'Candidatus Liberibacter solanacearum' Is Tightly Associated with Carrot Yellows Symptoms in Israel and Transmitted by the Prevalent Psyllid Vector Bactericera trigonica

Carrot yellows disease has been associated for many years with the Gram-positive, insect-vectored bacteria, 'Candidatus Phytoplasma' and Spiroplasma citri. However, reports in the last decade also link carrot yellows symptoms with a different, Gram-negative, insect-vectored bacterium, 'Ca. Liberibacter solanacearum'. Our study shows that to date 'Ca. L. solanacearum' is tightly associated with carrot yellows symptoms across Israel. The genetic variant found in Israel is most similar to haplotype D, found around the Mediterranean Basin. We further show that the psyllid vector of 'Ca. L. solanacearum', Bactericera trigonica, is highly abundant in Israel and is an efficient vector for this pathogen. A survey conducted comparing conventional and organic carrot fields showed a marked reduction in psyllid numbers and disease incidence in the field practicing chemical control. Fluorescent in situ hybridization and scanning electron microscopy analyses further support the association of 'Ca. L. solanacearum' with disease symptoms and show that the pathogen is located in phloem sieve elements. Seed transmission experiments revealed that while approximately 30% of the tested carrot seed lots are positive for 'Ca. L. solanacearum', disease transmission was not observed. Possible scenarios that may have led to the change in association of the disease etiological agent with carrot yellows are discussed. [Formula: see text] Copyright © 2018 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Direct quantification of unadsorbed viruses in suspensions of adsorbing colloids with the enzyme-linked immunosorbent assay

Quantification of two plant viruses in suspensions of homoionic Ca-bentonite was conducted by applying to the enzyme-linked immunosorbent assay plates either the virus-clay mixture (direct method) or the supernatant obtained after the clay was allowed to settle (classic method). Both methods showed a similar dependence of free virus content on clay concentration. A higher content at equilibrium was measured for both viruses by the direct method. The advantage of using the direct over the classic method is discussed.

Diversity of citrus tristeza virus strains indicated by hybridization with cloned cDNA sequences

Cloned cDNA sequences 500-2000 base pairs long, derived from a severe citrus tristeza virus (CTV) isolate, were used to study sequence homology with RNA of nine other serologically indistinguishable CTV isolates which differed in their biological properties. Six of the nine CTV isolates hybridized positively with the tested cDNA clones, while three others hybridized differentially with these cDNA clones. The potential use of cloned viral sequences for typing of virus strains is discussed.

Molecular cloning of complementary DNA sequences of citrus tristeza virus RNA

Complementary DNA (cDNA) of citrus tristeza virus (CTV) RNA, synthesized using calf thymus DNA random primers, was converted to a double-stranded form and inserted into the PstI site of the Escherichia coli pBR322 plasmid by the G-C tailing method. Bacterial clones harbouring virus-specific sequences were detected by colony hybridization with a 32P-labelled viral RNA probe. Hybridization patterns of denatured virus RNA revealed the presence of three types of specific clones: those hybridizing with a distinct narrow band corresponding to the full-length virus RNA, those hybridizing with a broader band of virus RNA sequences, and those hybridizing with several distinct virus-related RNA bands. Similar patterns were obtained when these clones were hybridized to purified double-stranded RNA from CTV-infected plants. None of these cDNA clones hybridized with similarly treated preparations extracted from healthy plants. The origin of variation among the CTV clones is discussed.

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.

5'-coterminal subgenomic RNAs in citrus tristeza virus-infected cells

Three unusual 5' coterminal positive-stranded subgenomic (sg) RNAs, two of about 0.8 kb and one of 10 kb (designated LMT1, LMT2, and LaMT, respectively), from Citrus spp. plants and Nicotiana benthamiana protoplasts infected with Citrus tristeza virus (CTV) were characterized. The 5' termini of the LMT RNAs were mapped by runoff reverse transcription and found to correspond with the 5' terminus of the genomic RNA. The LMT 5'-coterminal sgRNAs consisted of two modal lengths of 744--746 and 842--854 nts. The 3' of the LaMT RNAs terminated near the junction of ORF 1b and ORF 2 (p33). None of the 5' sgRNAs had detectable amounts of corresponding negative-sense RNAs, as occurs with the genomic and 3' coterminal subgenomic RNAs of CTV. The abundance of the short and long 5' sgRNAs differed considerably in infected cells. The LMT RNAs were considerably more abundant than the genomic RNAs, while the larger LaMT RNA accumulated to much lower levels. The kinetics of accumulation of LMT1 and LMT2 in synchronously infected protoplasts differed. The larger RNA, LMT1, accumulated earlier with a strong hybridization signal at 2 days postinfection, a time when only traces of genomic and 3' sgRNAs were detected. The lack of corresponding RNAs, that could be 3' cleavage products corresponding to the 5' coterminal sgRNAs and the lack of complementary negative strands, suggest that these sgRNAs were produced by termination during the synthesis of the genomic positive strands.

Both point mutation and RNA recombination contribute to the sequence diversity of citrus viroid III

Field-grown citrus trees often harbor complex mixtures of 4-5 different viroid species, and the presence of citrus viroid III (CVd-III) has been shown to reduce the rate of tree growth without inducing disease. To more fully define the structure of its quasi-species, we have examined nine citrus viroid complexes for the presence of previously undescribed sequence variants of CVd-III. Analysis of 86 full-length cDNAs generated from these nine viroid complexes by RT-PCR revealed the presence of 20 new CVd-III variants. Chain lengths ranged from 293-297 nucleotides, and sequence changes were confined largely to the lower portions of the central conserved region and variable domain. The previously described variants CVd-IIIa (297 nt) and CVd-IIIb (294 nt) were clearly predominant, but phylogenetic analysis indicated that certain isolates may contain representatives of two additional fitness peaks. At least one group of CVd-III variants appears to have arisen as a result of RNA recombination. Populations recovered from diseased/declining trees were the most diverse, but even dwarfing isolates originating from old line Shamouti trees showed considerable variability.

Amplification of Citrus tristeza virus from a cDNA clone and infection of citrus trees

Isolates of the Closterovirus, Citrus tristeza virus (CTV), are populations of disparate genotypes and defective RNAs developed during long periods of vegetative propagation of citrus trees. Because it has not been possible to obtain pure cultures of the virus, it is not known what components of the population are primarily responsible for induction of diseases. We previously developed an infectious cDNA clone from which in vitro-produced RNA transcripts could infect protoplasts (Satyanarayana et al., 1999, Proc. Natl. Acad. Sci. USA 96, 7433-7438). However, neither the RNA transcripts nor virions from transcript-infected protoplasts were competent for infection of citrus trees. Using a green fluorescent protein-marked virus as inoculum, we found that the approximately 20-kb RNA from virions or transcripts of cDNA infected only a small percentage of protoplasts ( approximately 0.01%), but virions could infect more than 80% of the protoplasts. Based on this information, we amplified the virus from the cDNA clone (recombinant virus) by successive passages in protoplasts using virions in crude sap as inoculum. By the third to seventh passages in protoplasts, maximal amounts of recombinant progeny virus were produced, which were used for inoculation of small citrus trees by slashing stems in the presence of virion preparations. A relatively high percentage of plants became infected with the recombinant virus from protoplasts, resulting in the first defined pure culture of CTV in plants. The comparative biology of the pure culture of recombinant CTV with that of the parental population in planta demonstrated that the recombinant virus retained through all of the recombinant DNA manipulations the normal functions of replication, movement, and aphid transmissibility, and had a symptom phenotype indistinguishable from that of the parental population. Additionally, fulfilling Koch's postulates of the first pure culture of CTV in plants suggested that the major genotype of the CTV T36 population is the primary determinant of the symptom phenotype. We could distinguish no biological contributions resulting from the minor genotypes and defective RNAs of the parental population.

The p20 gene product of Citrus tristeza virus accumulates in the amorphous inclusion bodies

Citrus tristeza virus (CTV) has 10 3' open reading frames (ORFs) of unknown function except for the two coat proteins. The highest produced subgenomic RNAs are those of the major coat protein gene (p25) and the 3' most genes, p20 and p23. The proteins from three ORFs, p25, p27, and p20, were examined in the yeast two-hybrid assay for the interactions between themselves and to one another. The p20 protein exhibited a high affinity for itself, suggesting that it might aggregate in infected cells. The cytopathology of CTV infections includes characteristic paracrystalline and amorphous inclusions in the phloem elements of infected citrus. Polyclonal antiserum raised against the bacterial expressed p20 gene product detected a protein of approximately 22-23 kDa, which accumulated to relatively high levels in CTV-infected citrus, but not in healthy citrus. Immunogold localization using antibodies to p20 protein showed strong and specific labeling of the amorphous inclusion bodies present in CTV-infected cells. Mesophyll protoplasts of Nicotiana benthamiana transfected with a CTV mutant containing the green fluorescent protein (GFP) ORF fused in-frame to the 3' end of p20 protein ORF expressed high levels of GFP. The fusion protein was concentrated in one specific area in the cytoplasm and lacked an organized shape. Accumulation of high levels of p20 protein in infected tissue, specific localization of the p20-GFP fusion protein, immunolocalization of p20 protein into amorphous inclusions, and strong homologous p20 protein-p20 protein interactions in the yeast-two-hybrid assay suggest that the p20 protein of CTV is a major component of the amorphous inclusion bodies present in CTV-infected cells.

An engineered closterovirus RNA replicon and analysis of heterologous terminal sequences for replication

Citrus tristeza virus (CTV) populations in citrus trees are unusually complex mixtures of viral genotypes and defective RNAs developed during the long-term vegetative propagation of the virus and by additional mixing by aphid transmission. The viral replication process allows the maintenance of minor amounts of disparate genotypes and defective RNAs in these populations. CTV is a member of the Closteroviridae possessing a positive-stranded RNA genome of approximately 20 kilobases that expresses the replicase-associated genes as an approximately 400-kDa polyprotein and the remaining 10 3' genes through subgenomic mRNAs. A full-length cDNA clone of CTV was generated from which RNA transcripts capable of replication in protoplasts were derived. The large size of cDNA hampered its use as a genetic system. Deletion of 10 3' genes resulted in an efficient RNA replicon that was easy to manipulate. To investigate the origin and maintenance of the genotypes in CTV populations, we tested the CTV replicase for its acceptance of divergent sequences by creating chimeric replicons with heterologous termini and examining their ability to replicate. Exchange of the similar 3' termini resulted in efficient replication whereas substitution of the divergent (up to 58% difference in sequence) 5' termini resulted in reduced but significant replication, generally in proportion to the extent of sequence divergence.

D-RNA molecules associated with subisolates of the VT strain of citrus tristeza virus which induce different seedling-yellows reactions

Citrus tristeza virus (CTV) strains were previously catalogued as seedling-yellows (SY) and non-SY (nSY) types, according to their yellowing and stunting effects on indicator seedlings. Among subisolates of the VT strain, which were selected from chronically infected Alemow plants, there was a correlation between the presence of 2.4-, 2.7- and 4.5-kb D-RNAs, and SY and nSY reactions, respectively. Similarly, plants infected with Mor-T subisolates, which cause SY, contained D-RNAs of 2.6 to 2.8 kb, while nSY subisolates from recovered sour orange tissue contained a major D-RNA of 5.1 kb. Plants harboring the 2.7-kb D-RNA were protected against challenge inoculation with a subisolate harboring the 4.5-kb D-RNA. This study suggests that the nSY reaction results either from the absence of SY gene(s) in the genomes of certain CTV strains or through the suppression of the effects of SY gene(s) by D-RNAs with 5' parts larger than 4000nt.

Involvement of a subgenomic mRNA in the generation of a variable population of defective citrus tristeza virus molecules

The fusion sites between the termini of naturally occurring defective RNAs (D-RNAs) from three citrus tristeza virus (CTV) isolates were sequenced. Seven of eight clones showed a common 3' terminus of 940 nucleotides (nt) fused to 5' termini with different sizes. An extra cytosine nucleotide was found at the junction site of the majority of the common 3' D-RNAs. Molecular analysis of the plus and minus strands of the 0.9-kbp double-stranded RNA, corresponding to the CTV open reading frame 11 subgenomic RNA (sgRNA), showed that they were identical in length and sequence to the common 3' sequence of the D-RNAs. These results imply that viral sgRNA messengers also function as building components for genomic rearrangement and exchange of complete viral genes.

Booster immunization with a partially purified citrus tristeza virus (CTV) preparation after priming with recombinant CTV coat protein enhances the binding capacity of capture antibodies by ELISA

Groups of rabbits and young lambs were immunized subcutaneously and intramuscularly with a recombinant citrus tristeza virus (CTV) coat protein (rCTV-CP) antigen. Three weeks after primary immunization the animals were divided into two groups that were boosted either with rCTV-CP or with a partially purified preparation of CTV particles (ppCTV). Twelve and 15 days after the last injection, the animals were bled and the binding capacity of the antisera for CTV detection was examined for capture antibodies by the indirect ELISA. Considerably higher ELISA titers were obtained from animals that were boosted with ppCTV than with rCP. Boosting with partially purified native antigens after priming with recombinant antigens is expected to extend the applicability of the antisera for detecting other structural and non-structural viral antigens by trapping ELISA.

A cDNA clone from a defective RNA of citrus tristeza virus is infective in the presence of the helper virus

A naturally occurring defective RNA of 2379 nt (D2.3) from the VT strain of citrus tristeza closterovirus (CTV) was cloned and sequenced. The D2.3 RNA is a fusion of two regions of 1521 and 858 nt from the 5' and 3' ends of the CTV genome, respectively. A cDNA clone of D2.3 RNA was tagged by the insertion of a 0.47 kb chimeric DNA fragment and the recombinant cDNA was inserted downstream of the cauliflower mosaic virus 35S promoter. The resulting construct was bombarded into CTV-infected tissue, which was then grafted onto virus-free plants. The presence of recombinant RNA in systemically infected leaves was demonstrated by RT-PCR. Sequencing the RT-PCR products synthesized from double-stranded RNA confirmed the presence of the chimeric segment used for tagging. This is the first report of an infectious cDNA molecule derived from CTV D-RNA.

Unusual sequence relationships between two isolates of citrus tristeza virus

The complete, 19,226 nt sequence of the RNA genome from VT, a seedling yellows strain of citrus tristeza virus (CTV), was determined and found to have a genome organization identical with that of the previously determined CTV-T36 isolate, except that ORF 1 of CTV-VT was 70 nt shorter due to two widely separated 18 nt deletions. Sequence comparison of CTV-VT and CTV-T36 revealed approximately 89% identity throughout the ten 3' ORFs, but only 60-70% identity throughout ORF 1. The 5' nontranslated regions were only 60% identical whereas the 3' nontranslated regions were 97% identical. The transition between regions of similarity and deviation was gradual, suggesting that the sequence similarities and differences compared to CTV-T36 were unlikely to have arisen from a recent recombination event between a close T36 relative and a distantly related CTV isolate. This is the first attempt to compare in detail the variation between the genomes of two strains of a member of the closterovirus group. The observed deviation between the large RNA genomes of the two CTV strains is greater than that among different viruses of most other groups, raising the question of how to define the taxonomy of these viruses.

Multiple species of defective RNAs in plants infected with citrus tristeza virus

Alemow (Citrus macrophylla) and sweet orange (C. sinensis) plants infected, respectively, with several Israeli and Florida isolates of the citrus tristeza virus (CTV) were found to contain multiple species of RNA molecules with features similar to defective-interfering RNAs. Northern blot hybridizations of dsRNAs extracted from serial passages of the Israeli VT isolate (CTV-VT) and from different plants infected with a single source of inoculum showed considerable variation both in the presence and in the relative abundance of the defective RNA (D-RNA) bands. The D-RNA molecules were found to be encapsidated in the CTV particles. Sequence analysis of two VT D-RNA molecules of 2.7 and 4.5 kb revealed that they were composed of two regions corresponding to 1818 and 4036 nucleotides from the 5' and 938 and 442 nucleotides from the 3' termini of the CTV-VT genomic RNA, respectively. A short (ca. 0.8 kb) nonencapsidated single-stranded positive-sense RNA species was also found in infected plants. This ssRNA, which copurified with dsRNAs, was shown by hybridization to encompass the 5'-terminal part of the CTV genome and might have an extensive secondary structure.

Defective RNA molecules associated with citrus tristeza virus

Preparations of single-stranded (ss) RNA extracted from particles of the Israeli VT strain of citrus tristeza virus (CTV-VT), and ss- and double-stranded (ds) RNA preparations extracted from infected Alemow (Citrus macrophylla) plants, contained a population of molecules with features that suggest that they are defective RNAs. The prototype of 2424 nt was cloned and sequenced and was found to be composed of two genomic regions corresponding to the 5' (1151 nt) and the 3' (1259 nt) termini of the genomic CTV-RNA, with two perfect direct repeats of eight nucleotides of unknown origin at the junction site. Northern hybridization analysis demonstrated that this 2.4-kb defective RNA is an abundant species among the other CTV-specific ss- and ds-RNAs in infected plants. The 2.4-kb RNA was found encapsidated by the CTV coat protein indicating that the CTV origin of assembly is located close to the 5' or 3' terminus. This is the first defective RNA to be reported for a member of the closterovirus group.

Populations of citrus tristeza virus contain smaller-than-full-length particles which encapsidate sub-genomic RNA molecules

Plants infected by citrus tristeza virus (CTV) contain, in addition to the 2000 nm full-length thread-like virions, a heterogeneous population of smaller particles. The CTV particles and RNA extracts from purified CTV preparations were fractionated by sucrose gradient centrifugation and the RNA molecules from different fractions translated in a reticulocyte translation system. Fractions containing predominantly an RNA band of approximately 3.2 kb directed the synthesis of CTV coat protein (CP), which in SDS-PAGE had an estimated molecular mass of 28 kDa. Three additional polypeptides, with estimated sizes of 21 kDa, 23 kDa and 27 kDa, were translated from a range of RNA molecules smaller than 3.2 kb. Hybridization with cDNA to the CP gene (CTV-CPG) and with a 350 base clone complementary to the 3' and 5' termini of the genes for CTV p20 and p23.5, respectively, indicated that preparations of CTV particles contain, in addition to the genomic (20 kb) RNA, two sub-genomic RNA molecules of 3.2 kb and 2.4 kb and probably also two smaller molecules of 1.6 kb and 0.9 kb. Only the 3.2 kb RNA, its corresponding dsRNA molecule and populations of larger RNAs, including the 20 kb genomic RNA, hybridized with a CTV-CPG probe, thus conflicting with our previous assignment of the CTV-CPG to the 0.8 kbp dsRNA. Based on these results we propose that distinct populations of CTV particles encapsidate smaller RNAs which were formed as a nested set of subgenomic RNAs. Sequence analysis of 2540 nucleotides downstream to CTV-CPG of strain VT revealed four open reading frames (ORFs) potentially encoding in the 5' to 3' direction, 18 kDa (p18), 13 kDa (p13), 20 kDa (p20) and 23.5 kDa (p23.5) proteins. The CTV-VT ORFs showed variable but usually close levels of homology with the corresponding ORFs of CTV-T36 from Florida.

The persistence of engineered Agrobacterium tumefaciens in agroinfected plants

Several plant species, including tomato (Lycopersicon esculentum), Gynura aurantiaca, avocado (Persea americana), and grapefruit (Citrus paradisi) grafted on Troyer citrange (Poncirus trifoliata x C. sinensis) were "agro-infected" with Agrobacterium tumefaciens strain LBA-4404, carrying a mini-Ti plasmid with a dimeric cDNA of citrus exocortis viroid (CEVd). Extracts prepared from tissues of the agroinfected plants 38-90 days after inoculation were plated on selective media and found to contain large amounts of the engineered bacteria. These observations suggest the need for more stringent quarantine measures when handling A. tumefaciens cells harboring constructs for "agroinoculation" with plant viruses or viroids.

Nucleotide sequence of the coat protein gene of citrus tristeza virus: comparison of biologically diverse isolates collected in Israel

The sequences of the coat protein genes of four seedling yellows (SY) and four non-SY (NSY) of citrus tristeza virus (CTV) isolates, which were collected in Israel over a period of 30 years, were analyzed. Pairwise comparisons showed extensive similarities in the nucleotide and amino acid sequences of six isolates designated the VT group. This group consists of three NSY isolates that cause a very mild CTV reaction on the sensitive combination of sweet orange (SwO) grafted on sour orange (SO), and three SY isolates that cause severe SY and SwO/SO reactions. MT, a CTV isolate that is consistently nontransmitted by Aphis gossypii, was found to be different in two amino acids (Val 103 and Glu 113) from each of the A. gossypii transmissible CTV isolates. Sequencing of the cDNA clones obtained from ST, a variably transmitted CTV isolate, showed extensive sequence variation among the tested clones. The sequence information indicates that the current CTV epidemics in Israel are caused by at least two CTV subspecies (VT and HT) displaying extensive differences in their coat protein genes.

Rapid isolation of double stranded RNA segments from disulphide crosslinked polyacrylamide gels

A simple method was developed to isolate dsRNA segments from disulphide crosslinked polyacrylamide gels. The dsRNA preparations from the P4 killer virus strain 77 of Ustilago maydis containing 7 genomic segments with molecular sizes ranging from 0.36 to 6.7 kbp, the 20 kbp dsRNA associated with the '447' cytoplasmic male sterility in Vicia faba and the 23 kbp genomic dsRNA of citrus tristeza virus (CTV) were separated on disulphide crosslinked polyacrylamide gels. After UV visualisation, the dsRNA bands were excised from the gel and dissolved using 2-mercaptoethanol. The dsRNA were then purified from the solubilized fractions by specific adsorption on microgranular cellulose and elution with a small volume of water. The method is rapid, simple and convenient for the isolation of all the tested dsRNAs segments.

Enzyme-linked immunosorbent assay on nitrocellulose membranes (dot–ELISA) in the serodiagnosis of plant pathogenic bacteria

The usefulness of enzyme-linked immunosorbent assay on nitrocellulose membranes (dot–ELISA) for diagnosis and identification of plant pathogenic bacteria was tested. Five pathovars of Xanthomonas campestris and two antisera, one produced against pv. vesicatoria and the other against pv. translucens, were used in a model system. A 10-min incubation of the bacterial cells, dot blotted on membranes, in diluted sera, followed by either alkaline phosphatase conjugated protein A or goat antirabbit globulin, resulted in a specific reaction between the homologous serum and bacteria. Populations of 1000–2000 cfu per spot (ca. 0.3 cm2) could be detected with these reagents. The streptavidin–biotinylated peroxidase complex produced a definitive reaction with as few as 800 cfu, but cross-reactions became evident at the higher cell concentrations among all five pathovars in tests with both antisera. Cell-free extracts, obtained by centrifugation of boiled bacteria, reacted similarly to live cells. Unrelated bacteria did not react with either antiserum. Extracts of lesions from tomato and pepper leaves infected with X. campestris pv. vesicatoria reacted positively with the antiserum produced against this pathovar but not that produced with pv. translucens. Samples of supernatants from boiled lesions reacted with similar intensity as those from homogenized tissues.

Detection of avocado sunblotch viroid by hybridization with synthetic oligonucleotide probes

Two short (20 and 17 nucleotides) DNA hybridization probes, complementary to avocado sunblotch viroid (ASBV) RNA nucleotides 68-87 and 88-104 respectively (Symons, R.H., Nucleic Acid Res. 9, 6527, 1981) were synthesized. The sensitivity and specificity of these radioactively labelled probes for hybridization with RNA of several ASBV isolates are demonstrated.

Detection of avocado sunblotch viroid (ASBV) by dot-spot self-hybridization with a [(32)P]-labelled ASBV-RNA

RNA was extracted from plants infected with avocado sunblotch viroid (ASBV) and was analyzed by electrophoresis in polyacrylamide gel. The ASBV related fraction was eluted from the gel, labelled with [(32)P] using polynucleotide kinase and used as a probe for hybridization with a purified ASBV-RNA preparation dot spotted on nitrocellulose paper. Positive self-hybridization indicated a high degree of internal complementarity. Dot spots of whole cell RNA and of leaf sap from ASBV infected plants were shown to hybridize with the labelled probe. This hybridization procedure proved to be 16-64 times more sensitive in diagnosing ASBV when compared with polyacrylamide gel analysis.

A simple procedure for the extraction of double-stranded RNA from virus-infected plants

A simple procedure for the isolation of double-stranded (ds) RNA from virus-infected plants is described. The method is based on grinding plant tissue in 4% p-aminosalicylic acid and recovery of ds RNA by phenol extraction and precipitation with 30% ethanol. The presence of both negative and positive virus RNA strands in RNA fractionated in agarose gels was verified by Northern blot hybridization with polynucleotide kinase labelled genomic RNA or complementary DNA (cDNA) probes. The procedure enabled detection of three major ds RNA species (MWs 4.2, 1.05 and 0.48 X 10(6)) and at least 4 minor bands with estimated MWs of 3.5, 2.5, 2.2 and 2.0 X 10(6) in Nicotiana tabacum plants systemically infected with tobacco mosaic virus (TMV). Cucumber mosaic virus (CMV)-infected Pachystachys coccinea plants contained 2 minor bands of MWs 0.49 and 0.35 X 10(6) in addition to the previously described 4 major ds RNAs and ds CARNA 5 (MW 0.22 X 10(6)). The patterns of ds RNA are useful for diagnosing natural infections of CMV and TMV in N. glauca plants and of citrus tristeza virus in Citrus spp.

Translational competition between related virus RNA species in cell-free systems

The competition between the RNAs from two tobacco mosaic virus (TMV) strains for translation in rabbit reticulocyte lysate and wheat germ cell-free systems was investigated. The virus translational products were distinguished by the size difference of the I2 polypeptides. The RNAs of the two strains were translated simultaneously if added to the translation system together at the beginning of the reaction. Each of the TMV RNAs could inhibit translation of the other when the reaction was initiated by the RNA of one strain and that of the other strain was added after active translation had commenced. In contrast, translation of alfalfa mosaic virus (AMV) RNA was not appreciably inhibited when its RNA was added after the start of translation of TMV RNA; the addition of TMV RNA to an AMV RNA-initiated reaction also resulted in independent synthesis of the products of both virus RNAs.

Hen egg yolk as a source of antiviral antibodies in the enzyme-linked immunosorbent assay (ELISA): a comparison of two plant viruses

A method of raising antibodies against plant viruses in hen egg yolk is described. Laying hens were immunized with citrus tristeza virus (CTV) or tobacco mosaic virus-avocado isolate (TMV-A). Anti-viral antibodies in the yolks of sequentially laid eggs as well as in the serum were titrated by the (heterologous) antiglobulin double antibody sandwich form of the enzyme-linked immunosorbent assay (HADAS-ELISA). Antibodies first appeared in yolk 7 days after injection and peak levels were attained on day 9-11; these levels persisted for about 6-12 days. Non-specific yolk antibodies were removed by absorption with an extract of uninfected plant tissue. Using the HADAS-ELISA technique we found that yolk titres were equal to, or higher than those in serum. The benefits of using laying hens over conventional laboratory animals as a source of antiviral antibody are discussed.

Rhizobium strain identification in Arachis hypogaea nodules by enzyme-linked immunosorbent assay (ELISA)

The technique of enzyme-linked immunosorbent assay (ELISA) was used for serological identification of peanut Rhizobium strains both in cell suspension of pure culture and in single root nodules of groundnut (Arachis hypogaea) plants. Antisera of three peanut Rhizobium strains were tested against eight different Rhizobium isolates. Three serogroups identified by agglutination and immunodiffusion tests were confirmed by ELISA. In this experiment ELISA was more sensitive by four to six orders of magnitude than the agglutination and immunodiffusion tests and enabled the detection of Rhizobium antigens in cell suspensions of 10(4)-10(5) cells per millilitre. The reactions of culture and nodule antigens were identical for all strains investigated. ELISA enabled the precise typing of rhizobial isolates in single small root nodules. The minimum fresh weight of nodule tissue necessary to perform the ELISA test was 0.4 mg crushed in 1 ml of phosphate-buffered saline (PBS). ELISA was also successfully used for strain identification in mixed inoculated plants. One of the strains in each pair formed most of the nodules examined.

Initiation factor eIF-4B (IF-M3)-dependent recognition and translation of capped versus uncapped eukaryotic mRNAs

Translation of capped and uncapped eukaryotic mRNAs is stimulated by addition of eIF-4B to an mRNA-dependent reticulocyte lysate system. m7G5 ppp inhibits translation of capped but not uncapped mRNAs and reduces translation of capped vaccinia mRNA to the level obtained with uncapped vaccinia mRNA. Exogenous eIF-4B but no other initiation factor reverses inhibition of protein synthesis by m7G5'ppp. Both capped and uncapped mRNAs interact directly with eIF-4B to form a stable complex, which can be detected by a simple nitrocellulose filter binding assay. However, addition of a 5'-cap to beta-eliminated globin mRNA or satellite tobacco necrosis virus RNA (normally uncapped) increased binding affinity of these mRNAs for eIF-4B and causes binding of these mRNAs to become sensitive to inhibition by m7G5'ppp. These results indicate that the role of the mRNA 5'-cap in translation is related specifically to the function of eIF-4B in forming a complex with mRNA (prior to association of mRNA with the 40 S ribosomal subunit) and that both cap and non-cap sequences participate in this process.

Spiroplasmas are the causal agents of citrus little-leaf disease

A spiroplasma isolated from citrus with little-leaf disease was grown in a cell-free medium and injected into leafhoppers (Euscelis plebejus). Injected leafhoppers, but not those fed on infected plants, transmitted the spiroplasma to white clover (Trifolium repens cv. S100) and sweet orange (Citrus sinensis cv. Valencia). Infected clover plants were severely stunted; infected sweet orange plants showed typical symptoms of citrus little-leaf disease. The spiroplasma was detected in clover and sweet orange plants by electron microscopy; the helical morphology of the organisms was most easily recognizable in sections 150-200 nm thick. The organism was re-isolated in cell-free media both from infected plants and from injected E. plebejus. The original isolate and those re-isolated from experimentally infected clover and sweet orange appeared by morphological, cultural, biochemical and serological criteria to be identical to each other and to the R8-A2 (type) and C-189 strains of Spiroplasma citri. Serological tests and electrophoretic analysis of protein preparations indicated no relationship to Acholeplasma laidlawii, although this organism survived for at least 10 wk after injection into E. plebejus. Our results show that the causal agent of little-leaf disease is related to S. citri.