The Rysto immune receptor recognises a broadly conserved feature of potyviral coat proteins

Knowledge of the immune mechanisms responsible for viral recognition is critical for understanding durable disease resistance and successful crop protection. We determined how potato virus Y (PVY) coat protein (CP) is recognised by Ry_sto , a TNL immune receptor. We applied structural modelling, site-directed mutagenesis, transient overexpression, co-immunoprecipitation, infection assays and physiological cell death marker measurements to investigate the mechanism of Ry_sto -CP interaction. Ry_sto associates directly with PVY CP in planta that is conditioned by the presence of a CP central 149 amino acids domain. Each deletion that affects the CP core region impairs the ability of Ry_sto to trigger defence. Point mutations in the amino acid residues Ser₁₂₅ , Arg₁₅₇ , and Asp₂₀₁ of the conserved RNA-binding pocket of potyviral CP reduce or abolish Ry_sto binding and Ry_sto -dependent responses, demonstrating that appropriate folding of the CP core is crucial for Ry_sto -mediated recognition. Ry_sto recognises the CPs of at least 10 crop-damaging viruses that share a similar core region. It confers immunity to plum pox virus and turnip mosaic virus in both Solanaceae and Brassicaceae systems, demonstrating potential utility in engineering virus resistance in various crops. Our findings shed new light on how R proteins detect different viruses by sensing conserved structural patterns.

Long-Term Efficacy and Safety of RNAi-Mediated Virus Resistance in 'HoneySweet' Plum

Interfering RNA technology has been established as an effective strategy to protect plants against viral infection. Despite this success, interfering RNA (RNAi) has rarely been applied due to the regulatory barriers that confront genetically engineered plants and concerns over possible environmental and health risks posed by non-endogenous small RNAs. 'HoneySweet' was developed as a virus-resistant plum variety that is protected by an RNAi-mediated process against Sharka disease caused by the plum pox virus. 'HoneySweet' has been approved for cultivation in the United States but not in countries where the plum pox virus is endemic. In this study, we evaluated the long-term efficacy of virus resistance in 'HoneySweet,' the nature and stability of its sRNA profile, and the potential health risks of consuming 'HoneySweet' plums. Graft-challenged 'HoneySweet' trees carrying large non-transgenic infected limbs remained virus-free after more than 10 years in the field, and the viral sequences from the non-transgenic infected limbs showed no evidence of adaptation to the RNAi-based resistance. Small RNA profiling revealed that transgene-derived sRNA levels were stable across different environments and, on average, were more than 10 times lower than those present in symptom-less fruits from virus-infected trees. Comprehensive 90-day mouse feeding studies showed no adverse health impacts in mice, and there was no evidence for potential siRNA off-target pathologies predicted by comparisons of the most abundant transgene-derived sRNAs to the mouse genome. Collectively, the data confirmed that RNAi provides a highly effective, stable, and safe strategy to combat virus diseases in crop plants.

Single amino acid changes in the 6K1-CI region can promote the alternative adaptation of Prunus- and Nicotiana-propagated Plum pox virus C isolates to either host

Plum pox virus (PPV) C is one of the less common PPV strains and specifically infects cherry trees in nature. Making use of two PPV-C isolates that display different pathogenicity features, i.e., SwCMp, which had been adapted to Nicotiana species, and BY101, which had been isolated from cherry rootstock L2 (Prunus lannesiana) and propagated only in cherry species, we have generated two infective full-length cDNA clones in order to determine which viral factors are involved in the adaptation to each host. According to our results, the C-P3(PIPO)/6K1/N-CI (cylindrical inclusion) region contains overlapping but not coincident viral determinants involved in symptoms development, local viral amplification, and systemic movement capacity. Amino acid changes in this region promoting the adaptation to N. benthamiana or P. avium have trade-off effects in the alternative host. In both cases, adaptation can be achieved through single amino acid changes in the NIapro protease recognition motif between 6K1 and CI or in nearby sequences. Thus, we hypothesize that the potyvirus polyprotein processing could depend on specific host factors and the adaptation of PPV-C isolates to particular hosts relies on a fine regulation of the proteolytic cleavage of the 6K1-CI junction.

Sequence variability, recombination analysis, and specific detection of the W strain of Plum pox virus

Plum pox virus (PPV), a member of the genus Potyvirus, is the causal agent of Sharka, the most detrimental disease of stone-fruit trees worldwide. PPV isolates are grouped into seven distinct strains. The minor PPV-W strain was established recently for the divergent W3174 isolate found in Canada. Here, the partial or complete genomic sequences of four PPV-W isolates from Latvia have been determined. The completely sequenced isolates LV-141pl and LV-145bt share 93.1 and 92.1% nucleotide identity, respectively, with isolate W3174, with two regions of higher (>20%) divergence in the P1/HC-Pro and NIa (VPg) regions. Further analyses demonstrated that these two regions correspond to two independent recombination events in the W3174 genome, one involving PPV-M (approximate genome positions 692 to 1424) and the other PPV-D (nucleotides 5672 to 5789). The LV-141pl and LV-145bt isolates appear to be representatives of the "ancestral" PPV-W strain, not affected by recombination. The PPV-W intrastrain variability is substantially higher than that of all other PPV strains, with potential implications for the serological detection of PPV-W isolates. A PPV-W-specific primer pair has been developed, allowing the specific reverse-transcription polymerase chain reaction detection of all five presently available W isolates. The characterization of these new PPV-W isolates sheds light on PPV-W evolutionary history, further supports the hypothesis of its East-European origin, and opens the way for the biological and epidemiological characterization of this poorly known PPV strain.

Evaluation of several RT-PCR primer pairs for the detection of Apple stem pitting virus

Detection of Apple stem pitting virus (ASPV) using RT-PCR based methods was studied in infected apple and pear trees. Three virus-specific primers (ASPF1CP, ASPF2CP, ASPR3CP) were designed to target the most conservative regions of the coat protein gene of 10 virus isolates in Poland and 7 other ASPV sequences available in GenBank. The suitability of the primer pairs ASPF1CP-ASPR3CP and ASPF2CP-ASPR3CP for detection of 19 virus isolates was checked. Both new primer pairs initiated amplification of a specific product from all sources tested. From 1 to 11 isolates were not detected with the primer sets published previously. Detection of the virus in the samples collected in March, using ASPF1CP-ASPR3CP primer pair, was possible up to 512 times dilution. For the samples collected in July, virus was detected in the extracts from infected plants diluted eight times. More than 100-fold increase of sensitivity could be obtained by semi-nested PCR with primers ASPF2CP-ASPR3CP following the first round with ASPF1CP-ASPR3CP. Identification of virus isolates with different number of deletions in the coat protein gene was possible using RT-PCR with newly designed reverse primer ASPDEL in combination with the published primer ASPV7956. Besides, the comparative analysis of silicacapture-RT-PCR (SC-RT-PCR) versus immunocapture-RT-PCR (IC-RT-PCR) assays was carried out. Few ASPV isolates escaped detection by IC-RT-PCR, while all isolates tested were detected using the SC-RT-PCR with the new primers.

Field Trials of Plum Clones Transformed with the Plum pox virus Coat Protein (PPV-CP) Gene

Transgenic clones C2, C3, C4, C5, C6, and PT-6, of plum (Prunus domestica L.) transformed with the coat protein (CP) gene of Plum pox virus (PPV), PT-23 transformed with marker genes only, and nontransgenic B70146 were evaluated for sharka resistance under high infection pressure in field trials in Poland and Spain. These sites differed in climatic conditions and virus isolates. Transgenic clone C5 showed high resistance to PPV at both sites. None of the C5 trees became naturally infected by aphids during seven (Spain) or eight (Poland) years of the test, although up to 100% of other plum trees (transgenic clones and nontransgenic control plants) grown in the same conditions showed disease symptoms and tested positively for PPV. Although highly resistant, C5 trees could be infected artificially by chip budding or via susceptible rootstock. Infected C5 trees showed only a few mild symptoms on single, isolated shoots, even up to 8 years post inoculation. These results clearly indicate the long-term nature and high level of resistance to PPV obtained through genetically engineered resistance.

Stability of gene silencing-based resistance to Plum pox virus in transgenic plum (Prunus domestica L.) under field conditions

Plum pox virus (PPV) is one of the most devastating diseases of Prunus species. Since few sources of resistance to PPV have been identified, transgene-based resistance offers a complementary approach to developing PPV-resistant stone fruit cultivars. C5, a transgenic clone of Prunus domestica L., containing the PPV coat protein (CP) gene, has been described as highly resistant to PPV in greenhouse tests, displaying characteristics typical of post-transcriptional gene silencing (PTGS). We show in this report that C5 trees exposed to natural aphid vectors in the field remained uninfected after 4 years while susceptible transgenic and untransformed trees developed severe symptoms within the first year. C5 trees inoculated by chip budding showed only very mild symptoms and PPV could be detected in these trees by IC-RT-PCR. The PPV-CP transgene in C5 was specifically hyper-methylated with no detectable expression. These results indicate both stability and efficiency of PTGS-based PPV resistance in plum under field conditions.

Preliminary report on the apparent breaking of resistance of a transgenic plum by chip bud inoculation of plum pox virus PPV-S

Five transgenic clones of Prunus domestica L. containing plum pox virus (PPV) coat protein (CP) gene and one non-transformed control clone were challenged with PPV-S in the field. Symptoms developed on C2, C3, C4, C6 and B70146 but not C5 trees inoculated by chip budding (CBI) (2/2, 2/2, 1/1, 2/2 and 2/2, positive/inoculated) in the first summer after inoculation. However, in the second year, symptoms appeared on CBI C5 trees. The presence of the virus in the plants was confirmed by enzyme-linked immunosorbent assay (ELISA) and reverse transcription-polymerase chain reaction (RT-PCR) amplification of a fragment of viral polymerase gene. During two years, symptoms of infection developed on 3 to 4 of 8 non-inoculated trees of clones C2, C3, C4, C6 and B70146. Eight non-inoculated C5 trees remained symptomless and ELISA-negative as of spring 1998, in spite of the presence of aphid vectors and inoculum sources.

Expression of the plum pox coat protein gene in transgenic Nicotiana tabacum plants

Plant expression vector pBI 121 containing the gene encoding coat protein of Plum Pox Virus of the Skierniewice isolate (CP PPV-S) was prepared (clone pCM1). The construct was used for transformation of Nicotiana tabacum plants using an Agrobacterium tumefaciens based system. About 82% of kanamycin resistant plant lines contained a transgene (the sequence of CP PPV-S) but only 81% of them actively expressed the PPV-S coat protein gene as measured by RT-PCR.

Expression of the plum pox coat protein gene in transgenic Nicotiana tabacum plants

Plant expression vector pBI 121 containing the gene encoding coat protein of Plum Pox Virus of the Skierniewice isolate (CP PPV-S) was prepared (clone pCM1). The construct was used for transformation of Nicotiana tabacum plants using an Agrobacterium tumefaciens based system. About 82% of kanamycin resistant plant lines contained a transgene (the sequence of CP PPV-S) but only 81% of them actively expressed the PPV-S coat protein gene as measured by RT-PCR.

Nucleotide sequence of the coat protein gene of the Skierniewice isolate of plum pox virus (PPV)

The coat protein (CP) gene of the Skierniewice isolate of plum pox virus (PPV-S) has been amplified using the reverse transcription--polymerase chain reaction (RT-PCR), cloned and sequenced. The nucleotide sequence of the gene and the deduced amino-acid sequence of PPV-S CP were compared with those of other PPV strains. The nucleotide sequence showed very high homology to most of the published sequences. The motif: Asp-Ala-Gly (DAG), important for the aphid transmissibility, was present in the amino-acid sequence. Our isolate did not react in ELISA with monoclonal antibodies MAb06 supposed to be specific for PPV-D.