suggest that 76-77% nt identity is the optimal species demarcation criterion for the CP. The study has also helped to allocate the correct virus name to some sequences from the international databases that currently have incorrect or redundant names. The taxonomic status of the current genus Rymovirus and of three unassigned species in the family is discussed. Significant discontinuities in the distributions within and between the currently defined species suggest that the continuum of variation that is theoretically available is constrained or disrupted by molecular barriers that must have some biological significance.

Cassava mosaic geminiviruses in Africa

Cassava mosaic disease (CMD) caused by cassava mosaic geminiviruses (CMGs) (Geminiviridae:Begomovirus) is undoubtedly the most important constraint to the production of cassava in Africa at the outset of the 21st century. Although the disease was recorded for the first time in the latter part of the 19th century, for much of the intervening period it has been relatively benign in most of the areas where it occurs and has generally been considered to be of minor economic significance. Towards the end of the 20th century, however, the inherent dynamism of the causal viruses was demonstrated, as a recombinant hybrid of the two principal species was identified, initially from Uganda, and shown to be associated with an unusually severe and rapidly spreading epidemic of CMD. Subsequent spread throughout East and Central Africa, the consequent devastation of production of the cassava crop, a key staple in much of this region, and the observation of similar recombination events elsewhere, has once again demonstrated the inherent danger posed to man by the capacity of these viruses to adapt to their environment and optimally exploit their relationships with the whitefly vector, plant host and human cultivator. In this review of cassava mosaic geminiviruses in Africa, we examine each of these relationships, and highlight the ways in which the CMGs have exploited them to their own advantage.

The new plant virus family Flexiviridae and assessment of molecular criteria for species demarcation

The new plant virus family Flexiviridae is described. The family is named because its members have flexuous virions and it includes the existing genera Allexivirus, Capillovirus, Carlavirus, Foveavirus, Potexvirus, Trichovirus and Vitivirus, plus the new genus Mandarivirus together with some related viruses not assigned to any genus. The family is justified from phylogenetic analyses of the polymerase and coat protein (CP) sequences. To help to define suitable molecular criteria for demarcation of species, a complete set of pairwise comparisons was made using the nucleotide (nt) and amino acid (aa) sequences of each fully-sequenced gene from every available accession in the family. Based on the distributions and on inspection of the data, it was concluded that, as a general rule, distinct species have less than ca. 72% identical nt or 80% identical aa between their entire CP or replication protein genes.

Structural characterization of Tobacco etch virus coat protein mutants

The assembly of Tobacco etch potyvirus (TEV) coat protein (CP) and truncated mutants in Escherichia coli was studied. CP from which 28, 63 or 112 amino acids were deleted from the N-terminus polymerized into potyvirus-like particles (PVLPs). These structures were more rigid and progressively smaller in diameter than those produced by full length TEV-CP. CP from which 175 N-terminal amino acids were removed, failed to polymerize. A fragment containing amino acids 131 to 206 of TEV-CP is sufficient for PVLP assembly in E. coli. To determine the function of the highly conserved amino acids Ser152, Arg154, and Asp198 point mutants were generated. The mutant CPDelta63(Asp198Glu) exhibited different spectral properties following circular dichroism analysis showing a lower amount of alpha-helix compared to the wild type molecule. No differences were observed in spectra obtained from fluorescence spectroscopy. The point mutants bind RNA in vitro to the same degree as the wild type protein. However, while the wild type and the Arg154Gln mutant CP were each able to form PVLPs in E. coli, the Asp198Glu and the double mutant Ser152Pro/Arg154Gln mutants did not. These results suggest that the Asp198Glu mutation has an altered secondary structure which affects the capacity of the protein to polymerize but did not affect in vitro protein-RNA interactions.

Tomato leaf curl Gujarat virus, a New Begomovirus Species Causing a Severe Leaf Curl Disease of Tomato in Varanasi, India

ABSTRACT The biological and molecular properties of Tomato leaf curl Gujarat virus from Varanasi, India (ToLCGV-[Var]) were characterized. ToLCGV-[Var] could be transmitted by grafting and through whitefly transmission in a persistent manner. The full-length genome of DNA-A and DNA-B of ToLCGV-[Var] was cloned in pUC18. Sequence analysis revealed that DNA-A (AY190290) is 2,757 bp and DNA-B (AY190291) is 2,688 bp in length. ToLCGV-[Var] could infect and cause symptoms in tomato, pepper, Nicotiana benthamiana, and N. tabacum when partial tandem dimeric constructs of DNA-A and DNA-B were co-inoculated by particle bombardment. DNA-A alone also is infectious, but symptoms were milder and took longer to develop. ToLCGV-Var virus can be transmitted through sap inoculation from infected tomato plants to the above-mentioned hosts causing the same symptoms. Open reading frames (ORFs) in both DNA-A and DNA-B are organized similarly to those in other begomoviruses. DNA-A and DNA-B share a common region of 155 bp with only 60% sequence identity. DNA-B of ToLCGV-[Var] shares overall 80% identity with DNA-B of Tomato leaf curl New Delhi virus-Severe (ToLCNDV-Svr) and 75% with ToLCNDV-[Lucknow] (ToLCNDV-[Luc]). Comparison of DNA-A sequence with different begomoviruses indicates that ToLCGV-[Var] shares 84% identity with Tomato leaf curl Karnataka virus (ToLCKV) and 66% with ToLCNDV-Svr. ToLCGV-[Var] shares a maximum of 98% identity with another isolate of the same region (ToLCGV-[Mir]; AF449999) and 97% identity with one isolate from Gujarat (ToLCGV-[Vad]; AF413671). All three viruses belong to the same species that is distinct from all the other geminivirus species described so far in the genus Begomovirus of the family Geminiviridae. The name Tomato leaf curl Gujarat virus is proposed because the first sequence was taken from an isolate of Gujarat, India.

Interactions Between Geminiviruses in a Naturally Occurring Mixture: Pepper huasteco virus and Pepper golden mosaic virus

ABSTRACT Pepper huasteco virus (PHV) and Pepper golden mosaic virus (PepGMV) are found in mixtures in many horticultural crops in Mexico. This combination constitutes an interesting, naturally occurring model system to study several aspects of virus-virus interactions. Possible interactions between PHV and PepGMV were studied at four levels: symptom expression, gene expression, replication, and movement. In terms of symptom expression, the interaction was shown to be host-dependent because antagonism was observed in pepper, whereas synergism was detected in tobacco and Nicotiana benthamiana. PHV and PepGMV did not generate viable pseudorecombinant viruses; however, their replication is increased during mixed infections. An asymmetric complementation in movement was observed because PHV was able to support the systemic movement of PepGMV A whereas PepGMV did not support the systemic distribution of PHV A. Heterologous transactivation of both coat protein promoters also was detected. Several conclusions can be drawn from these experiments. First, viruses coinfecting the same plant can interact at several levels (replication, movement) and in different manners (synergism, antagonism); some interactions might be host dependent; and natural mixed infections could be a potential source of geminivirus variability by generating viable tripartite combinations that could facilitate recombination events.

A New Begomovirus Species Causing Tomato Leaf Curl Disease in Varanasi, India

In November 2001, a leaf curl disease of tomato, manifested by yellowing of leaf lamina, upward leaf curling, leaf distortion, shrinking of leaf surface, and stunted plant growth was observed in tomato-growing areas in the Varanasi and Mirzapur districts of eastern Uttar Pradesh, India, which caused yield losses up to 100%. The causal agent was infective to tomato cv. Punjab Chuhara by whiteflies and grafting. Inoculated plants developed symptoms observed in naturally infected tomatoes. Viral DNA was isolated from artificially inoculated tomato plants using 1% CTAB (2) followed by a concentration of supercoiled DNA by alkaline denaturation (1). A geminivirus was confirmed by polymerase chain reaction using DNA-A degenerate primers (3), and a 550-bp amplified product was obtained from artificially and naturally infected plants. Full-length viral genomes of DNA-A and DNA-B were cloned in plasmid pUC18 at HindIII and XbaI sites, respectively. Partial tandem dimers of the viral clones were infective to Nicotiana benthamiana and tomato cv. Organ Spring through particle bombardment. Infected N. benthamiana plants exhibited downward and upward leaf curling, big veins, leaf puckering with interveinal chlorosis, and stunting. On tomato, symptoms were the same as those seen on naturally infected plants. Cloned DNA also infected Capsicum annuum cv. California Wonder (upward leaf curling and stunting) and tobacco cv. Xanthi (leaf curling and crinkling) but failed to infect Phaseolus vulgaris, okra, cotton, and N. glutinosa. The Varanasi isolate was sap transmissible (0.1 M potassium phosphate buffer, pH 7.0) from the bombarded plants to N. benthamiana and tomato cv. Organ Spring. DNA-A alone infected N. benthamiana (upward leaf curling and big veins) and tomato cv. Organ Spring (mild leaf curl), but symptoms were delayed and milder. Full-length genome sequencing revealed DNA-A (AY190290) contained 2,757 nt and DNA-B (AY190291) contained 2,688 nt. DNA-A of the Varanasi isolate shares 98.4% identity with a DNA-A sequence (AF449999) obtained from a tomato showing leaf curl symptoms from the same region and 97.1% identity with an isolate from Gujarat (900 km from Varanasi). All three sequences represent isolates of the same species, herein called Tomato leaf curl Gujarat virus, based on the priority of submission of the DNA sequence for the Gujarat region (ToLCGV; AF 413671). All isolates noted were obtained from GenBank. However, except for the DNA-A sequence, no other information is available for these ToLCGV isolates. DNA-A of the ToLCGV-Varanasi isolate shares 66.8% identity with Tomato leaf curl New Delhi virus, severe strain (ToLCNdV-Svr) (U15015), and 84.1% with Tomato leaf curl Karnataka virus (U38239). No DNA-B has been reported for these two ToLCGV isolates, and no infectious clone proving the etiology of the disease has been constructed, except for ToLCGV-Varanasi. DNA-B of ToLCGV-Varanasi shares 79.2% homology with ToLCNdV-Svr and 84.1% with ToLCNdV-Luc (X89653). These results suggest that the isolate from Varanasi belongs to ToLCGV, a previously undescribed geminivirus species causing a devastating tomato leaf curl disease in Gujarat and Uttar Pradesh. References: (1) H. C. Birnboim and J. Doly. Nucleic Acids Res. 7:1513, 1979. (2) K. M. Srivastava et al. J. Virol. Methods 51:297, 1995. (3) S. D. Wyatt and J. K. Brown. Phytopathology 86:1288, 1996.

Pepper huasteco virus and Pepper golden mosaic virus are Geminiviruses Affecting Tomatillo (Physalis ixocarpa) Crops in Mexico

Whitefly-transmitted geminivirus diseases cause important losses in several horticultural crops in all areas in Mexico (1). Tomatillo is important in the Mexican diet since it is widely used to prepare many types of salsas and other dishes. As a result, tomatillo, also known as tomate verde (green tomato), is cultivated in 29 of 32 states in Mexico, with the main production areas located in the states of Morelos, Puebla, and Michoacán. Leaf samples of 105 tomatillo plants exhibiting yellowing, yellowing mosaic, leaf curl, bunchy top, and stunting were collected from the states of Puebla, Morelos, Estado de México, and Sinaloa. Symptomatic plants were associated with the presence of whiteflies in many fields and suggested a viral etiology. Total DNA extracted from symptomatic tomatillo plants was used as a template in a polymerase chain reaction (PCR)-based geminivirus detection procedure. MP16 and MP82 primers (2) were used to direct the amplification of a segment from the stem-loop structure in the intergenic region (IR) to a conserved region in the coat protein (CP) of begomoviruses (2). Sixty-nine percent (72/105) of the samples produced the expected PCR fragment (400 to 450 bp). Similar results were obtained with a dot-blot hybridization procedure using as a probe the component A of Pepper huasteco virus (PHV) under low stringency conditions. More than 50 PCR products were cloned and sequenced. Sequence analysis (nucleotide level for the IR; amino acid level for the CP) revealed that the tomatillo-infecting geminiviruses clustered into two main groups. The first group showed a high percent identity (average of 95.3% at the CP N terminus) to PHV, whereas the second showed a similarly high percent (average 93.8%) identity to Pepper golden mosaic virus (PepGMV, previously called Texas pepper geminivirus. Both PepGMV and PHV were found in all sampled areas. Although mixed infections (differentiated by the respective IR probes) of PHV and PepGMV were common (61%), single infections were also detected (PHV 27%; PepGMV 10%). The presence of begomoviruses in tomatillo crops has been previously reported (1); however, their identity as PHV and PepGMV was not confirmed. References: (1) I. Torres-Pacheco et al. Phytopathology 86:1186, 1996. (2) P. Umaharan et al. Phytophatology 88:1262, 1998.

Expression of the oligomerization domain of the replication-associated protein (Rep) of Tomato leaf curl New Delhi virus interferes with DNA accumulation of heterologous geminiviruses

The minimal DNA binding domain of the replication-associated protein (Rep) of Tomato leaf curl New Delhi virus was determined by electrophoretic mobility gel shift analysis and co-purification assays. DNA binding activity maps to amino acids 1-160 (Rep-(1-160)) of the Rep protein and overlaps with the protein oligomerization domain. Transient expression of Rep protein (Rep-(1-160)) was found to inhibit homologous viral DNA accumulation by 70-86% in tobacco protoplasts and in Nicotiana benthamiana plants. The results obtained showed that expression of N-terminal sequences of Rep protein could efficiently interfere with DNA binding and oligomerization activities during virus infection. Surprisingly, this protein reduced accumulation of the African cassava mosaic virus, Pepper huasteco yellow vein virus and Potato yellow mosaic virus by 22-48%. electrophoretic mobility shift assays and co-purification studies showed that Rep-(1-160) did not bind with high affinity in vitro to the corresponding common region sequences of heterologous geminiviruses. However, Rep-(1-160) formed oligomers with the Rep proteins of the other geminiviruses. These data suggest that the regulation of virus accumulation may involve binding of the Rep to target DNA sequences and to the other Rep molecules during virus replication.

Recombination, pseudorecombination and synergism of geminiviruses are determinant keys to the epidemic of severe cassava mosaic disease in Uganda

The molecular variability of cassava geminiviruses occurring in Uganda was investigated in this study. Infected cassava plants and whiteflies were collected from cassava plantings in different geographical areas of the country and PCR was used for molecular characterization of the viruses. Two complete sequences of DNA-A and -B from African cassava mosaic virus (ACMV), two DNA-A sequences from East African cassava mosaic virus (EACMV), two DNA-B sequences of EACMV and the partial DNA-A nucleotide sequence of a new virus strain isolated in Uganda, EACMV-UG3, are reported here. Analysis of naturally infected cassava plants showed various assortments of DNA-A and DNA-B of the Ugandan viruses, suggesting the occurrence of natural inter- and intraspecies pseudorecombinations and a pattern of cassava mosaic disease (CMD) more complex than previously reported. EACMV-UG2 DNA-A, which contains a recombinant fragment between ACMV and EACMV-UG1 in the coat protein gene that resembles virus from Tanzania, was widespread in the country and always associated with EACMV-UG3 DNA-B, which probably resulted from another natural recombination event. Mixed infections of ACMV-UG and EACMV-UG in cassava and whiteflies were detected in most of the regions where both viruses occurred. These mixed-infected samples always showed extremely severe CMD symptoms, suggesting a synergistic interaction between ACMV-UG and EACMV-UG2. The first demonstration is provided of infectivity of EACMV clones to cassava, proving conclusively that the pseudorecombinant EACMV-UG2 DNA-A+EACMV-UG3 DNA-B is a causal agent of CMD in Uganda.

Structure of native and expanded sobemoviruses by electron cryo-microscopy and image reconstruction

Rice yellow mottle virus (RYMV) and southern bean mosaic virus, cowpea strain (SCPMV) are members of the Sobemovirus genus of RNA-containing viruses. We used electron cryo-microscopy (cryo-EM) and icosahedral image analysis to examine the native structures of these two viruses at 25 A resolution. Both viruses have a single tightly packed capsid layer with 180 subunits assembled on a T=3 icosahedral lattice. Distinctive crown-like pentamers emanate from the 12 5-fold axes of symmetry. The exterior face of SCPMV displays deep valleys along the 2-fold axes and protrusions at the quasi-3-fold axes. While having a similar topography, the surface of RYMV is comparatively smooth. Two concentric shells of density reside beneath the capsid layer of RYMV and SCPMV, which we interpret as ordered regions of genomic RNA. In the presence of divalent cations, SCPMV particles swell and fracture, whereas the expanded form of RYMV is stable. We previously proposed that the cell-to-cell movement of RYMV in xylem involves chelation of Ca(2+) from pit membranes of infected cells, thereby stabilizing the capsid shells and allowing a pathway for spread of RYMV through destabilized membranes. In the context of this model, we propose that the expanded form of RYMV is an intermediate in the in vivo assembly of virions.

3D domain swapping modulates the stability of members of an icosahedral virus group

BACKGROUND

Rice yellow mottle virus (RYMV) is a major pathogen that dramatically reduces rice production in many African countries. RYMV belongs to the genus sobemovirus, one group of plant viruses with icosahedral capsids and single-stranded, positive-sense RNA genomes.

RESULTS

The structure of RYMV was determined and refined to 2.8 A resolution by X-ray crystallography. The capsid contains 180 copies of the coat protein subunit arranged with T = 3 icosahedral symmetry. Each subunit adopts a jelly-roll beta sandwich fold. The RYMV capsid structure is similar to those of other sobemoviruses. When compared with these viruses, however, the betaA arm of the RYMV C subunit, which is a molecular switch that regulates quasi-equivalent subunit interactions, is swapped with the 2-fold-related betaA arm to a similar, noncovalent bonding environment. This exchange of identical structural elements across a symmetry axis is categorized as 3D domain swapping and produces long-range interactions throughout the icosahedral surface lattice. Biochemical analysis supports the notion that 3D domain swapping increases the stability of RYMV.

CONCLUSIONS

The quasi-equivalent interactions between the RYMV proteins are regulated by the N-terminal ordered residues of the betaA arm, which functions as a molecular switch. Comparative analysis suggests that this molecular switch can also modulate the stability of the viral capsids.

Sequence parameters that determine specificity of binding of the replication-associated protein to its cognate site in two strains of tomato leaf curl virus-New Delhi

The DNA binding sites for the replication-associated protein (Rep) of two strains of tomato leaf curl virus from New Delhi (ToLCV-Nde) were identified using electrophoretic mobility shift assays (EMSAs). The Rep proteins of the two strains were found to exhibit sequence specificity in recognition of their cognate repeat motifs (iterons) in the origin, despite the fact that they share 91% sequence identity. Using a series of synthetic oligonucleotides as probes in EMSAs, the interaction of Rep protein with its binding site was found to be dependent on number, size, and sequence of the two iterons. Mutations in the sequence of the repeat motifs or alteration in the arrangement of the motifs compromised the ability of Rep protein to bind the DNA sequence and reduced accumulation of viral DNA in protoplasts, suggesting that binding of Rep protein to its cognate iterons is an essential step in virus replication. In addition, a difference in sequence of two base pairs in the binding site of two ToLCV-Nde strains was found to affect DNA binding by the corresponding Rep protein and replication of the virus DNA in protoplasts.

Evidence of synergism between African cassava mosaic virus and a new double-recombinant geminivirus infecting cassava in Cameroon

Stem cuttings were collected in Cameroon from cassava plants displaying cassava mosaic disease (CMD) symptoms. The nature of the viruses present was determined by using the PCR with primers specific for the coat protein (CP) genes of African cassava mosaic virus (ACMV) and East African cassava mosaic virus (EACMV). All samples were infected by ACMV and eight of the 50 samples were infected by both ACMV and an EACMV-like virus. The complete nucleotide sequences of DNA-A and -B of representative ACMV and EACMV-like viruses were determined. The DNA-A component of the EACMV-like virus contained evidence of recombination in the AC2-AC3 region and DNA-B also contained evidence of recombination in BC1. However, both components retained gene arrangements typical of bipartite begomoviruses. When Nicotiana benthamiana plants were doubly inoculated with these Cameroon isolates of ACMV and EACMV (ACMV/CM, EACMV/CM) by using sap from cassava plants or infectious clones, the symptoms were more severe than for plants inoculated with either virus alone. Southern blot analysis of viral DNAs from infected plants showed that there were significantly higher levels of accumulation of both ACMV/CM components and, to a lesser extent, of EACMV/CM components in mixed-infected plants than in singly infected plants. These results strongly suggest the occurrence of a synergistic interaction between the two viruses.

Possible emergence of new geminiviruses by frequent recombination

Although exchange of genetic information by recombination plays a role in the evolution of viruses, the extent to which it generates diversity is not clear. We analyzed genomes of geminiviruses for recombination using a new statistical procedure developed to detect gene conversions. Geminiviruses (family, Geminiviridae) are a group of plant viruses characterized by a genome of circular single-stranded DNA (approximately 2700 nucleotides in length) encapsidated in twinned quasi-isometric particles. Complete nucleotide sequences of geminiviruses were aligned, and recombination events were detected by searching pairs of viruses for sequences that are significantly more similar than expected based on random distribution of polymorphic sites. The analyses revealed that recombination is very frequent and occurs between species and within and across genera. Tests identified 420 statistically significant recombinant fragments distributed across the genome. The results suggest that recombination is a significant contributor to geminivirus evolution. The high rate of recombination may be contributing to the recent emergence of new geminivirus diseases.

Identification of replication specificity determinants in two strains of tomato leaf curl virus from New Delhi

We used two strains of tomato leaf curl virus from New Delhi to investigate specificity in replication of their cognate genomes. The strains share 94% sequence identity and are referred to as severe and mild on the basis of symptoms on tomato and tobacco. Replication assays in tobacco protoplasts and plants showed that a single amino acid change, Asn10 to Asp in the N terminus of Rep protein, determines specificity for replication of the two strains based upon its interaction with the origin of replication (ori) sequences. The change of Asp10 to Asn in Rep protein of the mild strain coupled with point mutations at the 3rd and 10th nucleotides of the 13-mer binding site altered its replication ability, resulting in increased levels of virus accumulation. Similarly, changing Asn10 to Asp in Rep protein of the severe strain impaired replication of the virus and altered its severe phenotype in plants. Site-directed mutations made in ori and Asn10 of Rep protein suggested that Asn10 recognizes the third base pair of the putative binding site sequence GGTGTCGGAGTC in the severe strain.

Molecular characterization of a plant mitochondrial chaperone GrpE

Escherichia coli DnaK (Hsp70) cooperates with DnaJ and GrpE in its essential role as a molecular chaperone. Function of mitochondrial Hsp70 (mHsp70) in protein folding and organellar import in eukaryotes is critically dependent on GrpE. We cloned two genes from tobacco (Nicotiana tabacum) BY2 cells based on peptide sequences from a purified protein. The predicted amino acid sequences of both clones resembled that of GrpE from E. coli and its homologues from eukaryotes, and a cDNA clone from Arabidopsis thaliana. One gene (Type 1) encoded a deduced protein that was identical to the purified protein while the other (Type 2) encoded a deduced protein that has 80% sequence identity to Type 1. Both tobacco and Arabidopsis thaliana GrpE homologues bound to DnaK and ATP inhibited this binding. The tobacco GrpE homologue contained a typical N-terminal mitochondrial target presequence of 64 residues and the presequence directed the green fluorescent protein to tobacco mitochondria. The tobacco GrpE homologue also associated with mHsp70 when reintroduced into BY2 protoplasts, and this association was disrupted by ATP. A three-dimensional structure for the tobacco GrpE homologue was modeled based on the X-ray structure of E. coli GrpE complexed with DnaK. The modeled structure has the same overall structure as E. coli GrpE. We propose that the tobacco GrpE homologue interacts with mHsp70 in a manner analogous to E. coli GrpE with DnaK and designate it as tobacco mitochondrial GrpE (NtmGrpE).

A phage single-stranded DNA (ssDNA) binding protein complements ssDNA accumulation of a geminivirus and interferes with viral movement

Geminiviruses are plant viruses with circular single-stranded DNA (ssDNA) genomes encapsidated in double icosahedral particles. Tomato leaf curl geminivirus (ToLCV) requires coat protein (CP) for the accumulation of ssDNA in protoplasts and in plants but not for systemic infection and symptom development in plants. In the absence of CP, infected protoplasts accumulate reduced levels of ssDNA and increased amounts of double-stranded DNA (dsDNA), compared to accumulation in the presence of wild-type virus. To determine whether the gene 5 protein (g5p), a ssDNA binding protein from Escherichia coli phage M13, could restore the accumulation of ssDNA, ToLCV that lacked the CP gene was modified to express g5p or g5p fused to the N-terminal 66 amino acids of CP (CP66:6G:g5). The modified viruses led to the accumulation of wild-type levels of ssDNA and high levels of dsDNA. The accumulation of ssDNA was apparently due to stable binding of g5p to viral ssDNA. The high levels of dsDNA accumulation during infections with the modified viruses suggested a direct role for CP in viral DNA replication. ToLCV that produced the CP66:6G:g5 protein did not spread efficiently in Nicotiana benthamiana plants, and inoculated plants developed only very mild symptoms. In infected protoplasts, the CP66:6G:g5 protein was immunolocalized to nuclei. We propose that the fusion protein interferes with the function of the BV1 movement protein and thereby prevents spread of the infection.

Rice tungro bacilliform virus open reading frame 3 encodes a single 37-kDa coat protein

Rice tungro bacilliform virus (RTBV) is a plant pararetrovirus and a member of the Caulimoviridae family and closely related to viruses in the Badnavirus genus. The coat protein of RTBV is part of the large polyprotein encoded by open reading frame 3 (ORF3). ORF3 of an RTBV isolate from Malaysia was sequenced (accession no. AF076470) and compared with published sequences for the region that encodes the coat protein or proteins. Molecular mass of virion proteins was determined by mass spectrometry (matrix-assisted laser desorption/ionization-TOF) performed on purified virus particles from three RTBV isolates from Malaysia. The N- and C-terminal amino acid sequences of the coat protein were deduced from the mass spectral analysis, leading to the conclusion that purified virions contain a single coat protein of 37 kDa. The location of the coat protein domain in ORF3 was reinforced as a result of immunodetection reactions using antibodies raised against six different segments of ORF3 using Western immunoblots after SDS-PAGE and isoelectrofocusing of proteins purified from RTBV particles. These studies demonstrate that RTBV coat protein is released from the polyprotein as a single coat protein of 37 kDa.

Distribution and diversity of geminiviruses in trinidad and tobago

ABSTRACT Seven crop and eight weed species from 12 agricultural locations in Trinidad and Tobago were assayed for the presence of whitefly-transmitted geminiviruses (WTGs) by using dot blot hybridization and polymerase chain reaction (PCR) amplification of the N-terminal coat protein sequence with degenerate primers. The amplified fragments were cloned and analyzed by restriction enzyme digestion to determine fragment length polymorphism among the cloned fragments. Representative clones were then sequenced and subjected to phylogenetic analysis to determine the sequence similarity to known WTGs. WTGs were found in every location sampled and in 10 of the 15 species investigated: Lycopersicon esculentum(tomato), Capsicum annuum (pepper), Capsicum frutescens (sweet pepper), Abelmoschus esculentus (okra), Phaseolus vulgaris (beans), Alternanthera tenella, Desmodium frutescens, Euphorbia heterophylla, Malva alceifolia, and Sida acuta. The geminiviruses infecting these plants were closely related to potato yellow mosaic virus from Venezuela (PYMV-VE) and tomato leaf curl virus from Panama (ToLCV-PA). However, in pepper, sweet pepper, okra, Alternanthera tenella, Euphorbia heterophylla, Des-modium frutescens, and in one sample of tomato, a PYMV-VE-related virus was found in mixed infections with a virus related to pepper huasteco virus. Full-length infectious DNA-A and DNA-B of a tomato-infecting geminivirus from Trinidad and Tobago were cloned and sequenced. DNA-A appears to be a recombinant derived from PYMV-VE or ToLCV-PA, and Sida golden mosaic from Honduras. The implications of these findings in the control of WTGs are discussed.

Expression and inheritance of multiple transgenes in rice plants

The ability to control integration, inheritance, and expression of multiple transgenes is a prerequisite for manipulating biosynthetic pathways and complex agronomic characteristics in plants. One hundred and twenty-five independent transgenic rice plants were regenerated after cobombarding embryogenic tissues with a mixture of 14 different pUC-based plasmids. Eighty-five percent of the R0 plants contained more than two, and 17% more than nine, of the target genes. Plants containing multiple transgenes displayed normal morphologies and 63% set viable seed. Multigene cotransformation efficiency was correlated with the ratio in which the plasmids were mixed with respect to the selectable marker. All target genes had an equal chance of integration, indicating that the nature of the coding region had no effect on the efficiency of integration. Three plant lines containing 11, 10, and 9 transgenes, respectively, were analyzed for patterns of integration and inheritance until the R3 generation. Integration of multiple transgenes occurred at either one or two genetic loci, with inheritance conforming to a 3:1 Mendelian ratio. Coexpression of four marker genes was investigated until the R2 generation.

First Report of the Presence of East African Cassava Mosaic Virus in Cameroon

Cassava mosaic disease (CMD) occurs in all cassava-growing regions of Africa, India, and Sri Lanka. Characterized by mosaic and distortion of cassava leaves and reduced plant growth, causing high yield losses, CMD is caused by geminiviruses (genus Begomovirus, family Geminiviridae) transmitted through infected cuttings or by the whitefly, Bemisia tabaci. Three such geminiviruses have been described: African cassava mosaic virus (ACMV) occurs in most of the cassava-producing zones of Africa; East African cassava mosaic virus (EACMV) in East Africa; and Indian cassava mosaic virus (ICMV) in the Indian subcontinent (1). The two components of ACMV and ICMV genomes, DNA-A and DNA-B, have been sequenced; only DNA-A of EACMV has been identified and sequenced. Variations in symptom expression and severity within the same cassava variety have been observed in Cameroon. To determine the nature of the virus species inducing such variations, 50 samples were collected from CMD-infected plants in the savannah and rainforest zones of Cameroon: 2 from the sahel/savannah plain, 13 from the western highland savannah, and 35 from the main cassava-producing belt of the southwestern rainforest. There is a high incidence of CMD in the rainforest region, with some farms completely infected, while in the savannah regions farms generally have less than 25% incidence. Variation in symptom expression was more common in the rainforest region. Samples were collected from plants with distinct symptoms and/or different extents of symptom severity, then analyzed with the polymerase chain reaction (PCR) with specific primers: JSP1, ATG TCG AAG CGA CCA GGA GAT; JSP2, TGT TTA TTA ATT GCC AAT ACT; and JSP3, CCT TTA TTA ATT TGT CAC TGC. Primer JSP1 anneals to the 5' end of the coat protein (CP) of ACMV and EACMV; primers JSP2 and JSP3 anneal to the 3' ends of ACMV and EACMV, respectively. Virus identification was based on presence of an amplified fragment of either virus. ACMV was detected in all 50 samples; EACMV was detected in 8. All samples infected with EACMV were from the southwestern rainforest of Cameroon and were more severely affected by the disease than single infected plants. Previous reports have limited occurrence of EACMV to East Africa (1). This is the first report of the occurrence of EACMV in West Africa. The CP gene of three isolates of EACMV from Cameroon (EACMV/CM) was sequenced from cloned PCR products. There was a high CP nucleotide sequence identity (>99%) with only two amino acid differences among all three EACMV isolates. In contrast, there was a rather low sequence identity (94%) with EACMV/TZ from Tanzania (2), suggesting they may belong to a previously undescribed West African strain of EACMV. This indicates the geminiviruses causing CMD in Africa are more widely distributed than previously reported. None of the Cameroon isolates showed the type of recombination of the EACMV isolate from Uganda (EACMV/ UG) (having the CP core segment the identical to the corresponding ACMV CP sequence) (2). This emphasizes the need for characterization of the viruses causing CMD in different cassava-growing regions of Africa since appropriate control strategies depend on adequate knowledge of disease etiology. References: (1) Y. G. Hong et al. J. Gen. Virol. 74:2437, 1993. (2) X. Zhou et al. J. Gen. Virol. 78:2101, 1997.

Regeneration of transgenic cassava plants (Manihot esculenta Crantz) through Agrobacterium-mediated transformation of embryogenic suspension cultures

A protocol was developed for Agrobacterium-mediated transformation of embryogenic suspension cultures of cassava. The bacterial strain ABI containing the binary vector pMON977 with the nptII gene as selectable marker and an intron-interrupted uidA gene (encoding β-glucuronidase) as visible marker was used for the experiments. Selection of transformed tissue with paromomycin resulted in the establishment of antibiotic-resistant, β-glucuronidase-expressing lines of friable embryogenic callus, from which embryos and subsequently plants were regenerated. Southern blot analysis demonstrated stable integration of the uidA gene into the cassava genome in five lines of transformed embryogenic suspension cultures and in two plant lines.

The role of AV2 ("precoat") and coat protein in viral replication and movement in tomato leaf curl geminivirus

We analyzed various mutants of tomato leaf curl virus-India to investigate the role of ORFs AV3, AV2, and coat protein (CP) in viral replication, movement, and symptom development. The results of these studies indicate that ORF AV3 does not encode a protein. Plants inoculated with infectious DNA which contained deletions in AV2 developed very mild symptoms and accumulated only low levels of both single-stranded (ss) and double-stranded (ds) viral DNA, whereas inoculated protoplasts accumulated both ss and dsDNA to wild-type levels, showing that AV2 is required for efficient viral movement. However, both plants and protoplasts inoculated with substitution, frameshift, and other similar mutations in AV2 accumulated low levels of viral DNA. The low levels of accumulation of DNA of these mutants were apparently not due to a defect in AV2 synthesis. Mutations in the CP caused a marked decrease in ssDNA accumulation in plants and protoplasts while increasing dsDNA accumulation in protoplasts. Mutations in both AV2 and CP behaved like AV2 mutants in plants and like CP mutants in protoplasts. The results demonstrated that multiple functions provided by AV2, BV1, BC1 are essential for viral movement, and that changes in A-component virion-sense mRNA structure or translation affect viral replication.

Characterization of cassava common mosaic virus and a defective RNA species

The genome of cassava common mosaic potexvirus (CsCMV) has been sequenced from cDNA clones and consists of 6376 nucleotides (nt). A 76 nt untranslated region (UTR) at the 5' terminus was followed by ORF1 which potentially encodes a protein of 1449 amino acids (aa). ORFs 2, 3, and 4 were predicted to encode proteins of 231, 112 and 97 aa, respectively. ORF5 potentially encodes a 229 aa protein of 25 kDa that is similar to the coat proteins of other potexviruses. The 3'-terminal UTR of 114 nt was followed by a poly(A) tail. The genomic organization of the CsCMV genome is similar to that of other potexviruses. A cDNA clone that was apparently obtained from a defective RNA species contained both the 5' and 3' UTRs and an ORF that potentially encodes the first 263 aa of ORF1 and the last 33 aa of the coat protein. Defective RNA species were found both in purified preparations of the virus and in total nucleic acid isolated from CsCMV-infected plants.

Selection of large quantities of embryogenic calli from indica rice seeds for production of fertile transgenic plants using the biolistic method

The microprojectile bombardment of immature embryos has proven to be effective in transforming many indica rice varieties. One of the drawbacks of using immature embryos is the requirement of a large number of high quality immature embryos, which itself is a tedious and laborious process. To circumvent these problems, we have developed a procedure, using indica variety TN1 as a model that generates highly homogenous populations of embryogenic subcultured calli by selectively propagating a small number of regeneration-proficient calli derived from seeds. Thousands of embryogenic calli were produced from 50 seeds within 10 weeks. Ten to 20 independent R0 transgenic lines were regenerated per 500 embryogenic calli bombarded. The convenience and reliability offered by this transformation system has made transformation of indica rice a routine procedure.

Mass spectrometry and viral analysis

BACKGROUND

Electrospray ionization (ESI) mass spectrometry is a powerful new approach for analyzing biomolecules and biomolecular complexes. Previous studies have provided evidence that non-covalent biomolecular complexes can be observed by ESI mass spectrometry; it is not clear, however, whether the native conformation of the biomolecules is maintained throughout the ionization and analysis process. We set out to address this question using live viruses.

RESULTS

Viral ions have been generated in the gas phase using electrospray ionization mass spectrometry. These ions have been collected, following ion filtering through the mass analyzer, and then analyzed by transmission electron microscopy. Transmission electron microscopy revealed that rice yellow mottle virus and tobacco mosaic virus retained their respective spherical and rod-like ultrastructure. The viability of the isolated tobacco mosaic virus was confirmed by inoculation and infection of tobacco plants.

CONCLUSIONS

These results demonstrate the utility of electrospray for supramolecular complexes with molecular weights of over 40 million Da and offer conclusive evidence that native biomolecular structures can be conserved through the electrospray process.