Coat protein gene replacement results in whitefly transmission of an insect nontransmissible geminivirus isolate

Short deletions in nuclear targeting sequences of African cassava mosaic virus coat protein prevent geminivirus twinned particle formation

Coat proteins (CPs) of geminiviruses are multifunctional proteins. Using transient expression experiments, we have recently identified putative sequence motifs of African cassava mosaic virus (ACMV) CP involved in nuclear import (NLS) and export (NES) (Virology 286 (2001) 373). Here, we report on the effect of corresponding deletion mutants in the context of infecting viruses. Since NLS and NES may overlap with DNA binding and multimerisation domains, we have investigated their effect on viral infection, particularly, on particle formation. All deletion mutants were infectious in Nicotiana benthamiana when co-inoculated with DNA B, but poorly sap-transmissible. Some of the mutants showed reduced levels of viral single-stranded DNA (ssDNA), whereas the amount of double-stranded DNA (dsDNA) was not greatly affected. None of these CP mutants was able to produce stable virus particles. In contrast, viruses with CP fused to Flag epitopes at the N- or C-terminus (CP:Flag or Flag:CP) were readily sap-transmissible and formed amorphous nucleoprotein particles but only few geminate structures. The relevance of the identified sequences in replicating viruses with reference to nuclear import and export as well as to particle stability and DNA binding is discussed.

Exchange of three amino acids in the coat protein results in efficient whitefly transmission of a nontransmissible Abutilon mosaic virus isolate

Geminiviruses are transmitted in a circulative manner by whiteflies, leafhoppers, or treehoppers. The whitefly species Bemisia tabaci (Genn.) is the vector for members of the genus Begomovirus. The closely related bipartite Central American begomoviruses Abutilon mosaic virus (AbMV), Sida golden mosaic virus originating from Costa Rica (SiGMV-CR), and Sida golden mosaic virus originating from Honduras (SiGMV-Hoyv) were used to study transmission by their insect vector. The AbMV isolate is defective in transmission, whereas the two Sida-infecting viruses are readily transmitted by B. tabaci. These three viruses are able to form pseudorecombinant viruses by exchange of genomic components. The pseudorecombinant virus SiGMV-Hoyv A/AbMV B was transmissible, whereas the reciprocal pseudorecombinant virus AbMV A/SiGMV-Hoyv B was not transmitted, indicating that DNA B is not involved in the transmission defect. However, the uptake of the pseudorecombinant virus AbMV A/SiGMV-Hoyv B was much better than AbMV itself, indicating that DNA B or DNA B gene products enhance uptake of viral DNA. Exchange of AbMV coat protein with that of SiGMV-CR resulted in a transmissible chimeric AbMV. Mutagenesis of the AbMV coat protein showed that the exchange of two amino acids, at positions 124 and 149, was sufficient to obtain a whitefly-transmissible AbMV mutant. However, when amino acid 174 was altered in addition to amino acids 124 and 149 AbMV was readily transmitted by B. tabaci. From this we conclude that it is not a concise motif, such as the amino acid triplet, aspartate-alanine-glycine (DAG), involved in aphid transmission of potyviruses, that determines transmissibility of begomoviruses by B. tabaci. Instead it is the composition of the coat protein domain from amino acid 123 to 149, as a minimal transmission domain, with the contribution of amino acids 149 to 174 for efficient transmission.

DNA forms indicate rolling circle and recombination-dependent replication of Abutilon mosaic virus

Geminiviruses have spread worldwide and have become increasingly important in crop plants during recent decades. Recombination among geminiviruses was one major source of new variants. Geminiviruses replicate via rolling circles, confirmed here by electron microscopic visualization and two-dimensional gel analysis of Abutilon mosaic virus (AbMV) DNA. However, only a minority of DNA intermediates are consistent with this model. The majority are compatible with recombination-dependent replication (RDR). During development of naturally infected leaves, viral intermediates compatible with both models appeared simultaneously, whereas agro-infection of leaf discs with AbMV led to an early appearance of RDR forms but no RCR intermediates. Inactivation of viral genes ac2 and ac3 delayed replication, but produced the same DNA types as after wild-type infection, indicating that these genes were not essential for RDR in leaf discs. In conclusion, host factors alone or in combination with the viral AC1 protein are necessary and sufficient for the production of RDR intermediates. The consequences of an inherent geminiviral recombination activity for the use of pathogen-derived resistance traits are discussed.

The size of encapsidated single-stranded DNA determines the multiplicity of African cassava mosaic virus particles

Transgenic Nicotiana benthamiana plants harbouring a defective interfering (DI) DNA of African cassava mosaic virus (ACMV) and control plants were inoculated with ACMV. Virus particles were purified from infected plants, separated in sucrose gradients and fractions were analysed by Southern blotting. Transgenic plant-derived virus particles taken from the top fractions of sucrose gradients contained DI DNA, middle fractions contained a mixture of genomic and DI DNA and bottom fractions contained a mixture of multimeric, genomic and DI DNA. Virus particles from selected top, middle and bottom fractions were analysed by electron microscopy. In fractions containing only DI DNA, isometric particles of 18-20 nm were detected. In fractions containing DI DNA as well as genomic size DNA, isometric and geminate particles were found. Fractions containing multimeric size DNA were found to comprise particles consisting of three subunits adjacent to geminate particles. From these data, it is concluded that the size of encapsidated DNA determines the multiplicity of ACMV particles.

Rate of Tomato yellow leaf curl virus Translocation in the Circulative Transmission Pathway of its Vector, the Whitefly Bemisia tabaci

ABSTRACT Whiteflies (Bemisia tabaci, biotype B) were able to transmit Tomato yellow leaf curl virus (TYLCV) 8 h after they were caged with infected tomato plants. The spread of TYLCV during this latent period was followed in organs thought to be involved in the translocation of the virus in B. tabaci. After increasing acquisition access periods (AAPs) on infected tomato plants, the stylets, the head, the midgut, a hemolymph sample, and the salivary glands dissected from individual insects were subjected to polymerase chain reaction (PCR) without any treatment; the presence of TYLCV was assessed with virus-specific primers. TYLCV DNA was first detected in the head of B. tabaci after a 10-min AAP. The virus was present in the midgut after 40 min and was first detected in the hemolymph after 90 min. TYLCV was found in the salivary glands 5.5 h after it was first detected in the hemolymph. Subjecting the insect organs to immunocapture-PCR showed that the virus capsid protein was in the insect organs at the same time as the virus genome, suggesting that at least some TYLCV translocates as virions. Although females are more efficient as vectors than males, TYLCV was detected in the salivary glands of males and of females after approximately the same AAP.

The GroEL protein of the whitefly Bemisia tabaci interacts with the coat protein of transmissible and nontransmissible begomoviruses in the yeast two-hybrid system

We have previously suggested that a GroEL homolog produced by the whitefly Bemisia tabaci endosymbiotic bacteria interacts in the insect hemolymph with particles of Tomato yellow leaf curl virus from Israel (TYLCV-Is), ensuring the safe circulative transmission of the virus. We have now addressed the question of whether the nontransmissibility of Abutilon mosaic virus from Israel (AbMV-Is) is related to a lack of association between GroEL and the virus coat protein (CP). Translocation analysis has shown that, whereas TYLCV-Is DNA is conspicuous in the digestive tract, hemolymph, and salivary glands of B. tabaci 8 h after acquisition feeding started, AbMV-Is DNA was detected only in the insect digestive tract, even after 96 h. To determine whether AbMV-Is particles were rapidly degraded in the hemolymph as a result of their inability to interact with GroEL, we have isolated a GroEL gene from B. tabaci and used a yeast two-hybrid assay to compare binding of the CP of TYLCV-Is and AbMV-Is to the insect GroEL. The yeast assay showed that the CPs of the two viruses are able to bind efficiently to GroEL. We therefore suggest that, although GroEL-CP interaction in the hemolymph is a necessary condition for circulative transmission, the nontransmissibility of AbMV-Is is not the result of lack of binding to GroEL in the B. tabaci hemolymph, but most likely results from an inability to cross the gut/hemolymph barrier.

Insect transmission of plant viruses: a constraint on virus variability

Genetic diversity in viruses is shaped by high rates of recombination and is constrained by host defenses and the requirements of transmission. Recent studies of insect-transmitted plant viruses demonstrate highly conserved molecular motifs in viral genomes that regulate the specificity of insect transmission. In contrast, advances in our understanding of host plant response to virus infection reveal some generalized patterns of host defense to a diversity of viruses.

A GroEL homologue from endosymbiotic bacteria of the whitefly Bemisia tabaci is implicated in the circulative transmission of tomato yellow leaf curl virus

Evidence for the involvement of a Bemisia tabaci GroEL homologue in the transmission of tomato yellow leaf curl geminivirus (TYLCV) is presented. A approximately 63-kDa protein was identified in B. tabaci whole-body extracts using an antiserum raised against aphid Buchnera GroEL. The GroEL homologue was immunolocalized to a coccoid-shaped whitefly endosymbiont. The 30 N-terminal amino acids of the whitefly GroEL homologue showed 80% homology with that from different aphid species and GroEL from Escherichia coli. Purified GroEL from B. tabaci exhibited ultrastructural similarities to that of the endosymbiont from aphids and E. coli. In vitro ligand assays showed that tomato yellow leaf curl virus (TYLCV) particles displayed a specific affinity for the B. tabaci 63-kDa GroEL homologue. Feeding whiteflies anti-Buchnera GroEL antiserum before the acquisition of virions reduced TYLCV transmission to tomato test plants by >80%. In the haemolymph of these whiteflies, TYLCV DNA was reduced to amounts below the threshold of detection by Southern blot hybridization. Active antibodies were recovered from the insect haemolymph suggesting that by complexing the GoEL homologue, the antibody disturbed interaction with TYLCV, leading to degradation of the virus. We propose that GroEL of B. tabaci protects the virus from destruction during its passage through the haemolymph.

Recognition and receptors in virus transmission by arthropods

Fundamental knowledge of the molecular mechanisms underlying virus transmission by arthropods is a prerequisite for the creation of new strategies to modulate vector competence. There have been several recent advances in identifying the viral and vector determinants involved in virus recognition, attachment and retention.

NATURAL GENOMIC AND ANTIGENIC VARIATION IN WHITEFLY-TRANSMITTED GEMINIVIRUSES (BEGOMOVIRUSES)

Begomoviruses have circular single-stranded DNA genomes, cause many diseases of dicotyledons in areas with warm climates and are transmitted by whiteflies of the Bemisia tabaci complex. Their genomic and antigenic variation represents geography-related lineages that have little relation to host range. Genomic variation resulting from mutation is amplified by acquisition of extra DNA components, pseudo-recombination and recombination, both intraspecific and interspecific. Recombination, especially interspecific recombination, seems the key mechanism for generating novel virus forms, for enhancing biological fitness of pseudo-recombinants derived from closely related species and for maintaining the flow of genetic material among different geminiviruses occurring in the same geographical region. Recent begomovirus epidemics reflect favorable conjunctions of plant, vector, and viral (e.g. emergence of a novel recombinant virus) factors. Such epidemics typically result in co-infection of plants with different begomoviruses, leading to the appearance of further variants, especially recombinants. In their patterns of variation and evolution, begomoviruses differ greatly from plant viruses with RNA genomes.

Identification of genes directly and indirectly involved in the insect transmission of African cassava mosaic geminivirus by Bemisia tabaci

The inability to transmit progeny virus resulting from the cloned components of an isolate of African cassava mosaic virus originating from Kenya (ACMV-K) has been shown to be due to defects in both genomic components. This was achieved by the production of infectious pseudorecombinants between ACMV-K and the cloned components of a whitefly-transmissible ACMV isolate originating from Nigeria (ACMV-NOg). The exchange of gene fragments between ACMV-K and ACMV-NOg has been used to demonstrate that the defects responsible for lack of transmissibility reside on the coat protein and DNA B C1 gene of ACMV-K. The significance of these finding with respect to the present understanding of the function of these gene products are discussed.

Amino acids in the capsid protein of tomato yellow leaf curl virus that are crucial for systemic infection, particle formation, and insect transmission

A functional capsid protein (CP) is essential for host plant infection and insect transmission in monopartite geminiviruses. We studied two defective genomic DNAs of tomato yellow leaf curl virus (TYLCV), Sic and SicRcv. Sic, cloned from a field-infected tomato, was not infectious, whereas SicRcv, which spontaneously originated from Sic, was infectious but not whitefly transmissible. A single amino acid change in the CP was found to be responsible for restoring infectivity. When the amino acid sequences of the CPs of Sic and SicRcv were compared with that of a closely related wild-type virus (TYLCV-Sar), differences were found in the following positions: 129 (P in Sic and SicRcv, Q in Sar), 134 (Q in Sic and Sar, H in SicRcv) and 152 (E in Sic and SicRcv, D in Sar). We constructed TYLCV-Sar variants containing the eight possible amino acid combinations in those three positions and tested them for infectivity and transmissibility. QQD, QQE, QHD, and QHE had a wild-type phenotype, whereas PHD and PHE were infectious but nontransmissible. PQD and PQE mutants were not infectious; however, they replicated and accumulated CP, but not virions, in Nicotiana benthamiana leaf discs. The Q129P replacement is a nonconservative change, which may drastically alter the secondary structure of the CP and affect its ability to form the capsid. The additional Q134H change, however, appeared to compensate for the structural modification. Sequence comparisons among whitefly-transmitted geminiviruses in terms of the CP region studied showed that combinations other than QQD are present in several cases, but never with a P129.

Genetic determinants of host-specificity in bipartite geminivirus DNA A components

Geminiviruses are small, ssDNA-containing plant viruses. Bean golden mosaic virus (BGMV) and tomato golden mosaic virus (TGMV) have bipartite genomes, the components of which are designated A and B. Although they are closely related, BGMV and TGMV nevertheless exhibit distinct host-specific phenotypes, with BGMV being well adapted to beans and TGMV being well adapted to Nicotiana benthamiana. A previous study showed that the two open reading frames (ORFs) of DNA B only partially determine the host-adapted phenotypes of BGMV and TGMV. We have now investigated the contributions of A component ORFs to host adaptation. Co-inoculated TGMV DNA A enhances the accumulation of BGMV in N. benthamiana. Using mutant and hybrid TGMV A components, the determinant of this phenotype was mapped to a region encompassing the overlapping AL2 and AL3 ORFs (AL23). BGMV- and TGMV-based hybrid A components containing the heterologous AL23 region each displayed host-specific gain-of-function phenotypes, which indicates that these sequences contribute to host adaptation in both viruses. In N. benthamiana, al2 and al3 mutants of either virus can be complemented in trans by the heterologous A component, so adaptation of the AL23 region to this host is likely mediated through a virus nonspecific, trans-acting factor. In beans, however, co-inoculated BGMV A does not affect the accumulation of TGMV, and TGMV did not complement BGMV al2 or al3 mutants. Thus host-adaptation of the AL23 region may have a different mechanistic basis in beans than it does in N. benthamiana. Although our experiments did not reveal significant host adaptation of the coat protein, which is encoded by the AR1 ORF, a virus-specific effect on viral ssDNA accumulation was observed.