Virus and helper component interactions favour the transmission of recombinant potato virus Y strains

In recent years, several recombinant strains of potato virus Y, notably PVYNTN and PVYN:O have displaced the ordinary strain, PVYO, and emerged as the predominant strains affecting the USA potato crop. Previously we reported that recombinant strains were transmitted more efficiently than PVYO when they were acquired sequentially, regardless of acquisition order. In another recent study, we showed that PVYNTN binds preferentially to the aphid stylet over PVYO when aphids feed on a mixture of PVYO and PVYNTN. To understand the mechanism of this transmission bias as well as preferential virus binding, we separated virus and active helper component proteins (HC), mixed them in homologous and heterologous combinations, and then fed them to aphids using Parafilm sachets. Mixtures of PVYO HC with either PVYN:O or PVYNTN resulted in efficient transmission. PVYN:O HC also facilitated the transmission of PVYO and PVYNTN, albeit with reduced efficiency. PVYNTN HC failed to facilitate transmission of either PVYO or PVYN:O. When PVYO HC or PVYN:O HC was mixed with equal amounts of the two viruses, both viruses in all combinations were transmitted at high efficiencies. In contrast, no transmission occurred when combinations of viruses were mixed with PVYNTN HC. Further study evaluated transmission using serial dilutions of purified virus mixed with HCs. While PVYNTN HC only facilitated the transmission of the homologous virus, the HCs of PVYO and PVYN:O facilitated the transmission of all strains tested. This phenomenon has likely contributed to the increase in the recombinant strains affecting the USA potato crop.

Genetic Diversity of the Ordinary Strain of Potato virus Y (PVY) and Origin of Recombinant PVY Strains

The ordinary strain of Potato virus Y (PVY), PVYO, causes mild mosaic in tobacco and induces necrosis and severe stunting in potato cultivars carrying the Ny gene. A novel substrain of PVYO was recently reported, PVYO-O5, which is spreading in the United States and is distinguished from other PVYO isolates serologically (i.e., reacting to the otherwise PVYN-specific monoclonal antibody 1F5). To characterize this new PVYO-O5 subgroup and address possible reasons for its continued spread, we conducted a molecular study of PVYO and PVYO-O5 isolates from a North American collection of PVY through whole-genome sequencing and phylogenetic analysis. In all, 44 PVYO isolates were sequenced, including 31 from the previously defined PVYO-O5 group, and subjected to whole-genome analysis. PVYO-O5 isolates formed a separate lineage within the PVYO genome cluster in the whole-genome phylogenetic tree and represented a novel evolutionary lineage of PVY from potato. On the other hand, the PVYO sequences separated into at least two distinct lineages on the whole-genome phylogenetic tree. To shed light on the origin of the three most common PVY recombinants, a more detailed phylogenetic analysis of a sequence fragment, nucleotides 2,406 to 5,821, that is present in all recombinant and nonrecombinant PVYO genomes was conducted. The analysis revealed that PVYN:O and PVYN-Wi recombinants acquired their PVYO segments from two separate PVYO lineages, whereas the PVYNTN recombinant acquired its PVYO segment from the same lineage as PVYN:O. These data suggest that PVYN:O and PVYN-Wi recombinants originated from two separate recombination events involving two different PVYO parental genomes, whereas the PVYNTN recombinants likely originated from the PVYN:O genome via additional recombination events.

Retention of Rice dwarf virus by Descendants of Pairs of Viruliferous Vector Insects After Rearing for 6 Years

Rice dwarf virus (RDV) is characterized by its unusual ability to multiply in both plants and leafhopper vector insects and by its transovarial mode of transmission. Colonies of Nephotettix cincticeps, derived originally from pairs of leafhoppers infected with an ordinary strain of RDV, were maintained for 6 years in the laboratory and were found, at the end of this time, still to harbor RDV. Moreover, the isolate of RDV, designated RDV-I, obtained from these colonies retained the ability to infect rice plants. When we raised leafhoppers separately from eggs that had been placed individually on pieces of water-soaked filter paper and reared them in the presence of healthy rice seedlings, we found that all of these leafhoppers harbored RDV. This observation suggested that RDV-I had been maintained in the leafhoppers by transovarial transmission. Two further observations, namely, the low rate of acquisition of RDV by virus-free insect nymphs on symptomless plants on which viruliferous insects had been reared, and the fact that only 2 to 5% of plants had symptoms when rice seedlings were inoculated via RDV-I-viruliferous insects, confirmed that the maintenance of RDV-I by any other mode of transmission through plants and insects was unlikely. This efficient and long-term maintenance of RDV in a population of viruliferous insects might explain the prolonged duration of rice dwarf disease in the field, once there has been a serious outbreak.

Genetic mapping of the compatibility between a lily isolate of Cucumber mosaic virus and a satellite RNA

Five isolates of Cucumber mosaic virus (CMV) from Lilium sp. (lily), which were isolated from specimens in Japan, Korea and Taiwan, were unable to support satellite RNA (satRNA) accumulation. In order to map the CMV sequences that are involved in satRNA support, HL-CMV (Japanese lily isolate), Y-CMV (ordinary strain) and Y-satellite RNA (Y-sat) were used as the source material. The pseudorecombinants between Y-CMV and HL-CMV revealed that RNA1 was essential for satRNA replication in lily. The results of chimeric constructs and various mutations showed that two amino acid residues (at positions 876 and 891) in the 1a protein were the determinants for the inability of HL-CMV to support a satRNA. Specifically, Thr at position 876 had a more pronounced effect than Met at position 891. Specific changes in RNA sequence were also detected in the 3′ terminus of Y-sat and these particular alterations allowed it to be supported by HL-CMV. It is believed that, through evolution, the adaptation of CMV to lily resulted in the introduction of amino acid changes in the 1a protein, changes that coincidentally affected the ability of lily CMV to support satRNAs.

Examination of the Leaf-Drop Symptom of Virus-Infected Potato Using Anther Culture-Derived Haploids

Necrotic lesions and vein necrosis characteristic of the hypersensitive response (HR) controlled by the dominant resistance gene Ny develop in potato cv. Pito after infection with potato virus Y ordinary strain (PVYº) at a low temperature (16/18°C night/day). In contrast, at high temperatures (19/24°C night/day), large coalesced lesions develop in the lower infected leaves, which wither and remain hanging from stems forming the leaf-drop symptom; mosaic symptoms with no necrosis also develop in the top leaves. The genetic basis of the leaf-drop symptom and its dependence on temperature were examined using a novel approach involving 58 haploids (2n = 24) derived from ‘Pito’ (2n = 48) through anther culture. These haploids and ‘Pito’ were graft-inoculated with PVYO at 19/24 to 25°C (night/day). Necrotic symptoms were expressed in 28 haploids, of which 18 haploids (phenotype class N) developed top necrosis, vein necrosis, or both and necrotic lesions that are characteristic of HR. Ten haploids showed leaf drop similar to ‘Pito’ (phenotype class LD). Thirty haploids were susceptible and showed only mosaic symptoms (phenotype class S). These data indicated that necrosis was induced by a single dominant gene, Ny, in the simplex condition. However, the three distinct phenotypic classes (N, LD, and S) among the haploids grown under the same environmental conditions showed that another locus (gene) was involved in modifying the HR triggered by Ny. Data suggested that this locus contains a dominant temperature-dependent modifier (Tdm) gene that alters the expression of PVY-induced HR at higher temperatures, resulting in leaf drop.

Symptom expression and accumulation of potato virus Y (PVYO) and potato leaf roll virus in thirteen potato cultivars

Necrotic local lesions developed in cvs. Matilda, Ostara, Record, Satuma, Stina, Hankkija’s (Hjan) Tanu and Hjan Timo and local ring spots in Olympia and Sieglinde (Siikli) following sap inoculation with the ordinary strain of potato virus Y (PVY0). Secondarily infected cvs. Ostara, Pito, Siikli and Hjan Timo developed leaf drop. No infected progeny was produced by Matilda, Saturna and Hjan Tanu. In contrast, Bintje, Puikula and Sabina developed neither local lesions nor systemic necrosis, but showed mosaic symptoms following primary and secondary infection by PVYO. The ELISA absorbance values for potato leafroll virus (PLRV) in Ostara, Pito and Saturna were less than 10% of those in the PLRV-infected Siikli. The ELISA values for PLRV in Olympia, Stina, Hjan Tanu and Hjan Timo were not significantly different from those of Siikli. The severity of the symptoms did not correlate with the concentration of PLRV in the potatoes.