P. E. Thomas

Reduced field spread of potato leafroll virus in potatoes transformed with the potato leafroll virus coat protein gene

Russet Burbank potato was transformed with plant expression vectors containing the potato leafroll luteovirus (PLRV) coat protein (CP) gene. Transgenic potato lines contained a gene expression cassette with two copies of a PLRV CP gene in which the nucleotide sequence was modified to improve expression of the gene. In addition, the two copies of the PLRV CP gene were each driven by a different promoter. Field test screening for PLRV resistance identified 15 lines which showed moderate resistance to PLRV infection and virus titer build-up and a longer incubation period for systemic infection. By conducting field resistance assays during a period when the vector of PLRV was not present, it was possible to test whether the observed resistance was sufficient to restrict aphid transmission of PLRV in a field test. Two years of field testing demonstrated that PLRV-spread from an infected plant to adjacent healthy plants of the same line was severely restricted in nearly all the transgenic lines in the field. These lines have useful resistance to PLRV and could aid in managing PLRV disease in Russet Burbank potato.

Detection of tobacco rattle virus in tubers exhibiting symptoms of corky ringspot by polymerase chain reaction

Tobacco rattle virus (TRV)-specific products were obtained by reverse transcription-polymerase chain reaction (RT-PCR) of nucleic acid extracts prepared from tubers exhibiting symptoms of corky ringspot (CRS) disease. Viral origin of amplification products was confirmed by hybridization of PCR products with a nonradioactively-labeled complementary DNA probe prepared from purified TRV RNA. Results suggest that TRV is not uniformly distributed in CRS-affected tubers and appears to be in low concentration in tuber tissue.

A search for evidence of virus/transgene interactions in potatoes transformed with the potato leafroll virus replicase and coat protein genes

A search was conducted to detect evidence for interactions between potato leafroll virus (PLRV)-derived transgenes expressed in Russet Burbank potato and viruses to which the transgenic plants were exposed and by which they were infected. More than 25000 plants in 442 lines transformed with 16 different coat protein gene (CP) constructs and nearly 40000 plants in 512 lines transformed with seven different replicase gene (Rep) constructs of PLRV were exposed to field infection over a 6-year period. These plants were individually inspected for type and severity of virus symptoms. Heterologous viruses found infecting the plants were identified and examined for alterations in transmission characteristics, serological affinity, host range, and symptoms. Selected isolates of PLRV from field-infected plants were examined for unusual symptoms produced in diagnostic hosts and for alteration in sedimentation properties in density gradient tubes. Viruses that were propagated in selected transgenic lines in a greenhouse were examined for similar alterations. Transmission characteristics and serological properties were not altered when they replicated in potatoes containing CP constructs in the field or greenhouse. Potato plants expressing CP or Rep constructs of PLRV were not infected in the field or in the greenhouse with viruses that do not normally infect potato. New viruses or viruses with altered sedimentation characteristics, symptoms, or host range were not detected in field-exposed or greenhouse-inoculated potato plants expressing CP or Rep gene constructs of PLRV.

Weed Hosts of Paratrichodorus allius and Tobacco Rattle Virus in the Pacific Northwest

The ability of several weed species to serve as hosts for tobacco rattle virus (TKV), the causal agent of corky ringspot disease of potato (CRS), and its nematode vector,Paratrichodorus allius, was investigated in greenhouse studies. ViruliferousP. allius multiplied on 24 out of 37 weed species tested, indicating they were suitable hosts of the vector. However, only 11 of these weeds were infected with TRV, as determined by ELISA. The nonhost status of a given weed species was not changed whether the viruliferous vector population originated from CRS problem fields in WA, OR, or ID. Several weeds served as hosts for the vector and virus including kochia, prickly lettuce, henbit, nightshade species (black, hairy, and cutleaf), common chickweed, and annual sowthistle. Virus-freeP. allius acquired TRV from the three nightshade species, volunteer potato grown from TRV-infected tubers, and prickly lettuce, and subsequently transmitted the virus to ‘Samsun NN’ tobacco indicator plants. Thus, some weeds may play a role in the epidemiology of CRS by perpetuating TRV and its vector in a problem field.ResumenLa capacidad de diversas especies de malezas para servir como hospedante del virus rattle del tabaco (TRV), agente causal de la enfermedad de papa conocida como anillo corchoso (CRS) y de su vector, el nematodoParatrichodorus allius ha sido investigada en estudios de invernadero. ElP. allius se multiplicó en 24 de las 37 especies de malezas probadas, lo cual indica que son hospedantes apropiados del vector. Sin embargo, solamente 11 de estas malezas se infectaron con TRV de acuerdo a la determinación por el método ELISA. La condición de no hospedante de una especie dada de maleza no cambió aún en el caso de que la población del vector virulífero se hubiera originado en campos con problema de CRS de WA, OR, o ID. Diferentes malezas sirvieron como hospedante del vector y del virus incluyendo cochia-alfalfa, escarola, henbit, especies de hierba mora (negra, pilosa y de hoja cortada), hierba pajarera común y lechuguilla silvestre anual.P. allius libre de virus adquirió el TRV de las tres especies de hierba mora, de las plantas voluntarias de papa provenientes de tubérculos infectados y de la escarola y subsequentemente transmitió el virus a las plantas indicadoras de tabaco Sansum NN’. Es así como algunas malezas pueden jugar un rol en la epidemiología del CRS, perpetuando el TRV y su vector en un campo problema.

Pathogenicity of Washington and Oregon isolates of tobacco rattle virus on potato

Soil samples from corky ringspot (CRS) problem fields of potato in the states of Washington and Oregon were collected and planted withNicotiana tabacum ‘Samsun NN’ tobacco to bait tobacco rattle virus (TRV) and to increaseParatrichodorus allius populations, the vector of TRV. Pathogenicity of three isolates of TRV was assessed on Russet Burbank and Russet Norkotah plants usingP. allius as the vector. The most severe CRS tuber symptoms were with TRV from Pasco, WA, followed by Umatilla, OR, and Mattawa, WA, indicating a distinct variation in virulence among virus isolates. The lowest number ofP. allius that transmitted TRV resulting in CRS symptoms on Russet Norkotah was three nematodes per 250 cm3 soil. When potato plants were exposed toP. allius at different times in their growth, tubers on older plants were more resistant than younger tubers to CRS. Severity of CRS tuber symptoms was correlated with age of potato plants in pot culture. Reproduction ofP. allius on potato roots did not influence the incidence and severity of tubers symptoms. Under similar conditions, Bintje, a variety known to be resistant to CRS, served as a host forP. allius, yet remained asymptomatic. The protocol employed in these studies is appropriate for testing potato germplasm for resistance to CRS.

Distribution of tobacco rattle virus in tubers of resistant and susceptible potatoes and systemic movement of virus into daughter plants

Fifty-nine potato cultivars or breeding clones were planted near Umatilla, OR and/or Pasco, WA, in fields known to be infested with tobacco rattle virus (TRV) and vector nematodes,Paratrichodorus allius Jen. (Sid.). Tubers from these field plots were cut and examined for corky ringspot (CRS) symptoms. Reverse transcription-polymerase chain reaction (RT-PCR) for TRV was conducted on tissue samples from symptomatic and asymptomatic tubers. Sixty-five percent of the symptomatic and 42% of the asymptomatic tissue samples from CRS symptomatic tubers contained detectable TRV. Approximately 2% of plants grown from either symptomatic or asymptomatic tubers contained TRV when tested by ELISA, whereas 20% and 12% of plants grown from symptomatic and asymptomatic tubers, respectively, were positive for TRV by RT-PCR. These results suggest that RT-PCR is a more sensitive assay for detection of TRV. Systemic infections by TRV were detected more often in foliage of CRS-susceptible genotypes. Daughter tubers exhibiting symptoms of CRS, and which contained RT-PCR-detectable TRV, were produced on plants of three genotypes, including one from an asymptomatic parent tuber.

Overwintering and monitoring of potato leafroll virus in some wild crucifers

A potato leafroll virus (PLRV) isolate has been successfully transmitted to and recovered from two wild crucifers,Sisymbrium altissimum L. (Jim Hill or tumble mustard) andCapsella. bursa-pastoris (L.) Medic. (shepherd’s purse) by the green peach aphid (GPA),Myzus persicae (Sulzer). Virus antigen in both plant species was found to be higher in root tissue than in foliar tissue, based on enzyme-linked immunosorbent assay (ELISA) determinations.C. bursa-pastoris was apparently a relatively poorer source of inoculum for the GPA thanS. altissimum. Using two geographically-separated biotypes ofC. bursa-pastoris, a Washington biotype was found to contain higher antigen titer in both leaf and root tissue than a California biotype, as determined by ELISA. Field studies demonstrated that both weed species can serve as overwintering sources of PLRVCompendioUn aislamiento del virus del enrollamiento de la hoja de la papa (PLRV) ha sido exitosamente transmitido y recobrado de dos crucífieras silvestres,Sisymbrium altissimum L. (Jim Hill o mostaza postrada)y Capsella bursa-pastoris (L.) Medic, (boisa de pastor) por el áfido verde del melocotonero (GPA),Myzus persicae (Sulzer). Basándose en determinaciones efectuadas con la prueba inmunológica ELISA se encontró que el antígeno del virus en ambas especies vegetales era más alto en los tejidos de las raíces que en los de las hojas.C. bursa-pastoris fue aparentemente una fuente más o menos pobre de inóculo para el GPA, en comparación conS. altissimum. Utilizando dos biotipos geográficamente separados deC. bursa-pastoris, se encontró que un biotipo de Washington tenía, tanto en los tejidos de la hoja como en los de la raíz, una concentratión más alta de antígeno que un biotipo de California, tal como quedó comprobado con la técnica ELISA. Estudios de campo demostraron que ambas malezas pueden servir como fuentes de invernacion del PLRV.

Impact of wheat and corn as rotational crops on corky ringspot disease of Russet Norkotah potato

Tobacco rattle virus (TRV), the causal agent of corky ringspot disease of potato (CRS), was detected by reverse transcription-polymerase chain reaction (RT-PCR) in root tissues of field- and greenhouse-grown wheat (two) and corn (10) cultivars. TRV was also detected in some of the same samples by enzyme-linked immunosorbent assay (ELISA), but in a low frequency. Compared to ‘Samsun NN’ tobacco, wheat and corn appeared to be poorer hosts of TRV. However, these crops served as inoculum reservoirs of TRV. A nonviruliferous population of the nematodeParatrichodorus allius, the vector of TRV, acquired the virus from infected wheat and corn roots, and transmitted it to potato. ‘Russet Norkotah’ potatoes grown following TRV-infected corn and wheat, were blemished with arcs, concentric rings and diffuse browning, characteristic symptoms of CRS. The observations presented here suggest that wheat and corn as rotational crops will sustain TRV in a CRS problem field and the virus may cause severe damage on subsequent potato crops.ResumenEl virus “rattle” del tabaco (TRV en inglés), agente causal de la enfermedad de la papa denominada mancha de anillo corchosa (CRS), fue detectado por transcriptión inversa de la reacción en cadena de la polimerasa (RT-PCR) en el tejido de las raíces de dos cultivares de trigo y diez cultivares de maíz cultivados en campo y en invernadero. El TRV también fue detectado en algunas de las mismas muestras mediante el ensayo de inmunoabsorción con conjugados enzimáticos (ELISA), pero en baja frecuencia. Comparado con el tabaco de la variedad “Samsun NN”, el trigo y el maíz parecen ser hospederos más precarios de TRV. Sin embargo, esos cultivos sirvieron como reservorios de inóculos de TRV. Una población no virulenta del nematodoParatrichodorus allius, el vector del TRV, adquirió el virus de las raíces infectadas de trigo y maiz, y lo transmitió a la papa. Las papas de la variedad “Russet Norkotah” que crecieron a partir de la infestatión del TRV al maíz y trigo, estuvieron manchadas con arcos, anillos concéntricos y presentaron color marrón difuso, sintomas característicos de CRS. Las observaciones que aquí se presentan sugieren que el trigo y el maíz como cultivos de rotación mantendrán el TRV como un problema de campo, pudiendo este virus causar daños severos en los subsiguientes cultivos de papa.