Potato Leafroll Luteovirus Research Articles

Expression patterns of vascular-specific promoters RolC and Sh in transgenic potatoes and their use in engineering PLRV-resistant plants.

The expression patterns of GUS fusion constructs driven by the Agrobacterium rhizogenes RolC and the maize Sh (Shrunken; sucrose synthase-1) promoters were examined in transgenic potatoes (cv. Atlantic). RolC drove high-level gene expression in phloem tissue, bundle sheath cells and vascular parenchyma, but not in xylem or non-vascular tissues. Sh expression was exclusively confined to phloem tissue. Potato leafroll luteovirus (PLRV) replicates only in phloem tissues, and we show that when RolC is used to drive expression of the PLRV coat protein gene, virus-resistant lines can be obtained. In contrast, no significant resistance was observed when the Sh promoter was used.

Potato leafroll virus (PLRV): its transmission and control

This review provides a wide assessment of the present state of knowledge about the potato leafroll luteovirus (PLRV), a pathogen which is seriously devastating potato crops in many parts of the world. The main biological, physical and chemical properties of this virus are described. The transmission of PLRV by aphids, which are the only transmitters of this virus under natural conditions, is characterized. Special attention is given to the control of PLRV through the use of resistant potato cultivars. Recent advances in obtaining resistant transgenic plants are outlined.

Abstracts, Meeting of the Plant Pathology Society of Alberta, Lethbridge, Alberta, November 1995

Abstract Noteworthy diseases diagnosed on plant samples from Alberta submitted to Brooks Diagnostics Limited in 1995. Khalil I. Al-Mughrabi and Barbara J. Penner. Brooks Diagnostics Limited, P.O. Box 1701, Brooks, Alberta TIR IC5. The influence of cultivar and soil pH on take-all of wheat. M. A. Anderson, T.K. Turkington, K.G. Briggs, I. Evans, and D. Penney. Alberta Agriculture, Food and Rural Development, Lacombe, Alberta T4L 1W8, and (I.E. and D.P.) Edmonton, Alberta T6H 4P2; and (K.G.B.) University of Alberta, Edmonton, Alberta T6G 2P5 Development of a multiplex PCR-assay for detection of bean blights. Patrice Audy, André Laroche, Gilles Saindon, and Henry C. Huang. Agriculture and Agri-Food Canada, Research Center, Lethbridge, Alberta TU 4B1. Enzyme production by Helminthosporium solani, the causal agent of silver scurf disease on potato, and its relationship to thiabendazole sensitivity. Dee Ann Benard and Piara S. Bains. Soil and Crop Diagnostic Centre, 6909-116 St., Edmonton, Alberta T6H 4P2; and (P.S.B.) Crop Diversification; Centre North, R.R. 6, Edmonton, Alberta T5B 4K3. Genomic in situ hybridization to identify Haynaldia villosa chromatin in wheat lines carrying resistance to the wheat curl mite. Qin Chen, R.L. Conner, and A. Laroche. Agriculture and Agri-Food Canada Research Centre, Lethbridge, Alberta TU 4BL Impact of fungicide application on the incidence of blossom blight in alfalfa in 1995. B.D. Gossen, L.M. Harrison, and J. Holley. Agriculture Canada Research Centre, Saskatoon, Saskatchewan S7N 0X2; (L.M.H.) Alberta Agriculture, Fairview, Alberta TOH 1L0; and (J.H.) Crop Diversification Centre, Brooks, Alberta TO] 0J0. Genetic differences revealed between two isolates of Paulownia witches' broom phytoplasma in Korea by the RFLP analysis of 16S rDNA sequence. S.S. Han, L. Jiang, A.H. Khadhair, C. Hiruki, W.H. Yeo, and Y.H. Kim. Department of Forest Resources, Cheonbuk National University, Korea; (L.J., A.H.K., and C.H.) Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta T6G 2P5; and(W.H.Y.) Forestry Research Institute, Seoul, Korea Sclerotinia sclerotiorum causes blossom blight of alfalfa. J.D. Holley, R. Linowski, B. Gossen, and L. Harrison. Crop Diversification Centre, Brooks, Alberta TIR IE6; (R.L.) Medicine Hat College, Brooks Campus, Brooks, Alberta TIR IE6; (B.G.) Agriculture and Agri-Food Canada Research Centre, Saskatoon, Saskatchewan S7N OX2; and (L.H.) Provincial Building, Alberta Agriculture Food and Rural Development, Fairview, Alberta TOH ILO. Infection of canola pollen by Sclerotinia sclerotiorum. H.C. Huang, E. G. Kokko, R. S. Erickson, and R. K. Hynes. Agriculture and Agri-Food Canada Research Centre, Lethbridge, Alberta TU 4B1; and (R.K.H.) Agrium Inc., 402-15 Innovation Boulevard, Saskatoon, Saskatchewan S7N 2X8. Differential display of mRNA identifies three cDNA clones that are related to the pathogenesis in tobacco stunt virus-infected tobacco leaves. L. Jiang and C. Hiruki. Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta T6G 2P5. Attenuation of potato leafroll luteovirus infection and disease symptoms in genetically modified potato plants. L.M. Kawchuk, D.R. Lynch, G.C. Kozub, R.R. Martin, and B Farries. Agriculture and Agri-food Canada, PO Box 3000 Main, Lethbridge, Alberta TU 4B1; and (R.R.M.) Agriculture and Agri- Food Canada, 6660 NW Marine Drive, Vancouver. British Columbia V6T 1X2. Molecular diagnosis of alfalfa witches'-broom phytoplasma in three leafhopper species and alfalfa plants from the Peace River area. A.H. Khadhair, S.F. Hwang, and C. Hiruki. Alberta Environmental Centre, Bag 4000, (S.F.H.) Vegreville, Alberta T9C 1T4; and (C.H.) Department of AFNS, University of Alberta, Edmonton, Alberta T6G 2P5. Fungicidal control of entomosporium leaf and berry spot of saskatoon. Ralph M. Lange, Piara S. Bains, and Ronald J. Howard. Crop Diversification Centre, North, R.R.6, Edmonton, Alberta T5B 4K3; and (R.J.H.) Crop Diversification Centre, South, SS 4, Brooks, Alberta TIR 1E6. Identification of a DNA fragment linked to the bunt Bt- 10 resistance gene and its utilisation for marker-assisted selection in hexaploid wheat lines. André Laroche, Tigst Demeke, and Denis A. Gaudet. Agriculture and Agri-Food Canada, P.O. Box 3000 Main, Lethbridge, Alberta TU 4B1. Hybridization among pathotypes of Ustilago bullata . G.D. Turnbull and B.D. Gossen. Agriculture Canada Research Centre, Saskatoon, Saskatchewan S7M 0X2; and (B.D.G.) Alberta Environmental Centre, Vegreville, Alberta T9C JT4. The influence of tillage and nitrogen fertilizer on barley foliar diseases. T.K. Turkington, G.W. Clayton, and P.A. Burnett. Alberta Agriculture, Food & Rural Development, Lacombe, Alberta T4L 1W8; Agriculture & Agri-Food Canada, Alberta (G.W.C.) Beaverlodge, Alberta T0H 0C0; and (P.A.B.) Lacombe, Alberta T4L JWl. Preservation of dehydrated phytoplasma-infected Paulownia leaf tissue by quick drying with a microwave oven for molecular analysis. Keri Wang, AH. Khadhair, and C. Hiruki. Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, T6G 2P5.

Ultrastructural study of acquisition and retention of potato leafroll luteovirus in the alimentary canal of its aphid vector, Myzus persicae Sulz.

The chronology of PLRV acquisition and retention by Myzus persicae was investigated using electron microscopy. Examination demonstrated a rapid translocation of the virus through the intestine into the haemocoel. Indeed, viral particles could be observed in the intestinal epithelial cells, then in the haemocoel, 4 and 8 h, respectively, after their arrival in the lumen of the alimentary canal. However, the virus accumulated in the intestinal epithelial cells. In these cells, the first viral particles were seen enclosed in isometric or tubular isolated vesicles; a few hours later, they were present in tubular aggregated vesicles and also in lysosomes or multivesicular bodies. After a 40 h acquisition period, all studied intestinal epithelial cells exhibited high numbers of viral particles which were consistently distributed throughout these cell structures. When aphids were removed from viral source, viral particles were detected in intestinal lumen for a further three days and in intestinal epithelial cells for a total of eight days. Virus content in these cells began to decrease from the second day. Areas with tubular aggregated vesicles were maintained for seven days following aphid removal from viral source, but progressively became smaller and fewer. The accumulation and the persistence of PLRV in the intestine are discussed.

Assembly of virus-like particles in insect cells infected with a baculovirus containing a modified coat protein gene of potato leafroll luteovirus.

DNA encoding the coat protein (P3) of a Scottish isolate of potato leafroll virus (PLRV) was inserted into the genome of Autographa californica nucleopolyhedrovirus (AcNPV) such that the coat protein was expressed either in an unmodified form or with the addition of the amino acid sequence MHHHHHHGDDDDKDAMG at the N terminus (P3-6H). Insect cells infected with these recombinant baculoviruses accumulated substantial amounts of P3 and P3-6H. P3 could not be recovered from cell extracts unless it was denatured in SDS but a proportion of the P3-6H was recoverable in a soluble form in non-denaturing conditions. Immunogold labelling of sections of infected cells showed that P3 accumulated in nuclei in large amorphous bodies. In contrast, although much of the P3-6H also accumulated in nuclei, it formed virus-like particles (VLP) which were often grouped in close-packed, almost cystalline arrays. When electron microscope grids coated with antibodies to PLRV were floated on cell extracts containing P3-6H, VLP were trapped which were indistinguishable from PLRV particles trapped from extracts of PLRV-infected plants. The VLP co- sedimented in sucrose gradients with PLRV particles which suggests that the VLP contained RNA. VLP collected from sucrose density gradient fractions contained protein which reacted with nickel chelated to nitrilotriacetic acid, a histidine-specific reagent. Cells infected with either recombinant baculovirus also synthesized a protein, with an Mr of about 17000, which was shown to be the translation product of the P4 gene which is in the +1 reading frame within the coat protein gene. This protein was also found in the nuclear fraction of infected cells but was more readily soluble than was P3.

Non-canonical translation mechanisms in plants: efficient in vitro and in planta initiation at AUU codons of the tobacco mosaic virus enhancer sequence.

The 5' untranslated leader (Omega sequence) of tobacco mosaic virus (TMV) genomic RNA was utilized as a translational enhancer sequence in expression of the 17 kDa putative movement protein (pr17) of potato leaf roll luteovirus (PLRV). In vitro translation of RNAs transcribed from appropriate chimeric constructs, as well as their expression in transgenic potato plants, resulted in the expected wild-type pr17 protein, as well as in larger translational products recognized by pr17-specific antisera. Mutational analyses revealed that the extra proteins were translated by non-canonical initiation at AUU codons present in the wild-type Omega sequence. In the plant system translation initiated predominantly at the AUU codon at positions 63-65 of the Omega sequence. Additional AUU codons in a different reading frame of the Omega sequence also showed the capacity for efficient translation initiation in vitro. These results extend the previously noted activity of the TMV 5' leader sequence in ribosome binding and translation enhancement in that the TMV translation enhancer can mediate non-canonical translation initiation in vitro and in vivo.

Synthesis of a Full-Length Infectious cDNA Clone of Cucurbit Aphid-Borne Yellows Virus and Its Use in Gene Exchange Experiments with Structural Proteins from Other Luteoviruses

A full-length cDNA of cucurbit aphid-borne yellows virus (CABYV) has been constructed and expressed either as anin vitrotranscript, under control of a bacteriophage T7 RNA polymerase promoter, orin vivo,under control of the cauliflower mosaic virus 35S promoter in an agroinfection vector. The biological activity of the cloned cDNA was demonstrated by the ability of itsin vitrotranscript to replicate in protoplasts and of the agroinfection vector to infect agroinoculated plants. Virus in the agroinfected plants could be transmitted by the aphid vectorsMyzus persicaeandAphis gossypii.The specificity of luteovirus RNA packaging was investigated by replacing (1) the CABYV coat protein gene (and the overlapping ORF5) by the corresponding region of potato leafroll luteovirus or (2) the CABYV readthrough domain by the readthrough domain of beet western yellows luteovirus. The resulting chimeric transcripts replicated in protoplasts and produced virions.

Search for sources of early blight (Alternaria solani) field resistance not associated with vegetative late maturity in tetraploid potato germplasm

A total of 934 potato cultivars and clones from different breeding programmes across the world were tested for their reaction to early blight (Alternaria solani) in Brasilia-DF, Brazil. This field trial was set up in order to identify tetraploid potato genotypes with useful levels of resistance to early blight (EB) that were not correlated with late maturity or lateness. Artificial inoculation (spread ofA. solani infected leaves over plant canopy) was employed even though EB occurs naturally at epidemic levels in the test area. Four disease assessments were made every 10 days, beginning 45 days after planting. Based on the evaluation made 65 days after planting, only 27 clones were identified that had less lesioned leaf area than the resistant standard (cv. Aracy). Area under disease progress curve (AUDPC) was also used as a criterion of resistance. Using AUDPC as a parameter, 22 genotypes were identified presenting values lower than or identical to cv. Aracy. Differences in the commercial yield were recorded in genotypes with identical response to EB, suggesting that distinct levels of tolerance to EB also exist. In spite of the strong correlation between EB resistance and late maturity, some genotypes derived from the parental materials ‘NDD 277-2’, ‘Kufri Jyoti’, ‘CIP 377888-7’, ‘Maine-28’, and the clones ‘CFS 69.1’ and ‘I-853’ were identified as having good levels of EB field resistance not linked with the negative phenotypic trait of lateness. These materials presented medium to medium-early maturity with a vegetative cycle of around 90–95 days. Some selected genotypes presented more acceptable agronomic characteristics including resistance toPhytophthora infestans, potato leaf roll luteovirus (PLRV) and potato virus Y (PVY). The EB resistance sources identified in this work can be recommended as parental materials for widening the genetic base for EB disease resistance in breeding programmes for highland tropical and subtropical areas.

Characterization of the Virus x Temperature Interaction in Secondarily Infected Potato Plants Using EPIVIT

The model EPIVIT, designed for contact- and aphid-transmitted viruses of tuber crops, simulates the percentage of infected tubers harvested from a potato field. It includes a module for tuber infection of plants with a tuberborne (secondary) infection (efficiency of autoinfection). This module postulates a monomolecular function for the relation between the efficiency of autoinfection and developmental heat, providing a theoretical basis for understanding how an infectious, systemic virus and the environment, represented by temperature, are interacting. The module was calibrated with temperature and autoinfection data obtained with the modern potato cultivar Yungay (Solanum tuberosum ssp. tuberosum x S. tuberosum ssp. andigena) in five contrasting environments in Peru. Model estimates for potato X potexvirus (PVX), Andean potato mottle comovirus (APMV), potato Y potyvirus (PVY), or potato leafroll luteovirus (PLRV) were obtained. They were more accurate when temperature-sensitive growth rates were used for heat accumulation than with constant accumulation rates. The bell-shaped relationships obtained between heat accumulation rates and apparent temperature differed for each virus, with optimum heat accumulation rates at 28°C for PVY, and between 18 and 28°C, 20 and 25°C, and 23 and 28°C for PLRV, PVX, and APMV, respectively. With PLRV and PVY data, high precision levels (P < 0.05) were only obtained when the parameter trigger developmental heat was included. This parameter represents a threshold amount of developmental heat accumulated any time temperature fluctuates into the range between developmental cardinal temperatures, before heat becomes effective for the efficiency ofautoinfection. This calibration supports EPIVIT's assumptions regarding the influence of temperature on virus behavior in the host plant. With complete verification of this model component, validation is still needed for final confirmation of the model, as well as an elucidation of the biological mechanisms that underlie efficiency of autoinfection and virus behavior at different temperatures by analytical research.

Nucleotide sequence of RNA of a Polish isolate of potato leafroll luteovirus.

The nucleotide sequence of the genomic RNA of a Polish isolate of a potato leafroll virus was determined. Some variations between the determined sequences were observed. A comparison of the frequency of sequence variants in particular regions of the genome is presented.

The effects of temperature on the availability and acquisition of potato leafroll luteovirus by Myzus persicae

SummaryThe concentration of potato leafroll luteovirus (PLRV) did not differ in potato plants with secondary infections grown at 15°C or 27°C. Detached leaves of plants grown at 15°C or 27°C were used as sources of PLRV for peach‐potato aphids (Myzus persicae Sulz.) both at 15°C and 27°C. At comparable temperature during virus acquisition, aphids which fed on leaves of plants kept previously at 15°C contained more viral antigen detected by ELISA than aphids which fed on leaves of plants grown at 27°C. The aphids which acquired PLRV at 27°C contained evidently more viral antigen than those which acquired PLRV at 15°C. The greatest amount of PLRV was found in the aphids which acquired the virus at 27°C from the leaves of plants kept at 15°C. The ability of M. persicae to transmit PLRV to Physalis ftoridana Rydb. generally decreased with decrease in the amount of PLRV in vectors.

Changes in the Amino Acid Sequence of the Coat Protein Readthrough Domain of Potato Leafroll Luteovirus Affect the Formation of an Epitope and Aphid Transmission

Potato leafroll luteovirus (PLRV) is transmitted naturally by aphids, but two isolates (15 and V) are known to be only poorly transmissible (PAT); these isolates are also distinct in that their particles lack an epitope present in transmissible (HAT) isolates of PLRV (Tamada et al., Ann. Appl. Biol. 104, 107-116, 1984; Massalski and Harrison, J. Gen. Virol. 68, 1813-1821, 1987). Virus cultures propagated vegetatively for several years in potato plants from the source of isolate V were shown still to be poorly transmissible by Myzus parsicae. Moreover, when isolate V was transmitted by aphids, the subisolates obtained (V4 and V31) were no more readily transmissible than was the stock culture PLRV-V. The subisolates were also unchanged antigenically by the transmission. Thus the inefficient transmission had not restored the properties of HAT isolates. Sequence comparisons among the coat proteins and the readthrough domains of HAT and PAT Scottish isolates suggested that either or both of two amino acid changes in the C-terminal part of the readthrough protein are responsible for the poor transrmission and the loss of an epitope. The readthrough protein is therefore thought to play a role in the circulative aphid transmission of PLRV.

Mutational Analysis of the Nucleic Acid-Binding 17 kDa Phosphoprotein of Potato Leafroll Luteovirus Identifies an Amphipathic α-Helix as the Domain for Protein/Protein Interactions

The 17 kDa protein (pr17) of potato leafroll luteovirus is translated from a subgenomic PLRV RNA by internal translation initiation and binds to single-stranded nucleic acids (E. Tacke, D. Prüfer, J. Schmitz, and E. Rohde, 1991, J. Gen. Virol. 72, 2035-2038). Chemical crosslinking of in vitro expressed pr17 provided evidence for the preferential formation of pr 17 homodimers which were also detected in PLRV-infected potato plants and isolated from potato lines expressing the PLRV pr17 transgene. Mutation analysis identified an amphipathic α-helix within the acidic amino-terminus of pr 17 which acts as the domain for protein/protein interactions. Pr17 was predominantly associated with subcellular fractions enriched for nuclei, chloroplasts, mitochondria, and membranous structures. In addition it was shown that pr17 was phosphorylated in planta and that this modification did not inhibit binding of the protein to nucleic acids.

Resistance to potato leafroll luteovirus can be greatly improved by combining two independent types of heritable resistance

SummaryTwelve potato clones were exposed to infection by aphids with potato leafroll luteovirus (PLRV) in three field trials in order to assess their resistance to infection. Up to 92% of the plants of some clones became infected, although other clones were relatively resistant to infection and one clone remained virus‐free in all three trials. The resistance of the same 12 clones to PLRV multiplication was assessed in glasshouse‐grown plant: lants were graft‐inoculated and their daughter tubers were used to grow plants with secondary infection. High concentrations of PLRV were found in some clones (c. 1700 ng/g leaf) while in others much less virus accumulated (as little as 60 ng/g leaf). However, clones in which little virus accumulated were not necessarily those which were most resistant to infection in the field, and there was no association between the two types of resistance. Nevertheless, both types of resistance were found in some clones. The clone G8107(1), which remained virus‐free in all the field exposure trials, was also the most resistant to PLRV multiplication. The combination of these two types of resistance in cultivars should help to eliminate the spread of PLRV in crops.

Evaluation of resistance to potato leafroll virus in selected potato cultivars under field conditions

To test the resistance of potato cultivars to potato leafroll luteovirus (PLRV) in the field, small plots each containing 3 rows of 3 plants of the same cultivar were planted next to a row of infector plants. Harvested tubers were tested for PLRV infection by enzyme-linked immunosorbent assay. Most of the PLRV spread (85%) was sequential rather than random (i.e. related to distance between test and infector plants). Twenty-six potato cultivars were screened for resistance to PLRV in 6 trials during 3 years at 3 locations. Summer, autumn, or winter plantings were made. There were significant differences between cultivars in the degree of PLRV resistance and susceptibility, independent of location, season, or year. Of the cultivars included in at least 2 trials each, the most resistant were Delcora, Omega, Spunta, and Whitu. Most susceptible were Cadima, Denali, Elliot, Renova, Russet Burbank, and Tor. Contrary to earlier reports, Spunta was resistant, while Delaware was relatively susceptible. Four Australian-bred cultivars (Cadima, Elliot, Geographe, and Tor) that were tested for the first time had little or no resistance, and 2 (Bremer and Toolangi Delight) were intermediate. The incomplete (quantitative) nature of resistance to initial infection with PLRV by viruliferous aphids was demonstrated in a seventh field trial where aphids were more abundant and all plants became infected, including resistant cultivars Delcora and Omega.

Structural requirements for efficient translational frameshifting in the synthesis of the putative viral RNA-dependent RNA polymerase of potato leafroll virus.

The putative RNA-dependent RNA polymerase of potato leafroll luteovirus (PLRV) is expressed by -1 ribosomal frameshifting in the region where the open reading frames (ORF) of proteins 2a and 2b overlap. The signal responsible for efficient frameshift is composed of the slippery site UUUAAAU followed by a sequence that has the potential to adopt two alternative folding patterns, either a structure involving a pseudoknot, or a simple stem-loop structure. To investigate the structure requirements for efficient frameshifting, mutants in the stem-loop or in the potential pseudoknot regions of a Polish isolate of PLRV (PLRV-P) have been analyzed. Mutations that are located in the second stem (S2) of the potential pseudoknot structure, but are located in unpaired regions of the alternative stem-loop structure, reduce frameshift efficiency. Deletion of the 3' end sequence of the alternative stem-loop structure does not reduce frameshift efficiency. Our results confirm that -1 frameshift in the overlap region depends on the slippery site and on the downstream positioned sequence, and propose that in PLRV-P a pseudoknot is required for efficient frameshifting. These results are in agreement with those recently published for the closely related beet western yellows luteovirus (BWYV).

Detection of Potato Leafroll and Strawberry Mild Yellow-Edge Luteoviruses by Reverse Transcription-Polymerase Chain Reaction Amplification

DNA primers were constructed based on the nucleotide sequence of the coat protein gene of potato leafroll luteovirus (PLRV) and utilized for cDNA synthesis and polymerase chain reaction (PCR) amplification of a 487-bp DNA fragment from nucleic acid extracts of PLRV-infected tissue, and an approximately 500-bp DNA fragment from the luteovirus associated with strawberry mild yellow-edge-infected tissue. The amplified DNA fragments were identified by hybridization analysis with a cDNA probe for the coat protein gene of PLRV and differentiated by restriction fragment length polymorphism analysis. Reverse transcription (RT)-PCR assays were developed for the detection of PLRV in nucleic acid extracts of infected potato leaves and tubers, and in viruliferous aphids (.)

Analysis of epitopes on potato leafroll virus capsid protein

Pepscan hexapeptides prepared to the capsid protein amino acid sequence of potato leafroll luteovirus (PLRV) were tested against both polyclonal and monoclonal antibodies. Twelve continuous epitopes were identified: 11 were detected by two different PLRV polyclonal antisera, but only 4 were detected by both antisera. The 12th epitope reacted with monoclonal antibody BG3. The location of most of the epitopes did not correlate well with antigenic areas predicted by computer algorithms. Comparison of the amino acid sequences of PLRV and southern bean mosaic virus capsid proteins allowed a preliminary assignment of epitopes 4–12 to different regions of the putative S domain of the PLRV subunit. Five out of 14 monoclonal antibodies and both of the polyclonal antisera reacted with epitope 1 at the N-terminus. ELISA data indicated that even though the N-terminus is hydrophobic, it is exposed at the surface of the particles.

Spread of potato leafroll virus is decreased from plants of potato clones in which virus accumulation is restricted

SummaryTubers of eight potato clones infected with potato leafroll luteovirus (PLRV) were planted as ‘infectors’ in a field crop grown, at Invergowrie, of virus‐free potato cv. Maris Piper in 1989. The mean PLRV contents of the infector clones, determined by enzyme‐linked immunosorbent assay (ELISA) of leaf tissue, ranged from c. 65 to 2400 ng/g leaf. Myzus persicae colonised the crop shortly after shoot emergence in late May and established large populations on all plants, exceeding 2000/plant by 27 June. Aphid infestations were controlled on 30 June by insecticide sprays. Aphid‐borne spread of PLRV from plants of the infector clones was assessed in August by ELISA of foliage samples from the neighbouring Maris Piper ‘receptors’. Up to 89% infection occurred in receptor plots containing infector clones with high concentrations of PLRV. Spread was least (as little as 6%) in plots containing infectors in which PLRV concentrations were low. Primary PLRV infection in guard areas of the crop away from infectors was 4%. Some receptor plants became infected where no leaf contact was established with the infectors, suggesting that some virus spread may have been initiated by aphids walking across the soil.

The restricted distribution of potato leafroll luteovirus antigen in potato plants with transgenic resistance resembles that in clones with one type of host genemediated resistance

SummaryThe distribution of virus‐infected cells was examined, by fluorescence microscopy, within plants of a range of potato clones infected with potato leafroll luteovirus (PLRV). This range included nine PLRV‐resistant clones, of which four were transgenic lines carrying the PLRV coat protein gene and five were conventionally bred. Plants of these clones were resistant to PLRV multiplication and accumulated less virus antigen in leaf tissue than did susceptible clones. Indirect fluorescent antibody staining of thin sections from carbodiimide‐fixed petiole tissue revealed that in plants of PLRV‐susceptible clones, virus‐infected cells were abundant within both external (abaxial) and internal (adaxial) phloem bundles. In plants of the PLRV‐resistant conventionally bred clones and in resistant transgenic lines of cv. Pentland Squire, virus‐infected cells were much fewer in number and largely restricted to internal phloem bundles. In resistant transgenic lines of cv. Désirée, this restricted distribution of PLRV antigen was only detected in petioles of young leaves. The results suggest that the transgenic and a host‐mediated type of resistance that restricts virtis multiplication have underlying similarities.